Abstract 56 - Two Billion Years of Tectonics and Mineralisation Perth Sept 1999 by GSAustralia

Geological Society of Australia

ABSTRACTS Number 56

Two Billion Years of Tectonics and Mineralisation

Perth 6-7 September 1999

GEOLOGICAL SOCIETY OF AUSTRALIA ABSTRACTS NO 56 Two Billion Years of Tectonics and Mineralisation, Perth, September 1999

ISSN 0729-01IX © Geological Society of Australia Incorporated 1999 Copies of this publication may be obtained from the Geological Society of Australia Incorporated, 1203 Wynyard House, 301 George St, Sydney, NSW, Australia 2000 Example citation for papers in this volume: Rosen, O.M., Sukhanov, M.K., Zhuravlev, D.Z., Bibikova, E.V. and Zlobin, V.L., 1999, Late Palaeoproterozoic amalgamation of the North-Eastern Siberian Craton: Ancient terranes, shear zones and granites melt out (a synthesis of Sm-Nd geochemistry and U-Pb dating). Geological Society of Australia, Abstracts No 56, pp 69.

Two Billion Years of Tectonics and Mineralisation Tectonics Special Research Centre Conference Proceedings School of Applied Geology , Curtin University of Technology, Perth, WA.

September 6th & 7th, 1999

Abstract Volume

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Editors: G. R. Watt & D. A. D. Evans

Geological Society of Australia Abstracts Series #56 ISSN 0729-011X

Two Billion Years of Tectonics & Mineralisation Abstracts Convergence, interaction and boundaries in mineralised terranes J. BAXTER

Source terranes for Laurentian margin sediments: constraints from U-Pb dating in the Newfoundland Appalachians. P. A. CAWOOD & A. NEMCHIN

Neoproterozoic extension and Early Paleozoic orogeny on the Scottish Promontory of Laurentia: paleogeographic and tectonic implications. I. DALZIEL & J. SOPER

Processes relating Proterozoic supercontinents, ice ages, and Superior-type BIFs. D. EVANS

Newly discovered early metamorphic history of the Scottish Caledonides: The place of the Moine Supergroup in the Taconic. C. R. L. FRIEND, K. A. JONES & I. M. BURNS

Folding as a result of lower crustal flow during rifting: A new technique of centrifuge modelling applied to the study of high-grade gneiss terrains. L.B. HARRIS & H.A. KOYI

Landsat thematic mapper as a regional mapping and exploration tool in the Late Archaean to Palaeoproterozoic Hamersley Province, W.A. D. HOLLINGSWORTH, R. HICKEY & P. A. CAWOOD

Palaeomagnetic Evidence for united North and West Australian Cratons by ca. 1.7 Ga. Z. X. LI

Deformation and metamorphism during the c.2000 Ma Glenburgh Orogeny and ca. 1800Ma Capricorn Orogeny. S. A. OCCHIPINTI, S. SHEPPARD, I. TYLER & D. NELSON.

Geology, mineralisation and geodynamic evolution of the Palaeoproterozoic Yerrida and Earaheedy Basins, W.A. F. PIRAJNO

Tectonics Special Research Centre, School of Applied Geology, Curtin University of Technology, September 1999

Australia. S. PISAREVSKY & L. HARRIS 40

Ar/39Ar laserprobe dating: Applications to metamorphic and deformation

histories. STEVE REDDY Late Paleoproterozoic amalgamation of the north-eastern Siberian craton:

ancient terranes, shear zones and granites melt out (a synthesis of Sm—Nd and U—Pb dating). 0.M. ROSEN, M.K. SUKHANOV , D.Z. ZHURAVLEV, E.V. BIBIKOVA & V.L. ZLOBIN Granites of the southern Capricorn Orogen, Western Australia

S. SHEPPARD, S. A. OCCHIPINTI, D. R. NELSON, & I. M. TYLER Palaeoproterozoic orogeny in Western Australia

1. M. TYLER The spatial distribution of mineralization

J. VEARNCOMBE AND S. VEARNCOMBE Early Neoproterozoic metamorphism in central East Greenland: implications

for Rodinian reconstructions. G. R. WATT & K. THRANE SHRIMP U-Pb analysis of baddeleyite: crystal orientation effects and

implications for geochronology. M.T.D. WINGATE & W. COMPSTON High-pressure metamorphism of mafic granulites from the Trans-North China

Orogen: Implications for Palaeoproterozoic amalgamation of the North China Craton. G. ZHAO Reconstructions of global 2.1—1.8 Ga collisional orogens and associated

cratons: implications for two pre-Rodinia supercontinents? G. ZHAO, P. A. CAWOOD & S. A. WILDE Paleoproterozoic khondalites in the western block of the North China Craton:

P-T-D path and tectonic implications. G. ZHAO, S. A. WILDE & P. A. CAWOOD

Two Billion Years of Tectonics & Mineralisation

Convergence, interaction and boundaries in mineralised terranes. JOHN BAXTER Continental Resource Management Pty Ltd. Ore deposits occur as anomalous zones within

sis for creating a model of the rheological re-

the earth's crust. Mineralisation is localised by

sponse of a package of mineralised rocks is to

convergence of processes that have demon-

examine the following aspects:

strated interdependence. The fascination of

•

studying mineralised terranes is the recognition

the more strongly cleaved rock will be the more ductile;

that the whole is greater than the sum of the parts, there is emergence of ore deposits in re-

•

or chemical interactions. Patterns form that are

•

recognised in many deposits, although often elements of the pattern will appear unique to a particular deposit when viewed in isolation. There are

extension veins and extension shears will develop in the strongest rock unit;

sponse to the combined effect of simple physical

larger wave length and lower amplitude folds develop in strong rocks;

•

faults in brittle rocks may be accommodated by folds in ductile rocks.

numerous factors that contribute to the formation of the patterns eg. differential stress, hydrostatic

The Mohr diagram is a simple graphical tech-

pressure, periodicity and duration of seismic ac-

nique to examine the effects of rock strength and

tivity, fluid chemistry and host rock reactivity. It is

the types of fracture likely to develop under vari-

not uncommon for the mineralising process to be developing in conditions far from equilibrium, even though conditions are relatively stable.

ous differential and effective stress conditions. As differential stress increases, cleavage develops in the weaker rock units approximately perpendicular to Si while no deformation is apparent in the

In this paper I will empirically examine some of

stronger unit. With increasing fluid pressure there

the toolkit aspects of the process and some of the

is a reduction in the effective stress until a critical

resulting geometry. I will endeavour to identify

state is reached when the rock will rupture. The

patterns which can be observed and which assist

Mohr diagram predicts that rupture will occur in

the process of discovering mineralisation.

the stronger rock unit.

