Abstracts 09 - 6th AGC Lithosphere Dynamics & Evolution of Continental Crust Canberra 1983

Geological Society of Australia

ABSTRACTS Number 9

LITHOSPHERE DYNAMICS AND EVOLUTION OF CONTINENTAL CRUST Sixth Australian Geological Convention Canberra 1983

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GEOLOGICAL SOCIETY OF AUSTRALIA ABSTRACTS SERIES NUMBER 9

SIXTH AUSTRALIAN GEOLOGICAL CONVENTION CANBERRA, 1983

THEME:

Lithosphere Dynamics and Evolution of Continental Crust

ii

TABLE OF CONTENTS Symposium

Title

Page

Convention Lectures

1

1(a)

Recent plate movements and deformations

Kb)

Lithosphere transect studies of the

in the Australasian region

5

Australian continent

9

1(c)

Evolution of fold belts

23

1(d)

Metamorphism and geothermal gradients

49

1(e)

Palaeomagnetism and lithosphere dynamics

77

1(f)

Evolution of Precambrian terrains

87

Kg)

Tectonics of cratons and platform covers

103

Kh)

Continental margin evolution

121

Ki)

Oceanic lithosphere studies around Australia

133

2(a)

Intraplate igneous activity in Australasia

135

2(b)

Magmatism and crustal evolution (oceanic lithosphere, ophiolites, island arcs, fold belts)

141

2(c)

Chemical and isotopic constraints on the

3(a)

Palaeoenvironments and lithosphere dynamics

evolution of continental crust

167

(geomorphic-depositional, bathymetric, climatic)

195

3(b)

The surficial mantle of cratons

213

3(c)

Lithosphere dynamics, biogeography and

3(d)

The geological time-scale:

faunal and floral provinciality

219

biostratigraphy,

magnetostratigraphy and geochronology

233

4(a)

Metallogeny and crustal evolution

251

4(b)

Lithosphere dynamics and the accumulation of coal, oil shale and fluid hydrocarbons

277

4(c)

Kimberlites and carbonatites

281

SGI

Structure, tectonics and ore genesis at Cobar, NSW

301

SG2

Man as a geological agent

319

SG3

Evolution and biogeography of early vertebrates

SG4

SG5

Analysis and management of multivariate data in geology

325

Geological conservation in Australia

335

CONVENTION LECTURES

THE INTERNATIONAL LITHOSPHERE PROGRAM R.A. Price Geological Survey of Canada^ Ottawa.

No Abstract provided

PROGRESS AND PROSPECT IN SCIENTIFIC OCEAN DRILLING E. L. Winterer Scripps Institution of Oceanography, La Jolla, California, USA The extraordinary growth during the past 15 years in our understanding of the Earth is in no small measure due to results flowing directly from drilling by Glomar Challenger, and neither the rate of growth or our science nor the role that scientific ocean drilling plays in helping place that growth show any sign of diminishing.

Recent Challenger results include: 1) Sampling young oceanic crust to a depth of more than one kilometer, well within a sheeted dike complex; 2) Penetration into the zone of decollement separating the downgoing oceanic slab from the overriding plate, off Barbados; 3) Sampling of Triassic salt and Early Jurassic platform carbonates deep on the Atlantic margin off Morocco; 4) Successful implanting in the oceanic crust of a long-term advanced seismic station; 5) Documenting of the Cretaceous rift-stage evolution of a passive margin off western France; 6) Establishing the non-accreting, erosional, nature of the active margin off Central America; 7) Documenting, in the exquisite detail available in the Hydraulic Piston Corer, the spatial and temporal variations in Cenozoic productivity, dissolution and deep-sea erosional processes in the equatorial Pacific.

Looking toward the future, an international Conference on Scientific Ocean Drilling (COSOD) held in Austin, Texas, in November, 1981, to examine the question of how ocean drilling and associated programs can be organized to attack the most important scientific problems, recognized that a world-wide program of drilling is an essential component of research in the earth sciences and recommended a long-term continuance of scientific drilling, using the best new technology for drilling, for geophysical surveys and for downhole instrumentation. The COSOD report emphasized that the integration of continental with marine geology should progress through scientific ocean drilling, for example through geophysical and drill-site transects across well-chosen continental margins. This is but one of the many ties between ocean drilling and the International Lithosphere Program.

The Planning Cominittee for the Joint Oceanographic Institutions Deep Earth Sampling program (JOIDES), the international consortium that provides scientific guidance to the drilling program, has prepared a guideline document, designed to serve as a broad framework for the detailed scientific and logistical planning that will be required to carry out the basic scientific program for the first eight years of drilling beyond 1983. These plans, together with the COSOD recommendations, have received strong endorsement from the U.S. National Academy of Sciences and the U.S. National Science Board. Further action is required by the U.S. Office of Management of Budget, by the Congress and by each of the non-U.S. JOIDES member institutions to assure timely and adequate funding to carry out the recommended new program.

JOIDES seeks additional new partners, both to assist in carrying the financial load and to broaden the scientific and technical base of the program.

Scientific topics of high priority in the new program include: 1.

Origin, Evolution and Tectonics of Ocean Crust • character and composition of the deep portion of the ocean crust • mechanics of building of the volcanic pile that forms the upper part of the ocean crust • dimensions and characteristics of the hydrothermal systems on ridge crests, versus those on ridge flanks.

2.

Origin and Evolution of Marine Sedimentary Sequences • three dimensional anatomy and sedimentation history of gravity-driven (e.g., slides, debris flows, deep-sea fans) and deep ocean-current sedimentation systems (e.g., contourite drifts, mudwaves) • sedimentation in oxygen-deficient oceans • response of marine sedimentation to sea-level fluctuations • global mass balancing of sediments.

3.

Tectonic Evolution of Continental Margins • early rifting history of passive margins, and the origin of seaward-dipping seismic reflectors beneath the sedimentary cover • dynamics of forearc evolution, and the deformation and pore-water characteristics of sediments at accreting and erosional margins • space and time relations of forearc subduction, accretion and erosion; backarc spreading and island-arc volcanism.

4.

Long-term Changes in the Oceans, Atmosphere, Cryosphere, Biosphere and Magnetic Field • response of shallow and abyssal ocean circulation patterns to changing ocean boundaries and passageways • response of the atmosphere and oceans to variations in the earth's orbital parameters • patterns of evolution of marine organisms • detailed history of magnetic reversals and intensity changes during the past 200 m.y.

Many of the recommended scientific objectives can be accomplished with Glomar Challenger, but the extended capabilities of Explorer make it the preferred vessel. Conversion costs for Explorer are now estimated at about $75 to 90 million (to be borne by the U.S.).

Operating costs for Challenger and Explorer are estimated to be about the same (ca. $50 thousand/day).

Actual ship's schedules and tracks are yet to be decided upon, but discussions about these matters are underway in JOIDES. Drilling in the Australasian region has many attractions, and timely advice and participation of knowledgeable scientists from this region will be welcome.

4

Symposium 1(a) Recent plate movements and deformations in the Australasian region Convener: Professor K . Lambeck

RECENT CRUSTAL MOVEMENTS IN NEW ZEALAND R. Walcott Geophysics Division, DSIR PO Box 1320, Wellington, NZ. No Abstract provided.

SPACE GEODETIC TECHNIQUES FOR MONITORING TECTONIC DEFORMATION IN THE AUSTRALIAN REGION A. Stolz School of Surveying, University of New South Wales, Kensington, N.S.W.

In many parts of the world, including the Australian region, the predicted tectonic plate motions and deformations are such that they can be measured.

The classical terrestrial geodetic methods of triangulation, trilateration and levelling are capable of measuring relative positions with an accuracy of a few centimetres over distances in the range 100-200 km.

Longer intraplate and interplate measurements with compatible accuracy can only be made using space techniques such as laser ranging to satellites and very-longbaseline radio interferometry (VLBI).

It is also anticipated that in the near future signals from the satellites of the Global Positioning System can be used to measure relative positions over a thousand or so kilometres with an accuracy of a few centimetres using relatively inexpensive and highly mobile receivers.

The space geodetic measurement techniques will be reviewed.

Results of measurements from laser stations at Orroral in the east^and Yarragadee in the west of Australia will be described as will^ results obtained from VLBI measurements between sites at Tidbinbilla, Parkes, Sydney, Hobart and Alice Springs.

EARTHQUAKES, STRESSES, AND DEFORMATIONS OF THE INDO-AUSTRALIAN

PLATE

David Denham Bureau of Mineral Resources, Geology and Geophysics PO Box 378, Canberra City, ACT, 2601, Australia.

Stresses in the Indo-Austra1ian Plate cause deformation of the lithosphere and earthquake activity.

Current evidence from earthquake focal mechanisms, recent surface faulting, in-situ overcoring

and hydrofracturing measurements, and borehole deformation observations, indicate that a compressive regime is dominant - at least in the upper crust.

At present the earthquake activity appears to take place in poorly defined areas, rather than well delineated zones. There is no obvious correlation between the present regions of activity and the main geological features, except perhaps in Western Australia where there is a tendency for most earthquakes to be associated with craton boundaries. Of the 26 earthquakes for which focal mechanisms have been obtained, only two, which occurred at the edge of the continental shelf, were associated with normal faulting.

The majority (although there is some scatter in the azimuths of the pressure axes) indicate a predominantly east-west compressive stress acting throughout the crust.

The focal mechanism results are consistent with the faulting associated with the Meckering (1968), Calingiri (1970) and Cadoux (1979) earthquakes and also with the results of the shallow overcoring measurements taken in the South West Seismic Zone (SWSZ) of Western Australia.

The stress levels observed in the overcoring measurements are very high (greater than 20MPa at 5-lOm) in the SWSZ and the directions of the maximum principal stress axes are parallel to the pressure axes obtained from the earthquake focal mechanisms.

The borehole deformation measurements made in oil wells off the northwest coast confirm a similar stress field with high compressive forces being in evidence.

Although there could be several factors causing the high intraplate stresses it appears that the interaction with subduction and collision zones along the northern margin of the plate may be the most significant factor.

In this scenario the differences in the boundary conditions between a normal subduction zone and a continent/continent collision will induce stresses within the plate that cause deformations and earthquakes.

EROSION-INDUCED UPLIFT AND STRESSES IN SOUTHEASTERN AUSTRALIA Randell Stephenson Research School of Earth Sciences, Australian National University, Canberra ACT. The uplift of southeastern Australia is investigated as the isostatic response to post-tectonic erosion of Lachlan Fold Belt topography. The continental lithosphere is modelled as a linear viscoelastic rheoid with effective parameters D and T characterizing its elastic and viscous behaviour respectively. Erosion rate of topography is assumed to be proportional to topographic height, the

proportionality factor being the reciprocal of the erosion time constant O. The input data to the models are present-day topographic heights.

The comparison of uplift observations with models resolves values of the effective parameters D, T, and O in accord with those found independently elsewhere. Other predictions, presented as functions of time and geographic locality, include rates of uplift and erosion, paleotopography and uplift, gravity anomalies and stresses. Upper crustal erosion-induced stresses in southeastern Australia may be as large as 180 MNm"^ (1.8 kbar). Cenozoic extrusive rocks tend to occur where these stresses are tensile whereas seismicity tends to occur in regions of predicted compressive stress.

RECENT CRUSTAL MOVEMENT FROM REPEAT SURVEYING Peter Wellman Bureau of Mineral Resources, P.O. Box 378, Canberra City A.C.T. 2601

The rate and style of recent crustal movements can be measured by repeat horizontal or vertical geodetic surveys, or from the amount of Pleistocene crustal movement. In southwestern Australia repeat horizontal and vertical geodetic su)-veys suggest widespread crustal movement within theSouthwest Seismic Zone and in the adjacent area.

There is good evidence that the movement is concentrated along several narrow zones of weakness. In the Sim^pson Desert a line of magnetude 6 earthquakes is associated with a 20 m vertical displacement of a Pliocene pediplain. The displacement is restricted to a zone 300 km long and 50 km wide. In eastern Australia, in the Canberra and Brisbane areas, repeat horizontal surveys suggest that shear strain is of uniform magnetude over the areas investigated with a rate of about 50 10""9/yr. There is no evidence that this (elastic) strain is concentrated along zones of weakness.

8

Symposium 1(b) Lithosphere transect studies of the Australian continent Convener: Professor R.W.R. Rutland

A TRANSECT OF THE SOUTHERN CANADIAN CORDILLERA Raymond A., Price^ R.M. Clowes, R. Currie, T. Hoy, R.D. Hyndman J.W.H. Monger, V.A. Preto, R.D. Riddihough, P.S. Simony, J.O. Wheeler, G.J. Woodsworth and C.J. Yorath ^Geological Survey of Canada, 601 Booth Street, ^ttawa, Ontario KIA 0E8, Canada The Canadian B2 Transect Team The transect extends from the Alberta plains to the Juan de Fuca plate.

In the Rocky Mountain Belt (RMB) a continental terrace wedge of Proterozoic to Upper Jurassic rift and passive margin deposits and an Upper Jurassic to Paleogene cordillera-derived molasse wedge have been detached from their basement and displaced eastwards up to more than 200 km on imbricate listric thrust faults.

The Omineca Crystalline Belt (OCB) west of it is characterized by mid-Mesozoic to early Teritary polyphase deformation, metamorphism and intrusion, superimposed on the zone of overlap between upper Paleozoic to mid-Mesozoic allochthonous terranes and the distal part of the ancestral continental terrace wedge.

The Intermontane Belt (Int.B) comprises mainly a lower Mesozoic allochthonous magmatic arc-subduction complex, parts of which can be traced into the OCB, Mesozoic granites and mid-Cretaceous, Eocene and Neogene continental volcanics.

Westwards, across the right-lateral Fraser River Fault Zone lies the Coast Plutonic Complex (CPC), dominated by late Mesozoic and early Tertiary granitic intrusions emplaced in the east into lower Mesozoic oceanic strata and upper Mesozoic arc-related clastic rocks, and in the west into strata similar to those of the Insular Belt (Ins. B.) and coeval volcanic rocks.

Neogene and Holocene volcanic and intrusive rocks represent the northern extension of the Cascade volcanic chain.

The Ins. B. comprises the late Paleozoic and early Mesozoic arc:rift:arc sequence of an exotic terrane-Wrangellia, and, farthest west, a late Mesozoic accretionary prism possibly related to magmatism of the CPC. Offshore structures reflect subduction of the Juan de Fuca plate beneath Vancouver Island. Depth to Moho ranges from 40 km under the plains through 50 km under the western RMB to 36 km under the Int. B. and 30 km or less under the CPC and Ins.B.

SEISMIC FEATURES OF THE LITHOSPHERE UNDER THE CENTRAL EROMANGA BASIN — A CONTRAST WITH ADJACENT TECTONIC PROVINCES. D.M.Finlayson & S.P.Mathur Bureau of Mineral Resources, Geology & Geophysics, Canberra.

MAGSAT observations at heights of 400 to 450 km and regional gravity anomalies serve to distinguish the Eromanga Basin region from its neighbouring tectonic provinces. Seismic refraction and coincident seismic reflection profiling data from a 300 km traverse across the central part of the Basin quantify some of the features \(fhich contrast this region with the North Australian Craton to the northwest, the Lachlan Fold Belt to the southeast and the Anakie Inlier to the northeast.

10

Seismic refraction record sections indicate strong seismic velocity gradients at basement depths (about 2 km) and at a mid-crustal horizon (a Conrad discontinuity?), and lesser velocity gradients at the crust/mantle boundary (Moho) and in the sub-crustal lithosphere.

The P~wave velocity in the basement increases from 5*0-5*4 km/s at about 2 km depth to about 5.9 km/s at 5-7 km, and at greater depths does not exceed 6.1 km/s. Velocity decreases are evident at depths below about 9 km and these are interpreted in terms of the alpha-beta quartz transition at high temperatures (Kern, 1982).

Strong super-critical reflections evident as second arrivals at distances less than 100 km are interpreted as coming from a prominent velocity gradient at depths of 19-26 km. This mid-crustal horizon is in contrast to mid-crustal features from some other geological provinces of Australia. In other continents it has been referred to as the Conrad discontinuity.

The Moho deepens from about 36 km in the west to about 41 km in the east with the sub-crustal lithosphere having a velocity of 8.15+0.04 km/s. Energy returning after the first arrivals beyond 220 km indicates that there is a velocity gradient at depths of about 55-60 km, possibly with a low-velocity zone above it.

The seismic reflection section along the western half of the traverse can be divided into four 2-way reflection-time zones based or! the character of events. Between 0 and 2 s there are fairly uniform, coherent and continuous events which correlate with the Late Palaeozoic and Mesozoic sediments. The zone from 2 to 7 s is relatively free of primary reflections although a few multiples of the stronger sedimentary reflections exist. The absence of reflections in this zone suggests that the upper crust (6-20 km) is seismically homogeneous and may represent crystalline igneous rocks or highly deformed metasedimentary/metavolconic rocks.

Relatively low resistivity values for basement rocks favour the latter rock type. Without any recognizable reflections or diffraction patterns in this zone, it is difficult to say whether faulting observed in the sediments extends deep into the crust.

The zone from 7 to 13 s shows numerous short, discontinuous reflection events with variable amplitude and varying, sometimes cross-cutting, dips. Model studies made elsewhere suggest that such events can be produced by laminae of alternating high and low velocity, the layer thicknesses being less than the seismic wavelengths (about 100 m). The reflection-free zone below 13 s corresponds to the upper mantle, which therefore seems to be free of acoustic impedence contrasts. The lower crust with velocities of 6.3-7.0 km/s and numerous reflection segments could consist of finely interlayered rocks of intermediate composition.

The Moho under the Canaway Ridge is at a depth of about 34 km and is characterised by a sharp velocity increase. There does not appear to be a mid-crustal velocity discontinuity under the Ridge. Thus the Ridge could be a major horst at least to the depth of the Moho but it also has an intra-crustal structure different from the surrounding region. No deep reflection data is available yet from the Ridge area.

The crustal thickness of the region is less than that of the North Australian Craton (50-55 km) (Finlayson, 1982) and of the Lachlan Fold

11

Belt (43-50 km) (Finlayson & McCracken, 1981) but is similar to that interpreted for the Drummond (Cull & Riesz, 1972) and Bowen Basins to the east (Collins, 1978; Leven, 1980), The distinct velocity gradients of the lithosphere under the central Eromanga Basin are in in contrast to the gradational velocity increases interpreted for the North Australian and Lachlan provinces. The reflection characteristics of the deep crust in the Eromanga region are significantly different from those of the North Australian Craton, the Lachlan Fold Belt and the Anakie Inlier.

The crustal structure is consistent with a pericratonic or arc/ backarc basin being cratonised in an episode of convergent tectonics in the Late Proterozoic - Early Palaeozoic. The contrast in seismic reflection characteristics of the upper and lower crusts could indicate that the latter developed in a different tectonic environment.

REFERENCES COLLINS,C.D.N.,1978 - Crustal structure of the central Bowen Basin, Queensland. BMR J. Aust. Geol. Geophys., 3, 203-209.

CULL,J.P., & RIESZ,E.J.,1972 - Deep crustal seismic reflection/ refraction survey between Clermont and Charters Towers, Queensland 1971. Bur. Miner. Resour. Aust., Record 1972/97.

FINLAYS0N,D.M.,1982 - Seismic crustal structure of the Proterozoic North Australian Craton between Tennant Creek and Mount Isa. J. Geophys. Res., in press.

FINLAYSON,D.M., & McCracken,H.M.,1981 - Crustal structure under the Sydney Basin and Lachlan Fold Belt determined from explosion seismic studies. J. Geol. Soc. Aust., 28, 177-190.

KERN,H.,1982 - Elastic-wave velocity in crustal and mantle rocks at high pressure and temperature: the role of the high-low quartz transition and of dehydration reactions. Phys. Earth Planet. Int., 29, 12-23.

LEVEN,J.H.,1980 - The application af synthetic seismograms to the interpretation of crustal and upper mantle structure. PhD Thesis, Australian National University, Canberra.

MATHUR,S. ,1983 - Deep reflection studies in the central Eromanga Basin. Tectonophysics, in prep.

PROPOSED ACORP PROFILE ACROSS THE EASTERN TASMAN OROGEN: A PROFILE FROM COONABARABRAN TO PORT MACQUARIE Peter A. Cawood^, Evan C. Leitch^ and Erwin Scheibner^ ^Dept of Geology and Geophysics, University of Sydney and ^Geological Survey of New South Wales, Dept of Mineral Resources.

(Published with permission of the Secretary, NSW Dept of Mineral Resources) In order that deep seismic reflection profiling in Australia attracts financial support from a wide spectrum of funding agencies it is important that initial profiles yield results that (i) can be readily interpreted in terms of known upper crustal geology, (ii) make

12

important contributions to the understanding of major regional problems, (iii) are of general significance in terms of crustal evolution and (iv) have at least some implications for broad-scale resource assessment.

In addition profiles should traverse regions where earth scientists of differing expertise and approach, and from a variety of institutions, are currently active.

These prerequisites are well satisfied by a profile extending from the eastern margin of the Lachlan Fold Belt near Coonabarabran across the Gunnedah Basin and the southern part of the New England Fold Belt. We urge that serious consideration be given to traversing this profile, or a section of it, at an early stage in any ACORP programme.

As well as the strictly scientific advantages of the profile, outlined below, it is also favoured by possessing few logistic problems with a network of roads allowing an uninterrupted traverse approximately normal to the structural grain. The surface geology along the proposed profile is well known; most of it has been mapped on a scale of 1:100,000 and critical sections on scales of 1:10,000 and greater. In its western section where flat-lying strata. Quaternary cover and little relief limit the value of surface mapping, a seismic survey and drilling have provided good control on the nature of the shallow crust.

Recent detailed aeromagnetic surveys cover the eastern two-thirds of the profile but only regional gravity surveys have been published.

There is widespread agreement on the major upper crustal structural units that can be recognised along the profile (Fig. 1).

Neighbouring units show contrasts in depositional history, metamorphic and structural style and late- to post-orogenic igneous history.

Faults bound the units and the contrast across several of these, their minimum displacements, and their regional extent, indicate that they can be appropriately termed geosutures. The Mooki Thrust and the Peel Fault are clearly of major importance One aim of a seismic profile is to establish their shape and extent at depth; knowledge of this would restrict interpretation of their movement histories, aid in palinspastic reconstructions, and, in the case of the Mooki Thrust, may yield information on the extent and structure of that part of the hydrocarbon-bearing Gunnedah Basin hidden beneath the upper plate.

Nowhere is the contact between the Lachlan and New England Fold Belts exposed but quite clearly this occurs along the profile, for Lachlan rocks occur north of Coonabarabran.

East from here the Permian strata of the Gunnedah Basin rest on silicic and mafic volcanics of Late Carboniferous or earliest Permian age that extend at least as far east as the Mooki Thrust, the conventional western limit of the New England Fold Belt.

An extensive shelf existed in the western Gunnedah Basin throughout the Permian, and its eastern limit, or a major liiieament visible along Cox's Creek, could be the upper crustal manifestation of the contact between the two Fold Belts. This contact is probably marked by a change both in the character of the rocks and their attitude.

Profiling between Coonabarabran and the Mooki Thrust should indicate the disposition and thickness of the Late Palaeozoic volcanics that were extruded over much of what was to become the floor of the Sydney and Gunnedah basins; this information would constrain ideas on the formation of this basin system.

The depositional and early orogenic history of the Tamworth and Hastings units are very similar and the latter is probably a section of the former, moved north on a major transcurrent fault along which the Yarras Complex evolved.

These two blocks differ however, in that the Hastings Block contains abundant I-type granites whereas such

13

intrusions are rare in the Tamworth Belt.

Comparison o£ their deep crustal structure should reveal significant differences if generation of I-type granites is an important factor in crustal evolution.

Rocks of the Macdonald and Yarrowitch blocks are much more strongly deformed than those to the east and west, have undergone regional metamorphism of from sub-greenschist to amphibolite facies grade, and have been intruded by S-type granites. If the Peel Fault dips steeply at depth so that these blocks are autochthonous on a crustal scale, then their lower crustal levels probably contrast markedly with those of adjacent units.

Tectonic interpretations of the rocks along the profile are less unanimous than structural interpretations, in part because of the absence of information on Lachlan-New England relationships. However a wide body of opinion supports an arc - fore-arc basin - subduction complex scheme for the New England section, and we hold that it is one of the best and most complete examples of an accretionary convergent margin preserved in the geological record. The deep structure of such margins and their evolution as they are incorporated into continental crust are little known.

The eastern Tasman Orogen profile would contribute much to the understanding of these processes and thus of Phanerozoic continental growth in general.

Deep crustal structures of a very general kind were yielded by the LISPB programme across the Caledonian system in Britain, but otherwise deep crustal profiling of any detail is lacking across cratonised arc - fore-arc terrains.

WEST

L a c hIan

EAST C o x s Ck Linament

Mooki Thrust

\ ^^ \

7 ? ?

Peel Fault

Tamworth

Nowendoc Fault

Yarras Complex

Cowarra Fault

Yarrowitch

Macdonald

Hastings

10 0km Fig. 1:

Schematic cross-section from Coonabarabran (west) to Port Macquarie (east) showing major structural units of the upper crust.

14

PROPOSAL FOR A SEISMIC REFLECTION PROFILE OF TASMANIA E. Williams

1

& R.J.G. Lewis

2

2 ^Geological Survey, Mines Department, Hobart, Tasmania Department of Geology, University of Tasmania, Hobart, Tasmania INTRODUCTION The northern coastline of Tasmania is an ideal site for a seismic reflection traverse in South-East Australia. The proposed traverse extends from Marrawah [306468 - co-ordinates refer to 1:500 000 map grid] in the north-west to near Eddystone [613461] in the north-east via Beaconsfield [482441]. The traverse, which is some 350 km in length, intersects at approximately right angles all the major structural features of Tasmania (Williams, 1978). The northern coastline and its hinterland are well serviced by main and subsidiary roads, and have been mapped geologically at various selected scales ranging between 1:5000 and 1:20 000.

MAJOR GEOLOGICAL UNITS TRANSECTED AT THE NORTH COAST Arthur Lineament The western area of the extensive Precambrian Rocky Cape region [440360] of north-west Tasmania consists of a substantial unit of supermature orthoquartzitic sequences, separated by an 8 to 15 km wide belt of metamorphic rocks of the Arthur Lineament [393462] from. a thick unit of turbidite quartz-wacke successions to the east (Gee, 1977). Undoubtedly the metamorphic rocks of the Lineament, which are derived from the surrounding rocks, are of a fundamentally important deep crustal structure. However, its significance in terms of western Tasmanian geology is unknown.

Dial Range Trough The ?Eocambrian-Cambrian Dial Range Trough occurs between the Rocky Cape region to the west and the small Precambrian Forth region [427445] to the east. The Trough (Burns, 1965) is only 5 km wide at the north coast. To the south it extends into the sedimentary sequences of the meridional Dundas Trough [375370] and associated rocks of Eocambrian and Cambrian ages. The Dundas Trough deposits, into which was emplaced a large ultramafic mass, are associated to the east with a considerable pile of dominantly acid to intermediate volcanic material with accompanying economically important metalliferous ore deposits (Solomon and others, 1981). Crustal sutures if they existed within the Dundas Trough would continue northwards probably into the Dial Range Trough, and knowledge of their existence would be of fundamental importance in future evaluations of the geology of western Tasmania. An additional seismic traverse of some 50 km in length along newly constructed roads from the Arthur Lineament [345375] across the Dundas Trough to the eastern volcanic rocks [387375] would be of enormous benefit.

Tamar Fracture System In western Tasmania the ?Eocambrian-Cambrian Troughs were filled

by Early Ordovician to Early Devonian spread over most of the region.

15

There is an abrupt change in pre-Middle Devonian rock-types and structural characteristics resulting from Middle Devonian deformation between western Tasmania and the deeper water turbidite quartz-wacke sequences of north-eastern Tasmania, east of the Tamar River [500430].

The Tamar is probably the site of a fracture along which lateral movement brought the contrasting regions into juxtaposition (Williams, 1978). Information obtained from the proposed seismic profile would be vital in assessing the importance of the suggested Tamar fracture and its possible role in the development of Bass Basin to the north which is being actively explored for oil and gas. The southern extension of the postulated fracture may be examined buried beneath Permian and younger rocks in an additional west to east transect of some 100 km in length along good roads from the Central Plateau [480360] to Swansea [590335] on the east coast.

North-East Tasmanian Granites Results of a seismic reflection profile at the north coast would be of great benefit in determining the shape of Late Devonian granite masses in north-east Tasmania and would be of enormous help in exploring for associated minerals of economic importance.

GEOPHYSICAL INFORMATION Geophysical knowledge of the area indicates that deep structural information would be obtained from the proposed seismic reflection profile. The most relevant geophysical information to the proposal is a seismic refraction profile (Richardson, 1980), essentially coincident with the proposed traverse. The refraction profile shows that the depth of the mantle varies significantly and indicates that the Tamar Fracture System extends through the crust. The comparatively high resolution of the proposed reflection study should provide a considerable amount of additional information on the fundamental structures of Tasmania.

REFERENCES Bums, K.L. 1965. One mile geological map series.

Devonpor t. Explan.Rep.geol.Surv.Tasm.

K/55-6-29.

Gee, R.D. 1977. Geological atlas 1 mile series.

(8015N), Burnie. Explan.Rep.geol.Surv.Tasm.

Zone 7 sheet 28

Richardson, R.G. thesis.

1980.

University

Crustal Seismology. of

Unpublished Ph.D.

Tasmania.

Solomon, M. and others. 1981.. A special issue on the Geology and Mineral Deposits of Tasmania. Econ.geol. v 77, 2.

Williams, E. 1978. Tasman Fold Belt System in Tasmania.

Tectonophysics v 48, p. 159-205.

16

STRUCTURAL WESTERN

TRANSECTS

THROUGH

AUSTRALIAN

THE

SHIELD

M.J, Bickle & R.D, Gee Western Australian

LITSAC

Group

Carefully selected seismic reflection profiles, integrated with other data could provide information on a number of critical problems concerning early crustal evolution and structure. Two of these would be the three-dimensional shape of Archaean greenstone belts, and the geometry of Proterozoic mobile-belt boundaries with Archaean cratons.

THREE-DIMENSIONAL SHAPE OF ARCHAEAN GREENSTONES On available evidence there is conflict between geophysical and geological interpretation on the depths to which Archaean greenstones and within the vast sea of granitoid. Structural and stratigraphic evidence indicates depths in excess of 10 km, yet gravity profiling over greenstone suggest depths in the range of to 7 km. Although there remain uncertainties with the geological constructions, and the bulk densities are not precisely determined, this conflict seems real. The three-dimensional shape of greenstone belts therefore remains one of the outstanding enigmas of Archaean geology. A single profile across any greenstone belt in the Pilbara or Yilgarn Blocks would contribute to this problem.

SOUTHERN MARGIN OF THE PILBARA CRATON In the Western Australian Shield, the feature to stand out as most enigmatic, despite reasonable geological and geophysical control is the deep crustal nature of the southern margin of the buried Pilbara Block (i.e. the Pilbara Craton). The Pilbara Craton is eggshaped and its regional geology is now fairly well understood.

It had largely stabilised by 3.0 b.y. ago, and is overlain in its southern half by cratonic cover sediments in the Hamersley Basin of age about 2.7-2.5 b.y. During this latter time the Yilgarn Block was still going through its stabilisation process. The evolution of the Yilgarn and Pilbara cratons as separate entities, or indeed other Archaean nucleii in other Precambrian shields of the world, is still problematical. However it is evident that the Yilgarn and Pilbara cratons, and the intervening area, had their own characteristic crustal features.

The southern margin of the Pilbara Craton is expressed by (i) a major unconformity beneath Capricorn Orogen sediments (Wyloo Group); (ii) an arcuate zone of normal faults suggestive of isostatic rebound of the craton; (iii) distinct change in the Bouguer pattern, particularly the appearance of an arcuate high; (iv) a deep depositional trough (Ashborton Trough); (v) sudden thinning of the cratonic two-layer seismic crustal structure; and (vi) the appearance of a third lower crustal layer under the Capricorn Orogen.

GSWA are currently undertaking a major study of the Ashburton Trough. BMR have previously undertaken regional seismic refraction

17

and gravity studies of the Pilbara region, including one line that crosses the margin. Geological evidence to date indicates a crustal downwarp at the craton edge, rather than overthrusting or collision tectonics, however the possibility of low-angle thrusts cannot be ruled out.

A 200-km long traverse between Nanutarra and Tom Price would cross a declining metamorphic gradient, the synclinorial axis of the Ashburton Fold Belt, two unconformities, a major fault zone, two domes of Archaean basement and mildly deformed Hamersley Group.

This traverse would lie at 45° to the regional strike, but is confined to a wide sealed road. Such a traverse would be expected to contribute to (1) nature of Archaean crustal segments; (2) style of the earliest Proterozoic tectonism; (3) the depocentre of the Hamersley Basin; and (4) the potential for buried Rand-type sequences beneath the Hamersley Basin.

A TRANSECT THROUGH THE ANAKIE INLIER, BOWEN BASIN AND YARROL PROVINCE R.W. Day Geological Survey of Queensland, Brisbane Qld The Palaeozoic record in central Queensland contains repeated and superimposed episodes of marine and continental sedimentation, calc-alkaline volcanism, granitic to ultramafic intrusion, deformation, and metamorphism (Dickins & Malone, 1973; Murray & Kirkegaard, 1978; Day et.al., 1978).

Existing geological and geophysical data suggest that integrated investigations along a transect through this region would provide information fundamental to our understanding of the geological evolution of the eastern part of the Australian continent.

The accompanying figure shows the location of the proposed transect which is approximately 300 km in length.

From west to east the transect is planned to cross the Anakie Inlier and subcrops of the Drummond Basin of the eastern part of the Cambrian (?Precambrian) to Carboniferous Thomson Fold Belt, the Permian to Triassic Bowen Basin, the Tertiary Duaringa Basin, and the Connors Arch, Strathmuir Synclinorium, Gogango Overfolded Zone, and the Marlborough and Coastal Blocks of the Silurian to Permian Yarrol Province of the New England Fold Belt. The Thomson and New England Fold Belts display discordant structural trends and contain interpreted remnants of volcanic and island arcs, fore-arc basins, continental slopes and abyssal plains, and slices of oceanic crust and mantle. The relationship between the two fold belts is concealed by the Bowen Basin which is itself a tectonically complex foreland basin. Investigations along the proposed transect would test the validity of current structural and tectonic reconstructions and provide data applicable in coal, petroleum and mineral exploration.

Day, R.W., Murray, C.G., & Whitaker, W.G., 1978: The eastern part of the Tasman Orogenic Zone. Tectonophysics, 48, 327-64.

18

D i c k i n s , J . M . , & M a l o n e , E . J . , 1973: Geology of the Bowen B a s i n , Queensland. Bull.Bur.Min.Resour.Geol.Geophys.Aust., 130.

M u r r a y , C . G . , & Kirkegaard, A . G . , 1978: The Thomson Orogen of the Tasman Orogenic Zone. Tectonophysics, 4 8 , 299-325.

150°

STRATHMUIR SYNCLINORIUM

H

PROPOSED

TRANSECT

19

THE WAGGA-BATEMANS BAY GEOTRAVERSE M.J. Rickard & K.A.W. Crook Department of Geology, Australian National University, CANBERRA ACT

The four major tectonic zones recognised on this geotraverse from west to east are: 1. The Wagga Metamorphic Belt; 3. The Yass-Canberra zone; 2. The Tumut Trough; 4. The Monaro zone.

Zones 1 and 2, and 2 and 3 are separated by faults or ultramafics, and zones 3 and 4 by the boundary between S-type and I-type granites.

The Wagga Belt comprises mainly regionally metamorphosed Ordovician quartz-rich flysch intruded by extensive S-type granites.

Much of the flysch is of chlorite grade. Higher grade rocks generally occur as narrow zones associated with granite contacts. Metabasics, probably remnants of an Ordovician volcanic arc, extend along the eastern side of the belt. All the rocks are strongly deformed.

The Tumut Trough contains ophiolitic mafic volcanics and schistose amphibolites, overlain by Lower to Upper Silurian cherts, quartz-poor and quartz-rich greywackes and shales, succeeded by felsic volcanics and quartz-intermediate flysch derived therefrom.

The eastern and western sides of the trough are of contrasting facies - volcaniclastics with limestone blocks, and flysch respectively and the margins are faulted. The trough sequence may have accumulated in a deep rift of marginal sea character^ or as a fore-arc accretionary prism^. Sedimentation in the trough was teiminated by deformation and emplacement of small granite plutons in the Early Devonian. Lower Devonian ignimbrites and red paralic sediments lie unconfomably on the trough sequence.

The Young Batholith intrudes the eastern Coolac serpentinite belt and marks the boundary between zones with NW trends and zones with meridional trends. The Yass-Canberra zone is characterised by a horst and graben structure with extensive Silurian and Devonian terrestrial ignimbrites, volcaniclastics and shallow marine shelf sediments including carbonates. The Silurian sequences preserved in the separate grabens are somewhat different. The horst blocks expose deformed Middle Ordovician to Lower Silurian quartz-rich flysch and large S-type granite batholiths. Extensive granitic plutonism occurred in the Middle to Late Silurian (420 m.y. to 410 m.y.).

The junction between S- and I-type plutons approximately follows a zone of strong faulting (Late George Fault) and deeper water, Middle to Late Silurian sediments preserved in the Captain's Flat Graben and its northward extension in the Goulburn Synclinorium (Fig. 6). Some shallow water Silurian and Lower Devonian sediments occur further east associated with I-type felsic volcanics.

In the Monaro zone, low-grade highly deformed Middle to Upper Ordovician flysch predominates, but Upper Ordovician chert and melange units with occasional pillow lavas occur along the present coastline. Small I-type granite pluton clusters were intruded in the Early Devonian. The Monaro tectonic zone contains a major Late

20

Devonian graben (the Eden-Comerong-Yalwal Rift) filled with rhyolite, basalt and minor fluvial and lacustrine sediments, overlain by a red paralic to fluvial sequence. Similar Middle to Upper Devonian molassic sediments probably filled intramontane troughs westwards across the orogen; only remnants are now preserved.

The deformational geometry and history are similar for each zone, and are summarized in a general time-space plot. The Ordovician flysch is generally tightly folded into upright folds with a strong segregation, slaty or crenulation cleavage. Except locally in the Yass-Canberra zone, earlier, recumbent folds were also developed across the profile area; some have extensive (5 to 10 km) inverted limbs yielding downward-facing F folds^. These folds probably developed by gravity collapse in a colossal flysch wedge of BengalFan proportions^. Late F^ folds are associated in different places with granite intrusion, faults and deformation of the Late Devonian molasse. Silurian to Lower Devonian sediments are mildly deformed, with disharmonic behaviour against thick volcanic layers. A mild unconformity occurs between Lower and Middle Silurian in the YassCanberra zone (Quidongan Orogeny^), but the major deformation and upright folds in this zone probably accompanied Late Silurian granite emplacement. Middle Devonian folding is mild, increasing in intensity southwards. Similarly, post-Late Devonian folding is mild, except in the Comerong Rift where tight folds with a slaty cleavage are developed; this increases in intensity northwards^. Reactivation of 'basement' faults imposes kink-like crossfolds on the molassic sediments in the Wagga district.

Geophysical profiles show a gravity low over the Southern Highlands where the crust is 50 km thick. Bouguer anomalies rise towards the coast, as the crust thins towards the rifted continental margin. The aeromagnetics show distinct highs over the I-type (magnetite bearing) granites and the mafic and ultramafic rocks.

Three seismic refraction lines cross the profile corridor^. Two upper crustal low-velocity zones occur at 14 kms in the Wagga Belt and 20 kms beneath the Tumut Trough with the Moho at 40 kms. The third, reversed profile, shows two upper crustal low-velocity zones at 10 kms and 20 kms, beneath the Yass-Canberra and Monaro zones with a transition from crustal to mantle velocities extending from 35 to 50 kms. Chemical and isotopic studies® indicate that Late Proterozoic crust or sediments occur beneath the Ordovician flysch west of the S-I boundary. In spite of this evidence, there is little structural expression of this buried slab, and the pluton geometry and spacing shows little change across the S-I boundary^.

1.

Ashley, P.M. et

, 1979. Jl. geol. Soo. Aust.. 26^ 45-60.

2.

Crook, K.A.W. et

, 1980.

3.

Stauffer, M.R., & Rickard, M.J., 1966. Jl. geol. Soo. Aust. 13^

Jl. geol. Soo. Aust.y 27^ 215-32.

419-38.

4.

Cas, R.A.F.

5.

Crook, K.A.W. £t al., 1973.

, 1980. Jl. geol. Soo. Aust. 27y 19-32.

6.

Powell, C. McA. ^

, 1977.

7.

Finlayson, D.N.

, 1980. Phys. Earth Vianet. Inter. 21^

8.

9.

321-42.

White, A.J.R. ^ , 1976. Jl. geol. Soo. Aust.., 23^ 105-12.

Rickard, M.J. & Ward, P., 1981. Jl. geol. Soo. Aust., 28, 19-32.

Jl. geol. Soo. Aust., 20^ 113-38.

Jl. geol. Soo. Aust., 23^ 407-21.

21

A PROPOSED SEISMIC PROFILE AND GEOTRAVERSE ACROSS THE AMADEUS BASIN - ARUNTA COMPLEX M.W. McElhinney and others Division of Geosphysics, Bureau of Mineral Resources, Canberra No Abstract provided

22

Symposium 1(c) Evolution of fold belts Convener: Associate Professor C.McA. Powell

23

ACTIVE FORELAND FOLDS AND THRUSTS IN WESTERN PAPUA NEW GUINEA H.L. Davies Bureau of Mineral Resources, Canberra ACT

In western Papua New Guinea, continued convergence after Oligocene and earliest Miocene collisions has led to compressive fracturing of the crust, and thrusting, folding and uplift of supracrustal sediments. The locus of tectonic activity has migrated southwards, involving progressively younger strata, and currently coincides with the thrust belt which forms the southern slopes of the main cordillera (the Papuan Fold Belt of other authors). It is marked by crustal-level seismicity (the Southern Highlands seismic zone of Ripper and McCue, in press, BMR Journal) and by active thrusts and folds which have caused diversion of the Mubi River, disruption of the aprons of Kerewa, Sisa and Bosavi volcanoes, and horizontal displacement of mineralised intrusive rocks at the Futik prospect. Within the thrust belt,tectonic style varies according to thickness and character of the sedimentary column, from broad overthrust anticlines, with or without basement involvement, in regions of thick limestone cover (Darai Limestone), to overthrust strike ridges where the limestone is thinner, to saucer-like thrust synclines where limestone and shale are interbedded, and boat-like synclines where thin limestone overlies thick pelite.

THE BANDA ARC - SUMBA TO TIMOR - A REVIEW A.E. Grady, M.J. Abbott, F.H. Chamalaun and C.C. von der Borch. Institute for Australasian Geodynamics, Flinders University, Adelaide, S.A. The Banda Arc system is an arcuate structure connecting the Sunda Arc to the west, with a complicated structure associated with New Guinea and the Sula Spur to the northeast (subducting oceanic lithosphere combined with intense left-lateral strike-slip faulting).

Unlike the Sunda Arc, where oceanic lithosphere is being subducted beneath a volcanic arc, the Banda Arc is the site of a collision betv/een continental crust (Australian) and a volcanic island-arc subduction zone. This paper focuses on the western sector straddling the junction of the Sunda and Banda Arcs, and more particularly the islands of Sumba and Timor.

Much of the review involves comparing the development of Sumba with that of Timor, and comparing the volcanic petrology of the associated Inner Arc volcanics, such as Flores, due north of Sumba and that of Atauro and Wetar, due north of East Timor.

The Quaternary histories of Sumba and Timor are broadly similar, being dominated by fringing reef development, widespread

24

block faulting, and consistent uplift, which has been greater in Timor than in Suiriba. Volcanic activity has been continuous since the Miocene in western Flores, but ceased during the Miocene-Pliocene in Atauro and Wetar.

Earthquake seismicity of the region indicates a steep, north-dipping Benioff Zone which in broad terms is continuous across this sector. However, analysis of foci distribution could be interpreted to reveal a hitherto unrecognized lateral discontinuity in the Benioff Zone immediately east of Sumba, close to the change from subducting oceanic to continental lithosphere.

The Neogene development of this sector of the arc shows similarities and dissimilarities. Miocene sedimentation in Sumba and Timor shows basic similarities (lower Miocene reef limestones and upper Miocene pelagic marl-tuff sequences) with important differences (lower to middle Miocene submarine fan sequence in East Sumba, plus lower Miocene volcanics in Sumba, both of which are absent in Timor; uppermost Miocene-Pliocene palagic-turbidite sequence throughout Timor and absence of Pliocene sediments in Sumba; widespread occurrence of the Bobonaro Complex in Timor and its complete absence in Sumba).

A major dissimilarity at this time was in the intensity and style of deformation; open upright folding and possible block faulting in Sumba, in contrast with the widely reported reverse-fault imbrication and associated folding throughout Timor.

Volcanism in the Inner Arc apparently commenced in the Miocene. Flores is characterised by basaltic andesites and 2-pyroxene andesites (incompatible element-enriched arc-tholeiites to calcalkaline) with low 87s^/86sr ratios (c.a. 0.704). Atauro has calcalkaline basaltic-andesites, andesites and dacites with slightly higher ^'^Sr/^^Sr ratios (0.705 to 0.707). Wetar is distinguished by cordierite-bearing lavas and a more common appearance of rhyolites.

An extremely large range of ^''sr/^^Sr values (0.704 to 0.723) on Wetar is matched by marked co-variance of Nd isotope ratios, indicating interaction of mantle-derived components with continental sialic crust.

The Palaeogene of Sumba is characterised by localised deposits of volcanics (lavas and diamictites), widespread, generally intermediate composition igneous intrustions of probable Palaeocene age, and shallow water sediments of Eocene age. On the other hand, the Palaeogene of Timor consists of pelagic deep-sea sediments (interpreted autochthon), flysch type partly volcanoclastic sediments and volcanic rocks (lavas and diamictites) (interpreted allochthon) and scattered intermediate composition igneous intrusions in northwest Timor. Neither island contains any well established Oligocene rocks.

Throughout Sumba the Neogene sequence rests on a well developed uppermost Cretaceous-Palaeocene unconformity, whereas in Timor sedimentation seems to have been mainly continuous between the Cretaceous and Neogene. A significant unconformity does occur between meditmi to high grade metamorphic rocks and upper Cretaceous deep sea sediments within the interpreted allochthon on Timor.

The oldest known rocks on Sumba comprise a sequence of probable mid to late Cretaceous continent-derived submarine fan

25

arenites, shales and volcanic diamictites, very similar in many respects to the Palaeocene-Eocene Haulasi Formation of West Timor.

Timor contains extensive sequences of Mesozoic and Permian rocks plus widespread massifs of metamorphic rocks (greenschist to upper amphilolite facies) all of which are unknown from Sumba.

The results of palaeomagnetic work (not yet completed) indicate that Timor was in close proximity to Australia in the PermoTriassic and Tertiary and that some segments of its geology (specifically volcanic rocks on the northern coast and Cretaceous sediments near the south coast) have probably been subjected to significant (? local) tectonic rotations. Data from Sumba indicate that the island was part of an independent micro-continent (subjected to significant rotations) between the Palaeogene and upper Miocene, if not earlier, but they do not critically distinguish between possible origin from Sundaland or from Australia.

Although both islands are now in close proximity, and would appear to form a single tectonic unit, geological and geophysical data indicate that their histories were quite different prior to the Neogene. It is suggested that while Timor evolved as the leading edge of the Australian continental margin, Sumba has been a more or less independent micro-continental fragment, at least since the Jurassic.

INTERACTION DURING THE PLIOCENE BETWEEN THE AUSTRALIAN AND PACIFIC PLATES IN WESTERN IRIAN JAYA, INDONESIA D.B. Dow Bureau of Mineral Resources, Australia, Bandung, Indonesia Recent geological mapping done by the Indonesian Geological Research and Development Centre and the Australian Bureau of Mineral Resources in western Irian Jaya has provided important information on the results of late Tertiary convergence between the Australian continent and oceanic crust of the Pacific Plate.

As seen in Figure 1, Australian continental crust underlies the southern half of Irian Jaya and almost the whole of Kepala Burung, while north of the Sorong, Koor, and Yapen Fault zones the rocks are entirely ophiolites and island arc volcanics of the Pacific Plate.

All constituent formations of the Mesozoic and Tertiary platform succession have been mapped from the Australian continental block across southeastern Kepala Burung, so it is postulated that the Kepala Burung continental fragment has remained essentially in its present position since the late Palaeozoic. The distribution of the early Tertiary New Guinea Limestone group which was deposited without break along the

26

northern edge of the Australian continent from Papua New Guinea to the western extremity of Kapala Burung lends strong support to this postulate.

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Figure 1 : Major structural units of western Irian Jaya, Indonesia.

Compressive tectonic activity which started in the late Miocene folded, faulted, and overthrust the platform sediments and was accompanied by high

Figure 2 : Major structures in western Irian Jaya, showing postulated direction of maximum convergence between the Australian and Pacific Plates.

27

grade metamorphism in places. The resulting major structures, which in western Irian Jaya are highly transgressive to the general east-west structural grain of the rest of New Guinea, are explained as the result of convergence between the Australian and Pacific Plates. It is postulated that only convergence along a south-west-northeast direction can explain the known structures (Figure 2).

Such a direction of convergence is in broad agreement with that postulated in the recent Circum Pacific Plate Tectonic Map, though a more southerly vector is indicated by the present work.

SIGNIFICANCE AND AFFINITIES OF CONTINENTAL CRUST IN EAST INDONESIA Rab Sukamto

1

and D.S. Trail

?

Geological Research and Development Centre, Bandung, Indonesia Bureau of Mineral Resources Australia, Bandung, Indonesia 5

TALAUD

0 MOLUCCA

f&

Late

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Cainozoic

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pressure mefamorphics

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EAST INDONESIA Continental crust represented by sialic metamorphic rocks and Mesozoic clastic sediments is exposed in most of the large island groups of East Indonesia. The low-grade metamorphic rocks are Siluro-Devonian in western Irian Jaya Paleozoic in Misool and Buru-Seram, and Permo-Carboniferous in Banggai-Sula. In both Irian Jaya and Banggai-Sula they are intruded by Permo-Triassic tin-bearing granite, and on several islands the metamorphic basement is succeeded by a post-Triassic rift-drift sequenced Sumba is also believed to be a continental fragment.

In the paired arcs forming Sulawesi, Halmahera and the Banda Arc the volcanic arcs are Tertiary to Quaternary magmatic zones while the non-

28

volcanic arcs consist largely of strongly deformed sediments and associated ophiolitic rocks. However, Paleozoic and Mesozoic basement of continental origin is exposed in both the non-volcanic arcs in Timor, Kai, Seram, Obi, Bacan and Buton, and in the volcanic arc in Sulawesi.

Magmatism began in the Mid-Miocene in the inner Banda Arc, in the Oligocene in western Halmahera and in the Paleocene in western and northern Sulawesi; all three are still active. The outer Banda Arc is largely imbricated Paleozoic and Mesozoic sediments with ophiolites and Neogene olistostromes^, eastern Halmahera and Waigeo consist of highly deformed ophiolitic rocks associated with Jurassic-Cretaceous pelagic deposits and Tertiary sediments, and deformed sediments associated with ophiolite in eastern Sulawesi and the Talaud-Mayu ridge have been traced under the intervening Molucca Sea^. Buton Island, off southeast Sulawesi, has affinities with Timor and Seram in the outer Banda Arc^ .

Geoscientists working in the Banda Arc believe that the pre-Tertiary basement fragments separated from the northwest margin of Australian in Mid-Jurassic times^"^. However, they now exist in a very complex tectonic framework recently revealed by marine geophysical surveys. The Timor trough is underlain by continental crust up to 40 Km thick , and convergent movement between the Australia-India and Southeast Asia plates has been o taken up by compression and uplift in Timor .

A spoon-like lithospheric plate subducting beneath the Banda Basin and extending to a depth of 600 Km has been inferred from the distribution of earthquakes^, and the Banda Sea and Sawu Sea appear to cover a single oceanic plate of Early Cretaceous age on which the volcanic Banda Arc began to develop in mid-Miocene t i m e s w e s t of Flores, the Quaternary volcanoes of Lombok and Sumbawa also rest directly on oceanic crust^l.

Another plate is sinking southwards beneath Seram^, and the young acid volcanic and granitic rocks of Ambon are probably derived ^XQ^ partial melting of the continental crust over this subducting plate .

Beneath the Molucca Sea, an inverted U-form oceanic plate is overlain by highly deformed low density material about 15 Km thick, representing an arc-arc collision zone^. This deformed material has been thrust upwards over the aprons of the adjacent volcanic arcs and the Banggai-Sula continental fragment.

REFERENCES Pigram, C.J. & Panggabean, H., 6th Aust.Geol.Conv. (1983) Audley-Charles, M.G., Carter, D.J., Barber, A.J., Norvick, M.S. & Tjokrosapoetro, S. Jour.Geol.Soc.Lond. 136, 547-566 (1979) 3.

Silver, E.A. & Moore, J.C.

4.

Chamalaun, F.H.

5.

Bowin, C.O., Purdy, G.M., Johnston, C.R., Lawver, L., Hartono, H.M.S., & Jezek, P. Amer.Assoc. Petrol.Geol.Bull. 64, 868-915 (1980).

Johnston, C.R.

Jl.Geophys.Res. 83, 1681-1691 (1978).

Earth Plan.Sci.Lett. 34, 107-112 (1977).

Spec.Publ.Indon. R. & D. Centre 2, 199-216 (1981).

Jacobson, R.S., Shor, G.G., Kieckhefer, R.M. & Purdy, G.M. Assoc. Petrol.Geol.Mem. 29, 209-222 (1979).

Berry, R.F. & Grady, A.E. 171-181 (1981).

Amer.

Spec.Publ.Indon.Geol. R. & D. Centre, 2,

29

9.

Cardwell, R.K. & Isacks, B.L. 31-47 (1981).

10.

Larue, M., Harjoyo, H., Lapouille, A., Tachjudin, I., Indriastomo, J., & Pramuwijoyo, S. Sari Himp. Ahli Geofisika Indon. 7, 12 (1982).

11.

Foden, J.D. & Varne, R.

12.

Barber, A.J., Davies, H.L., Jezek, P.A., Hehuwat, F. & Silver, E.A. Spec.Publ.Indon.Geol. R. & D. Centre 2, 7-28 (1981).

152.

Spec.Publ.Indon.Geol, R.& D. Centre 2,

Spec.Publ.Indon.Geol.R & D. Centre 2, 135 -

THE CORDILLERAN FORELAND THRUST AND FOLD BELT OF SOUTHERN CANADA R.A. Price Geological Survey of Canada, 601 Booth Street Ottawa, Ontario KIA 0E8, Canada The foreland thrust and fold belt, a NE-tapering wedge, locally 20 Km thick, comprises miogeoclinal, platformal and foreland basin rocks that have been scraped off the under-riding North American plate, by an over-riding tectonic collage of foreign terranes that "collided" with and became attached to North America.

It is an accretionary prism that was tectonically prograded northeastward across the western margin of the North American craton in two major episodes.

The outboard part of the Cordilleran miogeocline (a passive margin, continental terrace wedge) was over-ridden by a tectonic collage of oceanic terranes, and descended to depths of 25 Km during mid-Jurassic prograde synkinematic regional metamorphism, before heating restored its latent buoyancy.

The ensuing Late Jurassic-Early Cretaceous compression within the miogeocline led to outward verging thrusting on either side of an central uplift and to tectonic loading and isostatic flexure of the lithosphere beneath the foreland basin where molasse accumulated. Mid-Cretaceous granitic plutons that rose through this tectonically thickened suture zone cut the outward verging structures on both sides of it. During the second episode accretion of another collage of oceanic terranes, outboard from the first, involved oblique, right-'lateral convergence.

Much of the Late Cretaceous-Paleocene overthrusting in the southern Canadian Rockies was transformed northward into right-lateral strike-slip along the Tintina-Northern Rocky Mountain Trench (T-NRMT) transform fault zone. Volcanism which occurred along the newly formed suture (Coast plutonic complex) during the northeasterly subduction of the Farallon plate, continued during the northerly subduction of the Kula plate; but convergence across the overthrust belt ended; and Early and Middle Eocene ductile crustal stretching occurred in the zone of en echelon overlap between the T-NRMT and the Eraser River fault zone. Late Eocene termination of this crustal stretching and transform faulting, may mark the return of the Farallon plate.

30 THE C A M B R I A N - O R D O V I C I A N OF L A N C E F I E L D , V I C T O R I A M A R G I N A L S E A , F O R E - A R C , O R C R U S T A L FRACTURES? A.H.M. VandenBerg G e o l o g i c a l Survey D i v i s i o n , D e p a r t m e n t of M i n e r a l s a n d E n e r g y , 140 Bourke S t r e e t , M e l b o u r n e , V i c t o r i a 3000 The C a m b r i a n m e t a b a s a l t s o f L a n c e f i e l d form the southern h a l f of the Heathcote 'Axis', and are the b e s t - k n o w n of a l l the V i c t o r i a n greenstone b e l t s in terms of s t r a t i g r a p h y .

Recent tectonic m o d e l s , b a s e d e n t i r e l y o n g e o c h e m i s t r y , h a v e p l a c e d it in e i t h e r a m a r g i n a l sea o r fore-arc s e t t i n g .

Both m o d e l s require that the greenstone is an ophiolite.

Detailed mapping has shown that the M o u n t William M e t a b a s a l t at L a n c e f i e l d has little in c o m m o n w i t h ophiolites: it is a simple m e t a b a s a l t sequence w i t h no interbedded c h e r t , no sheeted d y k e s , no gabbroic o r u l t r a m a f i c i n t r u s i v e s , a n d no s e r p e n t i n i t e s . The t o t a l thickness o f p o s t - g r e e n s t o n e sediments w e s t o f the Heathcote "Axis" i s , at m o s t 3 to 4 k m , only a s m a l l fraction of the t o t a l t h i c k n e s s of the present-day crust.

This a n d o t h e r evidence suggests that the g r e e n s t o n e is u n d e r l a i n b y P r e c a m b r i a n rock of c o n t i n e n t a l t y p e . It is suggested t h a t the greenstones w e r e e r u p t e d onto this crust v i a deep linear fractures.

T E C T O N I C D E V E L O P M E N T OF THE L A C H L A N F O L D B E L T AND C O N S T R U C T I O N OF T H E C O N T I N E N T A L C R U S T OF SOUTHEASTERN A U S T R A L I A Anthony J . Crawford D e p a r t m e n t of G e o l o g y , U n i v e r s i t y of M e l b o u r n e , P a r k v i l l e , V i c t o r i a , 3052 G e o c h e m i c a l and istotopic studies of L o w e r and Middle Palaeozoic S-type g r a n i t o i d s from the Eastern Highlands region of the L a c h l a n F o l d b e l t suggest the p r e s e n c e of a b l o c k of Proterozoic ("1400 m y old) sialic crust b e n e a t h this a r e a , p r o b a b l y at depths g r e a t e r than 2 0 k m . The M O H O b e n e a t h this region lies b e t w e e n 45 a n d 52km d e p t h and shallows to a r o u n d 35km in c e n t r a l V i c t o r i a , w h e r e seismic v e l o c i t i e s t y p i c a l of sialic c o n t i n e n t a l crust o c c u r from 2 0 - 3 5 k m . This evidence strongly suggests t h a t the L a c h l a n F o l d b e l t d e v e l o p e d u p o n c o n t i n e n t a l crust.

H o w e v e r , the occurrence of orogenic a n d e s i t e s , MORB-type tholeiites, 'ophiolite' and extensive flysch in Victoria and s o u t h e a s t e r n N e w South W a l e s imply t h a t the Lachlan F o l d b e l t e v o l v e d in a W e s t P a c i f i c - t y p e s e t t i n g , involving i s l a n d a r c s , b a c k a r c b a s i n s and subduction complexes.

A n a t t e m p t to solve this remarkable p a r a d o x has involved a d e t a i l e d p e t r o l o g i c a l - g e o c h e m i c a l study of the a p p a r a n t b a s e m e n t rocks t h e m s e l v e d , the o l d e s t rocks in m a i n l a n d southeastern A u s t r a l i a , the C a m b r i a n g r e e n s t o n e s in V i c t o r i a .

These o u t c r o p in three w i d e l y s e p a r a t e d , n a r r o w elongate b e l t s w h i c h strike p a r a l l e l to the N - N W s t r u c t u r a l g r a i n of the L a c h l a n F o l d b e l t . New data p e r t a i n i n g to the s t r a t i g r a p h i c relationships and affinities of the m e t a - v o l c a n i c s in these g r e e n s t o n e b e l t s are summarized in Figure 1 .

31

HEATHCOTE MT WELLINGTON GREENSTONE GREENSTONE BELT BELT

STAVELY GREENSTONE BELT Mt.Stavaly

Mt.Camel

Mt.Dryden

Ladys Pass

LIcola

Howqua

1km

Fault

L.Ordoviclan Quartz-ricf Flysch

MORB Tholelltic Lavas Sills,Hyaloclastltes

Pelagic

Low-TI

Sediments

(cherts,black

shales)

FossHlferous

Sediments

Middle

FIGURE

Cambrian

1; Stratigraphy

Lavas

•

Andesltes

^

Bonlnites

A r c L a v a s (mainly calc-alk. andesltes)

of Victorian Greenstone

belts

The most important features of these greenstone belts are:(i) (ii)

the occurrence of low-Ti, high-Mg lavas (ie boninites, boninitic andesite) in each belt, and the occurrence of MORB-type tholeiitic lavas conformably above the low-Ti lavas in the Heathcote and Mount Wellington Greenstone Belts.

A tectonic model is proposed to account for the temporal and spatial variation in magma compositions now represented by lavas in the greenstone belts. Attenuation and rifting of the eastern margin of the Australian craton about 800 my ago led to the opening of a mjor ocean basin. Westward subduction of oceanic crust at the western margin of this ocean basin commenced about 600 my ago and produced a typical West Pacific island arc-backarc basin setting.

Average subduction velocities are postulated to have been greater than the average half-spreading rate on the adjacent oceanic spreading centre, so that the spreading centre was subducted in the Lower Cambrian.

Diapirs associated with the subducted ridge provided the heat source necessary to generate low-Ti lavas from hydrous, shallow sub-backarc basin and sub-arc upper mantle. Partial melting accompanying continued ascent of diapirs derived from the subducted spreading centre generated abundant MORB tholeiites, which were erupted just after, and upon, the low-Ti lavas.

As a consequence of ridge subduction, progressively older crust was subducted, leading to an attenuated block of Proterozoic sialic crust (which rifted off the craton during the Late Proterozoic ocean opening) approaching the trench, and being subducted.

The attentuated continental crust choked the subduction zone and subsequent convergent motion was teken up by the over-riding, arc backarc basin

32

plate passively trusting over the Proterozoic sialic block.

This arc-continent collision is thought to have occurred in epi-Orodovocian times, producing the Benambran deformation; the underthrust sialic block produced crustal thickening and was the source of S-type granites across the Lachlan Foldbelt.

It is argued, therefore, that the Lachlan Foldbelt is essentially rootless, and may be regarded as a mega-allochthon.

Attempted subduction of continental crust may be a very important process in the construction of Phanerozoic foldbelts. This hypothesis reconciles the conflicting view regarding the nature of the substrate of the Lachlan Foldbelts.

TECTONIC RELATIONSHIP BETWEEN LATE ORDOVICIAN AND LATE SILURIAN PALAEOGEOGRAPHIES OF THE LACHLAN FOLD BELT C.McA. Powell School of Earth Sciences, Macquarie University, North Ryde, NSW

The Late Ordovician palaeogeography of the Lachlan Fold Belt was an island-arc and marginal-sea system similar to the present-day Andaman Sea region.

The N- to NW-trending andesitic volcanic arc in eastern New South Wales and Victoria (= Andaman/Nicobar island chain and submerged volcanic seamounts) was separated from the cratonic land mass to the west (= Malaysian Peninsula) by a youthful marginal sea (= Andaman Basin), and faced an oceanic realm to the east (= Bay of Bengal).

Basaltic andesite and associated volcaniclastics, together with calcareous deposits, were derived from within this fold belt, but the main sediment source was quartzose clastics from land masses to the south present-day Irrawaddy and Ganges/Brahmaputra sediment inputs to the Andaman system.

This configuration developed by Middle Ordovician 480 Ma) and lasted until Early Silurian 435 Ma).

In the mid-Silurian (425 ± 5 Ma) there was widespread deformation with development of the NNW-trending Wagga Metamorphic Belt over the site of the former marginal sea, and by Late Silurian (420 Ma) the second palaeogeographic configuration was established.

A horst and graben system similar to the Basin and Range terrain of western U.S.A. and Mexico developed east of a line from Cobar to Sale, with a foreland basin onlapping the exposed older Australian continent to the west.

The Cowra, Tumut and Hill End Troughs and Molong and Capertee Highs were part of the eastern horst and graben system, in which individual meridional grabens trended 20° oblique to the NNW-trending western bounding fault, suggesting dextral shear during their formation.

Silicic volcanics and associated clastics of both terrestrial and marine fades and minor, but important, high-K mafic extrusions were widespread throughout the eastern horst and graben provinces.

Extensive limestones reflected the low palaeolatitude.

In the western foreland basin, a quartzose clastic wedge prograded from the southwest so that Early Devonian deep-marine conditions near Cobar were replaced by fluvial environments by the Middle Devonian.

33

By comparing the late Cenozoic evolution of the Andaman Basin with the distribution and orientation of facies, structure and magmatic belts in the Lachlan Fold Belt, possible explanations for a number of hitherto puzzling features arise.

1.

The Early Ordovician metamorphism and plutonism in western Victoria might be the equivalent of the buried Mergui Ridge in the Andaman Basin, which is arguably the pre-Middle Miocene magmatic arc prior to the younger sea-floor spreading.

2. East of this western Victorian metamorphic belt, the Wagga Basin might have had an oceanic basement similar to the mid-Miocene to Recent ocean floor in the western half of the Andaman Basin.

3. By matching the positions of the Late Ordovician volcanic arc and the Andaman-Nicobar chain, the mid-Silurian Wagga Metamorphic Belt lies over the zone of high heat flow and submerged volcanic seamounts in the Andaman analogue.

4.

The early latitudinal fold trends in the Lachlan Fold Belt may be explained by various aspects of the geometry of oblique divergence in the Wagga Basin and oblique convergence in the accretionary prism, all being accompanied by dextral shear in the magmatic arc.

5.

The ophiolitic basement to the Tumut Trough was formed by local asthenosphere rising in a small pull-apart basin opened during regional dextral shear.

6.

The I-S line in eastern NSW corresponds with the leading edge of the Late Ordovician island arc which might well have incorporated a thin strip of older continental crust split off the Gondwanan continent during its formation.

In addition, the Late Ordovician island arc might have been underthrust by an old oceanic plateau in latest Ordovician or Early Silurian.

All these features are explicable in terms of modern island-arc and marginal—sea evolution, and account for the change from a largely ensimatic character to the Lachlan Fold Belt in the Late Ordovician to an ensialic character by Late Silurian.

EVOLUTION OF THE LACHLAN FOLD BELT IN NEW SOUTH WALES: AN ALTERNATIVE INTERPRETATION A.N. Yeates Bureau of Mineral Resources, Canberra Ordovician pelagic sedimentary sequences, with mixed hydrothermally-deposited and detrital constituents, are interspersed with These rocks constitute sparser basaltic to andes itic lava piles. the oldest of those known in the NSW portion of the Lachlan Fold Belt.

Each of these lava provinces has its own uniqueness (Crook & Felton, 1975) and most did not further evolve notable acid constituents in later times.

34

Many new volcanic rises developed in Silurian and Early Devonian times when deep vertical fracturing occurred, especially in the east.

Adjacent to each rise, deposits accumulated in fast-subsiding troughs.

This subsidence compensated for uplift and growth of the rises.

Variable, and especially shallower water depths, and eruption of variably andesitic to rhyolitic lavas and characteristic pyroclastics, produced a much greater diversity of rock types than in Ordovician times.

The isotopic systems of rising granitoids equilibrated when they became extinct, mainly in Silurian and Devonian times.

The initiation and growth of volcanic rises and granitoid emplacement caused uplift and eventual emergence of a sub-continentally-sized region in Mid- to Late Devonian times.

By this time volcanism and granitoid emplacement had waned and become restricted to the far east of the region.

Consequent rapid erosion of upland areas took place.

Red-bed sequences characterise many of the preserved deposits of this erosion.

Their deposition took place in both old graben, and newly initiated graben thought to be a further compensating consequence of the uplift and emergence of the region.

Granitoids now occupy about a quarter of the Fold Belt.

Batholiths mostly intrude sedimentary sequences, whilst stocks intrude volcanic rises.

The granitoids show broad relations to several designated, regional sedimentary facies (Yeates, 1982).

Within several of these, there are more frequent occurrences, and thicker beds of hydrothermally-deposited sediments surrounding the places the plutons now occupy.

This relation, and that of the granitoids' preference of place according to their size are interpreted to indicate their presence as magmas at depth during deposition of the sediments that now surround them, despite their apparently younger isotopic ages and intrusive relations.

Granitoid emplacement could therefore have been a passive process involving simple diapiric rise of magma beneath sea-floor swells in areas of deep ocean with high confining pressure.

Slow leakage of volatiles from these magmas is thought to have released the chemical constituents of many regional sedimentary facies.

Slow rise would tilt the adjacent unlithified sediments, deform them by gravity collapse (at least in the first instance) and produce apparently-intrusive relations.

On the other hand, magmatic reservoirs of batholithic size that were able to expel volatiles rapidly, became lava and tuff piles in which there was only enough residual granitoid magma to form stocks.

Intrusions of mafic magma are relatively sparse compared to granitoids.

They commonly occur as stocks and have a preferential location in Ordovician sedimentary sequences.

In terms of the Fold Belt's evolution, the following two features recognised by Packham (1960) indicate that vertical tectonics has played the dominant role: the emergence of a deep-sea region to form a sub-continental landmass via progressive lithogenesis in gradually shallowing environments; deep vertical faulting bordering troughs in which subsidence allowed thick sequences to develop.

Plate tectonics, a concept originally developed for much younger portions of crust of somewhat differing lithology and environment.

35

is not required to explain the evolution of the NSW portion of the Lachlan Fold Belt.

As well, lateral compression also appears to have been minimal.

Despite the intensity of folding, particularly in Ordovician sedimentary sequences, and in numerous graben, nappes are uncommon, mainly of outcrop- to hillside scale, and they lie entirely xvithin their structural domains.

There is no widespread repetition of stratigraphy by thrusting, as in the Appalachians Fold Belt (Williams, 1978).

In consequence, the NSW portion of the Lachlan Fold Belt is considered to be a "fold belt" because of the abundance of finegrained and thin bedded sedimentary rocks it contains, and the dominantly vertical tectonic regime induced by rising hydrothermal and magmatic reservoirs, and the consequent tilting and gravity collapse of adjacent unlithified sequences, especially between the lava and tuff piles.

References Crook, K.A.W., & Felton, E.A., 1975 - Tasman Geosyncline greenstones and ophiolites.

Journal of the Geological Society of Australia 22(1), 117-131.

Packham, G.H., 1960 - Sedimentary history of part of the Tasman Geosyncline in south-eastern Australia.

21st International Geological Congress, Norden, 12, 74~83.

Yeates, A.N. (compiler), 1982 - Lachlan Fold Belt: Ordovician, Silurian and Devonian sedimentary and volcanic facies in New South Wales.

1:1,000,000 Scale Preliminary Geological Map. Bureau of Mineral Resources, Canberra (unpublished).

Williams, H., (compiler), 1978 - Tectonic lithofacies map of the Appalachian Orogen. Memorial University of Newfoundland, St. Johns, New Foundland, Canada.

CONTRASTING STYLES OF MELANGE DEVELOPMENT IN THE NEW ENGLAND OROGEN K.C. Cross and C.L. Fergusson Department of Geology, University of New England, Armidale, N.S.W. 2351 Three principal types of melange have been recognized in the New England Orogen, namely: (1) serpentinite-matrix melange, which is often associated with major fault systems; (2) melange derived largely from the tectonic disruption of previously coherent marine strata; and (3) tectonically disrupted olistostromal deposits. These melanges are largely confined to rock associations lying within, or to the east of the Peel Fault System (i.e. within Zone B as originally

36

defined by Leitch, 1974). Reconnaissance and detailed investigations now suggest that such structurally incoherent rocks are more widespread throughout Zone B than generally recognized in the past.

Serpentinite-matrix melange (Type 1) occurs mainly along the Peel Fault System and in the Nowendoc, Port Macquarie, Yarras and Baryulgil areas. This type is dominated by autoclastic melange derived largely by the tectonic disruption of serpentinized harzburgite and lesser dunite. Exotic blocks are dispersed throughout the pervasively sheared matrix. This melange grades locally into ophiolitic melange which is characterized by an abundance of tectonically moulded mafic and ultramafic blocks embedded in a matrix of sheared serpentinite and/or other ophiolitic material. Here, the predominant block lithologies include a range of largely serpentinized xenoblastic and cumulate ultramafics, and a broad spectrum of variably altered, typically subalkaline, olivine-bearing and olivine-free gabbroic and basaltic rocks. In contrast with many other ophiolitic assemblages, no mesoscopic or largerscale pre-melange tectonite fabric elements have been recognised in these rocks.

Type (2) melange is widespread throughout much of the Woolomin Association and has also been recognised in parts of the younger Sandon Association. It has been examined in some detail in the Upper Manning area (Myra beds) and in the central Coffs Harbour Block (Gundahl Complex) .

The principal lithologies in the Myra beds are chert, impure, chert, and jasper. Siliceous argillite, argillite and basaltic rocks may be locally abundant. The main lithologies of the Gundahl Complex include bedded cherts, greyi^^ackes, thin-bedded turbidites, bedded argillite-tuff and greenstones.

In gross structural terms these Type 2 melanges largely consist of steeply dipping mesoscopic- to kilometre-sized blocks and slabs of the more resistant lithologies arranged in a more-or-less imbricate fashion in a matrix of highly sheared argillite (argillite-matrix melange) . Compared with the enclosed blocks the argillite matrix is often poorly exposed. The blocks and slabs have experienced varying degrees of internal deformation which ranges from incipient bedding-parallel shear, to significant stratal disruption and the localized development of autoclastic melange. Coherent and disrupted folding may be evident, especially in the blocks- of thin-bedded highly siliceous sediments.

The predominant direction of shear is sub-parallel to the general trend of the more substantial tectonic slabs.

An example of Type (3) melange from the Woolomin Association has been described by Leitch and Cawood (1980).

LEITCH, E.G., 1974: The geological development of the southern part of the New England Fold Belt. Geol. Soc. Aust., J., 133-156.

LEITCH, E.G. & CAWOOD, P.A., 1980: Olistoliths and debris flow deposits at ancient consuming plate margins: an eastern Australian example. Sediment. Geol., 25, 5-22.

37

MEGA KINK FOLDS AND RELATED STRUCTURES IN THE U P P E R DEVONIAN M E R R I M B U L A , G R O U P , SOUTH C O A S T , NEW SOUTH WALES 1 L . K . Rixon , W . R . Bucknell

9

and M . J . R i c k a r d ^

^CSIRO, Division of Mineral P h y s i c s , PO B o x 136, North R y d e , N S W 2113 2 Department of G e o l o g y , The A u s t r a l i a n N a t i o n a l U n i v e r s i t y Interbedded sandstones and red mudstones of the Upper Devonian Merrimbula Group w e r e mildly deformed during the Carboniferous in a 'Transitional* Tectonic r e g i m e . A remarkable variety of structures w e r e developed. Mudstones preserve pedogenic features (mottling, b a s i n - l i k e joint structures and calcrete). Tectonic shortening of about 5% was accommodated by kink-like monoclines and box folds w i t h wide-spaced solution-transfer cleavage in s a n d s t o n e s , reticulate cleavage in siltstones, and slaty cleavage in m u d s t o n e s . Faults and conjugate shear zones w e r e developed in competent beds followed by the regional j o i n t i n g . A b u n d a n t quartz v e i n s , derived by pressure-solution, are associated with bedding p l a n e s , faults and shear z o n e s . The geometry, development and interrelationship of these structures is d e s c r i b e d . The low strain allows certain k i n e m a t i c and dynamic conclusions; the structures w e r e developed under high p o r e w a t e r pressures at a high level in the crust and are related to a single long deformational episode under essentially horizontal E-W compression accompanied by eastward simple s h e a r . The stress configuration contrasts with that necessary for the extensional rifting postulated for the period immediately prior to the deposition of the Merrimbula G r o u p .

THE EVOLUTION OF EIGHT SUPERPOSED EASTWARD MIGRATING FOLD SYSTEMS IN THE NEW ENGLAND YARROL FOLD BELT

by H . J . Harrington Bureau of M i n e r a l R e s o u r c e s , C a n b e r r a , ACT In the Sydney-Bowen Basin section of the w e s t e r n part of the New England-Yarrol Fold Belt there are only macroscopic folds.

Further east early folding episodes produced penetrative small-scale folds, but later episodes produced large folds and successor b a s i n s .

The belts of small-scale folding moved eastwards from the Permian to the Cretaceous and the belts of large-scale folding trailed along behind t h e m .

Because of that pattern some areas have three sets of superposed small folds and a further three or four sets of large folds of decreasing intensity.

The small folds correspond with the orogenic stage in the development of the fold b e l t , and the large folds with the Transitional s t a g e .

Most of the granitoids were intruded during the transitional s t a g e .

They were intruded at the same time as successor basins such as the I p s w i c h , Moreton and Maryborough basins formed on top of the true o r o g e n .

This

38

evolutionary pattern of migrating fold systems is clearer in the New England-Yarrol Fold Belt than in any other fold belt that is known to the writer.

References KORSCH, R.J. & HARRINGTON, H.J., 1981: Stratigraphic and structural synthesis of the New England Orogen.

J. geol. Soc. Aust., 205-226, (Especially Table 1, pp. 218-219).

HARRINGTON, H.J. & KORSCH, R.J., (in Press): Tectonics of the New England Orogen, I, Devonian to middle Permian.

HARRINGTON, H.J. & KORSCH, R.J., (in Press): Tectonics of the New England Orogen, II, Late Permian to Cainozoic .

TECTONIC SIGNIFICANCE OF THE WOLLONDILLY-BLUE MOUNTAINS GRAVITY GRADIENT ZONE I.R. Qureshi School of Applied Geology, University of New South Wales, Box 1, Kensington, N.S.W. 2033 The most prominent gravity feature in the Sydney Basin is the Wollondilly-Blue Mountains Gravity Gradient Zone. It lies on the western flank of the basin and forms the western limb of an elongated gravity high. The zone runs north-south and overlies the Lapstone Lnocline, the Kurrajong Fault and the 'hinge line' (Bembrxck et al., 1980). To the north it splits into two part, one leading northeastwards over Lochinvar Anticline and the other continuing northward in a subdued form and perhaps suffering a w e s t w a r d displacement at Mount Coricudgy to join up with the Meandarra Gravity Ridge which runs northwards for a distance of 500 km (Mayne et al 1974). The gravity high parallels a gravity low to the east. The over a broad synclinal feature in the Sydney Basin and the low over the Kulnura Anticline has posed a puzzle.

Some 500 new gravity measurements have been made with a Worden gravity meter, at a spacing of 2-3 km, principally fo^g five profiles crossing the gradient zone. Height control is based ^ench marks and aneroid altimeters. Terrain corrections out to a radius of 20 km are applied and these reach as much as 6 mgal in rugged areas. The accuracy of reduced Bouguer anomalies is aimed at 1 mgal.

The anomaly map shows that near Katoomba, the gradient.zone can be divided into two parts. Across the steep eastern part, there is a change of about 40 mgal over a distance of about 20 km while across the less steep western part a change of 10 to 20 mgal occurs over an equal distance. The gravity anomalies increase eastwards from a minimum of -35 mgal at Bathurst to a maximum of +35 mgal at the apex of the

39

gravity high and then drop to +20 mgal at the base of the low before rising across the coastal gradient zone to +60 mgal.

A quantitative study of the anomalies has been initiated along the longest profile between Bathurst and Mona Vale, along which 87 gravity stations have been observed. An Airy-type isostatic model has been constructed for the Blue Mountains and the transition zone between the continent and the ocean. This model predicts a smooth eastward increase in gravity from a minimum of -70 mgal over the mountains to +13 mgal at the coast. The effect of the Sydney Basin sediments is computed on the basis of the seismic data and this shows a steady decrease, east of the "hinge line" with a maximum gradient over the Lapstone monocline. The effect reaches a minimum of -41 mgal over the thickest part of the basin and increases only slightly (-39 mgal) over the Kulnura Anticline.

When these effects are accounted for, the residual anomalies show a prominent, nearly symmetrical gravity high centred at the same position as in the uncorrected Bouguer anomaly profile and a small eastward increase in gravity. The latter may reflect a deep fault or an eastward thinning of the crust at a slightly greater rate than allowed in the isotatic model.

The gravity high turns out to be the main feature rather than the gradient zone and it completely masks the effect of the basin sediments.

It has an amplitude of 46 mgal and half-width of 32 km. The source of the high lies probably within the upper crust. The probability that it is the continuation of the Meandarra Gravity Ridge is quite high. It may represent the buried volcanic arc which was presumably active in the Carboniferous times.

It marks a zone prone to subsidence and a zone of weakness along which many Tertiary volcanic centres are placed.

Bembrick, C., Herbert, C., Scheibner, E.& Stuntz, J., 1973. Structural subdivision of the New South Wales portion of the Sydney-Bowen Basin. Q. Notes geol. Surv. N.S.W., 11, 1-13.

Mayne, S.J., Nicholas, E., Bigg-Wither, A.L., Rasidi, J.S. & Raine, M.J., 1974. Geology of the Sydney Basin - A Review, B.M.R. Bulletin No. 149.

STRESS HISTORY AND SOLUTION TRANSPORT DURING FOLDING AT CAPE LIPTRAP, VICTORIA P.G. Lennox^ & M.A. Etheridge^'^ ^Department of Earth Sciences, Monash University, Victoria Now at: Bureau of Mineral Resources, Canberra, ACT At Fold Stack near Cape Liptrap, Victoria, folding, microfabric development and quartz veining took place during late Middle Devonian regional deformation and metamorphism of a quartz arenite/shale sequence.

40

Quartz veins provide information on the orientations of principal stresses, and place some limits on their magnitudes.

We have examined the relationship of the orientations and locations of overprinting sets of quartz veins to the progressive development of folds in these excellent, three-dimensional exposures.

The folds approximate to a chevron style, and they have been subdivided about the finite neutral surface and inflection points, into inner, outer, hinge zone and limb zone blocks.

Each of these blocks has a characteristic vein geometry and history which can be used to constrain the stress history of folding.

An example of the progressive vein development in the hinge region of a fold is shown in figure 1.

Comparison is made with a number of theoretical folding models.

The presence of syntectonic, crack-seal veins indicates that p^ = + X (p^ = fluid pressure, a^ = minimum principle compressive stress, T = tensile strength of rock) throughout a substantial proportion of the deformation episode.

In order to understand the role of this high pressure fluid, we have attempted to model the mass transport path involved in microfabric development.

Approximately 10% of the total rock volume (mostly silica) has been removed from cleavage zones, whereas the volume of microfabric sinks (e.g., pressure shadows, beards) is negligible, and the total volume proportion of vein quartz does not exceed 1%.

There thus appears to have been a net loss of silica from these rocks during deformation.

The mass transport took place on a scale of at least 10^ m, and must have involved a significant component of solute transport in an advecting fluid.

41

LOWER ORDOVICIAN BENDIGO TROUGH SEQUENCE, CASTLEMAINE AREA, VICTORIA - DEFORMATIONAL STYLE AND IMPLICATIONS FOR THE TECTONIC EVOLUTION OF THE LACHLAN FOLD BELT S.F. Cox\

J. Ceplecha\ V.J, W a l l \ M.A. Etheridge^'^ R.A.F. C a s ^ R. Hammond^ and C. Willman^

^Department of Earth Sciences, Monash University, Clayton, Vic. ^Now at: Bureau of Mineral Resources, Canberra, A.C.T. Detailed structural and lithostratigraphic mapping, controlled by a well defined biostratigraphy in the folded and low grade metamorphosed Bendigo Trough sequence of the Castlemaine area of central Victoria, has placed significant constraints on the structural and tectonic evolution of this part of the Lachlan Fold Belt.

The Bendigonian to Yapeenian sequence comprises approximately 1000m of quartz wacke turbidites, siltstone, and mudstone deposited in a broad relatively deep-marine clastic apron marginal to the Ordovician Australian continental mass.

Sediments were deposited by subaqueous mass flows and hemi-pelagic processes, and the turbidites show sedimentary structures typical of the Bouma sequence.

There are rapid lateral and vertical lithofacies changes between sandstone intervals and pelite intervals throughout the succession, producing a grossly inhomogeneous sequence (see Cas et al., this volume).

The Ordovician sequence is folded about upright to steep west dipping axial surfaces.

Folds are close to tight and typically have wavelengths between 150m and 600m.

Throughout much of the region fold hinges are gently north plunging, though reversals in plunge and moderate to steep plunges occur in localized high strain areas.

The overall enveloping surface is gently north dipping.

The development of a penetrative cleavage has been associated with fold growth, and is most strongly developed in pelitic and semi-pelitic units, though arenites can have a pronounced divergent fanning solution cleavage in fold hinge zones.

Cleavage development has resulted in up to 30% mass loss by solution transfer processes in parts of the sequence.

The folds are cut by abundant steeply to moderately west dipping reverse faults which have strikes parallel to fold axial surface traces.

Throws usually range up to several tens of metres, but some major fault zones having displacements in excess of lOOm have been recognized.

Easterly dipping reverse faults are less common than westerly dipping ones, and appear to be conjugate to the latter and essentially synchronous with the late stages of fold growth.

Major gold mineralization is related to the development of some of the high angle reverse fault zones in the region (see Cox et al., this volume).

In the Castlemaine-Elphinstone region three structural domains, separated by major faults and characterized by significant differences in fold style and strain, have been recognized.

The central domain has a simple structural style characterized by the uniform development of angular upright folds having interlimb angles of 30°-40°. Fold wavelengths average 200m to 300m, and the fold enveloping surface is nearly horizontal.

To the east and west of the central domain fold

42

styles are very variable.

Immediately east of the central domain strain is low and folds are open.

Further east strain increases and folds become tighter by decrease in wavelength and increase in amplitude.

With increasing strain the eastern limbs of anticlines become overturned, and fold axial surfaces attain steep westerly dips.

The relatively thin blanket of Lower Ordovician sediments in the Castlemaine region exhibits fold styles, strain patterns, and a fault geometry similar to that of many other slate belts and foreland fold and thrust belts which have developed in a thin-skinned tectonic regime.

A thin-skinned deformation model for the central Victorian Ordovician sequence requires that strain, which at the presently exposed surface level is strongly partitioned towards folding and cleavage development, together with lesser shortening on high angle reverse faults, be taken up largely by movement on listrie reverse faults, low angle thrusts, and decollement surfaces at successively deeper crustal levels.

A major decollement/thrust surface may be present near the base of the Ordovician sequence (a present depth of four to six kilometres) and/or at the base of the Cambrian sequence (about five kilometres deeper).

The fault bounding the eastern margin of the Heathcote belt may represent the outcrop of a major listric fault derived from the latter decollement.

The fold vergence patterns in the Lower Ordovician sequence of the Castlemaine area are consistent with fault asymmetry and indicate eastwards tectonic transport in the region.

DIFFERENT STYLES OF FOLDING IN PROTEROZOIC OF VICTORIA BASIN (NT) PERMIAN OF BOWEN BASIN (QLD) TERTIARY OF EAST COAST BASIN (NZ) A.C.M. Laing 3319 Moggil Road, Bellbowrie Qld 4070 Geological cross sections across each basin and structural maps of each basin are presented at a uniform scale to enable visual comparison to be made.

These have been prepared after extensive fieldwork and basin studies in each basin by the author.

The style of folding in each basin is discussed and the origin of the folding pattern deduced by reference to the stratigraphy and geological history.

In the Victoria Basin NT the geological section consists of 1,200 metres of quartz sandstone siliceaous siltstone, shale and limestone of Upper Proterozoic age.

The quartz sandstones are friable with little or no siliceous growths indicating little induration.

South of the Victoria River Fault these beds generally dip very gently less than 5° except for long narrow zones of more steeply dipping beds less than ikm width of outcrop aligned around diamond shaped blocks about 30km across. There is no distinct direction of fold axes.

These zones are not aligned in any particular direction.

Along these zones there are two types of structure, monoclines or anticlinal domes.

The monoclines have an elbow with an abrupt change of dip from 15° to 40° on the upthrown side with a more gradual change on the downthrown side.

43

The domes are faulted at an acute angle to their long axis with horizontal throw up to 130 metres in either sense. Away from the axes they have a sharp transition from the 25° or more flank dips to dips of beds 10° or less in the surrounding beds.

In the Bowen Basin Qld the geological section consists of on the eastern side, 6,000 metres of lithic sandstone and shale with minor tuff and limestone and on the western side 4,000 metres of interbedded quartz sandstone and shale. These beds are folded on the east to a large syncline, the Mimosa Syncline, 90km wide containing 5,400 metres of Triassic lithic sandstone and shale quartz sandstone in addition to the Permian sediments. The dips on this syncline are fairly uniformly 20° in the east and 10° in the west. There is major overthrusting in the eastern side also. During the Permian terrigenous sedimentation was from the western side of the basin, volcanic sedimentation from the east.

On the western side there are a series of major Permian folds with axes about 10km apart. These folds, the most important of which is the Serocold Anticline, have flank dips of about 15°. All the fold axes are aligned in a north south direction.

In the East Coast Basin, New Zealand, there are more than 9,000 metres of Pleistocene limestone and mudstone and tertiary and Upper Cretaceous mudstone and greywacke sandstone.

These sediments are folded in two styles of folding. the Pleistocene sediments are folded in chevron type folds with a relatively gentle (20°) west dip abruptly changing to a steep (50°) east dip.

The Tertiary and Upper Cretaceous sediments are folded in growing anticlines with thinned section on the crests, vertical dips in the core and steep (30 - 60°) dips on the flanks. There is present day wrench faulting passing through the basin. In the northern part there is a large area (at least 1,200 sq km) of overturned section; two oil exploration wells in the Ruatoria area having each established Eocene under Cretaceous.

If it is accepted that there is a single cause of rock folding on the earth a cause must be found which provides a greater degree of folding and greater rate of sedimentation in New Zealand than in the Bowen Basin than in the Victoria River Basin.

It may illustrated thus East Coast Basin NZ

Bowen Basin Qld

Victoria River Basin

Cretaceous

Thick Sediment 20° - 50°

Thin Cretaceous in Surat Basin less than 5° dips

Thin veneer flat dips

Permian

In basement intense folding

20° thick section

Nil

Proterozoic

None known

None known

0 - 5 ° thin section

Any tectonic theory should take this sort of pattern into account.

If

44

the Indian Ocean had expanded to the same degree as the Pacific Ocean the Victoria River Basin should be more intensely folded. It should also be taken into account in any tectonic theory that the whole Australian continental plate has been folded and contracted.

DEVONIAN AND EARLY CARBONIFEROUS PALAEOGEOGRAPHY OF THE LACHLAN FOLD BELT C.McA. Powell School of Earth Sciences, Macquarie University, North Ryde, NSW

By earliest Devonian there were two tectonic regimes in the Lachlan Fold Belt. From the Wagga Metamorphic Belt eastward there was a terrain of horsts and grabens dominated by high-K silicic magmatism, while to the west there was a foreland basin filled mainly by quartzose clastics prograding from the Gondwanan craton. The silicic magmatic province in the east lasted until early Middle Devonian, when widespread deformation centred on southern N.S.W. and northern Victoria uplifted the region to form the Tabberabberan Highlands. In the western foreland basin, sedimentation continued uninterrupted apart from local intrabasinal block movements.

In the Late Devonian a new volcanic arc was established to the east of the presently exposed Lachlan Fold Belt, but west of the Tamworth Trough in New England. Frasnian sediments in the eastern Lachlan Fold Belt are quartzose, being derived from the uplifted Tabberabberan Highlands to the south and west, and deposited in fluvial to shallow-marine environments during the Frasnian transgression that extended as far west as Parkes and the Minuma Range. The typical Lambian quartzites derived from the southwest were deposited at this time.

Higher in the section in the eastern Lachlan Fold Belt, the rocks are volcanolithic, and were derived from the magmatic arc that lay to the east.

By the latest Devonian or earliest Carboniferous, the eastern Lachlan Fold Belt was a foreland basin adjacent to this arc, very much as the basin west of Cobar was a foreland basin to the eastern magmatic province in the Early Devonian. The eastern foreland basin probably existed until Visean when widespread folding occurred throughout eastern Australia and terminated sedimentation in the Lachlan Fold Belt.

These conclusions are formed from regional palaeocurrent and provenance studies, and are illustrated on a poster as a series of palaeogeographic maps and palaeocurrent sections.

45

STRUCTURAL SECTIONS ACROSS THE NORTHERN PART OF THE MELBOURNE TROUGH AND ADJACENT BELTS A.H.M. VandenBerg\ G. Gibson^, A.J. Crawford^ I.A. Nicholls'', C.J.L. Wilson^, L. Thomas^ and A.J.R. White^ ^Geol. Surv. Div., Dept. of Min. & Energy, 140 Bourke St., Melb., 3000. ^Seis. Res. Centre, Phillip Inst, of Tech., Plenty Rd., Bundoora,3083. ^Dept. of Geology, Univ. of Melbourne, Parkville, 3052. '^Dept. of Earth Sciences, Monash University, Clayton, 3168. ^Dept. of Geology, Latrobe University, Bundoora, 3083.

An east-west profile across Victoria provides probably the most varied and potentially useful seismic profile across the Tasman Fold Belt, from possibly old continental crust in the west, to a possible Ordovician accretionary prism (Wagonga Beds) in the east.

The Melbourne Trough in the centre has a very thick (ca. 10km) and continuous Lower Ordovician to uppermost Lower Devonian sequence, unique in the Tasman Fold Belt.

It is flanked by narrow belts of Cambrian mafic to rare intermediate volcanics and mafic-ultramafic intrusives and condensed sediments, totalling about 3-5km, associated with strike faults that have vertical displacement of up to 12-13km.

The tectonic setting of the Cambrian belts is open to several interpretations, yet is crucial in understanding the subsequent history of a large part of the fold belt.

Models for the emplacement of the Cambrian igneous rocks include a marginal sea crust, a fore-arc basin setting, and deep fracturing of unexposed continental crust.

West of the Melbourne Trough is the Stawell-Bendigo Sedimentary Belt ("Ballarat Trough") whose eastern part contains a surprisingly thin (3-4km) Cambrian-Ordovician sedimentary cover lying apparently directly on the Cambrian volcanics, although there is no direct evidence that these volcanics extend much beyond their present linear outcrop.

The Ordovician sedimentary rocks are tightly folded but structurally fairly simple.

The structure farther west may be similar but has not been thoroughly investigated.

The Grampians Trough, west of the Stawell-Bendigo Belt, contains a thick sequence of broadly folded Lower Devonian non-marine to very shallow marine sediments (Grampians Group) overlying a large sheet of undated Rocklands Rhyolite.

These overlie the amphibolite grade Glenelg Metamorphic Complex, probably correlative of the Kanmantoo Group, and forms the transition between the Kanmantoo and Lachlan Fold Belts.

This appears to be the only place where these two fold belts are juxtaposed.

East of the Melbourne Trough is a broad belt of Ordovician sediments which are more complexly deformed than those further west, often with quite different structural trends, and which include the Omeo Metamorphic Complex, a broad belt of schist, gneiss and granitoids similar to the Wagga Belt of N.S.W. The poorly dated rocks are at least as old as Early Ordovician and may even include Proterozoic metasediments and granitoids.

Seismological interpretation suggests a crustal thickness ranging from about 35km to about 45km below the metamorphics in the Snowy Mountains region, with quite low seismic velocities in the lower crust.

46

GEOLOGICAL CHARACTERISTICS OF THE EASTERN TASMAN OROGEN: A TRANSECT FROM COONABARABRAN TO PORT MACQUARIE Peter A. Cawood and Evan C. Leitch Department of Geology and Geophysics, University o£ Sydney The geology of the eastern Tasman Orogen is typically displayed along a latitudinal transect extending 350 kilometres from Coonabarabran to Port Macquarie in northern New South Wales.

Geological characteristics of units recognised along the transect are summarised in Table 1 (over).

The crust in this region mostly comprises Palaeozoic rocks of the New England Fold Belt but in the west material of the earliercratonised Lachlan Fold Belt is present. The boundary between these two elements is masked here as elsewhere in New South Wales. Permian and Triassic rocks of the Gunnedah Basin extend east to the west-dipping Mooki Thrust, the western limit of exposed New England rocks.

Mafic and silicic volcanic rocks of early Permian and (?)Late Carboniferous age underlie the sedimentary sequence of the Gunnedah Basin and either represent the latest activity along the magmatic arc that bordered the New England Fold Belt, or are a rift-related sequence developed above the arc during the formative stage of basin development.

East of the Mooki Thrust the gross structure is that of major fault-bounded crustal blocks.

The rocks exposed along the transect carry implications for the nature of deeper crustal material. However, uncertainty concerning the attitude of the major fractures at depth and hence the extent to which the various blocks are autochthonous on a crustal scale, is a major constraint on discussion of the nature of the lower crust. Suggestions that the Peel Fault, flattens eastward and that rocks of the Macdonald and Yarrowitch Blocks have been superimposed on an extension of the Tamworth Belt entail a considerably different crustal makeup from that implied by a uniformly steeply dipping Peel Fault. Likewise the significance of tectonic inclusions found within serpentinite bodies is uncertain; are eclogite, amphibolite and blueschist blocks representative of the lower crust at the time of the rise of the protrusions, or was their formation restricted to the high stress environments along which the serpentinites rose?

Events likely to lead to differentiation and development of the crust were not uniformly spread along the transect. Arc magmatism dominated the region between the Lachlan Fold Belt and the Mooki Thrust throughout the Middle-Late Palaeozoic; intermediate and mafic intrusions probably bolstered the lower crust in the early stages of development, and later partial melting of lower crustal rocks aided intracrustal differentiation. Similar magmatic activity within the Tamworth Belt may have contributed to the basement rocks of this block.

Any ophiolitic basement in this region must be of pre-Middle Cambrian age. type plutons have formed by partial melting of supracrustal rocks and the lower crust beneath the Macdonald and Yarrowitch Blocks must have been modified as a result. Tectonic interpretation of these units suggests that little crustal segregation will have been present prior to this event.

There is no obvious upper crustal manifestation of the source rocks for the "I" type granites and their rise may have been preceded by underplating of the crust by a mafic igneous layer.

Changes in crustal structure since the Palaeozoic may have involved phase changes engendered by variations in heat flow and by mafic intrusive activity complementary to widespread Tertiary volcanism.

TABLE 1.

I-ASTERN LAC! 1 LAN

GUNNEDAH G OXLEY

BELT

MACDONALD BLOCK

?Early-mid Age range of Palaeozoi c sedimentat ion

Pcrmian-Jurfiss i c

Cambri an-Earl y Permian

Early Palaeozoic - Early Permian

Quart zose G 1 i th i c sandstone, shale G coal.

Similar to Arc-dcrivcd quartz- Repeated upward coarsening sequence Macdonald Block. poor clastics; of chert, si 11stone minor limestone.

6 in thicker and younger slices arcderived clastics; olistostromes and pebbly mudstone.

Arc source.

Contemporaneous igneous activity

Jurassic alkaline intrusives G extrusives

Tholeiitic-calc alkaline magmatic arc activity in and west of belt.

Depositional basement

Volcanics in eastern § central parts of basin; Lachlan in west.

Sedimentary 1ithologies 5 source

Fine-grained continental derived quartzrich clastics.

Granites

Internal structure 5 regional metamorphism

Intense deformation, penetrative foliation; greenschist facies metamorphism.

Minor regional warping, essentially unmetamorphosed.