Mohr diagram

Immediately after rupture the differential stress is

If a shear zone, or fault controls mineralisation

reduced, the fluid pressure relieved and the effec-

the rheology contrast in the geological sequence

tive stress increased as the rock unit reverts to a

is often the primary focus of ore deposit. Assess-

relaxation state. The fault formed during rupture

ing the difference in strength of rocks is difficult,

will now be a cohesionless plane in the terrane.

as it is not the absolute strength that is important,

Continued invasion of hydrothermal fluids into an

but the contrast with adjacent rock units. The ba-

environment characterised by lower differential

Tectonics Special Research Centre, School of Applied Geology, Curtin University of Technology, September 1999

stress will see failure occurring on the cohesion-

ding will plot on a girdle with an error of

less fault and possibly shear veins within the

±20°;

stronger units. Although the mechanism is un-

•

clear it appears that once the fault planes develop

stretching lineations, or slip directions will plot on this girdle;

the differential stress state that can be maintained by a rock mass is significantly reduced. As a re-

•

sult any subsequent build up of hydrostatic pressure will have the effect of causing failure in the

•

minerals.

all fabrics will have consistent kinematic and metamorphic characteristics.

rock mass with tension and develop dilation sites into which mineralising fluids may deposit ore

related intersection lineations and fold hinges will plot near the pole to the girdle;

When these simple criteria are met, which is very often in mineralised terranes, it can be assumed

The fault planes provide the perfect fluid access

there is a single, probably progressive, deforma-

to the dilational shears. The effect of the second,

tion event. Under normal brittle-ductile conditions

third, and subsequent ruptures will be to system-

mineralisation will favour a dilational orientation

atically reduce the deviatoric stress. This sequen-

and shoots will develop sub-parallel to the pole to

tial decrease in deviatoric stress when accompa-

the girdle.

nied by increasing fluid pressure and decreased effective stress will ultimately lead to extensional structures late in the deformation history.

As our mapping database increases it is becoming more obvious that this simple pattern is only one part of the story. Frequently it can be demon-

Geometry and the stereographic net

strated that unmineralised transfer faults are de-

Faults and shears are rarely planar. More often

veloped in the terrane with kinematic, metamor-

there a number of fracture planes oriented in a

phic and age criteria similar to the mineralised

predictable array. Orientation measurements of

structures. It is common for the stereographic net

fabrics within fault zones can be displayed for

arrays of the mineralised and transfer structures

analysis on a stereographic net. In a homogene-

to share a common plane that is often an unmin-

ous rock type there is simple development of a

eralised fault sub-perpendicular to Si. The result-

Tchalenko pattern with common angles between

ing bow and arrow pattern on a stereographic net

different predicted planes. However, most miner-

is often the first indication of this developing.

alisation occurs where there is marked variation in rock type and consequent heterogeneous deformation. The result is usually seen as significant noise on the predictable distribution. The Tchalenko pattern provides a strange attractor to the distribution of observations developing simple patterns. Effective interpretation on a stereographic net can be made using the following guidelines: •

poles of faults, veins, cleavage and bed-

Two Billion Years of Tectonics & Mineralisation

Some observations of fabrics and effects of fluids Prior to rupture rocks respond to applied stress by deforming in an either elastic or visco-plastic manner. The resultant fabric from elastic deformation is not observed. Visco-plastic deformation produces cleavage and folds when the deformation is homogeneous. However, if the deformation is inhomogeneous (the most common case) planar zones of mylonitic fabrics will develop. The

only mineralisation likely to be affected by this de-

the fluids and rocks in these deposits are far from

formation is that which is pre-existing eg. VMS

equilibrium even though in an induced stable

deposits. Mineralising fluids are not likely to be

state.

concentrated into an ore body in these conditions. At the time of rupture the combination of the dif-

The Mohr diagram highlights the importance of faults in the transfer of hydrothermal fluids in a

ferential stress state and the hydrostatic pressure

mineralising system. The spatial distribution of

instigate fault planes or shear zones within the

faults is affected by:

rock. The effect is development of cohesionless

•

rheology of the host rocks;

fault planes that have a predictable angle from the principal applied stress Si within specific rocks

•

provide excellent channelways for fluid to be pumped into the system during deformation. However, it is expected the fluid will be expelled at the conclusion of the seismic event as there are no inherent dilation sites on these faults.

variation of rheology and unit thickness within the stratigraphic pile;

identified from the Mohr diagram. These faults •

orientation of pre-existing planes with respect to the applied stress field in the host rock mass.

Co-axial and non-coaxial fault patterns form in response to these factors within relatively homoge-

For the duration of the hydrothermal event,

neous rock types. In a package of rocks with vari-

50,000 to 800,000 yr (Cathles et al., 1997), there

ous lithologies it is often necessary for heteroge-

will be repeated pulses of hydrothermal fluids ac-

neous deformation to accommodate movement

companied by increase in the hydrostatic pres-

on the mineralised fault arrays by development of

sure. The seismic events will be separated by pe-

transfer faults.

riods of ductile deformation within the host rock. The net result is observed as a complicated shear zone displacement history such as that seen in the Golden Mile, Kalgoorlie, Western Australia (Phillips, 1986; Baxter et al, in prep). Significantly whenever the faults are present they provide a

It is a common observation in ore deposits controlled by or associated with development of faults, that the fault planes are not mineralised and often alteration (chlorite or carbonate) is limited. From the Mohr diagram it can be seen that the early faults, which are cohesionless surfaces,

fluid pathway, and with lower acquired or induced

always have positive normal stress acting on

differential stress they are the first zones to fail

them and consequently are unlikely to dilate.

thus providing fluid access to rock units which have rheology that causes deformation with dilatancy. An excellent example of this process is seen in the Mount Charlotte gold mine. The unmineralised Flanagan, Neptune and Shea faults provide fluid access to a rock unit (Unit 8 of the Golden Mile Dolerite) which deforms by dilation during the peak of the hydrothermal event and highest fluid pressure. It is worth observing that