Intracratonic basin.

j YARROWITCH

BLOCK

Early Palaeozoic - Early Permian

HASTINGS BLOCK

PORT MACQUARIE

Devoni an-Early Permian

Early Palaeozoic - Early Permian

Similar to Tamworth Belt.

Similar to Macdonald Block • mid-Palaeozoic volcaniclastic succession.

MORB-type basalts Basalt - probably G dolerites and in similar setting rarer alkalic to that in basalts at base of Macdonald Block. sedimentary slices.

Basaltic and andesitic arc type act ivi ty.

Basalt - simi1ar to Macdonald Block + mid-Palaeozoic andesite.

Oceanic crust

crust

Contentious

?Oceanic crust

Not along transect; Both S- and I-type. rare I-type elsewhere in east.

Both S- and I-type.

I-type only

?I-type (in subsurface)

Simple deformation, Intense deformaincreasing to east; tion, one penetraopen folds; zeolite tive foliation; prehnite-pumpelto prehnitelyite facies pumpellyite facies metamorphism. metamorphism.

Intense deformation, several foliations; pumpellyite-actinolite to amphibolite facies metamorphism.

Simple deformation, open folds G major faults; prehni te-pumpellyite facies metamorphism.

Intense deformation, penetrative foliation; prehnitepumpellyite to greenschist facies metamorphism.

Major slumping, Probably same as widespread imbric- Macdonald Block. ate thrust faulting § associated melange formation along fault zones.

Minor

slumping.

Probably same as Macdonald Block.

Oceanic crust and Same as Macdonald trench incorporated Block. into subduction complex; outer fore-arc and slope basin deposits.

Same as Tamworth Belt.

Same as Macdonald Block (in part).

Contentious

Minor

Syndepositional deformation

Inferred depositional £ tectonic setting

TAMWORTH

slumping.

Magmatic arc and fore-arc basin.

?Oceanic

48

STRUCTURAL STYLES OF INTRUSION:

PROFILE IB, NORTHEAST QUEENSLAND

E.J. Heidecker^ iDepartment of Geology and Mineralogy, University of Queensland, Brisbane Qld Tasman Orogen Profile lb of Working Group 9 of the International Geodynamics Project runs northeast across the Orogen to Townsville, Queensland. This profile has been selected for integrated consideration of the following structural characteristics of intrusions in a highly mineralised section of the Tasman Orogen: 1.

FORM.

Structural features of the Silurian Lolworth Batholith indicate that this intrusion is a layered slab overlying infrastructures which are continuous with peripheral country-rock structures. This batholxth was preceded by clustered dyke swarms with monoclinic symmetry, netveined complexes, and 'necklace' and multiple dykes, all intruding an Ordovician granodiorite complex with gneissic zones. Subsequent Devonian sub-volcanic intrusions are orthogonal dykes and sills and phacoliths. Carboniferous intrusions include radial dyke swarms and cylindrical intrusions with collars of dragged country rocks.

2.

EMPLACEMENT.

Emplacement styles span a time-dependent spectrum from induced to passive, permissive, and forcible styles.

3.

LINEAMENT CONTROL.

The geology of northeastern Queensland reflects a persistent framework of lineaments established by Early Palaeozoic times. This framework is remarkable for its survival through periods of substantial tectonic and igneous activity and for persistent genetic links with magmatism. From Palaeozoic to Recent times magmatism has recurred at nodes in the framework and along its constituent lineaments, particularly those trending northeast.

4.

SUBSIDENCE.

Deep mines at Charters Towers and excellent exposures along the Burdekin River together provide exceptional insights into the degree of subsidence at centres of passive intrusion, including those into contemporaneous folds. Low-angle normal faults, slumping, directions of sediment transport, brecciation, and passive sheet intrusion point to subsidence beneath igneous centres.

The characteristics listed above are rationalised in terms of tectonic subsidence directed towards removal of crustal density heterogeneities in an evolving lithosphere.

Symposium

1(d)

Metamorphism and geothermal gradients Convener: Associate Professor R.H. Vernon

49

WHAT METAMORPHIC PETROLOGY CAN AND CANNOT TELL US ABOUT THERMAL BUDGETS OF OROGENIC BELTS Philip England Department of Geological Sciences, Harvard University, 20 Oxford Street, Cainbridge, MA 02138, U.S.A. While it is relatively easy to find transient, or even steady state, geotherms that account for pressure-temperature (PT) conditions that are inferred from metamorphic mineral assemblages, it is considerably more difficult to reverse this process - that is, to use these data to place constraints on the thermal and tectonic regimes that produced them. One reason for this is that (unless we are to resort to ad ^ modelling) we can only treat the therm.al development of a metamorphic belt with geometrically simple initial and boundary conditions; this is so, not because the more complex geometries are intractable (they are not), but because, in general, there are no data available to constrain more complex models. Consequently, calculations are usually made on systems where the temperatures are sensitive only to the largest scale perturbations; yet in an actual belt, mineral assemblages may be set under conditions that differ considerably from those that could be considered as 'average' in a regional sense.

These considerations have naturally lead to the view that the information on tectonic and thermal history which may be extracted from metamorphic petrology data will primarily be on the large scale, and an obvious precaution, which is usually taken, is to avoid using PT estimates that are only of local relevance. For example, we may expect information on the regional rate of heat supply during metamorphism, but should not expect to account for the origin of every occurrence of anatexis; we might hope to estimate the average depth of burial of the present erosion surface - even to determine a regional trend in it but should not try to explain crossing of an isograd on this surface in terms of a regional thermal history.

Even when these precautions are observed, the fundamental ambiguity of PT data makes inference of regional thermal and tectonic histories difficult.

However, it may be shown that, for many metamorphic belts, the thermal evolution is dominated by a few major controls - again, provided that only the large scale is considered.

These are: the total amount of heat generated within the crust, the total heat supply from below (including magmatic transport), the erosional history of the belt and the initial conditions.

The pressure-temperature-time (PTt) paths which would be followed by rocks during metamorphic episodes governed by different combinations of these controls display certain characteristics which remain relatively unaffected by details of the thermal history and may be diagnostic of different tectonic styles. A review is given of the PTt paths followed during regional metamorphism for a number of geologically plausible thickening having Cenozoic analogies in the western United States, the Alpine-Himalayan Chain, Tibet and the Andes. It is suggested that, while individual PT points may be of limited use in inferring regional thermal budgets and tectonic style in older fold belts, PTt paths, when they can be disentangled from small scale activity, can provide useful information.

50

ASPECTS OF THE POSTDEPOSITIONAL HISTORY OF THE SYDNEY BASIN P. Hitchcock^ V.J. Wall^

S.L. Koul^

^Dept. of Earth Sciences, Monash University, Clayton, Vic., 3168. ^Chemical Physics, C.S.I.R.O., Clayton, Vic., 3168.

Evaluation of the postdepositional history of sedimentary basins is particularly relevant to understanding their tectonic evolution, coalification processes and the development, migration and trapping of fluid hydrocarbons.

In recent years investigations of organic maturation parameters have provided significant insights into such matters.

However, diagenetic mineralogy can also be useful in the elucidation of thermal conditions, porosity-permeability controls and fluid migration.

A temporal framework is required for modelling of postdepositional thermal histories.

In this paper we examine secondary mineral development in Permian rocks of the northern and southern Sydney Basin and apply fission tract dating techniques to clarify the thermal evolution history.

Analcite-, dawsonite-, zeolitic-- and authigenic clay mineral assemblages are widely developed in the northern Sydney Basin.

On the basis of our experimental and thermodynamic calibrations the phase equilibrium relations of these assemblages indicates maximum burial temperatures of 100-150°C. Despite the shallower burial of the southern Sydney Basin Permian sequences these rocks contain albite-, pyrophyllite-, laumontite-, prehnite and pumpellyite bearing assemblages, indicative of temperatures between 150-200°C. These inferred differences in geothermal gradients are compatible with vitrinite reflectance measurements (Cook, 1969; Diessel, 1975).

The extreme paleo-geothermal gradients in the southern Sydney Basin cannot be explained by basement characteristics (cf. Facer et al., 1980) and imply a significant contribution of magmatic heat.

Fission track work, currently in progress, should help clarify the timing and duration of the thermal event(s) and provide essential constraints for tectonic evolution models.

Authigenic mineral assemblages in coals, clastic sediments and volcanogenic rocks commonly vary markedly on a local scale and imply considerable variation in pore fluid compositions (^QQ ^ pH and dissolved salt components) as well as a complex history of rock-rluid interaction.

For example, dawsonite-bearing assemblages in the Singelton Coal Measures and Western Sydney Basin indicate that C02-rich and H20-rich pore fluids with limited miscibility were present.

Similar analcite-rich in the Newcastle Coal Measures formed during burial in the presence of H2O and CHi+ rich pore fluids, even though vitrinite reflectance data indicates lower levels of organic maturation of the Newcastle rocks relative to the Singelton sequence.

Secondary mineral assemblages in the Permian volcanic and volcaniclastic rocks of the southern Sydney Basin formed by interaction with pore fluids relatively poor in CO2.

Redox equilibria indicate that these fluids also lacked appreciable quantities of hydrocarbons.

Rock porosities were severely reduced by zeolite formation.

Many clay rich rocks of the coal measures had a substantial volcaniclastic component but were apparently flushed by relatively acid groundmass during early diagenesis.

Authigenic silicate-carbonate-oxide assemblages in coals throughout the Sydney Basin are commonly indicative of relatively C02-rich fluids

51

attending their development.

The origin of these fluids is problematic as most of the coals have attained ranks well above those in which substantial CO2 evolves (Stach et al., 1975), The CO2 may have been generated by interaction of aqueous fluids with the carbonaceous material, with or without bacterial agents.

More data on authigenic mineral assemblages (especially fluid inclusion and stable isotope investigations) interpreted in the light of textural and thermal evolution is required to understand pore fluid migration and chemical evolution.

METAMORPHISM IN TIMOR:

AN EXAMPLE OF INHERITED GEOTHERMAL

GRADIENTS IN A COLLISION MARGIN R.F. Berry Department of Geology, University of Tasmania, Hobart, Tasmania The orogenic belt exposed on Timor is the result of an island arccontinent collision.

Since the collision is less than 15 Ma old and the present average altitude is less than 2000 m, the total erosion from this orogen is less than 3 km.

A finite difference model of the thermal evolution of Timor shows that rocks in the top 3 km of the orogen reach a uniform geothermal gradient of 18°C/km in less than 5 Ma, The rocks presently exposed on Timor have not been heated past SO^'C as a result of their burial in the orogen.

The widespread occurrence of montmorillonite-bearing rocks (Audley-Charles, 1968) confirms this interpretation.

All rocks reported from Timor which have been metamorphosed at temperatures greater than 80°C, are the result of events which predate the collision.

There are three potential sources of rocks in an arc-continent collision: the arc-trench gap, the ocean floor and the continental margin.

Each of these is characterised by a distinctive metamorphic, deformational and sedimentological history.

Passive margin thermal models (e.g. Royden et al., 1980) postulate a simple metamorphic history for these rocks while the arc-trench gap melange should have a complex structure and high pressure metamorphism.

Ocean floor rocks have a distinctive composition and a simple thermal history.

The Aileu Formation has had a complex deformation history but only one climactic phase of metamorphism.

This medium-pressure metamorphic event ranged from lower greenschist facies, 350°C, to upper amphibolite facies, 700°C and 7 kbars.

The earliest structural element is a penetrative foliation everywhere subparallel to the compositional layering.

Microstructural evidence indicates that the metamorphic peak occurred after the development of this early foliation.

No evidence was found of first generation folds.

The second deformation caused widespread transposition of layering and the microstructures indicate that the temperature dropped rapidly during this deformation from temperatures less than the peak value.

Later deformations show a

52

continuing decrease in temperature.

The highest grade metamorphic rocks cooled through the hornblende (K/Ar) closure temperature at 11 Ma and the biotite closure temperature at 6 Ma. The hornblende closure temperature was reached during the second deformation phase.

Since the second deformation phase is associated with cooling during the collision event, the precollision history is a simple thermal peak which postdates the cleavage development.

The medium pressure metamorphism can occur in a rift valley or early spreading environment.

Sixty percent extension of average continental crust produces a peak temperature of 700°C at 7 kb and these rocks cool to 400°C, 6.5 kb (no sedimentation) - 500°C, 8 kbars (maximum sedimentation) in 150 Ma. The model thermal history of a passive margin predicts that the highest grade rocks in the Aileu Formation will remain open to Ar loss until cooling occurs due to the arc-continent collision.

The upper greenschist facies conditions observed for the initiation of the second phase of deformation (Berry & Grady, 1981) closely agrees with the temperature predicted by the passive margin model.

This interpretation also predicts that the first deformation in the Aileu Formation was due to crustal extension.

The metamorphism of other probable continental margin facies (Maubisse Formation, Permian - lowest greenschist to prehnitepumpellyite facies; Cribas Formation, Permian - prehnite-pumpellyite facies; Early Triassic limestones - 60 ± 25°C) are consistent with passive margin metamorphism.

A second group of rocks for which a detailed metamorphic history is available in the Mutis Complex.

Earle (1981) reported two metamorphic events in these rocks: an early 670°C, 7 kb event and a later 700°C, 4.5 kbar event.

The later metamorphism is related to a gabbroic intrusion.

These temperatures were obtained from politic gneisses but this unit also contains serpentinite metagabbro, amphibolites and graphitic phyllites.

There are two early generations of tight folding which are related to these metamorphic events.

The whole sequence is strongly deformed with intense retrogression and cataclasis near a Cretaceous unconformity.

The overlying sediments are also metamorphosed to very low grade.

Both groups have been intruded by Tertiary dykes.

The Mutis Complex is also unconformably overlain by Eocene and Early Miocene sediments.

The Mutis Complex and overlying sediments have a more complex geological history than the Aileu Formation.

The presence of a major Cretaceous unconformity provides circumstantial evidence that the Mutis Complex was the crystalline basement on the Northwest Shelf and was exposed during the spreading from this margin, but similar unconformities are known from Sulawesi (Haile et al., 1979).

The high geothermal gradients in the Mutis Complex does not fit an arc-trench gap environment but the stratigraphy of overlying rocks suggests correlation with Sumba.

The Mutis Complex may be the basement of a microcontinent which was north of the trench before the Mio-Pliocene arc-continent collision.

REFERENCES Audley-Charles, M.G., 1968.

Soc. Lond. 4, 1-76.

The geology of Portuguese Timor.

Berry, R.F. and Grady, A.E., 1981.

Mem. geol.

Deformation and metamorphism of the

53

Aileu Formation, north coast. East Timor and its tectonic significance. J. Struct. Geol. 3, 143-167.

Earle, M., 1981. The metamorphic rocks of Boi, Timor, Eastern Indonesia.

In: Geology and Tectonics of Eastern Indonesia (ed. Barber, A.J. § Wirysujono, S.) Geol. Res. 5 Dev. Centre Spec. Pub. 2.

Haile, N.S., Barber, A.J. and Carter, D.J., 1979. Mesozoic cherts on crystalline schists in Sulawesi and Timor. Geol. Soc. Lond. 136, 65-70.

Royden, L., Sclater, J.G. and von Herzen, R.P., 1980. Continental margin subsidence and heat flow: Important parameters in formation of petroleum hydrocarbons. Am. Assoc. Pet. Geol. Bull. 64, 173-187.

A REVIEW OF THE METAMORPHIC ROCKS OF TIMOR AND THEIR TECTONIC SIGNIFICANCE M.M. Earle BP Petroleum Development Australia No Abstract provided

GRANULITE FACIES METAMORPHIC CONDITIONS DURING THE ARCHAEAN EVOLUTION AND LATE PROTEROZOIC REWORKING OF THE VESTFOLD BLOCK, EASTERN ANTARCTICA 1

Kenneth D. Collerson , John W. Sheraton

2

and Pieter Arriens

I

^Research School of Earth Sciences, Australian National University Canberra ACT ^Bureau of Mineral Resources, Canberra ACT The Vestfold Block in the eastern Antarctic Shield is characterised by tectonically inter-layered tonalitic to granitic orthogneisses (Mossel gneiss, MG) and garnetiferous paragneisses (Chelnok supracrustal assemblage, CSA) as well as subordinate units of mafic granulite (Tryne metavolcanics, TMV). This sequence is cut by a second suite of orthogneisses (Crooked Lake gneiss, CLG), ranging in composition from gabbro-diorite to tonalite and granite which were emplaced synchronously with the last major phase of folding deformation. Rb-Sr and Sm-Nd isotopic data for the MG, CSA and TMV yield ages ranging from ^2800 3100 Ma. The CLG is clearly younger and was emplaced and metamorphosed ^2400 - 2500 Ma ago. These ages are believed to date a number of

54

granulite facies tectono-thermal events in the evolution of the gneiss complex. The earliest metamorphic assemblages (augite-orthopyroxene, antiperthite-mesoperthite and augite-antiperthite-quartz) record temperatures and pressures of ^ 1000° - 1100°C and 8 - 1 0 kbars with low water fugacity. However, most of the granulite facies assemblages in the terrain reflect equilibration at somewhat lower temperatures (760°- 850°C - two pyroxenes; 700°- 760°C - garnet-biotite; 780°C garnet-clinopyroxene; 700°- 760°C - garnet-orthopyroxene) at pressures ranging from 5 to 8.5 kbars (garnet-orthopyroxene-plagioclase-quartz and garnet-cordierite). These P-T estimates indicate that granulite facies metamorphism in this Archaean Block occurred in relatively thick crust ( 24 - 30 km) under a moderate geothermal gradient ( 35°45°C/km). The Archaean thermal history of the terrain probably reflects near isobaric cooling from ^1100°C (^3000 Ma ago) to 850°C by ^2800 Ma, then by a decrease in both temperature and pressure to ^ 700°C and 5 kbars by ^2400 Ma. During the Proterozoic, the gneiss complex was cut by a plethora of tholeiitic dykes. Most of these are unmetamorphosed, however, in the south-western part of the Vestfold Block, garnet-clinopyroxenequartz-plagioclase bearing assemblages were developed in the mafic dykes by static metamorphism during a late Proterozoic thermal event ^1100 Ma ago. Mineral equilibria indicate that these assemblages formed at temperatures of 600°- 700°C and pressures of 6 - 7 kbars. Granulite facies gneisses which crop out south-west of the Vestfold Block, along the coast of Prydz Bay, show the regional effect of this Proterozoic tectono-thermal event.

In the Rauer Islands ^ 3 0 km south of the Archaean terrain, gneisses which are compositionally similar to the MG, TMV, CSA and CLG contain remnants of mafic dykes. The dykes occur as folded and/or boudinaged trains in gneisses,or xenoliths in anatectic melts. They provide clear field evidence that granulite facies lithologies in the Rauer Islands are reworked Archaean gneisses. This interpretation is supported by Sr isotopic data for sixteen gneisses from Filla Island which yield an isochron corresponding to an age of 1106 ± 114 Ma with initial ratio of 0.7086 ± 0.0013.

Granulite facies mineral equilibria indicate that this reworking occurred at temperatures of 800°- 950°C and pressures of 6 - 8 kbars, under a similar geothermal gradient to that which caused regional metamorphism during the Archaean. Rare orthopyroxene-plagioclase coronas around garnet in a metamorphosed mafic dyke (mafic-granulite) probably reflect partial equilibration during uplift at relatively high temperatures after the peak of metamorphism. The formation of garnetiferous dykes in the south—western corner of the Vestfold Block is interpreted to reflect metamorphism at a higher structural level than the Rauer Islands during this event.

Possible explanations for this late Proterozoic reactivation of large areas of the East Antarctic Shield include overplating with continental crust, or sediment loading in fault controlled basins.

High heat flow resulting from underplating by mantle-derived melts was probably a factor in the thermal history of such reworked terrains.

55

LOW-PRESSURE GRMULITE FACIES METAMORPHISM IN THE HALLS CREEK MOBILE ZONE, WESTERN AUSTRALIA J. R. Thornett Department of Geology, University of Western Australia, Nedlands, W.A. Both similarities and differences between Proterozoic and younger mobile zones have been emphasised in a number of recent papers (Burke and Dewey, 1971; Kroner, 1977; Rutland, 1981). Among criteria for comparison are the metamorphic P/T regimes indicated by mineral assemblages, and the possible heat sources. The Halls Creek mobile zone contains rocks ranging in grade from greenschist facies (principally in the south) to granulite facies to the north (Dow and Gemuts, 1969). This paper discusses an area centred on the Sally Malay Ni-Cu prospect, which lies well inside the first sillimanite isograd (as outlined by Dow and Gemuts, 1969). The area extends for 20 km eastwest across the central-western part of the mobile zone.

The rocks represented are mainly metasediments and volcanics of the Tickalara metamorphics, which are intruded by prolific mafic and felsic plutons. Four deformations are recognized in the Tickalara metamorphics, the third being temporally associated with peak, or near-peak, thermal conditions. Predominant in the Tickalara metasedimentary sequence are meta-aluminous gneisses and migmatites which contain biotite and cordierite in the west and pass eastward into garnet-bearing assemblages, before the virtual disappearance of biotite. The biotite consuming reaction is spatially associated with the appearance of obvious in situ melt in gneiss leucosomes, and can be represented as: biotite + cordierite + quartz + plagioclase

garnet + melt

Other rock types intercalated within the gneisses include calcsilicate/marble and mafic granulite. Interlayered calc-silicate and marble horizons invariably contain wollastonite to the exclusion of either quartz or calcite. The first occurrence of scapolite, midway across the area, ends the stable coexistence of plagioclase and calcite in marbles to the west. Mafic granulites contain two pyroxenes-plagioclase ± hornblende ± quartz assemblages. Hornblende is ubiquitous in these lithotypes in the west and becomes less abundant eastward, ultimately to disappear.

Barometry using the anorthite and cordierite breakdown reactions, for assemblages without sillimanite, constrains maximum pressures to about 4 kb in the west and kb in the east. Minimum pressure estimates, using the garnet-quartz barometer in the absence of orthopyroxene, range from about 3.5 kb in the west to slightly above 5 kb in the east. While these barometers would seem to impose tight pressure constraints, minima indicated by the garnet-quartz barometer are probably overestimated (Ashworth and Chinner, 1978).

K^-based thermometers give varying and often unrealistic temperature estimates, generally indicative of re-equilibration.

However, reintegrated compositions of two feldspar pairs in some apparently unmelted gneisses indicate temperatures in excess of 710°C in the west, and plagioclase-calcite-scapolite relationships suggest temperatures above 750°C in the centre of the area. In the west.

56

800°C maxima are indicated by considering the biotite-cordierite-quartz breakdown as a dehydration reaction.

Although temperatures are not particularly well-constrained, high temperature-low pressure conditions are clearly indicated and are supported by the assemblages. Implied high geothermal gradients point toward a magmatic heat source. The abundant mafic intrusives, the most obvious potential heat source, cannot be seen in this role, as they show evidence of emplacement during the P/T decline from peak conditions.

References Ashworth, J.R. and Chinner, G.A., 1978: Coexisting garnet and cordierite in migmatites from the Scottish Caledonides: Contr. Mineral. Petrol., 65, pp. 379-394.

Burke, K.C. and Dewey, J.F., 1971: Orogeny in Africa, in T.F.J. Dessauvagie and A.J. Whiteman (eds), African Geology, pp.

538-608. Univ. Ibidan, Nigeria.

Dow, D.B., and Gemuts, I., 1969: Geology of the Kimberley region.

Western Australia: The East Kimberley: Bull. geol. Surv.

West. Aust., 120.

Kroner, A., 1977: The Precambrian geotectonic evolution of Africa: plate accretion versus plate destruction. Precambrian Res., 4, pp. 163-213.

Rutland, R.W.R., 1981: Structural framework of the Australian Precambrian, in D.R. Hunter (ed.), Precambrian of the Southern Hemisphere, pp. 1-32, Elsevier, Amsterdam.

ISOGRADS, REACTIONS AND P/T GRADIENTS FOR LOW PRESSURE-INTERMEDIATE METAMORPHISM IN THE PROTEROZOIC ROBERTSON RIVER FORMATION, NORTH QUEENSLAND M.J. RUBENACH & T.H. BELL Geology Department, James Cook University, Townsville, Queensland.

Isograds mapped in the Robertson River Formation resulting from prograde metamorphism during D2 are superimposed on Fi folds and early-formed ¥2 folds, and have been themselves folded during the D3 to D5 events. Nevertheless they show a relatively simple pattern, reflecting a general pressure decrease for the northern part of the area compared with the south. Zonal sequences between the biotite and sillimanite zones for three traverses shown on the accompanying figure are as follows: A-A^

: Chloritoid zone (ctd-cte); garnet zone (gt-bio-cte and ctd-cte); staurolite zone (st-bio-gt).

57

B-B

:

Chloritoid zone (ctd-cte); staurolite zone (st-bio-gt); andaluslte zone (st-bio-and-gt).

C-Ci

:

Cordierite-andalusite zone (and-bio-cord); stauroliteandalusite zone (st-and-bio-gt).

Bio zone

The northern traverse (A-A^-) is clearly equivalent to the Buchan Zones of the Scottish Highlands. Either or both the other traverses appear to be equivalent to the Stonehavian Zones, but the characteristic chloritoid-biotite assemblage is almost absent. A tentative explanation for this is that the relatively high Mn contents of garnets in the Robertson River Formation displaced reactions such that the staurolite-forming reactions overstepped the chloritoid-biotite assemblage.

Above the staurolite isograd, garnet was progressively dissolved depending on the Mn zoning. Thus geothermometers and geobarometers based on garnet are of no use in determining P and T for most rocks in the area. Our only alternatives for determining P and T are some experimentally studied reactions, petrogenetic grids for the KFMASH system, and a comparison with zonal sequences in other areas.

Nevertheless it is clear that the temperature gradients with depth were very steep, and were obviously controlled by the rise of granitoid batholiths (e.g. the Forsayth Batholith) during D2. This

58

is typical of the North Queensland Proterozoic, which is dominated by low pressure-intermediate facies series metamorphism. Thus calculation of geothermal gradients for metamorphic peaks in such terrains reflects only the abnormal heat flow related to the rise of granitoids, which in turn was probably the result of transfer of heat from the upper mantle into the lower crust.

REGIONAL METAMORPHISM IN THE SNOWY MOUNTAINS Lesley A.I. Wyborn Bureau of Mineral Resources, Canberra ACT

The Snowy Mountains lie on the eastern edge of the Wagga Metamorphic Belt and contain the most diverse mineral assemblages yet observed in the Belt.

The metamorphism affects the quartz-rich greywackes as well as a heterogeneous sequence of mafic volcanics and volcaniclastic rocks.

The meta-greywackes are restricted in composition and have low CaO and Na20 contents as well as high K2O and AI2O3.

The volcanic sequences, in contrast have high CaO, MgO. and FeO and are low in AI2O3 and K2O. In lower greenschist grade rocks, muscovite and chlorite coexist in the greywackes, whilst actinolite develops in_ the mafic volcanics.

In upper greenschist grades biotite develops in both rock suites.

Lower amphibolite grade rocks are distinguished by knots of andalusite, whilst almandine, staurolite, diopside^and bluegreen hornblende appear in the mafic volcanics.

In upper amphibolite grade rocks fibrolite appears first followed by cordierite + K-_ feldspar in the greywackes, whilst green brown hornblende forms in the volcanic rocks.

Most reactions are divariant depending not only on P and T but also on the Fe/Mg ratio of the ferromagnesian phases.

In the greywackes it can be shown that their restricted composition forbids the formation of cordierite throughout most of the amphibolite grade and the alteration of this mineral, which has. been generally attributed to a retrograde phase of metamorphism,is probably caused by prograde removal.

Sillimanite prisms (as opposed to fibrolite) are also restricted by bulk rock composition to aluminous slates in upper amphibolite grade rocks.

K-feldspar was not found to coexist with andalusite as is common in the Cooma region.

In the mafic volcanics, a miscibility gap was observed in the amphiboles from quartz bearing epidote + albite + chlorite metabasites whilst in contrast, there was complete gradation from actinolite to hornblende in quartz free metabasites which contained primary plagioclase and limited epidote and chlorite.

The diverse mineral assemblages developed in the Snowy Mountains allow a precise estimate of the PT conditions of metamorphism, 2.4 to 4 kb and 600 to 750OC. These pressures are higher than has previously been considered for the Wagga Metamorphic Belt and the metamorphic style is no longer believed to be analogous to contact metamorphism.

59

The peak of metamorphism was believed to be at the end of the Ordovician^as in two localities chlorite grade lower Silurian sediments unconformably overly biotite bearing Ordovician rocks.

The culmination of the metamorphism led to the production of large scale S-type granite melts.

However, most of the granite in the Snowy Mountains Region is not derived from the Ordovician greywackes.

GRANULITES AND THE TECTONIC EVOLUTION OF THE ARUNTA BLOCK

R.G. Warren Bureau of Mineral Resources, Canberra ACT

Granulites occur in the central and northern zones of the Arunta Block. Mineral assemblages preserved from the granulite stage and overprinted younger assemblages show different conditions of metamorphism and different cooling paths for granulites from different areas within the Arunta Block, a result of differing tectonic regimes.

Granulites in the central zone formed at depths equivalent to 7-9 kbar, and cooled under near-isobaric conditions, as shown by the presence of magnesian garnet (Mg number 40-55), by intersection of the cooling path with reaction curves of low positive slope (e.g. Cd Opx + Sil + Q) , and by formation of kyanite and staurolite during hydration in shear zones.

Granulites from the Reynolds Range (northern zone) formed at lower pressures (shown by the sparse occurrence of garnet and widespread occurrence of cordierite with Mg numbers down to 70); but probably cooled isobarically, as indicated by kyanite and staurolite in shear zones which transect the area.

In other granulites, from east of Aileron and near Deep Bore in the northern zone, and in the Kanandra Granulite from the northeastern part of the central zone, inversion of garnet to cordierite-bearing assemblages indicates early uplift at high temperatures. Kyanite is absent from shear zones cutting these granulites. The overprinting during uplift makes estimation of pressure for these granulites difficult; such data as are available indicate shallower depths than for the central zone in the Strangways and Harts Ranges.

Granulites in the northern Arunta Block form basement to Late Proterozoic sediments, but the granulites of the central zone may be remained under isobaric conditions until the massive uplift along the northern edge of the Amadeus Basin in the midPalaeozoic .

60

PARTIAL MELTING AND CONDITIONS DURING HIGH GRADE METAMORPHISM V.J. Wall\ J.D. Clemens^ and S,R. Bohlen^ ^Dept. of Earth Sciences, Monash University, Clayton, Vic., 3168. ^Earth & Space Sciences, S.U.N.Y. Stonybrook, Stonybrook, N.Y. 11794, U.S.A. The widespread development of comparatively high temperature, crustally-derived granitoids (e.g. Wall et al., this volume) indicates that extensive partial melting is associated with granulite facies metamorphism.

This has significant implications for high grade metamorphic conditions, the role of volatiles in such deep crustal environments, as well as for their thermal and compositional evolution.

Experimental phase equilibrium studies together with geothermometry and geobarometric investigations on some granitoid suites (e.g. Clemens & Wall, 1981) imply that the magmas formed at temperatures > 800-850°C and had H2O contents in the range 2-4 wt.%.

For source region pressures between 5 and 10 kbars such melt H2O contents necessitate moderate water activities in the anatectic environment - in conflict with the commonly held view that granulite facies metamorphism is "dry".

Water activities below 1 may result from the presence of other components (notably CO2, CHi+) in metamorphic fluids or could reflect vapour absent conditions.

The development of large volumes of hydrous anatectic melt obviously required an adequate supply of H2O component in the source region - an aqueous fluid and/or crystalline hydrates.

Quartzo-feldspathic segregations with granitoid compositions and non-granoblastic textures are commonly developed on a mesoscopic scale in high grade metamorphic rocks.

The modal and chemical character of these segregations indicates that they were partially molten during high grade metamorphism, although they commonly depart from haplogranitic compositions due to diffusive exchange with their host rocks.

Application of recent experimental data for feldspar-qtz-H20-C02 systems (Bohlen et al. 1981, 1982), together with Burnham's (1979) models for hydrous melt thermodynamics provides a P-T-aj^^g ^^^ dehydration equilibria (e.g. Bohlen et al). Estimates of T-fj^^Q relations for high grade partial melting may be made.

We give examples for Broken Hill and other terrains.

Geological implications from applications of the se models and granitic petrology are: i)

H2O fugacities are substantially higher in many granulites than published biotite-equilibria based estimates would suggest.

ii)

Vapour absent conditions may be common in granulites and a pervasive CO2-H2O fluid is not universally present.

An H2Orich fluid commonly attends upper amphibolite facies metamorphism.

iii)

The extent of partial melting is largely controlled by the availability of H2O and H2O activities in high grade rocks are commonly controlled by dehydration-melting reactions.

61

iv)

Inferred melt segregations are conmion in metamorphic belts which show near isobaric, early retrograde cooling paths (e.g. Broken Hill), Such features suggest that decompression melting is not universal in crustal anatexis.

v)

Crystallisation of melt segregations during retrograde metamorphism releases aqueous fluids and may explain C02-H20-rich fluid inclusions in high grade rocks.

The volatiles released also cause retrograde adjustments in host rocks.

Retrograde partial melting and metasomatic reactions may signal introduction of externally derived fluids.

THE HYDROTHERIIAL HISTORY OF A SMALL GRANITE-GREISENENDOSKARN-EXOSKARN SYSTEM.

AN EXAMPLE OF A LARGELY

CLOSED CIRCULATION SYSTEM. W.M. Brown

1

& T.A.P. Kwak

2

^R.M.S., 168 Greenhill Road, Parkside, S.A. "Department of Geology, La Trobe University, Bundoora, Victoria.

The Hole 16 deposit, located 5 kilometres south of Mt Garnet, N. Queensland, consists of a zoned F-Sn-W skarn which overlies a greisenized granitic cusp. From the contact with the granite upward the exoskarn consists of: (a) massive (Sn-) garnet-magnetite ± pyroxene ± cassiterite ± fluorite (b) "wrigglite'' - alternating layers of magnetite vs vesuvianite ± fluorite or cuspudine ± fluorite vs magnetite ± Fe-Zn spinel (c) massive to coarsely layered vesuvianite + fluorite ± garnet ± magnetite ± scheelite. Alteration of the F-annite-bearing "A-type" leucogranite increases upward so that nearest to the skarn the endoskarn consists of essentially phengite + quartz + fluorite + calcite.

Fluid inclusions in all the units, including the granite, indicate (1) the solutions were extremely concentrated throughout. Quartz and fluorite in the granite average 69.34 and 62.13 wt. % total dissolved salts respectively while garnet have 54.78: clinopyroxene, 57.25; vesuvianite 57.94; skarn fluorite 54.77 and late skarn calcite 48.66.

(2) temperatures were close to magmatic (near or greater than 600°C) throughout except for those in late calcite (ICQ to 300°C). (3) except for unusual rare hypersaline fluids in granite quartz the fluids in both environments were similar in being high in KCl vs NaCl, CaCl2, CaF2 and ZnS. Granite quartz and fluorite have higher FeCl2 while skarn minerals have no FeCl2 but higher CaCl2. (4) both saline and CO2 gasrich fluid inclusions occur indicating the system was in a 2 phase region during most of genesis.

The data can best be interpreted to show that limited exchange by reaction in marble to produce exoskarn and granite to produce greisen and endoskarn occurred. Periodic loss of CO2 is indicated but dilution

62

from circulating meteoric water must have been extremely limited. The lack of such dilution has inhibited retrograde skarn reactions and the subsequent redistribution of Sn from garnet breakdown. This is probably the reason why many similar, characteristic skarns in the Mt Garnet area do not contain economic quantities of mineable Sn.

GEOBAROMETRIC SIGNIFICANCE OF b

VALUES OF K-WHITE MICAS o IN LOW GRADE METAMORPHIC ROCKS,

WINDAMERE DAM - CUDGEGONG AREA, N.S.W. R. Offler

1

& J. Pemberton

2

^Department of Geology, University of Newcastle

2

Department of Geology, University of Wollongong

The rocks in the Windamere Dam-Cudgegong area lie within the Capertee High. They show upper anchizonal to epizonal illite crystallinities and sub-greenschist to lower greenschist facies assemblages. Two metamorphic zones have been defined on the basis of mineral assemblages in the basic rocks, viz. epidote - actinolite - pumpellyite - prehnite and epidote - actinolite, with the former dominating most of the area.

The grade of metamorphism increases from northeast to southwest.

According to the geobarometric scale of Sassi & Scolari (1974) , the mean b value of K-white micas in pelitic rocks indicate a metamorphism intermediate between the low-medium pressure type of north New Hampshire, U.S.A. and the medium-high pressure Harrovian type of Otago, N.Z. The pressures suggested by the composition of the white micas is confirmed by the sporadic occurrence of pumpellyite in the epidote-actinolite zone and the wider distribution of this mineral relative to prehnite in the epidote - actinolite - pumpellyite prehnite zone. These results contrast with those obtained by other investigators in the Hill End Trough where low pressures are considered to have existed during metamorphism. The slightly higher pressures in the study area are possibly due to the existence of a very thick sequence of Lambie Group beds prior to folding.

Sassi, F.P. & Scolari, A., 1974: The b value of the potassic white micas as a barometric indicator in low-grade metamorphism of pelitic schists. Contrib. Mineral. Petrol., 45, 143-52.

63

THE 1:10 MILLION METAMORPHIC MAP OF AUSTRALIA A . J . STEWART & G . W . D'ADDARIO Bureau of M i n e r a l R e s o u r c e s , C a n b e r r a , A . C . T .

The 1:10 million metaraorphic map is being prepared for the BMR Earth Science Atlas of A u s t r a l i a . The map is based on the 1:5 million metamorphic map currently in course of publication by BMR for the Commission for the Geological Map of the W o r l d , and attempts to portray the part played by regional metamorphism in the tectonic evolution of A u s t r a l i a . Please inspect and criticise the map at your leisure.

THEORETICAL PHASE RELATIONS OF FERRO-MAGNESIAN MINEPJVLS IN THE SILICA UNDER SATURATED PORTIONS OF THE SYSTEMS Fe0-Mg0-Al203Si02 (FMAS), FMASH (ADDITIONAL H2O) AND KFMASH (ADDITIONAL K2O) FOR ALUMINOUS HIGH-GRADE AMPHIBOLITES AND GRANULITES B.J. Hansen School of Applied Geology, University of New South V/ales Kensington NSW The phases considered in this study are, in order of decreasing Mgnumber: cordierite (Cd), biotite (Bi), sapphirine (Sa), hypersthene (Hy), gedrite (Ge), staurolite (St), spinel (Sp), garnet (Ga), kyanite (Ky), sillimanite (Si), K-feldspar (Or), quartz (Qz), corundum (Co) and vapour (V).

Experimental work and petrogenetic grids in the iron-free system MAS have limited application to natural rocks, whereas the theoretical analysis of multicomponent Fe-Mg phase diagrams in systems with the number of phases exceeding the number of components (n) by four or more (i.e. n+4, n+5, etc. multisystems) is mathematically intruiging but petrologically fruitless. By combining experimental evidence and observations on natural mineral assemblages internally consistent pressure (P)-temperature (T) grids of petrological interest can be constructed for the following systems (Invariant points and higher variance reactions are identified by the missing phase(s)): 1)

FMAS - phases: Cd, Sa, Hy, Sp, Ga, Si, Co (n+5 system).

One of the stable invariant points in this system, the point (Co) has been described previously (the point (Qz) in Fig. 1 and Table 1 of Hensen, 1971). Three other invariant points are believed to be stable: (Sa), (Cd) and (Ga). These points are linked by the univariant reactions: Hy+Ga+Co = Sp+Si (Cd,Sa), Hy+Si+Co = Cd+Sp (Ga,Sa), and Hy+Co = Sp+Sa+Si (Cd,Ga). The system contains another

64

nine stable univariant reactions and twenty divariant reactions involving Co. 2)

KFMASH - phases: Cd, Bi, Sa, Hy, Sp, Ga, Si, Or, V (n+3 system).

This system has three Bi - present invariant points: (Cd), (Ga) and (Sa) linked by the reactions: Hy+Sa+Or+V = Bi+Si+Sp (Ga,Cd), Cd+Sp+Or+V = Bi+Hy+Si (Ga,Sa), and Hy+Sp+Or+V = Bi+Si+Ga (Cd,Sa).

A further nine univariant and twenty divariant reactions complete the stable portion of this grid.

3)

KFMASH - phases as in (2) + Co (i.e. (l) + (2): n+4 system).

Evidence from natural rocks and topological constraints suggest the invariant points (Ga,Hy), (Ga,Sp) and (Ga,Si) are stable.

The stability of other points is uncertain owing to lack of data from nature and experiment.

4)

F^4ASH - phases: Cd, Hy, Ge, Ga, Sp, St, Si(Ky), Co, V (n+4 system), A continuous net linking low temperature staurolite and staurolitegedrite reactions with high temperature hypersthene reactions has been derived. One of the stable closed sequences of invariant points is (Sp,St)-(St,Si)-(Hy,St)-(Hy,Ga)-(Hy,Sp)-(Sp,St). Most of these invariant points can be directly related to naturally occurring mineral assemblages.

5)

FMASH - phases (l)+(4): n+5 system.

The phases Sa and Hy do not occur with (St). Reactions involving Sa can be linked to some stable St-absent invariant points e.g.

(St,Si,Ga).

6)

KFMASH - phases (5)+(3): n+6 multisystem.

A complete net covering the temperature interval of 700^ to QOO^C should eventually emerge. At present, however, only some of the patches needed to complete the quilt are known with any degree of certainty. Further stitching is in progress.

Reference:

Hensen, B.J. Theoretical phase relations involving cordierite and garnet in the system MgO-FeOAl203-Si02. Contr. Mineral. Petrol., 33: 191-214 (1971).

METAMORPHISM IN THE E A S T E M MANN R A N G E , NORTHWESTERN SOUTH AUSTRALIA R . L . Oliver Department of G e o l o g y , University of A d e l a i d e , South Australia, 5000

In the Musgrave B l o c k , initial D ^ granulite f a d e s metamorphism and deformation of metasediments and volcanics producing layered gneisses is variously h e l d to have taken place between 165O and 1200 M a ago.

65

Widespread plutonic emplacement (both felsic and mafic) into the layered gneisses occurred during the period 1200-1100 Ma in a granulite facies environment.

At the eastern end of the Mann Range, within the Musgrave Block, an area of 100 sq. kms consists predominantly of an unfoliated to weakly foliated medium to coarse grained granitoid, a manifestation of the 1100 Ma plutonic activity referred to.

Present also, are areas (up to several hundred metres across) of more strongly layered gneiss of pelitic, intermediate and basic compositions.

These are considered to represent fragments of the regional gneisses contained as xenolithic relics within the meta-igneous granitoid.

Mineralogically, the bulk of the granitoid is perthite, plagioclase (Anp^ ) , some of which is antiperthitic, and quartz in approximately equal quantities, with subordinate biotite, orthophyroxene and garnet.

A similar mineralogy typifies the layered intermediate gneiss but with proportionately more ferro magnesian minerals.

The presence of sillimanite apparently altering to kyanite in more pelitic types is noteworthy.

Mafic gneiss contains plagioclase, ortho and clinopyroxene, opaque and variable hornblende, biotite and garnet.

Most of the above phases are considered to represent granulite facies metamorphism during D^ and D^ and during the period of granitoid emplacement.

Profuse basic dyke intrusion followed the granitoid emplacement.

A D event, involving both gneisses and basic dykes produced rather tight to open folds with NW-SE to E-W trend.

D^ is manifested by tight folding and abimdant development of E-W trending, south-dipping thrust faults and shears.

Typical examples of recrystallisation in the zones of high strain are the development of mortar textured feldspar porphyroblasts , "plattung" quartz,coronas of granular garnet and hornblende, secondary biotite and some epidote/clinozoizite.

The hornblende coronas typically mantle pyroxene but also biotite, 12-16^ ^^2^3 ^ ^ 1.2-2.0^ Na^O in the hornblendes indicate that this mineral, though secondary, crystallised at relatively high grade.

Garnet coronas are commonly peripheral to orthopyroxene in contact with plagioclase but perhaps more abundantly are found mantling ilmenite or magnetite.

Typical garnet composition, mantling orthopyroxene, is almandine h2 spessartine 19 pyrope 23 grossular l6 (assuming all Fe is in almandine).

Garnet peripheral to opaque is almandine 51 pyrope 21 grossular 20.

spessartine 10

The secondary minerals developed are indicative of amphibolite facies metamorphism. Application of Ferry and Spear»s biotitegarnet geothermometer suggests 640oc at 5 kbrs and 660^ at 10 kbrs.

66

The D^ - D^ granulite fades PT environment, suggested "by the coexistence of hornblende and orthopyroxene in "basic rocks and the presence of garnet rather than cordierite in felsic compositions, is ca 1000 C and 10 khrs.

It is suggested that cooling accompanying decrease of pressure due to uplift has taken place along a cooling gradient to intersect the sillimanite-kyanite "boundary at a temperature and pressure of ca 650^ and 7,0 khrs.

Such a pressure and temperature is close to an extrapolation of the orthopyroxene + plagioclase ^clinopyroxene + garnet for rocks of granitic or adamellitic composition and would he compatible with the presence of epidote/clinozoizite in rocks of appropriate composition.

-I 6

Prograde Retrograde

1 7 8 Temperature in

granulite

facies

c r y s t a l ! isat ion

1 1 9 10 C (x100 ) crystallisation

r 11

13

67

STRUCTURAL EVOLUTION OF THE HARTS RANGE GROUP, CENTRAL AUSTRALIA P. Ding, P.R. James, R.W. lawrence Department of Geology & Mineralogy, University of Adelaide, S.A. 5000 Detailed structural mapping on 1:25000 scale of the Harts Range Group shows that it comprises a thick reciambent sequence of laterally extensive units of as yet undetermined age (Figure 1), all of which are assumed to overlie the Strangways Metamorphic Complex. Following B.M.R. terminology four stratigraphic divisions are described each with markedly different structural histories.

The Entia Gneiss outcropping in the Entia Domal Structure and south of Mt. Ruby comprises quartzofeldspathic and calcic gneiss, amphibolite and pelitic schist.

The Bruna Gneiss is a homogeneous granitic gneiss characterised at its margins by the variable development of biotite rich mylonitic gneiss. The Irindina Gneiss can be subdivided into a number of laterally persistant members. Directly overlying the upper contact of the Bruna Gneiss, the Ruby Amphibolite Member (Ruby Layered Igneous Complex) is thickest (max. ca. 2,000 m) at Mt. Ruby and thins dramatically away from this. Overlying this the Irindina Gneiss is predominantly a monotonous pelitic gneiss.

The Naringa Calcareous Member and its equivalents are characterised by rapid lateral thickness and facies variations.

Overlying this member within the Irindina Gneiss, the Riddock Amphibolite (Paddock Layered Igneous Complex) comprises strongly layered mafic amphibolite, with near the base a significant leucogabbro-anorthosite unit (Entire Anorthosite).

In the northwest of the area, the overlying Brady Gneiss is parallel to a sharp discordance, trending locally NE-SW but curving around the north of the Entia Domal Structure, southeast of which Irindina Gneiss and its constituent marker members are severely thinned.

Intense layer parallel fabrics with strong linear components are pervasive within all lithologies.

These fabrics along with other structures developed during an early recumbent upper amphibolite facies tectonothermal event.

In the Entia Gneiss the fabric is commonly folded by very tight similar style to tight angular asymmetric minor folds and these are truncated along a major decollement parallel to the boundary of the overlying Bruna Gneiss.

In the north layer parallel blastomylonites are restricted to 10 m thickness at the upper and lower boundaries of the Bruna Gneiss; however they thicken to the south as the whole unit develops an intense LS high grade fabric which is mylonitic in part and contains many interlayered mafic bodies.

In the Irindina Gneiss at least three northerly trending recumbent fold generations (F^, F2 and F3) formed during this event, the major generation (D2) forming fold nappes on a scale of 10-20 km. Intense flattening features and a pervasive northerly trending (near horizontal) elongation lineation, regular and ubiquitous over the 50 km section mapped, attest to the magnitude and intensity of this ductile strain event.

A major flexure of the lineation from 015^ in the south to 320^ in the north may reflect early incremental strain variation.

F4 is tight and reclined VJ. The early folds and fabrics are complexly overprinted by a number of major interfering upright open fold generations which have led to the complex large and small scale outcrop patterns shown in figure 1.

F5, F^ and F-y trend NW, ENE and E respectively whilst the whole synformal sequence dips shallowly to the north. The sympathetic Entia Dome results from the interference of three generations of antiform (F5, F^ and F7).

68

RETROGRADE GREENSCHISTS

STRUCTURE OF THE

HARTS RANGE GROUP BRADY GNEISS V T T ^ IRINDINA GNEISS RIDDOCK AMPHIBOLITE MEMBER NARINGA CALCAREOUS MEMBER RUBY AMPHIBOLITE

MEMBER

BRUNA GNEISS r r ^

MYLONITE GABBRO

P ^ ^

ENTIA GNEISS

r r ^

INKAMULLA GRANODIORITE

^RANGWAYS METAMORPHIC COMPLEX [ R ^ BUNGITINA METAMORPHICS

^

FIG.1

HARTS RANGE GROUP, CENTRAL AUSTRALIA SCALE Km

5

lU I 23°00'-

69

SCAPOLITE-WOLLASTONITE-CALCITE ASSEMBLAGES IN GRANULITES FROM THE ARUNTA BLOCK AND ANTARCTICA R.G. Warren^ and B.J. Hansen^ ^Bureau of Mineral Resources, Geology and Geophysics, Canberra, ACT^ ^School of Applied Geology, University of New South Wales, Kensington,NSW

Scapolite bearing calc-silicate rocks form a minor but petrologically important component of granulite units in the Arunta Block, Central Australia and in the Napier province of Antarctica. Mineral assemblages in these rocks contain information on pressure, temperature and fluid composition during high grade metamorphism and subsequent cooling and uplift.

Calc-silicate from the central zone of the Arunta Block contains the high temperature assemblage, scapolite (Meionite: (An84Abi6)3. CaCOa), wollastonite, calcite, garnet (GrgsAndss) and diopside. Triple junctions of scapolite, wollastonite and calcite are common. Garnet porphyroblasts including scapolite appear in textural equilibrium also. A second generation of garnet (Gr7 5And2 5) occurs as discontinuous rims between scapolite and other high temperature phases, except calcite. Discontinuous quartz rims occur against wollastonite separating it from first generation garnet, from calcite, in one single instance, and from scapolite rimmed by garnet. Calcite is mostly in direct contact with wollastonite.

Partial breakdown of scapolite to plagioclase and calcite postdates the development of the rimming phases.

The Kanandra sample has scapolite (Ans oAb2 o)3';CaC03) o. 8 (NaCl) o • o 8 (CaSOk)o.i2 with diopside, wollastonite, calcite and quartz. Garnet (GresAnds?) forms rims separating scapolite from diopside, wollastonite and scapolite. Wollastonite is found abutting calcite directly or is separated from it by a quartz rim.

The sample from McLeod Nunatak from the border of the Rayner and Napier provinces, Enderby Land Antarctica contains scapolite (An82Abi8)3 CaCOa), wollastonite, plagioclase (An9iAb8.5 Oro.5), calcite and diopside.

Scapolite has partly broken down to plagioclase and calcite, and wollastonite to calcite and quartz.

In the calc-silicate assemblage diopside is the only mineral containing significant amounts of FeO and MgO. The stability of the other minerals can be modelled in the system Ca0-Al203-Si02-C02-H20, (Ellis, 1978) .

A series of temperature (T) versus fluid composition reveals the following:

diagrams

1) The phases grossular (Gr), wollastonite (Wo), meionite (Me), anorthite (An), calcite (Cc), quartz (Qz) and fluid (V) coexist at an invariant point estimated at 880^0, 9Kb and X^Q^ = 0 . 3 .

2) The assemblage Me-Wo-Cc is limited at low X q and towards lower P by the isobaric univariant reactions: Me + Wo + Cc = Gr + V and Me + Wo = Gr + Qz + V and at low T by: Me = An + Cc, which is insensitive to P.

70

3) In the natural assemblages the stability of Me is extended by Na-substitution. CI and S have a similar effect. The stability of Gr is extended by Fe^+substitution. This effect is partly offset by Na in plagioclase. The combined effect of these substitution moves the invariant point to lower P and T, and higher X^^ .

LU2 In the light of the above considerations we conclude the following: 1) The Yambah scapolite formed above 750^C (calibration from Ellis, 1978) in the isobaric trivariant field of the reaction: Cc+Wo+Me = Gr ss ss Introduction of water from an adjacent shear zone lowered the X and resulted in the formation of Grggpz rims by the reaction: Wo = o o Grgg + Qz. A drop in temperature during hydration ^s possible but conditions stayed in the Me^g stability field ( > 750 C) .

2) The Kanandra scapolite gontains only 80 percent Megg and therefore may have formed below 750 C. In view of the P-T history of the surrounding granulites (Warren, this convention) the garnet-quartz rims are believed to have formed during near isothermal uplift. Subsequent cooling caused: Wo 4- V = Cc + Qz .

3) In the Antarctic samples MCgg occurs with Angg and Cc, and Wo with Cc and Qz. These relationships indicate near isobaric cooling at constant, high X^^ .

Reference:

Ellis, D.E. Stability and phase equilibria of chloride and caifbonate bearing scapolite at 750^C and 4000 bar. Geochem. Cosmochem. Acta 42: 1271-1281.(1978)

* Published with the permission of the Director, Bureau of Mineral Resources, Geology and Geophysics.

KORNERUPINE-SAPPHIRINE-GRANULITES FROM THE WESTERN HARTS RANGES, ARUNTA BLOCK

B.J. Hensen

2

and R.G. Warren

School of Applied Geology, University of New South Wales, Kensington, NSW Bureau of Mineral Resources, Geology and Geophysics, Canberra, ACT

Kornerupine and sapphirine have been found in a silica undersaturated lens, closely associated with mafic two-pyroxene granulites.

The lens in chemically and mineralogically heterogeneous, with one side adjacent to a small shear zone. Euhedral prismatic green kornerupine

71

crystals up to 20 cm in length are locally abundant. Subhedral to euhedral sapphirine (up to 2 cm) is associated with kornerupine in some of the rocks.

One rock of particular interest contains the following minerals (abbreviations and chemical data from electron microbe_^analyses given in brackets): cordierite (Cd: (Mgi.sF^Lz) (AI3.9 sFe? . 0 2 ) (Alo-ozSii, .98)0i8) , sapphirine (Sa: zoned from: (Mg2 .ygFe^??) (A1^ . 2sFeo . 33) (Ali^. seSii .itu) O 2 0 to (Mg2.93FeL62) (Mit.31Feo.2O (Al4.55Sii.45)020, hypersthene (Hy: (Mgo.68Feo.26) (FegT,lAlo.0e) (Alo.07SI1.93)O3, kornerupine (Ko: (Mg2.65Feo .40) (Al4.72FeLf3) (Alo . 78^i3. 22^ (Sic . 2 sBo . 7 O (0,0H)^2; wetchem, gedrite (Ge:Nao . se (Mgit. 27Fei. 52Mno • 0 iCao . 02) (Ali.otFe o.!**) (Al1.74Si6.25) (0,OH)20, tourmaline (To), biotite (Bi), titanhematite (Heme 0 lint o),rutile (Ru), corundum (Co), sillimanite (Si: both Co and Si contain 1 - 1.5 W% FezOs) albite (Ab: An4-5).

Sapphirine crystals contain oriented hematite lamellae and are zoned with a Mg-rich rim. Only where sapphirine has a complete or nearly complete mantle of albite grains surrounding it, has it maintained a euhedral to subhedral outline. Even here it has a thin fine grained reaction rim completely surrounding it. This rim consists of an inner zone of corundum and sillimanite and an outer zone of hypersthene and sillimanite - biotite - rare tourmaline - titan hematite. These rims also wrap around portions of crystals that have been completely replaced by coarse intimately intergrown titanhematite,sillimanite and corundum,and minor rutile. Sapphirine is mostly or completely broken down,where it neighbours cordierite^tohypersthene, biotite and sillimanite.

Kornerupine is not affected by alteration. It occurs in a group of roughly aligned prismatic crystals within the fine breakdown products of sapphirine described above. It includes rare small irregular sapphirine grains, texturally and compositionally identical to sapphirine relics in the matrix, and oriented sillimanite. Sheaflike sillimanite appears to have nucleated on the surface of the kornerupine grains.

Similar sillimanite is found on larger sillimanite crystals and on opaque grains. In one instance where kornerupine abutts sapphirine a finegrained border zone contains kornerupine with sillimanite and corundum.

Hypersthene forms spectacular oriented intergrowths with sillimanite - biotite. These are often seen to invade cordierite. Overgrowths on and intergrowths with biotite are common. Its Mg-number shows no variation but AI2O3 varies: 4.8 - 7.3 w%.

Gedrite rarely occurs in the fine intergrowths where sapphirine has largely or completely disappeared. Like hypersthene its composition varies (Na20 = 1.6 - 2.2w%, AI2O3 = 13.4 - 18.4 w%) Cordierite and biotite are remarkably homogeneous throughout irrespective of textural variability. Titanilmenite (HemeoHm^o) is now exsolved with host (Hem7oIlm3o) and lamellae (Ilm96Hem4). It contains inclusions of rutile.

The following steps in the metamorphic history of these rocks are proposed: 1)

High grade dry granulite m^tamorphism giving rise to Sa-Cd-Na rich Alkali feldspar - Ru - Si.

72

2)

Formation of He lamellae in Sa. Oxidation of Sa by a reaction such as: (Mg Fe) Sa + Ru + O2 =(Mg)Sa + Han- Ilm + Co + Si.

3)

Hydration,probably contemporaneous with (2).

Formation of reaction rims and breakdown of Sa by the rer.ctions Sa + K-feldspar + H2O = Hy + Bi + Si + Co (rims) and Sa + Cd + K-feldspar + H2O = Hy + Bi + Si. These reactions are divariant in KFMASH for P^^Q < Ptotal- ^^^ ^^^ consistent with the observed relationships. The K-feldspar was either derived from a preexisting alkali feldspar, leaving nearly pure albite, or introduced by metasomatism.

4)

Formation of Ko. The Ko is B-bearing. Possibly B is introduced during hydration.

A reaction describing this is Cd + Sa + B = Ko + Si.

5)

Formation of Ge. (further hydration) probable reaction: Hy + Si + Ab + H2O = Ge + Cd.

6)

In some rocks, occuring close to the shear zone Ko breaks down: Ko + K-feldspar component = Bi + To + Si - Co. This reaction describes the observed partial and complete pseudomorphs after Ko found in the rocks, which have abundant biotite and lack cordierite, hypersthene, gedrite or sapphirine. K-metasomatism is believed to be responsible for the changes in mineralogy and chemistry.

^Presented with the permission of the Director, Bureau of Mineral Resources, Canberra.

CHEMOGRAPHIC RELATIONSHIPS IN SAPPHIRINE GRANULITES FROM THE N.W. STRANGWAYS RANGE, CENTRAL AUSTRALIA

D.P. Windrim & W.E. Cameron

Department of Geology, Australian National University Canberra ACT

Sapphirine occurs as a rock-forming mineral in the Lower Proterozoic granulite terrain of the northwestern Strangways Range, Northern Territory. Although sapphirine-bearing granulites comprise << 1% of the sequence, they occur widely as small lenses Im x 0.5m) or large irregular masses 20m x 7m) within or adjacent to the following rock types: cordierite ± orthopyroxene, garnet, sillimanite granulites, quartzofeldspathic garnet granulites, mafic, ultramafic and calc-silicate granulites and quartz-magnetite rocks. Sapphirine is most commonly accompanied by the phases orthopyroxene, cordierite.

73

spinel, sillimanite, corundum, magnetite, gedrite and phlogopite; sapphirine + garnet is rare, and the association sapphirine + quartz is incompatible.

In general, between 95 and 98% of the bulk composition of the sapphirine granulites can be specified by the 4 component system Fe0(F)-Mg0(M)-A1^0^(A)-Si0^(S), K^O(K) and H^O(H) being the most important additional components. Continuous mineralogical gradation from high (5) to low (0) variance assemblages (involving the above 6 components) on a scale of tens of centimetres is common and may be explained by either (i) syn-metamorphic diffusion/infiltration metasomatism in response to externally imposed chemical potential gradients or (ii) essentially isochemical metamorphism of unusual pre-existing compositional variations. Detailed mapping and both major and trace element geochemistry support the latter possibility; thus, variations of phase are interpreted in terms of repeated metamorphism of cm to m-scale domains of contrasted bulk composition.

Compositions of minerals (i.e. orthopyroxene, clinopyroxene, garnet, cordierite, sapphirine, biotite) stable in different bulk compositions have been used to estimate the physical conditions of granulite facies metamorphism. Comparison of observed mineral equilibria in the model systems MAS (H) , FMAS (H) , KMAS (H) and ICFMAS (H) with experimental data places additional constraints on peak metamorphic conditions (T = 850 C, P = 8Kb) and indicates an approximately isobaric cooling history with major hydration and oxidation reactions occurring under conditions appropriate to the almandine amphibolite subfacies.

EXPERIMENTAL CALIBRATION AND APPLICATION OF FOUR GEOBAROMETERS FOR GARNET-BEARING ASSEMBLAGES V.J. Wall\ S.R. Bohlen^, and A.L. Boettcher^ ^ ^Dept. of Earth Sciences, Monash University, Clayton, Vic., 3168. ^Dept. of Earth and Space Sciences, U.C.L.A., Los Angeles, Cal., 90024.

Accurate geobarometry is essential for evaluating geothermal gradients and can afford important insights into the tectonic and geo— chemical evolution of the Earth's Crust.

We present four newly calibrated geobarometers and discuss the systematics and results of their applications to crustal problems.

The following equilibria were tightly reversed (750-1100°C, low f02) and extrapolated thermodynamically to the lower temperatures: 1) 3 ilmenite + sillimanite + 2 quartz = almandine + 3 rutile 2) 3 hercynite + 5 quartz = almandine + 2 sillimanite 3) fayalite + anorthite + garnet (GriAlm2) whereas: 4) 3 ferrosilite + anorthite = garnet (GriAlm2) + quartz was calculated from 3) and 5) Fa + Qtz = Fs. 1) and 2) are applicable to

74

peraluminous compositions under amphibolite and granulite facies conditions while 4) and 5) are model garnet-granulite equilibria.

The first reliable free energy data for almandine have been retrived from 1) and one used to calculate other geologically significant equilibria.

Pressures calculated for natural assemblages are sensitive to the solution models for impure phases, particularly garnet.

Tests of a range of solution models involving equilibria 1) through 4) and comparisons with other well calibrated geobarometers are used to constrain the activity-composition relations.

The results indicate that our new geobarometers can yield pressure estimates as good or better than other garnet-based equilibria.

Results for magnesian- or manganese- rich compositions are less definitive.

Some geological implications from geobarometric applications are: i)

Geothermal gradients in the amphibolite facies vary over a wide range.

Application of equilibrium 1) is hampered by inadequate recording of oxide phase assemblages and compositions and 2) is restricted to zincian spinels.

ii)

Few exposed granulite terrains exhibit P > 10 kbars and thus may not be representative of Lower Crustal sections.

Some terrains dominated by mafic rocks and some inclusions in diatremes are exceptions.

iii)

Published pressure estimates for many medium and high grade terrains are too high - due to the use of less accurate geobarometers.

Our new estimates have the effect of increasing inferred geothermal gradients for these terrains.

iv)

Many granulite massifs are magmatically and tectonically thickened supracrustal arrays supporting the contention that thrust/nappe tectonics are dominant in the lower and middle crust.

v)

Retrograde P-T paths commonly approach near isobaric cooling, giving further weight to the view that development of low Pand many medium P metamorphic belts involves a significant contribution of magmatic heat.

vi)

Near isobaric cooling paths exhibited by some metamorphic belts which have apparently involved substantial crustal thickening raise important questions on the rate of isostatic adjustment and the role of the latter in uplift.

vii)

Although good geobarometers are now available there is a need for more accurate geothermometers applicable to high grade metamorphic situations.

75

A CHEMICAL AND OXYGEN ISOTOPE COMPARISON BIlTWEEN MAFIC GRANULITES FROM THE GEORGETOWN INLIER, QUE:!:NSLAND, AND MODERN OCEAN FLOOR BASALTS

N.J. McNaughton^ & Allan F. Wilson^

^Department of Geology, University of Western Australia, Nedlands 6009, WA ^Department of Geology and Mineralogy, University of Queensland, St Lucia 4067, QLD

Modern ocean floor basalts are often subjected to alteration

processes such as submarine weathering, hydrothermal alteration and low- to medium-grade metamorphism which may significantly modify their original chemical and isotopic composition. In ancient basaltic rocks, these changes often constrain our interpretation of their primary composition, particularly where high-grade metamorphism may also produce compositional modifications.

Here we discuss 1.6 Ga old mafic rocks from the Georgetown Inlier which have undergone granulite facies metamorphism to a hornblende-stable assemblage. The concentration of the "immobile" elements (Ti, P, Zr, Y, Nb), as well as highly mobile K and Rb, and the values of these mafic granulites are indistinguishable from modem low-K tholeiites to ocean floor basalts. The only significant chemical differences in the granulites relative to ocean floor basalts are (i) an increase in H20"^ which predates the high-grade event, (ii) a lower Fe^"^/ZFe, and (iii) a slight decrease in Na which occurs during the waning of the granulite facies metamorphism.

Thus, we conclude that the chemical and oxygen isotopic compositions of these mafic granulites have remained essentially unchanged since their emplacement, and were produced from a source area and by a magma generation process similar to that presently acting under the mid-ocean ridges.

76

Symposium

1(e)

Palaeomagrietism a n d l i t h o s p h e r e

dynamics

Convener: Dr B.J.J. Embleton

77 PALEOZOIC P A L E O M A G N E T I S M AND PLATE TECTONICS: THE ASSEMBLY OF PANGAEA Rob V a n d e r Voo Department of G e o l o g i c a l Sciences the Universith of Michigan Ann A r b o r , M I . 4 8 1 0 9 , U . S . A .

New paleomagnetic data have become available in the last decade that allow us to constrain b e t t e r the various models for the assembly of Pangea w h i c h took place during Paleozoic t i m e .

The c o n t i n e n t a l blocks involved in this assembly are G o n d w a n a , Laurentia (North America and G r e e n l a n d ) , Baltica (the Baltic Shield and Russian P l a t f o r m ) , S i b e r i a , Armorica (essentially c e n t r a l and southern E u r o p e ) , and less w e l l d e f i n e d b l o c k s and microplates in A s i a and in b e t w e e n the major blocks.

The paleomagnetic poles indicate that the last collisions in the assembly of Pangaea w e r e b e t w e e n Siberia and Baltica/Armorica/Laurentia (combined) in the Late Paleozoic and b e t w e e n Gondwana and Baltica/Amorica/Laurentia during the C a r b o n i f e r o u s .

The latter collision created the Hercynian-Alleghenian-Ouachita-Mauretanide b e l t , and caused i n t e r n a l deformation in the combined northern c o n t i n e n t s , w h i c h h a d come together e a r l i e r before the deposition of the Devonian O l d Red S a n d s t o n e .

This "Old Red" Continent assembled during the Ordovician through earliest Devonian: two models exist for the assembly w h i c h resulted in the T a c o n i c , Caledonian and Acadian orogenies.

A relative scarcity of paleopoles does not y e t allow a definite choice b e t w e e n these models; one m o d e l has Laurentia and Amorica colliding first in the Late O r d o v i c i a n , forming the Taconic m o u n t a i n b e l t , w h e r e u p o n Baltica joins the o t h e r blocks in the latest Silurian to form the Caledonian m o u n t a i n s .

The other m o d e l has Laurentia and Baltica collide first in the Late S i l u r i a n , followed by a collision of these two blocks w i t h Armorica in the Early Devonian (Acadian o r o g e n y ) . In the Early Paleozoic a l l m a j o r blocks appear to have b e e n located in independent p l a t e s , although there are strong arguments to suggest that Armorica and Gondwana m o v e d together as one plate in the latest Precambrian and perhaps the Early CaiTibrian.

Intervening microplates are beginning to be identified in the Appalachians; they appear to have b e e n caught in the collisions of the m a j o r cratonic b l o c k s of B a l t i c a , Laurentia and G o n d w a n a .

PERMO-TRIASSIC PALAEOMAGNETISM OF THE TARIM B L O C K , CHINA P . L . McFadden^ & M.W.McElhinny^ ^R.S.E.S., Australian N a t i o n a l U n i v e r s i t y , Canberra ACT ^Bureau of M i n e r a l R e s o u r c e s , Canberra A C T .

In July 1982 samples w e r e collected from two sections in the Tien Shan M o u n t a i n s , Tarim B l o c k . The aim of the study is to improve our understanding of the Chinese Permo-Triassic plate tectonics.

78

In the Biyulopaokutze section (42^08'N, 83^21'E) 116 cores and 56 blocks were collected from 25 horizons. The sequence is roughly 870 m thick and 54 "layers" have been identified. Each layer consists of several bands, the total thickness ranging up to 50 m, frequently being conglomerates interbedded with mudstones and sandstones, or vice versa. From the bottom upwards layers 1-25 are largely greenish sandstones and mudstones with conglomerates and so sampling was commenced at layer 26 and continued upwards. Fossil evidence puts the Permo-Triassic boundary at about layer 40.

In the Ohuobulhake section (42^14'N, 82^59'E) 64 blocks were collected from 12 horizons. The sequence consists of overturned Upper Permian rocks and is badly fractured and distorted in many places. Consequently it was difficult to obtain reliable samples but it is hoped that those collected will provide a fold-test to aid in identification of primary magnetization.

Preliminary results from these two sections will be presented.

THE INDIAN APPARENT POLAR WANDER PATH SINCE THE LATE PALAEOZOIC AND THE INDIA-ASIA COLLISION Chris T. Klootwijk Bureau of Mineral Resources, Canberra ACT Am overview is presented of the Indian apparent polar wander path (APWP) for the Phanerozoic and in particular for post-Late Palaeozoic times. This APWP is compiled on basis of available published and unpublished data from peninsular and extrapeninsular IndoPakistan and from DSDP cores from the Indian plate. It is characterized by four major loops and kinks; i.e. an as yet rather ill-defined Permo-Carboniferous loop, a well-established Triassic-Jurassic loop, and kinks spanning the Middle Jurassic-Early Cretaceous and Late Cretaceous-Early Tertiary intervals. The broad timing of these kinks and loops agrees with periods during which major changes in the geological regime occurred, and speculatively they can be interpreted to reflect changes in plate tectonic motions and orogenic events.

- The Permo-Carboniferous loop is the least well established and its significance is not clear. It may represent a change in plate movements associated with the Hercynian-Alleghenian collision of Gondwana with the North American-European continent, alternatively it may -represent the end phase of a right lateral shear movement of Gondwana relative to Laurasia.

- The Triassic-Jurassic loop is the more important and newly recognized feature of the APWP. It indicates a reversal from a Late PalaeozoicEarly Mesozoic northwards and counterclockwise rotational movement into a southwards and clockv/ise rotational movement during the Early to Middle Jurassic. A comparable loop has been recognized also in APWP*s from other Gondwana continents, notably Australia. It can be concluded from the shape and position of this loop that Eastern Gondwana had reached by Late Triassic times moderate to low southern latitudes, and that formation of the Neotethys resulted not only from northward drift of fragments broken off

79

Gondwana's northern perimeter but also from southward movement of Gondwana.

- The Middle Jurassic-Early Cretaceous kink may reflect the early separation of Greater India from Gondwana and coincides with the Neocimmerian orogeny in the northern Tethyan region.

- The Late Cretaceous-Early Tertiary kink may be associated with a change in plate motion resulting from a possible Late Cretaceous collision with and obduction of an island arc (Kohistan-Ladakh) onto Greater India.

Palaeomagnetic studies of extrapeninsular regions up to and north of the Indus-Tsangpo suture zone have been hindered by wide-spread presence of magnetic overprints, whose nature and significance became clear only once data had been accumulated from widely separated regions in the Himalaya, the Hindu Kush and the Pakistan Fold Belts. These overprints delineate two regionally confined age groups. Younger overprints (20-40my) predominate in the more extrenal thrust sheets and have been induced probably during thrusting of the suprastructure onto the Indian shield.

Older overprints (50-60my), in contrast, are found in the more internal zones both north and south of the Indus-Tsangpo suture zone. These overprints, and in particular data from Chitral in the eastern Hindu Kush, indicate that by Late Palaeocene-Early Eocene time contact had been established already between the Greater India-obducted island arc complex and the southernmost belts of Asian's accretionary margin, with the zone of collision then situated at equatorial to- low northern (0-10°) palaeolatitudes. Subsequent northward movement of Greater India over 3000 to 4000 km and a minor clockwise rotation and southward movement of southern Asia have been taken up by up to 3000 km of impingement of Greater India into southern Asia and probable large-scale underthrusting of Greater India along the Himalayan Main Central Thrust beneath southern Tibet. Within the Western Lower Himalaya post-collisional deformation resulted to clockwise rotation of thrust sheets.

Recent palaeomagnetic result from southern Tibet by Chinese and French workers cannot as yet be interpreted in full accord with Indian palaeomagnetic data. Some discrepancies will be pointed out.

OVERPRINT MAGNETISATIONS AS SENSITIVE TECTONIC EVENT DETECTORS P.W. Schmidt CSIRO, Mineral Physics, North Ryde, N.S.W, The study of fossil magnetism has traditionally involved cleaning techniques to eradicate secondary magnetisations in the relentless quest for primary magnetisations.

This has unfortunately meant the loss of some potentially very useful data.

Recently the detailed analysis of vector trends during cleaning has become routine, revealing the character of magnetisations with clarity.

Multi-component magnetisations are common with primary magnetisations often being overprinted during tectonic events such as thermal pulses, uplift and cooling. The intensity of sach events is reflected by the thermal stability of the overprints and may be theoretically estimated.

The pole position calculated from the magnetic direction of an overprint also enables the timing of the event to be assessed.

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CORRELATION OF LATE PROTEROZOIC SEQUENCES FROM NORTH AMERICA H.C. Palmer Division of Mineral Physics CSIRO North Ryde, N.S.W. Australia Apparent polar wander paths derived from rocks of the Grand Canyon Supergroup of the western United States and from the Keeweenawan rocks of the mid-continent of North America are similar in form, which, together with radiometric data, suggest time correlation between these major packages of rock. Detailed correlation in the 1.1 - 1.2 Ma interval however breaks down. During this time interval two (possibly three) reversals are recorded in the Keweenawan succession which have not yet been recorded from v/estern North America.

Because of the asym-etric nature of these reversals, considerable apparent polar wander is generated by the Keweenawan paleomagnetic record. Neither rapid drift nor unremoved secondary remanence, as documented by positive baked contact tests, can reasonably be invoked as an explanation for this brief and rapid episode of apparent polar wander.

APPARENT POLAR WANDER WITH RESPECT TO AUSTRALIA FOR THE PERIOD 2500 Ma-750 Ma : UPDATE J.W. Giddings Bureau of Mineral Resources, Canberra ACT It is now seven years since the last major reconfiguration of the Australian apparent polar wander path (apwp) for the period 2500 Ma750 Ma (McElhinny & Embleton, 1976). Since then, only four new results have been added for this period: two from the Pilbara Block (2280 Ma Black Range Dyke and the Cajaput Dyke), one from the Gawler Block (1525 Ma Gawler Range Volcanics), and one from the Adelaide Geosyncline (1350 Ma?/900 Ma? Wooltana Volcanics). Recent reviews of this apwp (Embleton 1981; McWilliams 1981) consider these results further: in general, within the age constraints available, the path requires no major modification of its shape to accommodate them.

However, preliminary new results are now available from 14 units whose ages range from 1720 Ma to 900 Ma. The majority of these results, which represent about a 50% increase in the pole dataset for the period 2500 Ma-750 Ma, do not fall on the existing path. The necessary modifications to the path to incorporate the new data are considered; they form the basis for a major redefinition of the apwp for this period.

The units for which preliminary results are available are: (a) (b) (c)

Edith River Volcanics (%1720 Ma, Pine Creek Geosyncline)^ Lower part of the Kombolgie Formation of the Katherine River Group (1700 Ma-1650 Ma, western McArthur Basin) Hobblechain Rhyolite through to the Emmerugga Dolomite of the upper Tawallah and lower McArthur Groups (^^1700 Ma, eastern

31

(d) (e)

McArthur Basin) Morav/a Lavas, the underlying Neereno Sandstone and the overlying Campbell Sandstone ('^1350 Ma, Yilgarn Block) Stuart Dykes (^^900 Ma, Arunta Block) Main features of the results are:

(i)

The new Edith River Volcanics pole is some 25° away from the results established over 20 years ago.

The eastern McArthur Basin sequence defines an unambiguous polar shift opposite in sense to that of the existing apwp for the same period. (iii) A polar shift can be identified in the lower Kombolgie Formation from the western McArthur Basin which is in the same sense as, and superposes the first part of, the polar shift found in the eastern McArthur Basin rocks suggesting, at this preliminary stage, that the two sequences are equivalent in age. Traditionally, the Kombolgie Formation has been regarded as equivalent in age to the lower, not the upper, part of the Tawallah Group. ^ • i, ^ (iv) The new Morawa Lavas pole is about 40 away from the published results. Furthermore, using results from the underlying and overlying sediments, a 35 path segment is defined which establishes that the sense of polar motion through the Morawa Lavas pole is opposite to that s h o ^ on the existing apwp. (v) The Stuart Dykes pole lies about 35 away from younger poles and assists in defining an older part of the Late Precambrian section of the apwp. (ii)

The preliminary new results fall mainly in periods for which the old apwp was poorly defined. The most significant conclusion of former Australian Precambrian apwp studies (Giddings 1974; McElhinny & Embleton 1976; Embleton 1981) therefore still holds for the redefined apwp: that the path is single and common to the various Precambrian blocks that constitute Platform Australia back to at least a.1800 Ma, and that, as a corollary and subject to the uncertainties inherent in the palaeomagnetic method, those blocks have remained essentially contiguous since that time.

References EMBLETON, B.J.J., 1981: A review of the palaeomagnetism of Australia and Antarctica; McELHINNY, M.W. & VALENCIO, D.A. (eds) Palaeoreconstruction of the Continents. Vol. 2. Geodynamics Series A.G.U. and Geol. Soc. Amer., 77-92.

GIDDINGS, J.W., 1974: Precambrian palaeomagnetism of Australia.

Thesis, ANU, 143pp.

Ph.D.

McELHINNY, M.W., & EMBLETON, B.J.J., 1976: Precambrian and Early Palaeozoic palaeomagnetism in Australia. Phil. Trans. Roy. Soc. Lond., A280, 4 17-431.

McWILLIAMS, M.O., 1981: Palaeomagnetism and Precambrian tectonic evolution of Gondwana; KRONER, A. (ed) Precambrian plate tectonics. Elsevier, Amsterdam, 649-687.

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ISLAND ARC ROTATIONS OF THE BISMARCK ARCHIPELAGO, PAPUA NEW GUINEA

Falvey, D.A.

1

and Pritchard, T.

2

^Bureau of Mineral Resources 2 The University of Sydney

The Bismarck Archipelago of northern Papua New Guinea shows evidence of an Eocene to Holocene island-arc system.

Originally north-facing, the arc reversed in the late Miocene, and was partly disrupted by marginal basin sea-floor spreading in the Bismarck basin from late Pliocene to Recent.

Palaeomagnetic data has been collected and analysed from over 200 sites throughout the archipelago.

Principal component analysis has shown moderate angular motion of New Britain, associated with back-arc spreading.

Large angular rotation of island-arc microplates has been detected from earlier epochs. These microplates appear not to be disrupted by internal block rotations, but appear amenable to conventional plate tectonic analysis techniques.

A NEW CENOZOIC APPARENT POLAR-WANDER PATH FOR AUSTRALIA

Mart Idnurm Bureau of Mineral Resources, Canberra ACT

Since 1977 the accuracy of the original Cenozoic APW path for^ ^ Australia, determined from remanence directions of volcanic rocks ' , has been repeatedly questioned. Two alternative paths have been proposed pending further work on dated Australian rock units: the first was derived from pole positions of the Indian sub-continent using sea-floor spreading data,"^ and the second from directions of remanence in weathered units The second approach does not give a dated path.

A new polar-wander path has now been constructed from principally rem.anence measurements on sedimentary rocks. The dominant features of this path are its straight course and a pronounced '*far sided" effect which strongly modulates the rate of pole movement. The non dipole component causing the effect was introduced during the Palaeogene and appears to have persisted with diminishing amplitude until recent times.

The derivation of the new path is described, and its implications are considered for the drift histories of the Australian and Antarctic continents.

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1.

Wellman, P., McElhinny, M.W., & McDougall, I. G e o p h y s . J . 18, 3710395 (1969).

2.

M c E l h i n n y , M . W . , E m b l e t o n , B.J.J., & W e l l m a n , P. G e o p h y s . J . 3 6 , 141-151 (1974).

3.

Klootwijk, C.T., & Peirce, J.W. Nature 2 8 2 , 605-607 (1979).

4.

Embleton, B.J.J., & McElhinny, M.W. P l a n e t . S c i . L e t t . 5 8 , 141-150 (1982).

A LEAST-SQUARES METHOD OF POLAR WANDER PATH MODELLING : ITS APPLICATION TO ARC ROTATION ANALYSIS AND MAGNETIZATION DATING R . J . Musgrave^ § D . A . Falvey^ ^Department of Geology S Geophysics, The University o£ Sydney, N . S . W .

2Bureau of Mineral Resources, Canberra, ACT A new objective method of interpolating an apparent polar wander path between virtual geomagnetic poles has been developed. Curves are fitted by means of a least-squares regression. Unlike previous applications of least-squares analysis to this p r o b l e m , data which are poorly constrained in age may be included in the regression. The technique involves the projection of the spherical polar co-ordinates of longitude and latitude on to a Sanson's sinusoidal projection. The transformed co-ordinates of position are then fitted by a polynomial function of low order, or by a series of cubic splines. No account is taken at this stage of any ages which may have been assigned to the poles. Modifications of the matrix solution used in least-squares analysis allow the position curve to be constrained to converge on the geographic pole and to be responsive to differences in the variance of the data. The temporal aspect of the data is included by the construction of a second curve, in this case representing age as a function of length along the position curve. Only those points with well-constrained ages are used to produce the age-length function, and the fitting is again by means of a simple polynomial or spline series.

The continuous nature of the two defining curves allows an age to be assigned to a pole by associating it with the closest point on the position curve and the corresponding age on the age-length curve.

Relative plate rotation analysis can be carried out by assessment of the closeness of fit of the path for the plate in question and that of a reference wander p a t h , after a trial rotation of the first path.

To illustrate the technique a new polar wander path for Australia for the last 100 Ma has been derived from data of both known and unknown age. This path is used to assign ages of magnetization to the formations yielding the undated poles (published weathered profile poles from Australia, and two new poles obtained by the authors from New Caledonian laterites) . A polar wander path for the New Hebrides Arc (including data from both Vanuatu and the Santa Cruz Group) is also derived, and an arc rotation analysis carried out to determine a pole describing the arc's rotation relative to the Australian Plate over the last 6 M a .

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FITTING AND MATCHING APPARENT POLAR WANDER PATHS

R.M. Clark Department of Mathematics, Monash University, Clayton, Victoria, 3168 This paper discusses a new mathematical method for constructing a best-fitting smooth apparent polar wander (APW) path, with confidence limits, from a set of dated palaeomagnetic pole positions.

The procedure involves fitting cubic splines to angular co-ordinates (co-latitude and longitude) by robust weighted least-squares. The degree of smoothing of the data is determined objectively from the internal evidence of the data alone, using the method of crossvalidation. The resulting APW paths are defined continuously, with continuous first and second derivatives. Confidence limits are readily obtained by applying a simple Normal approximation, assuming that the 'noise' in the data can be modelled by the Fisher distribution.

As an illustration of the method. North American and European (Baltic Shield) APW paths, and their 95% confidence bands, are calculated for the last 500 Ma. The rate and acceleration of APW, defined continuously for each path, are then easily calculated.

The method is applied to give a quantitative assessment of 6 alternative reconstructions of Gondwanaland. For each such reconstruction, the palaeomagnetic pole positions for each Gondwanan continent are first rotated back to their proposed Gondwanaland locations. The above curve-fitting procedure is then used to obtain the best-fitting APW path to these rotated pole positions. The residual mean-square-error (MSE) of the pole positions about the bestfitting path provides a simple quantitative measure of the goodnessof-fit of the available data to each Gondwanan reconstruction. Of the 6 reconstructions examined, that of Smith and Hallam (1970) explains the distribution of palaeomagnetic data most effectively.

However, the differences in goodness-of-fit of the 6 reconstructions are not statistically significant.

The method may be used not only to assess a given continental reconstruction but also to give a quantitative and mathematically reproducible continental reassembly using only palaeomagnetic data, by matching the APW paths of several lithospheric plates. The optimal continent reassembly is then that for which the residual MSE to the best-fitting APW path to the pooled data is a minimum. As an illustration, this procedure is used to obtain a North Atlantic reconstruction by matching the APW paths for North America and Europe over the period 370-170 Ma. References Clark, R.M. & Morrison, B.J., 1983. Normal approximation to the Fisher distribution, Geophys. J.R. astr. Soc.^ to appear.

Smith, A.G. & Hallam, A., 1970.

Nature, 226, 139-144.

The fit of the southern continents.

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Thompson, R. & Clark, R.M., 1981.

Fitting polar wander paths,

Phys. Earth Planet. Int., 27, 1-7.

Thompson, R. & Clark, R.M., 1982. A robust least-squares Gondwanan apparent polar wander path and the question of palaeomagnetic assessment of Gondwanan reconstructions. Earth Planet. Sc%. Lett., 57, 152-158.

Thompson, R. 6. Clark, R.M., 1983. appear.

Matching polar wander paths, to

CURVE FITTING ON THE SPHERE WITH APPLICATION TO FITTING POLAR WANDER PATHS N.I. Fisher CSIRO Division of Mathematics and Statistics c/- Division of Applied Physics P.O. Box 218 Lindfield, NSW, 2070 The question of how to fit an apparent polar wander path to a set of palaeomagnetic poles provides an interesting example of the more general problem of fitting a curve to an ordered sequence of points on the surface of the sphere.

Previous attacks on this problem have all involved "linearising" the variables (usually by projecting the data onto the plane) so that standard spline-fitting procedures in two or three dimensions can be applied.

The price paid for this simplification is that distortion occurs when the curves are projected back onto the sphere; further, the results are not necessarily independent of the choice of coordinate system.

This paper presents a method being developed in conjunction with Professor T. Lewis (Open University, U.K.) for fitting curves directly to points on the surface of the sphere.

It involves defining a set of spherical splines sufficiently flexible to model the sort of data sequences typically arising in APWP modelling problems.

THE AUSTRALIAN POLAR WANDER PATH AS AN INDICATOR OF STRESS DUE TO CURVATURE VARIATION IN THE EARTH'S SURFACE J.C. Dooley Bureau of Mineral Resources, Canberra As plates move around on a non-spherical Earth, stresses will be generated as they adjust to changing curvature of the earth's surface. These stresses may be large enough to cause fracturing or measurable deformation in the lithosphere.

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The palaeomagnetic polar wander path is a good indicator of the types of motion - changes in latitude, or rotations about an internal axis at low latitudes - for which the curvature changes are greatest. The stresses induced by a decrease in curvature should be compressional near the centre of a plate and tensional near its edge, and vice versa for an increase in curvature.

The effect, though significant, is small, and is difficult to isolate from effects of other stresses. An examination of the possible effects in Australia in the light of the most recent information on the Tertiary polar wander path, and comparison with observed stresses and deformations, illustrates the difficulties.

Nevertheless, the curvature effect should be taken into account in analysis of the causes of the stresses and deformation.

The curvature changes should have been larger in earlier geological times, as a faster rate of rotation of the Earth implies a larger equatorial bulge. However uncertainties in pole positions and in accurate dating of palaeomagnetic events, cause even greater difficulties in isolating such effects. However it is apparent that a large supercontinent could not drift through significant distances without some part of it experiencing substantial stress; thus the curvature effect may be a cause of break-up of the supercontinents.

Symposium 1(f) Evolution of Precambrian terrains Convener: Dr D.H. Blake

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TECTONIC SETTING OF THE DAVENPORT GEOSYNCLINE, NORTHERN TERRITORY A.J. Stewart Bureau of Mineral Resources, Canberra, A.C.T. The Davenport Geosyncline is situated in the centre of the Northern Territory, and is centred about 170 km southeast of Tennant Creek.

It is a sequence between 1850 and 1660 Ma old of sedimentary and volcanic rocks about 10 km thick.

The exposed part of the geosyncline occupies a northwesterly-trending area about 230 km long by 170 km wide, but its true extent is (or was) larger than this.

The sequence rests unconformably on the Warramunga Group (greywackes and felsic volcanics) to the northwest, and on schist and quartzite of the Arunta Block to the south; Arunta rocks also form a small inlier in an anticline near the centre of the geosyncline.

To the east and west, the geosyncline is unconformably overlain by extensive platform cover of the Georgina and Wiso Basins, but there is sufficient subsurface information to indicate that the geosyncline is bordered by sialic rocks.

Hence, the Davenport Geosyncline is intracratonic.

The sedimentary sequence in the geosyncline consists chiefly of feldspathic quartz arenite, clean quartz arenite, and conglomerate; siltstone, shale, and carbonate are rare.

Sedimentary structures such as ripple marks, mud flakes, ubiquitous cross-bedding, and stromatolites in the one extensive carbonate unit indicate shallowwater deposition.

Abundant detrital feldspar and conglomeratic rocks, numerous occurrences of prolapsed ('recumbently folded') cross-bedding, and the rarity of lutite and carbonate indicate rapid deposition.

Local unconformities, where the upper parts of some units were folded and eroded before deposition of the succeeding^ units, suggest slumping or mass flow, and hence significant initial dips on depositional surfaces.

The volcanic rocks in the Davenport Geosyncline are a bimodal assemblage of mafic (SiO less than 56%; andesite and basalt) and felsic (mainly rhyolite) flows and sills.

Lavas and pyroclastics are both present, and commonly crop out as thick volcanic sequences in the cores of major anticlines.

The geosynclinal sequence - the Hatches Creek Group - is divided into lower, middle, and upper parts, and further into numerous named formations and members.

The lower Hatches Creek Group is restricted to the central part of the geosyncline, contains the greatest proportion of volcanic rock, and the constituent formations interfinger laterally with one another.

The middle Hatches Creek Group is more widespread, and to the northwest it onlaps on to Warramunga basement, and to the south on to Arunta basement.

The upper Hatches Creek Group is found mainly in the east of the geosyncline.

Both middle and upper Hatches Creek Groups are more regularly layered than the lower part, and the upper part is essentially non-volcanic.

The Davenport sequence is typical of Assemblage II (bimodal volcanics-quartzite-arkose) of Condie's (1982) classification of Early and Middle Proterozoic supracrustal successions.

The wholly

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ensialic setting of the geosyncline, and the abundance of mafic volcanics low in the sequence suggest deposition of the lower Hatches Creek Group in a mantle-activated ensialic cratonic rift.

The rapid deposition of the sediments, the abundance of early mafic volcanics, and the wider extent of the middle Hatches Creek Group compared to the lower, are consistent with the 'pull-apart' or lithospheric dyking model of basin development of Royden et al. (1980).

In this scheme, flow in the asthenosphere causes cracking and extension of the continental lithosphere, followed by intrusion of dykes from the mantle.

This results in replacement of light crustal rocks by denser rock, thus causing subsidence which is initially rapid, and would be accompanied by mafic volcanism if the dykes reached the surface, as in the lower Hatches Creek Group.

This is followed (Dewey, 1982) by a period of thermal subsidence, i.e., thermal contraction of the previously warmed crust.

This takes place over a wider and wider area, thus producing the basin margin onlap shown by the middle Hatches Creek Group.

References Condie, K.C., 1982: Early and Middle Proterozoic supracrustal successions and their tectonic settings.

American Journal of Science, 282, 34 1-57.

Dewey, J.F., 1982: Plate tectonics and the evolution of the British Isles.

Geological Society of London, Journal, 139, 371-412.

Royden, L., Sclater, J.G., & von Herzen, R.P., 1980: Continental margin subsidence and heat flow: important parameters in formation of petroleum hydrocarbons.

American Association of Petroleum Geologists, Bulletin 64, 173-87.

STRUCTURAL GEOLOGY IN THE WESTERN MACDONNELL RANGES (NORTHERN TERRITORY) C. Teyssier Department of Earth Sciences, Monash University, Clayton, Vic. 3168

The study area comprises the Arunta Block terrains on the 1:100,000 MacDonnell Ranges sheet. This area consists of extensive quartzofeldspathic gneisses (mainly orthogneisses), micaceous gneisses, amphibolites and geometrically overlying metasediments and quartzites forming the Chewings Range. A strongly deformed zone (the Redbank Zone) forms the northern side of the MacDonnell Ranges. About 10 km further to the north, mafic granulites crop out at Mount Hay and Ceilidh Hill. The main structural feature is a pervasive foliation steeply dipping to the North. This foliation, S2, is axial plane to north plunging reclined F2 folds, and the lineation (L2) parallels

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the fold axis. There is evidence of rare earlier F2 folds re-folded by the F2 hinges. The D2 phase presumably represents the first event of the Chewings Range Orogeny (1800-1600 Ma).

In the Chewings Range area, S2 is parallel to interlayered schists and quartzites. These metasediments comprise kilometric scale, open, inclined folds (F3) with axes plunging shallowly to the east.

However mesoscopic hectometric scale F3 folds show curved axes. In some tight F3 folds, they may develop a strong axial plane schistosity which transposes the previous composite layering. Large lenses of orthogneisses incorporated within the quartzites and metasediments are involved in the F3 folding. When evident, vergence is to the south.

Westerly plunging inclined folds (FL^) have been recognised. Tliey occur sporadically throughout the area and are best exposed in the west. They are moderately open folds rarely developing an axial plane schistosity, and appear mainly as a crenulation corrugating the foliation planes. Late ductile deformation zones, mylonites and ultramylonite bands usually concentrate at the quartzite-gneiss interface.

In the gneisses south of Chewings Range, F3 folds deform the main foliation and lineation. They locally show curved hinges and may plunge to the west. F^ folds refold them. Later folds occur below the Heavytree Quartzite, related to quite late fault zones.

Easterly trending thick cataclastic zones (up to several metres) are frequent in the south of the area. North of the Chewings Range, kilometric scale open to tight folds plunge quite steeply (30°-50°) to the West; vergence is to the South. East plunging F3 folds appear proximal to the Chewings Range quartzite. Mylonite and ultramylonite zones and few cataclastic zones crosscut the area between the Chewings Range and the Redbank Zone.

The Redbank Zone consists of mylonites affecting mainly granitic gneisses and granites. The mylonitic foliation dips steeply to the north and the main lineation is parallel to the north plunging reclined folds. Isoclinal folds are well exposed in some quartzites which form a 4 km long outcrop within the Redbank Zone. Both gneisses and quartzites are involved in this folding. The strain intensity makes it difficult to relate these rocks to the Chewings Range or Heavytree (late Proterozoic) quartzites. They possess a strong quartz fabric (c-axis maximum subnormal to the foliation) and contain more than 90% quartz. Quartzites and gneisses underwent a non-coaxial deformation as suggested by the following microstructures - extensional crenulation cleavage (foliation boudinage), shear planes and asymmetric trails around porphyroblasts. The Redbank Zone results from a complex deformational and metamorphic history. Minerals such as garnet and pyroxenes occur in the quartz rich rocks. Further studies will give more accurate data on the metamorphic conditions.

Ultramylonite and pseudotachyline bands occur in the mylonites and less deformed orthogneisses. Internal foliation and lineation are weakly developed in these bands and can only be seen on weathered surface. They are usually parallel to the planar and linear fabrics observed in the mylonites. Thickness ranges from 3 mm to a few metres. The dark appearance of ultramylonite bands and their parallelism to the major foliation suggest that they occurred at a late stage of concentrated deformation producing narrow zones of very fine grained rock. The displacement of markers and the weak defJec-

90

tion of the mylonitic foliation along the north dipping bands indicate a movement to the South uplifting the northern block. A weak crenulation cleavage steeply dipping to the South deforms the mylonitic foliation. A gentle warping of north plunging axes folds the previous structures late in the history of the deformation.

EARLY PROTEROZOIC EVOLUTION OF THE PINE CREEK INLIER IN THE NORTH AUSTRALIAN CRATON R.S. Needham & P.G. Stuart-Smith Bureau of Mineral Resources, Canberra ACT Events in the Pine Creek Geosyncline in Early Proterozoic time define an earli-er depositional domain, and a later orogenic domain including mafic to felsic igneous activity and volcaniclastic sedimentation. The domains overlap in space but are separated in time between about 1940 and 1870 m.y. Together they range from crystallisation of Archaean basement at about 2500 m.y., to development of McArthur Basin Plat form Cover at about 1650 m.y.

An extensive absolute time framework is being developed (Page & others, 1980), which, owing to relatively straightforward rock relationships, may be applied as a type model for the evolution of other Early Protogoic provinces in the North Australian Craton.

A maximum aggregate thickness of 10-14 km of clastic,organic, and chemical sediments accumulated in an intracratonic basin developed on late Archaean granite. Island shorelines early in the depositional domain are indicated by coarse clastic wedges around presently exposed Archaean granite in some areas, and a northerly provenance (below the present Van Diemen Gulf) is indicated by an extensive fluvio-deltaic sequence near the middle of the succession.

Margins of the basin are concealed under younger sedimentary basins in most places. Sediments were mainly neritic, fluvial or interto supratidal, and greyv/ackes in the upper part of the sequence are probably also of shallow-water origin. The sedimentary record is punctuated by mild tectonic events represented by two low-angle unconformities, the lower with associated local mafic volcanism.

An episode of felsic volcanism accompanied faulting in the southeast near the middle of the sequence. The age of dolerite sills in the sediments indicate that the depositional domain ended by about 1940 m.y.

The orogenic domain is dominated by regional metamorphism and polyphase deformation of the earlier sediments, about 1800 m.y. ago, and by post-orogenic granite intrusion at about 1730-1760 m.y.

Volcanism took place before and after granite intrusion, and unconformity between these two rock series indicates tight folding attendant with intrusion. The volcanics are mainly felsic, and are centred on a rift related to the same fault system on which depositional domain felsic volcanism was focused. Outside the

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rift the pre-granite volcanics are interbedded with greywacke-rich sediments similar to the youngest depositional domain sediments, but can be distinguished by differing fold styles and interbedded tuffs and flows.

A pre-orogenic granite intruded about 1870 m.y. ago is known in the extreme northeast, where it was partially melted along with some of the country rock. This event probably dates the start of orogenesis some 70 m.y. before its culmination. The final major event in this domain was intrusion of dolerite lopoliths at 1690 m.y. Carpentarian cover rocks with interbedded 1645 m.y. intermediate to mafic volcanics near the base rest on rocks of both domains with marked regional unconformity.

The distribution of igneous rocks and metamorphic and deformational intensity indicate that the late Early Proterozoic orogen was centred northeast of the main part of the Pine Creek Inlier, east of the zone of rifting, and is now largely obscured by Carpentarian cover rocks. This zone is roughly coincident with the western margin of the Oenpelli Regional Gravity Complex, which may in part represent a mantle diapir related to the late Early Proterozoic orogen.

Page, R.W., Compston, W., & Needham, R.S., 1980 - Geochronology and evolution of the late-Archaean basement and Proterozoic rocks in the Alligator Rivers Uranium Field, Northern Territory, Australia in (eds. J. Ferguson & A.B. Goleby) URANIUM IN THE PINE CREEK GEOSYNCLINE IAEA, Vienna

A REVISED STRUCTURAL AND TECTONIC FRAIIEWORK FOR THE PINE CREEK GEOSYNCLINE J.D. Johnston^ B.W. Nisbet^, M.A. Etheridge^ & V.J. Wall^ ^Depart, of Earth Sciences, Monash University, Vic. ^Pancontinental Mining Ltd., Sydney, NSW ^Bureau of Mineral Resources, Canberra, ACT The Pine Creek Geosyncline may be divided into four regions of distinct structural style.

1.

The East Alligator River Region - characterised by amphibolite facies metamorphism, overprinted by a widespread greenschist f a d e s retrograde event. Both metamorphic events are accompanied by relatively flat-lying structures, and a west or northwest vergence predominates. The most widespread foliation in the mineralized regions is the retrograde schistosity.

2.

The South Alligator River Region - characterized by greenschist facies metamorphism with macroscopic structures dominated by northsouth trending upright folds. These folds postdate extensive zones of high strain, tens to hundreds of metres wide that are subparallel to bedding over tens of kilometres. The high strain zones represent bedding-parallel faults which have a W.S.W. transport direction.

92

3.

The Central Marakai Region - characterized by lower greenschist facies metamorphisin and simple structure dominated by upright northsouth folding, with no apparent early faults.

4.

The Western Rum Jungle/Waterhouse Region - characterized by middle to upper greenschist facies metamorphism and more complex structure. Early ductile thrust faults with a north westerly movement direction have been refolded heterogeneously by F2 northsouth folds and F^ north-westerly trending folds producing the Archaean basement domes.

The tectonic character of the eastern half of the Pine Creek Geosyncline resembles those of classical nappe complexes; with a root zone in Arnhemland, synmetamorphic faulting in the East Alligator River region and brittle low angle faulting in the South Alligator foreland region. The low angle faulting offers a structural explanation for some apparent unconformities, rapid changes in thickness of units and some lateral facies changes.

The D| thrusting and two phases of folding in the Rum Jungle region interfere to produce domes of Archaean granitoid basement, with an S| foliation and Lj lineation parallel to their margins over most of their circumferences. The Dj and D^ formations in the Western Region appear to be different in origin to the north-south F2 folding further east, and may be related to the Litchfield province.

The presence of highly anisotropic rocks in the D, high strain zones, both in the Rum Jungle and East Alligator Riveri uranium fields may have influenced the development of Post-Carpentarian structures which are key features in controlling the distribution of mineralization.

STRATIGRAPHY OF THE EARLY PROTEROZOIC WILLYAMA SUPERGROUP AND IMPLICATIONS FOR EVOLUTION OF THE BROKEN HILL BLOCK I.L. Willis, R.E. Brown, B.P.J. Stevens, W.J. Stroud Geological Survey of N.S.W., Department of Mineral Resources, Broken Hill

The metamorphic rocks of the Broken Hill Block (BHB) have been subdivided in detail by the GSNSW (Willis et al, in prep.; Stevens et al, in press) (Fig. l). The sequence provides a model for comparison with Willyama equivalents in the region (e.g. Olary, Mt. Painter), and places some constraints on the evolution of the BHB. Variably-bedded feldspathic metasediments (clastics and volcanoclastics) occur in the basal exposed sequence (up to Thackaringa Group), intercalated with quartz-albite rocks (sodic metavolcanics) and basic gneiss (high-Fe tholeiite).

Extensive

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felsic (granite) gneiss (dacitic-rhyolitic volcanics) of Alma and Rasp Ridge Gneisses are laterally equivalent to quartz~albite rocks in Lady Brassey and Himalaya Formations. Cues Formation comprises widespread basic gneiss, with associated minor garnet-rich felsic gneiss (dacite), leucogneiss (leucorhyolitic tuff) and stratiform Fe + Cu + Pb - Zn horizons. There is a marked change to wellbedded dominantly pelitic metasediments (thin-bedded turbidites) in Broken Hill Group. These are interbedded with basic gneisses (high/extreme Fe tholeiites), garnet-rich felsic gneisses (daciterhyodacite) and stratiform Pb-Zn-Ag horizons, in Parnell Formation.

An extensive horizon of garnet-rich felsic gneiss (rhyodacite) at the top of Purnamoota Sub-Group (Hores Gneiss) is laterally transitional to Silver King Formation, dominated by basic gneiss + metarhyodacite. The BH orebodies occur in a metasediment-rich interval in Hores Gneiss. The upper Willyama Supergroup comprises a pelitic-psammitic (thin-bedded turbidite) facies (Sundown Group), overlain by carbonaceous pelites and fine-grained psammites (finegrained turbidites, contourites) of Paragon Group. The succession at Olary is dominated by sodic quartzofeldspathic rocks (mainly Thackaringa Group equivalents), with thin metasediment equivalents of Broken Hill, Sundown and Paragon Groups. The basic and felsic gneisses of the Thackaringa and Broken Hill Groups are largely absent from the Olary sequence. The dominant stratiform mineralization in the Supergroup is transitional from Fe-Cu (Thackaringa Group and below) to Pb-Zn-Ag-(W) (Broken Hill Group).

Deposition of the Supergroup in an extensional (rift) setting is indicated by the presence of bimodal volcanics, including highFe tholeiite. An ensialic regime is suggested by the abundance of felsic volcanics. Coarse continental clastics and/or alkaline basic volcanics which might be expected with initial rifting, are absent. The basal exposed sequence (up to Thackaringa Group) was deposited in an unstable, shallow marine environment which included the Olary area. This was characterised by immature epiclastics/ volcanoclastics, and, in the BHB, by ferrobasalts (ocean floor basalt-low-K tholeiite) with associated possible sodic (peralkaline) volcanics, extrusion of dacites-rhyolites of calc-alkaline affinity and formation of exhalative Fe-Cu (minor Pb-Zn) horizons. This represents an advanced rifting stage (possible analogue : Afar region). The Broken Hill Group represents a late rifting stage, with deposition of thin-bedded turbidites in a deeper water, distal environment. This sequence progresses from paired basic (oceanic-low-K tholeiite)/felsic (?tholeiitic-calc-alkaline) airfalls or lavas (Parnell Formation), to mass-flow felsic volcanics (Hores Gneiss). The absence of tholeiitic and ?calcalkaline volcanics at Olary indicates that these products were not derived from the west; this area probably lay beyond the deep rift environment. The final stage is a transition to the non-volcanic deep marine conditions of the Sundown and Paragon Groups.

The BHB resembles an African-type intraplate Proterozoic mobile zone (e.g. Namaqua belt). However, evidence of continental crust to the east during deposition of the Supergroup is lacking, implying that the BHB lay towards a continental margin. The same problem applies to interpretation of the BHB as an intra- or interplate divergent regime in terms of plate tectonic models i.e. narrow continental rift, or intercontinental immature ocean

94

basin (e.g. Red Sea). However, the lithofacies, depositional history and aspects of the magmatism (e.g. tholeiites of oceanic affinity) are compatible with the latter model. Formation in a broad marine ensialic back-arc basin behind a continental margin is also compatible with the lithofacies, extensional setting,^ and a deepening, volcanically-active environment to the east. The extensional setting, and lack of andesites or abundant granitic intrusives, are incompatible with a convergent continental plate margin (c.f. ?Mt. Isa).

REFERENCES Willis, I.L., Brown, R.E., Stroud, W.J., Stevens, B.P.J., in prep.

The Early Proterozoic Willyama Supergroup : Stratigraphic subdivision of high- to low-grade metamorphic rocks in the Broken Hill Block, N.S.W. Submitted to G.S.A. Journal.

Stevens, B.P.J., Willis, I.L., Brown, R.E., Stroud, W.J., in press.

The Early Proterozoic Willyama Supergroup : Definitions of stratigraphic units from the Broken Hill Block. Geol. Surv. N.S.W., Records 21(2) (in press).

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EVOLUTION OF THE PROTEROZOIC MOUNT ISA INLIER, A REVIEW D.H. Blake Bureau of Mineral Resources, Canberra ACT The Mount Isa Inlier contains Proterozoic stratigraphic sequences many kilometres thick of mainly shallow marine, fluvial, and lacustrine sediment:- arenites, conglomerates, siltstones, shales, and calcareous, dolomitic and evaporite rocks - and volcanics which are intruded by granitic and doleritic bodies, tightly folded about mainly northerly trending axes, extensively faulted, and regionally metamorphosed to lower greenschist to upper amphibolite grades. Dated volcanics and intrusives range in age between about 1870 m.y. and 1600 m.y. (U-Pb

95

zircon ages), except for some cross-cutting unmetamorphosed dolerite dykes intruded between about 1200 m.y. and 1100 m,y. ago, and are markedly bimodal in composition - andesitic rocks are almost completely lacking. Several distinctive geochemical suites of igneous rocks have been recognised. Rb-Sr data indicate widespread regional metamorphism between 1670 m.y. and 1550 m.y.

Two partly conflicting hypotheses for the evolution of the Inlier have been proposed recently. In one hypothesis the Inlier is considered to consist of a central northerly trending basement belt flanked to the west and east by stratigraphically equivalent younger cover rocks commonly termed the western and eastern successions, and to have evolved on the eastern margin of the Australian craton. The basement belt is formed mainly of 1870-1850 m.y. old granite and felsic volcanics and 1810-1780 m.y. old felsic and basaltic volcanics. The western succession includes a major rift structure, the Leichhardt River Fault Zone, and contains the pre 1840 m.y. old arenaceous and basaltic Haslingden Group and the partly calcareous Quilalar Formation, and also the 1680-1670 m.y. old Fiery Creek Volcanics and Pb-Zn-Cubearing sedimentary Mount Isa Group. The oldest rocks of the eastern succession are basalts and arenites of the Malbon Group which are thought to be time equivalents of the Haslingden Group laid down on the opposite side of a long narrow landmass formed of basement rocks; overlying sedimentary rocks of the Mary Kathleen Group, including calcsilicate rocks mapped as Corella Formation, are regarded as lateral equivalents of the Quilalar Formation. No correlatives of the much younger Mount Isa Group are recognised in the eastern succession.

Deposition of the western and eastern successions was interrupted by several episodes of regional deformation and metamorphism associated with granitic intrusion.

According to the other hypothesis, which is that favoured by the author, the Inlier developed as an intracratonic basinal feature resulting from crustal thinning and downwarping well away from the margin of the Australian craton. The oldest cover rocks in the west, those of the Haslingden Group, were deposited in a trough bounded to the west and east by extensive land masses formed of 1870-1850 m.y. old igneous rocks and also pre-1900 m.y. old metamorphics which may include some of the calc-silicate rocks mapped as Corella Formation. The Haslingden Group is considered to be older than the Malbon Group to the east, and most of it is thought to pre-date 1780 m.y. old 'basement' volcanics.

Younger cover rocks in the east, mapped as Mary Kathleen Group, include correlatives of the Quilalar Formation and also some sediments and volcanics about 70 m.y. younger than the Mount Isa Group. No regionwide deformation or metamorphism took place between about 1900 m.y. and 1600 m.y., although there were several episodes of granite emplacement and periods of local non deposition, uplift, and erosion.

There is general agreement that the cover rocks were laid down during crustal extension, and that post-depositional tectonism resulted from subsequent east-west compression some time before the intrusion of dolerite dykes about 1200 m.y. ago. The Inlier has remained essentially stable tectonically throughout the Phanerozoic.

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EVOLUTION OF THE WONGA FOLD BELT NEAR MARY KATHLEEN, NORTHWEST QUEENSLAND R.J. Holcombe Department of Geology and Mineralogy University of Queensland ABSTRACT The Wonga Belt is a long, narrow zone of intensely deformed metasediments and meta-igneous rocks flanked by rocks of considerably less deformation and lower metamorphic grade. The belt is 5-10 km wide and extends for over 100 km in rocks of the Lower Proterozoic Mt. Isa Inlier. The core of the belt is occupied by the strongly deformed and foliated Wonga Granite. This paper is based on a study of about 40 km of the belt near Mary Kathleen.

The strain history within the belt was a uniformly oriented pure shear with maximum finite shortening values of 70-80% near Mary Kathleen, decreasing to about 60% north of the Wonga Fault. Original layering, at all scales, in all rock types, has been transposed by a single deformation into a new tectonic layering parallel to the belt.

Locally, late generations of structures, presumed to be related to faults, overprint the first generation fabric.

The lithologies within the Wonga Belt form intermingled layers of metasiltstone, quartzite, silicic metaporphyry, metagranite and amphibolite. From structural analysis it is argued that the silicic rocks originally formed a sill complex within a sedimentary pile following injection of a mafic dyke swarm. That is, sedimentation was followed by bimodal igneous activity.

On the basis of the geometry of the belt, the marked difference in lithology between the belt and the flanking regime, and the bimodal igneous activity, it is argued that the Wonga Belt was a narrow rift (about 20-50 km wide) that closed during subsequent regional shortening.

Strain was concentrated within the rift region producing a narrow fold belt and considerable uplift. It is interesting to speculate on the location of possible molasse products or of possible nappes that could have been produced by such a restricted uplift.

97

NAGSSUGTOQIDIAN MOBILE BELT IN EAST GREENLAND John S. Myers

Geological Survey of Western Australia, Perth

The Nagssugtoqidian niobile belt of East Greenland is an ensialic zone of intense deformation and dyke intrusion, 240 Km wide, which cuts sharply across the Archaean gneiss complex (Figs.1 and 2). The belt was established 2600 m.y. ago by the formation of vertical east-west shear zones and the syntectonic intrusion of basic dykes (Nag. 1, Fig. 3). Plutons of tonalite (Blokken gneisses) cut across these shear zones and dykes before a second major episode of deformation and metamorphism (Nag. 2) associated with thrusting fron the north (Nag. 2, Fig. 3) about 1900 m.y. ago. In the southern part of the mobile belt Nag. 2 structures are northerly dipping shear zones in greenschist facies. The metamorphic grade and ductility increase northwards and shear zones are replaced by open folds with northerly dipping axial surfaces (Fig. 2) and regional amphibolite facies metamorphism. Sheets of charnockite were emplaced in the northern part of the belt and are associated with a broad aureole of granulite facies metamorphism. Regional uplift occurred before the intrusion of high level posttectonic plutons of diorite and granite 1500 m.y. ago.

500 km Fig. 1.

Location of the Nagssugtoqidian mobile belt (thin lines), other Archaean and Proterozoic gneiss deformed and metamorphosed 1.9-1.7 b.y. ago (stipple) and Archaean gneiss stable since 2.7 b.y. ago (black).

98

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Diagram contrasting the main Nag. 1 and Nag. 2 tectonic movements. Unmodified Archaean gneiss - stippled; mobile belt - white with Nag. 1 schistosity marked by planes with vertical ruling.

99

HIGH GRADE METAMORPHISM AND CRUSTAL DEVELOPMENT IN THE ARCHAEAN NAPIER COMPLEX, EAST ANTARCTICA Michael Sandiford School of Earth Sciences, University of Melbourne The Napier Complex in Enderby Land, East Antarctica, consists of exceptionally high grade gneisses, derived in part from supracrustal precursors of early Archaean age. Implicit in previous discussions of the metamorphism of the Napier Complex is the proposal that the "Napier" geotherm could not be maintained to crustal depths much beneath the present level of exposure. This raises the question of the nature of the crustal material lying beneath the present levels of exposure in the Napier Complex. In this contribution consideration is given to constraints on the operative crustal thickness during the late Archaean metamorphism of the Napier Complex, and to their implications for the nature of the underlying crust.

Pressure sensitive assemblages crystallized during the late Archaean metamorphic culmination (M ) indicate that the Napier Complex gneisses presently exposed at Fyfe Hills formed at depths of about 30 km (Table 1). These gneisses are presently underlain by crust some 35 to 45 km thick, giving a cumulative crustal thickness of 70 km.

Assemblages crystallised during the M^ and M^ events, as well as corona and exsolution textures developed in the intervening static periods, indicate that the post M metamorphic evolution of the gneissic pile is dominated by a prolonged residence (extending over at least 1200 Ma) at deep crustal levels under a regime of near isobaric cooling (Table 1).

Table 1 Metamorphic event

M^

M^

M^

M^

Age (Ma) Temperature (°C) Pressure (kb)

>2500 870±70 10±1.5

>2350 700±60 8.5±1

>1190 660±50 7.5±1

>500 620±50 <5

The barometric data suggest that a condition of crustal isostatic equilibrium was established soon after M , and that the crust at the time of M was 0-15 km thicker than at M^. Furthermore the field and petrographic evidence suggest that isostatic equilibrium was maintained up until the time of excavation which was initiated during the M event. By analogy with modern shields, it may be inferred that within a period of 1200 Ma after effective cratonization (i.e. in this case by the time of M , as cratonization was complete by the end of M^) a "normal" shield thickness of 35-40 km would be attained. Thus ft^ crustal thicknesses of 40-55 km are considered probable.

The proposal that M crustal thicknesses were substantially less than (the cumulative) 70 km implies that a substantial component (at least 20 km or 50%) of the present crust was emplaced at the time of excavation.

A model of crustal underplating of low grade continental material beneath the Napier Complex is considered. This model is consistent with the rehydration and intrusion of alum.inous pegmatites locally enriched in B and Be that accompanied the excavation of the Napier Complex.

100

CRUSTAL EVOLUTION IN THE PRECAMBRIAN SHIELD OF NORTHWEST AUSTRALIA EVIDENCE FROM A GEOPHYSICAL STUDY OF THE DEEP LITHOSPHERE B.J. Drummond Bureau of Mineral Resources, Geology and Geophysics, Canberra

The early history of the Earth is known largely from the surface rocks of continents - their type, chemistry, metamorphic character and structures all tell us something about the early history of the Earth.

However, the geodynamic forces which drive the formation of continents originate within the mantle, not in the crust, and therefore probably left their imprints deep in the lithosphere.

Consequently, when studying the evolution of the crust, we should also-consider the information buried within the lower crust and upper mantle, and the best way of doing this is with geophysics.

This paper discusses the evidence of crustal evolution derived from a seismic and gravity study of structures in the crust and upper mantle of northwest Western Australia.

Seismic models of the crust and uppermost mantle are now available for several profiles which cross the Archaean Pilbara and northern Yilgarn Cratons, and the intervening Proterozoic Capricorn Orogenic Belt. They show that the crust of the Pilbara Craton, which is 2833 km thick, is much thinner than that of the northern Yilgarn Craton (>50 km thick). This, coupled with other evidence, such as the isotopic ages of granitoids and greenstones, and the sedimentary and structural styles and trends of greenstone belts suggest separate evolutionary histories for the two cratons, especially in the Archaean.

The Pilbara Craton is two layered, with the velocities in the layers increasing steadily with depth. The velocities within the crust indicate a crust of overall average granitic composition at the surface, becoming metamorphosed to felsic granulite at 12-15 km depth, with the mafic content and amount of garnet increasing at greater depths. The base of the crust is no more mafic than diorite, and the crust/mantle boundary is a sharp chemical discontinuity.

Thus the Pilbara Craton, with its thin crust and sharp crust/mantle boundary is different from other stabilised, but younger, non-Archaean continents around the world, in which the crust is usually thick, with a transitional crust/mantle boundary.

This suggests that the tectonism in the Archaean and early Proterozoic was somehow different from that of the late Proterozoic and Phanerozoic, with tectonism occurring in mobile belts around the margins of cratons, and unable to penetrate the sharp density contrast at the base of the crust within the cratons.

The Pn velocity under the Pilbara Craton has a + 2% aiisotropy, with the direction of maximum velocity 30^ east of north, at right angles to the axis of the Hamersley Basin and parallel to the direction of major dolerite dykes in the Pilbara Block. The minimum velocity axis parallels the axis of the Hamersley Basin. The coincidence of the minimum velocity direction with that of the axis of the Hamersley Basin suggests that the anisotropy formed by recrysta11isat ion or shear in the stress field resulting from lithospheric flexure during basin formation.

If this is so, the uppermost mantle must have been

101

in tension during the flexure, and the Archaean lithosphere must therefore have been much thinner than the present-day lithosphere.

The base of the Archaean lithosphere was probably at a seismic discontinuity about 15 km below the Moho.

The rocks below this subMoho boundary are also anisotropic, but with a different direction to the Pn anisotropy; the maximum velocity direction is between north and 40^ west of north.

The direction of maximum velocity below the sub-Moho boundary parallels basement structures (eg. Tangadee & Flint Hill Lineaments) of the Bangemall Basin within the Capricorn Orogenic Belt. The Bangemall Basin is younger than the Hamersley Basin, and the directions of velocity anisotropy in the sub-crustal lithosphere therefore imply younger palaeo-stress directions at greater depths.

Thus the lithosphere grew downwards in time as the continents cooled and acreted by underplating.

The base of the Archaean lithosphere is now at 45 km depth; the present-day base of the lithosphere is at about 200 km depth.

Lateral changes of velocity and density are superimposed on this subhorizontal mantle stratification.

Apparent velocities below the sub-Moho boundary are 8.4-8.5 km/s at 50 km depth on a north-south line, but just to the north along the same azimuth, the velocity at 100 km depth is 8.25-8.3 km/s, and it does not reach 8.4-8.5 km/s until below 200 km depth.

Comparison of the theoretical gravity effects of the crustal models from the seismic models with the observed gravity field, and with the condition that the area is in isostatic equilibrium, leads to the conclusion that the upper mantle under the Pilbara Craton is less dense than that under the Yilgarn Craton. Removal of an iron-rich fraction to form the Hamersley Basin rocks was the probable cause - the mass deficiency under the Pilbara Craton is sufficient to account for the entire volume of rocks of the Hamersley Basin.

Thus, vertical and lateral velocity changes, anisotropy correlating with major features in the surface geology, and density differences between provinces, have been mapped in the sub-crustal lithosphere of northwest Australia and these features have provided useful constraints on the theories of crustal evolution of the region.

102

Symposium 1(g) Tectonics of cratons and platform covers Conveners: Dr M.J. Rickard & Mr K.A. Plumb

103

PLATFORM AND BASEMENTS - THE AUSTRALIAN EXAMPLE

K.A. Plumb D i v i s i o n of C o n t i n e n t a l Geology Bureau of Mineral Resources, Canberra, ACT

Basement and cover are the fundamental elements of cratons.

Basement control of basin development and structure is virtually axiomatic; Australia is no exception. From a review of the Australian Precambrian platforms, a correlation is evident between the tectonic style and development of platform covers and the tectonic style and setting of their basements. Through reactivation of basement structures^ individual basins seemingly inherit their characteristic tectonic styles from the earlier styles which characterise their basements.

The basement terrains of the Australian Precambrian Craton comprise a number of discrete ancient cratonic blocks, separated by younger belts of intense deformation, metamorphism, and reactivation the Central Australian Mobile Belts (Plumb, 1979a). This basic framework provides the first-order tectonic control on subsequent development of platform covers and craton deformation.

Thus, the ancient cratons are characteristically overlain by very little platform cover (e.g. Yilgarn Block, Gawler Craton), or by extensive basins of moderate depth (ca. 5 km) which display laterally continuous sedimentary facies and only mild to moderate deformation (e.g. Pilbara Block, North Australian Craton). Platform covers become thicker and more deformed across craton-mobile belt junctions (e.g.

Ophthalmia Fold Belt, Lawn Hill platform, Torrens Hinge Zone).

Tectonic control of the North Australian Craton is by basement faults and lineaments, expressed as basin margins and ridges (Murphy, Urapunga Tectonic Ridges), as deep narrow sediment-filled grabens or rifts (Batten Trough, Halls Creek Mobile Zone), and as linear belts of block faulting and folding (Plumb 1979b; Plumb et al., 1981).

Hamersley Basin structures reflect greenstone belts and the reactivation and uplift of the characteristic granitoid domes of the underlying Pilbara Block (Gee, 1979).

In contrast, the platform covers above the mobile belts are everywhere very thick (up to 20 km), display marked lateral facies changes, and are moderately to intensely deformed. This reflects a continuing instability of their underlying crust.

In the Adelaide Geosyncline, the arcuate Mount Lofty-Olary Foldbelt is attributed to shortening related to the Delamerian continental margin, along trends controlled by basement structures; the intersecting patterns of block faults and folds of the Flinders Zone to the northwest reflect an intercontinental rift setting (Rutland et al., 1981). The Arunta and Ngalia Basins of central Australia are dominated by large thrust-nappe complexes, passing laterally into long Jura-type decollement folds, in response to reactivation of older basement overthrust zones in the Arunta and Musgrave Blocks (Plumb et al., 1981). Arcuate concentric folding of the Bangem.all Basin is^ related to discrete linear zones of strain within the basement, with a marked change in trend, (SE to NE) and en echelon thrusts, at the boundary between metamorphosed Early Proterozoic and reactivated Archaean of the underlying Gascoyne Province (Gee, 1979).

104

Finally, there are simple teraporal relationships, both vertically and laterally, between craton and basement development: successively overlying platform covers reflect increasing stability of underlying crustal blocks through time, and they relate directly to concurrent events in other platforms and orogenic belts.

The early unstable phase of platform cover, which immediately succeeds cratonisation of the underlying basement, is characterised by thicker sequences and moderate deformation, and commonly by basaltic volcanism and graben formation (e.g. McArthur Basin, Adelaide Geosyncline) (Plumb, 1979). Later stable phases are characterised by thinner gently-deformed sequences (e.g. Daly River-Georgina and Eromanga Basins, respectively). At the same time, space-time plots of tectonic evolution reveal widespread correlations between depositional cycles and orogenesis in fold belts and deposition and regional unconformities in concurrent platform covers (GSA, 1971).

References GEE, R.D., 1979: Structure and tectonic style of the Western Australian Shield. Tectonophysics, 58, 327-369.

GSA (Geological Society of Australia), 1971: and New Guinea, 1:5 000 000. Sydney.

Tectonic Map of Australia

PLUMB, K.A., 1979a: The tectonic evolution of Australia.

Science Reviews, 14, 205-249.

Earth

PLUMB, K.A., 1979b: Structure and tectonic style of the Precambrian shields and platforms of northern Australia. Tectonophysics, 58, 291-325.

PLUMB, K.A., DERRICK, G.M. NEEDHAM, R.S., and SHAW, R.D., 1981: The Proterozoic of northern Australia; in Hunter, D.R. (ed.)

Precambrian of the Southern Hemisphere. Developments in Precambrian Geology, 2. Elsevier, Amsterdam, 205-307.

RUTLAND, R.W.R., PARKER, A.J., PITT, G.M., PREISS, W.V., and MURRELL, B., 1981: The Precambrian of South Australia; ^ H u n t e r , D.R. (ed.). Precambrian of the Southern Hemisphere. Developments in Precambrian Geology, 2. Elsevier, Amsterdam, 309-360.

THE DETECTION OF CONTINENTAL SCALE STRUCTURAL PHEN0I4ENA USING I14AGE PROCESSING TECHNIQUES APPLIED TO DIGITAL GEOPHYSICAL AND REMOTELY SENSED DATA. R.F. Moore & C.J. Simpson Bureau of Mineral Resources, Canberra, ACT Any regional gridded data sets in digital form can be displayed on a high resolution TV monitor as an image. By applying digital image processing techniques either the total data set, or specific areas

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or intervals, can be coloured, enhanced, or subdued to obtain maximum differentiation of the phenomena of interest. These processes are not restricted to manipulation of a single type of data and if desired several different data sets can be displayed simultaneously as colour composite images to assess the coincidence of features apparent in individual images.

Image processing techniques have been applied to digital topography, gravity, aeromagnetic, and satellite data to display them in image form for improving the detectability of continental scale linear and circular features which may reflect major structure. Such techniques allow more flexible and rapid manipulation of data than is possible by conventional analogue methods and, in the case of topographic information, is the only known means of effectively assessing phenomena on a continental scale.

At the present time the reduced availability of some data sets in digital form is a limitation on this approach. For example only those regions surveyed by the BMR airborne magnetics program since 1972 have been directly recorded in digital form and thus in some instances only small areas may be available for merging with other types of regional data. Despite this, results to date indicate that image display and processing of multiple data sets is a powerful tool to assist in the detection of continental scale structural phenomena.

Though the approach can assist detection of structures it does not provide answers about their origin, or significance. At this early stage in the application of image processing techniques many more features can be observed than can be explained by the current level of knowledge of Australian continental scale structures.

ROLE OF FUNDAMENTAL FAULTS (LINEAMENTS) IN CRATONIC TECTONISM S. White Department of Geology, Imperial College, London Blocks of crystalline Precambrian continental crust exhibit long linear belts of intense deformation which often show evidence for later tectonic reactivation. Once formed, these zones act as planes of weakness in an otherwise rigid crust.C onsequently they have had a profound influence on subsequent deformation, igneous activity, mineralization and cratonic basin development. They are also the likely planes along which continents can split to form new oceans.

The weakness of these zones is due to the fault rocks that develop along them. It has been argued that these must be mechanically softer than the bordering country rocks (Watterson, 1975; White et al. 1980). There are a number of potential softening processes (White et al. op cit) but the most important appear to be retrogression and the formation of mineral shape and crystallographic fabrics which are orientated to provide easy flow within the zone.

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The latter is of major importance when minerals do not retrogress.

Shear heating, which is a dynamic softening mechanism, will play no part in initiating the reactivation process but could be important once reactivation has commenced.

There is little specific experimental rheological data from which the effects of weakening can be quantified. However there is sufficient published data to estimate the effect. These indicate that fault rocks may be an order of magnitude or more weaker than their parent rocks if softening is due to retrogression or less (factor of two) if it is due to geometrical effects.

The soft fault rocks remain after the movement has ceased. The result is that the deep continental crust has within it arrays of linear zones of weak rock as a result of earlier eposides of shearing. There is now a growing volume of literature suggesting that these are preferentially reactivated when the continental crust is restressed. The faults to be reactivated will depend upon their orientation to the newly imposed strain field and their structure with depth. New information on the last point is being provided by current reflection seismic programmes and from detailed field studies of old, eroded zones. The seismics have shown that, even in a small area of old continental crust, major fault zones may pass into the mantle or decoll within the deep crust or at the Moho. The Moine Thrust and Slide Zones in NW Scotland are an example. Field studies reveal that deep fault zones may have an appreciable width, for example the Amorican Shear Zone in Brittany is approximately 100 kms wide. These points indicate that the arrays of weak rocks have variable geometrical forms and may, at depth have widths approaching, or even greater than the thickness of the continental crust. As a result, geometrical constraints on reactivation are greatly relaxed. Thus during reactivation the new sense of movement need bear no relationship to the earlier sense for example a thrust may be reactivated as a normal fault, a strike slip as a reverse fault and so on. Nor need the entire zone be reactivated. In some, fault rocks from earlier periods of reactivation remain and provide a valuable insight into the tectonic history of fundamental faults.

An example is the Darling Fault in Western Australia.

References Gapais, D. and Le Corre, C. 1980. Is the Hercynian belt of Brittany a major shear belt. Nature, 288, 514-575.

Watterson J. 1975. Mechanism for the persistence of tectonic lineaments. Nature, 253, 520-523.

White, S.H., Burrows, S.E., Carreras, J., Shaw, N.D., and Humphreys, F.J. 1980. On mylonites in ductile shear zones. J. Struct. Geol. 2, 175-187.

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SIMILARITIES BETWEEN THE TECTONICS OF THE PRECAMBRIAN CRUST OF WESTERN AUSTRALIA AND THE STRUCTURE OF MYLONITES M.B. Katz School of Applied Geology, University of New South Wales, Kensington, N.S.W.

The Archaean - Proterozoic crust of Western Australia consists of two contrasting tectonic units; older Archaean cratonic blocks made up of granite-gneiss-greenstone terrains termed the Pilbara and Yilgarn Blocks and Archaean-Proterozoic mobile zones, fold belts and orogens which separate and tend to surround and flow around the cratonic blocks known as the Albany Eraser Province on the south southeast, the Western Gneiss Terrain and the Leeuwin and Northampton Blocks on the west, the Capricorn Orogen between the Yilgarn and Pilbara Blocks and the Paterson Province on the north northeast (Gee, 1979) (Fig. 1). The Yilgarn h20 DYKE MOBILE

ZONE

GRANITE-GNEISS GREENSTONE

Figure 1.

Craton - Mobile Belt Relationships in the Precambrian of Western Australia. Cratons Pilbara - P, Yilgarn - Y. Mobile Belts Albany Eraser - AF, Leeuwin - L, Western Gneiss WG, Northampton - No, Capricorn - C, Capricorn C, which includes the Gascoyne - G, Hamersley Basin - HB, Nabberu - Na, Bangemall - B; Paterson - Pa. Lower right inset shows Porphyroclast - Matrix relationships in a mylonite. Note scales.

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and Pilbara cratons are relatively rigid blocks but have a history of ductile (folds and SL fabrics) and brittle (fractures, faults, dyke emplacement) deformations. The surrounding mobile and fold belts are either Archaean - Lower Proterozoic high s-train, high grade metamorphic belts or Proterozoic folded basins. Thus the relatively rigid cratons are surrounded by more ductile zones of mobility.

The mega-scale relationships between the cratons and mobile belts can be compared to the much smaller scale micro-meso scale porphyroclast matrix structures found in mylonite zones (Katz, 1981). The cratons are scaled up analogs of mylonite porphyroclasts and the mobile belts can be compared to the scaled down mylonite flow matrix (Fig. 1). g These structural relationships are of vastly different magnitudes (10 ) and as similar anastomosing structures can be observed on all the intermediate scales (1 mm - 10 km) this suggests that they all may be related (Allegre et al, 1982). Their geometric similarities may be interpreted as having a common mechanical - rheological origin. It is speculated that the Pilbara, Yilgarn and other conjugate cratons in the Archaean were originally separate, although neighbouring ensialic miniplates of a single continent which moved relative to one another according to the prevailing Eulerian poles and this mutual jostling progressively deformed their common boundaries. The deformed boundaries are now the sites of the surrounding ductile and higher strain mobile zones while the cratons represent the more rigid and lower strain cores or relicts. After the Archaean the cratons stabilized and subsequent Proterozoic and even Phanerozoic deformations and events were mainly confined to the mobile zones.

References Allegre, C.J., Le Mouel, J.L. & Provost, A. (1982) Scaling rules in rock fracture and possible implications for earthquake prediction.

Nature, 297, 47-49.

Gee, R.D. (1979) Structure and tectonic style of the Western Australia Shield. Tectonophys. 58, 327-369.

Katz, M.B. (1981) A shear-mobile transform belt in the Precambrian Gondwanaland of Africa - South America.

Geol. Rundsch., 70, 1012-19.

CENTRAL AUSTRALIAN AND CENTRJ^.L ALPINE TECTONICS A COMPARISON R.O. Brunnschweiler 101 Jervois Street, Deakin, Canberra Micre are astonishing similarities in the tectonics of the central Alps in Switzerland with those of the Late Devonian and Early Carboniferous folds and thrust-nappes system in central Australia (Alice Springs Orogen).

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While the Alps are constructurally a great deal more complicated in detail and also show a lot more dynamometamorphism there is no doubt that the fundamental geometry and overall scale of some major structural elements of the two orogens are quite well comparable, and this thesis is illustrated by several cross sections.

The kinematics of the central Australian orogen are poorly known although the time spans involved for the whole of its history as well as for the various expansional and compressional orogenic phases within it have been reasonably well determined. However, in analogy to the kinematic history of the Alps some of these individual phases may have had a greater influence on the final structure than is currently recognized.

STRUCTURE AND EVOLUTION OF THE AMADEUS BASIN, CENTRAL AUSTRALIA Kurt Lambeck Research School of Earth Sciences, Australian National University, Canberra ACT The central Australian geology is dominated by a series of parallel sedimentary intra-cratonic basins whose history spans time from the Late Proterozoic to the Carboniferous. The basins are separated by regions of exposed Proterozoic crust that have a history of significant uplift. The formation cannot be explained by conventional thermal or stretching basin-forming mechanisms. Nor does the foreland basin model provide an adequate explanation for their occurrence. The geophysical observations, particularly gravity, point to a structure whereby the crust is considerably thicker under the basins than it is under the regions where basement rock is exposed.

This suggests that the crust as a whole is involved in the process that led to the basin formation. A mechanical model is developed which is based on the supposition that the lithosphere has been in a state of horizontal compression for long time intervals. The lithosphere is considered as a viscoelastic plate that has been subjected to a small normal load, distributed in an irregular manner. Under compression, initial deflections due to this load are magnified according to the plate's physical properties, the magnitude of the compressive force, and the wavelength spectrum of the initial perturbations. Because of the viscosity, the deflections grow with time at a rate that is determined by the balance of forces acting on the plate and by the lithosphere's rheology. Both the elastic and viscous deformations are much enhanced if erosion of the uplifted areas occurs, with the sediments deposited into the downwarps. These deformations take place at stresses that are much less than the buckling stresses. As bending stresses increase with time, failure by thrust faulting is predicted to occur within the basins, with the exposed basement being thrust over the basins. The model has been applied to the central Australian region starting at about 900 million years ago. The first basins to develop are the Officer and Amadeus Basins with uplift

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occurring in the intervening Musgrave Block. Initial rates of uplift and basin subsidence are small but they can increase rapidly with time if the compressive force is maintained. A point may be reached where the uplift becomes sufficient for gravitational instabilities to occur and for the crust as a v^hole to fail. This occurred about three hundred million years later and corresponds to the Petermann Ranges Orogeny. Further deformation is now shifted to areas where there has not been significant stress release at the time of failure.

This corresponds to the southern Arunta Block, and the northern part of the Amadeus Basin is further developed. The main sequence of events leading to the formation of the Ngalia Basin also takes place now. Early basin growth is again slow but increases rapidly with time to culminate in the Alice Springs Orogeny in Early Carboniferous time. The proposed model reproduces many of the geological and geophysical aspects of the region and provides a framework for discussing the geological history deduced from the basin sediments and basement metamorphics. It also predicts a crustal structure that is in broad agreement with the available data and which can be further tested by geophysical observations.

DEFORMATION STYLES IN PROTEROZOIC COVER SEQUENCES IN THE NORTHERN PART OF THE GASCOYNE PROVINCE,W.A. John L.

Baxter

Department o f Geology and G e o p h y s i c s , Western A u s t r a l i a n I n s t i t u t e o f Technology The p r e s e r v e d s t r u c t u r a l f a b r i c s and t h e d e p o s i t i o n a l h i s t o r y in t h e n o r t h w e s t e r n p a r t o f t h e G a s c o y n e P r o v i n c e p r o v i d e s a unique area to c h a r a c t e r i z e the r e g i o n a l d e f o r m a t i o n a l s t v l e . R e p e a t e d movement a l o n g p r a r a l l e l s h e a r z o n e s in t h e b a s e m e n t h a v e p r o d u c e d f o l d i n g and s h e a r i n g o f t h r e e w e a k l y metamorphosed P r o t e r o z o i c s e q u e n c e s , in the n o r t h w e s t e r n part o f t h e G a s c o y n e P r o v i n c e . The s t r u c t u r a l and s t r a t i g r a p h i c r e l a t i o n s h i p s h a v e been a s s e s s e d , and u s e d as a b a s i s for e s t a b l i s h i n g the s t r u c t u r a l sequence.

The s t r a t i g r a p h i c s u c c e s s i o n c o n s i s t s o f t h e L o w e r P r o t e r o z o i c W y l o o and W a r r a b i l l y G r o u p s and t h e M i d d l e P r o t e r o z o i c Uaroo Group. The o l d e s t s e q u e n c e , t h e W y l o o G r o u p , consists of eight f i n i n g upward c y l c e s of diamictite c o n g l o m e r a t e , w a c k e , s i l t s t o n e and banded i r o n f o r m a t i o n . The base has been r e c r v s t a l 1 i z e d during metamorphism, but toward the top of the s u c c e s s i o n most o f the primary f a b r i c s are p r e s e r v e d .

The W a r r a b i l l v Group i s a n e w l y r e c o g n i s e d s e q u e n c e o f c l a s t i c and s t r o m a t o l i t i c d o l o m i t e and s h a l e which shows l i t t l e s i g n o f r e c r v s t a l 1 i z a t i o n . The Uaroo Group has b e e n d i v i d e d i n t o f o u r f o r m a t i o n s b v v a n de G r a a f f e t al ( 1 9 8 0 ) and c o n s i s t s e s s e n t i a l l y o f a f l u v i a t i l e quartz a r e n i t e at the b a s e , o v e r l a i n by s u p r a t i d a l d o l o m i t e , beach quartz a r e n i t e , deeper water s h a l e s and d o l o m i t e . The U a r o o G r o u p a p p e a r s t o h a v e u n d e r g o n e

Ill

r e c r y s t a l 1 i z a t i o n o f the m a t r i x in medium and c o a r s e - g r a i n e d units* There i s no d i f f i c u l t y determining the facing throughout the s u c c e s s i o n as numerous sedimentary s t r u c t u r e s are preserved.

Four d i s t i n c t f o l d phases have been d e t e r m i n e d w i t h i n the d i s t r i c t . Rare e a r l y f o l d s (F^) appear to be about layer p a r a l l e l p l a n e s which have a weak d e v e l o p m e n t o f a f o l i a t i o n . This f o l i a t i o n , S p may be "mimetic", and the F^ f o l d s system.atic s o f t sediment slumping on a p a l a e o s l o p e . The second f o l d phase, l i k e the f i r s t , o n l y a f f e c t s the Wyloo Group, but u n l i k e the f i r s t p h a s e , c o n t r o l s the r e g i o n a l d i s t r i b u t i o n o f the s e d i m e n t a r y u n i t s . F^ f o l d s are open to c l o s e , s y m m e t r i c a l , v e r t i c a l t o s t e e p l y i n c l i n e d , and c o n t a i n a spaced a x i a l p l a n a r c l e a v a g e , which c r e n u l a t e s S p and p r o d u c e s a s t r o n g domainal f a b r i c i n c o a r s e r grained rocks and w e l l - d e v e l o p e d c r e n u l a t i o n cleavage in t h e u s u a l l y more p e l i t i c f i n e r g r a i n e d u n i t s . F3 f o l d s a f f e c t t h e W a r r a b i l l y Group. They have g e n t l e , s y m m e t r i c a l , u p r i g h t geometry and contain i r r e g u l a r l y developed spaced cleavage. F^ i s seen in Wyloo Group Rocks as k i n k i n g or c r e n u l a t i o n o f the S2 f a b r i c about e a s t e r l y trending s u r f a c e s . The d i s t r i b u t i o n of S^ in the rocks of the Wyloo Group i s e r r a t i c , and appears to occur in d i s c o n t i n u o u s z o n e s . The Uaroo Group has been d e f o r m e d by F^ f o l d s which are open f o l d s about v e r t i c a l axial surfaces p a r a l l e l to a s t r o n g l y domainal f a b r i c . Deformation a s s o c i a t e d with F^ in the W a r r a b i l l y and Wyloo Groups can be seen as zones 100 to 200 m wide c o n t a i n i n g c r e n u l a t i o n o f S2 and S^ c l e a v a g e and t r e n d i n g 130^ to 145^. These zones o f t e n have s h e a r s in t h e i r c e n t r e s a l o n g which d i s p l a c e m e n t o f up t o 1 km can be d e m o n s t r a t e d in pi a c e s .

Cleavage development within all of the supracrustal rocks i s s i m i l a r . In c o a r s e r g r a i n e d members t h e c l e a v a g e i s smooth and anastomosing with spacing varying between 0.2 and 2 mm. Generally the cleavage o c c u p i e s the e n t i r e unit tending to o b l i t e r a t e f i n e bed forms. Mica bands commonly o u t l i n e the cleavage domains which appear to have formed due t o s o l u t i o n p r o c e s s e s . In the f i n e r grained beds the cleavage i s more commonly d i s c r e t e with d i s t i n c t m i c a - r i c h bands s e p a r a t e d by q u a r t z - r i c h m i c r o l i t h o n s . The cleavage domains are c h a r a c t e r i z e d by c r y s t a l 1 i z a t i o n of w h i t e , p o s s i b l y p h e n g i t i c , mica, while i t i s usual to have c h l o r i t e as the p h y l 1 o s i l i c a t e in the m i c r 0 1 i t h o n s . H e t e r o g e n e i i t y o f cleavage development in d i f f e r e n t l i t h o l o g i e s can be demonstrated in the f i e l d and in thin s e c t i o n s t u d i e s .

A c h a r a c t e r i s t i c o f d e f o r m a t i o n in the d i s t r i c t i s t h e s i m i l a r s o u t h e a s t e r l y o r i e n t i a t i o n o f the the axial surfaces of F2 and F^, Zones o f high s t r a i n p a r a l l e l to t h i s d i r e c t i o n are common in the Gascoyne Province. The l o c a l i z a t i o n of s t r a i n and the zonal s u p e r i m p o s i t i o n o f y o u n g e r d e f o r m a t i o n e v e n t s on e a r l i e r f a b r i c s s u g g e s t s t h a t shear z o n e s in the basement Gascoyne P r o v i n c e c o n t r o l d e f o r m a t i o n w i t h i n the P r o t e r o z o i c supracrustal r o c k s . Tnis s t y l e of deformation i s s i m i l a r to that proposed by Muhling and Brakel (in p r e s s ) in the Bangemal1 Basin to the southeast.

References: Muhling P.C. and B r a k e l , A . T . , i n p r e s s . The g e o l o g y o f Bangemal1 Basin: West. Australian Geol. Survey B u l l .

the

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Van de Graaff, W.J.E., Denman, P.D.,Hocking, R.M. and Baxter, J.L., 1980, Yanrey-Ningaloo, W.A.: West. Auetralia Geo!.

Survey 1:250 00 Geol. Series Explan. Notes.

LOCAL AND REGIONAL TECTONIC-STRUCTURAL CONTROLS OF LATE PALAEOZOIC CRATONIC VOLCANISM IN NORTHEASTERN AUSTRALIA Brian Oversby Bureau of Mineral Resources, Canberra, A.C.T. In common with most other parts of what is now eastern Australia, the region between Bowen and Torres Strait, extending as far west as Croydon, was characterised by Early (but not earliest) Carboniferous through Permian calc-alkaline igneous activity with extensive volcanism.

This activity took place in an extensional tectonic environment, and was superimposed on a mosaic of crustal elements which coincided broadly with the present Georgetown-Coen, Hodgkinson, Broken River, Charters Towers, and Clermont (structural) Provinces, as defined by Henderson (1980). By the time that igneous activity began, the essential parts of these elements had undergone one or more major deformations, and were effectively cratonised. Igneous activity was preceded by a Late Devonian to earliest Carboniferous phase of differential vertical movement, with local accumulation of fluviatile to shallow marine, predominantly epiclastic, sedimentary rocks which are currently assigned to the older structural provinces, although they constitute a discrete stratigraphic entity of transitional (Geological Society of Australia, 1971) tectonic aspect.

Even though these rocks contain evidence for only sporadic, minor, and localised contemporaneous igneous activity, the areas of uplift which they indicate at the sites of the Georgetown-Coen, Hodgkinson, and central to southern Charters Towers Provinces, and the Anakie Subprovince of the Clermont Province, may have developed in response to precursor thermal expansion of the crust.

Two main styles of volcanism occurred in northeastern Australia during late Palaeozoic time. The most conspicuous of these was "mature", in developmental terms, and was characterised by extrusion of voluminous rhyolitic ignimbrite sheets, with development of a hierarchy of surficial trough- to basin-like volcanic and volcano-tectonic subsistence structures (Oversby, et. al., 1980), in association with the emplacement of major granitoid batholiths. This style occurred mainly in Carboniferous time at the sites of the Georgetown-Coen and westernmost Hodgkinson Provinces, as well as the eastern Clermont Province (Anakie Subprovince) farther south.

The second, developmentally "immature" or "senile" (depending on timing and location) style of volcanism was characterised by local accumulation of rhyolitic and andesitic lavas with mostly subsidiary rhyolitic ignimbrite intercalations; the style was associated with limited development of granitoid plutons. This second style, in its "immature" form, was dominant at the sites of the Broken River and Charters Towers Provinces throughout the late Palaeozoic, and probably also overlay the eastern Hodgkinson Province during Permian time. In its "senile" form.

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the style succeeded "mature" activity at the sites of the southern Georgetown-Coen and westernmost Hodgkinson Provinces, and the Anakie Subprovince, during Permian time.

To a first approximation, the development of "mature" as against "immature" activity in any area is believed to have been largely a reflection of the physical response of rocks at intermediate to high crustal levels to extensional stresses, and their ability to permit and sustain development of acid magma chambers of very large size at those levels.

Thus, rocks of the Georgetown-Coen Province, and of the Anakie Subprovince of the Clermont Province, had an appropriate combination of attributes, such as tensile strength and homogeneity, to facilitate development of major magma chambers, while those of the eastern Hodgkinson, Broken River, and Charters Towers Provinces did not. By implication, rocks at depth in the westernmost Hodgkinson Province were more like those of the Georgetown-Coen Province than those in the eastern Hodgkinson Province.

Superimposed on this postulated effect of bulk crustal strength was one of decreasing availability of acid material for melting at lower crustal levels, which contributed to the "senile" second style of activity in some areas.

In a more local context, the position, form, and evolution of surficial volcanic-related structures, and of associated intrusive bodies, in northeastern Australia during late Palaeozoic time were evidently controlled in part by semi-regional stress fields, and in part by local crustal inhomogeneities such as dislocation zones and contacts between older granitoid bodies and host rocks. To a large extent, these controls interacted in complex fashion in both space and time, and were probably responsible for, among other effects, systematic age/locality variations within some extrusive (and intrusive) sequences which are superficially suggestive of "hot-spot" activity. At the site of the Georgetown-Coen Province, and possibly also the westernmost Hodgkinson Province, Carboniferous volcanism and high-level intrusive activity took place under the influence of an approximately horizontal and north to south-directed maximum principal stress which was relatively easily reoriented by local inhomogeneities.

In contrast, the maximum principal stress during Permian time had a less easily varied, approximately north-west to southeast, orientation.

Farther south, effects of only the younger stress field are conspicuous. Although the time of its initiation is unclear, it was evidently replaced in Late and post-Permian time by stresses which gave rise to the structural Bowen and Drummond Basins.

GEOLOGICAL SOCIETY OF AUSTRALIA, 1971: Tectonic Map of Australia and New Guinea, 1:5 OOP OOP. Sydney.

HENDERSON, R.A., 198P: Structural outline and summary geological history for northeastern Australia. In: Henderson, R.A., & Stephenson, P.J. (Eds), The geology and geophysics of northeastern Australia, pp. 1-26. Geol. Soc. Aust. (Qld. Div.).

Brisbane.

OVERSBY, B.S., BLACK, L.P., & SHERATON, J.W., 198P: Late Palaeozoic continental volcanism in northeastern Queensland.

Henderson, R.A., & Stephenson, P.J. (Eds), The geology and geophysics of northeastern Australia, pp. 247-268. Geol.

Soc. Aust. (Qld. Div.), Brisbane.

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LANDSCAPE EVOLUTION OF THE NORTHERN MONARO, NSW AND ITS IMPLICATIONS FOR THE UPLIFT HISTORY OF THE SOUTH-EASTERN AUSTRALIAN HIGHLANDS

Gordon R. Taylor Department of Geology, Australian National University Canberra ACT

The elucidation of the landscape evolution of the Northern Monaro region of the Southern Highlands of NSW allows important conclusions to be made about the uplift history of the South-eastern Australian Highlands.

An ancestral southeastern Australian highland was created at some time between the mid Triassic and late Cretaceous. By the late Cretaceous to Palaeocene a broad, gently undulating palaeoplain had formed in the Monaro region. It formed a 35 km wide valley eroded into relatively soft Ordovician and Silurian sediments, with harder Devonian granites forming hills several hundred metres higher to the east and west. This palaeoplain is part of the widespread late Cretaceous or Palaeocene Monaro Surface.

Uplift of the highlands commenced at about 59 Ma and continued until approximately 35 Ma (The Bathurst Uplift). Widespread basaltic volcanism was associated with the uplift. In the Monaro region the uplift resulted in the incision of 60 to 100m deep V-shaped valleys into the Monaro Surface. Once cut, these valleys were inundated with basalt associated with the Bathurst Uplift.

No regional uplift has occurred since the close of the Bathurst Uplift in the early Oligocene, as indicated by the valleys being re-cut into the basalt to the same base level as the pre-basalt valleys. However large scale block faulting and warping has occurred since the Oligocene and is continuing at present. This tectonic phase has been named the Murrumbidgee Tectonic Period.

Overriding of the eastern highlands over the former ridge position of the Tasman spreading ridge, using a South America fixed reference frame explains the Bathurst Uplift and concurrent volcanism. This thermo-tectonic anomaly in the asthenosphere caused partial melting, basalt volcanism, and consequent uplift. A similar overriding of the thermo-tectonic anomaly associated with the former Coral Sea ridge has caused progressive southward migration of basalt volcanism in the middle and late Tertiary. This later overriding may provide a mechanism for the Murrumbidgee Tectonic Period faulting and warping.

This study provides a tectonic framwork for the Miocene lacustrine sequence at Bunyan presently being studied by G.M. Taylor and P.H. Walker, to whom with K.A.W. Crook, the author expresses thanks for assistance and supervision.

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THE STUART SHELF, SOUTH AUSTRALIA AND THE CONTINENTIAL MARGIN DURING LATE PRECAMBRIAN

GLACIATIONS

C. R. Dalgarno Seltrust Mining Corporation, Adelaide South Australia.

The Gawler Province as described by Rutland et al, (1981) encompasses the Gawler Craton and the Willyama domain including Broken Hill, Olary, Mt. Painter and other inliers within the Palaeozoic fold belt in South Australia.

The Stuart Shelf is the relatively stable platform developed on the eastern portion of the Gawler Craton during deposition in the Adelaide Geosyncline and subsequent to the development of rifted proto basins.

An analogous shelf or stable platform developed to the east on the Curnamona Nucleus at the same time. Both provide a comparable but incomplete record of deposition to that within the geosyncline.

Neither shelf area was influenced significantly by the Delamerian orogeny in the early Palaeozoic which resulted in the Adelaide Fold Belt and intrusive granites into the tectonic belt in the east.

The sequence of the Stuart Shelf is underlain by transitional tectonic units, notably felsic volcanics and intercalated sediments ranging in age from 1650 Ma to 1500 My. Post tectonic granites of the Hiltaba suite intrude both this sequence and the Early Proterozoic metamorphic/ granitic basement in a broad W-E belt across the Gawler Province.

The Stuart Shelf has remnant fault troughs preserving a protoAdelaidean transgressive continental deposit - the Pandurra Formation not known within the Geosyncline. Locally thicknesses of the order of 1 km are preserved. The early sequences of the Adelaide Geosyncline are not represented on the shelf areas, presumably having been removed by uplift and erosion during the Sturtian epoch. Thus the early rift deposits comprising mafic volcanic and shale-carbonate sequences of the Peak Denison and Willouran Ranges and the Mundallio Creek/Depot Creek region are not known farther west than Port Augusta.

The Sturtian glacial record of the Stuart Shelf is restricted to the Stuart Creek area northwest of Lake Torrens and to possible occurrences near Mt. Gunson. Movements of major faults along the margins, and initiation of fold structures within the geosyncline, commenced prior to the Sturtian Glaciation and resulted in angular unconformities and rapid thickness and facies changes in the glacial sequence and its marine equivalents. Piercement structures developed in the thicker sequences at this time and continued to penetrate to the depositional interface through to the Early Cambrian.

Following the Sturtian Glaciation, dark laminated shale and dolomite was deposited over very wide areas including local basins on the Stuart Shelf. A platform developed over what is now the central Flinders Ranges and deltaic conditions interfingered with shallow marine carbonates and siltstones in the deeper part of the trough.

Movement of faults during deposition resulted in sedimentary breccias over wide areas of the western part of the basin in late Sturtian t ime s.

116

Deposits of the Marinoan Glaciation are poorly developed in the western part of the Adelaide Geosyncline and are unknown from the Stuart Shelf area with the possible exception of local lag deposits and loess-type sands (Whyalla Sandstone, Reynella Siltstone facies). Much of the planation and removal of earlier Adelaidean deposits from the shelves occurred in this interval which was one of widespread glacial influence across Australia.

Flooding of the post-glacial basins and shelves resulted in a widespread marker dolomite representing the changing conditions.

Carbonate previously locked up in cold saline waters was apparently deposited by biological growth on the broad warm shelves after rise in sea level resulting from widespread melting of continental ice.

The broad zone of late Precambrian red beds represents fluviomarine deltaic deposition with thickness and facies of various units influenced by buried basement structures, and diapiric domes. Marine carbonates transgressed the shelf areas in the Early Cambrian and gave way later to sabkas and continental red bed deposits which have now been largely eroded.

Early Palaeozoic orogeny deformed the Geosyncline but the stable shelves to east and west preserved very thin and incomplete sequences of platform cover developed on the transitional tectonic suite of the Gawler Province.

Reference: Rutland R.W.R., Parker A,J., Pitt G,M,, Preiss W.V., and Murrell B. 1981 "The Precambrian of South Australia" in Developments in Precambrian Geology, 2. Elsevier, Amsterdam 1981.

SCHEMATIC DEVELOPMENT MODEL FOR THE GAWLER PROVINCE

I 500 Ma

JL-^ TRANSITIONAL TECTONIC

REGIME

lOOOMa

P R O T O - ADELAIDEAN

BASINS

750 Ma

LATE ADELAIDEAN STUART SHELF

500 Ma

EARLY

PALAEOZOIC

ADELAIDE FOLD BELT

PALAEOZOIC

TROUGH

117

THE POLDA TROUGH: PERIODIC SEDIMENTATION SINCE THE PROTEROZOIC? B.J. Cooper & C.G. Gatehouse S.A. Department of Mines & Energy, P.O. Box 151, Eastwood, South Australia 5063

The Polda Trough forms a well-defined, east-west graben within the Archean and Early Proterozoic crystalline rocks of South Australia's Gawler Craton. It is about 360 km long, 10-30 km across and over 60$? of the trough length lies under the Great Australian Bight. The trough probably contains in excess of 5 000 m of sediments which range in age from ?Middle Proterozoic to Cainozoic.

The complete sedimentary succession has not yet been drilled.

Where investigated, discrete sequences of strata are revealed, which are separated by long periods of erosion or non deposition.

The Middle Proterozoic Blue Range Beds (Flint & Parker, I981) may be regarded as the basal succession, although there is an enormous time gap between these deposits and overlying units. In outcrop, this unit includes pebble conglomerates and medium to coarse grained sandstones having a presumed fluvial origin. Early Palaeozoic sediments have been recorded by Flint & Parker (1982), however little information is available about them.

Extensive drilling of the onshore part of the trough since 1976 has revealed substantial thicknesses of Late Palaeozoic glacigene sediments (Cooper et I982). These have been named the Coolardie Formation. The overlying Polda Formation is fluvial in origin comprising sands, clays, and coals (Gatehouse & Cooper, 1982), and has been dated by palynology as Late Jurassic.

The Middle and Late Eocene Poelpena Formation (Harris, I966) is similar to the underlying Jurassic succession. However this sequence is less carbonaceous than the Polda Formation and differs in coal quality as well. It also shows increasing marine influence towards the west. Pliocene sediments have been recognised, following palynological investigations, at the eastern end of the trough (Harris in Gatehouse, I98I). The onshore region of the trough has been blanketed by a cover of Quaternary dune aeolianites and calcrete.

The Polda Trough has been subjected to mild intermittent tectonism throughout its long history. Activity has probably been concentrated along the bounding faults that have been determined on geophysical evidence. Deformation of the sedimentary sequences has been minimal. Depositional evidence from the Mesozoic and Cainozoic of this trough, when compared with that of adjacent synchronous basins, suggests that only the initial phase of regional sedimentation is recorded here. Consequently there is a notable absence of marine Cretaceous and a poor development of marine Tertiary sediments. It is thus concluded that sediments were

118

deposited in the Polda Trough during periods of maximum tectonic instability in the region, and subsequently preserved.

The Polda Trough contains significant reserves of coal within the Jurassic Polda Formation. The Lock coal deposit is located 15 km west of Lock and is considered by the Electricity Trust of South Australia as a potential coal source for power generation.

A source of potable groundwater within Quaternary aeolianites is used for domestic supply.

References Cooper, B . J . , Harris, W . K . & M e y e r , G . M . , I982.

The Late Palaeozoic Coolardie Formation, Polda Basin. Q . geol. N o t e s , geol. Surv. S . A u s t . 81:9-13.

F l i n t , R . B . & Parker, A . J . , I98I. The Blue Range B e d s , central Eyre Peninsula. Q . geol. Notes, geol. Surv Aust.

80:12-15.

F l i n t , R . B . & Parker, A . J . (Compilers) I982.

Tectonic M a p , South Australia, 1:2 000 000 scale. S_. A u s t . D e p t . Mines & Energy.

Gatehouse, C . G . , I98I. Tuckey No. 1 , W e l l Completion Report.

S . A u s t . D e p t . Mines & Energy Rept. B k . 81/19 (unpublished).

Gatehouse, C . G . & Cooper, B . J . , 1982.

The Late Jurassic Polda Formation, Eyre Peninsula. Q . geol. N o t e s , geol. Surv. S_.

Aust. 81:13-16.

H a r r i s , W . K . , 1966.

New and redefined names in South Australian Lower Tertiary Stratigraphy. Q . geol. Notes, geol. Surv.

S . Aust. 20:1-3.

LOCALITY MAP

ROCK RELATION DIAGRAM BLUE RANGE

ONSHORE POLDA TROUGH

?

?

BEDS

119

A NEW MODEL FOR CRUSTAL DEFORMATION P.M. James 4 Merrilyn St, Chapel Hill, Q. 4069 No Abstract provided

TOWARDS AN UNDERSTANDING OF THE WARBURTON BASIN IN SOUTH AUSTRALIA by Colin G. Gatehouse Department of Mines and Energy, 191 Greenhill Road, Parkside, South Australia, 5063

The Warburton Basin is an Early Palaeozoic sedimentary basin, known only in the subsurface; it is centred around the northeast corner of South Australia but also extends into the southern Northern Territory, southwest Queensland, and northwestern New South Wales (Fig. 1). It contains carbonate and volcaniclastic sediments of Cambrian age, Ordovician coarse and fine clastics, and volcanics of pre-Middle Cambrian age. Several other lithologies of uncertain age are also known.

The Gidgealpa-Merrimelia-Innamincka (GMl) Trend is the major structure of both the Warburton and overlying Cooper Basin. Faults and other linear structures, seismically mapped at the base of the Permo-Carbonifercus surface by Delhi Petroleum Pty. Ltd., show a preferred orientation dominated by a northeasterly grain. O'Driscoll's ( 1 9 8 2 ) lineament 3 and lineament 8 lie parallel to the southern and western margins of the basin.

East of the G M I Trend the geology is apparently complex with significant lithological changes occurring between drillholes only a few kilometres apart. By contrast west of the GMI Trend in a region of less intensive drilling there is less variation in rock type and each lithotj-pe appears to be more areally extensive.

Pre-Middle Cambrian volcanism extruded at least 900 m of andesite, trachyte, dacite and rhyodacite in the vicinity of the GMI Trend. Faunal evidence shows that volcanic activity (tuffs and agglomerates) persisted into Late Cambrian.

Platform deposits of Early Cambrian age (in McDills-l) and of Middle to Late Cambrian age (in Kalladeina-1, Coongie-1, and in the Gidgealpa area) show low-angle dips and relatively little disturbance.

Ordovician platform sediments of fine and coarse-grained clastics occur west of the GMI Trend and extend as far southeast as the Murteree Horst. Shallow-water clastics are replaced eastwards by a

120

basinal black pyritic shale facies. Coarse clastics reappear further east and suggest an eastern margin to the deep Ordovician trough.

Middle Cambrian sediments and volcanics, and shallow water Ordovician sediments are known further southeast at Mount /Vrrowsmith in New South Wales and have been included by Youngs & Moorcroft (1982) in the Arrowie Basin.

The enigmatic near-horizontal Innamincka "Red-Beds" have been variously interpreted as Devonian or Cambrian but direct dating evidence is lacking.

Several occurrences of granitic rocks (that possibly intruded the Ordovician) have been intersected east of the GMI Trend. Individual samples of granite in Moomba-1 ^ggge in age from 333 to 362 Ma (Rb/Sr whole-rock, with assumed 87Sr/®^Sr ratios). It has been suggested that these represent the age of last cooling but the age of intrusion is not known.

Ordovician sediments are folded and all show steep dips where intersected. Induration is extensive and may have reached the zeolite facies of metamorphic grade. Cambrian sediments apparently were not folded as dips of 5-20^ are common; at Gidgealpa-1 is exceptional and may be related to nearby faulting. The time of folding must be post early Late Ordovician (dated in Dullingari-l) and pre Late Palaeozoic from undated undisturbed sediments in Tinga Tingana-1. A Benambran or Tabberabberan age for this event is possible.

REFERENCES O'Driscoll, E.S.T., 1982. Patterns of Discovery - The Challenge for Innovative Thinking. PESA JOURNAL 1:11-31.

Youngs, B.C. and Moorcroft, E., I982. The Petroleum Potential of the Eastern Arrowie Basin and Frome Embayment. APEA JOURNAL, 1982: 82-101.

ROCK RELATION DIAGRAM LATE PALAEOZOIC SEDIMENTS

LOCALITY

INNAMINCKA "RED-BEDS" (DEVONIAN OR CAMBRIAN)

Yi^A'^NfyfYHMY^

Kalladeinal.

Gidgealpa Field • • Oullingari 1 •

\

\ I

CAMBRIAN

SA

I VI V

V

V

V

V

PRE-MIDDLE CAMBRIAN VOLCANICS

\

GRANITE

Moombo ]

^ \

V

MAP

.

Tinga Tingano 1 .

X i I NSW

' MT A R R O W S M I T H ^

,

WILLYAMA COMPLEX

100 km I

Fig.

Symposium 1(h) Continental margin evolution Convener: Dr D.A. Falvey

121

SUBMARINE

CANYONS

AND

ADE LAIDE

CANYON-FILLING

PROCESSES,

GEOSYNCLINE

C.C. von der Borch, A.E. Grady, R. Aldam, D. Miller, R. Neumann, and A. Rovira.

School of Earth Sciences, Flinders University of South Australia, Bedford Park, S.A.

A series of major infilled submarine canyons has incised progradational deltaic and slope sediments of the Proterozoic Wilpena Group, Adelaide Geosyncline, in the Flinders Ranges of South Australia.

These canyons are filled by mixed terrigenous and carbonate sediments of the Wonoka Formation.

All canyons contain a common vertical lithological sequence, in which channelled turbidite sandstones, levee sandstones, slumps, slump breccias, olistostromes and mudstones occur.

Palaeocurrent analyses, particularly using flute casts at bases, of turbidite sandstones, indicate great coherency of flow direction in each of five canyons. Without exception all flute casts in each of the east-west trending canyons, from south to north, alternate by close to 180^ in adjacent canyons. In addition most abundant and coarsest sandstones, as well as coarse slump breccias, occur preferentially adjacent to north walls of all canyons. The canyon outcrop patterns and the above observational data are best explained by proposing that all the incisions are part of a major meandering palaeochannel on a north-dipping palaeoslope. This proposed palaeochannel has undergone post-depositional folding and erosional truncation.

SEDIMENTARY FACIES ON A PASSIVE MARGIN : ROCKALL PLATEAU, DSDP Leg 81 J.B. Keene Department of Geology and Geophysics, University of Sydney, N.S.W.

Four sites were drilled by Leg 81 of the Deep Sea Drilling Project with the aim of understanding the evolution of passive continental margins associated with the opening of the North Atlantic. The sites form a transect across the south-west margin of Rockall Plateau. The most oceanward site (554) was located on the "outer high" in 2574 mof water and the most landward site (555) was some 65 km to the east on Edoras Bank in 1669 m of water. The two remaining sites (552, 553) were drilled between these locations in the Edoras sedimentary basin (water depth 2300 m). Sub-sea floor penetration of 209 m, 682.5 m. 326 m and 964 m was achieved in the transect and each site terminated in a sequence of basalts with interbedded sediments.

122

The sedimentary fades sampled reflect a depositional environment controlled by subsidence, sedimentation rate and the changing sediment sources.

The changing depocentre leads to an interfingering of the resulting facies. Water depths of deposition show a general upward increase.

Volcaniclastic sediments dominate the lower sedimentary sequence (Late Palaeocene to middle Eocene) and are overlain by biogenic pelagic facies.

These two major facies can be subdivided based on depositional processes, sedimentary structures and textures, particle composition (sediment source) and water depth all of which reflect the changing tectonic, volcanic and climatic history of the region.

The lower part of the sequence consists of ash-fall basaltic tuffs interbedded with reworked volcaniclastic sediments and detritus from a high-grade metamorphic terrain. The base of this facies was not reached but 300m was penetrated at site 555. The outer high crust at site 554 post-dates this facies. Most of this sediment accumulated in shallow marine, near-shore environments with one ash fall probably sub-aerial.

These sediments represent the rifting phase and correlate with the "dipping reflector" sequence. This was followed by a time of little or no pyroclastic activity and fine-grained carbonaceous terrigenous clastics accumulated at rates of 90 m/m.y in shallow water, marginal marine or estuarine conditions. The subtle interplay between subsidence and sedimentation is shown by interbeds of regressive arkosic sandstones, characterised by a metamorphic mineral assemblage.

Then followed a period of maximum volcanic activity (andesitic to dacite in composition) immediately prior to the onset of spreading. Overlying this is a glauconitic horizon representing reduced sedimentation rates and a marine transgression resulting in outer shelf depths. Finer-grained and less frequent basaltic tuffs were then deposited interbedded with calcareous biosiliceous sediments. The tuffs are probably related to volcanism associated with sea-floor spreading. Near-shore high energy volcaniclastic sandstones and conglomerates were deposited on the "outerhigh" at this time.

From the middle Eocene until the middle Miocene a period of very slow sedimentation occurred resulting in condensed sequences including hardgrounds, manganese crusts and several major disconformities. This period of slow sedimentation and erosion 30 m.y.) reflects the major subsidence from outer shelf depths to abyssal depths soon after the onset of spreading.

The result was a major change in facies from calcareous tuffaceous spiculite below to nannofossil - foraminiferal chalk above. This overlying pelagic facies can be subdivided into true pelagic facies, drift deposits (the Hatton Drift contourites) and glacial facies.

A terrigenous tubidite facies is lacking on this passive margin because there was never a subaerial source post-rifting. Because the rift facies indicates a fairly uniform water depth in this area (with tectonic highs to the west and east), the present day structural relief between the sites results from post-rift differential subsidence.

123

THERMAL EVOLUTION OF RIFTED CONTINENTAL MARGINS FROM FISSION TRACK DATING OF APATITES A.J.W. Gleadow, M.E. Moore, K.U. Ferguson, I.R. Duddy and J.F. Lovering Department of Geology, University of Melbourne, Parkville Vic 3052 Apatite fission track ages are easily reset by thermal and tectonic processes associated with continental rifting because of the low temperatures (100 ± 25°C) required to anneal fission tracks in this mineral over geological time. These effects result in a pattern of decreasing apatite age towards a rifted margin, the youngest ages near the coast often correlating with the onset of seafloor spreading in the adjacent ocean basin. This distinctive pattern has now been found along continental margins from a number of different parts of the world, and is particularly well developed in southeastern Australia.

The distribution of fission track lengths in these apatites also enables the discrimination of samples which have been completely reset from those that have suffered only partial overprinting of an older age. Fission track analysis thus provides a means of testing various hypotheses cibout the thermal and tectonic evolution of rifted continental margins.

In southeastern Australia apatite ages from granitic rocks decrease from 250-360 Myr about 100km inland to 80-125 Myr along the coast, the youngest ages correlating with the initiation of seafloor spreading in the Tasman Sea. Track lengths indicate that the youngest apatites have been almost completely reset by a relatively short-lived thermal event at that time. This heating of the continental margin is independently evidenced by alkaline magmatic activity and magnetic overprinting (Schmidt and Embleton, 1981).

Accelerated uplift and erosion along the flanks of the developing rift have emphasised the thermally-induced apatite age pattern by exposing basement rocks from initially deeper levels in the crust.

Along the southern margin of Australia there is evidence of a significant heating event at about 100-120 Myr in the basement rocks of King Island adjacent to the Otway Basin, which has a major volcanogenic fill of this age. There is no evidence of a younger thermal event of any significance affecting this area. These results are consistent with a revised interpretation of magnetic anomalies in the southern ocean (Cande and Mutter, 1982) suggesting a Cretaceous age (90-110 Myr) for the oldest ocean crust between Australia and Antarctica.

Apatites from coastal rocks between Eyre Peninsula and Albany show fission track ages with a minimum value of about 300 Myr which, along with skewed track length histograms, suggest relatively low thermal effects during ocean crust intrusion.

This implies a relatively "cold fracture" origin for most of the Australia-Antarctica rift west of the Otway Basin, in contrast to the thermal activity in southeastern Australia.

124

RIFTING OF THE NORTHERN MARGIN OF THE AUSTRALIAN CONTINENT DURING THE BREAKUP OF GONDWANA C.J. Pigram

1

and H. Panggabean

2

2 bureau of Mineral Resources, Australia, Bandung, Indonesia Geological Research and Development Centre, Bandung, Indonesia Continental Australia is surrounded on three sides by passive margins.

The geological histories of the eastern, southern and western margins are well documented. However the northern margin of the Australian continent which is now an active collision margin has rarely been examined in terms of its previous passive margin history. Hamilton's (1978) assumption that the rifted margin of western Australia continued northward into eastern Indonesia and then eastward through Irian Jaya into Papua New Guinea is correct. However the distribution of the rift drift sequence and the history of events related to the break up throughout the region are more complex than Hamilton (1978) envisaged.

The continuation of the rift drift sequence northward from the Joseph Bonaparte Gulf into the western Arafura Sea region seems likely, but the area is poorly understood due to a lack of data. The rift drift sequence is recognised again in the western central ranges of Irian Jaya.

The sequence then continues eastward into Papua New Guinea where the platform sequence exposed in the central ranges can be interpreted as part of a rift drift sequence. The rifting of the continental margin in Papua New Guinea region is thought to have formed the Kubor Basement High and Kutubu Trough during the Triassic to Early Jurassic.

Interpretation of the late Palaeozoic to Mesozoic platform sequence along this margin in terms of prebreakup, breakup and post breakup stratigraphic associations permits the recognition of a post breakup unconformity which is correlated with the onset of seafloor spreading. In the western central ranges of Irian Jaya through to western Papua New Guinea the post breakup unconformity is Bajocian (Middle Jurassic) in age, while in the eastern central ranges of Papua New Guinea, in the Kubor area, it is of Upper Liassic (Early Jurassic) age.

The younging of the breakup events from present day Papua New Guinea through Irian Jaya and southward into western Australia implies that these events were related to a continuous rifting event which, off western Australia, is related to the opening of the Indian Ocean. The recognition of this same event in New Guinea suggests that the opening of the Indian Ocean started earlier and much further east than previously suspected.

The presence of a rifted margin along the northern edge of the Australian continent raises the question of what was rifted from this margin and where is it now. Because the oceanic crust that was to the north of this margin has largely been destroyed, it is not possible to close the ocean again to identify what was rifted. An analysis of the land geology of continental fragments found in southeast Asia offers one possibility for reconstruction of this margin. A search has been made for continental fragments which display similar geological histories prior to the onset of spreading and contrasting divergent histories after breakup.

In this approach the age of the post breakup unconformity is an important constraint in placing fragments in their approximate former position adjacent to the continental margin. A continental fragment with an Early Cretaceous post breakup unconformity cannot have come from the northern margin of the Australian continent for example.

125

The four microcontinents of Buton, Banggai-Sula, Timor and western Irian Jaya have a post-breakup unconformity of upper Liassic age and their prebreakup and breakup sequences are similar to those around and to the east of the Kubor Anticline region of Papua New Guinea implying that they were rifted from this region. The microcontinents of Obi and Bacan appear to have similar sequences to Banggai-Sula and may have been rifted from eastern New Guinea also. While the Buru-Seram microcontinent appears to have a similar prebreakup and breakup sequence it has a post breakup unconformity of Oxfordian (Late Jurassic) age. This younger age for the post breakup unconformity implies on that this micro-continent was derived from N.W. Australia.

The recognition of rift drift sequences on the continental fragments found in Eastern Indonesia has several implications for tectonic reconstructions of the region. (a) In the past most reconstructions have resorted to large scale sinisteral transcurrent fault systems to move continental fragments westward. However the presence of a rift drift sequence on each of the microcontinents shows that the microcontinents were detached from Gondwana during the Jurassic and that sea floor spreading may have been an important factor in their westward displacement. The Banggai-Sula microcontinent (Sula Spur) for example was detached from the eastern New Guinea portion of Gondwana during the Early Jurassic, not from western Irian Jaya in the Cainozoic is usually assumed. (b) The western Irian Jaya microcontinent has a post breakup unconformity of Liassic age in the west and Bajocian age along its eastern margin. This younging is in the opposite sense to that along the northern margin of the Australian continent margin. If the western Irian Jaya fragment was derived from the Kubor region, then this opposite sense of younging suggests that this fragment has been rotated.

If the microcontinent found in Eastern Indonesia have all come from that part of the Australian continental margin now found in Papua New Guinea it leaves open the question of what was rifted from the Irian Jaya section of this former continental margin. The recent work of Cameron and others (1980) in north Sumatra has identified several continental fragments with Gondwana affinities and implies that Sundaland is a mosaic of allochthonous or "suspect" terrains. Further work in Sundaland will probably identify the fragments "missing" from the Irian Jaya sector of the former Australian continental margin within southeastern Sundaland.

Cameron N.R., Clarke M.C.G., Aldiss D.T., Aspden J.A. and Djunuddin A. 1980. The Geological evolution of northern Sumatra. Proceedings Indonesian Petroleum Association Ninth Annual Convention, 149-181 Hamilton, Warren, 1978. Tectonic map of the Indonesian Region.

States Department of the Interior - Geological Survey.

United

126

THE DEVELOPMENT OF THE CONTINENTAL MARGIN AROUND THE WESTERN CORAL SEA BASIN. P.A. Symonds^ & K. Hinz^ ^Bureau of Mineral Resources, Canberra, Australia ^Bundesanstalt fur Geowissenschaften und Rohstoffe, Hannover, F,R. Germany Surveys carried out across the western Coral Sea Basin during 1978 and 1980/81 by the Federal Institute for Geosciences and Natural Resources, F.R. Germany, in co-operation with the Australian Bureau of Mineral Resources, have provided new information on the development of the continental margin and the change from coi^tinental to oceanic crust in this area.

The small, enclosed Coral Sea Basin (CSB) lies to the south of the present-day complex zone of interaction between the Australian and Pacific Plates. The western part of the basin is bordered by three marginal plateaus - the Papuan Plateau (PP) , the Eastern Plateau (EP) and the Queensland Plateau (QP) (Figure), which is the largest of the marginal plateaus of the Australian continental margin and is one of the largest of this type of feature in the world.

The plateaus are flanked by a complex rift basin system which is well developed, and still intact, along the western part of the area in the Queensland Trough (QT), the Osprey Embayment (OE) and to the west of the Eastern Plateau. The rift basin is not so well developed beneath the opposing margins of the Papuan and Queensland Plateaus, where it was split during the episode of seafloor spreading which formed the Coral Sea Basin. Prior to breakup the rift basins would have formed a radiating pattern coalescing in the northern Osprey Embayment.

In section the Queensland Trough looks like a simple gr^ben structure, but in plan it appears to be underlain by a series of high blocks and depressions connected by anastomosing faults. This type of structural style is typical of wrench zones and may have been initiated by left-lateral movement between Australia and the Queensland Plateau during the early stage of development of the rift basin system.

Outer basement highs, which appear to have low angle contacts with the oceanic crust (Type I), occur within the rift basin on both sides of the Coral Sea Basin and possibly beneath part of the Queensland Trough. Basement highs with a somewhat different character (Type II), but lying at the same depth below sea level, occur beneath the Osprey Embayment and the westernmost margin of the Coral Sea Basin (Figure).

Various origins have been proposed for outer highs - listrically faulted and rotated continental blocks, late-stage uplifts of pre-rift rocks, massive accumulations of volcanic rocks and the earliest formed portion of oceanic crust.

We feel that the evidence favours a volcanic origin for the outer highs and suggest that they are probably the result of rift-phase ^volcanism which preceded formation of the Coral Sea Basin.

On the basis of seafloor spreading magnetic anomalies Weissel & Watts (1979) suggested a Paleocene age for the formation of the Coral Sea Basin. The amount of oceanic basement lying between magnetic anomaly 26 and the rift basin (Figure) , and our interpretation of pre-Paleocene sediments overlying this basement in the western Coral Sea Basin, suggests that the basin opened earlier than indicated by Weissel & Watts.

127

In general, the development of the Coral Sea Basin margin seems to have followed a commonly proposed sequence of passive continental margin evolution from uplift,' to rifting accompanied by volcanism, to seafloor spreading in one of the arms of the radiating rift basin system.

REFERENCE: WEISSEL, J.K., & WATTS, A.B., 1979 - Tectonic evolution of the Coral Sea Basin. Journal of Geophysical Research, 84(B9), 4572 - 4582.

154

143°

FIGURE CAPTION : Structural elements in the Coral Sea Basin area.

Feature abbreviations such as PP are explained in the text. Open stipple is rift basin - its boundaries are dashed where they are poorly defined or extrapolated. Pale grey area is outer high type I; dark grey area is outer high type II. Continuous lines in the Coral Sea Basin labelled 26, etc., are the seafloor spreading magnetic anomalies identified by Weissel and Watts (1979); the broken line is the negative free-air anomaly which they found to be associated with the extinct spreading centre and which we have traced to the western end of the basin.

BATHYMETRY AND FINE STRUCTURE OF THE NSW MARGIN G.H. Packham Dept of Geology and Geophysics, University of Sydney ISlo

Abstract provided

128

NEW SEISMIC DATA FROM THE GIPPSLAND AND OTWAY CONTINENTAL MARGINS John C. Branson Bureau of Mineral Resources, Canberra,

ACT

Five seismic lines were recorded over eastern and western Tasmania sections of the Australian continental margin as part of the Bass Strait Geophysical Survey, 1982. The Bureau of Mineral Resources, Department of National Development and Energy conducted this geophysical survey during the period 18 March to 2 May 1982 using a contract survey vessel. Lady Vilma operated by Geophysical Service International. A 96 channel cable of 3200 metres length and 65.6 litres airgun array were used to record 48 fold c.d.p. coverage over the eastern Tasmania margin and 24 fold c.d.p. coverage over the western Tasmania margin.

One line across the eastern Tasmania margin was recorded eastwest at about 29®30'S, north of Flinders Island. Four lines on the v/estern Tasmania margin form a "V" pattern across the slope and rise, west of King Island. Good seismic resolution reveals important structural differences in continental margin formation at these two margins.

In the eastern Tasmania area, Hayes and Ringis (1973) proposed that the continental boundary formed as a pull-apart margin about a north-south trending spreading ridge. The BMR seismic line in this region shows that all blocks are faulted and there are no massive intrusive features. The oldest sediment at the base of the slope overlies irregular oceanic material. The poor seismic expression of these sediments suggests that are poorly sorted silts. They appear to have been laid down over the new ocean floor and v/ere subsequently undisturbed. The Early Eocene hiatus in DSDP Site 283 in the Tasman Sea farther east, is probably responsible for the major unconformity detected over the oldest sediments at the base of the slope. Other slope features show that the rifting stage in the development of this margin did not produce more than 1 km of rift fill sediment. The slope appears to be stable from Early Eocene times to the present and other second order features can be attributed to slumping, contour currents and sea-level changes.

The four western Tasmania traverses were recorded along a segment of the Southeast Indian Ocean where Weissel and Hayes (1971) deduced that Antarctica and Australia separated by transcurrent movements. B14R lines show continental blocks are faulted and displaced in the upper slope and volcanic features can be recognised in the lower slope and ocean basin. These volcanic features form ridges, over 700'metres high creating basinal regions between successive volcanic forms. If these ridges are compared with volcanic ridges developed in similar shear zones like the Dead Sea rift, then these volcanic structures may have formed v/hen Australia and Antarctica first separated. Similar suites of en-echelon basins should occur along this western Tasmania margin as occurs in modern shear zones. Reflection character of these lowest sedimentary units may indicate the environment of deposition. Once these oldest sediments were deposited, tectonic movements have little or no effect on them and this implies that the east-west Southeast Indian Ocean spreading ridge did not disturb the base of the slope.

129

An Early Cretaceous age is considered likely for the sediment^ contained between the volcanic ridges in the western Tasmania margin.

An unconformity occurs within these oldest sediments which could be either Late Cretaceous or Paleocene in age. The Paleocene age is postulated here as it correlates with the initiation of slow spreading proposed for the Southeast Indian Ocean by Cande and Mutter (1982).

The major unconformity crossing the slope is correlated with the onset of rapid sea-floor spreading in the Eocene. Other sea-level changes can be determined from seismic sequences in the upper continental slope.

References Cande, S.C. and Mutter, J.C. (1982) - A revised identification of the oldest sea-floor spreading between Australia and Antarctica.

Earth and Planetary Science Letters, 58, 151-160.

Hayes, D.E. and Ringis, J. (1973) - Seafloor spreading in the Tasman Sea, Nature, 243, 454-458.

Weissel, J.K. and Hayes, D.E. (1972) - Magnetic anomalies in the Southeast Indian Ocean Hayes, D.E. ed. Antarctic Oceanology II, The Australian-New Zealand sector: Washington, D.C. American Geophysical Union, 19, 165-196.

TECTONIC SUBSIDENCE AND THE THERMAL AND STRATIGRAPHIC HISTORY OF THE GREAT AUSTRALIAN BIGHT REGION K.A. Hegarty & J.K. Weissel & J.C. Mutter All at:

Lamont-Doherty Geological Observatory, Palisades, N.Y.

The vertical tectonic history of the Great Australian Bight Region, determined from published and publicly-available exploratory well data, shows convincingly that the southern margin of Australia between about 124®E and 136''E underwent rapid subsidence during the early Late Cretaceous. This type of analysis provides valuable constraints for understanding the history of the evolution of Australia's southern margin and passive margins in general.

The subsidence history is mapped using a geohistory technique similar to that of Van Hinte (1978) such that observed stratigraphic thicknesses are decompacted (using an assumed porosity-depth relation) and the depth to this horizon (of known age) is plotted with respect to time. Each stratigraphic unit is treated similarly. An exponential porosity-depth relation is used and age control is provided by paleontological or seismic correlation. Accurately quantifying the magnitude of the cumulative subsidence through time requires knowledge of the sedimentological behaviour, paleodepth and sea level variation. At least six different lithologies can be identified in the sedimentary column of these wells and a different porosity-depth relation is used for each lithology in order to help

130

reduce accumulating errors. Sea level through time is assumed (Falvey and Deighton,1982). Paleodepth estimates are thought to introduce the greatest error, but maximum and minimum values can be identified. Both Airy- and flexural- behaviour of the basement have been considered suggesting that discrepancies of the order of hundreds of meters (about 10% of the cumulative subsidence) can be accrued if basement response is incorrectly assumed.

The five wells analyzed show that the most rapid subsidence occurred approximately 90 to 100 m.y.B.P.. The tectonic event controlling this subsidence appears to have been active at nearly the same time within the Duntroon Embajnnent, the Eucla Basin, the Great Australian Bight Basin and the Eyre Sub-basin. This timing is consistent with the newly revised age of the oldest ocean seafloor adjacent to the southern Australian margin (Cande and Mutter, 1982).

In light of these results, the Cretaceous tectono-stratigraphic units are re-interpreted. The faulting active during Albian time (Eraser and Tilbury,1979) probably represents the presursory stages of continental breakup and is not simply a response to rapid sedimentation. The transition from non-marine to marine conditions recorded in the Madura Formation of the Eucla Basin during the middle Late Cretaceous, the uplift of the Gawler Block during the late Cenomanian, the development of deltaic deposits and the deposition of coarse-grained rift-valley sediments during the Late Cretaceous each are probably a result of the earliest stages of rifting and possibly oceanic crust accretion during the Late Cretaceous.

Simple extensional models of the rifting process can explain the recovered tectonic-subsidence curves and indicate that both crustal and sub-crustal material extended by approximately the same amount during the Late Cretaceous. Conductive cooling of the lithosphere controls the later-stage subsidence.

REFERENCES Cande, S.C. and J.C. Mutter, 1982, Revised identification of the oldest sea-floor spreading anomalies between Australia and Antarctica, EPSL,58,151-160.

Falvey, D.A. and I. Deighton, 1982, Recent advances in burial and thermal geohistory analysis, APEA,21,65-81.

Fraser, A.R. and L.A. Tilbury, 1979, Structure and stratigraphy of the Ceduna Terrace Region, Great Australian Bight Basin, APEA, 19,53-65.

Van Hinte, J.E., 1978, Geohistory analysis - application of micropaleontology in exploration geology, AAPG, 62,201-222.

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GEOHISTORY ANALYSIS AND SUBSIDENCE MODELLING IN THE EYRE BASIN

Edwards, B. The University of Sydney

The method of burial and thermal geohistory analysis has been applied to the well Esso Jerboa-1 (Eyre Basin). Well developed pre- and post-breakup subsidence cycles are clearly developed, with a strong unconformity (non-depositional) extending from early Cenomanian to Eocene. Geohistory analysis is extended beyond to well location using seismic stratigraphic methods.

A number of "pseudo" well locations have been analysed which more clearly illustrate the structural evolution of the basin.

Predictive geohistory analysis provides a method for making a quantitative assessment of seismic stratigraphic interpretation and for correctly extrapolating data on maturation levels.

132

Symposium l(i) Oceanic lithosphere studies around Australia Convener: Associate Professor J . J . Veevers

133

INTRAPLATE DEFORMATION AND SEASAT ALTIMETER GEOID ANOMALIES IN THE NORTHERN INDIAN OCEAN

J. K. Weissel & W. F. Haxby Lamont-Doherty Geological Observatory of Columbia University Palisades, New York 10964 USA Much attention has been given recently to the problem of predicting seafloor topography from satellite altimeter data especially in those areas of the oceans poorly explored by surface ships. In previous work, it has generally been assumed that bathymetric features represent a load on the surface of the oceanic lithosphere. Consequently, the altimeter data obtained along individual satellite passes have searched for geoidal anomaly signatures characteristic of surface loads and their associated isostatic compensation. If this approach is used routinely, incorrect predictions of bathymetry will occur in those parts of the oceans where additional tectonic processes cause significant geoid height variations with amplitudes and wavelengths similar to those expected from surface loads. For example, in the northern Indian Ocean, the surface of the plate is deformed into undulations with wavelengths of 100 to 300 km and relief of up to 3 km. These undulations are one manifestation of intraplate deformation which has affected a large region of the Indo-Australian plate over the past 10 m.y. The theoretical relationship between undulatory deformation of oceanic crust and the resulting geoid height or gravity anomaly can be expressed in the spatial frequency domain as an admittance function, analogous to the admittance functions developed by previous workers to express the relationship between isostatically compensated bathymetry and the resulting gravity or geoid anomaly. A two-dimensional free-air gravity anomaly data set at a grid spacing of %10 km is constructed from SEASAT altimeter data obtained on individual satellite passes over the northern Indian Ocean. Using filters determined from admittance functions, we are able to successfully determine bathymetry from SEASAT-derived gravity anomalies which contain contributions from a) intraplate deformation and b) compensated surface loads such as the Ninetyeast Ridge and the Afanazy Nikitin seamount.

STRATIGRAPHY, STRUCTURE AND EVOLUTION OF THE CENTRAL BASIN, VANUATU Falvey, D.A. \

Greene, H.G.^ and Shipboard

^Bureau of Mineral

2

U.S. Geological

Party

Resources Survey

During 1982 a joint Australia-New Zealand-USA marine geological and geophysical research cruise was conducted in Vanuatu, southwest Pacific.

Multichannel seismic data collected aboard the R/V S.P.

134

LEE, and processed by Sefal Geophysical (Australia) have revealed probable pre- Middle Miocene sediments filling the Central Basin, which lies between the eastern and western chains.

The margins of the Central Basin appear to have been deformed in the Mid to Late Miocene forming insular shelves or plateaus on the margins of Malabula and Espiritu Santo.

Th^ trench to the west of Malabula and Espiritu Santo is nearly devoid of sediments. The forearc region is composed of rocks of high velocity (4-5 km sec"^) which dip away from the trench.

Oceanic crust can be traced beneath the forearc.

YOUNG OCEANIC CRUST OF THE EASTERN WOODLARK BASIN, AND ITS SUBDUCTION AND ASSOCIATED VOLCANISM Neville Exon\ Brian Taylor^, and Shipboard Party (ANZUS - SOPAC Cruise KK820316 - 4) ^Bureau of Mineral Resources, Canberra, ACT ^Hawaii Institute of Geophysics, Honolulu, Hawaii Seafloor spreading in the Woodlark Basin has occurred about an east-west trending rift valley, sinistrally offset by north-south fracture zones. Magnetic lineations show that it has been opening for at least 3.5 m.y., separating the formerly continuous Woodlark and Pocklington Rises at half rates averaging 3-3.5 cm/yr. The basin has been subducted eastward beneath the island-arc of the New Georgia Group (Solomon Islands) at more than 10 cm/yr. The current triple junction is in a triangular zone bounded (a) to the west by Simbo ridge, which is a transform fault and bathymetric high at 156.5^E, (b) to the northeast by a bathymetric trough west of Rendova and (c) to the south by an east-west ridge at about^ 8.7^S. South of this zone the oceanic crust has been buckled into broad west-northwest trending folds that overprint the spreading fabric and degrade the magnetic anomalies.

Woodlark Basin crust does not appear to bend down into a subduction zone south of New Georgia. Northwest of the triple junction the young crust of the basin is yet to be subducted, and rifted segments of the Woodlark Rise are being faulted down into the trench. Southeast of the triple junction the basin crust simply abuts the forearc lower slope, and there is no bathymetric trench.

Normal mid-ocean ridge basalts were dredged from the central rift of the basin. The sediments overlying the basaltic crust as it approaches New Georgia are generally 100-200 m thick and consist of nanno-rich clay, marl and calcareous ooze. Rocks from the fore-arc slope and from Kavachi submarine volcano on the forearc high are, like those of New Georgia, porphyritic basalts, volcaniclastics, and rare andesites. Rhyolites were dredged from a seamount on the east-west ridge at about 8.7 S; they have apparently moved laterally upward through fractures in the oceanic crust from beneath the island-arc.

Symposium 2(a) Intraplate igneous activity in Australasia Conveners: Dr R.V7. Johnson & Dr S.R. Taylor

135

THE ULTRAPOTASSIC ROCKS OF THE WEST KIMBERLEY REGION, W.A.: PRODUCTS OF DEEP-SEATED INTRAPLATE VOLCANISM A.L. Jaques Bureau of Mineral Resources, Canberra, ACT

Kimberlites represent a rare but important manifestation of intraplate igneous activity. Apart from their economic value as the primary source of diamond kimberlites have been erupted from much greater depths in the mantle than other volcanic rocks and commonly carry numerous xenoliths derived from the crust and mantle. Studies of these xenoliths have yielded important information concerning the mineralogy, petrology and chemical composition of the subcontinental mantle.

In Australia diamond-bearing kimberlitic rocks have been found in the east, west and north Kimberley regions and the Carnarvon Basin of Western Australia (Atkinson et al., 1982), and at Orroroo in South Australia (Danchin et al., 1982). An additional fourteen areas of kimberlitic to olivine nephelinitic intrusion occur in southeastern Australia (Ferguson, 1980). Marked differences among the intrusions in age, tectonic setting and chemistry indicate the existence of a number of discrete phases of magmatic activity dating from Paleozoic (Permian) to Cainozoic (Miocene). The location of the kimberlitic rocks in southeastern Australia has been ascribed to mid-plate magmatism controlled by continental extensions of oceanic transform faults (Stracke et al., 1979), and to hot-spot activity (Wellman, in press).

The diamondiferous kimberlitic rocks of the West Kimberley region are of particular interest since they occur intimately associated with the leucite lamproites described by Prider (Wade and Prider, 1940; Prider, 1960). Together they form a broad belt comprising more than 100 separate diatremes, plugs, sills and dykes which extends 125 km south from the southern margin of the Proterozoic Kimberley Block across the Lennard Shelf and Fitzroy Trough at the northern margin of the Canning Basin. K-Ar and Rb-Sr dating indicates that the suite is of Miocene age, ranging from slightly more than 20 m.y. (-22+ 2) in the north to 18-20 m.y. in the south. Several of the diatremes^have associated crater sediments indicating little erosion since their emplacement.

The West Kimberley rocks show a petrographic, mineralogical and chemical gradation from^phlogopite kimberlite to leucite lamproite.

Both the kimberlitic and lamproitic rocks have high to extreme K 0 contents (4-12%) and have high K 0/Al 0 (average 1.2) and K O/NS^O (>10) ratios. Together they foA a c6n^anguineous ultrapotagsic suite (Jaques et al., 1982). Rock types range from ultrabasic (40-4^0 SiO , >20% MgO) to felsic lamproite (>60% SiO ,<5% MgO) . Features of th^^uite are very high contents of Ba, Rb, s l Pb, Th, U, Ta , Zr, Nb, and light rare earth elements (LREE). Rare earth patterns are extremely fractionated with REE abundances of 500 to 2000 times, and HREE of 4 to 6 times, chondritic abundances. Additional fg^tures are very low abundances of CaO, Na 0, CO anc|^c. ^J/Sr and Sr ratios are very high (0.3-0.4. 0.7 1 1-8.720) ^and Nd ratios very low (e = -7 to -15), and indicate derivation of the suite from mantle wh?ch has undergone long-term enrichment in Rb relative to Sr and Nd

136

relative to Sm (McCulloch et al, in prep.).

The presence of diamond and other xenocryst minerals derived from garnet Iherzolite (e.g. pyrope garnet, chrome diopside, magnesiochromite) in both kimberlitic and lamproitic rocks indicates a deep-seated origin for the suite as a whole. Mineralogical and geochemical data from xenoliths and xenocryst minerals indicate the existence of depleted mantle beneath the region. This is in sharp contrast with the extreme enrichment in 'incompatible' elements observed in the host rocks. The suite is interpreted as having been derived by low degrees of partial melting under hydrous conditions of phlogopiterich, garnet-poor Iherzolite which has been enriched in 'incompatible' elements by metasomatic fluids.

Unlike some other ultrapotassic provinces (e.g. East Africa) the rocks of the West Kimberley region of Western Australia do not appear to be related to a major rifting episode. The Miocene magmatism clearly post-dates both formation of the Fitzroy Trough, where the major sedimentation took place in the Devonian-Carboniferous, and rifting associated with the breakup of Gondwanaland in the late Jurassic The apparent southward younging of the intrusions suggests a crude linear migration of magmatic activity with time analogous to volcanism associated with 'hot-spots'.

However, the major factor controlling the disposition of intrusions appears to have been structural: WNW-trending faults at the margin of the Fitzroy Trough, N-trending fractures in the Kimberley Block, Lennard Shelf and Fitzroy Trough, and E-W en-echelon faults and folds within the Trough. The age relationship of the ultrapotassic rocks of the West Kimberley to the rocks of kimberlitic affinity in the east and north Kimberley regions is not known. Many of the kimberlitic rocks of the kimberley region lie within Proterozoic mobile belts bounding the Kimberley Block but all must be considered as both intraplate and intracratonic since the entire Kimberley region was cratonised 1800 m.y. ago.

ATKINSON, W.J., HUGHES, F.E., & SMITH, C.B., 1982: A review of the kimberlitic rocks of Western Australia. Terra Cognita, 2, 204.

DANCHIN, R.V., HARRIS, J.W., SCOTT SMITH, B.H., & STRACKE, K.J., 1982: Diamondiferous kimberlites at Orroroo, South Australia. Terra Cognita, 2, 205. . . r .i, FERGUSON, J., 1980: Kimberlite and kimberlitic intrusives of southeastern Australia. Mineral. Mag., 43, 727-731.

JAQUES, A.L., GREGORY, G.P., LEWIS, J.D., & FERGUSON, J., 1982: The ultrapotassic rocks of the West Kimberley region. Western Australia, and a new class of diamondiferous kimberlite. Terra Cognita, 2, 251-252. , .

PRIDER, R.T., 1960: The leucite lamproites of the Fitzroy Basin. J. geol. Soc. Aust., 6, 71-118.

STRACKE, K.J., FERGUSON, J., & BLACK, L.P., 1979: Structural setting of kimberlites in southeastern Australia: in Boyd, F.R. & Meyer, H.O.A. (eds) Kimberlites, diatremes, and diamonds: their geology, petrology, and geochemistry. Proc. Second Intern. Kimberlite Conf., 1, 71-91, Am. Geophys. Un., Washington.