Discussion This simple demonstration of the interaction between rheology and deformation which can be shown graphically on a stereographic net and Mohr diagram identifies the feedback that occurs during the passage of a hydrothermal fluid along a fault zone. The manner in which the rocks, and the mineralised environment, respond to the episodic hydrostatic pressure build up and rupture

Tectonics Special Research Centre, School of Applied Geology, Curtin University of Technology, September 1999

appears to be a natural example of a complex adaptive system. Emergent seismic events during deformation are accompanied by different differential stress, different hydrostatic pressure and probably different reactivity of the fluid. The association between the heterogeneous deformation within fault, or shear, zones and the relatively simple distribution of related planes seen on stereographic nets suggests that the Tchalenko distribution is a strange attractor in the pattern of mineralised shears. These empirical observations described herein suggest there are a number of attractors that are identified in mineralised terranes. However, it is most likely that to form an ore body it is necessary for there to be well-established feedback between the fluid composition and hydrostatic pressure with the applied differential stress and rheology of the rock sequence. The next steps in development of this discussion will be to examine thermodynamic implications, metamorphic imprint, geochronology and the empirical observation of mixed results from fluid inclusions in many mineralised terranes. References Baxter, J.L., Sauter, P.C. and Laubsch, N.C., (in prep), Structural control of mineralisation in the Kalgoorlie district, Western Australia Cathles, L.M., Erendi, A.H.J, and Barrie, T., 1997, How long can a hydrothermal system be sustained by a single intrusive event: Economic Geology, v. 92, p.1661-1678. Phillips, G.N., 1986, Geology and alteration in the Golden Mile, Kalgoorlie: Economic Geology, v. 81, p.779-808

Two Billion Years of Tectonics & Mineralisation

Palaeoproterozoic laterites, supergene iron and managnese or^es and atmospheric oxygen. NICOLAS J BEUKES, JENS GUTZMER AND HERMAN DORLAND Department of Geology, Rand Afrikaans University, Auckland Park 2006, South Africa

In this presentation we would like to report on the

enriched Wolhaarkop paleoweathering profile in

discovery of widespread 2,2 - 2,45 Ga pedogenic

the Griqualand West area of the Transvaal basin,

and groundwater paleolaterites of the Pretoria

and the assumption that the Hekpoort paleosol is

Group of the Transvaal Supergroup and how that

older than that of Wolhaarkop (Rye and Holland,

change our understanding of soil forming

1998). However, our regional studies indicated

environments, supergene iron and manganese

that none of these assumptions may be valid and

deposition, and the history of atmospheric oxygen

that in actual fact the Hekpoort paleosol forms part

in the early Paleoproterozoic. The latter is a very

of a ferric laterite succession which is time

contentious issue at present with two opposing

equivalent to the ferruginous Wolhaarkop

models; one by Rye and Holland (1998)

paleoweathering profile; implying highly

suggesting that earth had an essentially anoxic

oxygenated conditions at time of formation. We

atmosphere with p02 < 0,4% of present

also found older ferricretes and groundwater

atmospheric level (PAL) prior to 2,25 Ga followed

laterites in the Pretoria succession indicating that

by a rapid rise in p02 levels to at least 15% PAL in

oxidized terrestrial environments were present to

the period 2,05 - 2,25. The other advocated by Ohmoto (1996) defines an oxic atmosphere with minimum p02 of 1,5% PAL but most probably equal to PAL as far back as 3 Ga. These estimates are largely based on the behaviour of iron in paleosol profiles and the conflicting results depend on models of soil formation applied and whether terrestrial organic matter was involved or not.

at least as far back as 2,45 Ga. The lateritic nature of the Hekpoort paleosol becomes apparent as it is traced north-westward in deep drill core intersections below the unconformity at base of the Dwaalheuwel Formation from near Potchefstroom in South Africa to near Gaborone in Botswana. In this area the paleosol is between 4 and 10 m thick and composed of a red banded and mottled

Our results have implications for both models of

ferruginous upper zone successively grading

atmospheric oxygen development but impacts

down through an iron-depleted bleached pallid

most dramatically on the first. The concept of a

zone and a grey-green saprolite into parent

sudden change from an anoxic to oxic

Hekpoort basalt. It used to be classified as a

atmosphere shortly after 2.25 Ga is based on a 3+

ferruginous zone of the laterite profile had been

comparison of the Fe

-depleted reduced

Hekpoort paleosol of the Pretoria Group, as it appears in the south-eastern Transvaal between Pretoria and Waterval Onder, with the Fe3+-

reduced paleosol simply because the upper

removed by erosion prior to deposition of the Dwaalheuwel Formation in the Pretoria - Waterval Onder area where previous studies took place.

The laterite profile is capped with sharp contact by

paleoweathering profile may have developed as

a reworked laterite composed of red lateritic clay

far back as 2,35 - 2,39 Ga.

clasts derived from the underlying paleosol. It is

Based on the regional correlation and information

intensely indurated with hematite present as

available from Griqualand West, a rather unique

matrix and cement between clay clasts and as

reconstruction is possible of lateral variations in

pisolitic coatings on clay clasts. As such the

soil-forming environments on continent-wide

reworked laterite is best described as a ferricrete

scale in the early Paleoproterozoic. In Griqualand

that draped the erosion surface before deposition 3+

West, close to the western margin of the Kaapvaal

of the overlying Dwaalheuwel Formation. Fe

retaining environmental conditions must have remained in place during formation of the Dwaalheuwel cover beds which we now know to represent a very extensive succession of fluvial red beds; thickening and coarsening northwestwards from the Carletonville-Pretoria area into Botswana. Towards the east and southeast the red beds interfinger with marine quartz arenite which directly overlies the Hekpoort paleosol at

craton, Transvaal strata were buckled into wide open folds before or during peneplanation along the Wolhaarkop-Hekpoort erosion surface so that ferruginized paleoweathering profiles are developed in a variety of rock types. This is in contrast to the eastern interior of the craton where the beds have only been gently tilted during uplift so that the erosion surface gradually transects Hekpoort lava in northern direction. Two types of paleoweathering profiles are

Waterval Onder.