WADE, A., & PRIDER, R.T., 1940: The leucite-bearing rocks of the West Kimberley area. Western Australia. Qrt. J. geol. Soc. Lond., 2^ — , W E L L M A N ! P., in press: Hotspot volcanism in Australia and New Zealand: Cainozoic and mid-Mesozoic. Submitted to Tectonophysics.

137

HOTSPOT IGNEOUS ACTIVITY IN AUSTRALIA Peter Wellman Bureau of Mineral Resources, P.O. Box 378, Canberra City A.C.T. 2601

Some late Cainozoic igneous provinces in eastern Australian and New Zealand-Cambell Plateau show an apparent migration with time that is consistent with recent models of lithosphere movement relative to a hotspot absolute frame of reference. The rocks showing this migration consist of those alkaline to mildly tholeiitic provinces with significant felsic rocks, and also leucite bearing lava provinces. The provinces have mean ages that agree to within experimental error (0.5, 1.5 m.y.) with the migration defined by a stationary great circle beneath the lithosphere that passes through the pole of rotation of the lithosphere plate. The Australian provinces are consistent within experimental error with linear rotation of the lithosphere around this rotation pole for the last 34 m.y. at 5.4°/m.y.; they are not consistent with the changing rate of rotation suggested from analysis of the sea floor magnetic anomalies. Both Australian and New Zealand hotspots appear to be associated with a shortlived 100 m of uplift at the time of yolcanism.

In southeastern Australia late Triassic to late Jurassic volcanism of the same type, and in a similar tectonic environment, shows a migration at 26 mm yr in a SSW direction. Further information of volcanic migration elsewhere is required to determine whether this actually represents hotspot induced volcanism.

PETROLOGY AND GEOCHEMISTRY OF THE CAINOZIC VOLCANIC ROCKS OF EASTERN AUSTRALIA A. Ewart Department of Geology & Mineralogy University of Queensland A broad belt of Cainozoic, intra-plate anorogenic volcanic rocks outcrop discontinuously along eastern Australia, extending for some 3000km. The youngest, and probably still active regions occur towards the northern and southern ends of this belt. Although the majority of volcanic centres comprise accumulations of numerous, relatively small volume lava flows, some regions are characterised by very infrequent, large-volume eruptions of mafic lavas, especially well developed in north Queensland.

Withing the north-east New South Wales and south Queensland regions, the rock types are strongly bimodal, although by volume are at least 7:1 in favour of the mafic relative to the silicic eruptives.

Further south, the compositions are not notably bimodal, there being an asymmetric distribution with generally decreasing abundances of trachytes through to rhyolites.

The mafic

lavas

from

the whole

region

are

predominantly

138

andesine-normative, with Mg-numbers typically in the range 30-65; their compositional spectrum spans continuously from ne-normative through to Q-normative. Relatively large variations in Ni and Cr abundances exist within both the alkaline and sub-alkaline lavas, implying extensive fractionation of the erupted lavas prior to eruption.

In fact, relatively few of these lavas are basalts in the strictest definitions, being predominantly hawaiite and tholeiitic andesites.

The silicic eruptives include both peralkaline and non-peralkaline compositions, often occurring together with the one volcanic centre.

All are characterised by relatively Fe-enriched silicates; in the non-peralkaline types, these commonly include fayalitic olivine and ferrohedenbergite, with rarer occurrences of ferrohypersthene, and Fe-enriched biotite and hornblendes. Accessories include chevkinite, allanite, zircon, and apatite. Fluor-arfvedsonite and aegirine-augite are characteristic of the peralkaline lavas. In terms of their trace and minor element geochemistry, the silicic lavas all show, to varying degrees, evidence of strong to extreme crystal fractionation. Examples are the depleted Mg, Mn, P, Sr, Ba, and Eu abundances, and the variable enrichments of such elements as Th, Rb, Nb, and LREE. The silicic lavas of north-east New South Wales and south Queensland are silica oversaturated; those occurring from further south include both silica oversaturated trachytes and a significant proportion of ne-normative trachytes, with relatively fewer rhyolites.

These differences are also evident in the associated mafic lavas, there being a relatively greater proportion of ne-normative lavas in Victoria, New South Wales and Tasmania.

Within the south Queensland region, isotopic data suggest that the silicic magmas have two distinct derivations: the rhyolites which are interpreted as localised crustal melts with superimposed crystal fractionation, and the majority of peralkaline lavas and non-peralkaline trachytes which are interpreted as being derived by crystal fractionation from basaltic parental magmas, mainly within the lower crust, but with variable degrees of country rock contamination.

It is presumed that similar petrogeneses may apply to the silicic lavas from the more southern centres.

The nature of the tectonic controls, or the interrelationships between tectonic environment and the cause of this eastern Australian magmatism are uncertain.

One particularly intriguing problem is the relationship, if any, between the volcanism and the uplift forming the Great Dividing Range.

139

INTRAPLATE VOLCANISM IN EASTERN CHINA AND AUSTRALIA - SIMILARITIES AND DIFFERENCES 1 2 3 R.J. Arcuius , S.Y. Wass and J. Ferguson ^Research School of Earth Sciences, Australian National University, Canberra, ACT; School of Earth Sciences, Macquarie University, North Ryde, NSW; ^Bureau of Mineral Resources, Canberra, ACT. The eastern continental margins of China and Australia are characterised by Cenozoic outpourings of basaltic and related volcanic rock types that have been erupted in an intraplate setting. There are some parallels in the long-term tectonic development of these continental margins, and some broad similarities between the chemistry of the volcanic rocks and the nature of xenolithic fragments of subcontinental lithosphere enclosed in the host volcanics. In detail, however, there are striking differences between specific tectonic environments of eruption, and the underlying causes of eruption in intraplate, "inactive margin" settings seem to be diverse.

Both continental margins record the accretion of volcanic/plutonic belts and the reworking of older terrains at intervals from the Proterozoic. In southeastern China for example, a Mesozoic Andeantype volcanic chain was developed partially on and partially adjacent to Hercynian and Caledonian arc-trench systems. This basement can be found also in the Philippines, Taiwan, the Ryuku Islands, Korea and Japan. Similarly, eastern Australia and New Zealand have been the locus of successive volcano-plutonic events in an active margin of some type since at least the Paleozoic.

In the upper Cretaceous to the Tertiary, marginal basins were developed between active arc systems and the trailing continental edges. The South China Sea, Sea of Japan and more recently the Okinawa trough have formed as a consequence of the migration of fragments of continental lithosphere and active arc systems away from mainland China. Closure of some of these basins has resulted in the present movement back towards China of the Philippines and Taiwan. The opening of the Coral and Tasman Seas has resulted in the shift of active margin volcanism away from mainland Australia, and a broadly similar history of active margin to trailing edge following marginal sea formation can therefore be discerned. This cycle of activity may have been repeated in both China and Australia on several occasions in the past.

Cenozoic volcanics are distributed along the entire eastern margin of Australia, and generally coincide with the position of the "Great Dividing Range", a 300km wide belt of highlands rising up to 1000m above sea level. The volcanics are mainly located about 100400km inland from the continental slope bordering the Coral and Tasman Seas. The distribution of Cenozoic volcanics in eastern China is more irregular but there are prominent exposures in the northern provinces of Heilungjiang, Liaoning and Mongolia, and in the south on the island of Hainan and Guangdong province. The volcanoes of Wudalienchi (last erupted in A.D. 1712) north of Harbin, and Chang Bai Shan (last erupted in A.D. 1702) on the border of China and Korea represent the latest activity. In addition, relatively small volumes but widely distributed outcrops of Cenozoic Volcanics are located near the coast in Shandong, Jiangsu, Zhejiang and Fujian provinces and inland in Hebei province. There is no regular development of a highlands uplift along the eastern margin of China, the volcanics are primarily associated

140

with northeast-trending fault systems and are located up to ISOOkm or more inland from the continental slope bordering the marginal seas.

One of the most important eruption loci is the Tancheng-Lujiang fault belt cutting NE-SW from Liaoning to Shandong provinces across the eastern part of the Bohai Sea. This fault belt has been prominent in the tectonic development of eastern China since the Paleozoic, was the locus of Paleozoic and Mesozoic kimberlite activity and apparently penetrates the crust to the Moho. In essence therefore, whereas uplift and volcanism appear to coincide in eastern Australia, crustal extension, basin formation and fault control appear to be important in China.

Northward migration of the Australian continent over a fixed hotspot (s) in the mantle (Wellman and McDougall, 1974; Sutherland, 1978) or the spreading axis position of the Coral Sea (Smith, 1982) have been proposed to explain the time sequence of central volcanic complexes and associated uplift. The existence of comparably recent activity in Queensland as well as South Australia suggests that even more complex heat sources may be involved. Molnar and Tapponier (1977) regard the overall north-northwest extension of northeastern China as resulting from the collision of India and Eurasia, with little compressive stress being transmitted across the active island arcs to the east. Lithosphere thinning and basin formation have been demonstrated for several regions of northeast China, and a combination of the marginal sea development together with major plate collision in the Himalayan region may account for this.

In both China and Australia, alkaline basalt types appear to be predominant but a diverse range of subalkaline basalts and more silicic rocks are also present in minor volumes. Isotopic evidence suggests that the continental crust was a significant component of many lavas ranging from a minor contaminant to forming the main source of some rock types (Ewart, 1982; Zhou and Armstrong, 1982). A similar suite of upper mantle-derived xenoliths including Cr-diopside and Al-augite type peridotites and pyroxene, olivine, anorthoclase and spinel megacrysts are found in many localities in both Australia and China, suggestive of comparable upper mantle characteristics. A much thicker (up to 55km) continental crust in eastern Australia than China (^40km) may account for the greater development of intermediate and silicic volcanics in central volcanic complexes of Australia. It is surprising, given the long history of active margin involvement, that the upper mantle sources of basaltic rocks of both regions show little evidence of the chemical signatures expected from components of subducted lithosphere.

Ewart, A. (1982) J. Petrology, 23, 344.

Molnar, P. and Tapponnier, P. (1977) Geology, 5, 212.

Wellman, P. and McDougall, I. (1974) Tectonophysics, 23, 49.

Smith, A.G. (1982) Nature, 296, 400.

Sutherland, F.L. (1978) Tectonophysics, 48, 413.

Zhou, X. and Armstrong, R.L. (1982) Earth Planet Sci. Lett. 58, 301.

Symposium 2(b) Magmatism and crustal evolution (oceanic lithosphere, ophiolites, island arcs, fold belts) Convener: Dr V7.E. Cameron

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EXPERIMENTAL PETROLOGY AND PETROGENESIS OF A MARGINAL BASIN BASALT A.L. Jaques

1

& D.H. Green

2

^Bureau of Mineral Resources, Canberra, ACT, ^Dept of Geology, University of Tasmania, Hobart, Tasmania Dredging and recent drilling of small ocean basins behind active island arcs has shown that these back-arc basins are floored by olivine tholeiite basalts of very similar composition to mid-ocean ridge basalts (MORB), and it is tactily assumed that they originate by similar mantle processes.

We report the results of an experimental study of a relatively 'primitive' olivine tholeiite from the Lau Basin (95-1) from 1 atmosphere to 22 kb.

The results show that olivine is the liquidus phase at low pressure (£12 kb) and it is joined by calcic plagioclase and clinopyroxene at lower pressure.

Calcic clinopyroxene is the liquidus phase at higher pressure (^12 kb) and there is no near-liquidus field of orthopyroxene. The l^w pressure experiments show that Lau Basin basalt 95-1 could be a derivative liquid by olivine fractionation from a more olivine-rich parent.

Olivine-addition experiments show that a picritic magma (95-1 + 18% olivine Mg ) containing approximately 27% normative olivine is saturated in olivine, orthopyroxene and clinopyroxene at '^^20 kb and 1450°C. These results are very similar to those obtained on magnesian MORB glasses from the major ocean basins and suggest that many MORBS, including some of the more magnesian glasses, are not primary magmas but derived by olivine fractionation from tholeiitic picrites.

Examination of olivine-liquid pairs over a range of temperatures (1200 - 1400°) and pressures (0-15 kb) indicates that Fe/Mg partitioning is pressure dependent at pressures above 5 kb.

Our experiments show variation in K from 0.29-0.30 at 5 kb, 0.30-0.33 at 10 kb to 0.33-0.34 at 15 kb.

PHASF FFLATIONSHIPS IN TFF SYSTEM HIGH-AL BASAIT WATER TO lOkb.:

IMPLICATIONS FOR THF FVOIOTION OF ISLAND ARCS. J.D. Foden^ and D.H. Green2

loepartment of Geology and Mineralogy, University of ;^delaide. S. A. 2Geology Department, University of Tasmania.

Hobart.

Tas. Recently performed experiments have been carried out on an Indonesian high-Al basalt.

These have been aimed at determining the direction of geochemical evolution of liquids in this system, with H2O contents between 0 and 9% and at pressures up to lOkb. In particular attempts have been made to determine the composition of liquids in equilibrium with amphibole.

The latter experiments have been made using layered capsules in order to avoid problems associated with quench modification of equilibrium liquids.

142

The results of these experiments suggest that liquids with compositions quite close to natural low silica andesites (53-56% Si02) are in equilibrium with the aluminous paragasite precipitated in the high~Al basalt system at 7-10 kb. and 1030-1050^0. At reasonable water contents (<5wt.%) the basalt does not have amphibole on its own liquidus and an interval of clinopyroxene and/or olivine precipitation exists between the basalt liquidus and the amphibole field.

If amphibole does play a significant role in the generation of calc alkaline andesite, then these experiments suggest that such a process must take place by equilibrium crystallisation at pressures close to those at the base of a 20-30 km. thick crust.

In summary, it is likely that considerable arc growth occurs due to basal accretion of crystalline material (pyroxenite, hornblendite and hornblende gabbro) formed by equilibrium partial crystallisation from rising, mantle-derived magma batches. Fvolved arc lavas result from tapping of liquid batches in equilibrium with these residues and by at least partial equilibration of subsequent basaltic magma batches with crystalline material emplaced at earlier stages.

MANUS ISLAND, PAPUA NEW GUINEA:

GEOCHEMICAL EVOLUTION FROM

ISLAND-ARC TO OCEANIC ISLAND A.L, Jaques Bureau of Mineral Resources, Canberra ACT

Manus Island lies at latitude some 300 km north of mainland Papua New Guinea from which it is separated by the Bismarck Sea. The island consists of Cainozoic volcanic and sedimentary rocks intruded by a large dioritic complex of Miocene age. The oldest rocks are middle Eocene to early Miocene basaltic and andesitic lavas, pyroclastic and volcaniclastic rocks (Tinniwi Volcanics) which are overlain by Miocene limestone which is overlain in turn by a thick sequence of Neogene (mostly Miocene) clastic sediments. In the eastern part of the island basaltic andesite lava (Lorengau Basalt) is partly intercalated with and partly overlies the Miocene clastic sequence.

The Neogene clastics in the western third of the island are overlain

by Pliocene andesite agglomerate (Tasikim Agglomerate) which in turn is overlain by Pleistocene basalt (Likum Basalt) erupted from the collapsed caldera of Southwest Bay. The Tertiary igneous rocks of Manus Island have many of the petrologic and geochemical features of rock suites found in island-arcs elsewhere including intermediate silica contents, low Ti02 contents (<1%), high AI2O3 contents ( i6-20%), moderate to high CaO contents and low Fe contents (Fe as FeO <CaO). MgO contents, Mg/(Mg+Fe) ratios and Ni and Cr contents are low. The Tinniwi Volcarics are of low-K basalt

143

and andesite composition (50-62% Si02, 0.06-0.6% K2O) and have high Na20/K20 ratios (>10, average 20). Abundances of Ba,Rb,Sr and Pb are very low (<100, <10, <450 and <2 ppm respectively), Zr and Nb contents are low (<60 commonly 30-40 ppm Zr, <2ppm Nb), and Zr/Nb ratios are high. The Tinniwi Volcanics have low abundances of RER (^lOx chondrites) and have La/Y enrichment factors close to 1. These features are typical of the island-arc tholeiite suite found in some ensimatic island-arcs with thin crust. The Miocene basaltic andesites (55-58% Si02) of the Lorengau Basalt have much higher contents of K2O (1.5-2.4% K9O), lower Na20/Ko0 ratios (1.6-3), and much higher abundances of Ba (200-300 ppm), Rb (20-30 ppm), Sr (700-800 ppm), Zr (100-200 ppa) and other incompatible elements (Th,U,Pb,Nb etc). K/Rb and K/Ba ratios are high (600-900 and 50-70) and the suite is enriched in LREE (La/Ye.f.-o^ at 60 to lOOx chondritic. Apart from having higher K/Rb and K/Ba ratios, lower Rb/Sr ratios and higher Zr contents the Lorengau Basalts resemble medium to high-K calc-alkaline basaltic andesites and andesites from other island-arcs. Similarly, the hornblende andesites of the Tasikim Agglomerate closely resemble m.edium-K calc-alkaline andesites from island-arcs elsewhere apart from having slightly higher K/Rb and Zr/Nb and lower Rb/Sr ratios. The intrusive rocks of Manus Island (Yirri Intrusive Complex) range from medium-K diorite to high-K quartz monzonite and compositions overlap those of the Tasikim Agglomerate.

The Quaternary lavas of Manus Island (Likum Basalt) contrast markedly with the Tertiary igneous suite. The Quaternary lavas are transitional to mildly alkaline basalts characterised by high T 1 O 2 contents (2-2.5%), high Fe (11-14% Fe as FeO), and low to moderate AI2O3 contents. Compared to the Tertiary rocks they are enriched in Zr, Nb and Y and have much lower Zr/Nb (10), Y/Nb (2-3cf<7) and Ba/La (4-6 cflO) ratios than the Quaternary rocks. The Quaternary rocks most closely resemble the 'incompatible' element-enriched tholeiites and transitional basalts of oceanic islands.

The change from island-arc-type i.e. plate margin magmatic activity to oceanic-island-type i.e. intraplate igneous activity, implies derivation from different mantle sources by different tectonomagmatic processes. Marked stratigraphic similarities exist between Manus Island, New Ireland and New Britain, and these islands are believed to have formed as a contiguous island-arc system at the Australian/Pacific plate boundary in the early to middle Tertiary.

However, Manus Island no longer lies at a plate margin but lies near the centre of a minor plate (North Bismarck plate) between the Pacific and Caroline plates to the north and the South Bismarck plate to the south. The Quaternary intraplate volcanism of Manus Island and the St Andrew Strait islands to the southeast contrasts with the Quaternary island-arc magmatism associated with the subduction of the Solomon Sea beneath New Britain at the southern margin of the Bismarck Sea. The change from plate-boundary-type volcanism in the Tertiary to intraplate volcanism in the Quaternary (possibly latest Pliocene) coincides with the opening of the Manus Basin and the formation of the WillaumezManus Rise. Present-day volcanism in St Andrew Strait southeast of Manus Island appears to be related to a mantle plume or hot-spot which may represent the youngest stage of intraplate activity controlled by northwest-trending fractures (parallel to the Willaumez-Manus Rise) through the Johnstone amd M'Buke Islands and Southwest Bay. Alternatively, the suggested alignment may represent a hot-spot trace.

144

ISOTOPIC M D TRACE ELEMENT DIFFERENCES IN LATE CAINOZOIC VOLCANIC ROCKS FROM WEST MELANESIA M.T. Perfit\ M.T. McCulloch & R.W. Johnson^ ^Research School of Earth Sciences, Australian National University Canberra ACT. 2Bureau of Mineral Resources, Canberra ACT.

Late Cainozoic volcanic rocks from island arcs in Papua New Guinea and Solomon Islands range mainly from highly mafic basalts, low-K tholeiitic basalts, and andesites through to silica-undersaturated alkaline compositions. Tholeiitic basalts from New Britain and the Witu Islands have relatively unfractionated REE patterns and ^"^Sr/^^Sr values of 0.70301 - 0.70358, whereas mildly light-REE-enriched rocks from the western Bismarck arc, Rabaul, Bougainville, and Solomon Islands have ^"^Sr/^^Sr values of 0.70343 to 0.70418. Transitional to alkali basalts from the Tabar-to-Feni Islands have relative enrichments in several incompatible elements and a higher range of ^"^Sr/^^Sr values (0.70365 to 0.70452). The increase in ^'^Sr/^^Sr from low-K to high-K basaltic and intermediate compositions of more than 70 rock samples corresponds to a fairly systematic increase in incompatible-element contents and light-REE fractionation (La]S[/YbN - about 0.8 - 13). values for 20 samples have a rather restricted range = 8.6 to 5.4), and correlate inversely with ^"^Sr/^^Sr values.

Many of the isotope data plot to the right of the Nd-Sr 'mantle array'.

Basalts that have the least fractionated REE patterns in general have the highest e^d values, but a single province may contain rocks having a wide range of eisfd- These data appear to correspond to mantle sources that have had similarly long-term depletions of light REE relative to heavy REE. However, systematic differences in major, trace, and isotope geochemistry and spatial relationships between different rock types, are consistent with the mantle in this region having been enriched - relatively recently and in different degrees - in some incompatible elements, although there may also have been an additional overprint caused by crustal contamination effects.

TI-RICH ACCESSORY PHASE STABILITY IN HYDROUS MAFIC-FELSIC COMPOSITIONS AT HIGH P,T T. H. Green

S N.J. Pearson

School of Earth Sciences, Macquarie University, North Ryde, NSW, 2113

Experiments on hydrous mafic, intermediate and felsic compositions at 7.5-30 kb and 900-1100°C allow delineation of Ti02 contents for a range of liquid compositions coexisting with a Ti-rich accessory phase. Four different starting compositions including unadulterated fused natural rocks and variously Ti-rich phase and REE enriched

145

compositions give mutually consistent results. For the fCOz) of the experiments the Ti02 content of liquids saturated in a Ti-rich accessory phase is shoWn to be primarily a function of temperature, pressure and Si02 content of the liquid, with water content having little observable effect.

Thus at 1000°C and pressures corresponding to the deep crust (7.5-12 kb) for mafic compositions'>'2.5% TiOj, for intermediate compositions'vl .5% Ti02, and for felsic compositions ~ 1 . 2 % Ti02, is required before a separate Ti-rich accessory phase may be expected to crykallize. At the same depth but at 950°C these Ti02 values change to ~1.5%, 1.2% and 0.9% respectively. At upper mantle depths (20-30 kb) the corresponding Ti02 saturation levels at 1000°C are'v 1.7%, 1% and 0.7% respectively.

In natural igneous rock series the contrasting Ti02/Si02 systematics in alkaline and tholeiitic series compared with calc-alkaline series are well established, as are the characteristic, though nonunique, low Ti02 values for rock series in convergent plate regions.

The present results allow experimental confirmation of observed and/ or predicted crystallization of Ti-rich phases in these rock series.

They also provide reasonable constraints on conditions under which TiOz contents of convergent plate magmas may be buffered by a residual Ti-rich phase in the source region, and indicate that Ti02contents of mafic parent magmas in these areas are unlikely to be controlled in such a manner.

Finally the results may be applied to fractionating calc-alkaline magmas. Thus the recorded common occurrence of sphene in andesiterhyolites of high-K calc-alkaline series from continental margins and its absence in other calc-alkaline sequences may be evaluated in terms of temperature of crystallization, £(0)2 and Ti, V vs. Si02 systematics.

L0W-Ti02 BASALTS OF MANAM ISLAND, PAPUA NEW GUINEA: PETROGENESIS OF AN ISLAND-ARC VOLCANO R.W. Johnson', A.L. Jaques', R.L. Hickey^, C.O. McKee^ & B.W. Chappell'^ 'Bureau of Mineral Resources, Canberra ACT. ^Department of Earth and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts, USA. 3volcanological Observatory, Rabaul, Papua New Guinea.

Department of Geology, Australian National University, Canberra ACT.

Mafic lavas of Manam Island, a major island-arc composite volcano in the western Bismarck volcanic arc of northern Papua New Guinea (McKee, 1981), have been assigned to the 'boninite-series' on account

146

of their amonalously low TiO^ contents (Meijer, 1980). Manam rocks are, however, compositionally uniform tholeiitic basalts and lowSiO^ andesites ranging in SiO^ contents from 50.44 to 53.93 weight percent, and they contain only moderate amounts of MgO (4.97 to 9.00 wt percent) and low amounts of Ni (27-81 ppm) and Cr (49-288 ppm) .

Nevertheless, analysed Manam rocks are fairly primitive for islandarc magma compositions (adjusted 100Mg/[Mg+Fe-as-ZFe ] values are mainly 60-66), and they therefore provide an opportunity to study the character of their source regions.

Phenocrysts and xenocrysts make up between about 20 and 50 percent of most Manam rocks and are dominantly plagioclase (An86-82 cores). Olivine crystals (Fo92-77) are typically embayed and resorbed, and xenocrystal olivine may be identified by its coarse grain size, the presence of kink bands, broken and embayed grain boundaries, and inclusions of Cr-rich spinel. They are also more Mgrich than olivine compositions predicted from Fe-Mg/olivine-liquid partitioning. Olivine-phenocryst compositions are mostly in the range Fo87-81, and define trends of increasing CaO and MnO, and decreasing NiO and Cr^O , as Fo contents decrease. Clinopyroxene phenocrysts are mainly Ca-augite, but diopside (Mg89-84, Al^O^ 1-22.6 wt percent, Cr^O. up to 0.96 wt percent) is found as relic cores in some grains. OrtRopyroxene is found in the groundmass and in some polymineralic aggregates, and the most Mg-rich composition (Mg83) is appreciably more fractionated than that of the co-existing phenocryst olivine. Spinel forms (1) tiny euhedra of chromian spinel in xenocrystal olivine (Fo92-90), (2) subhedra of chromian magnetite in phenocrystal olivine (Fo90-85), and (3) subhedra of titanomicrophenocrysts in the more fractionated samples. Groundmass compositions analysed using a defocussed electron beam (traversed slowly during micoroprobe analyses) are andesitic. Conditions of crystallisation have been estimated from several equilibria for coexisting minerals and mineral-melt pairs. The absence of hydrous minerals in any of the Manam rocks, the early crystallisation and modal preponderance of plagioclase over pyroxene, and the low H^OHcontents of analysed Manam rocks (mainly less than 0.5 percent) are evidence that Manam magmas must have crystallised under strongly water-undersaturated conditions.

Manam rocks are characterised by striking enrichments in K, Rb, Ba, and Sr relative to the light rare-earth elements (REE), and are enriched in light REE relative to Y and the heavy REE; (La/Yb) e.f. values range between 1.9 and 2.7, and (La/Sm) e.f. values range between 1.7 and 2.2. Sc, Y, V, Zr and Hf have generally similar relative abundance levels to the heavy REE on chondrite-normalised diagrams, but Ti and especially Nb are relatively depleted. The low Ti/V and Ti/Sc values are similar to those in boninites. l^^Nd/l^^Nd values for eight samples range between 0.51296 and 0.51303, and 87sr/86sr values are^between 0.70313 and 0.70338. Manam rocks therefore plot within the 'mantle array' of e -versus-e diagrams. 206pb/204pb and 207pb/206pb values in three Manam samples are similar to those for mid-ocean-ridge basalts. values do not covary with ^'^Sr/^'^Sv, but K/La, Rb/La, and Ba/La correlate positively with ^'^Sr/^^Sr, and ^^^Nd values correlate negatively with La/Nd. ^^Sr/^^Sr, K, Rb, Ba, and Sr appear to completely decoupled from ^^^Nd/^^'^Nd, REE, and high-field-strength elements. This decoupling is strong evidence that Manam rocks retain a geochemical history involving at least two components, and raises again the contentious issue of the role of subducted materials in arc petrogenesis.

147

McKee, C.O,, 1981. Geomorphology, geology, and petrology of Manam volcano- In: Johnson, R.W. (editor), Cooke-Ravian Volume of Volcanological Papers, 23-38. Geological Survey of Papua New Guinea Memoir 10.

Meijer, A., 1980. Primitive arc volcanism and a boninite series: examples from western Pacific island arcs. In: The Tectonic and Geologic Evolution of Southeast Asian Seas and Islands, 269-282, American Geophysical Union, Geophysical Monograph 23.

GEOCHEMICAL AND ISOTOPIC VARIATION WITHIN THE UPPER PILLOW LAVAS OF THE TROODOS OPHIOLITE, CYPRUS: SECOND-STAGE MELTS REVISITED W.E. Cameron

1

& M.T. McCulloch

2

^Department of Geology, Australian National University, CANBERRA ACT ^Research School of Earth Sciences, Australian National University, CANBERRA ACT

There are two principal hypotheses for the origin of the Troodos ophiolite. Conventional wisdom maintains that it is a piece of midocean ridge (MOR) generated crust; the alternative is a subductionrelated origin. In this study we present new geochemical and isotopic data from chemically primitive units within the Upper Pillow Lava (UPL) sequence. The Lower Pillow Lavas are more evolved and generally more altered.

In the past, magnesian UPL have been described as limburgites, alkali basalts, low-Ti ophiolitic basalts, komatiitic basalts and boninites. Fresh samples contain olivine ± orthopyroxene as microphenocrysts and groundmass acicular pyroxene set in glass which contains 5-6% H2O. Magnesiochromite (Cr/(Cr+Al)>0.7) is the only accessory phase.

Major element compositions show unusually low Ti02 and Na20 and high MgO, Si02 and H2O+.' All the primitive UPL are quartz-normative, and if recalculated anhydrous have Si02^53%. Three units can be distinguished (see accompanying table). On the basis of the "immobile" trace elements Ti and Zr, Unit I has the highest Ti and Zr contents and a near chondritic Ti/Zr ratio 100). Unit II lavas have lower Ti and Zr abundances and slightly higher Ti/Zr ratios. Unit III lavas have extremely low Ti and Zr values at equivalent Mg numbers but Ti/Zr ratios which vary from >200 to 70. Rare-earth element patterns for Units I and II lavas are light rare-earth element (LREE) depleted and approximately parallel those of typical MOR-basalts (N-type MORB). All REE patterns for Unit III lavas are U-shaped with variable LREE enrichement. In this unit the degree of LREE enrichment can be correlated with K2O, Zr, Rb, Sr and Pb but not with MgO, Ni, Cr, Ti02/ Sc, Y or heavy rare-earth element abundances. ratios are lower than for MORB, with £ ncI values from

148

+7.9 to +0.8. Plagioclase Iherzolite from the tectonised basement of the ophiolite has an value of -2 and a U-shaped REE profile.

Within Unit III lavas, enrichment in LREE correlates with decreasing On an vs. ^^Sr/^^Sr plot, samples lie in the field characteristic of island-arc rocks.

Our results indicate that the UPL represent magmas extracted from a progressively more depleted Iherzolite source. Unit I lavas are typical second-stage melts with compositions very similar to those of Archaean komatiitic basalts. Unit III lavas result from the interaction of a depleted Iherzolite source with an incompatible-element enriched component (Pmetasomatic fluid) which is isotopically distinct . Unit II lavas may have been derived from a depleted source which has interacted with a fluid of different geochemical and isotopic characteristics. A suitable environment for the mixing of incompatible element-enriched fluids with depleted peridotite is above a subduction zone in an island-arc.

Representative Chemical and Isotopic Compositions of UPL, Cyprus III

Unit

I

II

Si02 Ti02 AI2O3 Fe203 FeO MnO MgO CaO Na20 K2O P2O5 H2O+ H2O" CO2 Total

52.04 0.48 13.29 2.13 5.63 0.14 9.78 10.13 1.62 0.29 0.03 3.72 0.76 100.04

51.38 0.36 12.72 1.82 5.63 0.15 10.51 9.50 1.45 0.27 0.04 3.78 1.93 0.22 99.54

50.19 0.24 12.74 2.19 6.24 0.15 10.88 11.11 0.83 0.14 0.02 4.03 1.06 0.24 100.06

100.12

Mg# norm Q

72 6

74 5

72 5

78 3

Sc V Cr Ni Rb Sr Ba Pb Zr Nb Y LaM Yb

38 211 655 138 6 141 20 1.1 30 1 13 4.3 6.8

32 188 580 214 4 96 30 1.0 19 <1 8 3.4 5.5

43 233 550 279 3 38 15 0.8 6 <1 8 1.7 4.0

40 203 850 339 14 62 45 2.7 16 1 7 3.5 4.0

Ti/Zr La^y Yb..

K bi

96 0.63

114 0.61

240 0.34

71 0.86

87s^/86 Sr

.7062 7.1

.7048 5.7

.7062 4.2

.7056 1.9

^Nd

51.74 0.19 12.47 1.20 6.10 0.14 12.74 9.88 1.10 0.39 0.01 3.60 0.56 —

149

POTASSIUM-RICH ALKALINE MAGMAS IN ISLAND ARC SETTINGS PRODUCTS OF ENRICHED MANTLE, "WALL ROCK REACTION" OR BOTH? I.A. Nicholls^ & D,J. Whitford^ ^Department of Earth Sciences, Monash University, Clayton, Victoria ^Division of Mineralogy, C.S.I.R.O., North Ryde, N.S.W.

While Sr and Nd isotopic studies of K-rich lavas in some continental margin settings (e.g, Roman and Neapolitan regions, Italy) have indicated the involvement of mantle sources with long-standing enrichment in LIL elements and light REE (e.g. Hawkesworth and Vollmer, 1979; Cortini and Hermes, 1981), the few available data for similar island arc occurrences(Aeolian arc - Vollmer ^ al., 1981; Sunda Arc - Foden and Varne, 1980; Whitford et , 1981) suggest that enrichment processes immediately preceded or accompanied magma genesis.

This is also the case for K-rich rocks of the Muriah volcanic complex, central Java, for which ®^Sr/®^Sr ratios (0.7043 - 0.7049) are amongst the lowest recorded from the Sunda arc, and the single available ^"" ^Nd/^''Nd determination (0.51270) locates the relevant sample within the Sr/Ndisotope "mantle array".

At least two groups are represented amongst available samples from Muriah.

An apparent differentiation series, the Anhydrous Series (Nicholls and Whitford, 1982) is dominated by modal leucite and nephelinerich basanites and tephrites with up to 40% normative (Ne + Lc), and K20/Na20 weight ratios of 1.2-2.0.

The major phenocryst phases are olivine and tschermakitic clinopyroxenes (up to 20% A1 + Fe^"^ + Ti in Ml sites).

A second, much less homogeneous group, the Hydrous "Series" (characterised by relatively abundant phenocrysts of hornblende and biotite), includes modal feldspathoid-poor tephrites to andesites (20% normative Ne to 2% Qz) with K20/Na20 ratios of 1.1-1.6.

This group may represent the products of mixing between magmas of Anhydrous Series type and less strongly Si02 - undersaturated magmas similar to those in active "andesitic" volcanoes near the Muriah complex.

Evidence for this comes from LIL element abundances and ®"^Sr/®^Sr ratios intermediate between those of Anhydrous Series rocks and basalts and andesites from the active volcanoes.

Possible processes to explain the high LIL element and light REE abundances of the Anhydrous Series rocks include: 1) Introduction into the mantle source of a fluid which almost simultaneously caused enrichment and promoted melting; and 2) Enrichment of primary magmas by reaction with mantle wall rocks during transport through, or residence in, the mantle overlying the source.

Evidence from the trace element geochemistry and experimental liquidus phase relations of a primitive leucite basanite of the Anhydrous Series is more compatible with the latter alternative.

While the relatively high Ni content (140 ppm) and low HREE abundances (La 4 x chondrites; (La/Yb)^^^^ -40) suggest an olivine-rich, garnet-bearing source, the liquidus 'fields of olivine and garnet for this basanite do not overlap, even when 10 wt.% olivine seeds are added.

Instead, they are separated by fields of calcic clinopyroxene, and for X^^^ > 5 wt.phlogopite.

There is no liquidus field of orthopyroxene,^even with 10 wt.% crystalline orthopyroxene added, and in the presence of CO2.

These results suggest that if the magma underwent

150

little modification (apart from 5-10% olivine fractionation) after leaving its source, the source was not Iherzolitic, but rather a garnetbearing olivine-phlogopite clinopyroxenite, very different from "normal" mantle.

An alternative explanation is that the parental basanite magma for the Anhydrous Series was derived from a precursor less rich in LILE and LREE by reaction with slightly enriched, phlogopite-bearing Iherzolite wall rocks in the deep lithosphere.

Wall rock reaction and flow differentiation processes similar to those described by Irving (1980), are proposed.

Calculations using published REE partition coefficients show that if the flowing magma preferentially extracted the less refractory wall rock minerals phlogopite and clinopyroxene, and precipitated a more calcic clinopyroxene, Al-spinel and minor olivine, its REE pattern would remain almost unchanged and indicate a garnet-bearing source, while its liquidus phase relations would be modified by expansion of the phlogopite field at the expense of the fields of olivine, garnet, and particularly orthopyroxene.

References Cortini, M. & Hermes, O.D., 1981.

Sr isotopic evidence for a multisource origin of the potassic magmas in the Neapolitan area (S. Italy).

Contr. Mineral. Petrol., 7^. 47-55.

Foden, J.D. & Varne, R., 1980.

The petrology and tectonic setting of Quaternary-Recent volcanic centres of Lombok and Sumbawa, Sunda arc.

Chem. Geol., 201-226.

Hawkesworth, C.J. & Vollmer, R., 1979.

Crustal contamination versus enriched mantle: ^'^^Nd/^^^Nd and evidence from the Italian volcanics.

Contr. Mineral. Petrol., 151-165.

Irving, A.J., 1980.

Petrology and geochemistry of composite ultramafic xenoliths in alkali basalts and implications for magmatic processes within the mantle.

Am. J. Sci., 280-A, (Jackson Vol.), 389-426.

Nicholls, I.A. & Whitford, D.J., 1982.

Potassium-rich volcanic rocks of the Muriah complex, Java, Indonesia : Products of multiple magma sources?

J. Vole. Geotherm. Res. (Spec. Issue "Arc Volcanism"), in press.

Vollmer, R., Johnston, K., Ghiara, M.R., Lirer, L. & Munno, R., 1981.

Sr isotopic geochemistry of megacrysts from continental rift and converging plate margin alkaline volcanism in South Italy.

J. Vole. Geotherm. Res., _U, 317-327.

Whitford, D.J., White, W.M. & Jezek, P.A., 1981.

Neodymium isotopic composition of Quaternary island arc lavas from Indonesia.

Geochim. Cosmochim. Acta 989-995.

151

VOLCANISM FROM THE CONTINENT-ARC COLLISION ZONE OF FLORES, SUNDA ARC t

G.E. Wheller,

1

1 R. Varne^

2 J.D. Foden

3 and M. Abbott

1. Department of Geology, University of Tasmania, Hobart, Tas 7000 2. Department of Geology, University of Adelaide, Adeliade SA 5001 3. School of Earth Sciences, Flinders University, Bedford Park, SA 5042

The Indonesian island of Flores is the easternmost volcanic island of the Sunda Arc and is the only part of the arc where the volcanoes are little known. West of Flores, on Sumbawa, the active volcanoes gCi^f T^igDora and Sangeang Api are potassic and ne-normative but have low Sr/ Sr ratios. In contrast, volcanic rocks from the small ^^langg immediately east of Flores are tholeiitic and have high Sr/ Sr ratios. The compositional change occurs somewhere in Flores and is probably related in some way to the present collision between the northward-moving Australian continent and the otherwise oceanic Banda Arc which continues eastwards from Flores.

We have been investigating the nature, timing and cause of compositional variation among the Quaternary volcanoes of Flores and present here geochemical data from three active volc^apic ^gentres in southern central Flores, and K-Ar ages and initial Sr/ Sr ratios from reconnaissance samples collected nearby.

The lavas from the volcanoes Ija and Ineri are phenocryst-rich basaltic andesites and andesites which mostly contain clinopyroxene, orthopyroxene and abundant plagioclase. Olivine with orthopyroxene reaction rims occurs in some rocks and magnetite, although generally abundant, is absent or rare in olivine-rich samples.

The lavas from the third volcano, Ebulobo, are two-pyroxene andesites and all contain abundant magnetite phenocrysts.

Each of these lava suites is chemically distinct. Those from Ija and Ineri are tholeiitic but are considerably enriched in incompatible elements relative to most other arc-tholeiite suites. The Ebulobo suite is calc-alkaline. Chemical variation within each suite seems to have been dominantly controlled by fractional crystallization of the observed phenocrysts, although the Ineri suite appears strongly affected by crystal accumulation.

Crystallization and removal of magnetite after the magma reached about 52% SiO^ satisfactorily explains the lack of Fe-enrichment in the intermediate stages of each suite.

The chemical variation between the suites is examined using element concentrations normalized to 55% ^^ construct chondrite-normalized abundance versus element patterns. The patterns for the Ija and Ebulobo suites, least-enriched and most-enriched respectively, are generally parallel and show particular enrichment of Ba, Sr and REE relative to other incompatible elements. It is not clear how these systematic differences could have been produced by fractional crystallization.

Trace element ratios suggest that a process such as successive batch partial melting of a previously-enriched mantle source is more consistent with the patterns than addition of different proportions of an incompatible element-enriched component to an initially homogeneous source.

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SV 86 Sr/ Sr ratios of andesites from central Flores are low (0.7047 - 0.7053) and generally conform -^c^ th^^Sunda Arc pattern. They are, however, slightly higher than the Sr/ Sr ratios ( 0.704) from the Bali-Lombok-west Sumbawa sector of the Sunda Arc, which raises the possibility that the continental fragment of Sumba was contributed material to the source region of the volcanism.

The very young K-Ar ages (0.27 - 4.77 m.y.) of andesites from the same region are consistent with previous impressions that the eastern Sunda and Banda Arcs developed only since the late (?)

Tertiary. The formation of a new volcanic-arc and trench system may have been the result of collision between Sumba and the pre-Tertiary Sunda Arc.

NEODYMIUM, STRONTIUM, LEAD AND OXYGEN ISOTOPIC AND TRACE ELEMENT CONSTRAINTS ON MAGMA GENESIS IN THE BANDA ISLAND-ARC, WETAR. 1 1 2 M.T. McCulloch , W. Compston , M. Abbott

1 and A. Chivas .

^Research School of Earth Sciences, Australian National University Canberra ACT. ^School of Earth Sciences, Flinders University of South Australia.

The mechanisms and sources involved in the generation of islandarc magmas are complex and not well understood. Potential sources include: 1) subducted oceanic crust, with or without entrapped sediment; 2) mantle wedge overlying the subducted slab; and 3) the basement of an arc which may be pre-existing fragments of continental^ oceanic or older island-arc basement. To elucidate the role of these potential sources, we have undertaken a multi-isotopic and trace element study of samples from Wetar Island in the Banda Arc. The samples represent a spectrum of chemical types, ranging from basalt, andesite, dacite to rhyolite. ®^Sr/®^Sr analyses of fifty samples range from 0.70446 to 0.72227. This is the largest variation reported for an island-arc. The Nd and 0 isotopic compositions also show large variations with e^d values of from 4.3 to -12.0 and values of from 6.2 to 18.1 (also the highest reported value for an island-arc rock). Lead isotopic compositions are relatively radiogenic with ^^^Pb/^^^'Pb ratios of from 18.609 to 18.913.

There is a systematic covariation of Nd and Sr and to a lesser extent 0 isotopic compositions. Low samples have more positive values and generally lower values. The large covariance of Sr and Nd requires the interaction with large amounts of continental crustal components with high ®^Sr/®^Sr (>0.723), low e^d (<-12.0) and high (>16). Analyses of an Upper Triassic shale from the Timor Trough indicate that it has appropriate isotopic compositions (®^Sr/®^Sr = 0.74105, CNd = -13.4 and = 16.8) required for the crustal component. The nature of the interaction between the crustal and mantle derived components is constrained by the curvature of the isotope covariations. Direct mixing of

153

partial melts of subducted sediment-oceanic slab with the overlying mantle wedge would produce essentially linear Nd-Sr covariations.

However, the covariatibns have pronounced curvatures which together with trace-element constraints imply complex fractionationassimilation processes operating within the arc crust.

T—^ r ^ Vma'ntle '

Infra'

T r BANDA ISLAND ARC

array Oceanic Lesser

Arcs Anfiles

Taupo

WETAR

23 0 704

0708

0712

0720

0716

87sr/86Sr Nd and Sr isotopic compositions of Wetar volcanics, Banda Island-Arc.

ARC ANKARAMITES, SANGEANG API XENOIITHS AND CORDIILFRAN ULTRAMAFIC TO DIORITIC INTRUSIVE COMPLEXES:

AN UPDATED CONCEPT

OF ARC GROWTH AND DEVFLOPMENT. j.D. Fodenl g^d R. Varne2 iDepartment of Geology and Mineralogy, University of Adelaide.S.A. 2Geology Department, University of Tasmania. Hotart. Tas. in the central Sunda Arc and in south-western Sulawesi we have recognized that new phases of island arc-style volcanic activity are often heralded by the eruption of magma with high proportions of large clinopyroxene crystals (ankaramites). These clinopyroxenes are often zoned with abrupt changes in composition from core to rim.

Often cores are Al-, Na-poor, Cr-diopsides, while rims are augite or salite. Large corroded olivine, amphibole or phlogopite crystals may accompany the clinopyroxene.

154

Naturally, these r e f l e c t i n g t h e i r high Mg, ^^/Fe and Sc) but d i v e r s e , ranging from normative v a r i e t i e s . variation.

l a v a s have g e o c h e m i c a l c h a r a c t e r i s t i c s c l i n o p y r o x e n e c o n t e n t s (low AI2O3, h i g h Ca, i n o t h e r r e s p e c t s they are g e o c h e m i c a l l y normative t o h i g h l y u n d e r s a t u r a t e d ne and I c LIL element c o n c e n t r a t i o n s show v e r y l a r g e

At Sangeang Api v o l c a n o i n the c e n t r a l Sunda Arc m o d e r a t e l y u n d e r s a t u r a t e d l a v a s o f the a n k a r a m i t e - t y p e c o n t a i n x e n o l i t h s o f m a f i c and u l t r a - m a f i c r o c k s .

These i n c l u d e ; olivine c l i n o p y r o x e n i t e , c l i n o p y r o x e n i t e , h o r n b l e n d i t e and gabbro (many w i t h abundant m a g n e t i t e ) .

Some o f t h e s e x e n o l i t h s have a complex h i s t o r y and t h e i r component m i n e r a l phases a r e e i t h e r zoned o r a r e u n d e r g o i n g reaction.

C o n s i d e r a t i o n o f the a n k a r a m i t i c l a v a s and t h e Sangeang Api x e n o l i t h s r e v e a l s t h r e e important f a c t s : 1.

The x e n o l i t h assemblages c l o s e l y resembles a number o f i n t r u s i o n s o r groups o f i n t r u s i o n s , i n c l u d i n g ; t h e Alaskan Zoned U l t r a m a f i c Complexes, numerous u l t r a m a f i c t o d i o r i t i c i n t r u s i o n s i n the western S i e r r a Nevada and the Garabal H i l l - G l e n Fyne complex i n S c o t l a n d .

2.

The m e g a c r y s t s o f the ankaramite l a v a g e n e r a l l y have c o m p o s i t i o n s e q u i v a l e n t t o t h o s e found i n the x e n o l i t h s , but are q u i t e d i s t i n c t from the t r u e p h e n o c r y s t c o m p o s i t i o n s o f the same lavas.

In o t h e r words, the l a v a s have p r o b a b l y a q u i r e d much o f t h e i r " b i g c l i n o p y r o x e n e " c o n t e n t as a c c i d e n t a l i n c l u s i o n s during the p a s s a g e o f magmas through a c c u m u l a t i o n s o f t h e s e c r y s t a l s , perhaps near the base o f t h e c r u s t .

3.

The x e n o l i t h assemblages p r o b a b l y o r i g i n a t e p a r t l y by cumulate p r o c e s s e s and p a r t l y by i n s i t u e q u i l i b r i u m c r y s t a l l i s a t i o n a t the base o f the c r u s t .

Once e s t a b l i s h e d , t h i s p y r o x e n i t e u n d e r p l a t e t o the a r c ' s c r u s t has a b u f f e r i n g i n f l u e n c e on magmas f i l t e r i n g through and i s i t s e l f m o d i f i e d by r e a c t i o n w i t h l a t e r batches of l i q u i d .

4.

This p y r o x e n i t e m a t e r i a l may r e a c t w i t h subsequent l i q u i d b a t c h e s sometimes undergoing c o n v e r s i o n t o a m p h i b o l i t e , a r e a c t i o n which a t some s t a g e may a l s o be r e v e r s e d t o supply a l k a l i s t o l a t e r magma b a t c h e s .

This i s a p o s s i b l e s o u r c e of many high-K i s l a n d a r c s u i t e s .

C l i n o p y r o x e n e - r i c h l a v a s have a l s o been d e s c r i b e d from s e v e r a l other arc l o c a l i t i e s including; Kanaga and B o g o s l o f i n A l a s k a , the Solomon I s l a n d s and the I t a l i a n Roman P r o v i n c e .

Both i n t h e ' c a s e o f our Indonesian examples as w e l l as t h e s e o t h e r s , e v i d e n c e o f t e c t o n i c complexity e x i s t s .

There may be l a t e r a l o r a c r o s s - a r c f a u l t motion or i n t r a - a r c r i f t i n g .

We s u g g e s t t h a t where t e c t o n i c movements d i s r u p t an e s t a b l i s h e d a r c , f r a g m e n t a t i o n o f the u n d e r p l a t e m a t e r i a l ( p y r o x e n i t e / h o r n b l e n d i t e ) may take p l a c e and v o l c a n i c e r u p t i o n immediately f o l l o w i n g t h i s event may c a r r y t h i s d e b r i s t o the s u r f a c e .

155

EVOLUTION OF THE PROTEROZOIC CRUST IN THE MOUNT ISA INLIER

I.H. Wilson^ ^Geological Survey of Queensland, Brisbane, Queensland

The dominantly Middle Proterozoic Mount Isa Inlier is well known from detailed mapping and thorough petrological, geochemical, geochronological, and geophysical studies. Moderate to intense deformation and good exposure have allowed comprehensive spatial and temporal sampling of most rock units. A characteristic of the region is the abundance of felsic lavas, which have been reliably dated by U-Pb zircon techniques and are intercalated with mafic and intermediate volcanics and various sediments. Several phases of granites and dolerite swarms intrude the sequence. Almost all the Proterozoic rocks have been subjected to low-pressure greenschist to amphibolite facies regional metamorphism.

An extension of the Early Proterozoic Arunta Inlier has been inferred from geophysical surveys to underlie the Georgina Basin and probably forms the basement for at least the western edge of the Mount Isa Inlier. This basement may be exposed as the Yaringa Metamorphics and some of the gneissic rocks in the south of the Inlier, The highly metamorphosed shale, sandstone, and volcanics of these units may represent deposits on an unstable continental shelf at the eastern margin of an Archaean to Early Proterozoic craton.

A thick sequence of crustal rocks is believed to have existed when the felsic and intermediate volcanics of the Leichhardt Metamorphics were extruded (1865 Ma). The abundant felsic ignimbrites (and comagmatic granites) have chemical affinities with modern continental margin igneous rocks and probably were derived by partial melting of shale or older andesitic rocks at depths exceeding 10 km. The intermediate volcanics in the Leichhardt Metamorphics support an orogenic environment but are unusually sparse, especially in the uppermost part of the formation. However, some modern analogues such as the Andes and New Zealand also have a concentration of felsic lavas in their later stages.

This early igneous event was probably associated with metamorphism and uplift. Erosion had locally exposed the Kalkadoon Granite before a second igneous cycle began (about 1800 Ma) with extrusion of mafic to intermediate lavas in the Bottletree Formation and Magna Lynn Metabasalt. The basal sequence of the latter formation consists of andesite which may have formed through extreme fractionation of mafic magma during ascent impeded by a compressional environment. Tuffs and hyaloclastites mark the transition to mafic lavas which are more consistent with a tensional environment.

Numerous dolerite dykes in the older Leichhardt Metamorphics are probably related to this mafic volcanism.

The voluminous mafic magma rising through and being trapped in the lower crust is a likely heat source for further partial melting which is represented by the felsic lavas in the Argylla Formation and the upper part of the Bottletree Formation. These lavas have

156

exceptionally high contents of incompatible elements such as K and Zr, consistent with low degrees of partial melting or partial melting of an enriched source such as crustal rocks veined by subvolcanic equivalents of the Leichhardt Metamorphics.

The flat HREE patterns of the younger felsic volcanics are consistent with partial melting at shallower depths than the source of the Leichhardt Metamorphics.

The thickening of the crust by the addition of mantle-derived magma, upward migration of volatiles and incompatible elements which raised the melting temperature of the lower crust, the presence of a network of felsic and mafic intrusive rocks, and metamorphism contributed to stabilisation of the crust. The next igneous event was of mantle-derived magmas which were extruded in tensional environments. A major rift valley in the west of the inlier was partially infilled by 50 000 km^ of mafic lavas (Eastern Creek Volcanics) with the chemistry of continental tholeiites. Apparently coeval volcanics (Soldiers Cap Group) in the east of the inlier are low-K tholeiites somewhat similar to ocean-floor basalts. The abundant pillow lavas and intercalated turbidites are consistent with this conclusion and indicate the possible position of the continental slope at this time. Near the middle of the inlier, minor intermediate volcanism in the Marraba Volcanics and Mitakoodi Quartzite could represent a stratovolcano.

Following this mafic volcanism, the western rift was almost completely filled with clastic sediments and intruded and uplifted by anorogenic granite. Much of the east of the inlier was a marine shelf on which banded iron formations, carbonates, pelitic sediments, and minor volcanics were deposited.

Pillowed low-K tholeiite, transitional alkaline basalts, and felsic volcanics occur locally.

Granite intrusion and uplift also occurred in the east of the inlier at about this time.

The final major volcanic event in the western rift and on the adjacent platform occurred at about 1680 Ma when alkali olivine basalt, trachyte, and rhyolite (Fiery Creek Volcanics and Carters Bore Rhyolite) were extruded.

These volcanics are difficult to characterise because they have been extremely altered, possibly by weathering in an arid palaeoclimate which produced 'red bed' facies sedimentary rocks. The mafic rocks are attributed to small degrees of partial melting of the mantle at depths greater than those which formed the tholeiitic lavas in the Eastern Creek Volcanics.

Extreme fractional crystallisation of the alkali olivine basalt probably produced the trachyte whereas the rhyolite may represent a crustal melt.

Subsequent deposition was mostly restricted to a broad marine platform in the west of the inlier where dolomitic sediments and stromatolites are abundant.

Intermittent eruptions of felsic tuff from one or more of the volcanic centres continued until the end of this deposition at about 1650 Ma. The major deformation and regional metamorphism affected the inlier between 1600 and 1450 Ma. Folding and metamorphic grade generally decreases in intensity to the northwest.

The crust was thickened to between 40 and 50 km and subsequent erosion has left about 35 to 40 km of crust.

157

REE PARTITIONING BETWEEN SPHENE, ALLANITE AND CHEVKINITE AND COEXISTING INTERMEDIATE - FELSIC LIQUIDS AT HIGH P,T. T.H. Green

and N.J. Pearson

School of Earth Sciences, Macquarie University North Ryde, NSW 2113

REE are readily accommodated in sphene, chevkinite and allanite, and fractionation of these phases from crystallizing intermediatefelsic magmas could cause large changes in the REE content of derivative liquids even though the volume of the fractionating phase is small.

REE !?5uid partition coefficients (D) show a convex upward pattern, with a ten-fold enrichment of Sm, Ho over La and a five-fold enrichment of Sm, Ho over Lu and with for example, D for Sm of 18 for a liquid with 57% Si02 at 12 kb and 1000°C. D values are clearly sensitive to temperature, pressure and Si02 content, and possibly to H2O content. Thus Dincreases significantly with decreasing temperature and increasing pressure and Si02 content. A slight increase in D appears to occur with decreasing H2O content. This is the first detailed documentation of a pressure effect on D»s for REE for deep crustal and upper mantle depths. No significant compositional change occurs in the sphene with increasing pressure, and the pressure effect on D is attributed to changes in melt structure with increasing pressure. Similar P-sensitive changes in D for the REE in clinopyroxene also occur, for high levels of REE in the pyroxene structure, and if such changes extend to natural abundances of REE in pyroxene, then these will have important implications for geochemical modelling of magma generated in the lower crust and upper mantle.

D |llanite/liq ^^^^ ^^^^ ^REE enriched patterns with D values for La of ^30 to 120 and for Lu of ^0.5 to 6. The values increase with increasing Si02 content and decreasing temperature.

Chevkinite has a similar though less LREE -enriched pattern. Thus D for La is ^70 and for Lu is ^15.

Sphene, allanite and chevkinite are recorded fractionating phases in evolving intermediate to felsic magmas, and incorporation of these experimentally determined D values for these phases should allow more rigorous geochemical modelling of magma fractionation. In the case of sphene, they may also be applied to crustal melting processes where this phase may be residual in the deep crust.

The overall results on the accommodation of REE in sphene, allanite and chevkinite are also relevant to "sphene-based ceramics" proposed as storehouses for radwaste.

158

EXPERIMENTAL MELTING STUDIES ON CRUSTAL SOURCE MATERIALS FOR RHYOLITIC MAGMAS, TAUPO ZONE, NEW ZEALAND

I.A. Nicholls Department of Earth Sciences, Monash University, Clayton, Victoria

The melting relations of possible crustal "igneous" and "sedimentary" source materials for rhyolitic magmas of the Taupo Volcanic Zone, New Zealand, have been studied experimentally, using respectively, compositions based upon the average Taupo andesite and dacite (Cole, 1979) and an average dacitic greywacke from the Kawerau geothermal field (Reid, 1982).

The andesite and dacite are Di-normative (5.3 and 3.4% Di), while the greywacke has 2.7% normative C. Glasses of these compositions were crystallized at P^oad "" 0.5GPa, T - 700°-850°C, in the presence of H2O or H2O/CO2 vapour (Xg^R = 1.0 and 0.75) and fH2 - QFM. The products were analysed as com;^letely as possible by electron microprobe.

Crystalline assemblages for the andesite and dacite are dominated

by Ca-Al amphibole ("hornblende", 7-12% AI2O3), plagioclase and ilmenite, with biotite, quartz and alkali feldspar below 725°C, As expected, the mafic phases become more magnesian Mgito-Mgeo) and plagioclase more calcic (Anito-Anyo) with increasing temperature.

Low temperature assemblages for the more Fe-, Al- and K-rich greywacke include gedrite, alkali feldspar and quartz, in addition to the dominant biotite, plagioclase and ilmenite, but alkali feldspar and quartz are lost and orthopyroxene replaces gedrite by 750°C. Again the mafic phases become more magnesian (Mgso-Mgso) and the plagioclase more calcic (Anao-Anso) with increasing temperature.

In all three compositions, lowering of X^^Q from 1.0 to 0.75 causes the felsic minerals to persist to higher temperatures.

Amphiboles of cummingtonite-grunerite type do not occur in any assemblage at 0.5GPa. This is also the case for experiments on a Kawerau greywacke at 0.2GPa, Xh^O " carried out by Reid (1982).

In andesite assemblages produced at X^^^ = 1-0, the proportion of glass present, calculated from mass balance^ relationships using phase compositions, rises from < 10% at 700° to ^ 65% at 850°C. The corresponding proportions for dacite and greywacke assemblages are approximately 30-85%.

Glass compositions (Na contents calculated from mass balance) change from rhyolitic to rhyodacitic 75-68% Si02) over this temperature range.

Glasses in the andesite and dacite charges are slightly corundum-normative (2.3-0.1%C), becoming less so with increasing temperature, while those in greywacke charges are strongly Cnormative (5.3-3.2%C).

Direct comparison of these glass compositions with those of Taupo rhyolites, and comparison of the corresponding crystalline assemblages with experimental near-liquidus assemblages of representative rhyolites, indicate that while the greywacke is a suitable source for more strongly peraluminous and potassic rhyolite magmas,^both the andesite and dacite are too rich in Al and Ca (and hence normative An) to act as sources

159

for Di-normative rhyolites.

Contrasted rhyolites (1.4%C, 3,0%Di) from the Okataina centre, both of which carry cummingtonite phenocrysts (Ewart et al., 1975) h^ve been crystallized experimentally at 0.3-0.5 GPa, 700°-850^C, X^^^ = 1 . 0 and 0.75 and fH2 - QFM. Liquidus orthopyroxene and biotite of the C- normative rhyolite closely match the same phases in the experimental greywacke assemblages.

However, liquidus assemblages of the Di- normative rhyolite are dominated by a calcic amphibole much less aluminous (4-5% AI2O3) than those in andesite and dacite assemblages, and a highly sodic plagioclase ( A n i 5 - 2 o ) . Neither rhyolite crystallizes cummingtonite at pressures > O.SGPa, and it is probable that natural phenocrysts formed at pressures around O.lGPa. These results suggest that Taupo Zone rhyolitic magmas were produced by melting of greywackes of varying degrees of mineralogical and chemical maturity within the Mesozoic basement.

"Pristine" dioritetonalite sources, which could be located in an underplated deep crustal layer, are unlikely to have played a direct role in rhyolite magma genesis.

These conclusions are compatible with those reached on the basis of Sr and Nd isotopic data (Perfitt et al., 1981), which show that both rhyolites and greywackes have higher ® Sr/®^Sr and lower ^'^^Nd/^'^^Nd than the associated basalts and andesites.

References Cole, J.W., 1979.

Structure, petrology and genesis of Cenozoic Volcanism, Taupo Volcanic Zone, New Zealand.

N.Z. J. Geol.

Geophys., 631-657.

Ewart, A., Hildreth, W. & Carmichael, I.S.E., 1975.

Quaternary acid magmas in New Zealand.

Contr. Mineral. Petrol., 1-27.

Perfitt, M.R., McCulloch, M.T. & Froude, D., 1981.

Sr- and Ndisotopic variations in volcanic and plutonic rocks from the Aleutian Islands and Taupo Volcanic Zone, New Zealand.

Implications for island arc magma genesis.

Abstracts, lAVCEI "Arc Volcanism" symposium, 292-293.

Reid, F.R., 1982.

Geochemistry of central North Island greywackes and genesis of silicic magmas.

Ph.D. thesis, Victoria Univ., Wellington, 266 pp.

160

EVOLUTION OF THE CENTRAL VICTORIAN UPPER DEVONIAN MAGMATIC PROVINCE - A PALAEOZOIC BASIN AND RANGE ANALOGUE V-J, Wall\ J.D. Clemens \ W. Compston^ and M. McCulloch^ ^Dept, of Earth Sciences, Monash University, Clayton, Vic., 3168. ^Res. School of Earth Sciences, Aust. Nat. Univ., Canberra, A.C.T., 2600,

Volcanic - plutonic, silicic magmatic complexes can provide important insights into processes of magma evolution and emplacement in the Upper Crust and also into the character of, conditions in, and magma generation processes in the Lower Crust.

In this paper we examine the evolution of the Central Victorian, Upper Devonian magmatic province on the basis of field, petrological, geochemical, isotopic (Rb/Sr, Nd/Sm, 0) and phase equilibrium investigations.

Situated in the Tasman Fold Belt, occupying around 70,000 km^ and post-dating Tabberabberan folding, this province is dominated high level granitoid batholiths and large-volume, intra-caldera silicic ignimbrites.

Developed over a 20-25 m.y. period the province exhibits peraluminous (broadly S-type) volcanic and plutonic suites, metaluminous (broadly I-type) suites together with some mantle-derived mafic and hybrid rocks.

Our major conclusions regarding the evolution of the province are as follows: i)

The "S-type" magmas were relatively high temperature (>800850°C), H2O poor varieties (2-4 wt.% H2O) which exhibit evidence of some early deep crystallization (3-8 kbars) but major chemical evolution by fractionation/accumulation processes in subvolcanic chambers.

Geochemical and isotopic data indicate that individual plutons, and major ignimbrites were derived from differing source regions, dominated by metasediments (not pelites) but of mixed character and Cambrian and/or older ages.

ii)

The "I-type" magmas were similarly high temperature and relatively dry.

Extensive fractionation and, in some instances, hybridisation with crustal and mantle components contributed to chemical variation within the suites.

iii)

The mantle-derived basic magmas also exhibit evidence of fractionation and crustal contamination.

iv)

Lower crustal source regions of the silicic magmas were heterogeneous with major labile metasedimentary and mafic/intermediate components, their inferred age .and lithological character being incompatible with crustal models involving early Palaeozoic oceanic crust but constant with the seismic data of Gibson et al. (1981).

v)

The Devonian granulite facies metamorphism which resulted in extensive crustal partial melting was produced by crustal underplating and intraplating of mafic magmas, which rarely reached the surface.

The Central Victorian Upper Devonian province has modern analogues in the Basin and Range province and other continental extensional environments.

161

VERTICAL DISTRIBUTION AND FRACTIONATION OF HEAT PRODUCING ELEMENTS IN GRANITOIDS: ,SIERRA NEVADA BATHOLITH, CALIFORNIA, U.S.A. Wayne N. Sawka Department of Geology, Australian National University, CANBERRA ACT The late Jurassic, I-type, Tinemaha Granodiorite is the intermediate formation of a three member intrusive suite and occupies ^ 160km of the total ^ 200km along the eastern Sierra Nevada batholith. The pluton is horizontally zoned in the classical manner and is also vertically zoned, yielding independently consistent variations. The vertical zoning of the pluton grades upward from quartz monzodiorite to granite, over 8 00 metres, in which the colour index decreases from 23 to 13 with a corresponding decrease in specific gravity from 2.77 to 2.70.

Heat production increases rapidly upward in the pluton, fig. 1 Uranium and thorium concentrations almost double in less than a vertical kilometre (U=^3.5 to '^^6.8ppm: Th='\^15 to ^32ppm) , if this trend continued linearly downward then both elements would be absent in little more than a kilometre below the present surface. If the vertical distribution of heat producing elements (HPE) is governed by an exponential function (Lachenbruch, 1968) decrease would be more gradual extending the approach to zero concentrations to more reasonable geologic depths.

The systematic upward increase in thorium and potassium correspond to an upward increase in allanite and K-feldspar. However, the upward increase in whole rock uranium is not accompanied by an increase in any phase likely to concentrate the element. Mineral separates indicate that heavy accessory minerals decrease by over 35% upward in the pluton. Fission-track autoradiography shows that almost all uranium is in discrete phases, not along grain boundaries.

Everywhere in the pluton, sphene is the major site for uranium.

Groundmass sphenes are zoned with higher uranium concentration along the rims, while thorium is depleted at the rims. Uranium concentrations in sphenes are higher in the low elevation rocks with less uranium than those of the high elevation rocks that are higher in uranium. The controlling phase for the pluton's upward whole rock uranium increase is a late magmatic dendritic sphene that is optically continuous with the host groundmass sphene. Such dendritic sphenes contain up to 20 times the uranium of the host sphene and usually occur in myrmekite or K-feldspar. Thus, the distribution of HPE's is a magmatic process involving a migrating melt phase that becomes trapped below the magma chamber roof and does not require large scale recycling of groundwater.

The pluton's vertical heat generation variation makes it possible to calculate the characteristic depth scale (D) directly from the exponential heat generation equation. The calculated value for the pluton, D=2.2km, is significantly lower than that of the batholith, D=10.1km, from combined heat flow and heat generation (Lachenbruch, 1968). This result demonstrates that plutons within a heat flow province do not observe the same exponential law (same D value), but distribute HPE vertically according to other factors. The larger D value from combined heat flow and heat generation for the batholith is thus a mean value, reflecting the larger average size of the

162

plutons in the batholith relative to the Tinemaha Granodiorite.

The linear heat flow relationship implies that in a well established heat flow province, heat generation will be indicative of heat flow. Data from the Tinemaha Granodiorite combined with regional heat flow data indicate two possible geothermal models, either: 1) an abnormally high surface heat flow or 2) a normal surface heat flow with an additional heat source at depth below the pluton. In a composite batholith like the Sierra Nevada, an additional heat source at depth (probably a pluton) is the most reasonable.

It is suggested here that plutons within a crustal heat flow province do not have the same vertical distributions of HPE's (same D value) and that using combined heat flow-heat generation data from more than one pluton yields a mean regional value.

Lachenbruch, A.H., 1968. Preliminary geothermal model of the Sierra Nevada, J. Geophys. Res. 73, pp. 6977-6990.

5.0

7.0

9.0

11.0

HB\T PRODUCTION ( 10"''CAL. CMT'SECT' ) Fig. 1

163

AGE RELATIONSHIPS IN THE NEWER VOLCANIC PROVINCE OF SOUTHEASTERN AUSTRALIA E.B. Joyce Department of Geology, University of Melbourne VIC The Newer Volcanic Province of southeastern Australia includes extensive thin lava flows on the Western District Plains of Victoria, with local lava plains and thicker valley flows within the Central Highlands, Ash deposits are usually of small extent, and restricted to the vicinity of the numerous maar eruption craters, such as Mt. Gambier in southeastern South Australia (Joyce 1975).

Available K/Ar and radiocarbon dates indicate that the volcanic activity ranged from 6 m.y to mid-Holocene• However, many of these dates have only local significance, and tell us little about the ages of the rest of the province. Materials for further dating are difficult to obtain; carbon is rarely found below the younger deposits, partly because of lack of exposure on the flat plains.

Suitable fresh rock for K/Ar dating is difficult to obtain without drilling. Even if samples could be obtained, the dates would still often be of only local value.

It is possible to characterize the age of deposits by their morphology, including the degree of preservation of original features, and the degree of weathering and soil erosion. Such a technique is used in land systems mapping, and this type of mapping has been carried out by Soil Conservation Authority workers over parts of the province. The far west of the plain has been mapped in detail, and six land systems of different ages differentiated (Gibbons and Gill 1974).

Absolute dates can be inserted into such a scheme, and each landscape given an age range. In this way a few dates can be made to yield relationships over wider areas. Areas in need of further absolute dating can also be recognized.

Land systems maps of the Werribee-Keilor lava plains have been prepared recently by Jeffery (1980) and form a suitable basis for a study of age relationships in this part of the province.

Some detailed conclusions about the volcanic activity on the Werribee-Keilor plains and some general conclusions about the province as a whole can now be made. These include the distribution of volcanoes, type of eruption, geochemistry, and volume of products with time across the province. Related deductions concerning rates of weathering and soil development, and erosion and landforra development also become possible.

References Gibbons, F.R. and Gill, E.D., 1964. Terrains and soils of the basaltic plains of far western Victoria, Proceedings of the Royal Society of Victoria 77 : 387-395.

Jeffery, P., 1980. The study of the land in the catchments to the north of Melbourne. Soil Conservation Authority, Victoria.

Joyce, E.B., 1975. Quaternary volcanism and tectonics in southeastern Australia, The Royal Society of New Zealand, Bull. 13 : 169-176.

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DEEP-SEATED RESIDUAL GASES FROM VICTORIAN VOLCANOES A.R. Chivas^, I. Barnes^, T. Torgersen\ J.O. Stone^ & J. Lupton^ ^Research School of Earth Sciences, Australian National University, ACT U.S. Geological Survey, Menlo Park, CA, USA University of California, Santa Barbara, CA, USA

The origin of the COz-rich mineral springs of central Victoria, especially those of the Daylesford district has been long debated.

Previous suggestions include sources of carbon from (i) Ordovician slates, (ii) oxidation of organic matter in deep-leads, and (iii) residual degassing from Late Pleistocene volcanoes. Several wellpreserved volcanoes of the district probably have an age of 20 ka.

Waters from several of the mineral springs form the basis of the Australian mineral-water industry. Mineral waters for commercial sale are commonly artifically aerated to remove Radon (and CO2) and are recarbonated prior to bottling.

Waters issuing directly from the mineral springs have a pH from 5.95 to 6.35 and a temperature of 11.9®C to 17.8''C. These waters have been analysed for their Mg, Ca, Na, K, Si, B, Sr, Rb, Cs, Li, Cl", SOit^", HCO3" content and their 6^®0/6D relationship indicates that they are of meteoric origin, and are isotopically similar to other groundwaters and precipitation in the area.

Analyses of gases (by gas chromatography) confirm that CO2 is the major phase present (>70% by volume). Other gaseous constituents include nitrogen, methane, oxygen, argon, and helium. Gases not detected include H2 and^CzHe- The of gaseous CO2 (-6.0 to - 8 . 5 % o PDB) and dissolved HCO3 are consistent with values for deep-seated carbon. ^^^Rn in the gas phase ranges from 1 x 10^ to 36 x 10^ DPM/L of gas and is preferentially partitioned from the liquid phase by a factor of 6±3 [DPM/L gas]/[DPM/L H2O]. The exception is Glenluce Mineral Spring which has a partition factor of 22±2. The Rn content of the gas phase shows no discernible correlation with the CO2 content of the gas phase, although a weak bimodal relation exists for ^^^Rn vs. % Ar. Gas chromatographic He determinations are insufficient for He/Rn age calculations. A preliminary ^He/'^He ratio for Glenluce /[ ^He/'^He]^ . ^ =1.7, (high CO2, high Rn, low Ar) gives [ ^He/^He] with (He/Ne) /(He/Ne)^^^ = 490. This indicates the possibility of a slight mantle component by either direct leakage or from the ageing of an emplaced magma. The ageing of magmas from mantle to crustal He ratios usually takes -10^ years. Mixing of He from both mantle and crustal source regions would reduce the resultant He/ He. Thus the observed ^He/'^He, , of 1.7 is a minimum ratio for the non-crustal component. TnPMift^^ CO2, high Rn, low Ar contents and the He isotope ratio suggest the multistage transfer of mantle gases from a deep, fresh source.

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ZIRCONS FROM BASALTS: KEYS TO THE DEEP CRUST? Julian D. Hollis and F.L. Sutherland Department of Mineralogy and Petrology The Australian Museum, Sydney, N.S.W. Large Zircons (1mm - >2cm) are widespread in alluvials down the eastern highlands of Australia.

The ubiquitous sircon-illemenite-pleonastecorundum-magnetite assemblage can usually be demonstrated as coming from alkali basalts that have been erupted through granitic areas.

Zircon geochemistry supports diverse granitic origins, particularly peraluminous alkaline varieties. Six physical groups of large zircons are recognised: Group Name

Crystals

Colour Size mm

Host rocks and Locality

NEWBURY

101,100

Scarlet

0.5-17

Alk. ol. basalt, Newbury, Vic.

LYONVILLE

101,100,211

Orange

0.2-1

Bio-anorthocl trachyte, Blue Mountain, Vic.

INVERELL

Wide range

Sherry

1-25

Alk. ol. basalt, Inverell, NSW

101,110,211,100 Mauve-orange

1-12

Alk. basalt, Mt Moffatt, Qld.

NUNDLE

BOATHARBOUR

110,101

Scarlet

0.5-20

Basalt?, Boatharbour, Tas.

BULLENMERRI

101

Yellow-brown

1.5-12

Basanitic tuffs. Lake Bullenmerri, Vic

Pupin (1980) equates zircons having maximum pyramid-prism development with dry alkaline magmas of upper mantle origin that crystallised zircon at >850°C. He indicates an ordered sequence of forms that correspond to magma alkalinity and crystallisation temperatures.

Conversely, Caruba (1979) found no relationship between T. and zircon crystallography. Degrees of form development appear to be related to compositional changes during magma differentiation and cooling.

Xenocrystal zircons should help to identify deep crustal rocks produced by plate-marginal plutonism.

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PETROLOGY OF UPPER CRETACEOUS VOLCANIC ROCKS FROM THE CHATHAM ISLANDS, NEW ZEALAND Paul Morris Department of Geology and Geophysics University of Sydney, NSW 2006 Upper Cretaceous volcanic rocks on the Chatham Islands (900 km east of New Zealand) range from alkali olivine basalt to trachyte.

These rocks comprise flows, dykes, and localised pyroclastics, which outcrop on the southern one-third of the two main islands, Chatham and Pitt, with isolated flows on northern Chatham Island.

Field evidence, limited geophysics, petrography, whole rock and mineral chemistry, and subsequent computer modelling have been used to interpret these rocks in time and space.

It is inferred that the main volcanic sequence exposed on southern Chatham/Pitt Islands resulted from low-pressure mafic then felsic dominated fractional crystallisation of an alkali olivine basalt or hawaiite parent, with eruption from a vent system situated between the two islands in present-day Pitt Strait.

Variations in the REE and trace element chemistry of the least fractionated rocks, combined with Sr-isotope data suggest more than one period of melting of an isotopically homogeneous source, although identification of either the source and/or the degree of partial melting is obscured by the effects of low-pressure fractionation.

Symposium 2(c) Chemical and isotopic constraints on the evolution of continental crust Conveners: Dr W. Compston & Dr B.W. Chappell

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FIELD AND Sr-Nd ISOTOPIC CONSTRAINTS ON ARCHAEAN CRUST AND MANTLE EVOLUTION IN THE EAST PILBARA BLOCK, WESTERN AUSTRALIA K.D. Collerson & M.T. McCulloch Research School of Earth Sciences, Australian National University Canberra ACT Archaean geological relationships in the Eastern Pilbara Block are dominated by three main lithological units': (1) ovoid foliated granitic bodies up to 60 x 40 km in size, (2) extensive flanking volcanosedimentary sequences ("greenstone belts"), and (3) discordant posttectonic granites. The foliated granitic bodies are internally variable and complex. They are characterised by areas of compositely layered tonalitic-granitic gneiss containing metavolcanic and metasedimentary inclusions of supracrustal derivation. These crop out on scales ranging from 10's to 100's of meters in thickness and up to 10's of kilometers in strike length. Cutting these units are major bodies of tonalite, granodiorite and granite which generally exhibit a simple fabric and display structural relationships indicative of emplacement under synkinematic conditions.

Relationships between the greenstone belts and these gneissic domains are equivocal. Hickman and Lipple^ considered that the protolith of the oldest greenstone belt (Warrawoona Gp.)was deposited on a sialic basement. However, Glikson2 interpreted the Warrawoona Group as simatic oceanic type crust, and visualized the granites as younger anatectic partial melts. Recent structural studies by Bickle et al.^ along the western margin and central part of the Shaw Batholith have demonstrated that structural relationships between gneisses and greenstones involved an early period of recumbent folding ("D2") and thrusting, and a later period of upright folding ("Da") on N-S plunging axes.

This formed regional interference patterns, and controlled the subsequent development of N-S striking shear belts. In other areas (e.g. the western margin of the Tambourah or Western Shaw Greenstone Belt), gneissic granites have intrusive contacts with units which have strongly developed N-S striking S3 fabrics. These structural studies have provided a sound geological framework for interpreting the relationship between the greenstone belts, infolded supracrustal enclaves and the ovoid gneiss terrains. However, they have not as yet provided the critical field evidence necessary to resolve whether the greenstone sequences were deposited on sialic basement.

Studies by deLaeter and Blockley^, Pidgeon5>6^ Hamilton et al.^, Richards ^ al.8 and Cooper ^ al.9 have provided a geochronological framework for the Pilbara Block. In this paper, we present Sr and Nd isotopic data for gneisses and granites from the Shaw, Yule and Mt Edgar Batholith, with the aim of providing additional constraints on the early evolution of the terrain.

Sm-Nd whole rock isotopic data for the oldest identified layered gneissic components in the Shaw Batholith have insufficient spread in ^''^Sm/^'^'^Nd to yield an isochron. However, they exhibit a range of model T ^ h u R between 3260 and 3460 Ma which are significantly older than the T S U U T ^ age of a tonalitic gneiss with anastomosing schistosity from the northern part of the Shaw Batholith (3130 Ma). In the Mt Edgar Batholith,gneisses with a similar fabric give a slightly older range of Nd model ages (3280 - 3310 Ma). The Nd model ages are interpreted as

168

representing the primary age of formation of the gneiss protoliths.

This is based on the assumption that the gneiss protoliths were derived from chondritic Sm-Nd (CHUR) mantle reservoirs and have subsequently evolved as closed isotopic systems. The good agreement between U-Pb zircon age d a t a l O and a T^^yp^ age for a sample of an 3450 Ma gneiss supports this interpretation. Sr model ages calculated using analogous assumptions, with one exception, range from 3235 Ma to 3435 Ma for the older group of gneisses. This is in excellent agreement with '^CHUR gneisses with a more simple fabric give Sr model ages of 2950 Ma to 3290 Ma which are also consistent with the slightly younger Nd model ages for this group. Model ages for post-tectonic granites from the Shaw Batholith (Cooglegong Adamellite) and Mt Edgar Batholith (Moolyella Granite) give T^gyj^ ages (3200 and 3170 Ma, respectively) . These are significantly older than the age of 2500 - 2600 Ma indicated by Rb-Sr whole rock studies^j^^. Therefore, these bodies are clearly derived by melting of older sialic crust.

Samples of a dacite and rhyolite from the Duffer Fm. of the Warrawoona Gp. give Nd model ages of 3400 Ma and 3430 Ma respectively.

These are the same, within analytical uncertainty, as those obtained from the granitic-gneiss complex and are in agreement with the U-Pb zircon age of 3450 Ma from the dacite. Thus parts of the granitegneiss complex and the volcanic greenstones were formed contemporaneously from similar relatively undepleted reservoirs with CHUR-type (^Nd = 0.5 ±0.4) characteristics. This is somewhat surprising as the older Talga-Talga sub-Gp. of the Warrawoona Gp. was derived from a depleted source region (e^d = 1.8 ±0.3) at 3560 Ma.7 Extraction of magmas from a fixed volume of mantle would be expected to produce progressively more depleted m a g m a s T h i s suggests the involvement of markedly different reservoirs in the upper and lower parts of the Warrawoona Gp. This may arise from either heterogeneous mantle sources, or production of the Duffer Dacite magma by partial melting 100 Ma later of material equivalent to the Talga-Talga sub-Gp. The relationships between the greenstone terrain and the oldest gneisses remain equivocal. If the correlation of Hickman and Lipple^ is correct, viz. that the Tambourah Greenstone Belt comprises units above the Duffer Dacite, then the geochronological constraints are compatible with the oldest gneisses possibly forming a basement to the upper parts of the Warrawoona Gp. However, parts of the Warrawoona Gp. (Duffer Fm.) appear to be coeval with the oldest gneisses and the lower part of the Warrawoona GD. is older.

1. Hickman,A.H. & Lipple,S.L. 1978: Explan.Notes geol.Surv. W.A., SF50-8.

2. Glikson,A.Y. 1979: Earth Sci. Res, j^, 1-73.

3. Bickle,M.J., Bettenay,L.F., Boulter,C.A., Groves,D.I. &Morant,P. 1980: Geology, 8, 525-529.

4. deLaeter,J.R. & Blockley,J.G. 1972: J. geol.Soc.Aust., 10,363-370.

5. Pidgeon,R.T. 1978: Earth, planet. Sci. Lett. , 421-428.

6. Pidgeon,R.T. 1978: Proceedings 19 78 Archaean Geochem.Conf. Toronto, Ontario. 360-362.

7. Hamilton,P.J., Evensen,N.M., O'Nions,R.K., Glikson,A.Y. & Hickman,A.H. 1981: Spec.Pubis geol.Soc.Aust. , 187-192.

8. Richards,J.R., Fletcher,I.R. & Blockley,J.G. 1981: Mineral Deposits, 1^, 7-30.

9. Cooper,J.A., James,P.R. & Rutland,R.W.R. 1982: Precambrian Res., 18, 199-236.

10.Williams ^ This report.

11.deLaeter ^ al. Ann.Rep.geol.Surv.West.Aust., 1974, 73-79.

12.McCulloch,M.T. & Compston,W. 1981: Nature,294, 322-327.

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EARLY CRUSTAL COMPONENTS IN THE WESTERN AUSTRALIAN ARCHAEAN: ZIRCON U-Pb AGES BY ION MICROPROBE ANALYSIS FROM THE SHAW BATHOLITH AND NARRYER METAMORPHIC BELT Williams, I.S.I, Page, R.W.^, Froude, D.^, Foster, J.J.^ and Compston, W.l ^Research School of Earth Sciences, Australian National University Canberra ACT. ^Bureau of Mineral Resources, Canberra ACT. A major difficulty in dating the Archaean is to resolve primary igneous from metamorphic ages. The Sm-Nd method is more resistant to metamorphic changes than Rb-Sr but it is usually dependent upon a modelled source isotopic composition. Zircon U-Pb ages are much less modeldependent but zircons of different ages may not be separable when analyzing bulk samples. Dating single grains and cores within composite grains by ion microprobe obviates this problem.

This paper reports ion microprobe U-Pb dating of zircons from two samples critical to an understanding of the evolution of the Archaean crust in West Australia. The first is a homogeneous grey tonalitic gneiss from a pavement in Tambourah Creek near the western margin of the Shaw Batholith in the Pilbara Block. This sample is the oldest of several material components recognizable in the deformed gneisses at this exposure, The second is a banded gneiss from near Mt Narryer in the north-western area of the Yilgarn Block which has a model Sm-Nd age of 3630 ± 40 Ma (de Laeter^t al. 1981).

New orthodox U-Pb analyses of different zircon fractions from the Shaw Batholith gneiss average 60% discordancy and are little dispersed.

All fractions considered together scatter about a discordia chord having intercepts at 3300 ± 140 Ma and 1360 ± 150 Ma, but the least magnetic fractions can be fitted within error to a chord which intersects at 3560 ± 70 Ma and 1690±60 Ma. Normally 3560 ± 70 Ma would be taken as the original age of these zircons. However, analyses of individual grains by the RSES ion microprobe (SHRIMP) show that there are at least two different ages recorded by the zircons. Most analyses plot on a chord between 3485± 30 Ma and 1440 ± 110 Ma. Some are totally concordant within error and others are up to 90% discordant, thereby giving excellent definition of the younger intersection. In several grains, cores plot towards the upper intercept and rims plot towards the lower.

A minor, morphologically distinct component of the zircon population plots above the principal discordance line. Assuming that these zircons lost Pb at the same time as the remainder, their primary age is approximately 3200 Ma. This may record either a period of new zircon growth during metamorphism, or the age of a local igneous mobilizate. On the basis of the igneous-like morphology of the zircons, 3485± 30 Ma is considered to be the primary age of the igneous protoliths of the gneiss.

This is consistent with the Sm-Nd and earlier zircon U-Pb data. There is no evidence for a zircon component older than 3485 ±30 Ma, which suggests that the gneiss did not have a significantly older crustal precursor.

A major disturbance of the zircon U-Pb systems occurred at 1440 ± 110 Ma. No event which affected the region at that time has been identified by other isotopic studies.

170

ARCHAEAN CRUSTAL EVOLUTION OF THE DIEMALS A R E A , CENTRAL YILGARN BLOCK:

PB ISOTOPIC CONSTRAINTS

M . J . B i c k l e \ H . J . C h a p m a n ^ N . J . M c N a u g h t o n \ L.F. B e t t e n a y ^ D . I . Groves^ & J . R . de Laeter^

^Dept. of G e o l o g y , University of Western Australia, Nedlands 6 0 0 9 , W A ^School of Physics and Geosciences, Western Australian Institute of Technology, Kent S t , South Bentley 6102, W A

Pb-Pb whole-rock isotopic studies of Archaean granitic and gneissic rocks from the Diemals area in the Central Yilgarn granitegreenstone terrain places important constraints on crustal evolution of this area. The Pb isotopic data are consistent w i t h the hypotheses that the precursors to regionally extensive banded gneisses (Pb-Pb whole-rock age 2700±97 M a . Note: Errors quoted at 20; XRb = 1.42 x yr~^) represent possible source materials for the synkinematic plutons (Pb-Pb whole-rock ages of 2737±62 M a and 2700±100 M a ) . T h e synkinematic plutons are relatively enriched in heat producing elements, U , T h , K , and emplacement of these plutons at high levels within the crust implies that magmatic processes played the major role in crustal chemical fractionation in this area.

The Pb isotopic geochronology and isotopic compositions of banded gneisses, synkineraatic and post-kinematic plutons require that this crustal accretion-differentiation event was relatively short lived (<150 M a ) . Previous Rb-Sr geochronology in this area suggested a more extensive tectonic-magmatic history over at least -300 M a .

Discrepancies between whole-rock Pb-Pb and Rb-Sr ages are interpreted to imply resetting or later closure of the Rb-Sr systematics. More extensive perturbation to Rb-Sr systenatics is noted in whole-rock samples collected adjacent to a suite of pegmatites where whole-rock Pb-isotopic compositions record an age of 2682±53Ma, most probably the age of emplacement, but Rb-Sr isotopic compositions record an age of 2268±lA3Ma (initial ®'Sr/®®Sr = 0.99±.16) interpreted as reset. This study cautions against use of Rb-Sr isotopic studies alone to constrain the duration of Archaean crustal evolution and timing of component events.

Ion probe analyses of zircons from the M t Narryer gneiss are generally very close to concordant. T w o , possibly three, ages are represented.

Some Hole grains and cores of grains have a age of 3630±4 M a , some have an age of 3568±4 M a . More single grain analyses are required to establish whether or not these are distinct populations Some w h o l e grains and rims of grains have a ^o7pb/2oep^,* ^g^ ^f 3 3 1 2 1 4 Ma The zircon discordance pattern shows no evidence of any geological disturbance other than a minor recent loss of P b . 3630±4 Ma is in close agreement with the published Sm-Nd result. It suggests that the primary age of the gneiss-protoliths is 3630 ± 4 Ma and that the protolith was derived from a mantle source that had a chondritic composition. 3312 ± 4 Ma is identical to the published Rb-Sr total-rock age of the g n e i s s , and evidently denotes new growth of zircon at that time during recrystallization.

171

The zircon ages measured on both samples by SHRIMP agree closely with the earlier Sm-Nd results and suggest that the protoliths in both cases are not significantly older than earlier determined. The SHRIMP analyses do, however, provide direct, model independent and precise confirmation of the primary age of the two gneisses.

In addition, the present data confirm that the oldest known part of the Archaean crust in West Australia is at Mt Narryer, not in the Pilbara Block.

Reference: de Laeter, J.R., Fletcher, I.R., Rosman,. K.J.R., Williams, I.R., Gee, R.D. & Libby, W.G. Nature, 291, 322-4 (1981)

THE GEOCHEMISTRY OF GRANITOIDS FROM THE AGNEW DOME/ YII^ARN BLOCK. W.A.:

IMPLICATIONS FOR ARCHAEAN CRUSTAL GROWTH. J.D. Foden

Department of Geology and Mineralogy. University of Adelaide.

S.A.

The Agnew Dome is a complex batholith with at least three distinct granitoid groups, which intrudes the Lawlers - Mt. White greenstone belt. Group I. the tonalites, have high concentrations of LREE/ Rb. Sr and Ba and low concentrations of HREF and Y. '^^/Yh N ratios are high (100). The second group post-dates the first and comprises a suit of granodiorites, adamellites and trondhjemites and has flatter REE patterns than group I. The third and youngest group is a group of leucocratic, true granites. These have high K2O and Rb concentrations and very low 7r, Sr, V, Ba, Ti and MgO. They have flat REE patterns with variable Eu-anomalies. They are minimum melts formed in equilibrium with K-feldspar.

The tonalites appear to result from fusion of mafic rocks in the lower crust, leaving a garnet-poor^ two-pyroxene granulite residue.

Such a model however fails to account for the very high Ba. Sr. Rb and LREE contents of the tonalites at reasonable proportions of fusion (20%).

A model is proposed whereby the high incompatible element content of the tonalites are derived as a result of widespread scavenging of granulite regions of the lower crust. The component derived by this means may be trapped at the granulite-amphibolite boundary which will then respresent a source of contamination to plutons rising en-route to higher crustal levels.

The existence of three distinctly different granitoid suites at Lawlers appears to reflect the existence of a mixed crust pre-dating the 2.7-2.5 b.y. age of most central Yilgarn granite-greenstone associations.

It is suggested that large scale mantle-based igneous activity provides the source of heat required to initiate large scale crustal re-working (granitoid production) as well as contributing new mafic intrusive and extrusive rocks to the crust.'

172

A GEOCHEMICAL VIEW OF THE ARCHEAN-PROTEROZOIC BOUNDARY

S.M. McLennan & S.R. Taylor Research School of Earth Sciences, Australian National University, Canberra, A.C.T.

Clastic sedimentary rocks preserve a record of the chemical composition of the upper continental crust, for several elements which are relatively insoluble in natural waters. The distribution of the rare earth elements (REE), thorium and scandium have proven particularly useful and with the exception of first-cycle, volcanogenic sediments, there is remarkable uniformity for their trace element patterns in post-Archean sedimentary rocks. The average REE pattern of the present day upper continental crust, as estimated from sedimentary data, has La/Yb =13.6 and Eu/Eu^ = 0.65. Depletion in Eu results from its retention in Ca-plagioclase in the lower crust during intracrustal melting. The trace element geochemistry of sedimentary rocks is consistent with a granodiorite composition for the upper continental crust (1).

Archean sedim.entary rocks show fundamental differences in trace element distributions to post-Archean sedimentary rocks. REE patterns are highly variable with La/Yb ratios from <1.5 to >35.

Typically, these rocks do not possess Eu-anomalies and accordingly suggest that shallow intracrustal melting was relatively unimportant in differentiating the Archean crust. Trace element variations strongly suggest an origin by mixing of two components; end members are likely to be mafic volcanics and felsic igneous rocks (tonalites, trondhjemites, felsic volcanics), the components of the well documented Archean bimodal suite. The available data are consistent with typical exposed Archean crust being comprised of about equal proportions of the two end members and record an overall composition approximating to andesite (1) .

Plots of various trace element characteristics, for Australian sedimentary rocks, against time indicate a major discontinuity at the Archean-Proterozoic boundary. This is best exemplified by Eu/Eu-.

Ratios of highly incompatible elements to compatible elements (Th/Sc, La/Yb) also show this discontinuity and further record the less differentiated nature of the Archean upper crust. All of these data are consistent with no change in upper crustal composition during post-Archean times.

The change from Archean to post-Archean trace element signatures is recorded in early Proterozoic sedimentary sequences.

This is best documented in the Huronian Supergroup (2.6-2.2 Ae) on the north shore of Lake Huron, Canada. Here, sedimentary rocks from the base (McKim, Pecors Formation) display Archean-like REE patterns while those from the top (Gordon Lake Formation) have typical postArchean REE patterns (2). A similar pattern of REE evolution is also seen for early Proterozoic sedimentary rocks from the ca. 2.5-1.9 Ae Pine Creek Geosyncline, Australia (3). No simple pattern of REE evolution is seen for the Mount Bruce Supergroup of the Hamersley Basin (2.75-2.3 Ae) , however, most of this sequence is characterized by post-Archean REE patterns; thus the data are consistent with

173

a compositional break at the base of the succession. Such changes in sedimentary composition can be related to widespread production of K-rich granites during the Late Archean (2.7-2.5 Ae). These granitic rocks, which effectively form the upper crust, are characterized by negative Eu-anomalies. Isotopic evidence indicates that their source material, from which they were derived by partial melting, experienced only short crustal residence times. Accordingly there must have been a period of major crustal growth, as well as intracrustal melting at about the Archean-Proterozoic boundary (4).

In South Africa, the granitic activity proposed to be responsible for the change in upper crustal composition occurred much earlier (3.2-2.9 Ae). Accordingly, it is to be expected that sedimentary rocks from South Africa should also record the change in trace element characteristics much earlier. Sedimentary rocks from the Pongola Supergroup (3.3-2.9 Ae) show REE patterns strongly evolved toward those expected for the post-Archean and thus confirm this prediction. The South African data indicate that the period of crustal growth and differentiation related to the ArcheanProterozoic boundary is virtually episodic in anyone region but occurred over a protracted period world-wide during the period ca.

3.2-2.5 Ae. References (1)

Taylor, S.R. and McLennan, S.M. (1981) The composition and evolution of the continental crust: rare earth element evidence from sedimentary rocks. Phil. Trans. R. Soc. London^ Ser. A, 301, 381-399.

(2) McLennan, S.M., Fryer, B.J. and Young, G.M. (1979) Rare earth elements in Huronian (Lower Proterozoic) sedimentary rocks: composition and evolution of the post-Kenoran upper crust.

Geoohim. Cosmoohim. Aota^ 43^ 375-388.

(3) McLennan, S.M. and Taylor, S.R. (1980) Rare earth elements in sedimentary rocks, granites and uranium deposits of the Pine Creek Geosyncline. In: J. Ferguson and A.B. Goleby (Eds.), Uranium in the Fine Creek Geosynoline^ IAEA, Vienna, pp. 175-190.

(4) McLennan, S.M. and Taylor, S.R. (1982) Geochemical constraints on the growth of the continental crust. Joixr. Geol.^ 90^ 347-361.

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THE NATURE OF THE LOWER CRUST : CHEMICAL, ISOTOPIC AND SEISMIC VELOCITY CHARACTERISTICS R.J. Arculusl, M.T. McCullochl, B.W. Chappell^, S.R. Taylor^, J. Ferguson^, I. Jackson^. ^Research School of Earth Sciences, Australian National University, Canberra, A.C.T. ^Department of Geology, Australian National University, Canberra, ACT. ^Bureau of Mineral Resources, Canberra, A.C.T. The nature of the lower continental crust is currently a subject of strong controversy with different advocates favouring a variety of bulk compositions from silicic (>60% Si02) to basic (<53% Si02), and a range of tectonic models to account for the occurrence of their chosen rock type. The evolution of the crust has involved both major additions from the mantle, especially in the Archean and Proterozoic, and major reworking of crustal materials. Our understanding of the evolution of the upper crust is relatively well founded, given the representative sampling processes found in terrestrial weathering and sedimentation cycles (Taylor and McLennan, 1981). But it is obviously important to understand the composition, structure and evolution of the lower crust as far as any complementary relationship between lower and upper crust and the evolution of the crust-mantle system is concerned.

For a number of years, the majority view was that the lower crust must be of intermediate to silicic composition, possibly amphibolitized in localized patches, because of geophysical, geochemical and experimental petrologic arguments. These can be summarized by a) the strong compressional velocity break in seismic profiles at the Moho from about 6.5 to 8.0km sec"^ is incompatible with the presence of large amounts of basic granulite or eclogite in.the crust; b) a coherent model involving the uniformitarian lateral accretion of andesitic island arcs to continental nuclei, with subsequent intra-crustal differentiation can explain the evolution through time of upper-crustal-derived trace element patterns, and suggests a feldspar-rich, intermediate, andesitic residual lower crust; c) the extrapolation from relatively high P/T conditions of experimentally determined phase boundaries indicates that under most geothermal gradients, any basaltic material in the lower crust would be stable as eclogite, yet no evidence for the presence of this rock type is routinely found in seismic profiles. Furthermore, the gravitationally unstable situation of dense (p > 3.3gms cc"^) eclogite overlying less dense peridotite might be hard to preserve for geologically reasonable lengths of time.

More recent research in old shield areas tends to support this type of crustal model, as described by Drummond (1981) for the Pilbara and for a number of other Archean terrains by Smithson et al. (1981).

In fact, the latter authors argue that the bulk composition of these crustal sections requires crustal additions from the mantle that are more silicic than andesite (>60% Si02). However, other lines of evidence indicate that the continental lower crust is not composed everywhere of intermediate to silicic compositions. For example, suites of crustal derived nodules in kimberlitic and other types of explosive volcanism are typically basic in character (e.g. Southern Africa, southeastern Australia, Colorado Plateau and Front Ranges, south central France, northwest Africa and West Germany), and exposed sections of deeper crust are dominated by basic granulite (Fountain and

175

Salisbury, 1981). Some seismic profiles in regions where basic materials appear as xenoliths show that a velocity gradient, rather than a sharp break occurs between the lower crust and upper mantle. These profiles are compatible with the transformation at progressively deeper levels in the crust of relatively felspar-rich to garnet-rich basic compositions. Furthermore, laboratory-measured seismic velocities of the xenoliths coincide with inferred velocities in the depth range 2545km of about 7.0 to 8.0km sec"l.

There appears to be no doubt that the lower crust is diverse in character, and the key problems then are to understand the chemical, metamorphic and tectonic processes that have led to this heterogeneous state. We have been studying lower crustal xenolith suites from southern and eastern Australia and the Colorado Plateau. Chemical characteristics of some of these suites have been described (Lovering and White, 1969; Ferguson et al., 1979; Arculus and Smith, 1979) and broadly, the dominant lithologies are basaltic in character and mainly olivine-hypersthene-normative. Oddities in major, minor and trace element geochemistry do not support a simple origin via basalt underplating of or intrusions into the crust, cumulus processes or as restite following granite extraction. Multistage processes must be involved if the materials were once formed by such mechanisms. It should be noted however, that a number of authors do regard the granulite xenoliths found in southern Africa, France and northwest Africa as cumulates requilibrated in the lower crust (Rogers and Hawkesworth, 1982; Dostal et al., 1980). Rare earth element (REE) patterns span the range from relative light REE enrichment through unfractionated to light REE depleted. In the main, garnet does not appear to have been involved as a residual phase during any of the formation stages of these rock types. Characteristically, lower crustal granulites have high Sr, Ba and Pb abundances relative to light REE, depletions in U and Rb but erratic and occasionally high Th contents. Isochrons determined from Rb-Sr and Sm-Nd systematics indicate that the last equilibration events took place at about 2.5 billion years ago for South Australian suites, 1.7byr ago for the Colorado Plateau and 1.4byr for southern Africa. A recent phase change in these materials does not seem to have occurred and cannot be invoked to explain plateau and highland uplift (Smith, 1982). Long residence times in the lower crust are indicated and suggest dynamic instabilities can be preserved for these periods.

Intriguing problems are 1) What processes are involved in the formation of these lower crustal sequences? 2) is the granite-restite couple the best estimate we have of bulk lower crust composition? 3) has granulite (eclogite) been stripped from beneath some Archean terrains?

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GEOCHEMISTRY OF LOESS AND CONTINENTAL CRUSTAL COMPOSITION S.R. Taylor, S.M. McLennan and M.T. McCulloch Research School of Earth Sciences, Australian National University, Canberra, A.C.T. Loess deposits of Pleistocene age are widespread. Although it is occasionally suggested that they result from intense weathering episodes, the generally held view that they represent wind-blown deposits, derived from rock-flour produced by Pleistocene glaciers, is adopted here. The evidence for such an origin is particularly clear at the Banks Peninsula site. South Island, New Zealand. At this locality, thick loess deposits of granitic (S.L.) composition lie on the NW dip slopes of alkali basalt flows, facing the glacial valleys of the Southern Alps, 60 km NW. Prevailing NW Fohn winds carry large amounts of dust from these valleys even under prevailing climatic conditions.

The combination of glacial erosion, production of well-mixed rock flour, and its transport by wind over many kilometers provides a natural sampling process of the surficial crusts. A number of questions arise.

Is the sampling local? Is it biased? Are the more soluble elements removed? What is the winnowing effect on heavy versus light minerals? How important is grain size? What is the effect of post-depositional alteration on the abundances of the elements? Are the loess deposits sampling igneous or sedimentary rocks?

In order to test these ideas and to see whether loess deposits can provide us with a sampling of the surficial crust, we have obtained samples from Banks Peninsula, New Zealand, Nanking, China Kaiserstuhl, near Freiberg, Rhine Valley, Kansas, and Iowa, U.S.A. The major element compositions of loesses from all these localities is surprisingly uniform (expressed on a carbonate-free basis). The only samples containing major amounts of carbonate were the Rhine Valley samples. The New Zealand loesses have low K2O/ Na20 ratios (-0.9) reflecting derivation from greywacke terrains.

The other loesses have K20/Na20 > 1.5. The Kansas and Iowa loesses have Si02 contents about 80%; the remainder average 74%. These high silica contents probably reflect the ease with which quartz is degraded to silt size particles, which are readily transported by wind, hence biassing the compositions toward high Si02 values.

Zr and Hf abundances are very uniform. Th/U ratios are variable but generally less than 4, reflecting predominantly unweathered, rather than weathered source materials, as expected from glacial erosion. This interpretation is complicated by the mobility of U during post-depositional processes. La/Th ratios are constant, not unexpectedly for these relatively immobile elements. Even elements which are expected to be quite mobile in the surficial crust (Ba, Cs, Pb, Tl) are relatively constant and high. For example, Cs is typically 3-4 ppm and shows no sign of depletion. The REE patterns are notably uniform and are generally similar to PAAS (Post-Archean Average Australian shale).

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La/Yb ratios are 13(±3.6) compared with PAAS mean of 13.6.

ELREE/EHREE for the loesses average 9.4(±1.5) compared with PAAS values of 9.7.

Most notably, the Eu depletion as shown by Eu/Eu* is 0.67 ± 0.022 for the loess samples, which is indistinguishable from PAAS (0.65) NASC (North American Shale Composite) or ES (European Shale Composite Sample).

The equivalence of the REE patterns for the Banks Peninsula loess, derived mainly from the Torlesse Triassic-Jurassic greywackes of the Southern Alps, indicates that they are mature sediments, with europium depletions equivalent to shales.

The uniformity of the REE patterns for the widely scattered loess deposits and their similarity to PAAS, ES and NASC indicates that loess is providing the same information on REE abundances as clastic sediments.

These two independent sampling techniques thus

provide identical REE patterns, reinforcing the concept that both processes (glacial erosion and normal erosion and deposition) are providing an average sample of the upper continental crust.

The Nd and Sr isotopic compositions of the loess samples are variable with measured c^j(o) values of from -5.1 to -15.1 and ®^Sr/®^Sr ratios of from 0.70964 to 0.71850. Individual localities however have a restricted range with e ,(o) values of -5.1 and -5.6 for Banks Peninsula, -8.4 and -10.0 for Rhine Valley, -10.2 for Nanking, and -12.7 to -15.1 for Iowa and Kansas. Nd model ages calculated using depleted mantle parameters show a similar grouping.

The Banks Peninsula loesses have the lowest T^^ ages of 1060 m.y. and 1110 m.y., Nanking and Rhine Valley have intermediate ages of 1420 m.y. to 1510 m.y., while those from Kansas and Iowa have the oldest model ages of 1570 m.y. to 1700 m.y.

A surprising feature of these results are the relatively young Nd model ages. These ages are interpreted as the time of formation of the crustal segments from which the loesses were derived. The model ages therefore indicate a relatively young mean age of the upper crust (<1700 m.y.) or alternatively, the loesses are representative of localised regions. The latter explanation is clearly applicable to the Banks Peninsula samples which are biassed to young values by the young crustal formation ages of the local sources (Mesozoic greywackes) . The sources for the remaining loesses are not as well constrained but it is likely that they also dominantly reflect inputs of material from younger orogenic belts with high relief rather than flat cratonic regions. Equating sedimentary Sm-Nd model ages with mean crustal ages also assumes uniform sedimentary recycling, and accordingly such estimates will always provide a minimum mean age for the crust.

A final conclusion is that the composition of the upper crust has remained relatively constant since the Archean. This is shown by the fact that the REE abundance patterns are uniform despite the spread in model crustal ages.

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MODELS FOR GRANITOID EVOLUTION AND SOURCE COMPOSITIONS : RESTITE - R.I.P.?

J.D. Clemens^, V.J. Wall^ and B.D. Clarke^ ^Dept. of Earth Sciences, Monash University, Clayton, Vic., 3168. ^Dept. of Geol., Dalhousie Univ., Halifax, Nova Scotia, B 3H 355, Canada.

The geochemical and isotopic composition of granitoid suites contain important information on their progenitors and thus potentially on the development of the lithosphere involved in their formation.

According to the restite model of White and Chappell (1977) chemical variation in many granitoid suites results from the "unmixing" of restite (meltdepleted source material) from magmas comprising this and a melt fraction.

The model has far reaching implications for the petrogenesis of granites (e.g., the nature of granitic magmas, the character of and conditions in magma source regions, etc.).

In view of this, we suggest that a critical examination of the model is warranted.

Some granitoids (especially deep emplaced S-types) do contain a significant restite component; however, we contend that chemical variation in many granitoid suites is largely explained by source' related characters, crystalliquid fractionation and hybridisation processes.

Our evidence for this is: i)

The textures of xenoliths in many granitoids indicate their magmatic crystallisation - i.e. cognate inclusions or products of hybridisation.

Even where xenoliths are demonstrably non-magmatic (e.g. in S-types) they may not be representative of magma source materials.

Such xenolith populations may include wall rock material incorporated during ascent and emplacement.

Relatively refractory materials may be over-represented in source-related xenolith suites.

ii)

"Xenocrysts" (e.g. plagioclases with corroded calcic cores, pyroxenes, garnets and cordierite) commonly regarded as restite have magmatic explanations compatible with their textural and phase relations.

For example, garnets and pyroxenes in some granitoid suites have been shown by Clemens and Wall (1981, 1982) to have crystallised at relatively low pressures, well below pressures inferred for magma source regions.

The complex oscillatory zoning exhibited by plagioclases (and their cores) can only have originated by crystallisation from a silicate liquid.

Calcic plagioclase cores are commonly of compositions appropriate to near liquidus crystallisation as shown by the experimental studies of Clemens and Wall (1981).

In other cases plagioclase cores may result from hybridisation processes involving compositional and heating effects.

Early crystallised (near liquidus) phases can be difficult to distinguish from restite or refractory materials that have adjusted by interaction with the melt phase.

All of these may be accumulated in mafic variants of granitoid suites.

iii)

Near linear chemical trends - supposedly characteristic of restite unmixing are also exhibited by granitoid/volcanic suites which show no evidence for appreciable restite.

These

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trends can be largely accounted for by fractionationaccumulation and hybridisation processes (e.g. Vernon et al., this volume), which are also consonant with trace element and isotopic systematics.

Examples of the latter chemical variation mechanisms are given by Phillips et al. (1981), Vernon et al. (this volume) and Wall et al. (this volume).

Some implications for the evolution of granitic magmas and their source rocks are: i)

Evidence for restite controlled variation should be carefully assessed.

In particular, relatively Si02-poor (even xenolith rich) variants of granitoid suites cannot be assumed to be restite rich.

Such variants are commonly accumulates of early formed materials, distinct more mafic magmas or hybrids of the latter with more felsic magmas.

ii)

Some granitoids, such as those emplaced at comparatively deep levels, in metamorphic belts may represent low temperature melting fractions.

However few high level types are nearminimum melts and may depart substantially from haplo-granitic compositions.

These magmas are of relatively high temperature, deep crustal origin.

The development of such magmas has important implications for lower crustal compositions, conditions and processes (e.g. Wall et al., this volume).

iii)

Estimates of granitoid source rock compositions based on the restite model can be rather misleading, especially with regard to SiOa, Na20, K2O and CaO contents.

There is an urgent need for establishing the nature of "primary" granitoid magmas and modelling their derivation from a range of source materials.

iv)

Restite-rich granites may result from processes of magma extraction and ascent which differ from those responsible for restite-poor suites.

PETROGENESIS OF THE MURRUMBUCKA TONALITE:

A GRANITOID WITH BOTH

I- AND S-TYPE CHARACTER E.J. Reid,^ D. Stewart-Richardson^ and B.W. Chappell^ ^Department of Geology, University of Tasmania Beagle Street, Red Hill, A.C.T. ^Department of Geology, A.N.U. Canberra, A.C.T. Most mainland S.E. Australian granitoids fall into groups: i) those which can be generated by partial-melting of a crustal source (S-type granitoids) and ii) those which have upper mantle affiliations (I-type granitoids) (Chappell & White, 1974).

The Murrumbucka Tonalite is probably the best example of one small group of granitoids which have

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simultaneous I- and S-type c h a r a c t e r .

It is located on the ISline (loc.cit.) just to the north of the Cooma M e t a m o r p h i c C o m p l e x , Some features of this b o d y are: Feature

Granitoid-Type Character

Intruded w i t h i n an S-type granitoid O c c u r s in a h i g h - g r a d e m e t a m . b e l t Contains numerous hornblende-bearing xenoliths C o n t a i n s n u m e r o u s r e c r y s t a l l i z e d s e d . xenoliths Has a strong secondary foliation H o r n b l e n d e is common m a f i c p h a s e Has no m u s c o v i t e , aluminosilicates^ g a r n e t or c o r d .

Has (considerably) m o r e than 15 modal% b i o t i t e Has a c c e s s o r y allanite rather than m o n a z i t e Has ilmenite and no m a g n e t i t e Has a r e s t r i c t e d b u l k - c h e m i c a l c o m p . range Shows a h i g h degree of inter-element correlation N o sample has m o r e than 2.0 wt.% Na^O N o sample has m o r e than 206 p p m Sr M o l (Al203/(CaO+Na20+K20)) is less than 1.1 ®^Sr/®^Sr-initial ratio is 0.710 e^d is less than -8.6

I S I S

s I I

s I

s s I

s s I

s s

B a s e d on an evaluation of f i e l d , p e t r o l o g i c , and chemical data for this p l u t o n and drawing on known p h y s i c a l p r o p e r t i e s of siliceous silicate m e l t s , a m o d e l is p r e s e n t e d w h i c h c o m b i n e s (in v a r y i n g p r o p o r t i o n s ) the two m a g m a - e v o l v i n g p r o c e s s e s : restite-unmixing (e.g.

W h i t e & C h a p p e l l , 1977) and fractional c r y s t a l l i z a t i o n .

This dualp r o c e s s m o d e l p r o p o s e s t h a t suspended p r i m a r y (largely anhydrous) restite reacts w i t h m e l t at the level of e m p l a c e m e n t to form hydrous restite.

The r e s u l t i n g w a t e r b u d g e t d e f i c i t is m e t through simultaneous c r y s t a l l i z a t i o n of the m e l t :

T r a c e - e l e m e n t m o d e l l i n g (e.g. M c C a r t h y & H a s t y , 1976) has been used to p l a c e c o n s t r a i n t s on the nature and r e l a t i v e significance of each c o m p o n e n t m a g m a - e v o l v i n g p r o c e s s .

F i e l d , p e t r o g r a p h i c , c h e m i c a l and isotopic (RbSr & SmNd) data for the M u r r u m b u c k a T o n a l i t e and n e i g h b o u r i n g lithologies are e v a l u a t e d in the light of the r e l e v a n t m a g m a - e v o l v i n g p r o c e s s d i s c u s s e d a b o v e , in order to p r o j e c t b a c k to m a g m a - g e n e r a t i o n and the e n i g m a t i c source of this p l u t o n .

T h r e e m a g m a - g e n e r a t i n g m o d e l s are p r e s e n t e d and e v a l u a t e d : 1) S y n p l u t o n i c m i x i n g of b a s a l t i c and S - t y p e granitic m a g m a s , 2) P a r t i a l - m e l t i n g of c r u s t a l m a t e r i a l containing a s i g n i f i c a n t m a n t l e - d e r i v e d (e.g. volcanic) c o m p o n e n t and 3) T w o - s t a g e (progressive) p a r t i a l - m e l t i n g of crustal m a t e r i a l without a mantle-derived component.

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Some resulting implications for lithospheric dynamics and evolution of continental crust in S.E. Australia are: 1) That fractional crystallization probably accompanies restite-unmixing during granite-magma evolution, 2) That I-type granitoid characteristics are unlikely to be developed in a granitoid which has come solely from an S~type source (e.g. via progressive partial melting), 3) That magma-mixing of basaltic and granitic magmas may sometimes generate pluton-sized hybrids and that the IS-line may be the remnant trace of a link with the upper mantle or That the pre-Ordovician supracrustal rocks beneath S.E. Australia are either poor in mantle-derived components or that such mixed sources are generally too refractory to produce magmas, and further that the IS-line is either a narrow zone of high (palaeo) heat-flow and/or it traces a zone in which the pre-Ordovician succession contains significant mantlederived and calcareous sedimentary components.

REFERENCES Chappel, B.W., White, A.J.R. 1974 Pac. Geol., 8: 173-174.

Two contrasting granite types.

McCarthy, T.S., Hasty, R.A. 1976 Trace element distribution patterns and their relationahip to the crystallization of granitic melts, Geochim. Cosmochim. Acta, 1351-1358.

McCulloch, M.T., Chappell, B.W. 1982 Nd isotopic characteristics of S- and I-type granites. Earth Planet. Sci. Lett., 51-64.

White, A.J.R., Chappell, B.W. 1977 genesis. Tectonophysics,

Ultrametamorphism and granitoid 7-22.

THE GENESIS OF THE GRANITOIDS OF THE BLUE TIER BATHOLITH, N.E. TASMANIA N.C. Higgins, M. Solomon S R. Varne Department of Geology, University of Tasmania, Hobart, Tasmania.

Previous explanations for the genesis of the Blue Tier Batholith (BTB) have invoked in situ fractional crystallization, with development of crystal cumulates (McCarthy and Groves, 1979); fractional crystallization and/or restite unmixing (McClenaghan and Williams, 1982); and generation of plutons by partial melting of isotopically-distinct crustal source rocks, coupled with some simple mixing and/or fractionation processes (Cocker, 1982).

New geochemical data from the BTB, combined with Sr isotope data of Cocker (1982) provide constraints on previous genetic models.

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The BTB is composed of hornblende-biotite granodiorite (IR = 0.7061-0.7073), biotite granite (IR = 0.7070-0.7105), biotitecordierite-garnet granite (IR = >0.7119) and alkali feldspar granite (AFG) plutons.

The latter are intimately associated with Sn-W ore deposits.

In general the granodiorites are older than the biotiteand biotite-cordierite-garnet granites, and the alkali feldspar granites postdate all other granitoids.

In this study the Pyengana pluton (377 ma), the Poimena pluton (371 ma) and the Ansons Bay pluton (370 ma) represent the major rock types granodiorite, biotite granite and biotite-cordierite-garnet granite respectively.

The AFG are represented by the Anchor, Mt. Paris and Mt. William plutons (-370 ma).

The Pyengana, Poimena and Ansons Bay plutons are chemically distinct and each defines separate trends in major and trace element variation diagrams (e.g. Ti02, MgO, Zr, Rb, Sr vs Si02) suggesting within-pluton fractionation.

The AFG appear to be chemically related to their nearest field neighbours, i.e. the Mt. Paris and Anchor plutons with the Poimena pluton; the Mt. William with the Ansons Bay pluton.

Variation diagrams, combined with major and trace element modelling, provide some quantitative insight into the process of crystallization within these plutons. Fractional crystallization (=30-40%) of a solid composed of hornblende (73%), biotite (4%), plagioclase (22%), apatite and sphene (<1%) explain the geochemical trends observed in the Pyengana granodiorite.

The composition of this solid is similar to the composition of mafic xenoliths within the pluton suggesting they may represent fragments of cumulate rock.

Fractional crystallization (15-20%) of plagioclase (48%) K-feldspar (10%), biotite (41%) apatite, monazite and zircon (<1%) can account for the chemical variation in the Poimena pluton although the wide but coherent compositional bands observed in variation diagrams suggests incomplete separation of cumulate and liquid.

The high ^^Sr/^^Sr ratios of the biotite-cordierite-garnet granite plutons suggest they had a different source or followed a different emplacement history than the petrographically similar and temporally equivalent biotite granite plutons.

There is abundant field and chemical evidence for assimilation of Mathinna Beds country rock in the Ansons Bay biotite-cordierite-garnet granites.

Modelling suggests that the Ansons Bay granite could be generated by 30-40% fractional crystallization of the Poimena biotite granite combined with 10-20% assimilation of Mathinna Beds.

Preliminary isotope data support this hypothesis.

Alteration of the AFG masks their primary composition, however, the least altered samples plot as extensions of the Poimena and Ansons Bay trends suggesting they represent more evolved magmas derived by fractional crystallization (=35%) of biotite- and/or biotite-cordieritegarnet granite magmas.

This is supported by similar initial ratios for the least altered AFG (=0.7090) and the biotite granites.

Petrographic, chemical and Sr-isotope evidence suggests that the AFG are variably albitized soon after magmatic crystallization.

The degree of albitization is marked by variation from a quartz-feldspar phyric texture (least altered) to an equigranular texture (complete recrystallization), and enrichments in Li, Ga, F, Rb, U, Th and Sn and depletions in Sr, Ba and ZREE. In particular, the REE geochemistry emphasises the link between the AFG and the biotite- and biotitecordierite-garnet granites and effectively monitors the transition from

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magmatic to hydrothermal conditions during the cooling history of the AFG. In summary, geochemical modelling of the Pyengana granodiorite, the Poimena granite, the Ansons Bay granite and several alkali feldspar granites indicates that the following processes are important: (a) progressive upward melting of the crust to produce isotopically and chemically distinct granodiorite and granite magmas, (b) emplacement and in situ fractional crystallization, the end product of granite crystallization being alkali- and volatile-rich magmas (the alkali feldspar granites associated with the Sn-W deposits), (c) emplacement and fractional crystallization combined with assimilation of Mathinna Beds (the biotite-garnet granites), and (d) high temperature alkali metasomatism of the alkali feldspar granite followed by lower temperature greisenization and tin mineralization.

Restite unmixing and thermogravitational diffusion cannot be excluded but seem unlikely to be important.

The processes of crystal fractionation and assimilation cannot account for the compositional range of the alkali-feldspar granites, particularly the marked enrichments in Rb, Ga, Li and F and the depletion in Sr and REE. These features probably result from subsolidus metasomatism related to the evolution of a vapour phase.

REFERENCES Cocker, J.D., 1982. Rb-Sr Geochronology and Sr-isotopic composition of Devonian granitoids, eastern Tasmania. J. Geol. Soc. Aust. 29, 139-157.

McCarthy, T.S. and Groves, D.I., 1979. The Blue Tier Batholith, northeastern Tasmania. Contrib. Mineral. Petrol. 71, 193-209.

McClenaghan, M.P. and Williams, P.R., 1982. Distribution and characterization of granitoid intrusions in the Blue Tier area. Pap. Geol.

Surv. Tasm. 4, 32 pp.

A COMPARISON OF RESTITE CONTROL AND FRACTIONAL CRYSTALLISATION IN GRANITES D. Wyborn Bureau of Mineral Resources, Canberra.

The recent recognition of restite controlled fractionation in granitic magmas has given rise to a powerful tool for studying the composition of the lower crust. However accurate application of the method requires that little fractional crystallisation of the liquid has taken place, and some means of estimating the silica content of the average source material is also required. Most granitic magmas

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are derived at temperatures below SSC'C and under such conditions most of the Mg, Ki, Cr, Co, V, Ti, Ca, Sr and Fe are in the restite components. In these magmas the liquid contains over 70% Si02 and any fractional crystallisation will be of only minor importance compared to restite removal in changing the major element composition of the aagma. In relatively rare conditions higher temperature granite magmas wiiii liquid compositions as low as 60% Si02 and magma temperatures of over 1000°C may be generated in the lower crust.

Obviously applying restite unmixing models to determine the composition of the source will be greatly in error for such magmas so distinguishing criteria for them must be obtained.

The Early Devonian Boggy Plain Granitic Complex, south east N.S.W. is an example of a high temperature magma in which nearly all of its restite was left at the source. The complex was emplaced at a high level estimated on surrounding contact metamorphic assemblages to be 150 ± 50 mPa. Cumulate rocks (50% Si02) derived from the liquid are composed of plagioclase (An^Q), two pyroxenes and biotite and yield two pyroxene geothermometer temperatures of up to 1100°C. Inward fractional crystallisation produced progressively more felsic rocks and core compositions reached 75% Si02. Cryptic variation inwards occurs over the silica range 63% to 72%.

The most important characteristics of the Boggy Plain Granitic Complex which distinguish it from restite controlled granites are: 1.

2.

concentric zoning with some parts crypticly zoned. paucity of xenoliths and lack of correlation of xenolith abundance with mafic mineral content of the host.

3. decreasing K/Rb with increasing Si02.

4. high Ni and Cr suggesting derivation from a more mafic source.

High temperatures would be required before partial melting was high enough for magma extraction from a more mafic source, 5. minor element compositional zoning in mafic minerals such as increases in Mn, Fe3+, Sc, Ga, Li, Rb, Cs and W in biotites with fractionation.

6. plagioclase crystal zoning commonly more gradual from core to rim rather than the broad calcic cores and narrow albitic rims of plagioclases from restite controlled granites.

It is suggested that the source for the Boggy Plain magma was of gabbroic composition and was intruded into the base of the crust during the Ordovician. Evidence for this comes from 1.

2.

3.

low initial ratios indicating short crustal pre-history. chemical similarities such as high K, Sr and Ba to adjacent Ordovician basalts. the close spa'tial association of a meridional belt of similar high temperature Early Devonian granites and volcanics through central NSW with the Ordovician Molong Volcanic Belt.

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MAGMA MIXING IN THE DEVELOPMENT OF METALUMINOUS GRANITOID SUITES OF THE MORUYA BATHOLITH AROUND TUROSS HEAD, N.S.W. 1

R.H. Vernon , M.A. Etheridge

2

& V.J. Wall

2

ISchool of Earth Sciences, Macquarie University, North Ryde NSW 2Department of Earth Sciences, Monash University, Clayton Vic

Investigations of the field and microstruetural relationships of granitoid intrusive complexes can give insight into their physical and chemical evolution, and hence into processes involved in the evolution of continental crust. In this paper we examine such data, along with geochemical and petrological evidence, bearing on the development of metaluminous ("I~type") granitoid suites in the Devonian Moruya Batholith. Around Tuross Head gabbroid, dioritic, tonalitic and granodiorite variants are exceptionally well exposed. These show striking evidence for the presence of and interaction between several magma batches.

Contacts between most variants are commonly scalloped and may exhibit mutual chilling relations and even pillow structures, indicating contemporaneous intrusion of a range of fluid magmas at emplacement levels. Most variants contain a range of xenoliths, from mafic to leucocratic. These xenoliths have igneous, commonly chilled microstructures. Xenoliths in foliated granitoids may be very elongate, yet show no evidence of solid-state deformation—suggesting that both xenolith and host were in a fluid state. Geochemical and mineralogical data are compatible with mixing of various melt and solid magmatic components.

These data imply: (i)

Several dominantly fluid magmas were intruded essentially simultaneously. Relative ages inferred from contact relations are ambiguous.

(ii)

Xenoliths are essentially cognate, although they may relate to several magmatic variants or hybrids.

(iii)

Geochemical variation in the Tuross suite is better interpreted as due to fractionation and magma mixing processes, rather than to restite-controlled variation.

(iv)

The Tuross suite developed from high-temperature magmas of deep crustal derivation, with possible Devonian mantle components.

We suggest that our observations and conclusions may have application to other "I-type" suites, and that, in view of the relatively short time-span involved in the intrusion of many batholithic suites, magma mixing may be expected to be relatively common.

186

IMPLICATIONS OF VARIATIONS IN ISOTOPIC COMPOSITION OF PHANEROZOIC PLUTONS IN THE WESTERN UNITED STATES TO EVOLUTION OF CONTINENTAL CRUST

Ronald ¥. Kistler U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025 Permian (270 m.y.) to Miocene (14 m.y.) plutons in the western United States have the following isotopic characteristics: (1 ) Initial 87Sr/86Sr values from 0.7035 to 0.7572; (2) Initial 143Nd/l44Nd [eNd(T)] values from +7.8 to -17.3; (3) 206Pb/204Pb values from 16.2 to 19.8 and 207Pb/204Pb values from 15-3 to 15.8; and (4) 6 180 SMOW from +0.6 to 13»0. All isotopic systems show regular geographic variations that are independent of the ages of the plutons investigated.

The boundary between plutons with initial 87Sr/86Sr values greater than 0.7060 and less than 0.7060, interpreted to approximate the western limit of Precambrian sialic crust, is now well defined in the region. This isotopic boundary is approximately coincident x^ith the boundary between Paleozoic eugeosynclinal and miogeosynclinal rocks in the area.

Neodymium isotopes correlate well with strontium isotopes; positive eNd(T) values occur in plutons with low initial 87Sr/86Sr values and negative £Nd(T) values occur in plutons with high initial 87Sr/86Sr values. Oxygen isotopes have no simple correlations with the other isotope systems, but in some areas correlate well with Pb isotopes.

Mantle and crustal xenoliths contained in a Tertiary volcanic neck that intruded the central Sierra Nevada batholith have strontium, neodymium, and oxygen isotopic values equivalent to the entire range of these isotopic values in the plutonic rocks investigated in the western United States. This indicates, that in spite of massive melting events during the Mesozoic to produce the Sierra Nevada batholith, significant proportions of the underlying continental crust remain relatively unchanged in this area.

Combined strontium, neodymium, and oxygen isotopic characteristics of plutons with initial 87Sr/86Sr values less than 0.7060 are compatible with these granitoid rocks having been derived from basalts of ophiolite sequences. These same isotopic characteristics in plutons with initial 87Sr/86Sr values greater than 0.7060 are compatible with these granitoid rocks having sources of mafic lower continental crust about 1700 m.y. old, altered oceanic basalts, or mixtures of mantle-derived basalts and sediments with a cratonal provenance. Scattered muscovite-bearing granitoids with initial 87Sr/86Sr values greater than 0.7100, 5180 values greater than +10.0, and £Nd(T) values of -10 to -17 could have been formed from source materials composed entirely of continentally d eri V ed s edim ents .

Note change of title from that given in Program

187

Nd AND Sr ISOTOPE RELATIONSHIPS IN PALEOZOIC GRANITIC ROCKS OF SOUTH EASTERN AUSTRALIA. M.T. McCulloch, B.W. Chappell, H.D. Hensel and W. Compston The Australian National University, Canberra

ACT.

As part of our continuing program of isotopic and geochemical studies of granites from south-eastern Australia, the initial Nd and Sr isotopic composition has been determined for - 100 whole-rock samples from four major batholiths. These are the New England Batholith of the New England Fold Belt and the Kosciusko, Berridale and Bega Batholiths of the Lachlan Fold Belt. Rb—Sr and K—Ar mineral and whole-rock ages indicate that plutonism ranged from 380 to 420 Ma in the Kosciusko, Berridale and Bega Batholiths and from 260 to 310 Ma in the New England Batholith.

Batholiths from the Lachlan Fold Belt have an extremely large range in isotopic compositions whereas the New England Batholith has a relatively restricted range. The S-type granites from the Berridale and Kosciusko Batholiths of the Lachlan Fold Belt form a nearly horizontal array with a wide range of initial ®^Sr/®^Sr (0.709 to 0.718) but a relatively restricted range of negative e^d values (-6 to -9.8). These variations are attributed to derivation of the granites from predominantly Proterozoic ( - 1500 Ma - 1800 Ma) sources that have been through at least one cycle of weathering on the earth's surface. This has resulted in a large and somewhat variable fractionation of Rb relative to Sr, and a concomitant increase in ®^Sr/®^Sr. I-type granites from the Lachlan Fold Belt have the largest range of e^^ values (+4.4 to -9.0) which is strongly correlated with initial ®'^Sr/®^Sr (0.704 to 0.710). Although this correlation can be explained by mixing between a juvenile arc-like component and a relatively mature sedimentary S-type component, this explanation is not compatible with major and trace element data.

Instead the covariation of the Lachlan I-types is attributed to predominantly igneous protoliths having a spectrum of mantle derivation ages. Using a depleted mantle model, these ages (T^d) range from 400 Ma to 1600 Ma. Although both S- and I-type suites are present in the New England Batholith, they have a substantially more restricted range and overlap in Nd and Sr isotopic compositions with e^d of from - 1.7 to +4.6 and initial ®^Sr/®^Sr of from 0.7035 to 0.7061. The lack of strongly negative values indicates an almost complete absence of any Proterozoic or older material in the granite sources. This implies a unique tectonic setting for the New England Batholith.

Abundant Devonian or older volcanogenic sediments indicate that island arc systems existed to the west of the present New England Batholith and this may have prevented the influx of Precambrian material from the Australian craton. Isotopic and geochemical analyses of these island arc derived sediments indicate that the more pelitic components have a suitable composition for production of the S-type granites. The relatively primitive isotopic character of the New England S-types (e^d = -1.0 to +2.0 and ®^Sr/®^Sr = 0.705 to 0.706) is in sharp contrast to the Lachlan S-types and reflects both younger provenance ages and a shorter duration between weathering and granite plutonism. The isotopic data indicate that the proportion of recycled pre-existing continental crust in the granite sources is

188

highly variable. For example, the Lachlan S-type granites consist of essentially 100% recycled Proterozoic crust whereas those from New England are composed of dominantly new (i.e. Paleozoic) material.

0.702

0.7U

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0.718

NEW ENGLAND BATHOLITH (265-310 m.y.)

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^Nd _

l-Types (400m.y) S-Types (412m.y) BEGA BATHOLITH (400 m y ) l-Types -30

0

100

50

150

200

Sr Comparison of the initial Nd and Sr isotopic ratios of granites from the New England and Lachlan Fold Belts.

GRANITOID BATHOLITHS AND CRUSTAL EVOLUTION: IMPLICATIONS FROM A Nd-Sr ISOTOPIC AND U-Pb ZIRCON STUDY OF PENINSULAR MALAYSIAN GRANITOIDS T.C. Liew^, M.T. McCulloch^, R.W. Page^ & B.W. Chappell^ ^ Australian National University, Canberra ACT 2 Bureau of Mineral Resources, Canberra ACT Granitoid batholiths sample large volumes of crust and as such provide bulk images of middle and lower continental crust. Sm-Nd and U-Pb isotopic studies of batholiths can place constraints on the generation and evolution of these deep crustal regions.

Granitoid batholiths in Peninsular Malaysia are divided into an East Coast and a West Coast Province by a north-south trending medial collision suture. The East Coast Province batholiths are of Permian

189

to mid-Triassic age. They are predominantly I-type granitoids and resemble modern-day subduction-related continental margin plutonic arcs.

West Coast Province batholiths consist exclusively of felsic S-type plutons and represent the plutonic expressions of the late-Triassic continental collision between the Indochina Block to the present-day east and a large 'missing' craton to the present-day west.

U-Pb zircon dating of individual plutons belonging to a number of suites from the West Coast Province give precise ages ranging from 198 - 220 Ma. On a Concordia diagram, the zircons are either concordant or define short reverse discordance patterns. The concordant zircons correspond to emplacement ages while the reverse discordia are interpreted as being due to incorporation of old inherited zircon components from the granitoid source regions. The geological consistency and concordance of much of the U-Pb data suggest insignificant post-emplacement Pb loss. Consequently the upper intercept ages of the reverse discordia (1500 - 1800 Ma) may broadly represent the ages of the older components.

Initial and ratios for plutons of the West Coast Province define a sub-horizontal array on the G^d " ^Sr diagram (Fig. 1) with GNd = -6 to -10 and esr = +120 (.713) to +620 (.748). ^^ Nd source ages calculated using a depleted mantle evolution model (T^^) range from 1300 - 1800 Ma. These ages are generally in good agreement with the U-Pb zircon inheritance ages. Basement of this age has not been recognized in Peninsular Malaysia.

I-type granitoid plutons and scattered mafic plutonic bodies from the East Coast Province show an overlapping range of £Nd (-0.8 to -6) and esr (+10 to +80) values. T^^ ages range from 900 - 1400 Ma. A mafic I-type granodiorite with e^d = "6 has zircons defining a reverse discordia with an upper intercept corresponding to ^^^800 Ma. Zircons from an I-type adamellite (e^d = -1.5) define a normal discordance pattern with an upper intercept indicating an emplacement age of 230 Ma. The variable incorporation of old zircons and the e^d " ^Sr relationships suggest that these I-type granitoids represent complex and variable mixes of mantle and infra-crustal derived magmas. A volumetrically minor group of felsic S-type granitoids is present in the East Coast Province. These have BNd values of -3.5 to -6 (T^g = 1000 1700 Ma) and esr from +60 to +120. A reverse discordia with an upper intercept of ^ 1300 Ma and a lower intercept emplacement age of about 260 Ma are defined by zircons from one of these plutons.

Nd The general agreement of U-Pb zircon inheritance ages and Tj^^ ages are interpreted to correspond to Proterozoic 'crust formation' ages of the continental segments represented by the East and West Coast plutonic provinces. On an e^d ^^ time diagram (Fig. 2), Phanerozoic continental margin plutonic-volcanic belts show a ubiquitous crustal end-member component with Tg^ ages of 1300 - 1900 Ma. This relationship suggests that lower and middle continental crust tapped by voluminous Phanerozoic continental margin magmas may represent midProterozoic juvenile additions to the continental crust. The ubiquitous mid-Proterozoic 'crust formation' ages and the absence of Archaean signatures at continent margins imply that many continental blocks had reached their present sizes by the mid-Proterozoic.

Crustal accretion processes by Phanerozoic arc magmatism may have added little to the growth of continents.

190

References AlLegre, C . J . and O t h m a n , D.B. (1980). Nd-Sr isotopic relationships in granitoid rocks and continental crust development: a chemical approach to o r o g e n e s i s .

N a t u r e , v . 286, 335-342.

DePaolo, D.J. (1981). A Nd and Sr isotopic study of the Mesozoic caIc-alkaline granitic batholiths of the Sierra Nevada and Peninsula R a n g e s , C a l i f o r n i a .

Jour. G e o p h y . Res.,86, 10470-10488.

Hamilton, P . J . , O ' N i o n s , R.K. and Pankhurst, R.J. (1980). Isotopic evidence for the provenance of some Caledonian granites. Nature, v . _87, 279-284.

Hawkeswroth, C.J. (1979). ^''^Nd/^ "''Nd, ®^Sr/®^Sr and trace element characteristics of magmas along destructive plate margins. In: Origin of Granite batholiths (Eds. A t h e r t o n , M . P . and Tarney, J . ) , 76-89, Shiva Publishing L t d . , U . K .

M c C u l l o c h , M . T . and Chappell, B.W. (1982). Nd isotopic characteristics of S- and I-type granites. Earth Planet. Sci. Lett. 58, 51-64.

100

300

40Q Fig. I. Nd and Sr Initial ratios of Phaneroroic continental margin granitoids and their volcanic equivalents. primitive island arc basalts Ecuador Andean (Hawkeaworth. 1979>.

Peninsula Ranges batholith „ , Sierra Nevada batholith ^^^^ Peru-Chile Andean (Ailegre (. Othman, 1980, Hawkesworth, 1979).

British Caledonian Younger Granites (Hamilton et al 1980). ' Lachlan I- and S-type granites (McCulloch & Chappell, 1982) French Hercynlan (Ailegre & Othman, 1980) British Caledonian Older Granites (Hamilton et al, 1980).

Himalayan leucograni tes (.Mlegre & Othrear > MEC, MWC - Peninsular Malaysia East Coast and West Coast Provinces Solid bar denotes ' mantle array'.

Fig. 2.

i

i1 ^

M I L ^ / •hm

12

^^^ 1 «

20

. 2 ^ 6a

-Nd — Phanerozoic rocks listed in Fig. I. Abbreviations as for rig. 1 except AD - combined Andean data.

Continuous and dashed diagonal lines correspond to the evolution of probable crustal end-members of Phanerozoic continental margin aagmatic belts with most negative and least negative aeasured . Intersections of these lines with depleted aiantie evolution band (stippled) yield T^jJ ages.

191

CHEMICAL AND ISOTOPIC CONSTRAINTS ON THE SOURCE REGIONS OF THE KALKADOON AND EWEN BATHOLITHS, MOUNT ISA INLIER AND THEIR IMPLICATIONS FOR EARLY PROTEROZOIC CRUSTAL EVOLUTION IN AUSTRALIA L.A.I. Wyborn and R.W. Page Bureau of Mineral Resources, Canberra

ACT

The I-type Kalkadoon and Ewen Batholiths and their comagmatic extrusive equivalents, the Leichhardt suite, form an association covering at least 5000 km^ in the central Mount Isa Inlier. U-Pb zircon data and selected Rb-Sr total rock data show that these rocks crystallised from melts emplaced from 1840-1870 m.y. ago and are the oldest known igneous rocks,in the Inlier. Chemically and isotopically these granites are relatively uniform and, compared with most other Mount Isa granites, they have higher Sr and AI2O3, and lower Ti02, Zr, Nb, and Th. The source for the rocks of the Kalkadoon-Ewen-Leichhardt association was fairly fractionated and is estimated to have had an Si02 content of 55-60%. Relative to other large Palaeozoic and Mesozoic I-type batholiths, this Mount Isa association was enriched in K2O, Rb, Th, U, La, Ce, Zr, and Nb, and depleted in CaO, MgO, Ni, and Cr. The least disturbed Mount Isa granites have relatively low initial 87sr/S6sr ratios (about 0.704), and combined with their high Rb/Sr ratios it is implied that the age of the source for these melts was not much older than the age of their emplacement.

Large chemically and isotopically similar granite batholiths occur in other Proterozoic terrains of Australia and were emplaced between 1800 and 1920 m.y. Representatives include the Nicholson Granite Complex of the Murphy Inlier, Nimbuwah Complex of the Pine Creek Inlier, and the Bow River Granite of the Halls Creek Inlier. Other possible examples occur in the Arunta Inlier, The Granites-Tanami Block, the Tennant Creek Inlier, the King Leopold Belt, and the Gawler Block. It is inferred from chemical and isotopic data from these felsic melts that during the period 1900-2000 m.y. a significant mantle differentiation event took place, during which large volumes of material were accreted to the base of the crust.

In the Australian early Proterozoic it is believed that the accretion event was associated with rifting or extension. All terrains in which this 1900-2100 m.y. source is known or inferred to occur are now gravity highs which may suggest the presence at depth of fairly dense mafic material.

192

Nd AND Sr CHRONOLOGY OF STRANGWAYS RANGE GRANULITES: IMPLICATIONS FOR CRUSTAL GROWTH AND REWORKING IN THE PROTEROZOIC OF CENTRAL AUSTRALIA D.P. Windrim

1

& M.T. McCulloch

2

^Department of Geology, Australian National University Canberra ACT ^Research School of Earth Sciences, Australian National University Canberra ACT. Determination of the time difference between formation of Precambrian continental crust and subsequent high grade metamorphism of such crustal material is fundamental to understanding of both the crust-mantle relationship and the evolution of continental crust.

With this in mind, the isotopic composition of Nd and Sr in granulite facies rocks from the Strangways Range in the southern Arunta Block has been used to identify the time relationships between crustal growth and reworking events in the Proterozoic of Central Australia.

The Strangways terrain comprises well layered supracrustal rocks ranging in composition from ultramafic to rhyolitic, and the 'sequence' includes both metasedimentary and metaigneous units. The Nd isotopic data for all rock types indicate that this crustal segment differentiated from slightly LREE-depleted mantle 2015 ± 120 Ma ago (eNdC^) = +1.4, +0.7/-0.6). This result is supported by an age of 2085 ± 175 Ma for mafic granulites ^Sr/®^Sr(I) = 0.70249 ± 19). Taken together, these data indicate that the major crust forming event in the southern Arunta Block occurred at ^2000 Ma. There is no evidence for significantly older crust.

Rb-Sr data for felsic and calc-silic^te granulites showing minimal effects of retrogression define an age for granulite facies metamorphism of 1800 ± 25 Ma ^Sr/®^Sr(I) = 0.70722 ± 20). Field and petrographic observations indicate that this regional thermal event was preceded by intense deformation and major crustal shortening; thus, within a period of 200 Ma, material derived from the upper mantle had differentiated into the entire range of upper crustal rock types and had undergone a major intracontinental collision event which culminated in deep crustal metamorphism at ^1800 Ma. This history of rapid crustal differentiation and subsequent orogenesis is similar to that of several Precambrian terrains, for example, the Western Yilgarn Block (W. Australia), Lewisian of N.W. Scotland, Cape Smith Fold Belt (N. Canada) and Front Range terrain (Colorado), but differs markedly from that of other Precambrian terrains such as the Fyfe Hills (E. Antarctica), Singhbhum gneiss terrain (E. India), Lofoten-Vesteralen basement (N. Norway), and the Guyana Shield. These latter terrains reveal large (600 - 1400 Ma) time intervals between crust formation and high grade metamorphism and/ or tectonism. However, a better understanding of the relationships between deformation and polymetamorphism in these terrains is required before the significance of these variable time differences can be fully evaluated.

Granulite facies metamorphism in the southern Arunta Block was rapidly followed by widespread development of amphibolite facies mylonitic belts along which the granulites were uplifted, hydrated, and in

193

places partially melted. This major phase of retrogression occurred at 1730 - 1650 Ma under amphibolite facies conditions and was pervasive in the southern Arunta Block. Mineral and whole rock data document further retrograde events at 1400 and 1100 - 900 Ma, and finally, partial Sr isotope rehomogenisation at 350 Ma which resulted from the mild thermal effects associated with intense deformation of the southern margin of the Arunta Basement and formation of the Arltunga Nappe Complex. The Central Australian granulites record evidence, therefore, of episodic tectonothermal events over a period of ^1500 Ma. Although the crust in this region must have had a thickness of 35 km since 1800 Ma ago, and was effectively stabilised at that time, the continual mobility of the crust requires longlived thermal anomalies in the underlying mantle. This suggests that major, mantle-rooted tectonic features (geosutures) may have been an important control on initial localisation of crust-forming magmatism and subsequent tectonic and thermal reworking of this crust.

This type of tectonic control is consistent with the nature of geosutures in the southern Arunta Block: these major features appear to originate in the mantle, are marked by large displacements and striking gravity and magnetic anomalies, and separate large crustal blocks differing in composition, density, structural state and metamorphic character. These faults have been active for at least 1600 Ma and may represent the modified expressions of initial mantle perturbations which controlled both formation and destruction of Proterozoic crust in Central Australia. The rapid convergence of continental segments across such major boundaries shortly after crust formation may be a characteristic feature of early Proterozoic plate tectonic regimes.

194

Symposium 3(a) Palaeoenvironments and lithosphere dynamics (geomorphicdepositional, bathymetric, climatic) Convener: Dr J. Keene

195

CARBONATE SHELF SEDIMENTATION IN THE MIDDLE PROTEROZOIC BANGEMALL BASIN, WESTERN AUSTRALIA R.G. Chuck and B. Tavcar.

Alcoa of Australia Limited, PO Box 291 Applecross, WA The Bangemall Basin represents the final phase of sedimentation in a tectonically active zone which developed during the Proterozoic between the Pilbara and Yilgarn Archaean Blocks.

In the western half of the basin the Bangemall Group can be subdivided into three broadly similar sedimentary packages, each produced by its own carbonate-dominated shelf and turbidite-dominated basinal depositional environments. Erosion has removed most of the shelf sequences of the upper two cycles, but first cycle carbonate shelf sediments are exposed for several hundred kilometres along the basin's linear northern margin.

In the Irregully Creek area this basal carbonate sequence contains three distinct depositional units arranged vertically as follows : (1) basal red beds : red coloured terriginous clastics and algal dolomites representing alluvial fan and marginal marine environments, (2) interbedded unit : algal and clastic dolomites, wave rippled quartz arenites and mud cracked shales of tidal flat origin, and (3) carbonate complex : diverse terriginous-free carbonate facies of tidal flat and sub-tidal platform edge and foreslope environments.

We believe that these units represent three separate tectonosedimentary events reflecting both waning hinterland tectonism and progressive transgression.

Palaeogeography of the basin margin during the first two stages was dominated by fluvially-influenced tidal flats which produced sequences characterised by vertical complexity but lateral continuity.

As tectonism waned and terriginous input declined, uninhibited carbonate sedimentation produced more complex shelf morphologies.

Lithosomes of this stage include intertidal algal, tepeed and clastic carbonates, and subtidal algal, clastic and pelagic carbonates.

A feature of the carbonate complex is the presence of mounds of dololutite in the subtidal shelf-edge and foreslope environments.

These mounds are internally massive, sparry and virtually lacking in evidence of algal binding.

Their outer surfaces are steep, and inter-mound facies include algal laminated bindstones and clastictextured dolomites, including breccia aprons composed of angular fragments of mound rock. Platform-edge mounds are capped by stromatolitic bindstone, but foreslope mounds lack this facies suggesting a possible sub-photic position. These mounds closely resemble the Waulsortian "reefs" of the Carboniferous of western Europe, but to our knowledge carbonate build-ups of this type, which lack internal organic frameworks, have not previously been recorded from sediments of this age.

Their presence here therefore suggests that their formation is not necessarily dependent on skeletal or framebuilding organisms.

196

SEDIMENTATION OF THE EARLY EDIACARAN, FLINDERS RANGES, SOUTH AUSTRALIA. Victor A. Costin and Richard J.F. Jenkins Department of Geology, University of Adelaide, S.A. The Wilpena Group of the Flinders Ranges comprises two major sedimentary cycles, completing Precambrian deposition in the region (Fig. 1). Both cycles begin with thin dolomites which are locally of shallow water origin, overlain by basinal shales and turbidites indicating rapid subsidence. Subsequent parts of each cycle show progradation from turbidites to shelf sediments succeeded by sandstones of shallow tidal origin. Transgressions then led to formation of a starved basin (Bunyeroo Fm.), or open marine deposits of tidal and shelf origin (Rawnsley Quartzite).

The older cycle equates with the upper part of the Marinoan "Series" (Mawson and Sprigg, 1955, Aust. J. Sci. 13, 69-72) in its type area, immediately south of Adelaide, and extends throughout the Adelaide Geosyncline and the adjacent Stuart Shelf. The younger cycle is limited to the Flinders Ranges and has been nominated as the Ediacaran System (Jenkins, 1981, Trans. R. Soc. S. Aust. 105, 179-194). The latter includes the Ediacara assemblage and older indications of megascopic life, and reaches a cumulative thickness in excess of 6 km in the north-eastern Flinders Ranges. Cloud and Glaessner (1982, Science 217: 783-792) include both sedimentary cycles within their recently defined Ediacarian System.

Our new sedimentary investigation has concentrated on the transitional interval between the two cycles in the type area of these nominated Systems: Bunyeroo Gorge. Overlying the siliciclastic Brachina Subgroup are the extensive uniform shales of the Bunyeroo Formation, including a significant euxinic interval. A thin cupriferous dolomite (Wearing Dolomite equivalent) marks the base of the redefined Wonoka Formation and the start of the Ediacaran sedimentary cycle. This widespread dolomite locally contains beach conglomerates and stromatolites indicating shallow water. It is overlain by terrigenous turbidites that grade into distal shales and a basinal euxinic facies. These units indicate tectonic instability and rapid subsidence which initiated the cutting of very deep (1100m) submarine canyons into intrabasinal highs and basin margins. The regressive basin-filling stage is seen in the coarsening upward calcareous turbidite fan deposits, shallower slope pelloidal carbonates with slump beds and limestone slab conglomerates. These are capped by shallow marine and paralic sediments including oolitic and stromatolitic limestones, dolomicrites, red and grey sandstones, and lagoonal shales. The overlying Bonney Sandstone (Pound Subgroup) consists of coastal red mudflat, sandflat, and deltaic deposits.

The sedimentary cycles within the Wilpena Group resemble major repeated cycles recognized within classical geosynclines, suggesting that the depositional regimen of the Adelaide Geosyncline has been more strongly influenced by episodic tectonism than previously realised. Of the two cycles, the Ediacaran reflects the more intense tectonic influence; its base is at a useful regional marker sandwiched between conformable deep basinal deposits.

197

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198

THE DEEP, THE DARK AND THE DIRTY AN ORDOVICIAN SHELF TO BASIN TRANSITION IN SOUTHERN TASMANIA, AUSTRALIA Clive Burrett^, Bryan Stait^, and John Laurie^ ^Geology Department, University of Tasmania, Hobart, Australia 2 Bureau Mineral Resources, Canberra, A.C.T, Most of the major sections of Gordon Group carbonates (of LowerUpper Ordovician age)l were deposited in a dominantly peritidal regime that supported a strongly endemic fauna. We have recently studied Gordon Group sections in southern Tasmania that reveal a transition from typical shelf sediments and faunas to deep water carbonates and faunas.

2 Precipitous Bluff section consists of 130m of peritidal dolomicrites and possibly biohermal biocalcarenites (the New River Beds) containing corals and Calathiwn of Chazyan-Blackriveran age, overlain by 230m of subtidal siltstone/calcareous mudstones, and argillaceous limestones(The Precipitous Bluff Beds) containing bryozoan and trilobites of "Trentonian" age and brachiopods that occur in Fauna III (Eastonian) of N.S.W. Five kilometres south, the Prion Beach Beds contain a trinucleid (gen.nov. Banks) a raphiophorid and brachiopods found in N.S.W. Five kilometres southeast of Surprise Bay we have measured a section of more than 300m of alternating dark grey micrites and thin, often graptolitic, shale beds with more lensoidal, crinoidal, graded biocalcarenites (probably limestone turbidites) and phosphatic ironstones (the Shoemaker Beds). The dark micrites contain swarms of blind or very large-eyed trilobites such as Nanshanaspis sp.nov., Bulbaspisj Telephina (Telephina) and Shumavdia associated with other widespread forms such as Nileus and Tseudohasilicus. We suspect that this Middle Ordovician Nileid 'community' lived near to the base of the photic zone (~150m water depth). Imploded nautiloids from a phosphatic ironstone allow the use of Westermann's formula^ to calculate a minimum depositional depth of 300 ± 50m. Conodonts are rare in the dark micrites but occur in the biocalcarenites. In contrast to the MidContinent province type conodonts occurring at Precipitious Bluff and in the Prion Beach Beds those from most of the Shoemaker Beds belong to the North Atlantic Province and include Feviodon aouleatus^ Amorphognathus tvaerensis^ Protopanderodus livipipus^ Eoplacognathus elongatus, and Baltoniodus sp. and indicate an A. toaerensis zone age for most of the Shoemaker Beds. Mid-Continent conodonts, including Chirognathus monodactylus^ occur near the top of the section but may be reworked.

A new genus of lobate drepanellid ostracod occurs in calcarenites throughout the section and is the only demonstrably autochthonous faunal element in common with the major Gordon Group sections of central and N.W. Tasmania. Graptolites include a bispinose Climaoograptus and Dioellogvaptus but have not.as yet^been studied in detail.

This study emphasises the amazing faunal contrasts that occur in going from shallow to 'deep' (-300m) water over a (present) horizontal distance of about 8 km. Conodonts from Precipitous Bluff can be

199

correlated with Nevada/Ohio and those from Surprise Bay with Europe and the Appalachians. However, no direct correlations may be made between Precipitous Bluff and Surprise Bay. The widespread or cosmopolitan nature of the Shoemaker Beds fauna fits in well with similar transects published by Fortey3 for the Lower Ordovician of Spitzbergen and Cook and Taylor^ for the Upper Cambrian of Nevada. Biogeographically and^ ecologically, Telephina^ Hleus^ Bulbaspis^ Nanshanaspis and Shumardia may be thought of as part of the M.U. Ordovician psychrospheric fauna that inhabited the relatively cold^deep waters of the ocean basins separating North America and Australia and perhaps China.

1. Banks, M. and Burrett, C. (1980), Journal Geol.Soc.Australia^ 26: 363-376.

2. Burrett, C., Laurie, J., Stait, B. (1981), Pap.Proo.R.Soc. Tas.^ lis, 93-99.

3. Fortey, R. (1975), Fossils and Strata, 4: 331-352.

4. Cook, H. and Taylor, M. (1975), Geology, 3: 559-562.

5. Westermann, G.E.G. (1973), Lethaia, 6: 383-403.

Tasrr

ognathus

eyi .

Chleognathus

m o n o <1 a c I y I u s -

PhragfTiodos

sp.nov. _

Tetradtum

^

spp.

Foofitephyllurn Pycnolithu» I

C a I a ( h i u ni Gorbyocerai Ditcoccras My s t e r i o c e r

WATER DEPTH (m) C Iimaco9raptu9

^

Buniaslolde»

• ubtidal

intertidal

supratidal

Fig.

details

Diccllogfap*"^

tee

5 tor

200

— .

rbyella

(Sowerbyella)

cf.

lepta

— . » ^

Skenidioides Ptychopleurella C l i t a m b o n i t e s ( C I i t a m b o n i t c s) S o w e r b y e l l a ( S o w e r b y e 11 a ) c t .

— — — Beloclina

compres»a

Oulodus

sp.nov

. Pliomerina Amphillctias emopIeurIde s

SURPRISE BAY

POINT CECIL

P R E C I P I T O U S BLUFF

IDA BAY

200

LOWER SUBMARINE

ORDOVICIAN FAN

OR

TURBIDITES

BASIN

PLAIN,

OF

CENTRAL

VICTORIA:

AND

TECTONIC

SIGNIFICANCE

R.A.F. Cas, S.F. Cox, L. Bieser, B.E. Clifford, R.L, Hammond, G. McNamara & I. Stewart^^ Department of Earth Sciences, Department of Zoology*, Monash University, Clayton, Victoria, 3168.

The Lower Ordovician turbidite succession of central Victoria contains an abundant graptolite fauna which make it possible to correlate the sedimentary facies on a regional scale within a detailed time framework.

The succession consists of turbidite sandstones and siltstones with the characteristics of the Bouma sequence, and black to grey mudstones which are the usual hosts to graptolites, phyllocarids and rarer inarticulate brachiopods.

Open-framework sandy mudstones also occur throughout the succession but no coarse conglomeratic mass-flow facies are known.

There is also a notable lack of abundant large-scale tractional sedimentary structures, so supporting a sub-wave base, relatively deep marine depositional environment.

Transportationaldepositional agents include high concentration and low concentration turbidity currents and slurry flows.

The facies subdivision used is: coarse-very coarse massive amalgamated sandstone facies

-

FACIES 1

-

sandstone dominant facies

-

FACIES 2

-

mixed sandstone - mudstone facies

-

FACIES 3

- mudstone facies - FACIES 4 and is based on sandstone to mudstone ratios which decrease from FACIES 1 to FACIES 4.

In any one measured section the thickest sandstone sedimentation units (up to several metres thick) tend to occur in FACIES 1, or less commonly FACIES 2.

Exceptions occur however, and in particular FACIES 1 may consist of clusters of amalgamated sandstones that are relatively thin (< Im).

All facies are represented throughout the Lower Ordovician, no particular facies being more dominant at any stratigraphic level.

Moreover there are significant lateral facies variations within individual stages of the Ordovician.

These are reflected by lateral variations in sandstone to mudstone ratios and in the thicknesses of FACIES 1 and/or FACIES 2 intervals.

The gross geometry of sandstone intervals (FACIES 1 and 2) is often lensoidal or channel-form, and suggests a channelized deep-marine setting.

Channelized environments are currently most popularly equated with base of slope canyon-submarine fan settings.

For the Lower Ordovician of central Victoria however the only convincing criteria which identify fan settings - a gross upwards coarsening (including conglomerates), prograding sequence and a large lens/semi-cone/lobe of

201

coarse sediment within a predominant mudstone matrix succession - are lacking.

Even small scale upward thinning/fining or thickening/ coarsening cycles are rare.

Instead a channelized slope apron or base of slope, basin plain setting fed by multiple sediment influx points or a line source is suggested to account for the channelling and the diversity of current directions.

Throughout the succession, palaeocurrent indicators, largely Bouma £ cross-lamination, show a diversity of directions although westerly directions are rare.

South to north and southwest to northeast directions predominate.

Northwest to southeast and north to south directions, in order, are less abundant.

Sandstone sedimentation unit thicknesses are less in northern sections around Bendigo than they are further south around Chewton, the Lerderderg Gorge and the Brisbane Ranges.

The source terrain(s) was quartz-rich and included polycyclic, exceedingly well-rounded quartz grains.

These were ultimately probably of Precambrian origin but were introduced into central Victoria after the Delamerian Orogeny which affected South Australia, Tasmania and Antarctica - all regions where the nearest Precambrian outcrop belts are known.

No volcanogenic detritus is known and this is all consistent with a setting marginal to a passive continental margin.

The position of the palaeoshoreline nearest to the central Victorian succession is uncertain.

Shallow marine successions of Early Ordovician age are known in Tasmania and northwestern NSW. It is uncertain however whether a shoreline lay within Victoria.

The Glenelg Metamorphic Complex of far western Victoria is intruded by a lower Ordovician granitoid and can be related to Delamerian deformation of the Adelaide-Kanmantoo Fold Belts.

Far western Victoria may therefore have been an emergent source land.

East of the Glenelg Complex is a terrain of deformed, unfossiliferous flysch-like sediments associated with fault-bounded, greenstone belts of assumed Cambrian age.

These flysch-like sediments could be Cambrian or Ordovician.

If Cambrian, they could have been a source for the fossiliferous Lower Ordovician succession of Central Victoria which would imply a relatively nearby shoreline and would explain the relatively proximal character of the turbidites in FACIES 1 and 2.

However there is no significant constraint on the timing of deformation of the unfossiliferous terrain.

Alternatively, the proximal character of FACIES 1 and 2 turbidites of Central Victoria may be relatable to a very large-scaled submarine sediment distribution system sourcing from the more distant shallow marine regions cited above.

202

THE HYDROTHERMAL ORIGIN OF QUARTZITES, PEBBLY QUARTZITES, LATERALLY-EQUIVALENT MICACEOUS ROCKS, AND THE PROBLEM OF "QUARTZ-RICH FLYSCH" IN THE LACHLAN FOLD BELT OF NSW A.N. Yeates Bureau of Mineral Resources, Canberra Petrographic features, fabric, and field occurrence suggest that many quartzites and pebbly quartzites in the Early Palaeozoic sedimentary sequences of the Lachlan Fold Belt are not metamorphosed quartzose sandstones and conglomerates. Flow textures, occurrences of calcic plagioclase, lack of clast imbrication, lack of tractioncurrent sedimentary structures, their limited occurrence as dykes in granitoids, their gradation along bedding into volcanic rocks, and the occurrence of fossils identifiable to species level in interbedded rocks, are interpreted to indicate their hydrothermal chemical origin. Clasts where present are interpreted to have been derived from conduit walls, and brecciation of former plug rock.

Variable rounding of the clasts is interpreted to have been caused by attrition in the conduits.

In some areas within the Fold Belt, the quartzites and pebbly quartzites grade imperceptibly into micaceous variants suggesting the latter may have a similar chemical origin. Where abundant in local sections, they may indicate area of former hydrothermal discharge .

Rocks that apparently display textures transitional between obvious chemical deposits and apparently-detrital clastic textures will be illustrated. Some of the latter could be described as "quartz-rich flysch", a ubiquitous facies throughout many of the Fold Belt's Early Palaeozoic sedimentary sequences. In many places the quartz "grains" in these rocks lack grain coatings, lack rounding, and are variably lenticular in shape. Sub grains and grain boundary relations indicate that these lenticular bodies have coarsened from formerly finer material.

It is suggested that the finer material may have been silica gel, formed perhaps by diagenesis of the associated formerly clayey material, or as precipates during deposition of clay and silt detritus.

During compaction of such sediment, the density contrast between silica "gel" and clays could cause the two to segregate, and to develop some of the observed pinch-and-swell-type lenticularity, During lithification, coarsening by grain growth to sand size in the microlenses resulted in the formation of apparent "grains".

These textures appear metamorphic in appearance. But the occurrence of fossils identifiable to species level, and of undistorted primary sedimentary structures in interbedded (an often less competent) rocks, casts doubt on a metamorphic origin. The preferred explanation follows an hypothesis by Elliston (1963) who interpreted similar features in some sediments of the Warramunga Geosyncline, Northern Territory, to be due to the pre-lithification segregation

203

of colloidal material into clots.

It remains to be determined just how much of the quartzose flysch is really clastic, and how much is chemical.

Re ference Elliston, J.N., 1963 - Sediments of the Warramunga Geosyncline ^ Carey, S.W., (convenor), Syntaphral Tectonics and Diagenesis.

A Symposium. Geology Department, University of Tasmania, Hobart, Ll-45.

DEPOSITIONAL ENVIRONMENTS OF LATE ORDOVICIAN CARBONATES IN CENTRAL WESTERN NEW SOUTH WALES B. D. Webby Department of Geology and Geophysics, University of Sydney, N.S.W. In the lower parts of the Cliefden Caves Limestone Group (Fossil Hill Limestone) and equivalents in central western New South Wales, the deposits are broadly divisible into five main fades types: (a) lingulide facies which includes locally restricted, 'lithic' calcareous siltstones; (b) rhynchonellide facies characterized by the presence of abundant Rhynchotrema in 'lithic' sands; (c) Eodinobolus facies which includes shell banks; (d) Tetradium cribriforme facies typified by the skeletal lime sands with associated coral colonies of T. cribriforme; and (e) strophomenide and orthide-strophomenide facies characterized by a rich and varied fauna and flora, especially articulate brachiopods.

From the intertonguing facies relationships it is possible to establish these associations as representing a range of depositional environments, from onshore to offshore on the fringes of an 'oceanic' island.

For instance the Eodinobolus facies occupied a protected position inside the shoals, banks and bars of the corallinedominated Tetradium cribriforme facies.

In the lower parts of Bowan Park and Regan's Creek successions there are similar occurrences of the above-listed facies types, but in addition there are two markedly different, terrigenous-free carbonate associations - the Eodinobolus/ Alleynodictyon and the Chondrites facies - interpreted as having formed on a major offshore Bahamas-like platform.

Much of the succeeding massive bedded, poorly fossiliferous, terrigenous-free limestones (Belubula Limestone and equivalents) seem to have formed in 'shelf lagoonal' conditions.

The 'open shelf strophomenide facies is also well represented in the upper part of the Cliefden Caves succession (in the Trilobite Hill Limestone Member of the Vandon Limestone), and in the equivalent Quondong Limestone at Bowan Park.

Another distinctive facies occurs in carbonates at the base, and as allochthonous clasts in limestone breccia deposits, of the succeeding 'basinal' graptolitic Malongulli Formation.

This facies of laminated lime muds, with its characteristic sponge-radiolarian fauna, is interpreted as a peri-platform ooze formed in deeper waters of the slope.

204

THE NATURE OF THE LATE PALAEOZOIC GLACIATION OF SOUTH-EASTERN AUSTRALIA - EVIDENCE FROM VICTORIA. F.E. O'Brien Bureau of Mineral Resources, Canberra A.C.T. The Late Palaeozoic glacial sequences scattered across Victoria contain the following facies groups 1. Diamictites deposited as subglacial tills containing boulder pavements, striated clasts and folded sandstone bodies.

2. Bedded diamictites and pebbly mudstones deposited by icerafting into standing water.

3. Sandstones and conglomerates deposited as fluvial outwash and deltas.

4. Coarse diamictites and complex sediments deposited as supraglacial tills and outwash.

Subglacial tillites are the most abundant facies. They contain evidence for deposition from ice at pressure melting point sliding on its bed. The shapes of clasts in both subglacial and supraglacial tillites suggest that almost all debris was carried subglacially and therefore it is likely that the ice mass was a large sheet that overwhelmed most topography.

Palaeocurrent and ice movement indicators across south-eastern Australia and in North Victorialand suggest that, at its maximum, the ice formed a large mass centred on the North Victorialand coast.

Using the inferred radius in theoretical models of ice sheet profiles suggests that more than 1 kilometre of ice covered central Victoria during periods of maximum glaciation.

The number of glaciations is very difficult to determine because of the complex pattern of erosion and deposition beneath large ice sheets but eight thick tillites in the thickest section in central Victoria suggests that there were at least eight major ice advances across the area.

THE GROOTE EYLANDT MANGANESE DEPOSIT : A MODEL OF CHEMICAL SEDIMENTATION IN INTRA-CRATONIC BASINS B.R. Bolton

1

& L.A. Frakes

2

^Department of Geology, La Trobe University, Melbourne, Victoria. ^Department of Earth Sciences, Monash University, Melbourne, Victoria.

Mid-Cretaceous marine manganese-bearing strata are exposed along the western and southwestern margin of Groote Eylandt, Northern Territory.

205

The ores occur as primary pisoliths, ooliths and associated secondary ores in an undeformed sedimentary sequence of claystones and sandstones which overlie, unconformably Proterozoic sandstones and orthoquartzites.

The lowest Mesozoic strata are unfossiliferous quartz sandstones derived directly from Proterozoic quartzites of the island. These are overlain apparently conformably by a shallow-marine glauconitic clay succession, the top of which bears the primary pisolitic and oolitic ores and which in turn are succeeded by secondary ores, concretionary manganese and weathering products of various ages.

The pisolitic ore forms a sheet-to-wedge shaped stratiform body with a maximum known thickness of 9 m. Pisoliths and ooliths are spherical, ovoid to pellet-shaped grains composed of concentrically laminated manganese oxides (pyrolusite and cryptomelane) or, as in southwestern Groote, manganese carbonates (manganocalcite).

Accretionary grains, which frequently show a radial alignment of constituent microcrystals about a central nucleus, are visually analogous to similar grains described in the calcium carbonate system in such areas as the Persian Gulf. Pisoliths are set in a mixed matrix/cement of variable composition-including, kaolinite, quartz sand, pyrolusite or cryptomelane wad and montmorillonite.

Significantly manganese pisolite and oolite make up several graded cycles as discrete beds up to 8 m thick. Inverse grading (2 mm up to 25 mm) is the prominent feature of the lower part of the ore zone, but normally graded units characterize the upper part. Graded units commonly are separated by rubbly hardgrounds containing broken pisoliths and composite grains of cemented pisoliths. Cross-bedded units are also recognized in several parts of the main ore-bearing horizon.

We propose that the Groote Eylandt manganese accumulation originated through a history of concentration of dissolved manganese in the water column during times of widespread relatively anoxic waters in intracontinental basins in concert with high sea level (transgression), followed by release of manganese through oxidative precipitation when sea level fell (regression). Inverse grading in manganese pisolite is facilitated by increasing energy levels during basin shallowing coincident with increasing oxygenation and provision of particulate manganese. It is proposed that particulate manganese is concentrated at the depositional site following initial trapping of manganese in the coastal environment by the salinity-controlled "veil" effect, and the tidal lag or 'broom' effect which operates to drive newly oxidized particulate manganese shorewards. The primary source of manganese is thought to be rocks, containing manganese at average clarke values, and which comprise the basin hinterland.

The model presented may also have applicability to other chemical species (e.g. base metals, iron, phosphorous, barite, etc.) in which solubility is dependent on oxygen levels characterized by Eh values from slightly positive to moderately negative.

206

LATE QUATERNARY STRATIGRAPHY OF NORTHERN SPENCER GULF, SOUTH AUSTRALIA J.R. Hails^ , A,P. Belperio^ , V.A. Gostin^ , and N.B, Billing*^ ^Manager, Environmental Services, Coal Division, CSR Limited, 1 O'Connell Street, Sydney, N.S.W. 2001. ^Department of Mines and Energy, P.O. Box 151, Eastwood, S.A. 5063. ^Department of Geology, University of Adelaide, Adelaide, S.A. 5000 "^CSIRO, Division of Soils, Glen Osmond, S.a. 506i+ Five Quaternary marine transgressive units (four Pleistocene and one Holocene) have been identified in northern Spencer Gulf, South Australia, from sub-bottom profiles and 330 vibrocores.

Sediments associated with the oldest unit have been correlated with stage 7 (c. 220 ky) of the marine oxygen isotope record. During the last interglacial maximum, sediments of the Mambray Formation were deposited above present low water datum,and have been associated with isotope stage 5e (c. 125 ky).

Palaeosols, with well structured clay-rich B horizons, are characteristic of these two oldest units.

Carbonate accumulation horizons predominate in palaeosols developed in younger sediments during glacio-eustatic low sea levels.

Maximum sea levels, associated with the two youngest Pleistocene transgressive units and correlated with isotope stage 5c (c. 105 ky) and 5a (C 82 ky), reached levels -8 m and -ih m respectively below modern datum. Palaeoclimates inferred from the palaeosols suggest that a more humid climate existed before c. 125 ky than has occurred subsequently.

Aeolian reworking of younger plaeosols is indicative of aridity since c. 120 ky.

Table 1 Summary Stratigraphic Table of Sedimentary Strata in northern Spencer Gulf

Holocene

Formation Name

Environment

Germein Bay Formation

(Marine)

Pooraka Formation >H K CD C (D O O -P

Lowly Point Formation

Probable Age (ky)

(Alluvial) (Lagoonal-marine)

0-8

C.20

8h

False Bay Formation

(Marine)

105

Mambray Formation

(Marine)

125

Older Pleistocene Marine Beds

(Marine)

C.220

Hindmarsh Clay

(Alluvial)

> 220

• H

H 0) H EH fl. < (D cd

207

HOLOCENE SEDIMENTS AND SEAFLOOR MORPHOLOGY OF NORTHERN SPENCER GULF, S.A. V.A. Gostin^, A.P. Belperio^ & J.R. Hails^ ^Department of Geology, University of Adelaide, South Australia. ^Geological Survey of South Australia. %anager. Environmental Services, Coal Division, GSR Ltd., Sydney.

The seafloor morphology, and distribution and nature of Holocene sediments have been studied over an area of 2,000 km^ using 300 km of sidescan sonar records, 365 km of high resolution seismic profiles and 330 vibrocores.

The northern Gulf is extremely shallow with 33% of its area intertidal, 33% between low tide level and 10 m, and only 5% exceeds 20 m in the flat-floored channels. Ten major environments in three tidal zones have been identified. The supratidal zone contains bare flats, coastal dunes and stranded beach ridges. The intertidal zone includes bare or Zosteva covered sand flats, beaches, mangrove swamps, and samphire flats. The subtidal environment consists of extensive seagrass meadows to 10 m water depth. At greater depth, the floor may be either featureless, scoured or covered with sandwaves.

Seagrass meadows occupy broad shallow depositional platforms predominantly along the eastern side of the Gulf and form several discrete offshore banks. The vigorous growth of the seagrasses Posidonia australis and P. sinuosa in these banks has supported an extremely rich fauna, resulting in a complex, very poorly sorted, organically bound carbonate sediment (skeletal packstone). This detritus includes, in relative order of abundance of fragments: bivalve, gastropod, foram, echinoid, coralline algal, and bryozoal detritus. This facies is 4 to 6 m thick and dominates the Holocene stratigraphy.

In water deeper than 10 m, over most of the Gulf, the sea floor is acoustically smooth and is underlain by a thin 0.5 to 2 m blanket of mud and shell. However, in the vicinity of Point Lowly and in the northern, more estuarine part of the Gulf, strong tidal currents have scoured the channel floor and produced 200 to 500 m wide belts of megaripples orientated parallel to the channel margins. The crests of these megaripples are orientated normal to the tidal flow and are commonly 2 to 6 m in wavelength and less than 0.5 m high. Large megaripples have wavelengths from 8 to 20 cm and heights up to 1.3 m.

The size of these bedforms is probably related to both sediment availability and current strength. Many megaripples are symmetrical, but some with asymmetry indicate a prevailing tidal flow, such as an anticlockwise gyre that occurs around Middle Bank.

The total carbonate content of surface sediment on the sea floor increases from some 40% at the head of the Gulf, to 90% in the Whyalla-Port Pirie area. The skeletal component is dominantly sand and coarse silt size, and no marked grain size trends have been found.

A minor amount of well rounded and frosted quartz grains are derived from aeolian activity.

208

STRATIGRAPHIC EVOLUTION OF HOLOCENE COASTAL CARBONATE BANKS, NORTHERN SPENCER GULF, S.A. A.P. Belperio^, J.R. Hails^ & V.A. Gostin^ ^Geological Survey of South Australia, Adelaide, S.A. ^ Environmental Services, CSR Limited, Sydney, N.S.W. Geology Department, University of Adelaide, Adelaide, S.A. Peritidal carbonate banks, which fringe northern Spencer Gulf, were investigated by intertidal and subtidal vibrocoring.

Radiocarbon, amino-acid, thermoluminescence and lead-210 dating techniques provided chronological control.

Two Pleistocene marine units are recognised beneath an extensive Holocene cover. These have undergone significant pedogenesis prior to Holocene marine inundation.

The Holocene sequence is dominated by sediments of the Posidonia australis seagrass facies which form distinctive skeletal carbonate banks offshore of and fringing the coastline. Seagrass bank facies also extensively underlie the intertidal and supratidal zones.

Holocene sedimentation commenced prior to 6600 radiocarbon years B.P. and a 2.5 m higher relative sea level existed until cibout 1700 years B.P. The relative fall in sea level to present level probably resulted from local seismo-tectonism and created wide intertidal areas. Mangrove and samphire colonisation and beach ridge progradation occurred over the former seagrass bank surface, but sediment accumulation in these environments was limited. The seagrass environment is both the primary source of skeletal sediment and the major sedimentary sink in the northern gulf^ Accumulation rates in this environment vary from 0.2 to 2.7 mm yr , depending on the time scale over which the measurements are made.

AGE, ORIGINS AND EXPRESSIONS OF ARIDITY IN LATE CENOZOIC AUSTRALIA J.M. Bowler Department of Biogeography & Geomorphology The Australian National University, Canberra, A.C.T.

Aridity, like the glacial conditions it so characteristically accompanies, is a geologically late arrival on the Australian and, indeed, on the planetary scene. Not since Permian time has there been such an associatioa of glaciation, desert expansion and evaporite production as has occurred over the past few million years. Although the development of at least seasoaal water stresses may date frooi Oligocene time, the main development of global arid zones post-dates Middle Miocene.

In this context, the Australia continent, relatively unaffected by major tectonic disruption, provides perhaps the best place in the world in which to discriminate between those effects controlled by tectonism, eustatic sea level change and global climatic variations.

209

In the early Tertiary, global climatic environments were characterized by greatly expanded warm humid zones resembling the tropics of today. The sub-tropical high pressure belts, the main climatic elements controlling Australian and global acidity, ware located at much higher latitudes and were much weaker than today.

Meanwhile, the Australian continent, drifting north, a veritable arc with its Gondwana flora and fauna, experienced warm humid climates even to ^ its southernmost shores. Located equatorward of the weak high pressure cells, the entire continent lay under the influence of summer rainfall.

With the build up of ice in Antarctica and the slow but progressive cooling of Miocene seas by generation of Antarctic bottom water, the high pressure cells, responding to increased meridional temperature gradient, were intensified and displaced equatorward. Thus whilst the Australian continent continued its northerly drift it was overtaken from the south by the migrating high pressure belt.

Southern Australia, the first part of the continent to experience the major change, would have been subjected to seasonal stress, wet summers with long, dry winters.

The earlier, equable humid summer rainfall environment of Miocene time gave way to Pliocene drier coaditioas with greatly increased seasonality.

As the anticyclone belt moved even further north acco.npanied by additional intensification, the southern regions for the first time came under the influence of winter rainfall and the westerlies.

In the stratigraphic record of Lake George and the Murray Basin this event is dated to 2.5 m.y.; it is marked by a widespread sedimentary and geochemical discontinuity in the stratigraphic record. Earlier acidic environments represented by ferricceUes ^ere replaced by alkaline calcareous facies throughout the Murray Basin.

Lake George, responding to winter rainfall, began its pattern of oscillatory wet-dry conditions that has continued to present day.

From that time, the landscape, flora and fauna were subjected to intensified hydrologic stresses culminating in the major cyclic expansion and contractions that characterize the Quaternary history of Australia's desert regions, a pattern that is repeated throughout the deserts of the world.

The contrast between those geologically extraordinary environments of Quaternary time (glaciation and desert expansion) and the global changes of the Late Miocene which made them possible involve dramatic events in the geological record.

Their registration in the landscape, sediments and soils of Australia calls for detailed and systematic studies of our continental deposits. New research programmes are being des^eloped towards this end.

Bowler, J.M. 1982. Aridity in the late Tertiary and Quaternary of Australia p.35-46 in Evolution of the Flora and Fauna of Arid Australia, W.R. Barker and P.J.M. Greenslade '(Eds.')",'' P'e'acock Publications, Adelaide.

210

ENVIRONMENTAL AND TECTONIC IMPLICATIONS OF HERRING-BONE CROSS-STRATIFICATION IN MODERN FLUVIAL SEDIMENTS

1

M.M. Alam , K.A.W. Crook

1

. 2

& G.M. Taylor

1

Department of Geology, Australian National University, CANBERRA ACT ^Geology, Canberra College of Advanced Education, BRUCE ACT

Herring-bone cross-stratification occurs in sediments less than 150 years old in Warrena Creek nears its confluence with the Castlereagh River at Coonamble, NSW, some 1800 river kilometres from the sea. These streams are low-gradient, with straight to anastomosing channels which become sinuous and distributive downstream. Channel beds are sand but banks are almost exclusively mud which is strongly burrowed and extensively penetrated by roots. The lithofacies present resemble inter-tidal deposits and could easily be misidentified on the basis of herring-bone cross-stratification if they formed part of an ancient sedimentary sequence. The herring-bone crossstratification results from flow reversals in Warrena Creek during flood events depending upon the discharge and stage in the creek at any given time relative to that in the adjacent river. Herring-bone cross-stratification should be regarded as diagnostic of depositional environments in which current directions are principally determined by the gradient of the water surface, rather than regional paleoslope.

Such environments are likely to be characteristic of the interior of lithosphere plates.

THE ROTTNEST SHELF : HOLOCENE FACIES MODEL FOR TEMPERATE SEDIMENTATION ALONG PASSIVE MARGINS Lindsay B. Collins Department of Geology and Geophysics Western Australian Institute of Technology The Rottnest Shelf is a narrow, open shelf on the southwest continental margin of Australia. It is characterised by high energy conditions and a swell wave climate. The warm temperate **foramol" biota is dominated by calcareous red algae and bryozoans.

Holocene sediments overlie a subaerial exposure horizon, which developed during low stand conditions prior to transgression.

Blanketlike bodies of carbonate sediment up to Im thick have been deposited over the upper continental slope to inner shelf. Coastal bank, beach ridge and dune sediments have been deposited in shallowing - upward sequences up to 30m thick.

The inner shelf (0-60m) is characterised by wave current reworking.

Sediments are a basal transgressive lag, composed of material reworked from the underlying unconformity, and blanket-like algal-

211

bryozoan grainstone which is wave ripple cross stratified. Pleistocene ridges are encrusted by algal-serpulid boundstone. The outer shelf (6O-I7O111) and upper continental slope are characterised by bioturbation, skeletal breakdown and slow accumulation of a carbonate sediment blanket which fines seaward. Sediments are bryozoan grainstone to packstone (outer shelf) and skeletal wackestone (upper continental slope).

Cainozoic evolution of the passive carbonate margins of southwest Australia has been controlled by sea level oscillations. Four Cainozoic depositional cycles have been recognised (Quilty, 1980), in which bryozoan carbonates are common. Glacio-eustatic cycles (duration ca 30,000 years) have characterised Quaternary evolution, and longer term cycles (third order cycles of Vail et al, 1977, duration ca 5 m years) were common in the Tertiary. Data from the Rottnest Shelf suggest that glacio-eustatic cycles should consist of thin, unconformity bound sediment blankets and coastal sequences, whilst longer term progradational cycles should consist of thick (hundreds of metres) shelf carbonates, dominated by algal and bryozoan grainstone to wackestone.

References Quilty, P.G., 1980, Sedimentation cycles in the Cretaceous and Cainozoic of Western Australia. Tectonophysics 63: 349-366.

Mitchum, R.M., and Vail, P.R., Thomson, S., 1977, Seismic stratigraphy and global changes of sea level, Part 4 : global cycles of relative changes of sea level. Am. Assoc. Petrol Geol. Mem. 26: 83-88.

SEDIMENTARY FACIES AND ENVIRONMENTS OF THE YOGANUP SHORELINE, SOUTHERN PERTH BASIN Lindsay B. Collins & John L. Baxter Department of Geology and Geophysics, Western Australian Institute of Technology Cainozoic strandlines in the southern Perth Basin occur as sub-parallel shoreline deposits. The Yoganup and Capel shorelines, where mining has been concentrated, are linear to arcuate shorelines which are remote from the modern coast. Blanket-like yellow quartz sands overlie the strandline deposits. Transient exposures, produced and destroyed by mining, have been utilized to study sedimentary facies.

The Bunbury Trough lies between the Darling and Busselton Faults in the southern Perth Basin, and is a deep graben which contains 10000m of Late Jurassic to Quaternary sediments. The only pre-Cainozoic rocks exposed are the Lower Cretaceous Bunbury Basalt and the Leederville Formation. At the foot of the Whicher Scarp the Leederville Formation is overlain by 3 to 4m of Cainozoic Yoganup strandline sediments, and

212

this sequence is exposed at the Yoganup Extended mine site. The uppermost 3 metres of the Leederville Formation is composed of weakly lithified, poorly sorted fine to very coarse-grained clayey sand and sandstone. Fine-grained heavy mineral sand (80% heavy minerals) is present Im below the disconformity at the top of the formation.

The 27m Yoganup strandline, which disconformably overlies the Leederville Formation (Fig. 1) consists of : (i) pebble lag deposits (510cm) or fine to medium quartz sand (10-50cm), (ii) crossbedded, pebbly fine to coarse sand (20-60cm), (iii) plane-laminated, finegrained heavy mineral sand (50-170cm), (iv) discordantly-laminated fine-grained heavy mineral sand (30cm), and (v) homogeneous, finegrained heavy mineral sand (30cm). Strandline sediments were deposited as shallowing-upward sequences adjacent to a sea cliff (the Whicher Scarp) and along a rocky shore cut into the Leederville Formation. The facies are characteristic of a transgressive clastic shore with high wave energy (Clifton et al, 1971; Reward, 1981).

Sediment supply was low, and heavy minerals and quartz sand were derived partly from shoreface erosion of the Leederville Formation.

Heavy minerals were concentrated in upper shoreface, beach and backshore environments.

References Clifton, H.E., Hunter, R.E. and Phillips, R.L., 1971, Depositional structures and processes in the non-barred high energy nearshore.

J . Sediment. Petrol. 41: 651-670.

Heward, A.P., 1981, A review of wave-dominated clastic shoreline deposits. Earth Sci. Rev. 17: 223-276.

IDEALISED

SEQUENCE HEAVY M

FINE HEAVY MINERAL S A N D HOMOGENEOUS FINE HEAVY MINERAL S A N D DISCORDANT LAMINAE

2 5 BACKSHORE

MINERALS

80 80

UNIT

E D

2 0

FINE HEAVY MINERAL S A N D LAMINATED

15 FORESHORE

10 80 PEBBLY FINE - VERY C O A R S E SAND + GRANULES MULTIDIRECTIONAL CROSSBEDS DISCONFORMITY

F I G U R E 1.

SEDIMENTARY FACIES, YOGANUP

20

B "A"

SHORELINE

Symposium 3(b) The s u r f i c i a l mantle of cratons Convener: D r A . R . Milne

213

GRANITE WEATHERING AND SILCRETE FORMATION ON THE YILGARN BLOCK, WESTERN AUSTRALIA. C.R.M. Butt Division of Mineralogy, CSIRO, Private Bag P.O., Wembley, W. Aust. 6014 Weathering profiles developed on granitic rocks, exposed in the breakaways of the Barr-Smith Range and in exploration pits at Gabbin, in the Yilgarn Block of Western Australia, consist of kaolinitic granite-textured saprolites merging upwards into sandy grits. The grits may also form columns or dykes, penetrating downwards into the underlying saprolite.

The grits are cemented by quartz, anatase and zircon (QAZ) to form silcretes and/or by an alumino-silicate, either as siliceous allophane or partly crystallized as kaolinite and opaline silica. The profiles are characterized by low levels of alkalis and alkaline earths and most metals.

The QAZ-silcrete horizon may contain over 3% TiO^ and 1000 ppm Zr. The profiles evolved through at least 4 stages.

1.

Solution of kaolinite near the top of the profile, under very acid conditions, causing settling of resistant quartz grains.

2.

Precipitation of QAZ-cement, the TiO and SiO^ being derived partly by lateral migration from upslope.

3.

Precipitation of allophane, in the sandy grits and the saprolite.

4.

Erosion and exposure of the profiles by pedimentation.

The kaolinitic saprolite-sandy grit profile, probably formed under humid conditions, is the equivalent of ferruginous laterite developed on more basic rocks nearby and of lateritic bauxite in the Darling Range. However, the sand was a surface horizon and there is no evidence that there was ever a ferruginous zone at this geomorphological situation at least. The sequential precipitation of QAZ- and allophane-cements was a response to increasing aridity and reduced groundwater flow.

Allophane-cemented materials tend to disintegrate on exposure but they are probably more abundant than the more prominent QAZ-silcretes.

SILICIFICATION OF CONCENTRIC RIDGES, TERTIARY, STUART CK, SOUTH AUSTRALIA R.A. Callen Department of Mines & Energy, 191 Greenhill Road, Parkside, S.Aust. 5063.

A remarkable landscape of concentric silicified arcuate ridges extends for more than 200 km in the region north and northwest

214

of the tip of Lake Torrens (Ambrose & Flint, 1981). Secondary silica is developed in strips of varying morphology and intensity in a very homogenous sequence of Tertiary sediments, part of which contains the unique Stuart Ck flora (Ambrose et al. 1979). The distribution of silica is controlled by the presence of a series of concentric mounds developed within the uppermost part of the Tertiary sands.

These mounds may be related to the regression of a lake of the same age as the sediments, as suggested by Ambrose & Flint.

The silica cement has been studied by light microscope and XRD (the latter by Dr R. Brown of Amdel, Adelaide), revealing a change in silica type related to porosity, grain size and whether the cemented material was above or below the watertable. Cryptocrystalline quartz develops in porous (coarse) sediments within the zone of fluctuation of the water table, and opaline cement appears in addition to this in less porous (fine) sediments also within the zone of fluctuation of the water table.

Quartz overgrowth cement is commoner in sediments cemented whilst below the water table. Crystallisation of different silica types follows a definite order.

The sections have been accurately leveled, and the results of this and conclusions to be derived will be included in the talk.

The sequence bears a remarkable resemblance to the concentric ridges of the Fontainbleau Sandstone (Paris Basin).

Ambrose, G.J. & Flint, R.B. (1981). A regressive Miocene lake system and silicified strandlines in northern South Australia implications for regional stratigraphy and silcrete genesis. J. Geol. Soc. Aust. 28, 81-94.

Ambrose, G.J., Callen, R.A., Flint, R.B. & Lange, R.T. (1979).

Eucalyptus fruits in stratigraphic context in Australia Nature 280, 387-9.

SILICIFICATION IN CAINOZOIC LANDSCAPES OF ARID CENTRAL AUSTRALIA A.R. Milnes Division of Soils, CSIRO, Private Bag No.2, Glen Osmond, S.Aust. 5064.

Following studies on silcretes in the Lake Torrens area of South Australia some years ago, renewed research on silica mobility during weathering led to studies of silicification in the near-surface zones of Tertiary drainage landscapes south of Alice Springs in Central Australia. There, a mosaic of silicified sediments and possible soils crop out as relict duricrust on mesas that are significantly higher than the base level of erosion defined by the contemporary drainage.

From experimental data on silica in soil solutions, it appears likely that acid-weathering conditions in pedogenic environments near the margins of the Tertiary drainage basins may have

215

provided silica for soil waters and groundwaters moving slowly through the regolith into depositional zones in the lower reaches of the basins. The structural state of the silica indurating the variety of old regolith materials ranges from opal to quartz: observations of textures suggest heterogeneous precipitation and recrystallisation effects that could have occurred in response to quite local fluctuations in conditions such as pH and moisture regime.

As with the earlier studies, attention is given to the high concentrations of titanium in some silicified materials. An enrichment of zirconium and aluminium is associated with titanium: much of the titanium and at least some of the zirconium and aluminium appears to be of secondary origin and to have been deposited from solution.

SILCRETES IN THE COBAR AREA, AUSTRALIA R.A. Glen^ and J.T. Hutton^ ^Geological Survey of New South Wales, Dept. of Mineral Resources, Box 5288 GPO Sydney, N.S.W. 2001. ^12 Bellevue PI., Unley, S.Aust. 5061.

A recent, detailed geological survey of the Cobar area in central-western New South Wales has disclosed areas of silcrete development.

These silcretes closely resemble previously known occurrences found west of Cobar (Dolo Hills N.S.W.) and northwest of Cobar (Tibooburra area N.S.W. and adjacent area in southwest Queensland).

Localities, petrographic descriptions in both hand specimen and thin section, and chemical analyses are given to support this claim. Although Wasson et al. (1979) have rightly questioned the earlier report by Dury (1966) of silcrete occurrences around Cobar, the new data presented here enable the Cobar area to be retained as a silcrete locality.

Dury, G.H. (1966) Duricrusted residuals on the Barrier and Cobar pediplains of New South Wales. J. Geol. Soc. Aust. 13, 299-307.

Wasson, R.J., Hunt, P.A. & Clarke, M.F. (1979) A re-evaluation of the "silcrete" of the Cobar area, Australia. Geoderma 22, 137-159.

216

STUDIES OF LATERITE IN SOUTH AUSTRALIA R.P. B o u m a n Waite Agricultural Research Institute, Glen Osmond, S.Aust. 5064.

The following is a summary of continuing investigations of iron-rich layers, crusts and horizons together with deeply weathered zones, which may not necessarily be genetically associated with the iron crusts that are commonly termed 'laterite' in South Australia.

The study in progress involves the mapping of zones of iron enrichment and weathered mantles in order to establish at least the relative ages of the materials, together with detailed examinations of profiles, many of which have been interpreted as representing the 'normal laterite profile' of Stephens (1946). However, many of these profiles are not standard monogenetic profiles, but are composite and complex, revealing a long history of repeated (or continuous) weathering and iron mobility. This phenomena is further illustrated by examination of land surfaces with demonstrated age differences in the Mount Lofty Ranges as well as by stratiographic investigations of iron enrichment of sediments within the marginal to the ranges.

The iron-rich zones are being characterised on the basis of their macromorphology (e.g. vermiform, pisolithic, concretionary, massive, etc.), following which microscopic examination of samples and thin sections thereof facilitate identification of details of the individual crusts (e.g. iron impregnation of pre-existing sediments or basement rocks, chemical preparation of iron oxides forming bog iron ore, cementation of nodules and pisoliths) . XRD and XRF analysis is providing information on the chemistry and mineralogy of the iron-rich materials.

Iron enrichment and mobilisation occur in many forms, and it is essential that these be categorised physically and chemically before they can be used reliably as environmental or time-related indicators.

Field evidence suggests that iron mobility and enrichment have taken place over a vast period of geological time and is also extant today, indicating that local environmental conditions may be more important than climate in their formation.

Without careful examination of field and stratigraphic relationships together with detailed investigations of the physio-chemical characteristics of the crusts, their use as morphostratigraphic markers is fraught with uncertainty.

Stephens, C.G. (1946). Pedogenesis following the dissection of lateritic regions in southern Australia.

Lull. Coun. Sci. Ind. Res. Aust. No. 206.

217

PRE-BASALT TOPOGRAPHY IN THE COOMA A R E A AND ITS RELATION TO THE PRESENT DRAINAGE SYSTEM 1 2 Graham T a y l o r , G . R . T a y l o r , C . Foudoulis

1 1 and M . Bink

^Ca^berra College of A d v a n c e d E d u c a t i o n , B r u c e , A . C . T .

Australian National University, Canberra, A.C.T. Recent d e t a i l e d mapping of the Tertiary geology in the northeastern Monaro has resulted in a reinterpretation of the geomorphic and tectonic h i s t o r y of the a r e a . The geomorphic history of this area has long b e e n a m a t t e r for d i s c u s s i o n .

Early workers generally a g r e e d that the Murrumbidgee River flowed south across the p r e s e n t Great Dividing Range and formed the headwaters of the Snowy River s y s t e m . They p o s t u l a t e d a divide further north and suggest that it shifted south to its p r e s e n t p o s i t i o n during late Tertiary tectonism.

The p r e - b a s a l t i c valleys in the Cooma region consist of b r o a d open valleys cut b y steep v - s h a p e d valleys about 60 m d e e p .

These valleys are m e r i d i a n a l l y aligned and closely p a r a l l e l to the p r e s e n t drainage and the Palaeozoic b e d r o c k grain in the a r e a . The pre-basalt topography shows these v a l l e y s form a dendritic drainage p a t t e r n w h i c h is o r i e n t e d in the same d i r e c t i o n to the p r e s e n t s y s t e m . The average relief on the pre-basaltic surface w a s of the o r d e r of 150 m w i t h a m a x i m u m relief of 270 m .

These v a l l e y s , and in some cases the interfluves were b u r i e d b y b a s a l t during the Eocene to early Oligocene (36-53 m A) .

The b a s a l t s flowed d o w n the palaeovalleys towards the north and in the case of one flow filled the valley running north from Cooma and flowed into the Murrumbidgee V a l l e y . This substantiates the Murrumbidgee Valley is at least Eocene in a g e . Similarly b a s a l t flows from Murrumbucca have flowed south and w e s t into the Murrumbidgee V a l l e y .

Post-basalt valleys have for the m o s t p a r t b e e n eroded along the basalt-Palaeozoic contact and as a result closely p a r a l l e l the pre-basalt d r a i n a g e .

This study has shown that contrary to earlier h y p o t h e s e s , the p r e s e n t drainage system has its origins pre-Eocene and probably as far b a c k as the late M e s o z o i c .

TERTIARY STRATIGRAPHY AND WEATHERING E V E N T S , W E S T E R N M A R G I N OF THE G A W L E R C R A T O N , SOUTH AUSTRALIA M . C . Benbow D e p a r t m e n t of Mines & E n e r g y , 1 9 1 G r e e n h i l l R o a d , P a r k s i d e , S.Aust. 5063.

An outline w i l l be given of the Tertiary stratigraphy and w e a t h e r i n g events for the w e s t e r n m a r g i n of the G a w l e r C r a t o n , South A u s t r a l i a . Some success is being h a d in relating inland sequences w i t h b o t h c o a s t a l sequences and marine sequences of the Eucla B a s i n .

218

Sediments considered Early Oligocene are described along with two sequences of Pliocene-?Early Pleistocene age. Several silcrete and ferricrete events are separated by tectonism before the Middle Miocene.

The only truly pedogenic silcrete developed after sedimentation of the Nullarbor Limestone and the Garford Formation During the Pliocene. The relationship between duricrusts and bleaching will be discussed.

The onset of aridity during the ?Late Pliocene-Early Pleistocene after an initial wet period is marked by sediments with interlaminated gypsum and by a massive gypsum crust.

THERMOLUMINESCENCE DATING OF SURFICIAL MANTLES J.T. Hutton Department of Physics, University of Adelaide Many minerals, due to defects in their crystal lattice, can store energy in suitable electron traps after they have been exposed to ionizing radiation.

Calcite, feldspars and quartz are suitable minerals and the common radioactive elements, uranium, thorium and potassium, are present in most sediments. The stored energy can be released by heating to above 250°C and part appears as visible light.

Thus in any environment of constant radiation, the amount of thermoluminescence is directly related to the length of time of exposure.

Crystals when they were formed, e.g. precipitated calcite, have no inherent thermoluminescence and hence the dating of some calcite deposits is possible. Other events, such as heating to 500°C or exposure to ultraviolet light, can remove any stored energy and allow the process of accumulation to begin again. Thus if the surface exposed to sunlight is subsequently covered with another deposit, it may be possible from thermoluminescence studies to date the event.

A prehistoric site in a sand hill on the east bank ^^f the River Murray north of Blanchetown has been dated by C and thermoluminescence studies of the ovenstones (Prescott, Polach, Pretty and Smith 1981), and currently the quartz sands from various levels in the dune are being studied.

Prescott, J.R., Polach, , Pretty, G.L. and Smith, B.W. (1981) Comparison of C and thermoluminescent dates from Roonka, South Australia. PACT Journal (in press).

Symposium 3(c) Lithosphere dynamics, biogeography and faunal and floral provinciality Convener: Dr E.M. Truswell

219

CAMBRO-ORDOUICIAN PALAEOGEOGRAPHY OF AUSTRALIA Peter J. Cook Division of Continental Geology Bureau of Mineral Resources, Canberra ACT A recent compilation of Australian palaeogeographic data clarifies the nature and extent of climatic, sea-level and sedimentological changes during the Cambro-Grdovician. A total of ten intervals, distinguished on the basis of major time breaks, or fundamental changes in the style of sedimentation are used as the time framevi/ork for the ten palaeogeographic maps, summarised in Figure 1. The Grdovician maps compiled, proved to be somevi/hat similar to those produced by Webby (1978). The Cambrian maps represent the first attempt at a comprehensive series of Cambrian palaeogeographic maps, and are summarised by Cook (1982).

It is possible to relate features such as occurrence of phosphorites and organic rich sediments to high sea-level stands during the CambroGrdovician. The development of a transcratonic seavi/ay in the middle Cambrian, and again in the Grdovician was also a very important feature in the extension of nutrient-rich conditions far onto the craton. Some difficulty vi/as experienced in relating cratonic and "geosynclinal" events because of the lack of good biostratigraphic control in some parts of the sequence. Despite this, an attempt has been made to present an integrated palaeogeographic picture throughout the Cambro-Grdovician for all parts of the continent. Inevitably this is model-dependent in the "geosyncline". The palaeogeographic maps generated by this project are useful not only for elucidating the history of the continent but also for establishing the knovi/n and inferred distribution of sedimentary mineral deposits and fossil fuels.