preserved in Griqualand West namely ferruginous

This newly established association of laterites with

saprolite and karstic laterite. The latter is

red beds in the Pretoria Group is similar to that of

developed where the erosion surface transects

the ferruginous Wolhaarkop paleo-weathering

Campbellrand dolomite and comprises ancient

profile and overlying Mapedi/Gamagara red beds

manganese earth deposits and giant Sishen-type

in Griqualand West (Gutzmer and Beukes, 1998),

hematite ore deposits derived from supergene

and allows correlation of the successions. The

leaching of manganiferous dolomite beds

revised correlation implies that the Hekpoort and

(Gutzmer and Beukes, 1996) and Asbesheuwels

Wolhaarkop paleoweathering profiles are time

iron-formation (Van Schalkwyk and Beukes, 1986)

equivalent and developed below a single

respectively. In areas outside of the karstic

ferruginized erosion surface that must have

environment virtually all of the Wolhaarkop

covered an area of at least 500 000 km on the Kaapvaal craton in early Paleoproterozoic times. The absolute timing of the laterization event is somewhat uncertain. Previously it was thought that the Hekpoort paleosol formed shortly after outflow of the Hekpoort lavas at 2,22 Ga. However, recent geochronological data suggests that the Hekpoort-Ongeluk succession has an age of between 2,39 and 2,41 Ga (Romer and Bau, 1998) and thus that the Hekpoort-Wolhaarkop

paleoweathering profiles lack preservation of upper paleosol zones and only highly oxidized hematite-enriched saprolites between 5 and 250 m thick and retaining original textures of parent rock, are preserved below the Gamagara/ Mapedi red beds (Holland and Beukes, 1990). Eluvial breccias and/or alluvial conglomerates, composed of clasts derived from the underlying ferruginous saprolite and duricrust, form the base of the overlying red beds.

Vast amounts of ferric iron and alumina-rich clays

atmospheric composition. Published results from

thus accumulated in the Hekpoort-Wolhaarkop

Wolhaarkop indicate that atmospheric oxygen

paleosol, a situation typical of modern soils formed

levels must have been at least 1 5 - 2 0 percent

under oxidizing atmospheric conditions in humid

PAL (Holland and Beukes, 1990) to have retained

tropical climates, with a long dry season. A tropical

iron in saprolite derived from carbonate-oxide

setting is supported by paleomagnetic data

facies iron-formation. However, these are

indicating that the Ongeluk lavas below and the

absolute minimum levels permitted by the data

Gamagara/Mapedi red beds above the Hekpoort-

and for carbonate-facies iron-formation, oxygen

Wolhaarkop erosion surface formed within 119 + Q

5 of the equator (Evans et al, 1997). In this near

levels equal to or slightly above PAL are required to explain the retention of ferric iron in the

equatorial environment, lateral variation in the

saprolite at Wolhaarkop. Most important these

composition of paleoweathering profiles was

oxygen concentrations were calculated at pC02

apparently controlled by the nature of parent rock

levels of 1 - 10 PAL and if higher CO2 values are

and tectonic setting.

used, oxygen levels required for iron retention are considerably above PAL.

Although common in modern tropical environments, the origin laterite profiles with

Highly oxygenated tropical lateritic environments

reduced pallid zones are poorly understood.

were thus present on the Kaapvaal craton some

However, under oxidizing atmospheric conditions

time after extrusion of the Ongeluk/Hekpoort

the leaching of iron from pallid zones of laterite

lavas at _ 2,4 Ga. Such conditions may, however,

profiles can be explained by lateral flow of reducing groundwaters with organic acids as

have developed earlier as indicated by our studies of the 2,41 - 2,45 Ga hematite oolite ironstones of

reducing agents. Soil-forming environments and

the Timeball Hill Formation of the Pretoria Group.

processes along the Hekpoort-Wolhaarkop

The ironstones covers about 100 000 km and

erosion surface were thus highly variable and

are associated with delta front and delta ditributary

complex, similar to modern soils. Extreme care

channel sands on two major delta lobes in the

should therefore be taken before any attempt is

Timeball Hill Formation. They are composed of

made to link the composition of the paleosol with

hematite oolites mixed with hematite-coated

ancient atmospheric conditions. Certainly our new

quartz grains set in a hematite-rich matrix.

insights invalidate any earlier estimates of

Centimetred sized in-situ hematite-coated

atmospheric oxygen concentrations by Rye and

pisoliths, similar to that of modern groundwater

Holland (1998) that were based on the

laterites, are abundant in some of the oolitic

composition of the pallid zone of the Hekpoort

channel sand deposits. In addition a lateritic mud-

paleosol, without consideration of the overlying

clast conglomerate cemented by large hematite

hematite-indurated laterite zone and possible

pisoliths overlies delta plain muds with a sharp

effects of terrestrial organic matter and reduced

erosional contact in the Pretoria area. It is thought

groundwater. However, in well-drained

to represent a pisolitic ferricrete that developed on

paleoweathering profiles, like that of Wolhaarkop,

an abandoned delta plain and together with the

rain- and groundwater may have been in

oolitic and pisolitic hematite ironstones could only

equilibrium, and could provide clues on

have formed under oxygenated terrestrial environments in the period 2,41 - 2,45 Ga.

The above seriously question Rye and Holland's (1998) conclusion that the early Paleoproterozoic atmosphere contained extremely low oxygen

uraninite and siderite in Neoarchean Mesoarchean fluvials suggest lower atmospheric oxygen levels that at present.

levels based on the assumption that reduced paleosols of that time allow estimation of maximum atmospheric p02 levels from ratios of oxygen

References

versus acid demand in parent rock. Rather our

Evans, DA, Beukes, NJ and Kirschvink, JL, 1997,

data suggest that paleosols in a specific period of

Nature, 386, p. 262.

time in the Paleoproterozoic and/or along the

Gutzmer, J and Beukes, NJ, 1996, Econ. Geol.,

same unconformity may range from oxidized to

91, p. 1435.

reduced depending not only on the redox state of the atmosphere as Rye and Holland (1998) would like to have it, but also on composition of parent rock, tectono-environmental and climatic setting,

Gutzmer, J and Beukes, NJ, 1998, Geology, 26, p. 263. Holland, HD and Beukes, NJ, 1990, Am. J. Sci.,

soil water drainage patterns, abundance of

290A, p. 1.

terrestrial organic matter and/or erosional

Ohmoto, H, 1996, Geology, 24, p. 1135.

preservation of soil horizons. The true history of atmospheric oxygen remains speculative back in time from the Hekpoort laterites at 2,35 - 2,4 Ga when present levels appear to have been established. In older rock successions the composition of terrestrial alluvium may prove more valuable in fixing upper oxygen levels. For example the presence of detrital pyrite,

Romer, RL and Bau, M, 1998, Chinese Sci. Bui., 43 Supplement, p. 109. Rye, R and Holland, HD, 1998, Am. J. Sci., 298, p. 621. Van Schalkwyk, JF and Beukes, NJ, 1986, Mineral Deposits of Southern Africa, Geol. Soc. S. Afr., Johannesburg, pp. 931-956.