REFERENCES CGGK, P.J., 1982 - The Cambrian palaeogeography of Australia and opportunities for petroleum exploration. Journal Australian Petroleum Exploration Association 22(1), 42-64.

WEBBY, B.D., 1978 - History of the Grdovician continental platform shelf margin of Australia. Journal Geological Society Australia 25, 41-63.

220

Early C a m b r i a n T i l ( 575 - 545 m.y.)

Templetonian TI3 ( 541 - 538m.y.)

Floran — U n d i l l a n T I 4 (538 - 529 m.y.)

Boomerangian —mid-ldamayan T I 5 ( 5 2 9 - 520 m.y.)

M i d - l d a m a y a n — Payntonian 116(520 - 509 m.y.)

Datsonian— Bendigonian T I 7 ( 5 0 9 - 496 m.y.)

Castlemainian — Yapeenian 118(496 - 491 m.y.)

Darriwillian 719 (491 - 475 m.y.)

Late O r d o v i c i a n 1110(475 - 445 m.y.)

G

S

g/auconite

sulphates

Continental I Emergent

Figure

H

C

halite ^ ^

to

semi-emergent

carbonaceous

Epemc,

shallow

Deep

marine

1 - Cambro-Ordovician

P marine,

phosphorite open

shelf

Pa1aeogeography

221

BIOGEOGRAPHY OF ORDOVICIAN NAUTILOIDS & TRILOBITES FROM AUSTRALIA, MALAYSIA & THAILAND AND THEIR TECTONIC IMPLICATIONS. Bryan Stait and Clive Burrett Geology Department, University of Tasmania, Hobart, Australia 1 2 The position of the Shan-Thai tectonic block ' (consisting of the western half of Thailand, eastern Burma and Malaysia) relative to Australia remains controversial An Ordovician placement next to Africa, next to Australia, near to Australia or distant from Australia have all been proposed (Fig.l).

Taxonomic work on nautiloids and trilobites (by Stait) coupled with conodont biostratigraphy and sedimentology (by Burrett) on the sequences in the Shan-Thai block support an Early-Middle Ordovician juxtaposition or at least proximity with western or north-eastern Australia.

FIG. 1.

1?

Middle Ordovician geography from Burrett, with suggested positions for Shan-Thai block: 1. Ridd, 19713, 2. Stauffer, 1974^, 3. Haile, 198ol3, 4. Ziegler, 1981^, 5. Burrett, 198212, 6. McTavish and Legg, 19767. N.A., North America; E., Northern Europe; S., Siberian Block; K., Kazakhstan; C.N., North China; C.S., South China; J., N.E. Siberia; A., Australia; A.N., Antarctica; I., India: T., Tibet; A.F., Africa; S.A., South America.

The Ordovician of Shan-Thai occurs in a belt from Kuala Lumpur to northern Thailand to the Shan States of Burma. Based on conodonts and trilobites the Upper Cambrian-Lower Ordovician boundary occurs within the Tarutao Sandstone on Tarutao Island. The uppermost intertidal/high subtidal sandstones contain a newly discovered Tremadoc trilobite fauna (including pilekiid n.gen.,n.sp.) and are overlain by the five formations of the Lower Ordovician Thung Song Limestone Group. The Upper Cambrianl^ and Lower Ordovician trilobites of the Tarutao Sandstone are similar to those of Australia.

222

Hardmanoeeras is found in Upper Arenig open subtidal limestones on Tarutao island, and is also known from the Langkawi Islands of Malaysia, the Canning Basin (W.A.) and N. China. Six nautiloid genera have been discovered in mainland central and southern Thailand in dominantly peritidal Lower and Middle Ordovician carbonates. These have strong affinities with E. Australia and N. China. The Lower and Middle Ordovician of W.Queensland is dominated by Geovgina^ Armenoceras^ Discosorida n.gen.,n.sp. (Wade ms.), AoHnooeras^ with rare Wutinoceras. All these genera occur in S. and C. Thailand but not on Tarutao.

Discosorida n.gen.,n.sp., Armenoceras^ Wutinooeras^ Manchurooeras and Chaohuceras occur in C. and S. Thailand and in Tasmania. Wutinoeeras robustum occurs in the Langkawi Islands and in North C h i n a . S h a n Thai block has two nautiloid genera in common with Tibet, three with N. America and two with S. China.

The similarity of the nautiloid faunas of Shan-Thai with Australia and N. China suggests that reconstructions placing Shan-Thai at a great distance or in a widely different climatic belt are unlikely.

Either juxtaposition or proximity of Shan-Thai Block and W. and/or N.W. Australia is suggested by our data. This conclusion contrasts with that of Haile^^ based on palaeomagnetic work from the cleaved and thermally metamorphosed (>250°C) Setul Limestone of the Langkawi Islands.

REFERENCES: 1. Bunopas, S., 1982. Joint ASCOPE/CCOP workshop on hydrocarbon occurrenoe in carbonate formations (Indonesia). 2. Ounchanum, P., 1978, Journ.Soi.Fac. Chang Mai Uni.y spea.issue^ 77-95. 3.Ridd,M.F, 1971, Nature,, 234, 531. 4. Audley-Charles, M.G., Milson, J.S. and Carter, D.J., 1972, Nature, 239, 35. 5. Stauffer, P.H., 1974, Bull.

Geol.Soc.Malaysia, 7, 89-138. 6. Ziegler, A.M., 1981. In McElhinny, M.W. and Valencio, D.A., Paleoreoonstruction of the continents.

7. McTavish, R.A. and Legg, D.P., 1976, In Bassett, M.G., The Ordovician System (Cardiff), 447-448. 8. Griffiths, J.R. and Burrett, C.F., 1973, Nature, 245, 92-93. 9. Stauffer, P.H. and Gobbett, D.J., 1972, Nature, 240, 139. 10. Burrett, C.F., \^lk,Earth & Plan.Sci.Lett., 21, 181-189. 11. Smith, A.G., Hurley, A.M. and Briden, J.C., Phanerozoic Paleocontinental world maps (Cambridge Uni.Press). 12. Burrett, C.F., 1982. In Carey, S.W., The Expanding Earth Symp. (Uni. of Tasmania).

13. Haile, N.S., 1980, Earth & Plan.Sci.Lett., 48, 233-236. 14. Carey, S.W., 1976, The expanding Earth (Elsevier). 15. Kobayshi, T., 1957, Jour.Fac.Sci.Imp.Univ.Tokyo, Sec 2, 10/3, 367-382. 16. Stait, B.A. and Burrett, C.F., 1982, Alcheringa, 6/13, 193-196.

We thank S. Bunopas, T. Wongwanwich, S. Muenlek, W. Tansathein, for invaluable help in Thailand and Chen Junyuan (Nanjing), R. Flower (Socorro), P. Jell (Melbourne), C. Teichert (Rochester) and M. Wade (Brisbane) for advice and information.

223

MIDDLE PALAEOZOIC FAUNAL PROVINCIALITY IN RELATION TO CRUSTAL BLOCKS IN ASIA John A. Talent School of Earth Sciences, Macquarie University, North Ryde, NSW

Fair resolution has been developed for chronologies of fragmentation, dispersion and amalgamation of former continents, mini/micro-continents and island arcs since the Jurassic. In contrast, the Palaeozoic context, where there is no benefit of constraints imposed by sea-floor magnetic anomalies, resembles a morass. Palaeomagnetic data has enabled determination of the latitudinal location of major bits and pieces, and much has been learned about the timing and nature of collision between them from tectonic, petrologic, sedimentary and palaeontologic (biochronologic) data. Inadequacy of longitudinal constraints, however, has led to important differences in models proposed for the disposition and motions of continent and island arc fragments during the Palaeozoic, especially in pre-Carboniferous reconstructions, and particularly for Asia.

Protracted isolation of marine and non-marine floras and faunas, e.g. benthic shelf faunas separated by deep oceanic barriers, is expressed as increased provinciality; the approach and interplay of continental margins results in reduction and ultimately disappearance of provincial differences. Always bearing in mind blurring factors such as intervening island or submarine plateau staging points, land barriers, palaeo-oceanic circulation patterns, depth and palaeolatitude differences, differing lengths of pelagic larval life, and the inevitable problems of sampling biota from the past, the degree of provinciality, broached with caution, may be used therefore to provide an impression of relative juxtaposition or isolation at any given time. There are two ways of broaching this problem with past faunas: 1 (the more qualitative). From the spectrum of information presently available on pelagic larval lives of benthic organisms, it can be inferred that deep-sea barriers of 700-1000 km will produce such filtering out of brachiopods, echinoids, benthic molluscs and so on as will lead to sharply differentiated provinces in the course of time. This sort of data can be useful in setting approximate limits to former proximity of crustal blocks.

2 (the more quantitative). In order to develop some provisional _ equations connecting distance and degree of faunal dissimilarity, distribution data on living organisms in relation to the contemporary_system of biogeographic provinces can be computerized. On the assumption that proneness to provinciality of the individual phyla has not changed significantly through time, the resultant equations for genera and species can then be used to produce notional distances apart of the various ^ hypothesized blocks. An exercise of this kind has been carried out using the distribution data on all living brachiopods, echinoids and comatulid crinoids.

Though in some recent reconstructions Asia, apart from India, Iran and Turkey, continues to be treated as a single entity, it is now generally agreed that it is a composite of crustal blocks. More than

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twenty of these delineated by ophiolite belts, major geosutures, flysch wedges and radiolarite developments can be delineated between the Urals and Caucasus, Kalimantan and Chukotka. The accretionary history is complex, especially in the Altai-South Mongolian fold-belt where ophiolites of various ages from Vendian to perhaps early Mesozoic (in the east) are represented. In a general way, however, the age of suturing of successive major blocks decreases radially from the Angara Block, for example on a southern transect from Late Devonian/earliest Carboniferous on the Irtysh Suture (Kazakhstania with Angara) to Eocene (India with the South Tibet and Helmand/Central Afghanistan blocks).

Similarly there is a sequence of Permian to Late Triassic events in the suturing of the constituent blocks of eastern China and SE Asia.

It is against the above background that Palaeozoic and especially Middle Palaeozoic marine faunas of Asia are analysed in a progress report that utilizes both of the biogeographic approaches presented earlier. Not all blocks have adequate faunas for useful analysis, but many do for appreciable parts of the record; for these there is often a clear waning of provincial contrast prior to suturing. Despite areas of poor palaeontologic and tectonic information, the picture of Asia (Eurasia after the Late Carboniferous closure of the Urals Suture) that emerges is reasonably clear. Tethys and, by implication, the northern margin of East Gondwanaland changed markedly through the PalaeozoicTriassic interval as block after block moved towards ultimate aggregation with Asia. Tethys, if not a chimaera, is certainly chameleonici

PERMIAN MARINE BIOTIC PROVINCES OF THE GONDWANAN REALM: AN ASSESSMENT BASED ON THE BRACHIOPODA N.W. Archbold Department of Geology, University of Melbourne, Parkville, Victoria The Gondwanan Realm includes varied Permian faunas from a wide geographic region. Subdivision of the Gondwanan Realm into provinces is suggested by an analysis of not only brachiopod generic distribution and diversity but also of species diversity within genera from province to province.

Members of the Brachiopoda appear particularly sensitive, as shown by their distribution, to water temperature, and this, in turn, can be used to demonstrate thermal barriers prohibiting migration.

A review (in press) of described and illustrated species of Permian chonetidine brachiopods of the Gondwanan Realm and an analysis of generic and specific distribution, permits the identification of the five faunal provinces shown below. Analysis of other brachiopod groups tends to confirm the proposed provincial subdivisions.

The Andean Province shows a strong Grandian (Texan) relationship but faunas are of lower diversity than the tropical faunas of Texas.

Andean faunas are readily distinguishable from those of northern and

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eastern Gondwana where distinctive chonetidine, strophalosiidine, productidine, strophomenid and spiriferid genera are found.

Brachiopod faunas of the Paratinan Province are poorly known; chonetidines are apparently significant for demonstrating provincialism during the Early Permian (Asselian - Sakmarian) and the province is distinctive by the Later Permian as shown by endemic molluscan faunas and an absence of brachiopods.

The Austrazean Province, incorporating Eastern Australia and New Zealand, supports some 62 genera of brachiopods, some 42 of which are not known from the Westralian Province. Approximately 33 out of 47 genera from New Zealand are shared with Eastern Australia (where 48 genera are found) and this number will probably increase as Eastern Australian faunas become better known.

The Westralian Province, with some 61 genera of brachiopods, of which about 40 are not found in the Austrazean Province, is readily separated from the latter province. The genera shared between the two provinces demonstrate different species diversities between the provinces and in some cases the species of each province belong to different stocks within the genera.

The Cimmerian Province, much of which was rifted off from northern Gondwana in the Mesozoic, stretches from Tunisia to Irian Jaya and possesses complex, high diversity faunas, generically of much higher diversity than the faunas of the Westralian Province. Faunal links with the Westralian Province are significant, while links with the Austrazean faunas are relatively minor, although a little stronger in the Early Sakmarian. The Cimmerian Province represents the subtropical and tropical region of northern Gondwana during most of the Permian, following the initial cooler temperatures during the Asselian and Tastubian.

Marine Permian Biotic Provinces of Greater Gondwana

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THE PERMIAN BRACHIOPOD FAUNAS OF IRIAN JAYA, INDONESIA: GONDWANAN OR SOUTH EAST ASIAN? N.W. Archbold Department of Geology, University of Melbourne, Parkville, Victoria The extensive Permian sequence or Aifam Group of Irian Jaya has been a "Terra Incognita" until the recent mapping carried out by the Irian Jaya Geological Mapping Project (a joint project carried out by the Indonesian Geological Research and Development Centre and the Australian Bureau of Mineral Resources under the auspices of the Australian Columbo Plan). Marine Permian faunas have been collected from the lowest or Aimau Formation and the middle Formation or Aifat Mudstone, whereas the upper or Ainim Formation contains terrestrial plant fossils of the Glossopteris Floral Realm. A progression of faunas can be recognised although further collections are expected to amplify the succession.

The oldest fauna has been found in float material from the Aimau Formation and contains an impoverished assemblage with representatives of Streptorhynchus and Cyrtella (s.l.). The Streptorhynchus is a large, finely ribbed species recalling similar species from the Tastubian faunas of the Lyons Group of Western Australia and correlative faunas of Tasmania. Cyrtella (s.l.) is common in Early Sakmarian faunas of Gondwanan affinity. Rare, poorly known, Taeniothaerus is also present.

A number of localities in the overlying Aifat Mudstone appear broadly correlatable and yield faunas containing variably known representatives of Streptorhynchus, Neochonetes (Sommeriella), small Heteralosia, Taeniothaerus, Dyschrestia Stereochia, Neospirifer, Spiriferella, and "Martinia". Comparison of these faunas with the Sterlitamakian - Aktastinian faunas of Southern Thailand (Waterhouse, 1981) is indicated. As with those faunas links are present with the faunas of similar age from Western Australia and the added occurrence of Taeniothaerus in the Irian Jaya faunas (absent in Thailand) is noteworthy. Chonetinella andamanensis from Southern Thailand is close to specimens of Neochonetes (Sommeriella) pratti from the Sterlitamakian of the Canning Basin and to Neochonetes (Sommeriella) sp. nov. from Irian Jaya.

Younger faunas, from localities within the Aifat Mudstone, were described and dated by Archbold (1982) as being Latest Baigendzinian/ Kungurian in age. They include representatives of the genera Streptorhynchus, Linoproductus, Stereochia, Stictozoster, Stenoscisma, Cruricella and Hustedia that are close to species from the Rat Buri Limestone of Southern Thailand. Localities with Quinquenella magnifica may be a little older (Early Late Baigendzinian) by comparison with Western Australian Quinquenella. These younger faunas show strong Tethyan links and yet retain some relationships with warmer regions of Gondwana such as Western Australia and are the youngest Permian faunas known from Irian Jaya at present.

The faunal diversity and affinities of the faunas outlined above indicate a marked amelioration of the Permian water temperatures of Irian Jaya from the Tastubian to the Kungurian, as in Western Australia,

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The Early Permian faunas are strongly Gondwanan in affinity and although the younger faunas contain warm water Tethyan elements, links persist with other Gondwanan regions.

REFERENCES ARCHBOLD, N.W., 1982. Permian brachiopods from Western Irian Jaya, Indonesia. Geol. Res. Dev. Centre, Pal. Ser., 2: 1-25.

WATERHOUSE, J.B., 1981. Early Permian brachiopods from Ko Yao Noi and near Krabi, Southern Thailand. Mem, geol. Surv. Thailand, 4: 43-213.

AGE, ORIGIN AND EVOLUTION OF DEEP-SEA AND ANTARCTIC FAUNAS C.F. Hickman The Australian Museum, College St, Sydney No Abstract provided

ACRITARCH ASSEMBLAGE FROM THE KOCKATEA SHALE, LOWER TRIASSIC, PERTH BASIN, WESTERN AUSTRALIA K.K. Sappal Department of Geology and Geophysics Western Australian Institute of Technology, Bentley, W.A. The acritarch assemblage described in the paper is of Lower Triassic age separated from the subsurface samples of the Kockatea Shale, Perth Basin, Western Australia, encountered in the BMR NO. 10 Bore (Beagle Ridge, 29°49'38"S, 114°58'30"E) drilled in 1959. The well had penetrated over 300 metres of dark grey and green shale with occasional siltstone and sandstone described as the Kockatea Shale

by McTavish (1965). Australian arcritarchs of triassic age have previously been described by Balme (1963), Norris (1965), Playford (1965) and Medd (1966). The assemblage of acritarchs described from the Kockatea Shale is characterised by the relatively small size of individuals, the overall size of visicles ranges between 14 and 34 y, and the majority of forms is below 20 y in dimension. Another important feature is the presence of simple processes which usually communicate with vesicle interior. Details of the surface ornamentation on vesicles and processes can only be seen under the scanning electron microscope. Twenty two species of acritarchs representing acantho-

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morphs, polygonomorphs, netromorphs and sphaeromorphs are identified in the assemblage from intervals 659.3 to 1053.4 metres of the bore.

Acanthomorph Micryhstridium and polygonomorph Veryhachium are the most abundant genera in the assemblage.

The vertical distribution pattern of spores, pollens, Micrhystridium species, Veryhachium species and other minor acritarchs shows that the core intervals between 732.1 to 1053.4 metres are abundant in acritarchs and represent a distinctive stratigraphic zone which marks the presence of a marine transgression in the basin. The core intervals between 530.1 to 732.1 metres contain abundance of spores and pollens and represent second stratigraphic zone of marine regression in the basin. This Western Australian evidence of Lower Triassic marine transgression is in agreement with Kummel and Steele (1962) for world wide extensive Lower Triassic Seas. The Micrhystridium component of the assemblage is 42.5% in the core samples and the Veryhachium component is 43.0%. The near equal abundance of the two genera suggests that the environment of deposition for the Kockatea Shale to be marine not far offshore and not apparently enclosed.

The acritarch assemblage of the Kockatea Shale has similarity to the assemblage described by Sarjeant (1973) from the Mianwali formation of the Salt Range, West Pakistan. The similarity of the assemblages is of some stratigraphic significance for the correlation of the Lower Triassic succession of the Gondwanaland.

References Balme, B.E., 1963: Western Australia.

Plant microfossils from the Lower Triassic of Palaeantology, V. 6(1), p. 12-40.

Kumiael, B., and Steel, G. 1S62: Ammonites from the Meekoceras gracilialatus zone at Crittenden Spring, Elko Country, Nevada.

J. Paleontology, V. 36, p. 638-703.

McTavish, R.A., 1965: Completion Report B.M.R. 10 and lOA, Beagle Ridge, Western Australia. Rept. Bur. Min. Resour. Aust. Geol.

Geophys., 80.

Medd, A.W., 1966: The fine structure of some Lower Triassic acritarchs, Palaeontology, V. 9(2), p. 351-354.

Norris, G., 1965: Triassic and Jurassic miospores and acritarchs from the Beacon and Ferrar Groups, Victoria Land, Antarctica, N.Z. J. Geol.

Geophys. V. 8(2), p. 236-277.

Playford, G., 1965: Plant microfossils from Triassic sediments near Poatina,'Tasmania, J. Geol. Soc. Aust., Vol. 12, p. 173-210.

Sarieant W.A.S., 1970: Acritarchs and Tasmanitids from the Mianwali and Tredian Formations (Triassic) of the Salt and Surghar Ranges, West Pakistan. In Logan, S., and Hills, G. (Editors). The Permian and Triassic systems and their mutual boundary. Can. Soc. Petrol.

Geol. Mem. V. 2, 35-73.

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BUCHANOSTEIDS (ARTHRODIRA) IN THE NORTHERN HEMISPHERE E. Mark-Kurik Institure of Geology of ESSR, Academy of Sciences, Estonia USSR No Abstract provided

PRELIMINARY DATA ON NEW BRACHYTHORACID ARTHRODIRES FROM THE EMSIAN OF MOROCCO, AND THEIR BIOGEOGRAPHICAL IMPLICATIONS H. Lelievre Institute de Paleontologie, Museum National d'Histoire Naturelle, Paris, France No Abstract provided

EARLY (SILURIAN AND DEVONIAN) VERTEBRATE BIOGEOGRAPHY OF CHINA Pan Jiang (P'an Kiang) Museum of Geology, Beijing, China The early vertebrates in China are chiefly distributed in the south, east and west, but are entirely absent from the central region (the Sino-Korean Paraplatform) , and so far none has been reported in the Dzungar-Xingan (Khingan) region. They first appear in the Early Silurian (Upper Llandovery) marine formations of South China. The antiarchs are an important early vertebrate group in Late Silurian (or Middle Silurian) to Late Devonian strata, whilst the Eugaleaspida (Agnatha) are known only from China and are regarded as endemic early vertebrates. The antiarchs are numerous in south and west China; of about 20 described genera only 4 are cosmopolitan; Microbrachius (Givetian), Bothriolepis (Middle to Upper Devonian), and Asterolepis and Remigolepis (Upper Devonian). The other genera are all endemic.

Thus, the faunas under discussion consist mainly of genera unique to East Asia; in other words, they possess strong oriental provincial characteristics. In 1975-1982, the discovery of Coronocephalus rex, Kaila, Chu anqi anoproe tu s (trilobites) as well as Fardenia lauta, Nalivkinia, Nucleospira and Striispirifer cf. hsiehi (brachiopods) in the uppermost part of the Fentou and Goudingshan Formations of N. Anhui Province and near Wuhan City, Hubei Province, shifted the age of the Hanyangaspis goudingshanensis fauna (P'an et al. 1975) from Early Devonian to Middle Silurian. Such an age-correlation has been upheld for the past 7 years through much regional stratigraphical work in the lower Yangtze region.

It is clear now that Agnatha such as

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Hanyangaspiformes, and Sinacanthus, form the principal early vertebrate fossils of the Early to Middle Silurian in the Yangtze region and Qinling-Dabashan area, but so far none of them is known to persist into the Early Devonian.

On their morphology and histology, the hanyangaspid agnathans are neither typical Polybranchiaspiformes nor true Heterostraci. Recent evidence from graptolites indicates that at the top of the Cryptograptus lapworthi Zone in Ziyang, Shaanxi Province (W China), Monograptus priodon, Retiolites genitzianua, R. angustidens, etc. occur together. In the Stringocephalus Zone at the top of Qujing Formation (Givetian) of E. Yunnan, a new species of Microbrachius is the first of its kind to be discovered not only in China, but also in Asia. In this sequence the Middle/Upper Devonian boundary is at present put above the Microbrachius horizon.

No Stringocephalus is known to have persisted into the Upper Devonian in Yunnan. In both Hunan and Ningxia the first appearance of numerous species of Remigolepis is currently regarded as indicating the beginning of the Upper Devonian non-marine and marine series.

Associated with the fishes are Leptophloeum rhombicum, Sublepidodendron, etc.

On the basis of differences in biogeographic characteristics the early vertebrates are divisible into the following five principal biogeographic provinces andassemblages: I. Early-Middle-Silurian - Hanyangaspid-Sinacanthid Province includes the Yangtze region and adjacent areas of the Jiangnan (= south of the Yangtze) Province, S China Province and the Kunlun-Qinling Province. New evidence from Agnatha and Acanthodii indicate these also in the Rongxi Formation (Llandovery) of W Hunan.

II. Late Silurian. Antiarchs found near the base of the marine Late (or Middle) Silurian Kuandi Formation (Yunnan) and Xiaoxi Formation (Hunan), indicate that they appeared earlier in China than elsewhere in the world.

III. Early Devonian - Yunnanolepid-Eugaleaspid-Polybranchiaspid Province covers various Provinces in South China, especially in Yunnan and the lower Yangtze area, where the most famous locality is the Chuifengshan Hill, Qujing District, Yunnan.

IV. Middle Devonian - Bothriolepis-Quasipetalichthys-Hunanolepis Province.

On the basis of bio-/ and lithofacies. Middle Devonian fishes in China can be allocated to the following two regions: B. H. and Q. extend widely in south and south-west China, and B. and Q. occur in E Qilianshan.

V. Late Devonian - Sinolepis-Asterolepis, and Remigolepis Provinces. The former covers the Wutung Group in south-east China, the latter is represented by the Zhongning and Fsikuangshan Formations extending from west China to south China.

Presumably, during the Ordovician/Early Silurian to Devonian, south and east China were part of an isolated faunal province in which eugaleaspid, Acanthodian and Placoderm fishes experienced a major evolutionary radiation, independently from the rest of the world. The Yangtze area is where eugaleaspids, Acanthodii, and Antiarchi originated. South China is clearly a province where early vertebrates underwent a major radiation. A dispersal centre of gnathostomes in

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south China, with dispersal to Australia and Antarctica on the one hand, and to Europe and North America on the other, would account for the space-distribution of the Pisces, Since Sinacanthus and Antiarchi appear as early as Early and Late Silurian in the Yangtze Region, the geological time-distribution of the genera concerned is also explained.

BIOGEOGRAPHY OF DEVONIAN VERTEBRATES: A VICARIANCE APPROACH G.C, Young Bureau of Mineral Resources, Canberra ACT

At least five biogeographic provinces based on endemic vertebrate taxa can be defined for the Early Devonian (Euramerica, Siberia, Tuva, South China, East Gondwana). These are compatible with geological and/or palae©magnetic data indicating separate histories for these areas during part or all of Devonian time.

Given such a pattern it may be asked how the biotic affinities between such provinces can provide insight into the past spatial relations of the areas concerned. In general comparisons between faunal provinces based on overall similarity or taxa in common emphasise the dispersal capabilities of these taxa and the effectiveness of barriers, but the latter may or may not bear a direct relationship to distance between areas. Under an allopatric speciation model it is probable that contiguous faunal provinces separated by effective barriers may contain no low-level taxa in common. Relevant here is the question of how adjacent faunal provinces may have developed. The vicariance approach assumes that faunal provinces containing related taxa arise by allopatric speciation when a more extensive ancestral province is subdivided.

Biotic affinity is seen in terms of relationship, not similarity, and at least 3 biotas or areas must be compared for these to be meaningful. It is suggested that information about the relative recency of connection between different areas, based on the relative recency of their common ancestral biotas, may be obtained by analysing the phylogenetic relationships of taxa endemic to those areas.

In the Late Silurian/Early Devonian the major groups of early vertebrates were apparently widespread in shallow marine environments. The invasion of freshwater caused endemic forms to develop in different regions. Two episodes of biotic dispersal are indicated by the Devonian vertebrate record : at the end of the Silurian (between Laurentia and Baltica), and during the Late Devonian (between Gondwana and Euramerica). Preliminary analysis of placoderm taxa suggest a Gondwana origin for Phyllolepis and Wuttagoonaspis, and affinity between East Gondwana and South China (antiarchs).

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Symposium 3(d) The geological time-scale: biostratigraphy, magnetostratigraphy and geochronology Conveners: Dr J.H. Shergold & Dr I. McDougall

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EDIACARAN SYSTEM R.J.F. Jenkins Department of Geology and Mineralogy, University of Adelaide, S.A. The Ediacaran System^ comprises the sedimentary cycle marking the close of Proterozoic deposition in the Flinders Ranges. In its sequential development - tidal dolomite, locally euxinic deep basinal deposits, flysch, prograding slope and shelf carbonates, molassic red arenites, and terminal, transgressive orthoquartzite - it resembles the repeated 'geosynclinal cycles' recognised by K.J. Hsu^ in the Valley and Ridge Province of the Appalachians.

There is general agreement that the distinctive assemblages of soft-bodied organisms characterising the interval, and now known from twenty or so different world localities, are broadly coeval in time and provide a potential basis for biostratigraphic correlation^.

Sequential occurrences of early metazoan remains occur in the Flinders Ranges, Namibia, southeastern Newfoundland, central England, northern and southwestern Russia,and Siberia. In the Flinders Ranges and England the earliest finds are represented by discoid objects or problematic markings. A succeeding assemblage, including Charnia, branching frondose forms, and distinctive ' medusoids*, suggests a possible correlation between central England, southeastern Newfoundland and northern Russia. A biota closely resembling the type Ediacara assemblage of the Flinders Ranges occurs a short distance stratigraphically above the Charnia assemblage in northern Russia.

Trace fossils made by arthropods occur in the type Ediacara assemblage; repetitive, reticulate patterns of short scratch marks indicate the feeding activities of an arthropod with multiple, doubleclawed limbs. Early Cambrian sandstones which succeed the Ediacaran sequences are characterised by two sequential trace fossil assemblages, the oldest including Arenicolites^ Rusophycus and Didymcculichnus, and the second Soolithos and Diplocraterion.

Recent investigation of trace fossils'^ suggests that the greater part of the Nama Group of Namibia is of Cambrian age; however it is still likely that the distinctive Rangea,Fteridinium assemblage from the lower Nama Group is broadly coeval with other Ediacaran assemblages. The cribricyathids {Cloudina) and tubular constructions of other worm-like creatures present in the lower Nama Group remain of uncertain stratigraphic significance. The lower Nama Group was possibly intruded by syenite and granite at 553 i 13 Ma^ and part of the succession including Diplocvatevion and Soolithos underwent sinkinematic metamorphism at 530 - 10 Ma^.

Geological and geochronological data for southeastern Newfoundland"^ ' ° and southern B r i t a i n ^ s u g g e s t that the rocks forming the basement to the Ediacaran cover were chiefly volcanic and underwent deformation or metamorphism with local crustal granitisation at ^605-590 Ma; coeval or slightly later intrusion of magmatic granites occurred at ^595-?585 Ma. Older parts of the Ediacaran cover are tuffaceous and megascopic biogenic remains may both predate and postdate localised explosive volcanism with associated subvolcanic intrusion. The Charnia assemblage postdates this waning phase of volcanism. Renewed magmatic activity and volcanism at "^>565-555 Ma was probably related to tectogenesis,

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and a passage through sandstones into thick molassic sequences characterises the time of major uplift and cooling at about ?550 Ma. Post-tectonic, sub-Cambrian diorites and granitoids give ages as late as 533^13 Ma^^, providing a maximum dating for the later part of the Tommotian transgression. This dating is supported by U-Pb zircon work on sub-Cambrian intrusives in Morocco. Based on the above interpretation, the start of the Ediacaran cycle is at about 590-585 Ma and the transition between the Ediacaran and Cambrian is near ^540 Ma. There is no direct evidence for the age of the movements which lead to the beginning of Ediacaran deposition in the Flinders Ranges. However a significant disconformity occurs at the base of the Ediacaran succession in the northern Flinders Ranges and it is speculated that this may be related to the Petermann Ranges Orogeny of the southern Amadeus Basin. This orogeny gave rise to an extensive Ediacaran molasse apron in central Australia. Mineral ages from metamorphosed granites suggest that metamorphism associated with the Petermann Ranges Orogeny occurred at ^590-^570 Ma. Several mixtites of presumed glacial origin and likely of different age occur in the Ediacaran successions, and one of the inferred glaciations (Namibia) is Cambrian.

References ^Jenkins, R.J.F., 1981. ^Hsii, K.J., 1973. mentology". pp. 66-92.

Trans. R. Soc. South Aust. 105 : 179-194.

In Ginsburg (Ed.) "Evolving concepts in sedi(Johns Hopkins University Press : Baltimore)

^Cloud, P. & Glaessner, M.F., 1982.

Science 217: 783-792.

^Crimes, T.P. & Germs, G.J.B., 1982.

J. Paleo. 56 : 890-907.

^Allsopp, H.L., Welke, H., Kostlin, E.D., Burger, A.J., Kroner, A. & Blignaut, H.J., 1979. Trans, geol. Soc. S. 82 : 185-204. ^Ahrendt, H., Hunziker, J.C. & Weber, K., 1978.

719-742. "^Williams, H. & King, A.F. , 1979.

Geol. Rund. 67 :

Mem, geol. Surv. Can. 389 : 24p.

^Dallmeyer, R.D., Odom, A.L., O'Driscoll, C.F. & Hussey, E.M. 1981.

Can. J. Earth Sci. 18 : 699-707 ^Beckinsale, R.D. & Thorpe, R.S., 1979.

433-439.

J. geol. Soc. London 136:

^^Patchett, P.J., Gale, N.H., Goodwin, R. & Humm, M.J., 1980.

J. geol. Soc. London 137: 649-656. ^^Forman, D.J., 1966.

87: 54p.

Rep. Bur. Miner. Resour. Geol. Geophys. Aust.

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THE NUMERICAL AGE OF THE BASE OF THE CAMBRIAN W. Compston

1

and Zi-Chau Zhang

2

J-Research School of Earth Sciences, Australian National University Canberra ACT ^Yichang Institute of Geology & Mineral Resources, Yichang, Hubei China.

Rb-Sr age determinations have been made on several sedimentary rock units near Lientuo (east Yangtze Gorge area, China) that are relevant to the numerical age of the Cambrian-Precambrian boundary.

The materials analyzed were illite- and carbonate-bearing shales as total rock samples, carbonate (HCl-soluble) and bulk illite fractions, and illite concentrates of different grain size. Two well-defined ages for illite growth were measured: 700 ± 5 Ma (2a) in the upper Toushantuo Formation (late Precambrian ) and 573 ± 7 Ma in the Shuijintuo Formation (lower Cambrian). A less well-defined age of 602 ± 15 Ma was found in the Tientzushan member of the Dengying Formation (Tommotian E basal Cambrian). Illite fractions below 1 ym from all rock units registered the effects of a weak thermal or chemical event at 460 Ma. The above values for age use the Mclntyre ^ al_. Model 3 for interpreting geological scatter in Rb-Sr isochronsT" Numerical values that are only slightly different are given by modelling the initial ®^Sr/®^Sr of the illites on contemporary seawater, as registered by the acid-soluble fractions and by massive carbonate samples.

Small but real changes in seawater are observed between sedimentary units.

Our results for the Shuijintuo Formation are somewhat younger than Rb-Sr ages reported by Ma £t (1980) and Huang S. (1982).

This i^ due at least partly to analytical problems and possibly to the presence of detrital micas also. Whether the Shuijintuo Formation contains recycled illite has been clarified recently by Huang and Zhou (1982), who determined a well-fitted U-Pb isochron for this unit at 56 7 ± 15 Ma. U and Pb in the shales is held under reducing conditions by carbonaceous material, and is thus involved in a completely different geochemical system to Rb and Sr. The close agreement of the two ages implies that both correctly designate the period of chemical changes within the shale in early diagenesis.

The above dates of 700, 602 and 573 Ma probably measure the successive times of principal growth of illite in situ after the deposition, respectively, of the Toushantuo, Dengying and Shuijintuo Formations.

Thus, 602 ± 15 Ma would be regarded as a minimwn age estimate for the deposition of the Tientzushan member and subject to agreement over biostratigraphic definitions, for the Cambrian-Precambrian boundary also.

The recent suggestion by Odin and Gale (1982) of 530 Ma as the best estimate for the base of the Cambrian cannot be correct in view of the present results. Close examination of some of the evidence used by Odin and Gale as well as earlier work which they chose to ignore, is also strongly against the 530 Ma figure.

236

THE EARLY TO MIDDLE CMIBRIAN BOUNDARY IN AUSTRALIA P.A. Jell National Museum of Victoria, 285 Russell Street, Melbourne, Victoria 3000 Confusion over the Early to Middle Cambrian boundary is international and is still far from solved. In the Viking (Atlantic) faunal province the boundary has been placed at the first appearance of Paradoxides, in the Columban province it has been the last appearance of OlenelTus, and in the Tollchuticook province (Asia, Australia, Antarctica) it has been the last appearance of Redlichia. Opik has disputed the latter and insisted that Redlichia extended higher in the Australian section than it did elsewhere.

It now seems more appropriate to use miomeroid trilobites to correlate Cambrian sequences where ever possible.

I accept recent calls to abandon difficult to correlate, Cambrian Series boundaries in favour of intraseries time planes that are able to be correlated internationally using species as determinants. Nevertheless the time interval in question is important and deserves the same consideration that it would as a Series, Period, or Eon boundary.

I adopt as a tentative guide to a time plane at this stratigraphic level the first appearance of Pagetia which seems to follow immediately the disappearance of Pagetides.

This allows correlation between and within the Columban and Tollchuticook provinces and the former may be correlated with the Viking province using other miomeroids of the "Pagetides Fauna".

In Australia the Mcotwingee sequence is critical and recent excavation of that section suggests an unconformity between the upper Cymbric Vale Formation (containing Pagetides (=Discomesites Opik 1976) and the overlying First Discovery Limestone Member of the Coonigan Formation containing Pagetia.

Associated with Pagetia in this limestone are Redlichia amadeana Opik, Onaraspis somniurna Opik, Peronopsis longinqua Opik, new species of Xystridura and Dinesus and a number of ptychoparioids including Probowmaniella. This association includes species that are used by Opik as the zone fossils at the base of his Ordian Stage and the middle of his Templetonian Stage. It appears that his Ordian is a relatively short interval of time and is in fact Middle Cambrian; Redlichia is known to occur with Probowmaniella in the Middle Cambrian c F China" so Redlichia now appears to extend higher than previously thought in that country also.

Of other Australian sequences:- 1, discovery of a protolenid trilobite in the Heathcote volcanic sequence tends to confirm its Early Cambrian age.

2, Anadoxides in the Moodlatana Formation near VJirrealpa, South Australia suggests an Early Cambrian age which makes the underlying Wirrealpa Limestone with Redlichia, also Early Cambrian.

3, in northern Australia the common occurrence of Xystridura and Redlichia indicates a short interval of time for the Ordian also and suggests that the first appearance of Xystridura and Pagetia may be contemporaneous and furthermore may be at the base of the Middle Cambrian.

237

CAMBRIAN-ORDOVICIAN BOUNDARY AT MOUNT PATRIARCH, NEW ZEALAND A. J. Wright

1

& R. A. Cooper

2

^Department of Geology, University of Wollongong, Wollongong, 2 New Zealand Geological Survey, Lower Hutt, New Zealand

N.S.W.

Cambrian-Ordovician strata at Mount Patriarch, Northwest Nelson, New Zealand, yield two successive, different trilobite faunas.

About 30 species are known, most being represented by few, highly deformed specimens.

The lower fauna is from the Patriarch Formation (400m thick) which is dominated by calcareous siltstone and slates.

The most abundant species is Ruapyge hectori which, along with common but very poorly preserved asaphids, is often preserved as entire tests. Other members of this lower fauna include Pseudagnostus, L. (Lotagnostus), Amzasskiella, Proteuloma, a new shumardiid genus and fragments of Hedinaspis.

No conodonts are known from the exposures in the immediate vicinity of Mount Patriarch. Comparison with similar faunas from similar sequences (especially from S.E. China) suggests that the fauna lies close to the Cambrian-Ordovician boundary (i.e. close to the Olenidian-Tremadocian boundary).

The biogeographic affinities of the trilobites are with Asian (?Pacific) and European faunas.

The higher fauna from slatey and silty interbeds in the Summit Limestone includes Kainella meridionalis, Parabolinella, Apatokephalus, Plicatolina and Onychopyge, thus having elements from North and South America as well as Europe. The age assigned to this fauna on the basis of trilobites is late Tremadocian at the youngest.

Conodonts reported from the Summit Limestone by Cooper & Druce (1975) included the Drepanodus? gracilis-Scolopodus sexplicatus fauna to which the authors ascribed an early Arenigian age; this conodont fauna is currently thought to be from an interval which includes the Kainella occurrence.

The Cambrian-Ordovician boundary is provisionally located within the Patriarch Formation, at about 200m from the top of the unit.

References Cooper, R.A. & Druce, E.C., 1975. Lower Ordovician sequence and conodonts, Mount Patriarch, North-west Nelson, New Zealand.

Geol.

Geophys. 18, 551-82.

238

COMPARISON BETWEEN STANDARD BRITISH AND VICTORIAN ORDOVICIAN GRAPTOLITE ZONES A.H.M. VandenBerg Geological Survey Division, Department of Minerals and Energy, 140 Bourke St., Melbourne, Victoria 3000 By virtue of its historical priority, the British Ordovician graptolite subdivision has been widely used as a correlation 'standard'.

It has some serious drawbacks, however: below the N. gracilis Zone, the faunas consist of long-ranging forms that are almost all endemic to NW Europe and there is an almost total lack of the shortlived near-cosmopolitan forms that characterise the Victorian succession.

The more finely subdivided and easily correlated Victorian succession is gaining increasing acceptance as a correlation standard for regions outside NW Europe.

The Tremadocian and Lancefieldian stages have important genera in common Dictyonema \ Clonograptus, Anisograptus ) but lack of common species prevents correlation of zones.

The four zones of the Arenigian are equivalent to 4 entire Victorian stages (see Table), but there are few clues to correlation.

One of the few links was removed recently when it was discovered that protobifidus' is different the Victorian Chewtonian index ' Didymogr. from the British D. protobifidus.

The I . gibberulus Zone is correlated with the upper Castlemainian because I . v. lunatus occurs below I . gibberulus near Oslo.

The D. hirundo Zone contains the Yapeen-Darriwil species Pseudotrigonogr. ensiformis and also the short-ranging genus Undulograptus, indicating correlation with the Yapeenian and the Dal Zone of Undulogr. austrodentatus.

Bo

Ea

Gi Da

C? extraordinarius D. ornatus-C. latus Clim uncinatus Dicellogr. gravis Dicran. hians kirki Clim. baragwanathi TT. spiniferus n.ssp. (no Nemagraptus) Nemagraptus gracilis Ulypt. teretiusculus Pseudocl. decoratus ' Glypt.' intersitus Und. austrodentatus

C? extraordinarius Dicellogr. anceps Dicell.

Pleurogr.

Ashgill

complanatus linearis Caradoc

Diplogr.

Nemagr.

Didymog.

multidens gracilis murchisoni

Didymogr.

bifidus

Didymogr.

Llandeilo Llanvirn

hirundo

Yapeenian Isogr.

gibberulus

Didymog.

nitidus

Castlemainian Chewtonian Bendigonian

_Pj.dyjriog._ __ d e fi ^xus

Lancefieldian

JCJono^_._^neUus 'D.' flabelliforme

Arenig

Tremadoc

CORRELATION OF VICTORIAN AND BRITISH GRAPTOLITE ZONES

239

The Llanvirnian can be correlated with the rest of the Darriwilian with rather more confidence. 'Diplogr:' ellesae from the British D. bifidus Zone is here regarded as a subspecies of, and correlating with, the Da3 index Pseudoclimacogr. decoratus.

Skevington (1969) showed that the British Zones of G. teretiusculus and N. gracilis have essentially similar faunas and advocated that the former be dropped.

Unlike its British namesake, the Victorian G. teretiusculus Zone certainly does not contain Nemagraptids or Dicranograptids and may therefore correlate with the D; murchisoni Zone.

The entry of Dicranograptus and Dicellograptus , soon followed by Nemagraptus gracilis, marks the base of the Gisbornian and has been documented from many other places.

In Britain, it heralds the appearance of much more cosmopolitan faunas which permit much better correlation. The D. multidens Zone contains Climacogr. bicornis and thus correlates with the Gisbornian, but the entry of the typically Eastonian group of Clim. spiniferus , which is of global significance, has yet to be precisely documented in Britain.

The D. clingani Zone is probably wholly early Eastonian; it contains a typically Eal-Ea2 assemblage of Neurog. margaritatus, Dicr. nicholsoni,

D.

ramosus

and

Corynoides

.

The succeeding

P.

line-

aris Zone was formerly correlated with the early Bolindian which contains locally abundant Pleurograptus, but P. linearis is now known to range from Ea3 to upper Bolindian. Correlation of the P. linearis Zone with the Ea3 Zone of Z). hians kirki is indicated by the first appearance of Clim.

Dicellograptus

tubuliferus, Orth. elegans in both.

quadrimucronatus

spinigerus

and

The British Z). complanatus Zone contains a very poor fauna and has been very difficult to correlate.

The recent discovery of the Ea4 index Dicellograptus gravis with D. complanatus at Girvan, Scotland (S.H. Williams, pers. comm.) provides strong grounds for correlation the D. complanatus Zone has formerly been correlated with rather higher levels.

The Bol index species Clim. uncinatus is not known from Britain, and it seems likely that much of the early Bolindian correlates with the unfossiliferous interval between the Complanatus and Anceps Bands at Dob's Linn. The D. anceps Zone has recently been subdivided into two subzones, of which the upper Paraorthogr. pacificus Subzone has a fauna that is almost identical to the upper Bolindian Zone of D; ornatus and C. latus.

This fauna is succeeded by beds containing Climacogr? extraordinarius and very little else in both Scotland and Victoria; these in turn are followed by the basal Silurian index species Glyptogr?

persculptus.

REFERENCES Skevington, D., 1969: The classification of the Ordovician in Wales. in: The Precambrian

and

Lower

Palaeozoic

Rocks

of

Wales

(Ed.

A. Wood), p. 161-79, U. Wales Press.

Victorian subdivision from VandenBerg, 1981: Victorian stages and graptolite zones, in: The Zealand and Antarctica

Notes (Ed. B.D. Webby).

Ordovician Correlation

System Chart

in Australia, New and Explanatory

Publ. I.U.G.S., 6:2-7, and Fig. 2.

240

U-Pb DATING OF ZIRCONS FROM LOWER PALAEOZOIC TUFFS USING THE ION MICROPROBE W. Compston, I.S. Williams, D. Froude and J.J. Foster Research School of Earth Sciences, Australian National University Canberra ACT The ages of zircons from volcanic ash horizons within the British Ordovician and Silurian stratotypes were measured recently by Ross e^ al (1982) using the U fission-track method. U-Pb ages for zircons from the identical mineral concentrates are currently being measured at R.S.E.S. using.the high mass-resolution ion microprobe, both to independently check the fission-track results and to further assess the utility of the ion probe for dating young (=Palaeozoic) zircons.

238 206 The most precise ion probe results were from the ^^UPb system. This is due to the large amount of radiogenic present, compared with other Pb isotopes, which gives good ion-counting statistics and is insensitive to error in the correction for common Pb. The 20 7p^__2 35y system is less precise as the amounts of are lower by a factor of 15. The age is the least useful owing to the very small change in radiogenic since the Palaeozoic, as well as its dependence upon an accurate correction for common Pb. Our results for the Llandovery zircons from Ross e_t ad.'s sample of Birkhill Shale shown in Figure 1 illustrate the above. Analyses^of ^^^ eight zircons combine to give 431 ± 4 Ma (±population o) for Pb/ U, 430 ± 18 Ma for and 425 ± 80 Ma for with the observed scatter being due entirely to experimental error.

The accuracy of the U-Pb ages is dependent upon bulk U-Pb agedeterminations by isotope dilution on our primary zircon standard^^and upon an empirical calibration curve by which the measured Pb+Z U , as the ratio of sputtered ions, is corrected to as a concentration-ratio in the target zircon.

Our results for the Llandovery sample agree well with the fissiontrack and other age estimates for this horizon. Results for three other rock units are more complex, owing to the presence of inherited zircons and perhaps to post-depositional loss of Pb from some grains.

The Laidlaw Volcanics contains a lower 'group' of 10 grains that average 423 ± 3 Ma (a mean), in agreement with Rb-Sr and K-Ar agedeterminations . However, all results greater than 440 Ma must be attributed to xenocrystic zircons. The Pont-Y-Ceunant Ash analyses show a strong peak at 444 ± 1 Ma (a m) which is most simply interpreted as the age of magmatism, plus a few grains up to 650 Ma which are clearly inherited. However, the fission-track age for this sample is 460 ± 14 Ma and the Rb-Sr age of the Caradocian Kinnekulle Bentonite is 455 ± 2 Ma. Zircons from the Decorah Formation show the widest dispersion in age, with some xenocrystic cores indicating an original age of 1500 Ma. The inclusion of only a small amount of such material in the magmatic grains, e.g. as embedded zircon powder during sample polishing or as radiogenic Pb in minute inclusions, will seriously bias their apparent ages. The simplest interpretation of the present data is to assign the frequency peak at 430 ± 1 Ma to the magmatic age, and analyses greater than 440 Ma to contamination by inherited Pb. However, this interpret-

241

ation conflicts with the fission-track result of 456 ± 11 Ma and with the Caradoc biostratigraphic assignment. If the true age is in fact 456 Ma, it can not be recognized from the present ion probe results and the apparent 430 Ma peak must be attributed to post-magmatic Pb loss.

J

R

— R

R

'

BIRKHiLL SHALi-

DECORAH; FORMATION

PONT-Y-CEUNANT ASH

LAIDLAW

VOLCANiCS 1 1 !

4J0

a 4 0;

.fci. n

tEPW..... 1000 K^n

500

[

460

I

i Figure 1.

3 ages of Palaeozoic zircons as analyzed by the R.S.E.S. ion microprobe. Each square represents the mean of 3 successive analyses on one 25 ym spot, mostly within different zircon grains. The size of the square is approximately 1 standard error. The shaded squares signify magmatic zircons contaminated by old radiogenic Pb from inherited zircons within the sample and/or the inherited zircon itself. The triangles mark the ages expected for each unit based on stratigraphic and other isotopic data.

242

LATE SILURIAN AND EARLY DEVONIAN BIOSTRATIGRAPHY OF SOUTHEASTERN AUSTRALIA M. J . G a r r a t t ^Geological

Survey

^Department

of

1

& A, J .

Division, Department Melbourne, V i c .

Geology,

University

of

Wright of

2

Minerals

Wollongong,

and

Wollongong,

Energy, N.S.W,

A r e v i e w o f the c o r r e l a t i o n o f Late S i l u r i a n and E a r l y Devonian faunas o f S . E . A u s t r a l i a i s p r e s e n t e d .

The s t a r t i n g p o i n t used i s the work o f P h i l i p & Pedder (1967a, b ) which was the f i r s t comprehens i v e use o f c o n o d o n t s f o r E a r l y Devonian c o r r e l a t i o n s i n S . E . A u s t r a lia.

Late S i l u r i a n c o n o d o n t s were s t u d i e d by Link & Druce ( 1 9 7 2 ) .

S i n c e 1967 a d d i t i o n a l data have been hard-won, a t l e a s t p a r t l y r e f l e c t i n g the l a c k o f a c o n o d o n t - r i c h c a r b o n a t e sequence i n S . E . A u s t r a l i a o f the type d e v e l o p e d i n C z e c h o s l o v a k i a and Nevada.

Major a d v a n c e s i n c o r r e l a t i o n a r e now f a c i l i t a t e d by the f u l l e r documentation o f the Bohemian sequence o f f a u n a s , e s p e c i a l l y the c o n o d o n t s , p e r m i t t i n g a new w o r l d standard f o r c o r r e l a t i o n .

The main which 1.

thrust

of

our

contribution

is

based on p o s t - 1 9 6 7

data

include: N.S.W. a) Spathognathodus s p . c f . s h e l l y fauna i n the upper p a r t of

remscheidensis with a the Bungonia L i m e s t o n e .

varied

b) Eognathodus s u l c a t u s i n the T a y l o r s H i l l Formation a t Queens P i n c h , above N a d i a s t r o p h i a and Baragwanathia i n the Mullamuddy Formation. c)

Polygnathus perbonus

i n the

Sutchers Creek F o r m a t i o n ,

Queens

Pinch. d) Abundant Polygnathus a f f . perbonus and a s i n g l e specimen o f P. ? l a t i c o s t a t u s i n the J e s s e Limestone a t L i m e k i l n s .

This u n i t O v e r l i e s the R o s e d a l e S h a l e , from which Monograptus y u k o n e n s i s has p r e v i o u s l y been r e p o r t e d . e) At Mount Frome the o c c u r r e n c e o f the b r a c h i o p o d T a e m o s t r o p h i a w i t h R e c e p t a c u l i t e s and Polygnathus s p . c f . P. p e r b o n u s ; t h i s a s s o c i a t i o n s u p p o r t s the r e c e n t l y p r o p o s e d Z l i c h o v i a n age f o r the R e c e p t a c u l i t e s and Warroo Limestones a t Taemas.

Polygnathus c o s t a t u s p a t u l u s has been r e p o r t e d from the Mount Frome L i m e s t o n e , p r o b a b l y i n d i c a t i n g an f « f e l i a n age f o r the upper p a r t of the l i m e s t o n e .

2.

Victoria f) Monograptus a f f . u n c i n a t u s u n c i n a t u s and the Baragwanathia f l o r a o f the l o w e r p l a n t assemblage in i n t e r b e d d e d s t r a t a o f the Yea Formation a t Yea. g) Most i m p o r t a n t l y , r e c o g n i t i o n of the sequence of p l i i d faunas spanning the Late S i l u r i a n - E a r l y Devonian.

notano-

243

h)

Monograptids

within

the

i) Eognathodus sulcatus and strata at Thompson River.

"Yeringian"

sequence

Monograptus thomasi in

at

Lilydale,

interbedded

j) Recognition that the assemblages from the Loyola Limestone are diverse and link the highest faunas of the Lilydale sequence to the lower fauna of the Tabberabbera succession.

The occurrence of polygnathid conodonts in the limestone gives a maximum age of latest Pragian for the shelly faunas.

These selected items permit a refinement of previously accepted ages, in particular with reference to the better-known sequence of conodont faunas from Western Europe.

Further, the position of the Silurian-Devonian boundary in S.E. Australia can be determined with a greater degree of accuracy than before. A major thrust in our work is the progressive integration of shelly faunas (especially brachiopods and, to a lesser extent, trilobites) with the condodont-coral sequence recognised by Philip and Redder (1967a, b).

The amplified faunal sequence will be discussed in detail.

References Link, A.G. & Druce, E.G., 1972.

Ludlovian and Gedinnian conodont stratigraphy of the Yass Basin, New South Wales.

Bull. Bur. Min. Res. Geol. Geophys., 134.

Philip, G.M. & Pedder, A.E.H., 1967a. A correlation of some Devonian limestones of New South Wales and Victoria.

Geol. Mag. 104, 232-9.

Philip, G.M. & Pedder, A.E.H., 1967b. Stratigraphical correlation of the principal Devonian lim.estone sequences of eastern Australia.

Oswald, D.H. (ed.).

International Symposium on the Devonian System, Calgary, 1967, Alberta Soc. Petrol. Geol., Calgary, 1025-41.

SILURIAN-DEVONIAN GEOCHRONOLOGY

TEST CASE

FOR A NEW APPROACH TO ERECTING A TIME SCALE A.J. Wright, P.F. Carr & B.G. Jones Department

of

Geology,

University

of

Wollongong,

Wollonqong,

N.S.W.

Fossils from the Bungonia Limestone, N.S.W. (Fig. 1), indicate that the formation contains Late Ludlovian (Silurian) and Early Lochkovian (Devonian) strata. The limestone is overlain by the Tangerang volcanics and both formations are intruded by the Marulan Batholith (K-Ar age These data led to compilation and 397-1-7 Ma; Carr et a]^. , 1980). reassessment of biosTratigraphically well-controlled radiometric data for Silurian and Early Devonian strata (Jones ^ , 1981).

The quest for satisfactory integration of isotopic and biostratigraphic data to produce a geological time scale has intrigued qeologists since the initial development of radiometric dating techniques at the turn of the century.

In part, the problem stems from the general lack of rocks suitable for isotopic dating at, or very near to, the system boundaries as defined by biostratigraphically important fossil assemblages.

Most prior geochronological assessments have involved

244

listing the radiometric data in the standard biostratigraphic sequence and then making an intuitive judgment to determine the quantitative age of each individual stage, series or system boundary.

This intuitive method of data assessment is subjective in its approach so that, given the same data, various researchers may not necessarily determine the same age for a particular boundary.

Fig. 1 .

Simplified geological map of the Bungonia district showing localities of samples for age determinations.

A more objective approach to the problem of integration of these radiometric data with biostratigraphic data involves the use of a linear regression technique from which statistical errors are also derived.

This technique is applicable where the radiometric data are obtained from rocks whose age is well-controlled biostratigraphically (ideally confined to a single series or stage) ar^ where the relative durations of the systemic subdivisions have been estimated. Using these relative durations, it is possible to plot the radiometric ages (abscissa) against the cumulative relative duration (ordinate). In the case of the Silurian and Devonian systems, relative durations have been estimated by Boucot (1975) and Ziegler (1978) respectively, and there are 30 wellcontrolled radiometric ages for this time interval. Linear regression of these data yields an age of 402+4 Ma for the Silurian-Devonian boundary and the ages can be determined for each series and stage (Fig. 2a).

Where equal durations are assigned to the systemic subdivisions, the age of the Silurian-Devonian boundary only decreases by 0.4 Ma (Fig. 2b).

245

Fig. 2.

Plot of radiometric age against biostratigraphic

position.

Since palaeontologically-derived estimates for the relative durations of series or stages of many other systems are available, this method of geochronological assessment could be extended to other time intervals and eventually to the whole Phanerozoic.

It has the advantage of combining the palaeontological information, which is directly related to the biostratigraphic subdivisions, with the radiometric data by using a quantitative analytical technique.

Of course, the analytical framework has to allow for updating the estimates of relative duration as well as adding additional data points when available.

References BOUCOT, A.J.,

1975.

Evolution and extinction rate controls.

Elsevier,

Amsterdam, 427 pp.

CARR, P.P., JONES, B.C. & WRIGHT, A.J., 1980: Dating of rocks from the Bungonia district. New South Wales.

Proc. Linn. Soc. N.S.W., 104, 111-117.

JONES, B.C., CARR, P.F. & WRIGHT, A.J., 1981: Silurian and Early Devonian geochronology — a reappraisal, with new evidence from the Bungonia Limestone.

Alcheringa, 5, 197-207.

ZIEGLER, W., 1978.

Devonian. In Contributions to the Geologic Time Scale, G.V. Cohee, M.F. Glaessner & H.D. Hedberg, eds. Am. Ass. Petrol. Geol. Studies in Geology, 6, 337-339.

THE LAIDLAW VOLCANICS: A LATE SILURIAN POINT ON THE GEOLOGICAL TIME SCALE D. Wyborn

1

1 2 2 , M. Owen , W. Compston & I. McDougall

^Bureau of Mineral Resources, P.O. Box 378, Canberra City, A.C.T. 2601 (Australia) 'Research School of Earth Sciences, Australian National University, P.O. Box 4, Canberra, A.C.T. 2600 (Australia)

Mapping, mineralogical and chemical studies of an area west and northwest of Canberra, have shown that the Laidlaw Volcanics are

246

unique in the region. They carry a phenocryst assemblage of quartz, plagioclase (Anyo to An^g), biotite, orthopyroxene, sanidine, ilmenite, magnetite and allanite in a microcrystalline quartz-feldspar groundmass commonly preserving eutaxitic layering. Volcanics stratigraphically beneath the Laidlaw Volcanics such as the Hawkins, Ainslie, Mount Painter, Walker and Paddy's River Volcanics contain garnet and cordierite (mostly altered) phenocrysts - sanidine and allanite are absent. The Mountain Creek Volcanics, which unconformably overlie the Laidlaw Volcanics to the west, contain a very sparse phenocryst assemblage of plagioclase, two pyroxenes and biotite. The distinct phenocryst mineralogy of the various volcanics groups has been instrumental in setting up a unified sequence in a region where previously stratigraphic names had proliferated and correlations were not attempted.

On the basis of fifteen whole rock analyses of Laidlaw Volcanics^ Wyborn et al (1981) showed that the volcanics are richer in Ca, Na, Ba, Sr, La and Ce and lower in K, and Rb than the underlying Hawkins Volcanics and related units.

The Laidlaw Volcanics are tightly controlled stratigraphically as they lie above the Yass Formation and below the Euralie Limestone Member of the Silverdale Formation. Both these sedimentary units have been placed in the Neopioniodus excavatus conodant assemblage zone by Link & Druce (1972) which corresponds to the British graptolite zone 33 viz. the lowest part of the early Ludlow.

K-Ar mineral analyses, previously published Rb-Sr mineral analyses, and new Rb-Sr whole rock and mineral analyses together give the Laidlaw Volcanics an age of 420.7±2.2 m.y. This result for the Early Ludlow is identical numerically with a recently published age of 421 ± 3 m.y. for the Ashgillian (Late Ordovician) Stockdale Rhyolite (Gale et al, 1979), and suggests either that the time interval between the Ashgill and Ludlow does not exceed a few million years or that the age estimate for the Stockdale Rhyolite is too young.

An additional conformation of the reliability of the Laidlaw date has been obtained by the use of the ion microprobe at RSES. Magmatic zircons have been dated at 423 m.y.

References GALE, N.H., BECKINSALE, R.D., & WADGE, A.J., 1979 - A Rb-Sr whole rock isochron from the Stockdale Rhyolite of the English Lake District, and a revised mid-Palaeozoic timescale. J. Geol. Soc. London 136, 235-242.

LINK, A.G., & DRUCE, E.C., 1972 - Ludlovian and Gedinnian conodont stratigraphy of the Yass Basin, New South Wales. Bur. Miner.

Resour, Geol. Geophys, Aust. Bull. 32, 1-136.

VJYBORN, D., CHAPPELL. B.W., & JOHNSTON, R.M., 1981 - Three S-type volcanic suites from the Lachlan Fold Belt, S.E. Australia. J. Geophys. Res. 86, 10335-10348.

247

FORAMINIFERAL DATING OF SOIE SOUTH AUSTRALIAN MARINE TERTIARY SEDIMENTARY CYCLES IN RELATION TO PROPOSED GHDBAL SEA LEVEL CHANGES J. Murray Lindsay S.A. Departnent of Mines & Energy, P.O. Box 151, Eastwood, S.A. 5063 Current foraminiferal correlations suggest that the effects of most of the eustatic cycles proposed by Vail ^ al. can be detected in one or more South Australian marine Tertiary basins. Transgression associated with rising sea level can be recognised clearly in many instances near basin margins, but at times tectonism and more abundant sediment supply modified or even reversed this effect.

Despite relatively high global sea levels in the Early Tertiary, even marginal-marine Paleocene deposits with shelly faunas are only known in South Australia from the offshore Eucla Basin (Great Australian Bight Basin), the Otway Basin, and perhaps the Murray Basin.

Because of the tectonic and depositional setting, marine influence did not extend into the St. Vincent Basin until probably late in the Middle Eocene.

In the eastern St. Vincent Basin, the sequence from Tortachilla Limestcne to lower Port Willunga Formation comprising the Aldingan Stage, is on the best available evidence Zones P. 15 - P. 17, Late Eocene, and thus matches C^cle TE 3 of Vail et High in this sequence the marginal-marine Chinaman Gully Formation (including the Tandanya Sand Member) illustrates that regression can occur during rising sea level given sufficient influx of sediment, due for example to uplift and rejuvenation.

At the northeastern margin of the Eucla Basin, the peak of Late Eocene Cycle TE 3 is represented by a thin terminal Eocene marine intercalation in carbonaceous Pidinga Formation. There has been considerable progress towards local resolution of an Eocene/ Oligocene boundary zone by means of a succession of foraminiferal (mainly planktonic) datum levels - FADs and LADs. Similarities and a few differences are noted with respect to New Zealand datums.

The extent of Oligocene hiatus in South Australian onshore sequences is much less than sore authors have alleged or implied.

Marine deposition during the rising and relatively high global sea level of Cycle TO 1 (Early to Middle Oligocene) is recognised in the St. Vincent, Murray, and Otway Basins. Chert nodules are characteristic of this unit in the St. Vincent Basin (Ruwarung Member of Port Willunga Formation) and the Otway Basin (lower cherty unit of Gambier Limestone). The proposed dramatic midOligocene (Zone P. 21) fall in global sea level at the end of Cycle TO 1, estimated by Vail et al. to be of the order of 400 m, cannot be confirned at present by dramatic sedimentary events in South Australia, although for example i.n the St. Vincent Basin a disconformity within limestones at Kingscote, Kangaroo Island, probably represents this event; and the top of the cherty Ruwarung Member is approximately this age. In marked contrast, mid-Tertiary marine sedimentation commenced at this time in the I^^nga Valley, at the southeastern margin of the St. Vincent Basin, illustrating that sufficient basina] subsidence can counter the effects of a fall in sea level, producing locally a nett marine transgression.

248

Transgressive deposits corresponding to Late Oligocene Cycles TO 2.1 and TO 2.2 are reported fran the St. Vincent Basin, the Murray Basin, and the Ganibier Embayment. A limestone unit at Kingscote, K.I., is matched by foraminifera with Cycle TO 2.1.^ In SADME Oakvale - 1 corehole, northern Murray Basin, three thin glauconitic and calcarenitic bands which resemble Ettrick Formation lithologically, are intercalated in black, marginalmarine Geera Clay, and correlate by foraminifera with Cycle TO 2.1, and lower and upper phases of Cycle TO 2.2, respectively. In more marine parts of the Murray Basin, and in the Gambier Embayment, two depositional episodes characterised successively by Globigerina anqulisuturalis and the Turborotalia kugleri group seem to correlate with Cycles TO 2.1 and TO 2.2 respectively, as incursions reflecting relatively short-lived warmer and more open-marine conditions.

A further thin, 'Ettrick-like' intercalation in Geera Clay in SADME Oakvale - 1 bore, northern Murray Basin, is matched by foraminifera with earliest Miocene Cycle m 1.1. Four successive transgressive phases of Melton Limestone on northern Yorke I^ninsula may correlate with Cycles TM 1.3, m 1.4 (both Longfordian Stage), TM 2.1 (widespread Batesfordian - early Balcombian Lepidocyclina howchini Zone in St. Vincent and Murray Basins), and TM 2.2. Widespread marine carbonates with FlosculineJ.la bontangensis (Eucla and St. Vincent Basins), and above the Orbulina datum (St. Vincent and Murray Basins), hence Zone N. 9, early Middle Miocene, concluded the onshore South Australian midTfertiary marine record v/ithin Cycle TM 2.2.

Step-like fall of global sea level.s through the later Middle Miocene and the liate Miocene, plus probable uplift of the southern continental margin, combined to exclude further Miocene marine sedimentation from South Australia (if Bookpurnong Beds are taken to be Early Pliocene).

Transgressive clastic-carbonate deposits with Early Pliocene foraminiferal faunas (Kalimnan Stage), have been found in outcrop ai southern Kangaroo Island, and subsurface in the St. Vincent Basin and the southern Murray Basin. These units probably correlate with Cycles TP 1 and TP 2.

NUMERICAL TIME SCALES AND THEIR APPLICATION TO THE OCEAN BASINS

Ian McDougall Research School of Earth Sciences, Australian National University, Canberra, A.C.T.

Two types of numerical time scales, both based upon K-Ar isotopic dating of suitable rocks, have been developed, and these are widely applied in the ocean basins. The first is calibration of the

249

relative geological time scale, which is based essentially upon superposition of sediments, by dating suitable samples that are more or less precisely controlled as to their position within the relative time scale by means of biostratigraphy. The second is calibration of the geomagnetic polarity time scale, that is the history of reversals of the geomagnetic field, for the last 5 Ma or so, and its extrapolation to much greater ages based upon marine magnetic anomaly patterns.

Both types of time scale have been developed by isotopic dating mainly of igneous rocks cropping out on land, and then have been translated into the ocean basin environment. As a generalization direct isotopic dating of rocks from the ocean basins has been unsuccessful because most samples from this environment are altered, and thus do not meet the requirement of closed system behaviour, so yielding incorrect ages. The translation of the numerical time scales, based upon measurements on land-based samples, into the ocean basins, relies principally upon biostratigraphic and magnetostratigraphic methods. For the ocean basins we are concerned with approximately the last 200 Ma of geological time: over this interval, ages for the marine magnetic anomalies or the many boundaries in the relative geological time scale are known with varying degrees of accuracy, ranging from an uncertainty of a few percent for the last 10 Ma, to as much as 10% for some of the older anomalies and boundaries.

250

Symposium 4(a) Metallogeny and crustal evolution Convener Dr E. Scheibner

251

THE EVOLUTION OF CONTINENTAL CRUST AND METALLOGENETIC PROCESSES Richard J. Arculus Research School of Earth Sciences, Australian National University, P.O. Box 4, Canberra, A.C.T. 2600.

Integration of modern global tectonic hypotheses, widespread application of isotopic data gathering and the employment of sophisticated geochemical and dynamic modelling have dramatically advanced our knowledge of the evolution of the crust and mantle of the Earth.

In step with these advances has come an enhanced appreciation that ancient tectonic and magmatic processes were not identical to those presently in operation, even though many apparent similarities in some ancient and modern rock types and assemblages can be discerned.

To a certain extent, there has been a waning of the initial wave of enthusiasm with which plate tectonics and concepts of global crustal recycling were incorporated into metallogenetic hypotheses. Many geologists take a sanguine view of the insight which the current analyses of large-scale recycling processes can offer for understanding ore-forming systems in the continental crust.

For example, some workers espouse the viewpoint that the present day creation of new lithosphere at oceanic ridges, its ultimate destruction at trenches and partial melting in subduction zones gives rise to a one-way process of distillation of "silicic" or continentforming material from the mantle. The discovery of deep-sea metalliferous brines, active and fossil hydrothermal plumes, and the possibility that the associated sulphide-bearing sediments and oceanic igneous rocks could become involved in the genesis of "calcalkaline" rock suites of island and continental arcs seemed to be a logical hypothesis for the association of porphyry copper and molybdenum sulphide deposits with arc rock types.

However, many difficulties with recognizing more than a trivial chemical contribution (if at all) to the composition of arc volcanics from the subducted lithosphere have been demonstrated. It appears that the major portion of the source components of these melts is in the upper mantle wedge overlying the subduction zone, and the crust of the arc itself. In some ore deposits, such as those in southwestern U.S.A., isotopic analyses support the derivation of most of the lead in porphyry copper and molybdenum deposits from the subjecent continental crust. The ultimate fate therefore of subducted metalliferous lithosphere is a controversial subject, and the role of plate tectonics in the long-term magmatic evolution of continental crust is not clear.

It is now apparent that crustal evolution has been confined neither to a single Archean creation with subsequent extensive reworking, nor a quasi-steady accumulation throughout time. Isotopic, geochemical and tectonic evidence indicate major pulses of crust formation coupled with the present-day possibility of some degree of diminution of crustal volume. There are many features of ores and associated host rocks that point to an evolution of sedimentary, petrogenetic and tectonic processes. The restriction of volcanic peridotites and associated Ni-Cu sulphides to the late Archean is an instructive example indicative of 1) higher heat flow - degree of melting of the Archean upper mantle; 2) comparative enrichment of sulphur in the upper mantle source regions, and implications of secular chemical changes in the mantle.

252

Samples of the upper mantle found in kimberlites and basaltic diatremes record pervasive and complex metasomatic events. Although on a large scale it appears that a degree of regularity in the abundances of refractory and highly siderophile elements is present, localized enrichments in volatile, incompatible and chalcophile elements may be significant for the genesis of singular, sulphide concentrations as ore deposits, A voluminous literature attests to the changes in the Earth's atmosphere from essentially 02-free to thoroughly oxidizing. Dramatic consequences for the mode and nature of occurrence of iron and uranium ores can be cited as a consequence of this atmospheric evolution.

There is an intriguing possibility that subduction of relatively oxidized crust into theupper mantle may have long-term effects for the redox state of erupted basalt magmas and their inherent sulphur solubility characteristics. It may be that atmospheric feedback into the upper mantle has ultimately had an influence on the changing nature of ore deposition in the crust.

The inter relationship of the major crust-forming and differentiation events and metallogenetic processes is an intriguing one.

Considerable evidence exists for dramatic changes in the evolution of the crust in the approximate time period corresponding to the ArcheanProterozoic boundary. Upper crustal rock types changed from a dominance by bimodal mafic-silicic (Na-rich granite/trondhjemite) to K-rich granitoid terrains. However the consequences for ore-forming processes of this major intra-crustal differentiation are not obvious.

Many studies have demonstrated secular changes in ore occurrence linked with host rock type such as Archean greenstone belt or Proterozoic sediment-filled rift. Nevertheless the interplay between metallogenesis and continental crust evolution remains a fertile field for speculation and analysis.

THE EVOLUTION OF TIN DEPOSITS WITH TIME I.R. Plimer Department of Geology, University of New England, Armidale 2351, NSW The oldest known tin deposits are submarine exhalative ores associated with Archean submarine explosive acid volcanism (e.g. Abitibi Belt, Canada; Yilgarn Block, W.A.). Although minor cassiterite and stannite are commonly present in Palaeozoic submarine exhalative deposits associated with acid (e.g. Iberian Pyrite Belt) and mafic volcanism (e.g. Bleikvassli, Norway), enrichment in Sn in submarine exhalative ores is only present where B and/or F minerals are abundant (e.g. Kidd Ck., Sullivan, Canada). The solubility of Sn in chloride solutions below 300°C precludes Sn being a major component of most submarine exhalative ores but the association of Sn with tourmaline, fluorite, fluorapatite and fluro celadonite indicates lower temperature (<250°C) transport of Sn as B- and/or F- bearing complexes in exhalative systems.

Pegmatitic Sn deposits are characteristic of the Late Archean to Early-Middle Proterozoic. Albite-quartz pegmatites within high metamorphic grade sequences contain a great diversity of Sn, Y, Mn, Fe, Ta, Nb, Li, Be and phosphate minerals. In some places, these pegmatites have

253

a spatial relationship with granitic rocks (e.g. Tanco, Canada) whereas in most places (e.g. Bikita, Kamativi, Zimbabwe; Rhondonia, Brazil) these pegmatites have no spatial or genetic relationship to exposed granitic rocks. Although Proterozoic tourmaline-rich exhalites associated with mafic metavolcanics and exhalative W ± Sn deposits appear to be the source of Sn and W in stratabound pegmatites (e.g. Bushmanland Sequence, South Africa; Willyama Complex, N.S.W., Central Goias, Brazil), these pegmatites generally contain no albite or associated rare minerals and appear to be of metamorphic origin as they are commonly stratabound, associated with unusual chemical metasediments in felsic-mafic metasediment complexes, can commonly be related to the sillimanite-andalusite isograd, and often occur in structural domes.

The above stratiform, stratabound and pegmatitic tin deposits produce small quantities of cassiterite and constitute some 10% of primary tin deposits.

More than 80% of primary tin deposits are associated with Palaeozoic (e.g. E. Australia), Mesozoic (e.g. Nigeria, U.S.S.R.) and Tertiary (e.g. Bolivia) granitic rocks. Although much more data is needed, it is becoming established that many tin deposits are associated with an uncommon granitoid type (A-type). A-type melts generally are late stage to anorogenic, high temperature, low viscosity, cognate xenolith-free melts which crystallize to form alkali grante ring complexes (e.g. Nigeria), biotite adamellite (e.g. Blue Tier, Tas.) and biotite granite (e.g. New England, N.S.W.). These granitoids usually lack hornblende, contain anomalous zincian fluorannite (± muscovite), 3-looking quartz, possibly Baveno and Manebach twinned K-feldspar, accessory ilmenite, monazite, U and REE minerals, zircon, fluorite, fluorapatite and Sn±Ta±Nb phases. The melts are anhydrous with F being the dominant volatile and, when Cl-rich, tend towards peralkalinity.

These granitoids are characterized by high Si02, Na20, K2O, (F+Cl), Ga/Al, Rb/Sr, Rb, Th, U, Zr, Be, Li, B, Ta, Nb, Sn, Y, La, Ga and Zn and low Fe203, FeO, MgO, CaO, K20/Rb, Ba, Sr and Eu relative to other granitoids in the same province.

A-type granitic rocks commonly have a radiometric, gravity and magnetic anomaly; shed cassiterite, monazite, Ta-Nb minerals, ilmenite, topaz and tourmaline into alluvium; commonly produce positively correlated Zn-Sn stream sediment anomalies and enrich base waters in U and F. These unusual granitoids are emplaced in rifts, tensional fractures or above hot spots and commonly the control for granitoid emplacement is also a structural control for ore deposition.

Most A-type granites have no associated mineralization and the formation of a relatively brittle quenched carapace above cupolas or ridges, the late stage formation of anomalous magmatic-hydrothermal biotite, late stage fluid saturation followed by rock-fluid reactions, fracturing or re-opening of earlier fractures and the passive release of fluid are considered fundamental criteria for the formation of a hard rock tin deposit.

If the above takes place in proximity to carbonate rocks, Sn silicate skarn deposits form at or near the contact (e.g. Doradilla, N.S.W.) whereas a pyrrhotite-cassiterite skarn (e.g. Renison, Tas.) forms at a distance from the contact. A sheeted greisen zone in a cupola (e.g.

Cinovec, Czechslovakia) is more likely at low levels of intrusion or where the "trap rocks" are impermeable whereas a vein swarm deposit (e.g.

Taronga, N.S.W.) is developed associated with a cupola at high levels of intrusion. Porphyry and breccia deposits (e.g. Ardlethan, N.S.W.) result from the explosive release of fluids in a subvolcanic environment.

254

Sheeted griesen and vein swarm deposits are commonly vertically and laterally zoned with regard to vein mineralogy, vein morphology and vein density; early gaugue minerals contain three phase fluid inclusions; and the economic portions of vein systems in the one field is often at a constant height above sea level (i.e. ore precipitation at a specific hydrostatic pressure or from boiling).

Sheeted greisen and vein deposits are commonly spatially and temporally zoned. The most common zoning is W ^ Sn + W ^ W-Cu ± As ± Sn Zn Pb-Ag ± Sb carbonate with tourmaline ± topaz associated with the Sn-W, fluorite with base metal sulphides and barite with the carbonates (e.g. Mole Granite, N.S.W.). Reversal or partial reversal of zoning (e.g.

Cornwall, U.K.; Mt. Bischoff, Tas.) probably results from the partial inward collapse of the hydrothermal system.

Tertiary tin deposits include alluvial deposits (>60% world production e.g. S.E. Asia) and primary deposits associated with subvolcanic intrusives (e.g. Bolivia) and terrestrial volcanic piles (e.g. Mexico).

About 8% of tin deposits are related to Tertiary volcanics and little is known about the geochemistry of the specific rhyolitic flows related to the low temperature deposition of silica polymorphs and "wood tin".

Tin deposits show a world wide evolution in time from Archean stratiform deposits to Proterozoic stratiform, stratabound and pegmatitic deposits, to Palaeozoic and Mesozoic granite-related deposits to Cenozoic subvolcanic, volcanic and alluvial deposits. Cassiterite in the pegmatitic deposits is commonly associated with Ta, Nb, Li, Be, Y, Mn, Fe, and phosphate minerals whereas cassiterite in Phanerozoic granite-related deposits is associated with W, Cu, Pb, Zn, As, Sb, Ag, Bi, U, Hg, Ba, Fe and Mn minerals in polymetallic zoned deposits. Furthermore, in one particular tin province, older stratiform, stratabound and pegmatitic tin deposits are present in the same geographic area as granite-related and volcanic tin deposits (e.g. Erzgebirge, Bolivia, SE Asia, North Queensland) and hence suggestions of metallogenic inheritance must be considered in construction of models for the genesis graniterelated of tin deposits by fractural crystallization or partial fusion of unusual source rocks.

HYDROTHERMAL MOBILISATION, TRANSPORT AND DEPOSITION OF TIN

J. Taylor, V.J. Wall and M.S. Bloom Department of Earth Sciences, Monash University.

Evaluation of the physiochemical controls on the extraction of tin from granitoid magmas and subsequent hydrothermal transport and deposition is essential for understanding the development of tin deposits directly associated with the crystallisation of granitic plutons. In this paper we discuss: (i)

Our experimental investigations of cassiterite solubility in granitic melts and the partitioning of tin between such melts and various aqueous phases as a function of (T, fH 0, fo , aqueous phase chemistry). ^ ^

255

(ii)

Constraints on the more significant features of magmatic evolution and processes at the magmatic-hydrothermal stage relating to tin mobilisation, based on these data and information from natural systems.

(iii)

The modelling of thermal and chemical aspects of hydrothermal transport and deposition of tin on the basis of experimental data and high temperature aqueous geochemical calculations.

Finally, we shall apply this information to help clarify the evolution of greisen, vein and replacement tin mineralisation.

REGIONALLY DISTRIBUTED AND STRUCTURALLY CONTROLLED ALKALINE METASOMATISM IN GRANITES OF THE EMUFORD--IRVINEBANK--MT. GARNET AREA, N.E. QUEENSLAND W. K. Witt^

&

C. Johnston^

^Department of Geology, James Cook University Townsville, Qld. 2

Geological mapping over some 700 km of granite terrain in the Emuford-Irvinebank-Mt. Garnet area of N.E. Queensland has revealed a range of intrusive phases which m a k e up one or more differentiated series.

Principal members of the series, listed in probable order of intrusion are : 1) coarse grained biotite granite, 2) porphyritic biotite granite, 3) medium grained Li-mica granite, 4) granite porphyry and 5) microgranites, usually granophyric, and sometimes megacrystic.

The granites intrude folded Siluro-Devonian sediments of the Hodgkinson Formation and loci of intrusion have to some extent been controlled by major, crustal fractures and lineaments. Form of the various plutonic bodies is difficult to determine but limited drilling information and outcrop data indicate that some of the porphyritic granites in the Emuford area are sheet-like bodies intruded into the roof zone of a composite, coarse grained granite mass. Medium grained granites and microgranites are thought to occur as plutons of at least moderate vertical extent. Numerous dykes, sills and plugs of the latter unit also occur.

Alkali metasomatism is intensely developed as structurally controlled, albitic or microclinitic metasomatites in two localized districts.

One occurrence, near Emuford, is hosted by coarse grained granite but is in close proximity to several small microgranite bodies which may be the source of metasomatic fluids.

The other, more closely studied area, is situated south of Irvinebank within a body of medium grained granite. The distribution of fracture controlled zones of intense alkali metasomatism appears to be controlled by large scale, pre-intrusion, linear zones of weakness. The albite and microclinite products are commonly associated with high grade cassiterite mineralization. The source of the fluids responsible for this metasomatism is thought to be the host granites.

256

Textural evidence suggests that most of the granites have suffered varying degrees of auto-metasomatism on a regional scale by reaction with indigenous fluids. These effects reach a maximum in the medium grained granites, which host the structurally controlled felspathic metasomites.

Textural and cross-cutting relationships indicate the following sequence of structurally controlled alkali metasomatism which overprints the regionally distributed autometasomatism : 1) early microclinization, 2) early albitization, 3) greisenization, 4) late albitization, 5) late microclinization. Examples of structurally controlled late albites are rare.

Similar, though not identical, sequences have been observed by Beus and Zalashkova (1962) who studied similar rocks in the USSR. They explain the sequence as resulting from declining temperature and pH evolution of initially high temperature, alkaline fluids which promote the activity of different components of the fluids in successive stages.

Regionally distributed, autojnetaaomatic reactions observed in the medium grained granites and the textural evidence for these reactions are presented in Table I. The nature and timing of these reactions parallels those of the structurally controlled metasomatites. However, the relative timing of regionally distributed late albitization and greisenization are not clearly established. The late timing of rim and skeletal albite is based on their pristine nature in contrast to the mild to moderate serecitization of perthite albite and albitic plagioclase. lADUC.

I

Structurally trolled

Con-

Analogous

Regionally

Autometasomatlc

Metasomat-

Distributed

Textural

Evidence

somatlc

Reactions

for

Autometa-

Reactions

ism 1) E a r l y

Microcliniz-

Plagioclase

-

Relict

K-felspar

.T t i on

Biotlte

» K-felspar

Quartz

K-felspar

plagioclase

within

K - f e l s p a r grains;

velnlng

plagio-

clase

cores

K-felspar.

grains

Cleavage

of by

controlled

of

K-felspar

In

micas.

2) E a r l y

Albitizat-

Plagioclase

-

Irregular

ion

K-felspar

-

continuous

quartz

K-fcUpar

grains.

Albltlc

Albite

Greisenization

Dark

micas

primary

(ologioclase?)

mica

and

bearing

Felspars

Wliite

mica

Albitizat-

K-felspar

-

Quartz

5) L a t e

Microclin-

No

-

Albite

analogous

reactions

textures

asfoclated and

in

expulsion

per-

dark

of

Tl-

fluorite.

f l a k e s of w h i t e

felspars,

particularly

across

reun-

Ab:0n:90:10).

mica

cores

in

of

plagioclase. skeletal

rims of albite

ion

of

of w h i t e mica with

opaques

Pristine

Albite

of

composition.

Irregular albitic

4) Late

plagioclase

albite

(up t o

Interleaving

optically

(Ab 9 8 / 9 9 )

places

Replacenttnt

Albite

Micas - White

of

in m a r g i n s

plagioclase

known

thite

3)

Inclusions

perthite

grains

growing

and

Into

swapped and

grains.

Irregular

inclusions

of

continuous

quartz

rim and

in

optically Intcr-

observed

ization

Two mechanisms by which the late, autometasomatizing fluids may have permeated the medium grained granites on a regional scale are considered. Crystallization is considered to have proceeded from the roof and walls of the pluton inwards. Small amounts of fluid evolved at the crystallization front may become trapped in a zone of unknown width where crystallization is 80-95% complete. The trapped fluids equilibrate with the crystals as solidification is completed and temperature

257

falls, causing autometasomatism. Alternatively, a portion of fluids accumulated at depth may penetrate upwards through the solidified granite carapace via microfractures and grain boundaries at a comparatively advanced stage of pluton consolidation^ Advanced textural readjustment occurs with the onset of meteoric-hydrothermal convective circulation about the pluton.

Both models envisage an accumulation of volatile, alkaline rich fluids towards the centre of the pluton. Volatile accumulation resulting from continuous crystallization from margins and roof inward ultimately causes fracturing of the solidified granite carapace and overlying roof rocks. Aqueous fluids, channelled upwards through these fractures cause intense metasomatism of adjacent wall rocks, with total elimination of quartz and mica in favour of felspars.

Reference: Reuss, A. A. & Zalashkova, N. Ye. 1962 : "Post-Magmatic High Temperature Metasomatic Processes in Granite Rocks", Int. Geol. Rev. 6 (4) pp. 13-31. ~

INFERRED MANTLE DEGASSING, CONSEQUENT CRUSTAL EVOLUTION, AND THE PREDICTION OF SOME METALLIFEROUS SOURCE REGIONS FROM REGIONAL GRAVITY DATA IN THE LACHLAN FOLD BELT OF NEW SOUTH WALES A.N. Yeates & A.S. Murray Bureau of Mineral Resources, Canberra, ACT The upper crustal rocks in the NSW portion of the Lachlan Fold Belt formed as a consequence of intermittent volcanism, precipitation from discharging hydrothermal solutions, and equivalent clastic sedimentation. These events began in deep ocean; they were accompanied and succeeded by plutonism. Later-derived clastic sedimentation occurred when the region had emerged to form a subcontinental area.

The triggering mechanism which led to these events appears to have been the generation of, and subsequent expulsion of volatiles from mantle depths, over a 150 m.y. period. In deep water, the volatiles were released from sea-floor swells around which chemical siliceous and phyllosilicate-rich sediments accumulated with variable detrital pelagic admixture. Some swells became sites for granitoid plutons whose magmas did not completely degas until deposition had ceased. In shallower parts of the ocean, volatiles were expelled during volcanism.

One consequence of crust formation was the production of rocks having lesser density than mantle material (cf. Ringwood, 1975). The observed density changes can be explained by an increase in volume of the crustal material, and by degassing that accompanies calc-alkaline magmatism (Stanton, 1978), hydrothermal discharge, and batholith emplacement.

258

Evidence of volatiles having passed through magmatic and hydrothermal systems is rarely preserved in rocks. But calculations (Stanton, 1967), observations of some modern volcanoes and springs, and gravity data, all indicate that it can be considerable. As gravity relates entirely to mass, the "lost" fraction from magmas (and its consequential, compositional modification - see Stanton, 1978; Stanton & Ramsay, 1980) can be shown by negativity of Bouguer anomalies relative to an upper mantle density of about 3.

In the Lachlan Fold Belt, the most positive Bouguer anomalies coincide with regions of mafic rocks; chemically, these will be the least modified relative to underlying mantle material.

Conversely, the most negative anomalies coincide with acid igneous rocks and their associates; these are the most compositionally-modified relative to mantle material.

Gravity and magnetic data can be interpreted to indicate_ that the following areas of the Fold Belt have near-surface mafic igneous rocks considerably larger than their mapped extents indicate: the WantabadgeryJunee area; the Wyalong area; the Tomingley West area; the Trangie area; an area half way between Bobadah and Nyngan; a large area extending from Nyngan to Girilambone and then due north to the Bogan River, and the Byrock area.

In terrains that become calc-alkaline, several metallic elements show close ties to mafic magmas (Stanton & Ramsay, 1980), and any of the above-mentioned areas are accordingly interpreted from geophysical data, and known mineral deposits (Markham & Basden, 1974), to contain source rocks for Cu, Ni, Mn, Pt, Cr, and Au, and possible deposits.

Note.

, . 1 In this talk A.N. Yeates is responsible for the geological

content.

Both authors are responsible for the geophysical notes.

References Markham, N.L., & Basden, H., (eds.) 1974 - the Mineral Deposits of New South Wales. Geological Survey of New South Wales, Sydney, 682 p.

Ringwood, A.E., 1975 - Composition and petrology of the Earth's Mantle. New York, McGraw-Hill Book Company, 618 p.

Stanton, R.L., 1967 - A numerical approach to the andesite problem.

Koninkl. Nederl. Akademie Van Westenschappen, Proc. Series B 70, 176-216.

Stanton, R.L., 1978 - Mineralisation in island arcs, with particular reference to the southwest Pacific region. Introductory lecture. Proc. Australas. Inst. Min. Metall. 268, 9-29.

Stanton, R.L., & Ramsay, W.R.H., 1980 - Exhalative ores, volcanic loss, and the problem of the island arc calc-alkaline series: a review and an hypothesis. Norges geologiske unders^kelse, 360, 9-57.

259

REGIONAL META140RPHIC ORE DEPOSITS STRUCTURAL AND CHEMICAL CONTROL M.A. Etheridge^ and V.J. Wall^ ^Bureau of Mineral Resources, Canberra, A.C.T. ^Department of Earth Sciences, Monash University, Victoria

Regional metamorphism and deformation are fundam-ental processes in crustal evolution, and several important classes of ore deposits are formed as a result of these processes. This paper examines the relationship between deformation, metamorphism, fluid behaviour and the general characteristics of these classes of deposits. In particular, we will concentrate on the mobility of the fluid and its effectiveness as a scavenging medium, the "structural control" of ore distribution, and the chemistry of metamorphic fluids as it relates to transport and deposition of valuable metals.

Fluid mobility and source of metals - There is widespread evidence of tensile fracturing (veining) during regional deformation and metamorphism, indicating that pore fluid pressure (P ) commonly exceeds the minimum principal compressive stress (a^)- At these high fluid pressures, active microcracking is to be expected, with resultant significantly enhanced permeability. Permeabilities of the order of 10-15 to 10-18 ni2 (10"3 to lO""^ darcy) are not unreasonable under these conditions, and the metamorphic fluid will therefore be highly m.obile.

It may even be convectively unstable, and the resultant large scale fluid circulation would provide a very effective means of scavenging elements which are present in very low concentrations. The scavenging ability of this fluid will depend also on its composition, and we will discuss the transporting properties of a typical low salinity, CO^ - bearing, somewhat reduced metamorphic fluid.

Structural control of transport and deposition - A regionally metamorphosing and deforming terrain is expected to have widely varying permeability, and fluid flow may be strongly focussed into high permeability lithologies and/or structures. Several mechanisms for enhancing permeability and therefore focussing fluid flow will be discussed, In the case of structurally enhanced permeability, the geometry of fluid flow may be predictable, and therefore represent a useful exploration guide. The geometry of selected quartz vein/gold and unconformityrelated uranium deposits will be used to illustrate these points. The specific case of retrogressive metamorphism and deformation will also be discussed.

Chemical controls on deposition - Ore deposition within the fluid channelways can take place due to 1) changes in temperature, 2) changes in pressure, 3) fluid/wall rock interaction, or 4) mixing of migrating fluids with local rock fluids. Temperature gradients in regional metamorphic terrains are too small to provide an effective means of concentrating deposition within a small rock volume, but each f the o?Err'?JrL''^rocLses may be important. Again, thfs prograde and retrograde, will be discussed (see also. Cox e ^ . , this volum.e).

260

GOLD-QUARTZ MINERALIZATION IN SLATE BELTS : THE CASTLEMAINE - CHEWTON EXAMPLE S.F. Cox^ V.J. Wall^ M.A. Etheridge^' ^, S.S. Sun^ and T.F. Potter"" ^Dept. of Earth Sciences, Monash University, Clayton, Vic., 3168 ^Now at: Bureau of Mineral Resources, Canberra, A.C.T., 2600. ^C.S.I.R.O., Division of Mineralogy, North Ryde, N.S.W., 2113. ""Chewton Gold Associates, Chewton, Vic., 3451.

Auriferous quartz veins in low grade regionally metamorphosed rocks have constituted a major source of gold on a world-wide basis.

Ordovician slate belts of the Tasman Orogenic Zone have been particularly productive examples.

In this paper we examine the structural evolution and depositional controls of this association with particular reference to the Castlemaine-Chewton region of central Victoria.

Gold-quartz mineralization in the Castlemaine-Chewton area is hosted by a folded lower Ordovician quartz wacke/slate sequence, and is restricted to narrow north-south trending belts which are parallel to major fold axial traces.

Auriferous quartz-vein systems have developed in faultrelated and fold-related dilatant sites that were generated at very low (even negative) effective pressures (P.> P during regional deformation.

Faults with related gold mineralization are dominantly west-dipping reverse structures, though less common conjugate east-dipping faults are also developed.

Several types of quartz-vein systems have been recognized.

Dominant are within-fault veins developed by oblique opening along faults.

Large veins of this type have been formed where refraction of faults across folded structures has caused the local fault orientation to be inclined to the bulk displacement direction.

Flat-lying quartz-vein systems have been generated in extension fractures related to the modification of the local stress field during fault movement.

Their abundance is typically low away from fault zones, but increases markedly adjacent to fault or shear zones.

Saddle-reef structures and some bedding-parallel vein systems have developed during flexural slip and fold growth.

Vein microstructures indicate that vein growth has occurred by repeated hydraulic fracturing and the operation of crack-seal mechanisms over an extended period, with veins only one centimetre wide involving as many as 2 x 10^ crack-seal increments.

Apart from quartz, which usually occupies greater than 95% of vein volume, ankerite and lesser quantities of chlorite, phengitic mica, and albite are also present.

Arsenopyrite and pyrite are the major sulphide phases.

Sphalerite, galena, chalcopyrite, and pyrrhotite also occur, especially in association with high gold values.

Higher grade mineralization is commonly developed where vein systems abut carbonaceous and pyritic slates.

Wall rock alteration is minor and usually restricted to zones a few centimetres wide.

Silicate-carbonate alteration assemblages close to major vein systems involve quartz + ankerite + white mica ± chlorite, and are similar to those developed on a regional scale attending the prehnite-pumpellyite to lower greenschist facies metamorphism.

Disseminated arsenopyrite porphyroblasts occur in arenites up to several tens of metres away from major vein systems.

261

Sulphur isotope fractionation between galena and sphalerite from the Wattle Gully Gold Mine at Chewton indicates temperatures around 300-325 The sulphur isotopic compositions of pyrite and arsenopyrite, though essentially uniform throughout the mine (+1.5 to are significantly out of equilibrium with associated sphalerite and galena. S^^^SJ^QW values for vein quartz in the Wattle Gully vein systems vary between +15.6 to +18.0%o. Calculated values for aqueous fluids in equilibrium with the vein quartz at 300®C range from +8 to 10%o, and are typical of metamorphic fluids.

Vein carbonates have values ranging from -2.8 to -6.8%o . At 300°C the corresponding for fluids (as CO2) would be -1 . to -5%c Both aqueous and C02-rich primary fluid inclusions are present in auriferous quartz, indicating trapping in a two phase fluid regime.

Inclusions rarely contain mica and dawsonite daughter minerals.

Salinities of the inclusion fluids are low, and their homogenization temperatures are consistent with trapping around 300°C and 2kb pressure.

Micaalbite equilibria and fluid salinities imply weakly acid fluids under these conditions, may be estimated from sulphide equilibria, and f^^ limited by the apparent lack of carbon saturation in the vein systems.

The development of the auriferous quartz-vein systems during the regional deformation history (but predominantly late in fold growth and cleavage development), the lack of extensive wall rock alteration, the depositional conditions, as well as the isotopic and chemical compositions of the fluids all attest to the formation of mineralization from regional metamorphic fluids.

Gold precipitation in the vicinity of pyritic and carbonaceous slates may be ascribed to mixing of locally derived methanebearing fluids with more oxidized fluids traversing faults and associated vein systems.

Associated base-metal sulphide and arsenopyrite deposition reflect increaseda from the mixing of these fluids.

Quartz deposition is best explained by transient fluid pressure decreases following repeated hydraulic fracturing in dilation zones.

Based on experimental data for quartz solubility, estimates may be made of the mass of fluid necessary to form quartz-vein systems if transient fluid pressure decreases are limited to less than about 100 bars.

At 300°C and 2 kb total pressure, deposition of one gram of quartz requires a minimum of lO"^ gm of H2O. The development of major quartz-vein systems must thus involve the throughput of cubic kilometres of fluid and necessitates significant structural control and^ focussing of fluid transport.

Consideration of the total gold production from the Castlemaine region and gold content of the Ordovician sequence and the probable underlying rocks (Glasson & Keays, 1978) indicates that the source region volumes were of the order of 100 km3.

The large volumes of both fluids and source regions imply deep circulation of fluids which were ultimately derived from devolatization of the metamorphic pile.

The development of auriferous quartz-vein systems of widely varying geometry in the Castlemaine region reflects deformation under conditions of very high fluid pressure within a stress field substantially locally modified by developing folds and faults.

The restriction of auriferous veins to narrow belts within the lower Ordovician sequence reflects the temporal and spatial sequence of reverse fault development, the ability of these faults to tap auriferous fluids deeper in the pile, and the mixing of these fluids with more reduced fluids within the lower Ordovician sequence.

262

GOLD MINERALISATION IN A LATE DEVONIAN RIFT VALLEY, SOUTHEASTERN NEW SOUTH WALES

J.L. Stockley Gold Fields Exploration Pty, Ltd., Box 590, Fyshwick, A.C.T. Present address: Freeport of Australia Inc., Box 1314, Townsville, Qld.

The total precious metal production from the Wolumla gold field, 20 km south of Bega, New South Wales, was 672 kg of gold and 102 kg of silver.

The stratigraphic sequence in the Wolumla mine area comprises , from the top: Lake Devonian Merimbula Group

:

Boyd Volcanic Complex :

boulder conglomerate, red siltstone, feldspathic sandstone; amygdaloidal basalt, flow-banded rhyolite, volcaniclastic conglomerate, lahar, and basal arkose.

- Non-conformityEarly Devonian Kameruka Adamellite, Yurammie Granodiorite Ordovician Mallacoota Beds

:

micaceous quartzite, siltstone, greywacke.

Hornblende quartz diorite (trondhjemite) cropping out at Wolumla is petrographically similar to quartz diorite at Nadgee, south of Eden. The Maxwells Quartz Diorite (new name) is Late Devonian in age and coeval with the Boyd Volcanic Complex.

Sericitic biotite-quartz porphyritic rocks grade into altered rhyolitic crystal-lithic tuffs on the western side of Wolumla. Gabbroic rocks crop out at the base of amygdaloidal basalt flows northwest of Wolumla.

The intrusive rocks at Wolumla display alkaline affinity with the bi-modal volcanic rocks cropping out in the area.