Source terranes for Laurentian margin sediments: constraints from U-Pb dating in the Newfoundland Appalachians. PETER A. CAWOOD & ALEXANDER A. NEMCHIN TSRC, School of Applied Geology, Curtin University, GPO Box U1987, Perth 6845, W.A., Australia. The Humber Zone or miogeocline of the Appalachian/Caledonian orogen represents the eastern margin of Laurentia and preserves a record of continental margin initiation and destruction associated with opening and closing of the lapetus ocean. The Humber zone consists of crystalline basement of the Grenvillian province unconformably overlain by a cover sequence containing rift, continental margin and foreland basin units that are in places structurally overlain by a transported succession (Taconian allochthons) which includes facies equivalents of the cover sequence capped by ophiolitic rocks which represent vestiges of lapetan lithosphere (Dunnage Zone). The SHRIMP II ion microprobe at Curtin University was used for U-Pb dating of 341 detrital zircon grains from 6 samples from the Laurentian margin sequence in Newfoundland. These zircons range in age from 3592 ±5 Ma to 572 ±14 Ma. Three of the analysed samples are from the rift-related sequence (Summerside, Blow-MeDown and South Brook formations), two are from the drift sequence (Bradore and Hawke Bay formations), and one is from the foreland basin succession (Goose Tickle Formation). U-Pb data is divisible into 4 major age components: 1) Archean grains with maximum frequency between 2600 - 2850 Ma 2) Paleoproterozoic age detritus ranging from 1750- 1950 Ma

3) Predominantly Mesoproterozoic detritus ranging in age between 950 -1450 Ma 4) Neoproterozoic detritus between 570 - 760 Ma. The relative proportions of these four components vary between samples. Mesoproterozoic age detritus is present is all samples whereas Neoproterozoic detritus is restricted to rift-related samples and two samples, one each from the rift and drift successions, lack Paleoproterozoic and Archean detritus. Each of these age groupings can be linked to specific source regions within the Laurentian hinterland: Archean detritus corresponds with the age of major magmatic and tectonothermal pulses related to accretionary events in the Superior craton, Paleoproterozoic ages correspond with the orogenic belts marginal to the craton (e.g. Ungava, New Quebec and Torngat), Mesoproterozoic age detritus corresponds with the age of the Grenville Orogen, and Neoproterozoic detritus with rift-related igneous activity along the Laurentian margin within the Appalachian orogen. The youngest grains in the rift sediments are dated at 570-580 Ma and provide a maximum age for accumulation of these units. There is no evidence, or requirement, from the available age data to invoke input from crustal blocks which may have lain outboard of Laurentia prior to formation of the lapetus Ocean (e.g. Baltica and Amazonia).

Tectonics Special Research Centre, School of Applied Geology, Curtin University of Technology, September 1999

Neoproterozoic extension and Early Paleozoic orogeny on the Scottish Promontory of Laurentia: paleogeographic and tectonic implications. 1

I.W.D. DALZIEL & 2N.J. SOPER

1nstitute for Geophysics, University of Texas at Austin, 4412 Spicewood Springs Rd., Bldg. 600, Austin, TX 78759-8500 USA, Phone: 512-471-0431, Email: ian@utig.ig.utexas.edu. Gams Bank, Thresh field, Skipton BD23 5NP United Kingdom, Phone/Fax: +44 01756 752217.

The Hebridean shield, which constitutes the

setting in the Andes. Deformation and

northwest foreland of the Caledonian orogen of

gneissification took place during the early

Scotland, is a small fragment of Laurentia that

(Grampian) phase of the Caledonian orogeny.

became detached during the Cenozoic opening of

Our interpretation leads to a substantial

the North Atlantic Ocean basin and is now part of

simplification of the geologic history of the

Europe. It was located at the tip of a major

Scottish Promontory of Laurentia, comprising

promontory of the ancestral core of North

protracted Neoproterozoic extension followed by

America, Laurentia, between the Labrador and

Early Paleozoic Taconic-style arc-continent

Greenland margins. The history of this Scottish

collision. The protracted rift history was bimodal,

promontory is important to understanding of

with episodes of major extension in the Riphean

paleogeography and tectonics in late

and Vendian. These episodes coincide with the

Precambrian and Early Paleozoic times. Isotopic

two-stage breakout of Laurentia from the

ages and structural complexities in the Moine and

Rodinian and Pannotian supercontinents during

Dalradian supergroups of the Caledonian orogen

the Neoproterozoic.

have been interpreted over the past forty years as reflecting Neoproterozoic orogenic episodes overprinted by the Early Paleozoic deformation and metamorphism. The West Highland Granite Gneiss has been viewed as a synorogenic intrusion, and its -870 Ma U-Pb zircon age has been interpreted as dating a 'Knoydartian' orogeny. New field evidence shows that the granitic protolith of the gneiss was emplaced before a regional suite of tholeiitic dikes which carry all the regional deformation. The zircon age thus reflects the crystallization of an extensionrelated anatectic melt, not its subsequent gneissification. Melting resulted from advection of heat by emplacement of basaltic magma deep within the Moine sedimentary pile. This interpretation is supported by comparison with a Mesozoic analog in a well established tectonic

Two Billion Years of Tectonics & Mineralisation

Processes relating Proterozoic supercontinents, ice ages, and Superior-type BIFs. DAVID A. D. EVANS. TSRC, Dept. of Geology and Geophysics, UWA, Nedlands, WA 6907.

Proterozoic glacial deposits and Superior-type banded iron-formations (BIFs) are bimodally distributed with peaks during the early Paleoproterozoic and the mid-Neoproterozoic (Hambrey and Harland, 1981; Klein and Beukes, 1992). With such a coincidence at this broad temporal resolution, one might expect a common causal relation between the two types of deposits, and the most obvious candidate would be some

semi-quantitative geological comparisons (e.g., Hoffman, 1989; Roscoe and Card, 1993; Rogers, 1996; Aspler and Chiaranzelli, 1998) rather than paleomagnetism. An exception to this general rule is the decades-old, paleomagnetically based reconstruction by Piper (1982) that represents only one of many possible interpretations of the global database. Among common usage, a "supercontinent" is an

kind of tectonic forcing due to supercontinental

ill-defined entity. One can envisage a

evolution. Indeed, both glacial episodes seem to

supercontinent in the traditional, paleogeographic

have occurred during the waning stages of a

sense, whereby all or nearly all of the continents

long-lived supercontinent (Young, 1991).

are conjoined, encircled by a continuous shelfal

Stratigraphic relations between the glacial

area; such was Wegener's "Ur-Kontinent", or

deposits and the BIFs, however, are

Pangea, existing for a brief interval of early

fundamentally different between the

Mesozoic time. A paleogeographically defined

Paleoproterozoic and Neoproterozoic groups.

supercontinent will influence the biospheric

This paper explores these global-scale relations

evolution and production profoundly, blocking

and speculates upon their causes and effects.

oceanic currents and allowing direct contact among marine and terrestrial biota (c.f. Klein,

Supercontinents

1994). In another sense, supercontinents may be

Global paleogeography for the 95% of Earth

defined geodynamically. If a continental region is

history preceding Pangea is only beginning to

large enough, long-lived, and drifting slowly over

come into focus. Presently favored models for

the asthenosphere, its underlying mantle will be

the late Mesoproterozoic to early Neoproterozoic

isolated from subducted slab penetration and

supercontinent, Rodinia, were initially constructed

thus will not be cooled directly, for perhaps 100s

using geological constraints (Bond et al., 1984;

of Myr. This effect can generate true polar

Moores, 1991; Dalziel, 1991; Hoffman, 1991) and

wander (TPW) and, eventually, supercontinental

only subsequently verified quantitatively through

breakup (Anderson, 1994; Evans, 1998), both

paleomagnetism (Powell et al., 1993; Weil et al.,

processes indirectly affecting the biosphere

1998). Likewise, speculations of

through paleogeographic changes. Because of

Paleoproterozoic supercontinents are based on

our merely rudimentary constraints on Proterozoic

Tectonics Special Research Centre, School of Applied Geology, Curtin University of Technology, September 1999

supercontinental configurations, the geodynamic

supercontinental assemblies are extremely

definition is currently more useful - we may not

conjectural.

be able to re-position every cratonic block precisely, but we may be able to identify long-

Ice ages

lived and large crustal entities such as

It is intriguing that both glacial epochs of the

Gondwanaland and Rodinia, that influenced

Proterozoic Eon occurred during a time of

mantle convection in the manner described

apparent supercontinental fragmentation (Young,

above. Given the geodynamic definition of supercontinents, it appears that three such entities existed during the Proterozoic. The latest was Rodinia (ca. 1.1-0.7 Ga), whose paleogeography may be correct to first-order: at least we may be confident that Laurentia was near its center, given the well defined, -10,000 km of late Neoproterozoic rifted margins encircling that craton. Definition of the earlier supercontinents that begat Rodinia is more problematic. Rodinia assembled along late Mesoproterozoic orogenic belts, presumably at the expense of early Mesoproterozoic ocean basins. Farther back in time, a global preponderance of 2.0-1.8 Ga orogenic belts could suggest widespread late Paleoproterozoic continental amalgamation. There may have been, therefore, a supercontinent formed by 1.8 Ga ("Nuna"; Hoffman, in press), but the system of Paleoproterozoic-Mesoproterozoic rifted margins

1991). According to models of whole-mantle convection and the geoid, we would expect that a long-lived supercontinent in the geodynamic sense would lie atop mantle upwellings (induced by slab isolation) and migrate to the equator, via TPW, prior to breakup (Anderson, 1982,1994). According to paleomagnetic data, this appears true for Pangea as well as Rodinia (Evans, 1998), and if the low-latitude determinations for many Paleoproterozoic igneous suites are any indication, for Kenorland and Zimvaalbara as well. A long-lived supercontinent at low latitudes may induce glaciation through extensive tropical weathering of its exposed continental shelves, drawing down atmospheric C0 2 (Young, 1991). Alternatively, or in combination with that process, the albedo effects of a broad equatorial landmass may cool the planet sufficiently to generate extensive polar sea ice and, eventually, globally widespread glaciation. The paleoclimatological issue of low-latitude

manifesting its demise has yet to be identified

Proterozoic ice ages is widely debated (Hoffman

among the world's cratons. Preceding Nuna, the

et al., 1998; Williams et al., 1998) and will not be

earliest Paleoproterozoic is characterized by

dealt with extensively here; however, low

abundant mafic or bimodal igneous provinces,

paleomagnetic latitudes have been determined

possibly indicating protracted breakup (2.4-2.2

on both Neoproterozoic (Sohl et al., 1999) and

Ga) of one or two Neoarchean supercontinents

Paleoproterozoic (Evans et al., 1997) glaciogenic

(proposed names of Kenorland and Zimvaalbara;

deposits and stratigraphically adjacent strata.

c.f. Aspler and Chiaranzelli, 1998). Except in the

The so-called "Snowball Earth" model of globally

cases of amalgamations that have survived to the

engulfing ice ages does explain association of the

present, as in the Canadian Shield, for example,

Neoproterozoic glacial deposits with sedimentary

all hypothesized Paleoproterozoic

iron-formation, the latter occurring after a billion-

Two Billion Years of Tectonics & Mineralisation

year absence in the geological record (Kirschvink,

increased hydrothermal ferrous input to the

1992). But what about the Paleoproterozoic ice

oceans; or (4) more efficient upwellings of the

ages and BIFs? With the exception of the

deep oceanic iron reservoir onto the continental

recently described northern fades of the

shelves. Processes (1) and (4) would be strongly

Meteorite Bore glaciation on the Pilbara craton,

influenced by a supercontinent in the

where apparent dropstones penetrate laminated

paleogeographic sense, whereas (2) and (3)

Boolgeeda ferruginous mudstones (Martin, 1999),

could be spurred by a geodynamically defined

the world's Paleoproterozoic glaciogenic units are

supercontinent. If the Superior-type BIFs were

stratigraphically quite distinct from the voluminous

largely the result of global photosynthetic blooms

Superior-type BIFs. For example, on the

(1), then we would expect to see large, positive

Kaapvaal craton the glaciogenic Makganyene

del-13-C isotopic spikes associated with the BIFs.

diamictite postdates the Kuruman and

In fact, one or more major carbon-isotopic spikes

Griquatown BIFs by ca. 200 Myr (Trendall et al.,

exists in the Paleoproterozoic carbonate record,

1990), and on the Superior craton the Huronian

but the age range for these enriched results is ca.

glaciogenic levels precede the type Superior BIFs

2.2-2.1 Ga (Karhu and Holland, 1996), an interval

by at least 100 and perhaps 300 Myr (Morey and

that is devoid of extensive, Superior-type BIF

Van Schmus, 1988). Thus the Paleoproterozoic

preservation. Obviously, more detailed carbon-

ice ages are not intimately linked to BIF

isotope stratigraphy is required to resolve the

deposition like their Neoproterozoic counterparts.

details of Paleoproterozoic secular trends.