Basement dextral transcurrent faulting on northeast trends has occurred south of Wolumla. High angle, en-echelon dextral wrench faulting on north-south trends is developed in the mine area.

The faulting appears to be contemporaneous with the deposition and emplacement of the volcanic rocks and has controlled the distribution of the proximal sedimentary rocks.

Emplacement of the flow-banded rhyolite appears to be in part fault controlled. The lateral and vertical extent combined with the constant vertical flow-banding, zones of brecciation, and porphyritic texture suggest that this unit is a lava dome.

Onlapping of the basal arkose unit, overlapping of the basalt lavas, rapid thickening of the alluvial sedimentary units, and the position of

263

the lava dome, suggest that the Wolumla area was the location of several north-south elongate, volcanic basins or maars located within the major meridional Eden-Yalwal Rift Zone. The dimensions of the volcanic basins are estimated to be 1200 m by 800 m and 500 m by 300 m with depths of around 100 m. The precious metal mineralised zone is located within the smaller basin.

Extensive sericitisation, particularly of the basalt lava, chalcedonic banded quartz veining and stockworking, and pyritisation is associated with gold, silver and arsenic mineralisation in the Late Devonian rocks at Wolumla. The mineralisation appears to be directly related to phases of explosive rhyolitic volcanism that occurred in and along the margins of the smaller volcanic basin adjacent to the rhyolite lava dome.

Accompanying the rhyolitic volcanism were episodes of basalt eruption, and rapid, high energy influxes of voIcaniclastic and alluvial sediments.

The style and timing of the mineralisation is similar to that occurring at Yalwal and Yerranderie, New South Wales, where gold and silver mineralisation is hosted by proximal volcanic rocks.

The principal host rock at Wolumla is altered vesicular basalt.

Wolumla is an example of epithermal precious metal mineralisation occurring in basic volcanic rocks, but genetically related to terrestrial rhyolitic volcanism of Late Devonian age.

TECTONIC SETTING OF THE PORPHYRY COPPER AND GOLD DEPOSITS OF THE PAPUA NEW GUINEA MAINLAND H.L. Davies Bureau of Mineral Resources, Canberra ACT

Porphyry-type copper-gold mineralisation at Ok Tedi, Frieda River and Star Mountains, and gold mineralisation at Porgera, occur within, or are associated with, dioritic intrusives of Miocene and younger age. The intrusives are probably of mantle origin with some contribution from crust. The magmas developed during lithospheric shortening following earliest Miocene arc-continent collision, and probably are not subduction-related.

They decrease in age southward, from earliest Miocene on the line of the Sepik River, to middle Miocene in the Sepik foothills (Frieda complex), late Miocene and Pliocene on the main divide (Star Mountains and Porgera), and Pliocene and Pleistocene on the southern slopes (Ok Tedi).

Mineralisation is magma-related and may stem from a metal-enriched zone in the mantle.

264

GEOCHEMISTRY AND ORIGIN OF LOWER PROTEROZOIC SAPPHIRINE GRANULITES AND ASSOCIATED BASE METAL MINERALISATION IN THE STRANGWAYS RANGE, CENTRAL AUSTRALIA

D.P. Windrim

Department of Geology, Australian National University Canberra ACT

Sapphirine-bearing granulites exposed in the N.W. Strangways Range, Northern Territory are frequently associated with significant Cu-Pb-Zn (rare Ni) sulfide orebodies and minor Au, Ag mineralisation.

Field and petrographic studies show that these small deposits were metamorphosed along with their host rocks. The sapphirine granulites occur as small lenses Im x 0.5m) or large irregular masses 20m X 7m) within or adjacent to cordierite-quartz granulites, quartzofeldspathic garnet granulites, mafic, ultramafic and calcsilicate granulites and quartz-magnetite (or gahnite) rocks.

Chemical compositions of the sapphirine-bearing and mineralogically similar granulites cannot be matched with those of any 'normal' igneous or sedimentary rocks; they generally contain high MgO, FeO + Fe203, AI2O3, low Si02, K2O, Na20 and CaO, and exhibit highly variable trace element and rare earth element abundances.

Detailed mapping and whole rock geochemistry demonstrate that these unusual bulk compositions were developed throughout a (pre-metamorphic) stratified sequence of tholeiitic, dacitic and rhyolitic volcanics, immature clastic sediments, and thin siliceous limestones and dolomites, locally intruded by melanocratic cumulate gabbros. In terms of major element chemistry, there is no difference between sapphirine granulite protoliths developed in (ie derived from) felsic, mafic and ultramafic rocks, respectively.

More than 95% of the bulk composition of the majority of the sapphirine granulites is accounted for by the 4 components FeO (+Fe203), MgO, AI2O3, and Si02; the most important remaining constituent is K2O. On a molar basis, these compositions represent the following pre-metamorphic mineralogies: chlorite + quartz, chlorite + sericite + quartz, chlorite + haematite + quartz, while rare examples require the presence of an additional aluminous phase such as diaspore, boehmite, gibbsite or alunite. The process most likely to result in convergence of bulk compositions towards chlorite-dominated assemblages irrespective of starting material (ie felsic, mafic or ultramafic) is low temperature hydrothermal alteration.

Despite the similarity in major element chemistry, relative abundances of Ti, Zr and Cr in the sapphirine granulites may be used to clearly discriminate the pre-alteration rock type for individual sapphirine granulite protoliths; the usefulness of these discriminants is illustrated by the fact that the host rocks (= starting materials) mapped at individual sapphirine granulite localities are

265

those predicted by the Ti-Zr-Cr relationships of the sapphirinebearing rocks.

Development of chlorite-rich zones as a result of hydrothermal alteration was accompanied by leaching of Pb and Zn from felsic volcanic and sedimentary rocks and removal of Cu, Zn, Ni, Ag, Au (and S) from mafic and ultramafic rocks. The common association of magnesian granulites, calc-silicate granulites, quartz-magnetite rocks and sulfide mineralisation probably represents sulfide precipitation within chloritic, carbonate-rich cherty exhalite units close to a Proterozoic sediment/seawater interface.

STRUCTURE, STRATIGRAPHY AND MINERALISATION POTENTIAL OF THE ANORTHOSITIC WINDIMURRA GABBROID, YILGARN BLOCK, WESTERN AUSTRALIA A.L. Ahmat Department of Geology, University of Western Australia, Perth, WA Present address: Geological Survey of Western Australia, Perth, WA The Windimurra Gabbroid, measuring 85 x 35 km, is the largest single body of gabbroic rocks in the Yilgarn Block of Western Australia. It is characterised by, i) a high proportion of fresh rocks, ii) rhythmic layering, iii) differentiation, iv) a scarcity of ultramafics, v) limited systematic fractionation, and vi) a strong anorthositic affinity % AI2O3) . The body has been severely disrupted, intruded by several Proterozoic-Archaean dyke sets, and partly regionally metamorphosed at greenschist fades. Notwithstanding, many of the rocks are fresh. The main minerals are: plagioclase (Anss-ss)» Ca-rich pyroxene (Fe/Fe + Mg = 18-46 %), Ca-poor pyroxene (Fe/Fe + Mg = 22-48 %), and olivine (Fa20-68)• Magnetite is mainly an accessory phase and chromite (^32 % Cr203), and associated ferritchromit, occur predominantly as fine-grained, disseminated grains in several rare ultramafic occurrences.

Gravity and field data suggest that the Windimurra Gabbroid is a steep-sided, fault-bounded, slab-shaped (tabular) body with an average thickness of roughly 3.5-5 km. An elongate, deeper central 'root* appears to be present. Rb-Sr isotopic studies suggest it is older than ^2.67 Ga (with an initial ^"^Sr/^^Sr ratio <0.7012), and preliminary Sm-Nd isotopic data indicate it may be as old as ^3.05 Ga. These data pose the intriguing problem of what constituted the original 'country rocks'. The favoured hypothesis is that the Windimurra Gabbroid represents part of a much larger anorthositic body, principally disrupted by the emplacement of the granitoids which now surround it.

The fault-bounded Windimurra Gabbroid is immediately surrounded by major shear zones (up to 1 km wide) which grade outwards into weakly foliated granitoids. In the eastern shear (the Wyemandoo Shear), some remnants of a former greenstone belt occur in the tectonic zone. The Windimurra Gabbroid is either structurally, or unconformably, overlain

266

by the Kantie Murdana Volcanics, comprising predominantly very-low- to low-grade metamorphosed felsic volcanics/pyroclastics and meta BIF/ jaspilite. In many places, younger dolerite/gabbro sills and dykes complicate the contact relationships between the Gabbroid and the Volcanics.

Layering is a prominent feature of the Windimurra Gabbroid, and partly defines a crude, basin-shaped body, with dips that steepen towards the margin. The measured structural/stratigraphic sequence is up to ^8.5 km thick. However, the internal structure is complicated by faulting/shearing and by large discordant zones. For example, the Shephards Discordant Zone, which contains major magnetite deposits, is up to 1 km wide and 45 km long, and transgresses the general trend of the gabbroids. In addition, less than 7 percent of the complex is exposed for examination. The gabbroids generally crop out as isolated 'blocks', some separated from each other by 10 km or more. Most blocks show their own distinct, albeit limited, compositional/mineralogical ranges, and distinct stratigraphic sequences, and correlation between blocks is therefore equivocal. Good marker horizons have not yet been recognised to allow inter-block correlation, but in some places, euhedral magnetite can be used to distinguish 'higher' levels within the complex.

On a broader scale, three large sub-divisions have been established based on the presence and/or absence of either olivine or Ca-poor pyroxene: Upper Zone PI (^Anss), Cpx, 01 (Fa^^^^s) y Middle Zone PI (^Anse), Cpx, Opx (inv. pigeonite), Mt Lower Zone PI Unss-es)^ ^px, Opx, 01 (Fa20-50)These zones also correspond with distinct compositional differences, and are analogous to major sub-divisions made in other large stratiform complexes (e.g. Bushveld, Skaergaard). The Upper and Middle zones are restricted in areal extent, and the Lower Zone constitutes ^90 percent of the exposed complex. Olivine is locally absent within the Lower Zone, and where present, rarely exceeds % of the volume.

The Windimurra Gabbroid has many features common to other large, mineralised stratiform complexes (e.g. Bushveld, Stillwater), and significantly contains extensive vanadiferous titaniferous magnetitites (up to 1.2 % V2O5), and some chromitites. By analogy, it appears a suitable repository for platinum group minerals (PGM). However, the complex is both older, and more anorthositic, than many of the mineralised complexes, and is more similar to the low PGM-bearing Fiskenaesset-type of complex.

267

METALLOGENY M D

GEOBIOLOGICAL E V O L U T I O N , A R C H A E M - E A R L Y

PROTEROZOIC

Ian B . Lambert^ & David I. Groves^ ^ C S I R O , Baas Becking L a b . , BMR B u i l d i n g , C a n b e r r a , A C T . ^Department of G e o l o g y , University of W . A . , P e r t h , W A .

This overview of geobiological evolution and metallogeny in the first half of Earth history integrates conclusions from Lambert and Groves (1981) and Groves (1982). Figure 1 summarises the major trends and relates them to younger evolutionary trends.

G e o l o g i c a l , g e o c h e m i c a l , geochronological and experimental data imply that: (i) there w e r e major additions of granitic (s.l.) rocks to the crust during the A r c h a e a n , largely as a result of anatexis of simatic crust; (ii) there w a s diachronous accumulation of Archaean greenstone (supracrustal) s e q u e n c e s , with ultramafic and mafic igneous components formed by melting in the upper mantle; (iii) different tectonic styles are represented in greenstone s e q u e n c e s , particularly by their sedimentary c o m p o n e n t s , and these may reflect different stages of crustal evolution; (Iv) the Archaean hydrosphere was generally sulphate-poor and less s a l i n e , richer in Fe and Si, and probably warmer than present seawater; (v) the early atmosphere was relatively C 0 2 - r i c h , and is unlikely to have become oxygenated until the earliest P r o t e r o z o i c , commensurate w i t h an increase in the sulphate content of the hydrosphere; (vi) photosynthesis was established locally by 3.5 G a , the age of the oldest putative microfossils and s t r o m a t o l i t e s , but it is not clear whether this was an oxygenic process in the Archaean; (vii) bacterial sulphate reduction was not a significant process u n t i l ^ 2 . 8 Ga; (viii) the Early Proterozoic was characterised by widespread accumulation of shallow water to subaerial sequences comprising mainly mature to immature clastics derived from the Archaean c r a t o n s , (stromatolitic) c a r b o n a t e s , cherts and B I F , with variable proportions of mafic to felsic v o l c a n i c s .

Global-scale appraisal of mineralisation in Archaean terrains indicates that: (i) most important mineralisation occurs in greenstone belts and reflects the high incidence of subaqueous igneous and exhalative activity; (ii) styles of mineralisation vary with the tectonic settings in w h i c h the greenstone belts formed: sequences deposited in the m o s t stable environments contain porphyry-style Cu-Mo-Au m i n e r a l i s a t i o n , bedded barite and small volcanogenic Cu-PbZn-Ba d e p o s i t s , w h e r e a s sequences which accumulated in more tectonically a c t i v e , rift-like environments contain important volcanogenic Cu-Zn d e p o s i t s , komatiite- and komatiitic dunite-hosted Ni-Cu o r e s , and m e t a m o r p h i c Au c o n c e n t r a t i o n s , (iii) certain ore types are important in o n e , or perhaps t w o , Archaean cratons and virtually unknown in others; for e x a m p l e , a large proportion of volcanogenic Cu-Zn ores are in the Superior p r o v i n c e , magmatic N i ores are concentrated in the Yilgarn B l o c k , and magmatic Cr ores are largely restricted to Zimbabwe; (iv) economic deposits of Fe or Au associated with BIF,^ and Au ores in quartz veins and shears are relatively widespread in Archaean c r a t o n s , as are largely subeconomic pegmatite-associated deposits and minor m a g m a t i c Cr accumulations; (v) sulphate deposits and lead-rich m i n e r a l i s a t i o n are rare overall in the Archaean record and most porphyry Cu-Mo d e p o s i t s , and S n , W , Hg and Sb deposits formed in this era are m i n o r .

In c o n t r a s t , the main styles of mineralisation formed in the Early Proterozoic reflect the importance of epicratonic sedimentation and widespread stable environments: (i) the sedimentary sequences are hosts to Fe in Superior type B I F , Au and U in palaeoplacers in

268

quartz-rich clastics, U associated with unconformities and breccias, Mn associated with carbonates and BIF, and a few important Cu and PbZn deposits; (ii) large, layered, Mg-rich mafic intrusives, probably related to major deep-seated fractures, contain major concentrations of Cr, platinoids and Ni. References: Groves, D.I., 1982: The Archean and Earliest Proterozoic evolution and metallogeny of Australia.

Int. Symp. on Archean and Early Proterozoic Evolution and Metallogenesis. Revista Brasileira de Geoci§ncias (in press).

Lambert, I.B., and Groves, D.I., 1981: Early Earth evolution and metallogeny.

In K.H. Wolf (Ed.). Handbook of Strata-bound and Stratiform Ore Deposits, v. 8., p. 339-447 (Elsevier).

ARC'IJA'EAN

<ABCHAEAN/

I PHANEROZOIC I

AAA 6,7 8 9 Development of primordial, mainly $

CONTINENTAL CRUST

Growth of sialic continental crust Stable continental blocks, platform cover sequences, moDile belts Evidence for modern-style

plate tectonics

(predominantly CaC03)

Carbonaceous sediments ses, oligomictic conglomerates, orthoquartzitc Oxidized terrestrial sedimentary rocks (red beds)

Halite (casts in Proterozoic) SEDIMENTARY ROCKS AND ASSOCIATED MINERALIZATION

!S (Ca sulphates largely pseudomorphed in Precambnan)

Superior type BIF deposits Manganese deposits Uraninite. gold, pyrite placer deposits Other uranium deposits y copper deposits Sandstone type lead deposits Shale-hosted lead-zinc deposits Carbonate-hosted lead-zinc deposits

Residual deposits e sulphide deposits (copper, zinc ± lead) Gold deposits (all types) Nickel sulphide deposits (± copper, platinoids) Chromite deposits Porphyry (s.l.) copper deposits ( ± n OTHER TYPES OF MINERALIZATION

Tungsten deposits issociated deposits Mercury deposits Antimony deposits

Kimberlitic diamond pipes

Figure 1: General evolutionary trends for' the Earth, after Lambert and Groves (1981).

Thick line signifies important period of formation, thin line signifies occurrence in moderate abundances, dashed line signifies sporadic occurrences.

1 = oldest putative microfossils and stromatolites; 2 = earliest isotopic data suggestive of bacterial sulphate reduction; 3 = development of stable oxygenous atmosphere; 4 = oldest eukaryotic microorganisms; 5 = oldest megascopic algae; 6 = oldest animal burrows; 7 = oldest animal body fossils; 8 = oldest animals with skeletons; 9 = oldest vascular land plants.

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PROTEROZOIC STRATABOUND U R A N I U M DEPOSITS - A PROPOSED N E W CLASS D.J. Perkin B u r e a u of M i n e r a l R e s o u r c e s , Canberra A C T This p a p e r p r o p o s e s a change in the existing i n t e r n a t i o n a l system of nomenclature a n d categorisation of u r a n i u m d e p o s i t s .

It recommends replacing the category "Proterozoic Unconformity-related" w i t h a new class n a m e d Proterozoic Stratabound u r a n i u m deposits w h i c h w o u l d include not only a l l the deposits currently classified in the u n c o n f o r m i t y - r e l a t e d class b u t also s e v e r a l o t h e r similar deposits occurring in Z a m b i a , Z a i r e , G a b o n and C h i n a .

The new name is considered necessary because the existing name does not a p p e a r to suit m a n y o f the deposits c i t e d as t y p i c a l of the class and because the defining characteristics of the "Unconformity-related" class do not a p p e a r to b e p r e s e n t in m a n y of these e x a m p l e s .

The Rum Jungle and A l l i g a t o r Rivers u r a n i u m deposits are instances o f inappropriate u s e o f the existing n o m e n c l a t u r e . They w e r e originally c l a s s i f i e d b y the OECD N u c l e a r Energy A g e n c y / I n t e r n a t i o n a l Atomic Energy Agency as V e i n s and R e l a t e d Types and later as U n c o n f o r m i t y - r e l a t e d , referring to the overlying o r y o u n g e r (1600-1800 m.y.) u n c o n f o r m i t y . H o w e v e r , they are n e i t h e r vein-like n o r is there p r o o f of a genetic relationship w i t h the y o u n g e r u n c o n f o r m i t y . In fact it is suggested that from a genetic v i e w p o i n t , the Rum Jungle and A l l i g a t o r Rivers deposits are stratabound b o d i e s m o r e closely related to: .

the lower unconformity

(which is n e a r the crystalline source rocks)

a h y p e r s a l i n e evaporitic environment w h i c h is reducing in p a r t Examples of Proterozoic Stratabound u r a n i u m deposits are the Rum Jungle and A l l i g a t o r Rivers deposits in A u s t r a l i a , the S h i n k o l o b w e , Swambo and Kalongwe deposits in Z a i r e , and Nkana deposit in Z a m b i a , the M o u n a n a , B o y i n d i z i and Oklo deposits in G a b o n , and the Lianshanguan deposit in C h i n a . It is suggested that the so-called U n c o n f o r m i t y - r e l a t e d uranium deposits of S a s k a t c h e w a n , C a n a d a , are a distinctive group of the Proterozoic S t r a t a b o u n d class and not a separate t y p e .

The characteristics of the various deposits w h i c h g r o u p e d t o g e t h e r as the Proterozoic Stratabound type include:

are

now

occurrence of stratabound u r a n i u m m i n e r a l i z a t i o n in specific rock types of Proterozoic age the close s p a t i a l a s s o c i a t i o n of the u r a n i u m m i n e r a l i z a t i o n w i t h anomalous to economically important concentrations of b a s e , precious and o t h e r m e t a l s (polymetallic mineralization) p r o x i m i t y of the h o s t strata to o l d e r crystalline b a s e m e n t rocks (source of uranium) .

the nature of the h o s t sediments (evaporitic/reducing f a c i e s ) , and

270

a moderate level of tectonism and metamorphism Proterozoic Stratabound uranium deposits appear to be an important class of uranium deposits which account for about one fifth of Western world reserves and which are more widespread than has been generally recognised. The deposits usually exhibit mineral zoning and appear to be not only restricted to the Proterozoic era, but also restricted to specific and distinctive depositional environments characterised by epior intra-continental hypersaline sedimentary sequences unconformably overlying crystalline basement.

They are envisaged to have formed in a sabkha-like environment similar to that modelled for the benesis of the Zambian copper belt. Typically, the mineralization has been upgraded by subsequent deformation and metamorphism.

It is therefore concluded that the tectonic and depositional model most consistent with these genetic processes is the moderately metamorphosed epi- or intra-cratonic sabkha/evaporite model.

In view of the high grade and large size of these deposits, it is likely that the Proterozoic Stratabound class will continue to be economically attractive as an exploration target in a world characterised by a slow-down in the rate of increase of demand for uranium and declining and historically low (real) uranium prices.

PROTEROZOIC SANDSTONE HOSTED URANIUM DEPOSITS A BIMODAL CLASSIFICATION? M.B.M. Hochinan Department o£ Economic Geology, University of Adelaide, S.A. A radiation history study utilizing thermoluminescence (TL) has been made of the Westmoreland uranium deposits in northwest Queensland.

These are hosted by the Middle Proterozoic Westmoreland Conglomerate.

The study has shown that all of the 800 samples taken of the host rock (from the vicinity of the ore bodies and up to 15 km distant) have suffered major radiation damage. Such radiation damage suggests that the initial uranium content of the Westmoreland Conglomerate may have been as high or higher than 20 ppm. Such a high inherent uranium content for sandstone can be achieved if the host rock is derived from a uranium-rich source rock. Such a rock exists in the Middle Proterozoic Cliffdale Volcanics which underlie the Westmoreland Conglomerate these are a series of acid volcanic tuffs, lavas and ignimbrites which may have contained between 30 ppm and 50 ppm uranium considering their present uranium content and Th/U ratio, (Mitchell, 1976), the fact that recrystallized volcanic rocks lose up to 60% of their uranium, (Roshold and Noble 1969), and comparison with other similarly aged acid volcanics around Australia (Giles 1980). Petrological evidence (Manning, 1979) has shown that the Westmoreland Conglomerate has been derived from the Cliffdale Volcanics, thus strengthening the hypothesis of a high initial uranium content for the Westmoreland Conglomerate.

TL, petrological and field evidence also suggests that uranium was then remobilized during intrusion of dolerite dykes which set up a

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convective cell system resulting in transportation and precipitation of uranium close to the margins of the dykes and the overlying basaltic Siegal Volcanics where suitable reducing environments existed, i . e . the uranium deposits have been derived from within the sandstone itself rather than being introduced from an exterior source.

A literature survey of similarly aged sandstones and conglomerates in the Northern Territory (the Kombolgie Formation), Canada (the Athabasca Formation and Karzon Arkose) and Africa (the FA Series at Oklo), as well as a TL study of the Corunna Conglomerate in South Australia, has been undertaken, and tables of similarities and differences prepared.

Similarities include stratigraphic position and lithology of the host rocks and basement age. Differences include mineralization grade, type and stratigraphic position, basement lithology and provenance of the host rock.

From this it appears that two distinct types of Proterozoic sandstone hosted uranium deposits are discernable.

The major distinction between them is the provenance of the host rock.

In one case the host rock is derived from an underlying uranium-rich, usually Mid-Proterozoic acid igneous source rock resulting in a sandstone with an inherently high uranium content (20 ppm or more).

In the second case the host rock is derived from Lower Proterozoic uranium-poor metasediments resulting in a sandstone with an inherently low uranium content.

Such a dependence between host rock uranium content and source rock has also been deduced for younger sandstones and siltstones by Beeson (1980) for the PermoTriassic Beaufort Group, Cape Province, South Africa, and Ishihara et al. (1981) for sediments in Western Shikoku, Japan.

Given that such a relationship exists, the following table is a summary of the two types of Proterozoic sandstone hosted uranium deposits, which may prove helpful in exploration for such deposits.

Type 1 - Host rock derived from underlying uranium-rich LowerMiddle Proterozoic acid volcanics and/or granites.

- Host sandstone has high initial uranium content (20 ppm or more).

Therefore all host rock is radiation damaged (can be quickly verified by TL). uranium.

-

Ore deposits are formed by remobilization of precontained Examples are Westmoreland and Oklo.

- Other factors to be considered in exploration: Need a suitable capping rock; margins of basins appear most prospective; need a suitable reducing environment or fixing agent for the uranium; deposits usually occur at the top of the sandstone.

Type 2 Host rock is derived dominantly from Lower Proterozoic metasediments and (usually basic) metavolcanics.

Host rock has a low initial uranium content.

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Therefore not all of the host rock shows radiation damage (can be verified by TL).

- Deposits are epigenetic (and may be formed by remobilization or faulting of older deposits, e.g. ^'unconformity type" deposits) - Examples are Athabasca deposits, Karzan Arkose deposits.

Potential areas include the Kombolgie Formation and Corunna Conglomerate.

- Other factors to be considered in exploration are that such deposits usually occur at the base of the sandstone.

Bibliography Beeson, R., 1980:

Chem. Geol.

Giles, C.W., 1980: Ph.D. Thesis.

p.81-107.

Univ. of Adelaide.

Ishihara, S., Sakamaki, Y., Mochizuki, T., Terashima, S. and Endo, Y., 1981: Bull. Geol. Soc. Japan.

6, p.329-342.

Manning, R.A., 1979:

B.Sc. (Hons.) Thesis.

Mitchell, J., 1976:

B.M.R. Geol. Geophys. Aust. Record 1976/34.

Rosholt, J.N. and Noble, D.C., 1969: p.268-270.

Univ. of Adelaide.

Earth Planet. Sci-Lett.

URANIUM-LEAD AND LEAD-LEAD INVESTIGATIONS OF MINERALS FROM THE BROKEN HILL LODES AND MINE SEQUENCE ROCKS AND THEIR IMPLICATIONS FOR ORE GENESIS Brian L. Gulson CSIRO Institute of Energy and Earth Resources, Division of Mineralogy, P.O. Box 136, North Ryde, 2113 Australia The investigation was undertaken with three main objectives: (1) to determine the age of crystallization of apatite in the Mine sequence rocks and the Lodes, (2) to attempt to determine the age(s) of the Mine Sequence rocks by U-Pb analyses of zircons, and (3) to ascertain the possible volcanic or sedimentary percentage of the Mine Sequence rocks by observations of the zircon morphology.

Apatite separates were analysed from the Mine Sequence rocks, from banded iron formations (BIF) and from the Lodes. A regression analysis of the combined rock and Lode apatite data for the 207p|^/206p|3 ^g 204p|^/206pt5 pi^t- gives an age of 1565 ± 15 Ma (2a) which is similar, within the experimental error limits, to the U-Pb age for monazite and Pb-Pb age for sphene from the Mine sequence rocks of about 1590 Ma, and also the ^^Ar/^^Ar event dated by Harrison and McDougall (1981).

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The U-Pb data for the zircons from the Mine Sequence rocks define two populations. The main population contains zircons from two samples of Upper Granite gneiss and two of Potosi gneiss and whose data lie on a single array of apparent age 1660 ± 10 Ma (la). This age - identical with the Rb-Sr ages of Pidgeon (1967) and Shaw (1968), the Pb-Pb whole rock age of Reynolds (1971) and some ^^Ar/^^Ar ages of Harrison and McDougall (1981) - is interpreted as the time of granulite facies metamorphism. The least magnetic zircon fractions of samples close to mineralization contain from 2.2 ppm to 16 ppm of common Pb. The second zircon population is that of the Lower Granite gneiss. These data are quite discordant and the chord on which they lie requires a long extrapolation to concordia at about 1740 Ma. This apparent age may be either the time of crystallization or reflect an inherited component of older Pb which was not reset during granulite facies metamorphism. The latter interpretation is favoured in view of the other zircon data.

Morphological studies of the zircons indicate that they have undergone a complex history and are not simply derived in a one-stage process from a volcanic source.

The U-Pb and Pb-Pb data for monazites and sphenes respectively from the same rocks are, relative to the zircon data, more concordant and have an apparent age of 1590 ± 20 Ma, similar to the apatite age of 1565 ± 15 Ma. These ages are also similar to the ^^Ar/^^Ar measurements for some hornblendes (Harrison and McDougall, 1981). The possible differences in apparent age of the monazite/sphene and apatite may reflect the different 'blocking' temperatures of the U-Pb systems in the two minerals, i.e. the U-Pb systems in the monazite are 'closed' at a much higher temperature (e.g. > 600°C) than in apatite. From the isotopic data it is proposed that there have been two major metamorphic events at Broken Hill during the mid Proterozoic: the granulite facies at 1660 Ma and another at ^ 1580 Ma, which may correlate with the F, and F2 structural episodes of Laing et al. (1978). As Lode apatites are from coarse-grained intergrowths with silicates (K-feldspar, garaet, pyroxene) and sulfides, the 1580 Ma age is considered to represent the time of recrystallization of the orebodies.

REFERENCES Harrison, T.M. and McDougall, I. , 1981, Excess ^^Ar in metamorphic rocks from Broken Hill, New South Wales: implications for age spectra and the thermal history of the region.

Earth Planet. Sci. Letters V.55, p.123-145.

Laing, W.P., Marjoribanks, R.W., and Rutland, R.W.R., 1978. Structure of the Broken Hill Mine area and its signlficaace for the genesis of the orebodies: Econ. Geol. V.73, 1112-1136.

Pidgeon, R.T., 1967, A rubidium-strontium geochronological study of the Willyama Complex, Broken Hill, Australia: J. Petrology, v.8, p. 283-324.

Reynolds, P.H., 1971, A U-Th-Pb isotope study of rocks and ores from Broken Hill, Australia: Earth Planet. Sci. Letters, v.12, p.215223.

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Shaw, S.E., 1968, Rb-Sr isotopic studies of the mine sequence rocks at Broken Hill, in Broken Hill Mines - 1968 (ed. M. Radmanovich and J.T. Woodcock): Mbnograph Series, No. 3, Australas. Inst. Min. Metall., p.185-198.

SULPHIDE IMJyilSCIBILITY AND THE GENESIS OF HYDROTHERMAL ORE DEPOSITS: THE UPPER MANTLE CONNECTION Reid R. Keays and Paul R. Hamlyn Department of Geology, University of Melbourne, Parkville, Victoria, 3052.

A model is developed which accounts for the genesis of hydrothermal Au and volcanogenic Cu-Zn-Au deposits.

This model requires the emplacement of specialized, high magnesium magmas of upper mantle origin into rifted oceanic or continental crust. A feature of these magmas is that they become S-saturated at a late stage and that when solidified the ore metals are hosted by reactive immiscible magmatic sulphides.

Second stage melts such as boninites may be especially important as these are enriched in the ore-forming metals.

The recognition of these specialized magma types and their tectonic settings can be used to design exploration programmes.

It will be shown that sulphide immiscibility of mafic silicate melts plays a major role in the formation of not only "magmatic" sulphide deposits but also of many hydrothermal ore deposits.

Most silicate melts are S-saturated when emplaced into the upper crust and as a result are significantly depleted in many of the ore-forming chalcophile and siderophile metals such as Ni, Cu, Zn, Pt, Pd and Au. These metals have high partition coefficients in favour of sulphide melts and as a result of early S-saturation of silicate melts and as a result of early S-saturation of silicate melts are scavenged by immiscible droplets which remain either in the source regions of the magmas or in high level magma chambers.

Evidence for widespread S-saturation of basic silicate melts is provided by FeO and S relationships in MORE glasses as well as the platinoid metal content of these glasses and by the platinoid metal contents of upper mantle spinel Iherzolites, both in continental and oceanic regions.

Indirect evidence that sulphur saturation of partial melts must be a widespread phenomenon is provided by considerations of core-mantle equilibria.

If the Fe-Ni core of the earth contains FeS, as moment of inertia considerations and analogies with iron meteorites demand, then this must have segregated from a mantle in which the silicate melt fraction was S-saturated.

Hence, the partially solidified upper mantle consisted of silicate crystals sitting in a sulphide-saturated silicate melt.

During magma genesis all of this former "interstitial" melt is consumed and hence produces partial melts which are generally S-saturated.

The

enhanced

precious

metal

contents

of

high

temperature

275

magmas such as komatiites and picrites indicate that these magmas did not become S-saturated until they were emplaced into the upper crust.

The reason for this is that the S capacity of silicate melts increases dramatically with increasing temperature, each lOO^C increase in temperature produces a 5-7 fold increase in the S capacity of the magma (Haughton et al., 1974: Econ. Geol., v. 69: pp. 451-467). Boninites also have high precious metal contents. It is suggested that these second stage melts inherited sulphides left behind by S-saturated first stage melts.

Previous work (Keays and Scott, 1976; Econ. Geol., V. 71, pp.

705-720) has demonstrated that the mineralogical sitting of Au in rocks determines in large part the suitability of the rock as a source of Au. Only Au associated with sulphides is available for the ore-forming process; that which is locked up in oxide and silicate phases is generally inaccessible. The same principal applies to other elements such as Cu and Zn. There are several reasons why the mineralogical siting of the ore-forming metals in sulphide droplets is important.

The concentrations of metals in the sulphide are up to 1000 times greater than they are in the rock as a whole. The sulphides sit in highly accessible sites along grain boundaries.

They react very rapidly with pore fluids compared to the sluggish reactions of silicates. In the case of Au reaction of the sulphides with aqueous fluids produces Hs" which forms the highly stable AuHS complex. In contrast, reactions between pore fluids and silicates are not only sluggish but will buffer the composition of the fluids and may therefore prohibit metal transport.

For example, active serpentinization of ultramafic rocks produces conditions that are so reducing that sulphides in the rocks are often reduced to native Fe, Ni, Cu and platinoids. The reducing conditions are generated by the oxidation of ferrous iron in the ferromagnesium minerals to form ferric iron in magnetite and other oxides. Metal transport will generally be inhibited until reactions between pore fluids and the ferromagnesium minerals in mafic and ultramafic rocks have gone to completion.

276

Symposium 4(b) Lithosphere dynamics and the accumulation of coal, oil shale and fluid hydrocarbons Convener: Mr F. Jeffries

277

ASPECTS OF THE POST-DEPOSITIONAL HISTORY OF THE GIPPSLAND BASIN J . M . B o d a r d , V . J . W a l l , M . P a t o n , R . A . F . Gas Department of Earth S c i e n c e s , Ilouash U n i v e r s i t y , C l a y t o n , V i c t o r i a Investigations of diagenetic m i n e r a l o g i c a l and textural adjustments can yield important insights into the post-depositional history of sedimentary b a s i n s . Essentially complimentary to organic maturation studies, these investigations are required to elucidate controls on porosity/permeability r e l a t i o n s , fluid m i g r a t i o n and entrapment, and contribute independent constraints on thermal histories attending sediment b u r i a l . In this paper w e discuss preliminary observations of post-depositional processes in selected intervals of Strzelecki and LaTrobe Group strata w i t h i n the Gippsland B a s i n .

Formed in a h i g h energy regime fluvial s y s t e m , exhibiting high sand/mud ratios and of a largely penecontemporaneous volcanogenic p r o v e n a n c e , the Early Cretaceous Strzelecki Group is the oldest stratigraphic unit of the Gippsland B a s i n . The texturally submature and chemically labile character of Strzelecki Group lithologies has resulted in the extensive development of secondary m i n e r a l assemblages during its b u r i a l history including zeolites, chlorites and clay m i n e r a l s , c a r b o n a t e s , albite and rare e p i d o t e , prehnite and pumpellyite, These minerals replace framework clasts and m a t r i x , develop interstitially as cements w h i c h occlude primary p o r o s i t y , fill fracture veins and appear to b e the products of a complex growth history beginning during shallow d i a g e n e s i s . Forming consistent assemblages w h i c h vary on a regional s c a l e , the authigenic mineralogy indicates substantial b u r i a l in a region experiencing high geothermal gradients and comparatively high rates of s e d i m e n t a t i o n . This assessment is consistent w i t h the n a t u r e of carbonaceous matter w i t h i n Strzelecki sediments and the ensialic rift-type paleogeographic setting of the group.

The Late Cretaceous - Tertiary LaTrobe Group unconformably overlies Strzelecki strata and consists predominantly of non-marine/ paralic sediments from a m o r e m a t u r e provenance comprising largely granitic and metamorphic e l e m e n t s . Authigenic materials w i t h i n the LaTrobe Group are locally dependent on compositional components and include a range of clays and other p h y l l o s i l i c a t e s , Ca - M g - Fe carbonates and s i l i c a . Secondary porosity is common in sandstones as a result of the dissolution of early formed carbonate phases and/or the dissolution - alteration of detrital feldspars and to a lesser extent other framework and matrix components. In a d d i t i o n , forms of mechanically derived porosity are evident in some i n t e r v a l s . We shall discuss several models for pore fluid/rock interaction which may explain these diagenetic features.

W e suggest t h a t , at least in the onshore and nearshore Gippsland B a s i n , the unconformity between the LaTrobe and Strzelecki Groups represents a pronounced b r e a k in depositional and thermal histories the Strzelecki Group having undergone burial to zeolite facies alteration prior to an emergent period of erosion and the subsequent deposition of the LaTrobe G r o u p . This situation may not have been as marked in the distal offshore portion of the Gippsland B a s i n .

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TECTONIC SETTINGS OF CENTRAL AND EASTERN AUSTRALIAN PERMIAN COAL BASINS 1

H.J. Harrington , A.T. Brakel

1

and J. Hunt

2

^Bureau of Mineral Resources, Geology and Geophysics, Canberra ^Commonwealth Scientific and Industrial Research Organization, North Ryde An overview study of all the Permian coal basins of eastern Australia, but concentrating on the Sydney, Gunnedah, Bowen, Galilee and Cooper Basins, has been carried out, and new structure contour maps, isopach maps, and assemblies of data on coal rank and coal type have been prepared.

Other new data available to us include long BMR seismic profiles which cross some of the basins and extend vertically to below the M discontinuity.

The data provide a factual base for the examination of existing palaeogeographic and tectonic models and for the construction of new models.

Existing published models, and our own models, are applied to two groups of coal basins: (i) The Permian basins under the Eromanga Basin; and (ii) the Permian Sydney, Gunnedah and Bowen Basins.

There are four main groups of models that depend on the following concepts: (1)

Simple down-warping of the crust with no known cause.

(2) Thermal tectonic effects such as (a) the sinking of part of the crust following the heating and metamorphism of the lower crust to a greater density, or (b) the intrusion of a hot diapir from the asthenosphere followed by its cooling and contraction to form a basin.

(3) Compressional tectonism which involves terms like foredeep, fore-arc basin, and molasse basin.

(4) Extensional tectonic concepts which involve terms like failed third arm and marginal sea.

(5) basins.

Strike-slip tectonic concepts which involve transform

Seismic traverses across the Eromanga Basins by BMR show that the upper crust below the Permian and early Palaeozoic infrabasins is homogeneous and unlayered down to about 26 km (S.P. Mathur, pers. comm.).

It is possible that it formed originally as oceanic crust flooring the "Barcoo Marginal Sea" of Harrington (1974).

There is no indication of the later intrusion of an asthenospheric diapir or of metamorphism of the lower crust (as in model 2).

The basins in the Sydney-Bowen trough appear to have formed as transform basins along a major strike-slip zone active in the^ latest Carboniferous and Early Permian.

We call it the Mooki Fault Zone.

Components of it have been reactivated in more recent times as the Nepean Fault, Lapstone Monocline, Mooki Fault, Goondiwindi Fault and Dawson Tectonic Zone.

Probable splay faults branching off the main zone include those in the Gogango Overfolded Zone.

Early activity in the Sydney-Bowen trough was

279

along northwesterly trends, turning to northerly in the Denison Trough.

In the late Permian, activity changed to northerly trends, as typified by the Lochinvar Anticline, the Taroom Trough and the Gloucester Trough.

POST-DEPOSITIONAL EVOLUTION OF SEDIMENTARY BASINS: THERMAL HISTORY AND HYDROCARBON EVALUATION BY FISSION TRACK ANALYSIS A.J.W. Gleadow, I.R. Duddy and J.F. Levering Fission Track Group, Department of Geology, University of Melbourne, Parkville, Victoria 3052.

The petroleum potential of a sedimentary basin is controlled by a number of post-depositional factors which affect generation, entrapment and production of liquid hydrocarbons.

Generation is primarily controlled by the extent to which suitable source beds have been heated within the 60 to 130°C temperature window for geologically significant periods.

Conventional maturation indicators, such as vitrinite reflectance, can provide only qualitative information on maximum palaeotemperatures and give little idea of the variation of temperature with time. Fission track dating of some common detrital minerals is a new approach which can be used to reconstruct the past thermal regimes of a sedimentary basin under hydrocarbon resource evaluation.

Annealing of the radiation damage which constitutes a fission track, like the generation and maturation of hydrocarbons, is a function of both temperature and time. The temperature interval over which track annealing occurs in apatite is virtually identical (60 to 125°C) with that required for the maximum generation of liquid hydrocarbons. Fission tracks in apatide separated by a rock sample thus contain a record of it^ heating in the oil generation window. For a heating time of about 10 years the onset and end of track annealing in apatite corresponds to vitrinite R values of 0.55 and 1.3% respectively.

Four different fission track parameters can be used to give information on thermal history which is largely unobtainably by other methods. These are: 1.

Reduction of fission track ages.

2.

The shape of the apatite age profile with depth.

3.

Reduction in mean track length.

4.

The shape of the track length distribution.

The unique advantage of the fission track method is that it can give information not only on maximum palaeotemperatures, but also their variation through time.

280

The process of fission track annealing in apatite is increasingly well understood, under both laboratory and geological conditions.

Fissions track studies can now give a quantitative perspective on the temperature history of rocks, which could have an important impact on the techniques of petroleum exploration.

Apatite fission track studies also have an obvious application to the investigation of coal rank evolution. In addition, fission tracks in detrital zircon and sphene are stable to higher temperatures (200 - 300°C) enabling limits to be placed on maximum temperatures reached in sedimentary basins, as well as giving important information on sedimentary provanance.

Symposium 4(c) Kimberlites and carbonatites Conveners: Dr J. Ferguson & Dr A.L. Jaques

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THE ALKALINE ULTRAMAFIC CLAN AND THEIR ASSOCIATION WITH CRYPTOEXPLOSION SITES John Ferguson Bureau of Mineral Resources, Canberra ACT

The proposal that shock deformation uniquely fingerprints meteorite impact sites is controversial because some shocked^ cryptoexplosion structures and craters have associated alkalic and alkaline ultramafic intrusives.

Sometimes they lie on a common lineament and in other cases the alkalic body lies within the individual structure or crater.

A few workers have suggested that violent release of high fluid pressure, originating from ultramafic magma, was responsible for the shock deformation and cratering.

Several categories of disturbed structure are associated with carbonated alkaline ultramafic magmas.

They have a single linking factor - venting of highly compressed fluids, which were originally in an underlying source.

When there was rapid permeation of near-surface rocks by these compressed fluids, this led to a (mechanically) highly unstable condition, which was followed by cratering and formation of a cryptoexplosion structure.

If the high pressure fluid was in a superheated condition during disruption, then a detonation could relieve the fluid from its metastable condition and shock deformation of the rocks ensued.

The 38th parallel lineament in the east-central U.S. manifests long-continued activity by carbonated alkaline ultramafic magma; there is a deep asthenosphere-tapping fracture extending across this region.

Controls on magma generation included the detailed variations in local strain pattern along the fracture and the presence of either cupolas or dimples along the asthenospheric crest. ^ It has been shown that this major fracture also localises the formation of ore deposits; intensive study of such lineaments will throw light on the mechanism by which high pressure fluid is transferred to the earth's surface.

Intrusive carbonatite-generated complexes, located deeper in the crust than the cryptoexplosion structures, may play an important role in this transfer.

A REAPPRAISAL OF CERTAIN 'KIMBERLITIC' OCCURRENCES IN E. AUSTRALIA. Julian D. liollis and F.L. Sutherland Department of Mineralogy & Petrology The Australian Museum, Sydney, NSW 2000 The relevance of diatreme/maar-type alkali volcanic centres to diamond exploration is uncertain. Whilst showing certain kimberlitic features (eg. autolith-bearing breccias, pyropic garnets, high-Cr

282

spinels and other popularly held kimberlite indicators) they involved alkaline basaltic magmas that are unrelated to true kimberlites.

Xenoliths give pressures indicating origins seldom deeper than 70 km and their host magmas came from no deeper than the upper garnet Iherzolite levels, well inside the graphite stability field. New data are presented from Bow Hill, Tasmania; Lake Bullenmerri, Vic; Anakie Eastern Hill, Vic.; Ruby Hill (Bingara), NSW and localities in S.E. Queensland. This shows maximum xenolith and megacryst depths of about 90 km in a high heatflow regime that was established in eastern Australia from before the Mesozoic.

A reappraisal of classic kimberlite indicator minerals and their application to E. Australia shows them to be irrelevant, if not highly misleading. Garnets from inclusions frequently lie in a narrow field forming a stright line from Ca/Ca+Mg+IFe = 13 in garnet Iherzolite through 13.5-14 in feldspar-free pyroxenites to 14-14.5 in feldspathic granulitic pyroxenites. This appears to be pressure dependant for alkali basalt magmas. Strongly pyropic garnets (ie. Mg/Mg+IFe >70) show minimal Cr and often high Ti. This range, frequently falls in Dawson's kimberlite garnet fields for South Africa; but has little relevance to diamond facies in E. Australia. Garnet compositions outside these 'basaltic trend' fields, such as high Cr, Ca types may be significant. Some diatremes have xenolithic minerals with abundant inclusions of CO^. Abundant mantle CO^ maybe a prerequisite for maarproducing volcanism, which would be initiated suddenly by fissure propagation through suitably brittle or structured crustal rocks.

Although kimberlite-like in certain aspects, alkaline basaltic pipes in E. Australia are best treated separately.

THE GEOLOGY AND PETROLOGY OF SOME KIMBERLITES NEAR TEROWIE SOUTH AUSTRALIA D.M. Colchester Department of Applied Geology, N.S.W.I.T., Sydney

A Kimberlite province covering an area 45km x 30km occurs 224km north of Adelaide in the South Australian county of Kimberley.

Kimberlite dykes which outnumber the diatremes in a ratio of 6 to 1 are composed of bluegreen to green brown massive friable sandy textured micaceous hypabyssal facies Kimberlite. Their diameters vary from 10m down to 0.1m but averaging l-2m. The diatremes, by world standards, are small and have a very irregular outline often elongated along possible strike directions of feeder dykes. Most of them occur in pairs. The diatreme Kimberlites are green micaceous highly brecciated (typical diatreme facies) with abundant angular and rounded country rock xenoliths, rounded boulders of earlier formed Kimberlite, and occasional lower crustal garnet granulite and mantle eclogite xenoliths. The matrix is almost

283

entirely weathered to clay in contrast to the Kimberlite dykes which still contain much serpentine. Almost 1,700m of land surface deflation has occurred since emplacement 164-174m years ago. A large Wyomingite (leucite lamproite) dyke 1km long is the only other intrusive igneous rock in the area.

Their region of occurrence is interpreted to be a subsequently cratonized circum-cratonic orogenic belt (the Adelaide Fold Belt) sandwiched between the older Gawler and Willyama cratons. From African experience this belt should contain non diamondiferous kimberlites in contrast to some that may occur in the Gawler and Willyama cratons. Kimberlite dyke strike directions are controlled chiefly by country rock grain and regional lineament directions.

The physical and chemical properties of the megacryst minerals is similar to other Kimberlites, consisting of chrome pyrope, picroilmenite, chrome diopside, enstatite chromite and often with two generations of phlogopite. Garnets belong to cluster groups 3,5,9 of Dawson and Stephens (1975) and the ilmenite has a 29% geikieite component. Major element chemistry resembles * average* kimberlite closer than any other igneous rock type, despite only weathered samples being available for analysis.

Dawson J.B. § Stephens W.E., 1975, Statistical Classification of garnets from Kimberlite § Associated Xenoliths, Jour. Geol., Vol. 83, p.589-607

DIAMONDIFEROUS KIMBERLITES AT ORROROO, SOLTTH AUSTRALIA: AN INITIAL REPORT R.V. Danchin\ J.W. Harris^, B . H . S . Smith^ & K . J . ^Stockdale Prospecting L i m i t e d ,

60 W i l s o n S t r e e t ,

Stracke^

South Yarra

3141,

Victoria, Australia Department of Applied Geology, University of S t r a t h c l y d e , Glasgow, ^ Scotland Anglo American Research L a b o r a t o r i e s , P.O. Box 106, Crown Mines 2 0 2 5 , South Africa A s u i t e of J u r a s s i c k i m b e r l i t e dykes and a s s o c i a t e d blows ranging in thickness from a few millimetres to 30 metres has been discovered near the town of Orroroo, South Australia. At the surface the kimberlites are heavily altered but fresh material has been recovered at a depth of 60 metres. The kimberlites are hypabyssal and all have the distinct inequigranular texture typical of kimberlites with the usual two generations of olivine. Phlogopite is the most abundant groundmass mineral but diopside, calcite, perovskite and spinels are also present. Ihe geochemistry of the groundmass minerals is d i s c u s s e d in some d e t a i l . Although no u l t r a m a f i c x e n o l i t h s have been recovered, the kimberlites contain a profusion of meyacrysts. Microprobe analyses of several hundred garnets, ilmenites, diopsides and spinels are presented and discussed. Several of the kimberlites are weakly diamondiferous and details are given. Microprobe analyses for an enstatite and two magnesiowustite inclusions are given and discussed.

284

A REVIEW OF THE KIMBERLITIC ROCKS OF WESTERN AUSTRALIA W.J. Atkinson, F.E. Hughes, C.B. Smith CRA Exploration Pty Limited, Manager, Ashton Joint Venture, Western Australia

In the course of exploration for diamond, CRA Exploration Pty Limited and the Ashton Joint Venture have discovered four diamond bearing kimberlite provinces in Western Australia.

Three of these provinces are located marginal to the Kimberley Craton in the north of the state (Fig. 1), and one lies in the Carnarvon Basin, adjacent to the Yilgarn Block, some 1300 km to the southwest.

The kimberlites intrude rocks ranging in age from Lower Proterozoic to Permian, and are covered by sequences ranging in age from Cretaceous to Miocene.

The bodies range in size from dyke-like features less than one metre in width to pipes with a surface area of 128 hectares.

The bodies with larger surface area are volcanic crater deposits, champagne-glass shaped in cross section, the narrow stem corresponding to the pipe feeder.

The craters are infilled with air-fall and waterdeposited tuffs and epiclastic sediments.

A late-stage phase in many of the West Kimberley diatremes was the emplacement of massive, igneoustextured, magmatic kimberlite, rising to the surface in the shape of a lava-blister.

This magmatic kimberlite fills the central part of the craters, and overlaps the tuffs towards the margins.

Diamond content ranges from trace amounts to economic concentrations.

Feasibility studies being carried out on the Argyle AKI kimberlite pipe are currently envisaging a 2.25 million tonnes per annum operation, to come into production in 1985, producing some 20 million carats per year, while it is hoped that limited commercial production from associated alluvial deposits will commence in the latter part of 1982.

The exploration discoveries were facilitated by early recognition that the petrography, mineralogy and chemistry of the kimberlites varies from classical types resembling those of kimberly. South Africa, to unusual more highly fractionated, alkaline, silicic varieties having affinities with leucite-lamproite and composed essentially of phenocrysts of oline + clinopyroxene ^ phlogopite + glassy groundmass.

Mantle nodules recove7ed range from dunite to Iherzolite; graphic-textured intergrowths of picroilmenite and silica (after diopside?) occur at the Skerring pipe.

Heavy mineral concentrates from the kimberlites yield pyrope, picroilmenite, chrome-diopside, chromite and zircon, the former two minerals being more abundant in the classical types of kimberlite.

Recognition of the characteristics of the unusual kimberlite/ lamproite association strongly influenced exploration techniques, much use being made of such minerals as chromite, andradite and zircon as kimberlite indicator minerals during heavy mineral gravel sampling.

In regions where host rocks displayed a mild magnetic response it was found that the kimberlitic bodies produces recognisable magnetic .anomalies from detailed aeromagnetic surveys, and much use has been made of this technique in exploration.

285

286

THE ULTRAPOTASSIC, DIAMOND-BEARING ROCKS OF THE WEST KIMBERLEY REGION, WESTERN AUSTRALIA 1 2 3 1 4 A.L. Jaques , J.D. Lewis , G.P. Gregory , John Ferguson , C.B. Smith , B.W. Chappell^, and M.T. McCulloch^

1. Bureau of Mineral Resources, Canberra, ACT, 2. Geological Survey of Western Australia, Perth, WA, 3. Seltrust Mining Corporation Pty Ltd, Perth, WA, 4. C.R.A. Exploration Pty Ltd, Belmont, WA, 5.

Department of Geology, Australian National University, Canberra, ACT, 6. Research School of Earth Sciences, Australian National University, Canberra, ACT Diamonds have recently been found in the ultrapotassic rocks (kimberlites and lamproites) of the Fitzroy area of the West Kimberley region of Western Australia by CRA/Ashton Joint Venture. Over 100 separate bodies of kimberlite and lamproite occur as diatremes, plugs, sills and rare dykes in three main fields (Ellendale, Calwynyardah and Noonkanbah) within a broad belt which extends from the southern edge of the Proterozoic Kimberley Block across the Lennard Shelf and Fitzroy Trough at the northern margin of the Canning Basin. Petrographic and chemical similarities indicate that the kimberlites and lamproites form a consanguineous, ultrapotassic suite of Miocene age (17-25 m.y.).

The kimberlites range from phlogopite to phlogopitediopside to phlogopite-diopside-richterite types, and appear to grade petrographically into leucite-bearing lamproite.

The compositional ranges of olivine, diopside, phlogopite, amphibole and spinel in kimberlite overlap those in the more magnesian lamproites.

Both the kimberlites and lamproites are characterised by high K^O contents (4-12%), and high K2O/AI2O3 (average 1.2) and K^0/Na20 (typically >10).

MgO contents range from 18-28% (average 24%) in kimberlite to ~3% MgO in the most evolved lamproites.

The suite as a whole is saturated to oversaturated in silica, and follows a trend of increasing Si02 with decreasing MgO content and Mg/(Mg+Fe) ratio.

Characteristic features of the suite are the very high contents of Ba, Rb, Sr, Pb, Th, U, Ti, Zr, Nb and light rare earth elements (LREE). REE patterns in kimberlites and lamproites are very similar, being highly fractionated and LREE-enriched at 500-2000 x^ chondritic abundances, with low abundances of HREE (4-6 x chondritic).

Both the kimberlites and the lamproites have low to very low abundances of CaO, Na20 and 87sr/86sr ratios are high (%0.3-0.4, 0.711-0.726) and Nd/ Nd ratios very low (e^^^ = -7 to -15), and indicate derivation of the suite from an ancient, large-ionlithophile-element (LILE)- enriched mantle source.

The West Kimberley kimberlites differ significantly from 'typical' kimberlite in that they are intimately associated with leucite-bearing basic rocks.

Petrographic differences include the presence of groundmass amphibole, absence of primary carbonate, and rarity of garnet and picro-ilmenite.

Compared to 'typical' kimberlite, the West Kimberley rocks have higher Si02 contents, are enriched in K, Rb, Sr, Pb, Zr, Ti, Zr and LREE, and depleted in CaO and, to a lesser extent Al^O^, and have very low CO^ contents.

287

The West Kimberley rocks are thought to have been derived by low degrees of partial melting of phlogopite-rich garnet Iherzolite under hydrous ( % o»^C02) conditions.

The peridotite source had previously experienced long term enrichment in LILE. The comparative rarity of 'kimberlite indicator' minerals is thought to be due to the existence of depleted (garnet-clinopyroxene-poor) peridotite beneath the region.

Diamond occurs in both kimberlite and lamproite at grades ranging from 'trace' to sub-economic (Ellendale kimberlite) indicating the ultrapotassic kimberlite/lamproite association is of potential economic significance.

Nd AND Sr ISOTOPIC SYSTEMATICS IN KIMBERLITES AND LAMPROITES FROM WEST KIMBERLEY, WESTERN AUSTRALIA. 1 1 2 M.T. McCulloch , D.R. Nelson ,A.L.Jaques

and J.D. Lewis

3

^Research School of Earth Sciences, Australian National University Canberra ACT. Bureau of Mineral Resources, Canberra ACT. ^Geological Survey of Western Australia, Perth W.A. We report Nd and Sr isotopic compositions of lamproites and kimberlites from the West Kimberley area of Western Australia.

Samples of these ultrapotassic lavas have extreme light rare earth element enrichments (Nd ^ x 90 to x 500 enrichments relative to chondrites). esr -50

0

50

—I

1

100

150

200

MORB DEPLETED MANTLE (DM)

07o 0-8% ^

m

Samoa

15% o •o

(Sr/Nd)DM

Z

=2 (Sr/Nd)EM"

3% o 57o

?

(Sr/Nd)D; ENRICHED MANTLE (EM)

(St/NCDem"'^ " typical precision

I07o

5

H I007o

0705

07K)

0715

0720

Nd and Sr isotopic compositions of Western Australian kimberlites (•) and lamproites (•).

288

The ^^^Nd/^^^Nd ratios have a range corresponding to e^^ = -7.4 to -15.4 and high initial ®^Sr/®^Sr ratios of from 0.71055 to 0.71865.

These relationships are consistent with either contamination of lavas with substantially older continental crust or derivation from highly enriched (i.e. high Nd/Sm and Rb/Sr) portions of the mantle.

The latter alternative is preferred, due to difficulty of contaminating these highly light rare earth element and Sr enriched magmas with crustal materials. In addition, mineralogical evidence indicates equilibration at depths ^ 100 km. Mixtures of this enriched mantle with depleted MORE type mantle can account for both the enriched and depleted portions of the mantle as well as the apparent clustering near G^j ^ 0 of previously reported values for kimberlites. Possible mechanisms for producing enriched portions of the mantle will be discussed.

REDOX STATE EVIDENCE FOR THE XENOCRYST STATUS OF DIAMOND IN KIMBERLITE Richard J. Arculus

Research School of Earth Sciences, Australian National University, P.O. Box 4, Canberra A.C.T., 2600, Australia.

The oxidation states measured for peridotites, megacrysts and melts derived from the upper mantle have been shown to be quite diverse. There is currently a major problem in accounting for this diversity, but already the studies of intrinsic oxygen fugacity (f02) of these samples has provided considerable information on petrogenetic processes in the upper mantle. For example, there is a conflict of opinion regarding the phenocryst-xenocryst status of diamond in kimberlite. With oxygen-specific solid electrolyte cells (stabilized Zr02), it is possible to measure the intrinsic f02 of megacryst species and use the results to determine whether carbon (graphite/diamond) or carbonate is stable in kimberlite melts.

Over the past few years, we have measured the intrinsic f02 of a number of different spinel peridotite samples, megacryst spinels and ilmenites. In broad outline, it appears that the majority of type A (or so-called "chrome-diopside" type) spinel peridotites are surprisingly reduced and fall close to the synthetic iron-wustite (IW) buffer in f02-temperature space. These results are consistent whether the peridotites are from a continental environment (e.g. southeastern Australia) or from an island arc (e.g. Japan). Most petrologists^ regard the type A peridotites as representative of the major portion of the shallower part of the upper mantle. The stable form of carbon under conditions close to IW at pressures of about 20 kbar is graphite (and at higher pressures would be diamond). There is currently an unresolved discrepancy of 1 to 2 orders of magnitude between intrinsic f02's calculated from thermodynamic analysis of equilibria of the sort 6Fe2Si04 + 0 2 ^ 3Fe2Si205 + 2Fe30^, and measured (with solid electrolytes) values. It is possible that the measured, relatively reduced values result from minor non-stoichiometry (cation-excess) in the spinels, and imprecision in the assumptions of spinel character used in the theoretical analysis.

289

In contrast with the values of IW for type A samples, type B (or "aluminous augite" type) peridotites lie close to the synthetic fayalite-quartz-magnetite (FQM) buffer, or about 4 orders of magnitude more oxidized than type A. This oxidation level is also characteristic of the majority of erupted magmas. Unless some process such as H2 loss has occurred from the melts, it is impossible for the type A peridotites to be genetically related (i.e. restite or parental) to the erupted melts, whereas it is possible that type B^peridotites are genetically related to the melts.

Megacryst ilmenites of variable MgO and Fe203/Fe0 content from kimberlite pipes in southern Africa and the alnoitic breccia in the Solomon Islands are even more oxidized than type B peridotites. With a decrease in Fe203/Fe0 ratio, there is a fairly regular decrease in the intrinsic f02 of these ilmenites from values equivalent to the synthetic nickel-nickel oxide (NNO) to FQM buffers. The ilmenites apparently record redox conditions prevailing in the early stages of kimberlite magma generation. The stable form of carbon under redox conditions greater than FQM along probable geothermal gradients is as some type of carbonate. Consequently, diamond (or graphite) entrained in kimberlite must be of xenocryst character. There is thus no reason for diamond to be restricted to kimberlite hosts - any fast-travelling, deep-seated magma capable of transporting metastable xenocrysts to the surface can be a host.

The fundamental question why there is a dispersion between reduced (IW) and oxidized (NNO-FQM) peridotites and magmas is unresolved. A reduced upper mantle may be partly relict from core-mantle equilibrium and subsequent oxidation may have resulted from lithosphere recycling or upward volatile movement from redox equilibria in the lower mantle.

THE (CENTRAL

STPANGWAYS AUSTRALIA)

P.W. Crohn 1

&

RANGE CARBONATITE

D.H, Moore

1 Durham Road, Surrey Hills, Vic. 3127 Formerly Department of Mines and Energy, Darwin, N.T.

2 The Broken Hill Pty. Co. Ltd., P.O. Box 559, Camberwell, Vic., 3124 Formerly Department of Mines and Energy, Alice Springs, N.T. The Strangways Range Carbonatite is located near 23^ 01'S 124^ 15'E about 100 km north-north-east of Alice Springs.

It has been investigated several times, including diamond drilling by Geopeko Ltd. (Williams, 1967), and by the Mines and Water Resources Branch, North Territory Administration (Crohn, 1971).

The main occurrence consists of an irregular north-east trending lens of crystalline carbonate rocks with maximum dimensions of about 2000 by 700 metres. A second, much smaller lens occurs about 2 km to the south-west.

290

The carbonate rocks show prominent banding due to differences in texture and composition, with bands generally from a few centimetres to more than a metre in width. The general trend of this banding is parallel to the long axis of the occurrence and to the regional trend of the adjacent host rocks, which are schists and gneisses of the Lower Proterozoic Arunta Complex.

Either dolomite or calcite may predominate in any of the carbonate rocks. In outcrop, a group of crystalline carbonate rocks with subordinate apatite, magnetite, zircon, phlogopite, chlorite and soda-amphibole predominate. However drilling has shown that the most abundant lithology is a foliated micaceous carbonate rock, the foliated appearance being caused by pale brown phlogopite which may constitute up to 60 percent of the rock. Both these groups are thought to be the products of crystallisation from a carbonatite magma.

On the other hand, minor occurrences of feldspathic carbonate rocks characterised by a generally even-grained granular or slightly porphyroblastic texture and by the presence of sodic plagioclase, green-brown amphibole, brown biotite and subordinate clinopyroxene, are thought to represent contaminated or hybrid phases, and the same applies to a number of rocks composed almost entirely of amphibole and biotite in various proportions.

Minor late stage pegmatites also occur.

The carbonatite lies at the axis of a regional gravity high, and appears to be emplaced on a north-east trending shear zone complementary to the north-west trending Woolanga Lineament, which is a major structural feature of the region and passes about 8 km south-west of the carbonatite.

The carbonatite has also been affected by subsequent faulting and shearing, including one fault with an apparent horizontal displacement of about 300 metres.

The carbonatite has been dated at about 730 million years.

Fig. 1. Surface Geology of the Strangways Range Carbonatite Moore, in prep.)

(after

291

which does not coincide with any other known event in this general area (Black & Gulson, 1978). No other carbonatites are confirmed from the area. The alkaline Mordor Complex, (1200 m.y.) some 50 km to the south-east, contains veins and irregular masses of carbonates with higher Sr87: Sr86 ratios than are typical of carbonatites.

Niobium, rare earths and phosphate concentrations in the Strangways Range rocks are within normal limits for carbonatites, but well below economic values.

However, zircon crystals of gem quality are present in the soil and colluvium overlying the carbonatite, and attract numerous fossickers.

In addition, vermiculitic mica has formed in the 40 metre deep weathered zone.

References: Black, L.P. & Gulson, B.L. 1978. The age of the Mud Tank Carbonatite, Strangways Range, Northern Territory.

B.M.R. Jour, of Australian Geol. & Geophys., 3, 227-336 Crohn, P.W. 1971.

Investigations at the Strangways Range Carbonatite locality. Northern Territory, 1969-1970.

Mines Branch, Northern Territory Administration; Northern Territory Geological Survey Record 1971/1 (unpubl.)

Moore, D.H. In Prep. The Mud Tank Vermiculite Prospect, Alice Springs 1:250,000 Sheet Area SF 53/14. Northern Territory Government Department of Mines and Energy; Northern Territory Geological Survey Record (unpubl.)

Williams, B.T. 1967. Report on the investigation of the Enterprise 2 Group of Prospects, Strangways Range, Northern Territory.

Geopeko Ltd. unpublished report.

MINERALOGY AND GEOCHEMISTRY OF A BARIAN CARBONATITE AND ASSOCIATED ULTRASODIC FENITES FROM A LAMPROPHYRE DYKE SWARM, SOUTH WESTLAND, NEW ZEALAND A.F. Cooper Geology Department, University of Otago, Dunedin, New Zealand

A lamprophyre dyke swarm intruding Haast Schists in south Westland, New Zealand comprises camptonites, damkjernitic peridotites, tinguaites, trachytes, and carbonatites.

Although calcite-dolomite carbonatites occur, the majority contain ankerite or siderite, occasionally with a discrete Ba-rich phase.

The most extreme carbonatite composition occurs in a 1.2 m thick sill, symmetrically flanked by aegirine-riebeckite schist fenites.

The carbonatite is dominated by norsethite (BaMgCCO^)^) coexisting with ankerite, strontianite, Sr-calcite, albite, aegirine, pyrite, sphalerite, galena, monazite, and apatite.

Daqingshanite (approx. 3SrC03.REE PO4) occurs rarely as a rounded, possibly phenocryst, phase.

Chemically the carbonatite is Ba-rich (25.7% BaO) with high concentrations of other

292

typical carbonatite trace elements Sr (2.4% SrO) and REE (1.9% RE2O3).

Rare earth elements are dominated by Ce-earths, with an extremely high La/Yb ratio (approx. 3300).

Carbon and sulphur isotope ratios indicate magmatic affinities for these elements, while values suggest minor interaction with the metamorphic country rock.

In the fenite aureole country rock quartzofeldspathic schist is progressively converted over a distance of 8.5 m to an ultrasodic aegirine-albite schist at the carbonatite contact.

Fenitization occurs under approximately constant volume - constant oxygen conditions with addition to the schist of Na, Fe^^ and to a lesser extent C balanced by depletion of Si, K and possibly H. In terms of trace elements fenites are enriched in Ba, Sr, Nb, Zn and REE compared to parent schist.

Similar compositions of aegirine, albite, and carbonate in inner fenite and carbonatite suggest that the fenitizing agent, (a highly mobile, oxidising fluid) was in equilibrium with carbonatite, rather than an earlier silicate-igneous precursor.

The carbonatite crystallized in the temperature range 340-440^C at pressures close to 6 kb.

Low temperatures of crystallization combined with the mineralogical and geochemical characteristics indicate a latestage ferrocarbonatite.

However, as is probably the case in most carbonatites the crystalline product is not representative of the parent medium, which on the basis of the associated fenitization must have been a highly sodic barian carbonatite.

ORBICULAR RHYTHMIC LAYERING IN THE PALABORA CARBONATITE, SOUTH AFRICA Alan C. Moore BHP Exploration, 20 O'Connell Street Sydney, N.S.W., 2001 Australia The earliest stage of magmatic activity within the Palabora carbonatite was marked by the intrusion of phosphate-bearing pyroxenite. In good exposures in the northern pyroxenite, produced by open cut mining by Foskor (Phosphate Development Corporation Ltd.), large scale (2m diameter) orbicular structures are found.

These consist of regularly spaced alternating dark layers (phlogopite-rich) and light layers (diopside- plus apatite-rich) which, in hand specimen are very similar to the "inch-scale" planar layering which has been described in layered mafic intrusions. One of the purposes of this paper is to describe these unique features which are currently being destroyed by mining, as they form economic phosphate (apatite) concentrations.

The resemblance of the Palabora orbicules to Liesegang rings has led to the development of a qualitative model whereby the orbicules are regarded as having been formed by concentric periodic precipitation around central cores within dynamically quiet, isolated pockets of largely liquid magma. The controlling parameters are interpreted as

293

being the rates of growth of the constituent minerals and the ra^es of diffusion of the elements cr^_cial tc^^their growth, ie: K , A1 and (OH)" for dark layers, and Ca and p for light layers.

The presence of these spectacular structures indicates that at the pyroxenite stage of the intrusion of the Palabora carbonatite, the mechanism of intrusion was such that relatively stable conditions prevailed. Hence, the vertical large-scale banding in the pyroxenite may also be a function of diffusion controlled processes, similar to those proposed for Skaergaard by McBirney and Noyes (1979), rather than being caused by separate magma pulses.

McBirney, A.R. and Noyes, R.M. 1979: Crystallization and layering of the Skaergaard intrusion. J. Petrology, 20, 487 - 554.

I EiEGMATOID

PEGMATOID

f9$C Sketch, from a photograph, of block of pyroxenite with well developed orbicules.

A transgressive pegmatoid marks the right edge of the loose block.

FOSKOR open oit, Palabora.

294

THE NATURE OF THE LOWER CRUST/UPPER MANTLE TRANSITION IN EASTERN AUSTRALIA EVIDENCE FROM ECLOGITIC AND GRANULITIC XENOLITHS IN BASALTIC ROCKS Suzanne Y. Wass^ & J.D. Hollis^ ^School of Earth Sciences, Macquarie University, North Ryde, NSW 2113 ^Australian Museum, College Street, Sydney, 2000, Australia Mafic and ultramafic xenoliths in a basaltic cone at The Anakies in southeastern Australia are geochemically equivalent to continental basaltic magmas and cumulates. The xenolith microstructures range from recognizably meta-igneous for intrusive rocks to granoblastic for garnet pyroxenites. Contact relationships between different rock types within some xenoliths suggest a complex petrogenesis of multiple intrusive, metamorphic and metasomatic events at the crust/mantle boundary during the evolution of southeastern Australia. Unaltered spinel Iherzolite, typical of the uppermost eastern Australia mantle, is interleaved with or veined by the metamorphosed intrusive rocks of basaltic composition.

Geothermobarometry calculations by a variety of methods show a concordance of equilibration temperatures ranging from 880°C to 980°C and pressures of 12 to 18 kb (1200-1800 mpa). These physical conditions span the gabbro to granulite to eclogite transition boundaries. The water-vapour pressure during equilibration is estimated to be about 0.5% of the load pressure, using amphibole breakdown data. Large fluid inclusions of pure CO2 are abundant in the mineral phases in the xenoliths, and it is suggested that flux of CO2 from the mantle has been an important heat source and fluid medium during metamorphism of the mafic and ultramafic protoliths at the lower crust/upper mantle boundary.

The calculated pressures and temperatures suggest that the southeastern Australian crust has sustained a high geothermal gradient.

In addition, the nature of the mineral assemblages and the contact relationships of granulitic rock with spinel Iherzolite, characteristic of mantle material, suggest that the Moho is not a discrete feature in this region, but is represented by a transition zone approximately 20 km thick. These inferences are in agreement with geophysical data (including seismic, heat-flow and electrical restivity data) determined for this region.

The geochemistry of the lower crustal xenoliths suggests they originated as underplating of the crust by continental-type basaltic magmas. It is postulated that such addition of basaltic magma to the lower crust may represent an important alternative or additional mechanism to the conventional andesite model for crustal accretion.

295

NATURE AND ORIGIN OF THE UNDERSATURATED ROCKS OF THE MT DROMEDARY COMPLEX

Ian E.M. Smith^S Bruce W. Chappell^

^Department of Geology, University of Auckland, New Zealand ^Department of Geology, Australian National University, Canberra, ACT

The Mount Dromedary complex is a group of small (< 25 km^) plutons exposed near Narooma on the southern New South Wales coast.

K~Ar dating indicates an age of about lOOmyr the rocks thus represent a temporally isolated magmatic event which occurred within a spatially restricted area (about 250 km^) at a time immediately prior to rifting associated with the commencement of spreading to form the Tasman Sea basin. Although predominantly monzonitic, rock types of the complex range from ultramafic (pyroxenite) to felsic (> 70% SiO^) and include both critically undersaturated (normative nepheline 14%) and quartz normative rocks.

Some of the plutons show well developed compositional zoning over a wide compositional range (typically 50-65 wt% SiO^) which is interpreted as due to fractional crystallisation.

The undersaturated rocks include melanite bearing pyroxenites which are quite distinct from rocks typically associated with alkali basalt magma.

The compositions of the complex as a whole show systematic variations which can be interpreted as due to fractional crystallisation of an alkali-rich (TiO^ < 1 . 5 wt%) low Si magma. This magma appears to have provided the stem for an oversaturated series and an undersaturated series of monzonite. Elsewhere rock types comparable to those of the Mount Dromedary complex (e.g. southeastern Papua) have been linked to convergent plate boundary tectonics and so to island arc magmatic associations.

In this case the tectonic regime appears to have been tensional; Joplin (1971) has suggested a possible affinity with carbonatites.

The generation of the Mount Dromedary maqma is thought to be an isolated mantle event possibly linked to rifting associated with the break up of the Australia/New Zealand continental block.

Joplin G.A. 1971.

Petrography of Australian igneous rocks Angus & Robertson

(3rd ed.).

296

I^iEPhELlNE AMD QUARIZ-BEARIMG S^ENllE A1 JINGERA ROCK COMPLEX, SOUlHEASl AUSIRALIA S.D. Beams

B.W. Chappell^ and A.J.R. White ^

o 1 Minerals Department, Esso Australia Ltd., Sydney N.S.W., 3 Department of Geology, Australian National University, Canberra ACT Department of Geology, La Irobe University, Melbourne Vic.

Ihe Mesozoic Jingera Complex of southeast ISiSW consists mainly of quartz- and feldspathoid-bearing syenites.

Field and petrographic features of the complex and surrounding metamorphic aureole indicate a high temperature, near surface intrusion. hedenbergite-hastingsite-fayalite + Quartz Syenite forms a narrow (5 to 50m wide) outer rim around the complex and also occurs as a series of discontinuous screens within hastingsite-hedenbergite biotite nepheline syenite which forms the bulk of the complex.

Other major rock types present are biotite-hastingsite nepheline monzonite and inclusion bearing monzonites.

Ihe latter are hybrid rocks consisting of polycrystalline aggregates and single crystals or xenocrysts of aluminous titanaugite, plagioclase, titanomagnetite, spinel, microperthite, kaersutite, and alteration products after olivine and orthopyroxene? set in a matrix of hedenbergite-hastingsite microsyenite.

Dyke rocks cutting the major rock types of the complex include peralkaline quartz trachyte, peralkaline phonolite, sodalite phonolite, felspathoidal trachytes and fluorite-rich trachyte.

Ihe different rock types present cannot be related by crystal fractionation of a single parent.

Ihe composition of inclusions contained within both quartz and nepheline normative microsyenite preclude the derivation of the complex from typical mantle material.

Ihe inclusion assemblage suggests that the source of both syenites had a transitional basalt chemistry.

Partial melting of this source at about lOkb and 1100°C produced a series of melts with the observed range of silica saturation.

AN EXAMPLE OF LIQUID IMMISCIBILITY L.M. Barron Geological Survey of New South Wales, Mining Museum, Sydney N.S.W. An unusual basic dyke, Im wide and of probably Tertiary age trending at 030^/90^, occurs at GR 135680 (m) (Kusciusko 1:100,000) on the slopes of Mount Townsend, outcropping in a creek that runs eastwards into Lake Albina, about 3.5 km north-northeast of Mount Kosciusko. The dyke has minor flow-flattened vesicles to 10 mm but it also has abundant 1-5 mm silicate droplets which exhibit lobate protuberances, flow differentiation, crystal settling, and 1 mm round vesicles. The droplets (D) and host (H) are direct evidence for silicate magma immiscibility, and textures indicate that this process operated before and

297

during intrusion and solidification of the dyke. The dyke mineralogy has been studied by microscope and electron microprobe, indicating the presence of olivine (H), Cr-spinel (H), Ti-magnetite (H), Ti-augite (HD), Mg-biotite (HD), ferrohastingsite (D), aegirine (HD), and as the major constituent of the rock, quenched pseudoleucite (HD). Mineral distributions, textures and chemistry are vastly more complicated than would be expected from such a simple geological occurrence, see fig.

1, 2. Given the probably very short time for solidification of the dyke, droplet coalescence and flow migration must be very efficient processes. Such efficiency would destroy evidence of liquid immiscibility in coarser plutonic rocks so perhaps liquid immiscibility is more common than presently recognised in eastern Australia.

Area Fraction of Thin Section

03

01

10

04

H

-05

) ^^^ ^ 'Host or Droplet

D

>

o 1

06

07

08

> q"^

\ ^

30

40

'i-

Sample localities

y

y

I It Hosf/10

o

Olivine

H

D Droplets/lO

•

Opaques

mmmm Size of droplets (equivalent diameter in J^)

X LA.

Hornblende 12366

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ALKALINE PLUTONIC ROCKS IN THE WILLYAMA COMPLEX, NEW SOUTH WALES R.A. Binns

1

& B.J. Barron

2

^CSIRO Division of Mineralogy, North Ryde, NSW Consultant, formerly Geological Survey of NSW Recent mapping at Broken Hill by the Geological Survey of New South Wales has revealed numerous dykes and larger bodies of jacupirangitic pyroxenite, first reported by Binns (1966), and also one mass of ijolite.

Together with ijolite, nepheline syenite and alkali syenite recorded from South Australia by Bell et al. (1979), these indicate that a significant province of alkaline magmatism occurs within the Willyama Complex.

The pyroxenites are demonstrably discordant to foliation and lithological layering of metasedimentary country rocks, and are apparently younger than other intrusive rocks of the Willyama Complex apart from a set of uralite dolerite dykes. Their relationship to serpentinites at Rockwell and Thackaringa is unknown. The ijolite body (near Bald Hill, 10 km SW of the Pinnacles Mine) has unexposed contacts, but lies between two pyroxenite intrusions. It is cut by rare narrow veins of pegmatitic ijolite approaching urtite.

Individual exposures of pyroxenite and ijolite are relatively homogeneous except for variation in degree of metamorphic recrystallization. However altered jacupirangite is known from one pyroxenite body and others contain more metamorphosed rocks of melteigite and jacupirangite composition. Rare pyroxenite occurs among outcrops of the Bald Hill ijolite, where drilling indicates that metamorphosed pyroxenite and jacupirangite become predominant at depth.

The consanguinity between pyroxenite and ijolite evident from field relationships is supported by petrology, mineralogy, and geochemistry.

Virtually unmetamorphosed pyroxenites, ijolites, and ijolite-urtite pegmatites are preserved in parts of the intrusions. The pyroxenites contain close-packed, elongate euhedral prisms of clinopyroxene up to 2 cm across but more commonly about 5 mm long and 1 to 3 mm across, together with significant interstitial magnetite and accessory apatite and ilmenite. The ijolites contain similarly-sized elongate prisms and smaller grains of clinopyroxene, scattered magnetite and apatite grains, and stubby prismatic nephelines typically 0.5-1 mm across with euhedral growth patterns defined by clouds of inclusions. The pegmatite veins contain large transverse clinopyroxene needles, minor magnetite and abundant subhedral to anhedral nepheline. Flow alignment is characteristic of both pyroxenite and ijolite. Minor phases including pargasitic hornblende, phlogopitic biotite, cancrinite, and calcite are possibly magmatic or deuteric. The more common metamorphosed pyroxenites and jacupirangites are foliated and dominated by Fe-rich hornblende, sodic plagioclase, and sphene. Metamorphosed ijolites contain a range of hydrous and anhydrous assemblages, including hornblende, metamorphic clinopyroxenes, garnet, and epidote, plus metamorphic nepheline, sodic plagioclase, and scapolite. Relic igneous clinopyroxenes are common in both rock types.

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Igneous clinopyroxenes have similar compositions in both pyroxenite and ijolite, and display the Ca-rich hallmarks of alkaline affinity. Those in pegmatites appear more acmitic. Bulk-rock analyses of igneous and metamorphic variants are characterised by undersaturated normative phases such as nepheline, leucite, and larnite. Differentiation indices range from about 10 in pyroxenites to 55 in ijolites and 70 in pegmatites. Titania is characteristically high. On an AFM diagram the pyroxenite-ijolite suite forms a coherent alkali-enrichment series with little Fe/Mg variation. Fractionation in the normative Ne-Ks-Si02 system is minimal. Geochemical and mineralogical data suggest the main differentiation control was settling of earlycrystallized clinopyroxene and magnetite, with magmatic fractionation becoming significant only at the pegmatite stage.

The pyroxenite bodies and ijolite occurrence appear geographically and structurally related to retrograde schist zones. It is suggested they were emplaced and partially metamorphosed during a major period of retrogressive metamorphism in the Willyama Complex. A preliminary K-Ar study by Harrison and McDougall (1981) indicates emplacement at about 560 m.y.

Altered pyroxenites and ijolites contain traces of chalcopyrite.

However the main economic significance of the Willyama alkaline suite lies in the possibility of associated carbonatite intrusions, for which abundant scope exists in poorly exposed portions of the Complex.

References Bell, A.J., Croxford, N.J.W. , and Hemming, G.R., 1979. Quart. Geol.

Notes, Geol. Surv. S. Aust., 69, 4-9.

Binns, R.A., 1966. Aust. J. Sci., 28, 353 Harrison, T.M., and McDougall, I., 1981. Earth Planet. Sci. Lett., 55, 123-149.

LATE PROTEROZOIC PERALKALINE INTRUSIVES OF THE ALLIGATOR RIVERS REGION, NORTHERN TERRITORY

P.G. Stuart-Smith & R.S. Needham Bureau of Mineral Resources, Canberra, ACT

Swarms of phonolite dykes of the Mudginberri and Maningkorrirr Phonolites occur within 10 km of Mudginberri Homestead and over 8 km^ near the headwaters of Jungle Creek respectively.

They are discordant and intrude rocks ranging in age from Archaean (2500 m.y.) to late Early Proterozoic (~1800 m.y.).

The Mudginberri Phonolite yields an Rb/Sr isotopic age of 1316 ^ 40 m.y. (Page & others, 1980).

Although Middle Proterozoic (1645 m.y.) sandstone crops out extensively in the area contact relationships are not apparent.

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The dykes are subvertical, parallel sided, straight and in places bifurcate.

The Mudgenberri Phonolite dykes are generally less than 1 m wide but range up to 10 m; the Maningkorrirr Phonolite dykes range from 4 cm to 1 m wide.

Chilled margins are common and contact effects on the country rock are limited to a few millimetres.

The Maningkorrirr dykes range in composition from phonolite to nepheline trachyte, whereas at Mudginberri they are restricted to phonolite.

The phonolites are typically dark green-grey fine to medium g r a i n e d porphyritic rocks containing alkali feldspar, nepheline, sodic pyroxene, brown mica and apatite phenocrysts.

Small globules consisting mostly of alkali feldspar, present in a dyke at Granite Hill, may be interpreted as the products of liquid immisc-^ ibility.

Textures in the nepheline-trachyte dykes are variable with alkali feldspar being the only common phenocryst and in places constituting over 50% of the rock.

The dykes intruded the Alligator Rivers Region, part of the stable North Australian Craton and may represent late differentiates of an alkaline magma which failed to reach the surface, as there are no other known alkaline intrusive rocks in the region.

Alternatively their genesis may be related to incipient melting in the upper mantle and deep crust in an environment which could also have produced kimberlitic intrusives.

REFERENCE Page, R.W., Compston, W., & Needham, R.S., 1980 - Geochronology and evolution of the late Archaean basement and Proterozoic rocks in the Alligator Rivers Uranium Field, Northern Territory, Australia.

In Uranium in the Pine Creek Geosyncline. Proceedings of the International Uranium Symposium on the Pine Creek Geosyncline, Sydney 4-8 June 1979, IAEA Vienna, 39-68.

Symposium SGI Structure, tectonics and ore genesis at Cobar, NSW {SGTSG SGEG) Convener: Dr R. Glen

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TECTONIC EVOLUTION OF THE COBAR-MT HOPE-MINERAL HILL REGION AND ECONOMIC IMPLICATIONS* D.J. Pogson Geological Survey of N.S.W., Department of Mineral Resources, Box 5288, G.P.O. Sydney, N.S.W. The structural framevork controlling deposition and sulDsequent deformation of the Early Devonian Cobar Supergroup vas established during the development of the Girilamhone-Wagga-Omeo metamorphic "belt in the Early Silurian. The nature, spatial distribution and subsequent thermal history of the Early Palaeozoic sequences (e.g. Girilambone and Tallebung Groups) forming this metamorphic belt indicate these rocks were deposited in an active back arc basin (Wagga Marginal Basin) characterized by high heat flow. The character and interrelationship of deformation, metamorphism and igneous intrusion suggest that this metamorphic belt marks a collision orogen resulting from arc/back arc collision and consequent continental underthrusting (A-type subduction). Development of this collision orogen in the latest Ordovician to Early Silurian caused large scale imbrication (tectonic thickening) of the sedimentary sequences which (in the existing high heat flow setting) led to anatexis and subsequent widespread granitic intrusion.

A system of north-northwest to northerly trending high angle reverse faults and west northwest and northeast trending fractures was established within the Girilambone-Wagga-Omeo metamorphic belt during development of the collision orogen. Post kinematic intrusion of anatectic granitoid melts generated during development of the collision orogen was localized along the north-northwest to northerly trending faults during extension in the Mid Silurian.

Renewed crustal extension affected the Cobar-Mt Hope-Mineral Hill region at the end of the Silurian. This extension reactivated inherited faults as normal faults and these controlled the development of basins and troughs in which the Early Devonian Cobar Supergroup sequences accumulated.

The inherited fault systems represent deep crustal fractures and their reactivation in an extensional regime also facilitated mantle diapiric rise into the crust. The rise of these mantle diapirs is considered to be responsible for further higher temperature partial melting of the Early Palaeozoic metasediments and consequent widespread felsic volcanism associated with some of the Early Devonian basins.

This felsic volcanism is largely restricted to such areas as CanbelegoBobadah-Mineral Hill and Mt Hope where limited crustal extension generally inhibited rapid rise of mantle diapirs to high crustal levels.

Areas of greatest extension and hence greatest collapse are characterized by thick turbiditic sequences with little or no felsic volcaniclastics or lavas. This can be explained by the rapid rise of mantle diapirs to high crustal levels where temperature/pressure conditions were insufficient to melt the Early Palaeozoic metasediments.

High temperature mantle diapirs emplaced rapidly to high crustal levels in strongly extended areas such as the Cobar Basin may be potential generators of large, metal bearing hydrothermal systems of

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relatively long duration. Large, shale-hosted Cu-Ph-Zn ore bodies are possible in the Cohar Basin vhere these hydrothermal systems dehouche in areas of low terrigenous sedimentation, peripheral to turhidite fan lohes.

In less extended areas, lover temperature, high level felsic intrusives rather than mantle diapirs were prohahly the main generator of metal-hearing hydrothermal systems. These systems would "be smaller and of shorter duration than those generated "by mantle diapirs and consequently could he expected to produce much smaller ore bodies. Only dispersed mineralization would result where hydrothermal systems dehouche in association with active felsic volcanism. ^Published with permission of the Secretary, N.S.W. Department of Mineral Resources.

TECTONIC DEVELOPMENT OF THE SOUTHERN PART OF THE COBAR TROUGH IN THE MOUNT HOPE AREA*

Erwin Scheibner Geological Survey of N.S.W., Department of Mineral Resources, Box 5288, G.P.O., Sydney, N.S.W.

Deposition of the Cobar Supergroup in the southern part of the Cobar Trough in the Mount Hope area is thought to have commenced in the latest Silurian to Early Devonian. This area of the Cobar Trough is characterized by the Mount Hope Group (volcanic complex) and comagmatic intrusions.

Characteristic, mainly explosive felsic volcanics and typical bimodal volcanism (however, only weakly expressed) lead to the interpretation that at its initiation the southern Cobar Trough had the character of a volcanic rift or volcano-tectonic depression with typical rhyolitic volcanoes.

The region of the present Lachlan Fold Belt during the Silurian (post-Benambran) to Devonian (pre-Tabberabberan) time formed a back-arc area behind a frontal arc positioned just west of the Tamworth Trough. This back-arc area was characterized by meridionally oriented extensional features: volcanic rifts and troughs with turbidite sedimentation. At least one of these troughs, the Tumut Trough was partly floored by ophiolites. The formation of extensional features (crustal attenuation) indicates either operation of B-type subduction of Mariana-type or transtension (pull-apart structures), or probably both (Scheibner, I982).

The en-echelon arrangement of major Silurian granites in the Wagga-Omeo metamorphic belt is consistent with left-lateral transtension in post-Benambran time. A similar sense of movement has been proposed

by L. VJyborn (19TT) for the opening (Early Silurian) of the Tumut Trough.

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On the other hand during the closing (pre-Early Devonian) of the Tumut Trough, "besides east-west compression, right-lateral transpression is suggested. The Cohar Trough opened approximately during the time of closing of the Tumut Trough and it is speculated that the same rightlateral sense of strike-slip movement existed in "both areas, only that transtension was in operation in the Cohar region.

The subduction processes at the frontal arc during the Early Devonian were remote from the Cohar Trough and could hardly directly geochemically influence the volcanic rifting here. The volcanic rifting in the Cohar area could have been influenced "by subduction of the oceanic crust of the Tumut Trough, but hard evidence for this is lacking. Hence the volcanic rifting in the Cobar region was probably achieved along near meridional transform faults (perhaps "leaky")•during process of transtension and formation of pull-apart structures.

Based on results of recent mapping it is suggested that the volcanic rift had the form of a composite resurgent cauldron with rhyolitic volcanoes in the Mount Hope area. Evidence for this is: resurgence in volcanic activity; block structure which resulted in differences in individual block stratigraphy and development, and subsidence of the area after the initial volcanism. The main subsidence occurred after the volcanism waned, and in the western branch of the cauldron a narrow trough developed in which the Broken Range Group sediments (partly turbidites) accumulated.

The present remains of the cauldron measure about 50 km wide and TO km long. The cauldron had a central stable horst which was formed by the Walters Range Block. The Mount Hope Group accumulated in the western part of the composite resurgent cauldron, while the Ural Volcanics formed in the eastern part. Volcanics in the east are less voluminous and developed in more distal facies.

The apparently narrow, graben-like southern Cobar Trough was bounded by relatively stable blocks, the Thule Block in the west and Erimeran Block on the east. The Cobar Trough widened towards north into Cobar Basin of Glen (this Symp.). Shallow water shelf-type sedimentation occurred on stable blocks.

The deformation and metamorphism of the Cobar Supergroup rocks is variable. The most intense deformation seems to be localized by zones of previous rifting and subsequent shearing. The Mulga Downs Group (late Early to early Mid Devonian) overlies Cobar Supergroup rocks with relationships changing from conformity through disconformity to local unconformity. However, the Mulga Downs Group rocks are absent in areas of strongest deformation of Cobar Supergroup rocks.

Based on similar orientations of structures in the Mulga Downs Group and Cobar Supergroup rocks Glen (cf. this Symp.) suggested that the main and terminal deformation occurred during Carboniferous time and besides east-west compression, left-lateral transpression played a role.

References Scheibner, E., 1982. Some aspects of the geotectonic development of the Lachlan Fold Belt. Geological Society of Australia^ Abstracts 6, p.2. Geological Survey of N.S.W. Report, GS 1982/069 (unpubl.).

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WylDorn, L.A.I. , 19TT. Aspects of the Geology of the Snowy Mountains Region and their implications for the Tectonic Evolution of the Lachlan Fold Belt. Ph.B. Thesis^ Australian National University (unpuhl.). ^Published vith permission of the Secretary, N.S.W. Department of Mineral Resources, Sydney.

SETTING AND TECTONIC HISTORY OF THE COBAR MINING FIELD, N.S.W.*

R. A. Glen Geological Survey of N.S.W., Department of Mineral Resources, Box 5288, G.P.O. Sydney, N.S.W.

The Cobar Trough formed as a depositional structure around the Silurian/Devonian "boundary, and was filled with sediments and volcanics of the Early Devonian Cohar Supergroup. WNW trending lineaments of the Crowl Creek Set, part of the Lachlan River Lineament, passing north and south of Shuttleton divide the trough into two parts. The southern part - Cohar Trough South - has the character of a volcanic depression or rift (Scheihner, this Symposiiom). North of the lineament, the Cobar Trough widens into the Cobar Trough North- or Cobar Basin, which is characterized by turbidites and by a general absence of volcanics. The Cobar Basin is also characterized by major orebodies which occur in fine-grained rocks. These orebodies, stretching some 60 km in a northwest direction from Elura south to Queen Bee, constitute the Cobar Mining Field.

Basement rocks below the Cobar Basin are not known. Basement granite occurs in the southwest and the only evidence for basement west of the basin consists of a small outcrop of Ordovician hornfels. North and east of the Cobar Basin, basement rocks consist mainly of the ?

Cambro-Ordovician Girilambone Group which accumulated in a back-arc basin (Pogson, this Symposium). Deformation and metamorphism of these rocks in the Early Silurian were accompanied by a generation of granites which were emplaced from the Early to the Mid Silurian.

These Early to Mid Silurian events were followed around the end of the Silurian by generation of the Cobar Basin, with subsidence controlled by pre-existing faults. One such fault, the Rookery Fault System, marks the eastern boundary of the basin, separating it from an emergent block to the east on which local shallow-water sediments accumulated.

Rapid relative uplift of the block to the east led to deposition of conglomerate and thick to thin-bedded sandstones of the Chesney Formation, presumably in submarine fan systems, fringing the eastern edge of the Cobar basin. As uplift and erosion slowed, thin-bedded turbidites of the upper part of the Chesney Formation were deposited. These were succeeded

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l3y rocks of the Great Cobar Slate which in part, act as mud drape to the eastern distributary system. Some of the Great Cobar Slate may also be distal to a western distributary system which deposited the overlying Amphitheatre Group. Basal units of this group, including the C.S.A. Siltstone, are generally thin-bedded and were deposited west of the area being covered by the Nurri Group (Chesney Formation and Great Cobar Slate) which was derived from the east.

Subsequent sedimentation in the Cobar Basin was triggered by block movements in the west. This resulted in the progradation of sandy submarine fan lobes of the Biddabirra Formation into the previously silt-rich depositional basin. Distributary systems flowing from the northwest and southwest carried sediments across the basin, in the former case right up against the eastern margin. Retreat of these fans and reversion to more thinly bedded deposits of the upper Amphitheatre Group reflect levelling of the blocks west of the Cobar Basin. During this time, some shelf sedimentation (Winduck Group) took place along the southwestern fringe of the basin. With a slowing of basin subsidence, turbidite deposition terminated towards the end of the Early Devonian. During this time, emergent blocks to the west became finally submerged and the Winduck Group continued to be deposited.

Although Middle to Late Devonian tectonism is well documented in eastern Australia, in the Cobar area it is generally represented by an abrupt change from marine to fluviatile deposition, and by changes in the nature of sediments within the fluviatile Mulga Downs Group. Local angular relations at the base of this group reflect only minor movement on pre-existing basement fractures.

The lack of widespread unconformity at the base of the Mulga Downs Group and congruence of folds between the Mulga Downs Group and Cobar Supergroup would suggest that deformation of the Cobar Basin is Carboniferous in age. Orientation of structures in the cover suggest that deformation took place in a regime of shortening coupled with leftlateral simple shear. Distribution of structures was to a large extent controlled by basement fractures. ^Published with permission of the Secretary, N.S.W. Department of Mineral Resources.

COBAR SUPERGROUP DEPOSITS:POLYMODAL GENESIS DURING PROTRACTED TECTONISM

Brian Marshall, S.R. Sangameshwar, P.P. Plibersek and I.J. Kelso, Department of Applied Geology, N.S.W. Institute of Technology P.O. Box 123, Broadway, N.S.W. 2007 Debate over the primary origin and subsequent evoluton of base metal deposits in the Cobar Supergroup has ranged over hydrothermal veins of igneous derivation; ductile remobilization of

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probably stratiform syn-sedimentary exhalative mineralization; mobilization of dispersed syn-sedimentary mineralization; and transposition of a * typical exhalative deposit* . More recently poly-modal genesis has been advocated (Marshall et al., 1981; Marshall and Sangameshwar, 1982). Emphasis has been on progressive evolution concepts whereby the gross geometry and the fabric of discrete deposits reflect the interplay of episodic(?) primary exhalative and subhalative processes, and secondary syn- and post-cleavage physical and chemical remobilization accompanying metamorphic dewatering.

The deposits of the Cobar Supergroup show textural and chemical disequilibrium and thereby enable those so inclined to adduce evidence in support of simplistic genetic models. Such an approach is false. Emphasis should be placed on the percentage contribution of differing processes in forming economic mineral concentrations. Only in this way can one establish the complex evolutions of mesoscopically similar Cobar Supergroup deposits.

The gross geometry of Cobar Supergroup mineralization relative to bedding and cleavage orientations has enabled recognition of the Nymagee, C.S.A. and Elura subtypes (Marshall and Sangameshwar, 1982) . Each subtype displays mineralization that is variously synsedimentary, pre-Si, syn-Si and post-Si. The relative importance of each mineralization category, and thereby the processes that controlled these original- and re-distributions, largely explain the sub-type geometries.

Investigations at The Peak (P.P.P.) and Queen Bee (I.J.K.) deposits suggest a close affinity to the C.S.A. subtype. However, the dominance (particularly at The Peak) of post-Si metahydrothermal mineralization in faulted shear zones parallel to cleavage necessitates re-examination of the subtype. Emphasis had previously been placed on discordance (with respect to So and to lesser extent Si) of the subtype and the essential role of subhalative feeder mineralization over-printed and modified by post-Si hydrothermal processes. At The Peak and possibly Queen Bee, the need for pre-Si feeder mineralization has not been demonstrated. Subdivision of the subtype could therefore be appropriate.

Repeated mineralization by processes involving mobilization and remobilization in and adjacent to persistent dislocation zones (at least latest Silurian to late Early Carboniferous - some 70 million years) is the hallmark of the polymodal deposits of the Cobar Supergroup. The scenario involves re-energized hydrothermal and mechanical redistribution during tectonic evolution of the eastern Cobar Basin and Synclinorial Zone.

REFERENCES Brian Marshall, S.R. Sangameshwar, D.P.H. O'Connor and M. Bouffler (1981). Genetic aspects of C.S.A. and Q.T.S. mineralization, Cobar, N.S.W.: ^ **Sediments through the ages", fifth Australian Geological Convention Abstract No. 3, p.4, D. Grover (Editor).

Brian Marshall and Sangameshwar, S.R. (1982), commonality and differences in ores of the Cobar Super Group: in Geology and mineralization in the Lachlan Fold Belt, NS.W."Geological Society of Australia, Abstract No. 6, p. 15 .

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REGIONAL METAL ZONING AND ORE GENESIS, COBAR, N.S.W. P.K. Seccombe Department of Geology, University of Newcastle, N.S.W., 2308 Mineralisation within metasedimentary rocks of the Cobar Supergroup, although presently discordant, represents hydrothermal accumulation on or below the sea floor during at least two periods of deposition.