Superior-type BIFs

Summary

Discussion on the Paleoproterozoic BIFs has

There are currently no simple relations among

endured for decades (c.f. Cloud, 1968), but

Proterozoic supercontinents, glaciations, and

through recent and forthcoming geochronology

BIFs, except the association of ice ages with

and stratigraphy we are at the threshold of major

fragmenting supercontinents. Relating the

understanding. Within error, the major Superior-

Paleoproterozoic glacial deposits and BIFs, within

type BIFs of the southern hemisphere are

the context of carbon-isotope stratigraphy and

synchronous at ca.2.45-2.47 Ga (Trendall et al.,

hydro-atmospheric chemistry, will require better

1990), and those of the Canadian Shield are

constrained geochronology of the relevant

nearly coeval at ca.1.9-1.85 Ga (Hoffman, 1987).

volcanic-sedimentary successions. In

The former are found in passive tectonic settings,

combination with thorough paleomagnetic

whereas the latter are found in foreland basins

investigations, these detailed studies will yield

(Hoffman, 1987). Among other possibilities,

profound insights into Paleoproterozoic

Paleoproterozoic BIFs could be the result of (1)

supercontinents and their effects on Earth's

increased photosynthetic productivity, releasing

evolving hydrosphere, atmosphere, and

oxidants into Fe(ll)-saturated seawater; (2)

biosphere.

enhanced burial of non-Fe reductants such as organic carbon, increasing the interaction between oxidants and dissolved ferrous iron; (3)

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Tectonics Special Research Centre, School of Applied Geology, Curtin University of Technology, September 1999

Anderson, D.L. Geology 22, 39-42 (1994).

Klein, G.D., ed. Pangea: Paleoclimate,

Aspler, L.B. & Chiaranzelli, J.R. Sediment. Geol.

Tectonics, and Sedimentation During Accretion,

120, 75-104(1998).

Zenith, and Breakup of a Supercontinent, Special Paper 288 (Geological Society of America,

Bond, G.C., Nickeson, P.A., and Kominz, M.A.

Boulder, Colorado, 1994).

Earth Plan. Sci. Lett. 70, 325-345 (1984). Martin, D.McB. Geol. Soc. Am. Bull. 111, 189Cloud, P.E. Science 160, 729-736 (1968). Dalziel, I.W.D. Geology 19, 598-601 (1991). Evans, D.A. Earth Plan. Sci. Lett. 157, 1-8 (1998). Evans, D.A., Beukes, N.J. & Kirschvink, J.L. Nature 386, 262-266 (1997). Hambrey, M.J. & Harland, W.B, eds. EarthOs Pre-Pleistocene Glacial Record (Cambridge

203(1999). Moores, E.M. Geology 19, 425-428 (1991). Morey, G.B. & Van Schmus, W.R. U.S.Geol.Surv. Prof. Paper 1241 -F, F1-F31 (1988). Piper, J.D.A. Earth Plan. Sci. Lett. 59, 61-89 (1982). Powell, C.M., Li, Z.X., McElhinny, M.W., Meert J. G. & Park, J.K. Geology 21, 889-892 (1993).

University Press, 1981). Rogers, J.J.W. Geology 104, 91-107 (1996). Hoffman, P.F. in Proterozoic Lithospheric Evolution (ed. Krsner, A.), Geodynamics Series,

Roscoe, S.M. & Card, K.D. Can. J. Earth Sci. 30,

v.17, 85-98 (Am. Geophys. Union, Washington,

2475-2480 (1994).

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Sohl, L.E., Christie-Blick, N. & Kent, D.V. Geol.

Hoffman, P.F. Geology 17, 135-138 (1989).

Soc. Am. Bull. 111,1120-1139 (1999).

Hoffman, P.F. Science 252, 1409-1412 (1991).

Trendall, A.F. & 11 others, in 3rd Int. Achean Symp. Ext. Abst. 81-83 (1990).

Hoffman, P.F. in Micro, Meso, Macro-An Introduction to Deformation and Tectonics (eds

Weil, A.B., Van der Voo, R., Mac Niocaill, C. &

Van der Pluijm, B.A. & Marshak, S.), Wm C.

Meert, J.G. Earth Plan. Sci. Lett. 154, 13-24

Brown Publ., in press.

(1998).

Hoffman, P.F., Kaufman, A.J., Halverson, G.P., &

Williams, D.M., Kasting, J.F., & Frakes, L.A.

Schrag, D.P. Science 281, 1342-1346 (1998).

Nature 396, 453-455 (1998).

Karhu, J.A. & Holland, H.D. Geology 24, 867-870

Young, G.M. Geoscience Canada 18,100-108

(1996).

(1991).

Kirschvink, J.L. in The Proterozoic Biosphere: A Multidisciplinary Study (eds Schopf, J.W. & Klein, C.), 51-52(1992). Klein, C. & Beukes, N.J. in The Proterozoic Biosphere: A Multidisciplinary Study (eds Schopf, J.W. & Klein, C.), 139-146 (1992).