Ore shoots define individual submarine vents, variously active in time. The distribution of ore shoots suggests that hydrothermal venting was related to points of discharge close to the eastern margin of the Cobar Trough. Clusters of individual ore shoots and deposits distributed for as much as 6 km along the mineralised horizons may represent accumulation from a single convection cell underlying the basin floor. Variations in ratios of the metals Cu, Zn, Pb, Au and Ag amongst ore shoots within such clusters and along the known 60 km extent of mineralisation in the Cobar area may then be attributed to the generation and decay of a number of submarine geothermal systems. Ore shoots central within any geothermal field are likely to be Cu-rich, those at the periphery or where,^ ^ during the life of the field, temperatures have not exceeded 250 -300 C are likely to be Zn, Pb or Au and Ag-rich. The zoning patterns therefore show a control based on metal solubility and the manner in which the metal is complexed in the hydrothermal fluid. Igneous activity at depth is required to generate the thermal regime for individual convection cells. The distribution of ore shoots in the region and their particular mineralogy suggest areas of further ore potential.

IMPLICATIONS OF S AND Pb ISOTOPE DATA TO GENESIS OF MASSIVE SULPHIDE IN THE COBAR AREA Shen-Su Sun CSIRO Division of Mineralogy, North Ryde, NSW 2113 Various types of massive sulphide mineralization occur in the late Silurian-Early Devonian formations of the Lachlan Fold Belt. Some are stratabound and associated with felsic volcanism (e.g. Woodlawn, Captains Flat). Whereas others (e.g. Cobar CSA, Elura) are of discordant hydrothermal type (O'Connor 1980, Adams and Schmidt 1980), probably emplaced during the main phase of regional metamorphism (Carboniferous? D.Glen, pers. comm. 1982). Nevertheless, discordant sulphides from the Cobar-Elura area have very similar S and Pb isotope composition to conformable volcanic mineralization at Colo Creek, Woodlawn and Captains Flat far to the south east. This enigma must have important genetic implications.

Pyrite samples from Colo Creek, Woodlawn and Captains Flat have

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values +7 to +12^/oo (Burns and Smith, 1976^ Stanton and Raftir, I 9 6 6 ) . Associated barite beds have values of +29 to +30^/00 about 4°/oo higher, as expected, than the upper Silurian seawater (^26o/oo). Available Pb isotope data for Captains Flat and Woodlawn (e.g. Gulson and Vaasjoki, 1982) are very similar (^O^Pb/^^^Pb = 18.08 and 18.11 respectively). The degree of Pb isotopic homogeneity is comparable to the Miocene Kuroko deposits in Japan (Sato et al., 1981). By analogy this suggests derivation of metals from enclosing volcanic rocks.

Disseminated stratabound sulphide mineralization is widespread in the Cobar region, including occurrence of pyrrhotite in the CSA siltstone and Great Cobar Slate. Stratabound mineralization also occurs at Canbelego. The QTS mineralization at Cobar, stratagraphically lower than CSA mineralization, appears conformable and could be of exhalative origin. Minor conformable sulphide mineralization has also been observed in the Cobar CSA and Elura Mines, although the main phase of mineralization at both places is discordant and is considered epigenetic. values for pyrites from the Canbelego mineralization (+9 to +15°/oo), chalcopyrites from QTS ores (+8 to +14), disseminated pyrrhotite in the Great Cobar Slate (+8 to +10^/oo), and minor conformable sulphides at Elura and Cobar CSA (+8 to +IOO/00 for pyrites) are similar to one another. values for pyrites from the discordant ores at Elura (+8 to 120/oo) and Cobar CSA (Ca +7 to + 9 0 / 0 0 , Marshall et al. 1981) are again similar to the conformable ores in the Cobar area and resemble in general those at Colo Creek, Woodlawn and Captains Flat (+7 to +120/oo).

The Pb isotope compositions of Cobar and Elura ores (Ostic et al., 1967, Gulson and Vaasjoki, 1982) are very similar to those of Woodlawn deposit. Their 206pb/204pb difference is within 0.15%. It might be significant that at Cobar the Pb isotope data for the stratigraphically lower (host rocks) Great Cobar, Tharsis, Silver Peak and Queen Bee deposits have 206pb/204pb values about 0.15% lower than CSA ores (Ostic et al., 1967). Such a difference is consistent with a later emplacement of discordant mineralization at CSA and Elura. If formation of the discordant mineralization at CSA and Elura were some 30 million or more years later than the stratabound exhalative (?) type mineralization in this area, Pb isotope data would favour a model of hydrothermal remobilization of pre-existing mineralization from stratagraphically lower levels such as those in the Great Cobar Slate.

It is envisaged that all these massive sulphide mineralizatiors are related to thermal and tectonic activities in the Lachlan Fold Belt.

Scarcity of penecontemporaneous volcanic activity in the Cobar area would argue against a magmatic origin for the base metals. The metal source is most likely in the Ordovician-Silurian sediments of the environment. Discordant mineralization in the Cobar area could have resulted from hydrothermal remobilization of pre-existing mineralization probably of exhalative origin. Volcanics associated with Colo Creek, Woodlawn and Captains Flat mineralizations may have resulted from melting of the upper crustal rocks including the OrdovicianSilurian sedimentary beds which are widespread in the Lachlan Fold Belt. Lead in these volcanic rocks and associated ores could ultimately derive from the upper crustal sediments in the environment through crustal reworking and hydrothermal leaching.

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REFERENCES Adams, R.L. and Schmidt, B.L., 1980, Geology and the Elura Zn-Pb-Ag deposit. Bull. Aust. Soc. Explor. Geophys., v.11, p.143-146.

Burns, M.S. and Smith, J.W., 1976, ^^S/^^S ratios in some sulphides of the Lachlan Fold Belt, Bull. Aust. Soc. Explor. Geophys., V.7, p.43-48.

Gulson, B.L. and Vaasjoki, M., 1982, Lead isotopes in geochemical exploration, in Geochemical exploration in Deeply Weathered Terrain. Ed. R.E. Smith, Div. Mineral. CSIRO W. Australia p.145-151.

Marshall, B., Sangameshwa, S.R., O'Connor, D.P.H., Sun, S-S and Bouffler, M., 1981, Genetic aspects of the CSA and QTS mineralizations, Cobar, NSW. Abst. Geol. Soc. Australia, No. 3, p .4.

O'Connor, D.P.H., 1980, Evidence of an exhalative origin for deposits of the Cobar district. New South Wales, BMR Journ. Aus. Geol.

Geophys. v.5, p.70-72.

Ostic, R.G., Russell, R.D. and Stanton, R.L., 1967, Additional measurements of the isotopic composition of lead from stratiform deposits. Can. Journ. Earth Sci., v.4, p.245-269.

Sato, K., Delevaux, M.H. and Doe, B.R., 1981, Lead isotope measurements on ores, igneous and sedimentary rocks from the Kuroko mineralization area, Geochem. Journ., v.15, p.135-140.

Stanton, R.L. and Rafter, T.A., 1966, The isotopic constitution of sulphur in some stratiform Lead-Zinc sulphide ores. Mineral.

Deposita, v.l, p.16-29.

THE METAMORPHISM AND DEFORMATION OF STRATIFORM SULPHIDE ORES F.M. Vokes Department of Geology, The University of Trondheim - The Norwegian Institute of Technology, Trondheim, Norway The idea that sulphidic deposits of iron and base metals, of varying genetic types, have been subjected to metamorphism and deformation subsequent to their formation is one which has occupied ore geologists, in varying degrees, since before the beginning of the present century. The last 20 years, perhaps, have witnessed the greatest degree of interest in, and the greatest number of publications on, this subject.

Stratabound and stratiform deposits of this type are especially susceptible to being affected by subsequent metamorphic and deformational events. The typical environments of many of them are those unstable portions of the earth's crust originally termed 'geosynclines', that are now defined in relation to the margins of crustal plates.

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Many ore bodies of the type discussed here show a close spatial arid almost certainly genetical relation to volcanic, mixed volcanic sedimentary and sedimentary rocks which are known to form in such environments and which are prone to be affected by subsequent orogenic events such as plate collision, obduction and gravity nappe sliding.

As a consequence of this, sulphide ores of the stratabound/stratiform type formed at plate boundaries (as well as some formed in intraplate positions) have all been affected to greater or lesser degree during the orogenic stage of mobile belt development. As a general rule, the ores have been affected by the metamorphism to the same extent as have all the other rock units in their lithological environment. The metamorphic/deformational effects seen in the ores can in many respects be shown generally to parallel those observed in the enclosing rocks; in addition, there can be found many unusual features, resulting from the sulphide minerals' special physical and chemical properties.

The sulphide ores and their host rocks may be, of course, subjected to all the various forms of metamorphism and deformation which affect the rocks of the earth's crust, whether of local or of regional extent. However, in the present context, it is obvious that regional dynamothermal metamorphism, modified in places, perhaps by thermal contact metamorphism, is the dominating type to be considered.

The present discussion will therefore concentrate upon the effects of regional metamorphism on sulphide ores. Sulphide concentrations are not notable for the chemical-mineralogical changes taking place during metamorphism. They react only to limited extent with accompanying silicate and carbonate components. They are, however, notable for the physical changes produced - both as regards fabric and structure/morphology, due to their ready deformation and high capacity for annealing and grain growth, in many ways paralleling the behaviour of metals under similar conditions. The response to metamorphism of the ore masses as a whole is also often noticeably different from those of the enclosing silicate-dominated country rocks, a point which has often led to conflicting interpretations of the age relations between the two.

The effects of metamorphism/deformation on the sulphide bodies can be reviewed under the headings of changes in fabric, of mineralogy, and of structure or morphology and of mobilization of ore components.

Ore fabrics have been affected both by grain growth during prograde metamorphism and by deformation of both brittle and ductile aspects. Recrystallisation or annealing following deformation is widespread and may eliminate most signs of deformation in the sulphide fabrics.

Changes in mineralogy due to metamorphism are only exceptionally met with in sulphide ores due to the generally large stability fields of common sulphide minerals and the often relatively simple chemical composition of the ore mass as a whole. More importantly, high temperature phases, formed during metamorphic peaks, are easily reequilibrated under retrograde conditions.

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Structural or morphological changes brought about during m e t a morphism/deformation include folding and disruption of originally planar or lenticular bodies, as well as the often extreme elongation of the bodies parallel to the stretch directions of the surrounding rocks.

Metamorphism appears to be capable of selectively mobilizing components of pre-existing ore bodies and moving them varying distances, either within the orebodies themselves or into the surrounding wallrocks.

Two m a i n processes seem to be involved; solid state mobilization (creep, flow, etc.) and fluid state mobilization (metahydrothermal fluids or low melting point melts).

The distances involved appear to be relatively small, from m m to perhaps one or two tens of metres.

THE MOUNT BOPPY GOLD DEPOSIT DUCTILE AND BRITTLE DEFORMATION OF A STRATIFORM EXHALATIVE

DEPOSIT

L.B. Gilligan Geological Survey of New South Wales, Department of Mineral Box 5288, GPO Sydney, NSW 2001

Resources,

Canbelego, 40 km east of Cobar, is the centre of an old mining field that was active from the late 1890's until the 1920's.

Numerous small gold and base metal mines and prospects are known in the area although the Mount Boppy gold mine is by far the largest deposit, producing in excess of one million tonnes of ore grading 12 g/t gold.

The local geology consists of a deformed basement complex (Girilambone Beds) unconformably overlain by the Early Devonian Baledmund Formation and Florida Volcanics of the Cobar Super-Group.

The basement Girilambone Beds are a sequence of quartz-mica schist, psammitic schist, metagreywacke and metaquartzite which has experienced four cleavage-forming deformation events.

Dl resulted in a weak layer silicate fabric that has been largely destroyed by a metamorphically differentiated crenulation fabric (S2) of the regionally dominant D2 event.

D3 formed localized zones of crenulation cleavage (S2) , but did not cause regionally significant structures.

D4 caused a pervasive crenulation cleavage in the pelites and chevron and kink folds in the psammites.

Faulting occurred in the late stage of D4 parallel to S4.

The Baledmund Formation consists of a basal conglomerate, an overlying micaceous-lithic siltstone-sandstone sequence and a transitional volcano-sedim.entary sequence.

The Florida Volcanics are a suite of calc-alkaline rhyolitic and rhyodacitic tuffs and lavas.

The cover rocks have undergone one cleavage-forming deformation (Fl) which is correlated with D4 in the basement.

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The Mount Boppy gold deposit is situated in the basal part of the Baledmund Formation between the basal conglomerate and overlying siltstone and sandstone. The deposit consists of a breccia zone which grades up stratigraphy to a zone of quartz-filled net-vein fractures which in turn grades into unfractured micaceous siltstone.

The deposit is conformable and together with the Baledmund Formation defines a tight south-plunging syncline. The breccia zone consists of clasts of siliceous siltstone^ chert and minor massive Pb-Zn sulphides healed by quartz^ carbonate and minor sulphides. The primary mineralogy consists of sphalerite^ galena, minor pyrite and (?) arsenopyrite. Gold predominantly occurs in the particulate state associated with sphalerite. The massive sulphides in the clasts have undergone ductile deformation prior to their brecciation with the siliceous host rocks. Meta-hydrothermal remobilization of sulphides accompanied the brittle deformation.

Other deposits in the Canbelego area consist of fault-controlled gold mineralization in the basement and at basement-cover contacts and minor base metal mineralization in the basal cover rocks. Many of the mineralized faults are those that developed late in D4 - F1 and the mineralization was remobilized into these faults from the basalt Baledmund Formation mineralization which is believed to be volcanic exhalative.

The Mount Boppy gold deposit is a distal, volcanic exhalative deposit that consisted of stratiform pyritic chert-siliceous siltstone containing lenses of massive lead-zinc sulphides.

Synfolding brecciation of the deposit occurred during D4 - Fl. During relatively recent time the gold grade of the deposit was enhanced by supergene enrichment.

Published with the permission of the Secretary, NSW Department of Mineral Resources.

ELURA:

ORDBODY

AND WALLROCK

STRUCTURE

D.A.C. Archibald Electrolytic Zinc Co. of A/Asia Ltd., Cobar.

N.S.W.

The Elura Pb-Zn-Ag orebody lies 43 kilometers N.N.W. of Cobar, N.S.W., and is hosted by distal turbidites of the C.S.A. siltstone. The vertically orientated pipe-like massive sulphide deposit is grossly discordant to the surrounding metasediments. Host rocks in the mine area exhibit large open folds with shallow north and south plunges.

Exposures in recent mine development have shown later folding has occurred in close proximity to the sharply defined massive sulphide mineralisation. It is

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characterised by small to medium size structures which plunge steeply away from the ore with axes commonly radial to the ore deposit. Both lithology and the regional northsouth striking sub-vertical cleavage are folded. A subsequent local axial plane cleavage is sometimes superimposed .

There is evidence for ductile flow of massive sulphide in thin conformable sulphide beds close to ore. The pyritic layering has become transposed into vertically orientated pods upto a metre in length, with a long axis parallel to the dominant schistosity/cleavage close to the deposit.

Within the orebody, all three ore types display signs of mobilisation. Sulphide-rich siliceous ore can be seen to have invaded/replaced bedded metasediments, but without any rotation of the remnant fragments of shale. Further from the periphery of the deposit, large blocks of fine grain pyritic ore have been enveloped and veined by coarser grained material of the same composition. In the central region of the orebody pyrrhotitic ore often contains large, vertically orientated pods of unhanded sulphide which is usually non-magnetic. These are surrounded by well foliated magnetic ore with a streaky compositional banding which seperates to encompass the pod.

More plastic pyrrhotitic sulphides are interpreted as having flowed around more competent zones of pyritic mineralisation. It is not known whether the late stage folding close to the orebody and the mobilisation of the sulphides relate to the original emplacement of the ore or to a subsequent structural event.

ASPECTS OF MINERALIZATION AT ELURA B.L. Schmidt Electrolytic Zinc Co., Sydney, NSW The Elura Pb-Zn-Ag deposit is situated 43kin NNW of Cobar, NSW, and contains in excess of 27 million tonnes of ore at a grade of 5.8% Pb, 8.4 Zn and 130g/t Ag in a crudely elliptical body 115 x 210m in plan and over 500m deep that towards the top divides into two apophyses, one of which reaches the surface.

The deposit is hosted by the C.S.A. Siltstone, a distal turbiditic unit within the Amphitheatre Group of the Lower Devonian Cobar Super-Group, that was deposited in the area of maximum depression towards the eastern margin of the basin.

Deposition was terminated by the Mid Devonian Tabberabberan Orogeny that caused moderately intense deformation with areas of distinct fold styles, and metamorphism that reached lower greenschist grade.

Cobar Super-Group sediments were overlain by a sequence of shallow water Upper Devonian clastics.

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Amphitheatre Group sediments are irregularly deformed and a weak to moderately well-developed axial surface cleavage has formed.

The Elura orebody occupies the core of a small 100-200m wide anticline in an overall south plunging synclinorial structure. Structure in the immediate vicinity of the orebody is delineated by a slump marker horizon.

Away from Elura the host rock comprises dominantly a quartz + muscovite + chlorite + albite + calcite ± ankerite assemblage but around the orebody wall rock alteration caused first, the destruction of chlorite with the progressive formation of ankerite, then siderite, and albite destruction.

The zone of Fe-Mg-carbonate formation is coextensive with the 5-80m wide halo of megascopic siderite prophyroblasts and base metal sulfide enrichment.

Compositional changes in the host rock are limited to a major increase in CO and decrease in Na O, and more subtle variations of K^O and Mineralogical changes are locally more extreme adjacent to the orebody, with silicification, sericitization, ankeritization, and chloritization with commensurate chemical changes.

Three ore types are distinguishable on the basis of gangue mineralogy and content, and Fe-sulfide mineral abundances. Siliceous and massive ores are similar except that the former contains more abundant SiO and less CO . Both contain above average abundances of Ag, As, Sb, Sg but only traces of pyrrhotite. Pyrrhotite ore contains abundant pyrrhotite and siderite, little quartz and below average contents of Ag, As, Sb and Hg. Ore types have relatively distinctive compositions so that the orebody is compositionally and mineralogically zoned. Minerals in the orebody in approximate order of abundances are pyrite, sphalerite, siderite, pyrrhotite, quartz, galena, arsenopyrite, chalcopyrite and a suite of minor minerals including barite, Ba-feldspar, muscovite, chlorite, tetrahedrite, tennantite, enargite etc. Much of the silica in the orebody appears to be silicified rock.

Two megascopic preferred orientations are present, one a sub-vertical normally north-south striking gross mineralogical 'sulfide* layering and the other sub-horizontal predominantly gangue filled fracture or 'dilation' layering.

Four dominant varieties of pyrite are present and appear to have been deposited in sequence. Arsenopyrite commonly replaces early pyrite and both minerals are replaced by sphalerite, galena, chalcopyrite and pyrrhotite, which tend to be in textural equilibrium with each other and also quartz, siderite, chlorite, muscovite, tetrahedrite etc. Pyrrhotite replaced pyrite during a prograde event and the FeS content of sphalerite decreased during a retrograde event.

A complex series of late and post-depositional chemical and physical changes are recognised. Mineral assemblages in the orebody and host rock indicate formation probably in the range 254°-330° at an estimated pressure of 2.5 - 3.5kb.

Mineralization and wall rock alteration occurred prior to the termination of cleavage formation and were metamorphosed, possibly soon after deposition, thereby obscuring many depositional features. The extent to which features in the orebody and host rock are of metamorphic origin is unknown.

Two genetic models are proposed, but the lack of unequivocal diagnostic criteria prevents determination of the "correct" genesis. A modified syngenetic model proposes that the mineralization was

315

originally stratiform and forcibly emplaced into a favourable structural site during deformation.

Post-emplacement metamorphism is required to explain a variety of features, several of which appear unlikely to have been generated in such a way. An alternate epigenetic model proposes a replacement origin where rupturing of an early minor anticline allowed the escape of metal-bearing overpressured basinal brines with contemporaneous alteration, and is better able to explain many of the features without recourse to extensive metamorphic modification.

The deposits near Cobar have some similarities to the Elura mineralization but many differences of detail.

For example, the commodities produced were dominantly Cu and Au, and wall rock alteration involves extensive chlorite formation at the expense of muscovite and lacks the carbonate formation found around the Elura orebody. Genetic models proposed are similar to those for Elura, with the replacement model providing a better explanation of many observed features.

THE NATURE AND ORIGIN OF COBAR 'ELVAN' R.A. BINNS^ ^CSIRO Division of Mineralogy, North Ryde NSW Drill core and underground samples from the structural hanging wall of the Western System mineralization, CSA Mine, were studied to understand formation of the distinctive siliceous rock locally called 'elvan', to establish its genetic and temporal relationships to mineralizing processes, and to examine whether its geochemistry can be employed in exploration.

Earlier suggestions of an exhalative origin for 'elvan' are obviated by its discordance to bedding in the enclosing metasiltstone country rocks (mapping by Cobar Mines Proprietary Limited).

Progressively increasing silicification occurs towards the ore zone over distances of about 20 metres. At the locality studied in detail, the least altered hanging wall siltstones possess subangular clasts of quartz in a slaty quartz-chlorite matrix. They are laminated and graded, and are intercalated with thin beds of quartzite (formerly well-sorted quartzose sandstone). Silicification involves outgrowths on clastic particles and deposition of new quartz between chlorite flakes in the matrix. Quartz veins, which are partly recrystallized, also increase in abundance towards the ore zone.

'Elvans' closest to or within the ore zone contain flattened particles of pyrrhotite surrounded by white mica.

The SiO content of siltstones (sampled to exclude quartz veins and quartzite layers) increases from 66% to 84% approaching ore. Other components (Mg, Fe, Al, Ti, Mn, Cr, V, Zr) fall systematically in a manner consistent with dilution of chlorite and accessory phases by pure silicification. Pyrrhotite-muscovite bearing 'elvans' within the ore zone are enriched in S, K, and Rb, but Fe/Mg ratios are preserved

316

by a change to more magnesian chlorite compositions during relatively minor sulfurization.

Development of silicified siltstone and 'elvan' in the hanging wall of the Western System at the CSA Mine appears most intimately related to formation of copper ore. The evidence for dilation by as much as 100% during silicification and veining of country rocks in the hanging wall suggests a marked flux of siliceous fluids, immediately prior to the mineralizing event, under pressures exceeding the lithostatic confining pressure.

The 'least altered' siltstones referred to above are themselves geochemically anomalous relative to footwall siltstones and some that are more remote from ore in the hanging wall. These latter contain muscovite and a more ferruginous chlorite in their slaty matrix. While no useful primary geochemical halo (apart from silicification) occurs in the 'elvan' zone itself, it appears that a wider zone of alteration also surrounds mineralization. Research is continuing into its geometry and relationship to 'elvan' and ore-forming processes.

Assistance from staff of Cobar Mines Proprietary Limited is gratefully acknowledged.

GEOLOGY AND MINERALIZATION OF THE SHUTTLETON AREA, WEST OF NYMAGEE NSW by D.W Suppel & L.B Gilligan Geological Survey of New South Wales, Department of Mineral Resources Box 5288, GPO Sydney, NSW 2001

copper and minor lead-zinc mineralization is developed in a volcanosedimentary unit at Shuttleton, west of Nymagee. This unit is located in the lower part of the Shum.e Beds, a sequence of thickly bedded quartz sandstone and siltstone, and part of the Early Devonian Cobar Super-Group. Similar copper and lead-zinc mineralization occurs within extensive sequences of felsic volcanic rocks of the Cobar Super-Group in a composite rift structure to the south at Mount Hope. Mineralization at Shuttleton is stratabound on the western flank of northerly trending anticlinorium. Minor mineralization at Wirlong, 6 km to the east, is in felsic volcanic rocks at the same stratigraphic level.

Two small mines produced copper at Shuttleton, the Crowl Creek mine and the South Shuttleton mine. Mineral exploration in the 1970 s largely concentrated on the area of South Shuttleton and accordingly much of the sub-surface data are from this area. Two sequences, an eastern and western, are recognized. These are separated by a ma^or northerlv striking fault. The eastern sequence consists of:

317

(a) a volcanic unit consisting of felsic pyroclastics (rhyolitic and rhyodacitic crystal and lithic tuffs), volcaniclastic sediments and fine grained sediments, adjacent to the fault;

and

(b) an overlying sedimentary unit of interbedded shale, siltstone and massive sandstone.

The volcanic unit and the lower part of the sedimentary unit contain the sulphide mineralization. The western sequence is unmineralized and consists of massive sandstone and quartzite and underlying carbonaceous siltstone.

Sulphides occur in a variety of forms, the most common being: 1 Narrow massive bands, irregular veins, and disseminations of sulphides in the volcanic unit and immediately overlying shale.

Chlorite is a common gangue mineral, quartz is minor.

2 Minor disseminated sulphides in planar quartz veins traversing the overlying sandstone.

The dominant sulphides at South Shuttleton are pyrite, pyrrhotite, and chalcopyrite. Sphalerite and galena are far less abundant.

The strongest copper (-lead-zinc) mineralization is in the volcanic unit. Pyrite is most abundant in the volcanic unit and in the immediately overlying shales, pyrrhotite occurs stratigraphically higher. The sulphides display evidence of both ductile flow and cataclasis resulting from cleavage - forming deformation. Pyrrhotite, chalcopyrite and galena underwent ductile deformation whereas pyrite deformed brittly.

Chlorite composition in the ore zone in the volcanic unit and immediately overlying shale differs from that in stratigraphically higher rocks in having lower Fe/Fe+Mg values, reflecting enrichment in magnesium accompanying the mineralization.

Mineralization at Shuttleton is considered to be volcanogenic and to have undergone deformation resulting in local remobilization.

The mineralized quartz veins in the sandstone may have resulted by remobilization of stratabound exhalative mineralization from the underlying volcanic unit. Deformation has resulted in a structurally complex setting; the volcanic unit and overlying sedimentary rocks rocks are situated on the easterly dipping limb of a fold which has been truncated by a major fault, bringing stratigraphically higher, barren rocks of the western sequence into contact with the mineralized rocks of the eastern sequence.

Published with the permission of the Secretary, NSW Department of Mineral Resources.

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Symposium SG2 Man as a g e o l o g i c a l agent (EGSG) Convener: M r I . D . Hair

319

r4AN AS A GEOLOGICAL AGENT F.C. Beavis School of Applied Geology, University of New South Wales, Kensington NSV/ While maintaining its initial interest in the geological factors which influence the design and construction of engineering works, engineering geology, in the past decade, has shown a marked trend towards studying the effects of such works on the environment. As a result, environmental geology has emerged as a discipline in its own right.

By his activities, both constructive and destructive, man has changed, and continues to change at an accelerating rate, geological processes and products. Erosional and depositional activities of rivers and oceans may be modified to an alarming degree. The construction of large structures and the excavation of cavities at, and below, the surface create "unnatural" stresses, albeit locally, but the effects can be significant. Chemical, physical and biological changes are being induced in water bodies: rivers, lakes, ocean and subsurface. All of these point to man as an agent in the totality of geological processes.

Man is also creating his own stratigraphic column, and this term is used not in the archaeological, but the strict geological, sense. In the need to dispose of the waste products of his domestic and industrial activities, man is creating large waste disposal areas. Quite serious publications now present the stratigraphy of waste dumps I Man's importance as a geological agent cannot be overstressed. The important point, however, is that he does, now, appreciate his position, and some endeavour is being m.ade to ensure that he is a henefioial agent. Half a century ago, or even less, the geologist was concerned only with the foundations of a dam, for example. Now he is equally concerned with the stresses which this structure will impose; the effect on erosional and depositional regime of the stream; and the hydrological consequences both at and below the surface. It is essential that geologists approach the issues with objectivity.

Environmental issues all too often are approached subjectively, with the emotions, not calm scientific assessment, forming the basis for decision making.

HYDROGEOLOGICAL CHANGE IN AUSTRALIA G. Jacobson Bureau of Mineral Resources, Canberra, ACT

Groundwater occurs in aquifer systems which have a natural slow cycle of recharge, flow, and discharge. Over long periods of time, this

320

movement gives rise to chemical change in the water, from fresh to brackish to saline^ In Australia, since European settlement, the natural dynamic equilibrium of groundwater systems has been altered in significant ways. Land clearing has led to rising water tables over several decades and resulted in a widespread problem of dryland salinity, affecting over 4000 km of land in southwest Western Australia, Victoria and South Australia. Irrigation has raised water tables in the Murray valley and elsewhere, inducing salinisation that affects about 2000 km of land and the quality of Australia's largest river.

These salinity problems are of growing dimensions and have no easy solution.

In other parts of Australia, aquifers have been overdeveloped and groundwater is mined; this has led to saltwater intrusion in some places, either from the sea or from adjacent saline aquifers.

Artificial recharge of the aquifers is being undertaken in some of these localities to redress the balance. Groundwater pollution is an em-erging problem; an important aquifer in South Australia is contaminated with nitrate from sevjage and animal wastes. Other aquifers, especially near the cities, are contaminated by industrial effluent, urban landfill, or leaked petrol. The dewatering of large coal mines has led to pollution and land subsidence problems, and there is now an imminent hazard to certain groundwater systems as a result of proposals for solution mining.

CHANNEL CHANGES IN AUSTRALIAN RIVERS SINCE EUROPEAN SETTLEMENT R.F. Warner Department of Geography,

University of Sydney

NSW

Natural fluvial processes are not altered by man. However, in changing the natural surfaces of catchments and floodplains, and by altering channels, he can modify water and sediment discharges. These can alter channel form and dimensions through complex responses to process changes. Channel adjustments to both man-induced and natural changes of regime have been called river metamorphosis.

Catchment and floodplain changes associated with deforestation, grazing, cropping, urbanization, conservation., mining and afforestation affect basic hydrological processes, like interception, evaporation, transpiration, infiltration and, most importantly, runoff. Changes in the latter modify water and sediment delivered to adjacent stream channels.

Such changes are regarded as indirect modifications, whereas those which involve alteration of the channels are known as direct changes. These include: dredging operations, dams, weirs, channelization and other

321

so-called improvements. They tend to disrupt the continiuty of water and sediment movement, and frequently give rise to unforeseen environmental problems.

Examples from mainly New South Wales are used to illustrate man-induced channel changes. Such studies are dependent on the availability and quality of old surveys, together with maps and air photographs.

Comparisons between old and present-day surveys indicate what changes have occurred, but the causes may include a variety of indirect and direct man-induced modifications, as well as those associated with changes in the natural regime. For instance, a secular change in climate, involving an increase in precipitation, between the late 1940s and possibly the late 1970s is known to have caused many channel changes.

Frequent surveys at shorter time intervals, plus a detailed knowledge of catchment and channel modifications, as well as the nature of intervening fluvial events, may reveal more of channel responses to maninduced changes. The impacts of urbanization, various logging practices in deforestation and other changes have already been assessed in limited areas using such techniques.

flAN-MADE EROSION IN AUSTRALIA WITH A SPECIFIC EXAMPLE FROM THE BROKEN HILL DISTRICT R.J. Wasson & R.W. Galloway Division of Water and Land Resources, CSIRO, Canberra, ACT Soil erosion is by far the most significant part of man-made erosion. A Commonwealth and State Government Collaborative Soil Conservation Study (Anon. 1979, Pauli 1979) showed that half the land used in both arid and humid Australia requires some soil conservation treatment because human activity has accelerated erosion. Much of the treatment consists only of changes in management practises (e.g. reduced grazing pressure at critical periods) but nearly half the area requires earth works such as contour banks whose estimated cost in 1975 dollars was nearly $700 000 000. Some of the severest erosion has occurred in agriculturally valuable areas, notably the Darling Downs of southwestern Queensland.

The amount of sediment trapped in reservoirs offers a guide to the mean rate of anthropogenic plus natural erosion. However, it is usually difficult to assess the rate of natural erosion prior to the advent of Europeans for comparison. At Umberumberka Creek near Broken Hill such a comparison between Holocene and post-European rates of erosion is possible. The volume of a fan deposited on the plains west of the Barrier Range between 6500 and 3000 years ago has been assessed with a fair degree of confidence (Wasson 1979). It implies an average erosion rate of 10 mm per thousand years over the 450 km catchment.

Between 3000 B.P. and the arrival of Europeans, the rate was apparently slower and indeed virtually negligible.

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3 A reservoir at the head of the fan has trapped some 8 000 000 m of sediment since 1915. Assuming a trap efficiency of 90% and that the bulk density of the sediments is the same as the soil from which they are derived, the mean rate of erosion over the last 67 years has been 20 mm or at least 30 times faster than in the second half of the Holocene. The sedimentation record can be extended back to 1892 when the adjoining Stephens Creek reservoir was built.

The rate of sediment accumulation in Umerbumberka reservoir has apparently slowed since it was built. It is not year clear how far this is due to improved condition of the catchment, to erosion of the most easily removed material first, or to decreased trap efficiency of the reservoir as it silted up and consequently overflowed more often.

This comparison between Holocene and modern erosion rates leaves unanswered the intriguing question of whether the advent of Europeans coincided with a period of increased natural erosion but even if this were so it is clear that man's impact on soil erosion over the last century or two has been enormous. If this impact continues at comparable rates what will the landscape be like a few centuries from now?

REFERENCES Anon., 1978: Commonwealth and State Government Collaborative Soil Conservation Study, Report 1-14. Aust. Govt. Pub. Serv., Canberra, 1978.

Pauli, H.W., 1978: Land degradation and potential for its control.

In Hallsworth, E.G. and Woodcock, J.T. (Eds.) Land and Water Resources of Australia. Australian Academy of Technological Sciences, pp. 39-57.

Wasson, R.J., 1979: Sedimentation history of the Mundi Mundi alluvial fans, western New South Wales. Sediment. Geol. 22, pp. 21-52.

TERRAIN ANALYSIS OF THE SYDNEY AREA, NEW SOUTH WALES A.A. Finlayson CSIRO, Institute of Energy and Earth Resources, Division of Applied Geomechanics, Syndal, Victoria The opportunity to apply the P.U.C.E. System (Grant and Finlayson, 1978) for terrain analysis to the Sydney 1:250 000 topographic map sheet area evolved following the collaboration between the CSIRO Division of Applied Geomechanics and the School of Civil Engineering (University of New South Wales). This collaboration was connected with the establishment of a postgraduate course in terrain engineering at that University.

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Terrain analysis and classification The terrain of the Sydney area has been analysed, classified and described in terms of 17 provinces (geology), 46 terrain patterns (landscape) and 170 basic, topographical landform units, which were further subdivided on the basis of soil and vegetation differences into 1010 terrain units. Provinces and terrain patterns were mapped at a scale of 1:250 000, and all terrain units were described (Finlayson, 1982). It should be stressed that once the terrain classification has been conducted on any one area it always remains valid as a framework against which any scheme of assessment/ evaluation might be implemented.

Terrain assessment In the study of the Sydney area no single organization (as is usually the case) was responsible for the specification of the information to be collected against the terrain classification framework. An assessment scheme was initially suggested and evolved into its final form after discussions with various Sydney-based organizations. That scheme is presented here as only one example of any niomber of possible terrain assessments that could have been devised.

The remoteness, extent and access of every terrain pattern were determined. The following engineering geology and planning factors were specified and each terrain unit was systematically assessed in relation to them: (a)

Source of borrow for: (i) water retaining embankments (dams) (ii) non-water retaining embankments (road, rail) (iii) fill

(b)

Resources: (i) clean sand or gravel

(c)

Suitability for: (i) road sub-grade (ii) shallow building foundations (iii) trenching and/or tunnelling (iv) septic tanks (v) untreated roads

(d)

Urban development: (i) potential (ii) limiting factors

(e)

Route (road and rail) location

(f)

Liability to flooding in the natural state

(g)

Relative susceptibility to tunnelling or gullying type erosion, and

(h)

Slope stability REFERENCES

Finlayson, A.A. (1982). Terrain Analysis, Classification and an Engineering Geological Assessment of the Sydney Area, New South Wales. Volume 1: Terrain Analysis and Classification,

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258 pp. Volume 2: Terrain Assessment, 199 pp. CSIRO Australia, Division of Applied Geomechanics Technical Paper No. 32.

Grant, K. and Finlayson, A.A. (1978). The Assessment and Evaluation of Geotechnical Resources in Urban or Regional Environments.

Engineering Geology, Volume 12, No. 3, pp 219-293.

Symposium SG3 Evolution and biogeography of early vertebrates (AAP) Conveners: Dr G.C. Young & Professor K.S.W. Campbell

Abstracts for Symposium SG3 will be printed separately and will be available at the symposium

Symposium SG4 Analysis and management of multivariate data in geology (V7orkshop) (SGGMP) Conveners: Dr J.C. Van Moort & Dr M. Cameron

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F U T U R E OF P E T R O L O G I C A L

DATABASES

R.W. Le Maitre Department of G e o l o g y , University of M e l b o u r n e , P a r k v i l l e , Vic No A b s t r a c t

3052

provided

THE "CHEMROCK" DATA BASE A N D A M P H I B O L I T E S FROM BROKEN L.M. Barron* and J.S.

HILL

Stroud

G e o l o g i c a l Survey of New South W a l e s , Mining M u s e u m , Sydney

N.S.W.

The G e o l o g i c a l Survey of N . S . W . acquired the BMR's data base package for handling chemical analyses of rocks with the assistance of M . O w e n s .

The package has been recast into HP9845 BASIC and considerable enhanced to become the CHEMROCK package but is is still compatible with the BMR's data d i s c s .

It can handle chemical analyses of minerals as well as rocks.

The storage/correction programs have provision for a revised entry order of data base abundances, while the correction program can automatically insert the same partial character string into the identification string for all pertinent analyses.

The data base is maintained with two levels,namely the coded format and the package format.

The coded format corresponds to the BMR's format of data storage, while the package format c o m p r i s e s groups of analyses which have been specially selected from the coded format for future use.

Retrieval from the coded format can be made by index number of the analysis, or by searching for a character string in the identification string.

This search can be confined to a specific subfield (i.e. formation).

The plotting p r o g r a m s contain their own sieves that enable the user to selectively plot by symbol, or by a prearranged group of analyses within a p a c k a g e .

The plotting p r o g r a m s protect against the use of no-entry abundances while individual analyses in a p a c k age can be deactivated for the plotting session or permanently.

Analyses are plotted by symbol or as an index number or value of another variable, or some combination of these three.

Thus an XY plot can be hand-contoured for a third variable, and similarily for a fourth variable on a triangular XYZ plot.

Geographic plots by grid reference or drill hole plots by depth can be made.

Overlays showing, for example, map localities, or c o m p o s i t i o n a l fields can be added to a plot along with references lines and extra labels.

Multimodal d i s t r i b u t i o n s of a variable are decomposed using a cummulative probability plot while the operator interactively smooths the data.

Statistics are performed with a reduced major axis algorithm that is unbiased concerning errors of the variables.

A stereoscopic quaternary plot can be rotated to any one of sixteen

326

orientations along with an overlay of informative planes and labels.

The CHEMROCK package is very powerful and can be used interactively at any time in any of the programs because it is in BASIC. The CHEMROCK package is demonstrated for a suite of 500 analyses of amphibolites in and around the Broken Hill area. Discriminant functions, including several developed by trial and error, demonstrate that basically the amphibolites from the Broken Hill region are all low potassium ocean floor tholeiites regardless of their stratigraphic position. Amphibolites associated with nearby major mineralization at Broken Hill have a chemical drift from this original composition and this drift has been quantified. There are three other places in the Broken Hill region where a similar chemical drift has been found, and in each case significant mineralization is present.

This drift is thus a potential path finder for major mineralization.

DATA MANAGEMENT AND MISMANAGEMENT IN GEOCHEMICAL EXPLORATION THE CASE FOR PATTERN RECOGNITION N.J. Marshall N.J.Marshall and Associates Pty.Ltd.,2/113 Forest Road, Hurstville, NSW. 2220.

Interpretation of large quantities of multi-variate geochemical data should be considered under the topics of data quality,data relevance, acquisition and management,and interpretation techniques. The following philosophical discussion highlights some aspects which should be considered throughout the design and progress of a mineral exploration survey, in an interactive manner. The ad hoc application of number crunching techniques without a deeper insight into the total geological problem, including consideration of the geological environment and geochemical landscape, can lead to at best, trivial, and at worst, confusing solutions.

Data Quality; poor quality data may be difficult to recognize without an appreciation of the analytical techniques and known geochemical behaviour of the elements involved. This can be minimized by proper specification of techniques of sample collection and analyses, tested by orientation survey. Control samples usually only test precision,but high precision by some techniques may be illusory.

Data Relevance is something which a competent geochemist familiar with the geological-geochemical environment and exploration model must decide early in the survey. Shot-gun type approaches in the hope that "something will fall out" are at best cost-inefficient, and may add noise to the data structure where variables are non-diagnostic.

For example, extractable U as measured by fluorimetry is generally more diagnostic of economic mineralization than total U as measured by x-ray fluorescence.

Well chosen extractable elements, regressed to extractable iron and manganese content of stream sediments, may be more diagnostic than more complex multivariate surveys in lithologically varied terrain

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which do not consider the geochemical dispersion model.

Cu, Pb and Zn data do not help to find Carlin type gold deposits.

Data Acquisition and Management; Field data should be recorded on site in a computer compatible format which can enable simple sorting,screening and editing to be undertaken on variables such as the nature of the sample, without any statistical manipulation. Constant data for archival purposes (such as photo number) and derived data (such as grid reference from airphoto sample points, and lithologic unit) can be entered at base camp. The aim is to minimize paper work at the field site, and to this end, field data cards specific to the task, rather than universal formats of greater complexity, are advocated.Some examples are illustrated.

Field data must be merged later with analysis data and digitized sample coordinates, if the latter are machine derived.

Interpretation Techniques; The aim of the survey should be to depict, preferably in pictorial map form, areas of potential mineralization.

This may involve identification of background and mineralized sample populations, or taxonomic classifications of multivariate patterns spatially related to geologic features.

Many statistical practices in current use have been developed from the behavioural sciences and include probabilistic concepts whose application to exploration is naive - particularly when carefully documented data are processed without due consideration of their spatial relation to the geologic map, and when rigorous assumptions of normality and unimodality inherent in many statistical techniques, are ignored.

The concept of the 2h percentile of anomalous samples has no geologic significance, and can be misleading. It may be preferable to estimate background from a few carefully chosen samples rather than statistics which may include many subtle anomalies.

Correlations based on a global data set can destroy the very exceptions one is looking for, since a few correlated values spatially associated to a geologic feature can be downgraded by non-diagnostic random background.

Many natural geochemical distributions consist of aggregate populations, and attempts at log transformation may be self defeating in over-correcting skewness, and enhancing background populations at the expense of those representing mineralization.

Stacked line profiles are the simplest pattern recognition technique, do not distort data and assume spatial relationships, and are superior to contour maps in identifying subtle trends.

Interactive color graphics techniques offer scope for real time data interpretation by the geochemist, through presentation of map images which can be related to geological and geophysical maps using distribution free classification and the human interpreter's superior ability to identify significant spatial patterns-

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PROCEDURES FOR STATISTICAL ANALYSIS OF MULTI-ELEMENT GOSSAN DATA N,A. Campbell

1

and R.E. Smith

2

^Division of Mathematics and Statistics, CSIRO, Wembley, WA Division of Mineralogy, CSIRO, Wembley, WA

The approach adopted for the statistical analysis of multi-element gossan data can be set out in a number of steps, as follows.

(1)

Selection of appropriate reference groups.

These are decided on geological criteria from orientation data.

(2)

Characterization of each homogeneity of the group.

reference

group

and

examination

of

An appropriate power transformation for each element is determined, with the effect of extreme values being taken into consideration. Examination of each element separately via Q-Q plots of the transformed values is followed by a multivariate examination of the homogeneity of the material, using calculations which are insensitive to extreme values.

(3)

Review of transformation used.

for each element when all groups

are

The recommended transformation for each element is examined for each of the groups. A transformation that is suitable for all (or most) groups is sought, taking into account the large-sample confidence intervals for the power transformations.

(4)

Investigation of separation of reference groups.

Canonical variate analysis is used to determine the degree of separation or overlap of the multi-element characteristics of the reference groups.

(4a) Determination of optimum subsets of elements.

All-possible-subset calculations are used combinations that best separate the groups.

(5)

to

find

the

element

Test of allocation performance for each sample from the reference groups.

The probability of each sample belonging to each of the reference groups (the probability of group membership) is calculated, using a leave-one-out approach. In addition, an index of typicality is calculated to indicate whether the sample is in fact typical of any of the reference groups. The calculation of both these values involves the squared Mahalanobis distance for each sample to each of the reference groups.

(6)

Allocation of samples of unknown origin.

The probability of group membership and index of typicality with

329

respect to each of the reference groups are calculated (as in 5) for each unknown sample. This allocation procedure is used to determine the most likely origin of the unknown sample.

FINDING RELATIONS BETWEEN THE PETROGRAPHIC COMPONENTS OF COAL M.A. Cameron CSIRO Division of Mathematics & Statistics, Sydney, NSW

It is well-known that testing for association between members of a set of proportions is complicated by the constraint that their sum is one. Similar problems occur when testing for association between a number of sets of proportions. Spurious correlations between variables in the different sets may arise because of constraints within each set.

Such a problem arose in a project with M. Smyth, of CSIRO Division of Fossil Fuels, to investigate relationships, in samples from an oil well, between the compositions of dispersed organic matter in sediments and of microlithotypes from associated coal seams. It has recently been suggested (Aitchison, 1982) that relationships between such sets of proportions be investigated by considering relationships between the logarithms of the ratios of proportions. That procedure was found to be inappropriate for the data in this problem.

An approach based on the ideas of "neutrality of proportions" (see, for example, Darroch and Ratcliff, 1978) was devised. It involves the calculation of a type of partial correlation coefficient which eliminates the effect of the constraint and leads to easy interpretation, if there is no association between the proportions within a set.

When applied to the petrological data, the methods showed that some apparently significant correlations are artefacts induced by the constraint.

References Aitchison, J. (1982). The statistical analysis of compositional data.

Jour. Roy. Statist. Soc. Ser. B, 139-177.

Darroch, J.N. and Ratcliff, D. (1978). No-association of proportions.

Math. Geol. 10, 361-368.

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CHEMICAL ALTERATION AROUND AND ELEMENT ZONATION WITHIN THE ROSEBERY OREBODY W. Naschwitz Department of Geology, University of Tasmania, Hobart, Tasmania The Rosebery Orebody, a Zn-Pb-Cu bearing volcanic massive sulphide deposit, is located on Tasmania's west coast.

The deposit is hosted in shale within a sequence of acid pyroclastic rocks which form part of the Mt. Read Volcanic Group (Brathwaite, 1974).

The aims of the current project are twofold: (a) To examine the metal distribution within the northern ore lens, which is tilted 45 degrees to the east and shows otherwise only minor affects of post-depositional deformation. (b) To look for alteration haloes in the surrounding pyroclastic rocks. (a) Ore Metal Zonation The E.Z. Company at Rosebery routinely analyses for Pb, Zn, Cu, Ag, Au and Fe from drill core samples.

These metal assays including their position within the mine grid were stored on a data file.

For each sample its position had to be determined from the drill core survey.

The accumulated sample points representing a sheet-like ore lens were tilted back into a horizontal position using a computer program.

In order to smooth out a minor bending of the ore sheet, a stepwise turning procedure was used which interpolated for each sample the appropriate angle of dip and E-intercept.

The major data file was sorted and subdivided in smaller units representing slices from the horizontal ore lens.

These subfiles were run through a plotting program, called SURFACE II (R.J. Sampson, 1975).

Using either a nearest neighbour search algorithm or a 3rd order trend surface analysis, a zonation of the above mentioned metals could be plotted.

Green et al. (1981) described the metal zoning for the southern ore lens at Rosebery in a similar manner. (b) Alteration Haloes around the Deposit Samples were collected from the immediate vicinity of the orebody and as far as 4 km away.

The samples were analysed for major and minor elements by XRF-spectrometry and the analytical data stored on several data files which could be merged by demand and read into the same plotting program.

The contour plots indicate that the chemical alteration is in agreement with the mineralogical alteration.

Single major oxides, combinations of major oxides and trace elements show a distinct pattern of either depletion or enrichment which may represent a former channelway for hydrothermal solutions.

The alteration extends into the hangingwall of the ore deposit.

REFERENCES Brathwaite, R.L., 1974. The Geology and Origin of the Rosebery Ore Deposit, Tasmania. Econ. Geol. 69, 1086-1101.

Green, G.R., 1981. The Formation of the Volcanic-Hosted Massive Sulfide Ore Deposit at Rosebery, Tasmania. Econ. Geol. 76, 304-338.

331

Sampson, R.J,, 1975. SURFACE II, Graphics System (Revision 1).

Geological Survey, Lawrence, Kansas.

Kansas

Simple Conputer Analysis for the Interpretation of Geochemical Data in Mineral Exploration J.F. Gilfillan John F. Gilfillan & Associates Pty Limited Much geochemical data generated fran exploration projects throughout Australia still receives little more interpretive effort than a casual "eye balling" by project geologists.

This paper proposes that a far more useful and cost effective analysis of data can be undertaken, even in the field office, by project staff with access to straight forward user-oriented interactive computer programmes. These should allow data to be assembled into meaningful groupings for the determination of patterns of distributions and associations of elements, rocks, structural trends and other variables, and the display of these rapidly in symbol form at nominated scales.

The geochemical signature of a buried mineral deposit, particularly in a weathered environment, may be a canplex or subtle relationship of several elements, perhaps associated with geological factors.

Recognition of these may not only allow direct identification of prospective targets, but may also permit discrimination between true and false anomalies with significant cost saving in field follow up.

Few geologists have the time or inclination to go beyond the manual generation and examination of more than a few sinple statistical parameters for a limited number of elements. Most will recognise that this represents an inconplete study of the data: some will also know that this partial treatment can lead to totally incorrect conclusions.

The key to successful geochemical interpretation is the ability to over-view all the variables in the data set; analyses, rock types, structural information, alteration types and intensity, veining, magnetic response and so on. The study of sub-sets of data based on these variables allows a more thorough understanding of the elemental distributions and combinations and the development of patterns and trends of significant associations.

This type of interpretation requires input frcan the project geologist wDrking in direct association with the conputer as a mathematical tool. It can be undertaken by a skilled exploration geochemist familiar with the geology of the region but can rarely reach a satisfactory result if dumped on a distant Bureau.

Examples of the interpretation of field data used in this paper have been derived fron several Australian exploration projects. Data analysis has used Q-GAS programmes developed by Queens University, Kingston, Ontario, adapted for mini-computer from earlier work for the Geological

332

Survey of Canada with support fron the I.A.E.A, and CRAE. The ^ s t e m consists of eleven interactive programmes, plus an input editor, vdiich allows data management, univariate statistics with histograms, correlation analyses, X-Y plots, symbol maps and profiles, multiple linear regression, discriminant analysis and R-mode factor analysis.

Important aspects of the programmes are the facilities for: (a) Selection of sub-groups of data based on one or many selection criteria determined by other variables (b) Transformation of data to logs, ratios, complex arithmetical function, and (c) Inspection of progress results on a graphics screen with a facility to re-run progressively modified data to develop or highlight patterns.

It is proposed that significant improvements in the interpretation of geochemical data could result from the use by field staff of simple techniques v^ich could give an improved understanding of the complex interplay of elements which make up a characteristic geochemical signature for mineralisation.

DIGITAL-IMAGE DISPLAY OF STREAM-SEDIMENT GEOCHEMICAL DATA AND A S S O C I A T E D LITHOLOGICAL M A P P I N G A . A . Green & M . D . Craig CSIRO Division of M i n e r a l P h y s i c s , P . O . Box 136, N o r t h R y d e , N S W This paper d e s c r i b e s the m e t h o d s used to put the stream-sediment geochemistry for the Forsayth 1 : 100,000 sheet in a digital-image format that can b e numerically compared w i t h the mapped lithology for the same a r e a .

The raw data came from a BMR computer tape w i t h coordinates and measured concentrations.

These data w e r e combined w i t h a digitized d r a i n a g e - b a s i n m a p to produce an image w h e r e each sample-location had the picture elements in its associated d r a i n a g e basin coded w i t h a b r i g h t n e s s v a l u e proportional to the observed c o n c e n t r a t i o n .

In this way w e produced 24 images (one for each element) w h i c h could b e displayed in colour-coded f o r m a t , or as combinations of any three elements on the b l u e , green and red channels of a colour TV s y s t e m .

The lithologic m a p for the area w a s also displayed in image form a t in r e g i s t r a t i o n w i t h the geochemical i m a g e s .

The percentage of each rock-type in each d r a i n a g e basin w a s c o m p u t e d , and a p p r o x i m a t e estimates m a d e for the background concentrations due to each r o c k - t y p e o c c u r r i n g on the m a p sheet.

333

EDA - A PACKAGE OF SUBROUTINES FOR EXPLORATORY DATA ANALYSIS M.A. Cameron CSIRO Division of Mathematics & Statistics, Sydney, NSW

Over a number of years, J.W. Tukey has developed methods for investigating the structure of data which are insensitve to deviations from the usual normal distribution assumptions associated with many traditional techniques. Tukey's approach is described in detail in two books, Tukey (1977) and Hosteller and Tukey (1977).

More recently some of the techniques have been summarised by Velleman and Hoaglin (1981).

This book contains good Fortran subroutines for applying the techniques.

Both the general approach of Tukey and the specific techniques described by Velleman and Hoaglin seem to be useful for examining and summarising geochemical data.

In this workshop the programs of Velleman and Hoaglin will be described and some examples of their use presented.

References Mosteller, F. and Tukey, J.W. (1977). Data analysis and regression.

Addison-Wesley.

Tukey, J.W. (1977). Exploratory data analysis. Addison-Wesley.

Velleman, P.F. and Hoaglin, D.C. (1981).

Applications, Basics and Computing of Exploratory Data Analysis. Duxbury Press.

PROGRAMS FOR STATISTICAL ANALYSIS OF MULTI-ELEMENT EXPLORATION GEOCHEMICAL DATA N.A. Campbell^ division of Mathematics and Statistics, CSIRO, Wembley, WA The programs implement statistical techniques for examining and describing the variation amongst samples from a (supposedly) single group of data, and for describing the separation between several groups of samples and for allocating new samples to these reference groups.

The first suite of programs is designed to examine data from an orientation survey in which it is assumed that a majority of the samples are from background material, with some samples originating from the target area relating to the orebody. The statistical techniques are based on the assumption that the underlying distribution is multivariate Gaussian.

Graphical displays of the data are used wherever possible.

POWRTR can be used to determine the Box-Cox power transformation to Gaussian form for each element; options exist for quantilequantile (Q-Q) plots of the data, for deletion of the largest and/or

334

smallest observations and for robust calculations. The inclusion of options for deletion of observations and/or robust calculations allows the transformations to be determined for the main body of data - that is, for samples which can reasonably be expected to belong to a homogeneous population. When some of the values are below the lower limit of detection, CENSOR can be used to incorporate the information in the power transformation calculations. This program uses theory for censored values for Gaussian data to estimate replacement values.

After appropriate transformation of the data, WTHGRP can be used to examine the homogeneity of the samples using all elements simultaneously. Options exist for the implementation of a robust procedure which is insensitive to atypical observations to estimate means and covariances, and for a Q-Q plot of the associated Mahalanobis distances. An alternative approach is implemented in MIXTRE; here it is assumed that the samples arise from a mixture of Gaussian densities and the posterior probabilities of membership are estimated.

The second suite of programs is used when reference groups of samples related to mineralization and to background material are known. After appropriate transformation of the data, RBSTCV can be used to describe the separation of the reference groups along canonical variates. Options exist for the implementation of robust calculations, and for a plot of the canonical variate means. SUBSET can be used to determine the important elements effecting the separation between the reference groups. All-possible-subset calculations are used to give the likelihood ratio statistic (and canonical root and Mahalanobis D^ for two groups) for subsets of each size. When values are missing for some elements for some samples, MISVCV can be used to carry out a canonical variate analysis and provide estimates of the missing values.

ALLOCN can be used to calculate probabilities of group membership and typicality indices to allocate new samples to one or other of the reference groups. Options exist for using equal or unequal covariance matrices in the calculations, and for using multivariate Gaussian or multivariate Student densities to calculate the probabilities of group membership.

Leave-one-out calculations are used for each of the samples from the reference groups. The probabilities are written to a disc file to allow plotting of these values against sample coordinates.

Symposium SG5 Geological conservation in Australia Convener: Mr E.B. Joyce Session 1 Chairman: N.C. Stevens 11.00am Introduction by convener of seminar - E.B. Joyce 1.

11.15am 2.

Geological conservation and the Australian Heritage Commission - M. O'Brien & P. Matthews Divisional Reviews 1 Queensland 2 New South Wales 3 Commonwealth Territories 4 Victoria

W.F. Willmott R.A. Osborne J.M. Dickins E.B. Joyce

12.15pm General discussion.

12.30pm

LUNCH Session 2

1. 30pm 3.

Divisional Reviews (continued) 5 Tasmania 6 South Australia 7 Western Australia

C.R. Calver ri. McBriar D. Gee

2.15pm 4.

2.30pm 5.

2.45pm

Report by Federal Convener N.C. Stevens

Report on Workshop on Geological Conservation held in Canberra in November 1982 - E.B. Joyce General discussion and conclusion of seminar.

3.00pm

AFTERNOON TEA

335

GEOLOGICAL CONSERVATION IN AUSTRALIA GEOLOGICAL CONSERVATION AND THE AUSTRALIAN HERITAGE COMMISSION M. O'Brien & P. Matthews Australian Heritage Commission, Canberra, A.C.T. Since 1975 the Commonwealth Government through the Australian Heritage Commission has made grants under the National Estate Programme of well over $100,000 to the Divisions of the Geological Society of Australia Incorporated, to document significant geological features.

Eleven substantial reports have now been prepared, and over 800 sites described.

Nomination of geological places for the Register of the National Estate has proceeded slowly, as most attention until now has been devoted to the preparation of the Divisional reports. Some Divisions have also been reluctant to nominate more than a small number of places. Nevertheless, many places of geological and geomorphological significance now appear in the Register, having been nominated either by the Society or by others.

BILNEY, E., and others, 1981: The Heritage of Australia. The illustrated Register of the National Estate. The Macmillan Company of Australia, and the Australian Heritage Commission. 118 p. + 9 state and territory sections.

YENCKEN, D.G.D., 1982: The National Estate in 1981. A Report of the Australian Heritage Commission. Australian Government Publishing Service, Canberra, 225 p.

GEOLOGICAL CONSERVATION IN AUSTRALIA DIVISIONAL REVIEWS 1 . QUEENSLAND W.F. Willmott* Geological Survey of Queensland, Brisbane, QLD. The Subcommittee for the Preservation of Geological Monuments received a grant of J?6,000 in the National Estate programme for 1975/76, to prepare a listing of geological monuments of considerable areal extent, important for their scenic, recreational or aesthetic value to the general public, as well as for their scientific value to the profession, and to complete the detailed documentation of a number of these features with recommendations for their preservation.

The resulting report

(de Jersey, Stevens and Willmott,

336

1976) documented the larger features throughout the State that were considered suitable for National Parks (Category A). Many of these were in north and northwest Queensland. Features already within National Parks were mentioned only briefly, to concentrate resources on those not yet protected. The smaller Category B features of more purely scientific or educational value were listed briefly. Two hundred copies of the report were distributed to conservation groups, government departments and various members of parliament.

In 1978/79 Queensland received a second grant of $4,000, and documentation of the Category B features in southeast Queensland was undertaken. The resulting report (Willmott and others, 1981) also updated information on Category A features in southeast Queensland, and described the recommendations for geological conservation in Queensland in more detail. Only a limited number of copies of this report were distributed to administering authorities to avoid publicising fragile features.

An application has been made for a further grant under the .1982/83 National Estate programme to allow documentation of the smaller Category B features in coastal north and central Queensland.

DE JERSEY, N.J., STEVENS, N.C. and WILLMOTT, W.F. (eds), 1976: Geological elements of the National Estate in Queensland. Geological Society of Australia Incorporated, Queensland Division, 103 p.

WILLMOTT, W.F., WEBB, J.A. and WADE, M. (eds), 1981: Geological sites in southeast Queensland. Report 2, Geological elements of the National Estate in Queensland.

Geological Society of Australia Incorporated, Queensland Division, 119 p. * Convener, Geological Monuments Subcommittee, Queensland Division, G.S.A. Inc.

GEOLOGICAL CONSERVATION IN AUSTRALIA DIVISIONAL REVIEWS 2. NEW SOUTH WALES R.A.L. Osborne* Department of Geology and Geophysics, University of Sydney, N.S.W. The Australian Heritage Commission made a grant of $14,000 available in July 1978 to the Geological Sites Subcommittee. A limited distribution report has appeared (Percival 1979) which identifies and surveys places of geological and geomorphological significance in N.S.W. and recommends

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a p p r o p r i a t e m e c h a n i s m s f o r t h e i r p r e s e r v a t i o n , A r e v i e w of l e g i s l a t i o n in N e w S o u t h W a l e s a n d a d i s c u s s i o n of p r o t e c t i o n m e c h a n i s m s o p e r a t i n g in B r i t a i n is f o l l o w e d by d e t a i l e d d e s c r i p t i o n s a n d r e c o m m e n d a t i o n s f o r 47 s i t e s w h i c h w e r e selected from a p p r o x i m a t e l y 200 o r i g i n a l l y n o m i n a t e d . A b o u t 160 o t h e r s i t e s of s i g n i f i c a n c e a r e l i s t e d in an a p p e n d i x .

T h i r t y - o n e of t h e s i t e s a r e to be d e s c r i b e d in a b o o k to b e published soon. This will also contain a general Introduction on the p r o b l e m s of m a n a g e m e n t a n d p r e s e r v a t i o n .

A further grant f r o m the A u s t r a l i a n H e r i t a g e C o m m i s s i o n and a d m i n i s t e r e d by t h e N . S . W . P l a n n i n g a n d E n v i r o n m e n t C o m m i s s i o n w a s a n n o u n c e d in A u g u s t 1 9 8 0 . T h i s c o n s i s t e d of $ 5 , 0 0 0 f o r 1979/80 and $5,000 for 1 9 8 0 / 8 1 . The grant was given for the m a p p i n g a n d d o c u m e n t a t i o n of 34 s i t e s o v e r a s i x m o n t h s p e r i o d in 1 9 8 0 / 8 1 a n d t h e p r e p a r a t i o n of a r e p o r t f o r s u b m i s s i o n to t h e H e r i t a g e C o u n c i l of N . S . W . ( S t e v e n s o n 1 9 8 1 ) . A g r a n t to c a r r y o u t w o r k t o w a r d s a t h i r d v o l u m e w a s r e q u e s t e d in 1 9 8 2 .

T h e S u b c o m m i t t e e is i n c r e a s i n g l y a c t i v e in c o m m e n t i n g on draft local e n v i r o n m e n t a l p l a n s , and n a t i o n a l park m a n a g e m e n t plans. Certain geological sites have received special a t t e n t i o n . E v i d e n c e h a s b e e n g i v e n to the J o i n t P a r l i a m e n t a r y C o m m i t t e e on the A . C . T . A P a r l i a m e n t C o n s e r v a t i o n O r d e r h a s b e e n o b t a i n e d for o n e p a r t i c u l a r s i t e .

P E R C I V A L , I . G . , 1 9 7 9 : T h e g e o l o g i c a l h e r i t a g e of N e w S o u t h W a l e s . A r e p o r t p r e p a r e d for t h e A u s t r a l i a n H e r i t a g e C o m m i s s i o n a n d t h e P l a n n i n g and E n v i r o n m e n t C o m m i s s i o n of N e w S o u t h W a l e s . G e o l o g i c a l S o c i e t y o f A u s t r a l i a I n c o r p o r a t e d (NSW D i v i s i o n ) , 280 p . + a p p .

S T E V E N S O N , B . , 1 9 8 1 : T h e g e o l o g i c a l h e r i t a g e of N e w S o u t h W a l e s . A r e p o r t p r e p a r e d for the A u s t r a l i a n H e r i t a g e C o m m i s s i o n a n d the N e w S o u t h W a l e s D e p a r t m e n t of E n v i r o n m e n t a n d P l a n n i n g . G e o l o g i c a l S o c i e t y of A u s t r a l i a I n c o r p o r a t e d (NSW D i v i s i o n ) , 2 0 0 p . + a p p . * C o n v e n e r , G e o l o g i c a l Sites and M o n u m e n t s NSW D i v i s i o n , G . S . A . I n c .

Subcommittee,

G E O L O G I C A L C O N S E R V A T I O N IN A U S T R A L I A DIVISIONAL REVIEWS 3 . COMMONWEALTH TERRITORIES J . M . Dickins'^ B u r e a u of M i n e r a l R e s o u r c e s , C a n b e r r a ,

A.C.T.

A g r a n t of $ 4 , 5 0 0 w a s r e c e i v e d f r o m the A u s t r a l i a n H e r i t a g e C o m m i s s i o n in F e b r u a r y 1 9 7 9 to d o c u m e n t g e o l o g i c a l m o n u m e n t s in the A . C . T . a n d to n o m i n a t e s i t e s for the R e g i s t e r of the

338

National Estate.

A further grant of $3,500 for 1980/81 was received to complete this work. A report is now being prepared.

A grant of $6,500 for 1982/83 is to be used to commence documenting geological monuments in the Northern Territory, beginning in the Katherine-Darwin region. This will be the first time such work has been undertaken in the Northern Territory, although the N.T. Geological Survey is currently engaged in defining the geology, sites of special interest, and sensitive areas within National Parks for incorporation into their management plans.

The Heritage Commission has now registered the eight sites so far submitted to them. These are the State Circle Cutting, Tuggeranong Parkway Cutting, Yarralumla Brickpits, Cotter Caves and Mine, Pine Island Agglomerate, "Gossan Hill" near CCAE, Ginninderra Creek Road Cutting, and the Deakin Anticlines. They have also registered the Woolshed Creek fossil locality, a locality not submitted to them by the Subcommittee.

Signs have been designed for six selected geological monuments. These will not have geological information, but will carry reference to Tourist Bureau leaflets written by the Subcommittee.

The fate of the State Circle Cutting continues to cause concern, since Che effects of the construction of approaches to the new Parliament House are still uncertain. However the decision of the Heritage Commission to register the locality will greatly assist the Society's efforts to preserve the locality. It is also uncertain if it will be possible to make a new exposure of the unconformity within the grounds of the new Parliament House, to replace the former Monument on the top of the hill.

*

Convener, Geological Monuments Subcommittee, Commonwealth Territories Division, G.S.A. Inc.

GEOLOGICAL CONSERVATION IN AUSTRALIA DIVISIONAL REVIEWS 4. VICTORIA E.B. vJoyce.

Department of Geology, University of Melbourne, VIC. In 1977 the Australian Heritage Commission made a grant totalling ^10,500 to the Victorian Division. The Geological Conservation Subcommittee employed a consultant to draw up a complete inventory of geological features of the National

339

E s t a t e in V i c t o r i a a n d i n i t i a l l y s o m e 9 0 0 f e a t u r e s w e r e d o c u m e n t e d . U n d e r a l a t e r g r a n t of $ 4 , 0 0 0 in 1 9 7 8 / 7 9 t w o c o n s u l t a n t s c a r r i e d o u t d e t a i l e d d o c u m e n t a t i o n of 23 s e l e c t e d f e a t u r e s . T h e m a i n p r o j e c t c o n t i n u e d w i t h two f u r t h e r c o n s u l t a n t s , and some 200 f e a t u r e s w e r e d o c u m e n t e d and t a b u l a t e d , i n c l u d i n g t h e 23 m e n t i o n e d .

A r e p o r t w a s p r e p a r e d by t h e S u b c o m m i t t e e ( J o y c e a n d K i n g 1 9 8 0 ) w h i c h i n c l u d e d an a n a l y s i s of t h e i n v e n t o r y d a t a , a d i s c u s s i o n of the c o n c e p t s i n v o l v e d in t h e s u r v e y , d i s c u s s i o n of c u r r e n t s t a t e a n d f e d e r a l l e g i s l a t i o n , a n d r e c o m m e n d a t i o n s .

243 c o p i e s of the r e p o r t w e r e p r i n t e d a n d c o p i e s w e r e s e n t to Ministers, government departments, libraries, selected local geologists and p l a n n e r s . The report was corrected and r e p r i n t e d in A u g u s t 1 9 8 2 , a n d a f u r t h e r 2 0 0 c o p i e s p u t on s a l e .

A f u r t h e r g r a n t to c o m p l e t e the d o c u m e n t a t i o n of a l l f e a t u r e s in the f i l e , a n d p u b l i s h a b r i e f l i s t i n g a n d e v a l u a t i o n , has been r e qu e s t ed for 1 9 8 2 / 8 3 .

900

D i s c u s s i o n s h a v e b e e n h e l d w i t h m e m b e r s of t h e E n v i r o n m e n t a l L a w S e c t i o n of t h e L a w I n s t i t u t e of V i c t o r i a , w h i c h is w o r k i n g t o w a r d s a V i c t o r i a n H e r i t a g e A c t w h i c h w o u l d e s t a b l i s h a r e g i s t e r , s i m i l a r to t h a t of the A u s t r a l i a n H e r i t a g e C o m m i s s i o n , to p r o v i d e s t a t e c o n t r o l of n a t u r a l a n d cultural features. The Law Institute also strongly recommends t h e u s e of c o v e n a n t i n g p r o c e d u r e s u n d e r e x i s t i n g l e g i s l a t i o n for the f u t u r e p r o t e c t i o n of g e o l o g i c a l f e a t u r e s .

T h e M i n i s t r y f o r C o n s e r v a t i o n in V i c t o r i a h a s s p o n s o r e d a n u m b e r of d e t a i l e d s t u d i e s of g e o l o g i c a l a n d g e o m o r p h o l o g i c a l f e a t u r e s in r e g i o n s of V i c t o r i a , t h r o u g h c o n t r a c t s a w a r d e d by its E n v i r o n m e n t a l S t u d i e s D i v i s i o n , and these have s u p p l e m e n t e d the w o r k r e c o r d e d in J o y c e a n d K i n g ( 1 9 8 0 ) .

A m a j o r r e v i e w of t h e w o r k of the M i n i s t r y f o r C o n s e r v a t i o n is n o w u n d e r w a y , a n d m a y l e a d to the f u t u r e e m p l o y m e n t of g e o l o g i c a l a n d g e o m o r p h o l o g i c a l s t a f f in the M i n i s t r y .

T h e L a n d s c a p e C o m m i t t e e of t h e N a t i o n a l T r u s t of A u s t r a l i a ( V i c t o r i a ) h a s c o n t i n u e d i t s c l a s s i f i c a t i o n a n d r e c o r d i n g of local areas.

M a n y g e o l o g i c a l f e a t u r e s in V i c t o r i a a r e n o w on the R e g i s t e r of t h e N a t i o n a l E s t a t e a l t h o u g h o n l y a s m a l l n u m b e r h a v e y e t b e e n d i r e c t l y n o m i n a t e d by the S u b c o m m i t t e e . T h e c o n v e n e r h a s a s s i s t e d in the e v a l u a t i o n of n o m i n a t i o n s c a r r i e d o u t by the l o c a l N a t u r a l E n v i r o n m e n t E v a l u a t i o n P a n e l .

T h e S u b c o m m i t t e e c o n t i n u e s to w o r k w i t h the L a n d C o n s e r v a t i o n C o u n c i l , and other o r g a n i s a t i o n s and i n d i v i d u a l s , by p r o v i d i n g i n f o r m a t i o n on g e o l o g i c a l and g e o m o r p h o l o g i c a l f e a t u r e s in the s t a t e , a n d c o m m e n t i n g on m a n a g e m e n t p l a n s .

J O Y C E , E . B . and K I N G , R . L . ( e d s ) , 1980: G e o l o g i c a l F e a t u r e s the N a t i o n a l E s t a t e in V i c t o r i a . A n i n v e n t o r y c o m p i l e d f o r the A u s t r a l i a n H e r i t a g e C o m m i s s i o n , G e o l o g i c a l S o c i e t y of A u s t r a l i a I n c o r p o r a t e d , Victorian Division, 176p. + apps.

of

340

*

Convener, Geological Conservation Victorian Division, G.S.A. Inc.

Subcommittee,

GEOLOGICAL CONSERVATION IN AUSTRALIA DIVISIONAL REVIEWS 5. TASMANIA C.R. Calver.

Department of Mines, Rosny Park, TAS.

In 1974 the Environment and Conservation Subcommittee submitted a list of twenty-three important sites to the Committee of Enquiry into the National Estate, covering about 60% of the area of Tasmania.

In 1979 the Australian Heritage Commission made a grant of $10,000 to the renamed Geological Monuments Subcommittee to investigate and record geological sites selected by the S ubcomrni ttee • A report was prepared detailing sixty sites, outlining the criteria used to select sites as monuments, and making specific recommendations for their conservation (Eastoe, 1979), The report was tabled at the Division's meeting in June 198U, and after the addition of the Darwin Crater to the list of sites, copies were distributed to regional and other libraries, and organizations concerned with conservation. Only 50 copies were produced.

It was decided not to nominate sites for the Register of the National Estate, as it did not seem necessary for their pr.-ser\^ation, and might pre-empt other activities in the area.

EASTOE, C.J., 1979: Geological monuments in Tasmania. A report funded by the Australian Heritage Commission. The Geological Society of Australia Inc., Tasmania Division, 149 p.

*

Convener, Geological Monuments Subcommittee, Tasmania Division, G.S.A. Inc.

341

GEOLOGICAL CONSERVATION IN AUSTRALIA DIVISIONAL REVIEWS 6. SOUTH AUSTRALIA E.M. McBriar* Department of Geology, University of Adelaide, S.A. South Australia was one of the first two states to receive a grant to document and report on those areas in the state which could be regarded as geological monuments, and to make recommendations on actions necessary for their preservation. $11,500 was received in 1975/76 and the Geological Monuments Subcommittee began work with a full- time consultant in March 1 976 .

The grant came with a recommendation from the South Australian Director of Environment that the resultant report should not be a public document, to avoid increasing problems of site protection; only 15 copies were produced. The report presented detailed information about forty-eight geological monuments, mainly in the metropolitan and outer metropolitan areas, but also including areas elsewhere in the state. Also included were discussions of the concept of a geological monument and of state legislation, together with a series of recommendations for future work and forwarding geological conservation in South Australia (McBriar and Mooney 1977).

In 1977/78 the South Australian Division received a second grant for $7,000 to continue the documentation of geological monuments in South Australia. The report produced dealt with 39 sites, and is a private document of the Division with limited circulation to the same recipients as the earlier report (Toteff and McBriar 1979). In 1978-79 a grant of $15,000 was made to the Division to fund further work, and a third report was completed in late 1980 (McBriar and others 1980).

In 1979/80 a further grant of $9,000 was made and a fourth report was commenced. When funds were depleted early in 1981 a supplementary grant of $3,292 was provided to complete the work. The fourth volume was published late in 1981 (McBriar and others 1981). An index to the four volumes has now been prepared.

Other items of business for the Subcommittee here included comments on planning proposals, requests for the removal of graffiti and rubbish from geological sites, the maintenance of the right of access to sites, nominations to the Register, and advice to local councils. The South Australian Division has nominated seven major sites to the Register of the National Estate, and these have now all been listed.

A grant of $9,000 in the 1981/82 programme for further identification and documentation work has been received.

McBRIAR, E.M. and MOONEY, P.A. (eds), 1977: Geological Monuments in South Australia. Geological Monuments Subcommittee of the S.A. Division of the Geological Society of Australia Incorporated.

342

TOTEFF, S. and McBRIAR, E.M. (eds), 1979: Geological Monuments in South Australia, Part II. Geological Monuments Subcommittee of the S.A. Division of the Geological Society of Australia Inc. MCBRIAR, E.M., GILES, C.W. and MOONEY, M.D. (eds), 1980: Geological Monuments in South Australia, Part III. Geological Monuments Subcommittee of the S.A. Division of the Geological Society of Australia Inc. McBRIAR, E.M., GILES, C.W. and MOONEY, M.D. (eds), 1981: Geological Monuments in South Australia, Part 4.

Geological Monuments Subcommittee of the S.A. Division of the Geological Society of Australia Inc. *

Convener, Geological Monuments Subcommittee, South Australian Division, G.S.A.

GEOLOGICAL CONSERVATION IN AUSTRALIA DIVISIONAL REVIEWS 7. WESTERN AUSTRALIA R.D. Gee* Geological Survey of Western Australia, Perth W.A. From July 1978 to January 1979 some 60 geological sites within about 100 km of Perth were documented under the supervision of the Geological Monuments Subcommittee. The study was financed by a grant of $9,000 from the Australian Heritage Commission, and was to make recommendations to the Commission on sites suitable to become geological monuments, on problems of preservation, and what further action might be needed.

Fifteen sites were to be nominated for inclusion on the Register of the National Estate, and the Society's attitude on possible state legislation to protect sites was to be outlined for the Environmental Protection Authority. The final report was of limited circulation (Lemmon and others 1979).

In 1979 a Geological Sites Committee was set up by the state Department of Conservation and Environment in Western Australia to examine possible geological sites throughout the state. Members of the Committee include a representative of the W.A. Division of the Society, with other representatives from the Geological Survey of W.A., Australasian Institute of Mining and Metallurgy, Royal Society of W.A., the University of W.A., and the W.A. Chamber of Mines Inc. A further Heritage Commission grant of $10,000 was given for 1979/80 and 1980/81, to identify and document the best fifty geological sites in the state, and if appropriate, nomenate them as geological monuments for the Register of the National Estate. A contractor was employed to carry out the survey and prepare a report.

343

The Subcommittee sees no continuing operational role when the report underway has been completed. The Geological Sites Committee of the state government has embarked on a State-wide documentation scheme which, by mutual agreement, integrates with the Subcommittee's documentation.

The Geological Monuments Subcommittee has found that unofficial agreements between geologists and landowners has been the most effective approach to sites threatened with alternative uses that would preclude continued geological study.

It is hoped that a type of Heritage Act, such as those in operation in NSW and SA, but specifically to preserve geological sites, may be passed in Western Australia.

LEMMON, T.C., GEE, R.D., MORGAN, W.R. and ELKINGTON, C.R., 1979. Important Geological Sites in the Perth and South Western Area of Western Australia: a Report on their Scientific Significance and Future Protection.

Geological Society of Australia, Western Australia Division, 178 p. + app.

Convener, Geological Monuments Subcommittee, G.S.A. Inc.

GEOLOGICAL CONSERVATION IN AUSTRALIA REPORT ON WORKSHOP ON GEOLOGICAL CONSERVATION IN AUSTRALIA HELD IN CANBERRA IN NOVEMBER 1982 E.B. Joyce.

Department of Geology, University of Melbourne, VIC. Representatives of Divisional Subcommittees concerned with geological conservation met in Canberra on the 17th and 18th November 1982. The workshop was held at the offices of the Australian Heritage Commission, and was attended by Commission representatives.

The meeting began with reports on past and future work in each Division. Discussions were held on the concepts underlying geological conservation, including their definition, techniques used, filing systems, and nominations to the Register of the National Estate.

Work began on the preparation of a list of geological and geomorphological features or sites of National and World significance in Australia.

The possibility of an Australia-wide survey was discussed, as well as a publication on geological conservation in Australia.

344

A report on the Workshop is now available (Joyce, 1982)

JOYCE, E.B., 1982 : Report on Workshop on Geological Conservation in Australia, held in Canberra in November, 1982 (typescript).

345

Symposia

Page No

Name

Symposia

Name

Page No

2(b) 9

Abbott, M.

151

3(a)9

Billing, N.B.

206

2(b) 10

Abbott, M.

152

3(b) 6

Bink, M.

217

l(c)2

Abbott, M.J.

23

4(c) 16

Binns,

R.A.

298

4(a)ll

Ahmat, A.L.

255

SGI.11

Binns, R,A.

315

3(a)13

Alam, M.M.

210

l(f)6

Blake, D.H.

94

1(h) 1

Aldam, R.

121

4 (a) 3

Bloom, M.S.

254 277

3(c)4

Archbold, N.W.

224

4(b)l

Bodard, J.M.

3(c)5

Archbold, N.W.

226

1(d) 20

Boettcher, A.L.

73

SGI. 9

Archibald, D.A.C.

312

1(d) 10

Bohlen, S.R.

60

2(a)4

Arculus, R.J.

139

1(d)20

Bohlen, S.R.

73

2(c)6

Arculus, R.J.

174

3 (a) 8

Bolton, B.R.

204

4(a)l

Arculus, R.J.

251

3(b) 5

Bourman, R.P.

216

4(c)8

Arculus, R.J.

288

3(a)12

Bowler, J.M.

208

1(d) 5

Arriens, P.

53

4(b) 2

Brakel, A.T.

278

4(c)5

Atkinson, W.J.

284

1(h) 7

Branson, J.C.

128 92

2(b) 18

Barnes, I.

164

l(f)5

Brown, R.E.

4(c) 16

Barron, B.J.

298

1(d) 11

Brown, W.M.

61

4(c) 15

Barron, L.M.

296

Kg) 5

Brunnschweiler, R.O.

108

SG4.2

Barron, L.M.

325

Kc)ll

Bucknell, W.R.

37

i(g)7

Baxter, J.L.

110

3 (a) 3

Burrett, C.

198

3(a)15

Baxter, J.L.

211

3(c)2

Burrett, C.

221

4(c) 14

Beams, S.D.

296

3(b)l

Butt, C.R.M.

213

SG2.1

Beavis, F.C.

319

3(b) 2

Callen, R.A.

213

1(d) 7

Bell, T.H.

56

SG5.5

Calver, C.R.

340

3 (a) 9

Belperio, A.P.

206

SG4.5

Cameron W.E.

329

3(a)10

Belperio, A.P.

207

SG4.9

Cameron, M.A.

333

3(a)11

Belperio, A.P.

208

Kd)19

Cameron, M.A.

72

3(b) 7

Benbow, M.C.

217

2(b) 7

Cameron, W,E.

147

1(d) 3

Berry, R.F.

51

SG4.4

Campbell, N.A.

328

2(c)3

Bettenay, L.F.

170

SG4.10

Campbell, N,A,

333

Kb) 5

Bickle, M.J.

16

3(d) 8

Carr, P.P.

243

2(c)3

Bickle, M.J.

170

Kc)15

Cas, R.A.F.

41

3(a)4

Bieser, L.

200

3(a)4

Cas, R.A.F.

200

346

4(b)l

Cas, R.A.F.

277

4(c) 10

Cooper, A.F.

291

Kb) 3

Cawood, P.A.

11

K g ) 11

Cooper, B.J.

117

l(c)19

Cawood, P.A.

46

3(d) 4

Cooper, R.A.

237

l(c)15

Ceplecha, J.

41

Kc)15

Cox, S.F.

41

1(c) 2

Chamalaun, F.H.

23

3 (a) 4

Cox, S.F.

200

2(c)3

Chapman, H.J.

170

4 (a) 7

Cox, S.F.

260

2(b) 6

Ghappell, B.W.

145

SG4.8

Craig, M.D.

332

2(c)6

Chappell, B.W.

174

Kc)7

Crawford, A.J.

30

2(c)9

Chappell, B.W.

179

Kc)18

Crawford, A.J.

45

2(c)14

Chappell, B.W.

187

4(c)9

Crohn, P.W.

289

2(c) 15

Chappell, B.W.

188

Kc)10

Cross, K.C.

35

4(c) 6

Chappell. B.W.

286

Kb) 7

Crook, K.A.W.

19

4(c) 13

Chappell, B.W.

295

3(a)13

Crook, K.A.W.

210

4(c)14

Chappell, B.W.

296

Kd)13

D'Addario, G.W.

63

2(b) 10

Chivas, A.R.

152

K g ) 10

Dalgarno, C.R.

115 283

2(b) 18

Chivas, A.R.

164

4(c)4

Danchin, R.V.

3(a)l

Chuck R.G.

195

Kc)l

Davies, H.L.

23

l(e)10

Clark, R.M.

84

4 (a) 9

Davies, H.L.

263

2(c)8

Clarke, B.D.

176

Kb) 6

Day, R.W.

17

1(d)10

Clemens, J.D.

60

2(c) 3

de Laeter, J.R.

170

2(b)15

Clemens, J.D.

160

Ka)3

Denham, D.

2(c)8

Clemens, J.D.

178

SG5.3

Dickins, J.M.

3 (a) 4

Clifford B.E.

200

Kd)17

Ding, P.

67

4(c)3

Colchester, D.M.

282

Ke)12

Dooley, J.C.

85

1(d) 5

Collerson, K.D.

53

Kc)3

Dow, D.B.

25

2(c)l

Collerson, K.D.

167

Kf)10

Drummond, B.J.

100

3(a)14

Collins, L.B.

210

Kh)3

Duddy, I.R.

123

3(a)15

Collins, L.B.

211

4(b) 3

Duddy, I.R.

279

2(b) 10

Compston, W.

152

Kd)4

Earle, M.M.

53

2(b) 15

Compston, W.

160

Kh)9

Edwards, B.

131

2(c)2

Compston, W.

169

Kd)l

England, P.C.

49

2(c)14

Compston, W.

187

Kc)14

Etheridge, M.A.

39

3(d) 2

Compston, W.

235

Kc)15

Etheridge, M.A.

41

3(d) 6

Compston, W.

240

Kf)4

Etheridge, M.A.

90

3(d) 9

Compston, W.

245

2(c) 12

Etheridge, M.A.

185

3(c)l

Cook, P.J.

219

4(a)6

Etheridge, M.A.

259

5 337

347

4 (a) 7

Etheridge, M.A.

260

1(h)3

Gleadow. A.J.W.

123

2 (a) 3

Ewart, A.

137

4(b)3

Gleadow, A.J.W.

279

l(i)3

Exon, N.

134

3(b)4

Glen, R.A.

215

l(e)7

Falvey, D.A.

82

SGI.3

Glen, R.A,

304

l(e)9

Falvey, D.A.

83

3(a)2

Gostin, V.A.

196

l(i)2

Falvey, D.A.

133

3(a)9

Gostin, V.A.

206

l(c)10

Ferguson, C.L.

35

3(a)10

Gostin, V.A.

207

2(c)6

Ferguson, J

281

3(a)11

Gostin, V.A.

208

2 (a) 4

Ferguson, J.

9

1(c)2

Grady, A.E.

23

4(c)l

Ferguson, J.

174

1(h)1

Grady, A.E.

121

4(c)6

Ferguson, J.

286

l(i)2

Greene, H.G.

13

1(h) 3

Ferguson, K.U.

23

SG4.8

Green, A.A.

332

SG2.5

Finlayson, A.A.

322

2(b)l

Green, D.H.

141

Kb) 2

Finlayson, D.M.

9

2(b)2

Green, D.H.

141 144

l(e)ll

Fisher, N.I.

85

2(b)5

Green, T.H.

2(b) 2

Foden, J.D.

141

2(b)13

Green, T.H.

157

2(b) 9

Foden, J.D.

151

4(c)6

Gregory, G.P.

286

2(b) 11

Foden, J.D.

153

2(c)3

Groves, D.I.

170

2(c)4

Foden, J.D.

171

4(a)12

Groves, D.I.

267

2(c)2

Foster, J.J.

169

4 (a)15

Gulson, B.L.

272

3(d) 6

Foster, J.J.

240

3(a)9

Hails, J.R.

206

3(b) 6

Foudoulis, C.

217

3 (a)10

Hails, J.R.

207

3 (a) 8

Frakes, L.A.

204

3 (a)11

Hails, J.R.

208

2(c)2

Froude, D.

169

4(a)16

Hamlyn, P.R.

274

3(d) 6

Froude, D.

240

1(c)15

Hammond, R.

41

SG2.4

Galloway, R.W.

321

3 (a)4

Hammond, R.L.

200

3(d) 7

Garratt, M.J.

242

1(c)12

Harrington, H.J.

37

i(g)ii

Gatehouse, C.G.

117

4(b)2

Harrington, H.J.

278

K g ) 13

Gatehouse, C.G.

119

4(c)4

Harris, J.W.

283

Kb) 5

Gee, R.D.

16

l(i)l

Haxby, W.F.

133

SG5.7

Gee, R.D.

342

1(h)8

Hegarty, K.A.

129

Kc)18

Gibson, G.

45

1(c)20

Heidecker, E.J.

48

Ke)6

Giddings, J.W.

80

2(c)14

Hensel, H.D.

187

SG4.7

Gilfillan, J.F.

331

1(d)14

Hensen, B.J.

63

SGI. 8

Gilligan, L.B.

316

1(d)17

Hensen, B.J.

69

SGI.12

Gilligan, L.B

311

1(d)18

Hensen, B.J.

70

348

2(b) 6

Hickey, R.L.

145

SG5

3(c)6

Hickman, C.F.

227

Report

Joyce, E.B.

2(c)10

Higgins, N.C.

181

Kg) 4

Katz, M.B.

107

1(h) 5

Hinz, K.

126

4(a)16

Keays, R.R.

274

343

1(d) 2

Hitchcock, P.

50

1(h) 2

Keene, J.B.

121

4(a)14

Hochman, M.B.M.

270

SGI. 4

Kelso, I.J.

305

1(f) 7

Holcombe, R.J.

96

2(c) 13

Kistler, R.W.

186

2(b) 19

Hollis, J.D.

165

l(e)3

Klootwijk, C.T.

78

4(c)2

Hollis, J.D.

281

1(d) 2

Koul, S.L.

50

4(c)12

Hollis, J.D.

294

1(d)11

Kwak, T.A.P.

61

4(c) 5

Hughes, F.E.

284

1(c) 16

Laing, A.C.M.

42

4(b) 2

Hunt, J.

278

Kg) 6

Lambeck, K.

109

3(b) 4

Hutton, J.T.

215

4(a)12

Lambert, I.B.

267

3(b) 8

Hutton, J.T.

218

3 (a) 3

Laurie, J.

198

l(e)8

Idnurm, M.

82

Kd)17

Lawrence, R.W.

67

2{c)6

Jackson, I.

174

SG4.1

Le Maitre, R.W.

325

SG2.2

Jacobson, G.

319

Kb) 3

Leitch, E.G.

11

l(g)12

James, P.M.

119

Kc)19

Leitch, E.G.

46

1(d) 17

James, P.R.

67

3(c)9

Lelievre, H.

229

2(a)l

Jaques, A.L.

135

Kc)14

Lennox, P.G.

39

2(b)l

Jaques, A.L.

141

Kb) 4

Lewis R.J.G.

14

2(b) 3

Jaques, A.L.

142

4(c)6

Lewis, J.D.

286

2(b) 6

Jaques, A.L.

145

4(c) 7

Lewis, J.D.

287

Jaques, A.L.

286

2(c) 15

Liew, T.G.

188

4(c)6 4(c)7

Jaques, A.L.

287

3(d)10

Lindsay. J.M.

247

3(d) 3

Jell, P.A.

236

Kh)3

Lovering, J.F.

123

3 (a) 2

Jenkins, R.J.f.

196

4(b) 3

Lovering, J.F,

279

3(d)l

Jenkins, R.J.F.,

233

2(b) 18

Lupton, J.

164

2(b) 4

Johnson R.W.

144

3(c)8

Mark-Kurik, E.

229

2(b) 6

Johnson, R.W.

145

SGI. 4

Marshall, B.

305

4(a)4

Johnston, C.

255

SG4.3

Marshall, N.J.

326

l(f)4

Johnston, J.D.

90

Kb) 2

Mathur, S.P.

9

3(d) 8

Jones, B.G.

243

SG5,1

Matthews, P.

335

2(b) 17

Joyce. E.B.

163

SG5.6

McBriar, M.

341

SG5.4

Joyce, E.B.

338

2(b) 4

McCulloch, M.T.

44

349

2(b) 7

McCulloch, M.T.

147

4(c) 7

Nelson, D.R.

287

2(b) 10

McCulloch, M.T.

152

1(h) 1

Neumann, R.

121

2(b) 15

McCulloch M.T.

160

1(c)18

Nicholls, I.A.

45

2(c)l

McCulloch, M.T.

167

2(b) 8

Nicholls, I.A.

149

2(c)6

McCulloch, M.T.

174

2 (b) 14

Nicholls, I.A.

158

2(c)7

McCulloch, M.T.

176

l(f)4

Nisbet, B.W.

90

2(c) 14

McCulloch, M.T.

187

SG5.1

O'Brien, M.

335 •

2(c)15

McCulloch, M.T.

188

3 (a) 7

O'Brien, P.E.

204

2(c) 17

McCulloch, M.T.

192

1(d) 12

Offler, R.

62

4(c)6

McCulloch, M.T.

286

1(d)15

Oliver, R.L.

64

4(c)7

McCulloch, M.T.

287

SG5.2

Osborne, R.A.

336

3(d) 9

McDougall, I.

245

Kg) 8

Oversby, B.

112

3(d) 11

McDougall, I.

248

3(d) 9

Owen, M,

245

Kb) 8

McElhinny, M.W.

21

1(h) 6

Packham, G.H.

127

l(e)2

McElhinny, M.W.

77

2(c) 2

Page, R.W.

169

l(e)2

McFadden, P.L. :

77

2(c)15

Page, R.W.

188

2(b) 6

McKee, C.O

145

2(c) 16

Page, R.W.

191

2(c)5

McLennan, S.M.

172

l(e)5

Palmer, H.C.

80

2(c)7

McLennan, S.M. ;

76

3(c)10

Pan Jiang.

229

3 (a) 4

McNamara, G. '

200

1(h) 4

Panggabean, H.

124

1(d) 21

McNaughton , N.J

75

4(b)l

Paton, M.

277

2(c) 3

McNaughton , N.J

170

2(b) 5

Pearson, N.J.

144

1(h) 1

Miller, D.

121

2(b) 13

Pearson, N.J.

157

3(b) 3

Milnes, A.R.

214

1(d)12

Pemberton, J.

62

4(c) 11

Moore, A.C

292

2(b) 4

Perfit, M.R.

144

4(c)9

Moore, D.H

289

4(a)13

Perkin, D.J.

269

1(h) 3

Moore, M.E

123

1(h) 4

Pigram, C.J.

124

i(g)2

Moore, R.F

104

SGI. 4

Plibersek, P.F.

305

•

•

•

2(b) 20

Morris, P.

166

4 (a) 2

Plimer, I.

252

4 (a) 5

Murray, A.S.

257

l(g)l

Plumb, K.A.

103

l(e)9

Musgrave, R.J.

83

SGl.l

Pogson, D.J.

301

1(h) 8

Mutter, J.C.

129

4 (a) 7

Potter, T.F.

260 32 44

l(f)8

Myers, J.S

97

l(c)8

Powell, C.McA.

SG4.6

Naschwitz, W.

330

l(c)17

Powell, C.McA.

l(f)3

Needham, F..s.

90

4(c) 17

Needham, P..s.

299

1 Lecture Price, R.A. Price, R.A. l(b)l

•

1

9

350

l(c)5

Price, R.A.

29

2(b) 18

Stone, J.O.

164

l(e)7

Pritchard, T.

82

4(c)4

Stracke, K.J.

283

l(c)13

Qureshi, I.R.

38

SG4.2

Stroud, J.S.

325

2{c)9

Reid, E.J.

179

Kf)5

Stroud, W.J.

92

Kb) 7

Rickard, M.J.

19

Kf)3

Stuart-Smith, P.G.

90

l(c)ll

Rickard, M.J.

37

4(c)17

Stuart-Smith, P.G.

299

1(c) 11

Rixon, L.K.

37

Kc)4

Sukamto, R.

27

1(h) 1

Rovira, A.

121

4 (a) 7

Sun, S.S.

260

1(d) 7

Rubenach, M.J.

56

SGI. 6

Sun, Shen-Su.

307

l(f)9

Sandiford, M.

99

SGI.12

Suppel, D.W.

316

SGI. 4

Sangameshwar, S.R.

305

2(b) 19

Sutherland, F.L.

165

3(c)7

Sappal/ K.K.

227

4(c) 2

Sutherland, F.L.

281

2(b) 16

Sawka, W.

161

Kh)5

Symonds, P.A.

126

Kb) 3

Scheibner, E.

11

3(c) 3

Talent, J.A.

223

SGI. 2

Scheibner, E.

302

3(a)l

Tavcar, B.

195

SGI.10

Schmidt, B.L.

313

Ki)3

Taylor, B.

134

Ke)4

Schmidt, P.W.

79

3(a)13

Taylor, G.M.

210

SGI. 5

Seccombe, P.K.

307

Kg) 9

Taylor, G.R.

114

Kd)5

Sheraton, J.W.

53

3(b) 6

Taylor, G.R.

217

Kg) 2

Simpson, C.J.

104

3(b) 6

Taylor, Graham

217

4(c) 13

Smith, I.E.M.

295

4 (a) 3

Taylor, J.

254

4(c)4

Smith, B.H.S.

283

2(c)5

Taylor, S.R.

172

4(c)5

Smith, C.B.

284

2(c)6

Taylor, S.R.

174

4(c)6

Smith, C.B.

286

2(c)7

Taylor, S.R.

176

SG4.4

Smith, R.E.

328

Kf)2

Teyssier, C.

88

2(c) 10

Solomon, M.

181

Kc)18

Thomas, L.

458

3 (a) 3

Stait, B.

198

Kd)6

Thornett, J.R.

55

3(c) 2

Stait, B.

221

2(b) 18

Torgersen, T.

164

Ka)4

Stephenson, R.

6

Kc)4

Trail, D.S.

27

Kf)5

Stevens, B.J.P.

92

Ke)l

van der Voo, R.

77

2(c)9

Stewart-Richardson, D.179

Kc)6

VandenBerg, A.H.M.

30

Kd)13

Stewart, A.J.

63

Kc)18

VandenBerg, A.H.M.

45

Kf)l

Stewart, A.J.

87

3(d) 5

VandenBerg, A.H.M.

238

3 (a) 4

Stewart, I.

200

2(b) 9

Varne, R.

151

4(a)8

Stockley, J.

262

2(b) 11

Varne, R.

153

Ka)2

Stolz, A.

5

2(c)10

Varne, R.

±81

351

2(c) 12

Vernon, R.H.

185

3(d) 6

Williams, I.S.

240

SGI. 7

Yokes . F.

309

Kf)5

V^illis, I.L.

92

l(c)2

von der Borch

23

Kc)15

Willmon, C.

41

1(h) 1

von der Borch

121

SG5.1

Willmott, W.F.

335

l(a)l

Walcott, R.

5

Kd)21

Wilson, A.F.

75

l(c)15

Wall, V.J.

41

Kc)18

Wilson, C.J.L.

45 155

1(d) 2

Wall, V.J.

50

2(b)12

Wilson, I.H.

1(d)10

Wall, V.J.

60

Kd)19

Windrim D.P.

72

1(d) 20

Wall, V.J.

73

2(c) 17

Windrim, D.P.

192

l(f)4

Wall, V.J.

90

4(a)10

Windrim, D.P.

264

2(b) 15

Wall, V.J.

160

2 Lecture Winterer, E.L.

2(c)8

Wall, V.J.

178

4(a)4

Witt, W.K.

1 255

2(c)12

Wall, V.J.

185

3(d) 4

Wright, A.J.,

37

4 (a) 3

Wall, V.J.

252

3(d) 7

Wright, A.J.

242

4 (a) 6

Wall, V.J.

259

3(d) 8

Wright, A.J.

243

4(a)7

Wall, V.J.

260

2(c)ll

Wyborn, D.

183

4(b)l

Wall, V.J.

277

3(d) 9

Wyborn, D.

245

SG2.3

Warner, R.F.

320

Kd)8

Wyborn, L.A.I.

58

1(d) 9

Warren, R.G.

59

2(c) 16

Wyborn, L.A.I.

191

1(d)17

Warren, R.G.

69

Kc)9

Yeates, A.N.

33

1(d) 18

Warren, R.G.

70

3(a)5

Yeates, A.N.

202

2 (a) 4

Wass, S.Y.

139

4 (a) 5

Yeates, A.N.

257

4(c)12

Wass, S.Y.

294

3(c) 11

Young, G.C.

231

SG2.4

Wasson, R.J.

321

3(d) 2

Zhang, Z.C.

235

3(a)6

Webby , B.D.

203

1(h) 8

Weissel, J.K.

129

l(i)l

Weissel, J.K.

133

l(a)5

Wellman, P.

7

2 (a) 2

Wellman, P.

137

2(b) 9

Wheller, G.

151

1(c)18

White , A.J.R.

45

4(c) 14

White , A.J.R.

296

Kg) 3

White , s.

105

2(b) 8

Whitford, D.J

49

Kb) 4

Williams, E.

14

2(c)2

Williams, I.S

169