Two Billion Years of Tectonics & Mineralisation

Newly discovered early metamorphic history of the Scottish Caledonides: The place of the Moine Supergroup in the Taconic. C. R. L. FRIEND, K. A. JONES & I. M. BURNS Department of Geology, Oxford Brookes University, Oxford OX3 OBP; United Kingdom. Caledonian geology of northern Scotland The metasedimentary rocks above the Moine Thrust on the north coast of Scotland belong to the Neoproterozoic Moine Supergroup and were moved north-westwards during the main phase (D2) of the Ordovician-Silurian Caledonian orogeny (e.g. Butler & Coward, 1984). This is considered to have occurred as a series of foreland-propagating thrusts, older structures being carried in piggy-back fashion onto younger, successively lower grade thrusts (e.g. Butler 1986), at c. 440 Ma (e.g. Barr etal. 1986). Within this dominantly sedimentary pile are numerous struc-

DC10

tural inliers of gneissic rocks normally correlated with the Lewisian basement found in the foreland to the Caledonian (e.g. Holdsworth etal. 1994). Also occurring within some parts of the Moine Supergroup, quite distinct from the basement inliers, are numerous discontinuous strips of homogeneous amphibolite that are interpreted to represent an intrusive basic igneous component of the oth-

Fig. 1. Sketch map showing the disposition of thrusts and nappes in northern Scotland. The position of the high pressure granulite (HPG) assemblages in the Naver nappe is indicated with the sample numbers prefixed DC.

erwise metasedimentary sequence (e.g. Moorhouse & Moorhouse, 1988).

of earlier composite fabrics in this nappe, D1, are

The Lewisian, Torridonian and Cambro-

considered to be Proterozoic (e.g. Alsop et al,

Ordovician rocks of the Caledonian foreland were

1996), in line with earlier interpretations from fur-

overridden by deformed low-grade equivalents in

ther southwest (see refs. in Allison etal. 1988). A

the Moine Thrust Zone. This zone was overrid-

major metamorphic change occurs at the next,

den by higher grade, upper greenschist and am-

structurally higher thrust carrying the Naver

phibolite fades rocks (the Moine Supergroup) of

nappe. Here migmatites occur in appropriate

the allochthonous nappes above the Moine

compositions and rare sillimanite indicates middle

Thrust which floors the Moine nappe, dominated

to upper amphibolite fades conditions. Above the

by psammite with minor semi-pelite. Remnants

Naver Thrust there are also important changes in

Tectonics Special Research Centre, School of Applied Geology; Curtin University of Technology, September 1999

the lithologies, metamorphism and style of defor-

vious tectonic models were based either on geo-

mation. On the north coast, the Naver nappe

physical data e.g. the thick-skinned models of

comprises psammitic gneisses into which several

Watson & Dunning (1979) and Soper & Barber

heterogeneous groups of acid to basic meta-

(1982), or used analogues such as the North

igneous rocks were emplaced at different times.

American Rockies and Appalachians to support

Such igneous rocks comprise approximately 35%

the thin-skinned models e.g. Butler (1986) and

of the Naver nappe and one such component

Barr etal.,

comprises amphibolite sheets up to 3 m wide that

explaining the formation and exhumation of the

now occur as boudinaged strips with the chemical

newly discovered HPG rocks.

(1986). Both models have problems in

characteristics of Moine basic rocks (Moorhouse & Moorhouse 1988). Because there are occa-

New Field and metamorphic data

sional shallow discordances with the host psam-

New investigations revealed that the Caledonian

mites these amphibolites have been interpreted

metamorphic history of the Naver nappe is more

to represent post-sedimentary sills or dykes em-

complicated than previously recorded. The pelitic

placed into the psammitic to semi-pelitic host

rocks demonstrate early, clear garnet with quartz

rocks pre- to syn-D2. For example, the British

inclusion trails (D2) at a high angle to the enclos-

Geological Survey (1997) indicates that these ba-

ing D3 fabric. A second phase of garnet, syn-D3,

sic rocks cross-cut the Druim Chiubhe Psammite

is full of minute opaque inclusions and contains

Formation. Another important structural feature is

grains of biotite and quartz that define curved in-

that the rocks just beneath the Naver Thrust mark

clusion trails, and grows either in pressure shad-

the beginning of a zone of steepened fabrics

ows or as partial rims on D2 garnets. About

characterised by intense later, middle to lower

10km south of the coast, where the effects of D3

amphibolite facies, SSE-directed ductile oblique

are absent, the metamorphic conditions of D2 are

extension (D3). Only in rare areas of very low D3

constrained by the assemblage grt + bio + sill in

strain have any relics of D2 structures, usually at

melt-absent pelites.

high angles to each other, been observed. This

In the amphibolites two stages of green amphi-

strong D3 deformation has largely obliterated ear-

bole growth can be correlated with the metamor-

lier relationships and brought layers of amphibo-

phism associated with D2 and D3. Green amphi-

lite into a broad parallelism with the technically

bole grew parallel to the NW Caledonian trans-

transposed layering of the psammitic rocks. Because of the intensity of the Caledo-

port direction lineation (L2) and is associated with a pronounced foliation defined by biotite and

nian deformation and metamorphism, the tectonic

hornblende. This L2/S2 fabric was subsequently

and metamorphic history of the Moine Super-

overprinted by green amphibole growth aligned

group has been hotly debated (e.g. see Hold-

with the SSE-plunging L3.

sworth etal.

1994). In particular, it has been diffi-

cult to establish precisely which Neoproterozoic

Within the amphibolite layers in low D2 strain zones, occasional brownish-weathering

events have occurred, quite apart from establish-

relic cores up to 2 m in diameter occur that carry

ing the chronology of Phanerozoic events prior to

the peak assemblage grt (XAim 0.50, Xpyr 0.16,

the main phase of Caledonian movements. Pre-

XGrs 0.33, XSps 0.02) + di + plag (XAn 0.20) + rut ±

Two Billion Years of Tectonics & Mineralisation

the HPG assemblage. Grt-Plag-Cpx-Qtz barometry and Grt-Cpx thermometry constrain minimum P-Tconditions of this HPG assemblage to ~11 kbars at 650-700°C, and 10-11 kbars is provided by the GRIPS barometer for the rutile-bearing assemblages. These pressures correspond to a minimum of -35 km. The P-7"conditions of the early hydration, - 650°C at 10 kbar, have been constrained by application of the grt-hbl and amph-plag thermometers, and the Grt-Plag-Hbl-Qtz barometer and GRIPS with ilmenite. The L2 fabric associated with the NW-directed transport is widely interpreted to be that developed during the Caledonian excavation of the Moine rocks i.e. during the Ordovician - Silurian. Fig. 2. P-T-t-D paths for the Naver nappe during -467 Ma HPG event and the 'main phase' D2 NW-directed Caledonian transport.

That this fabric is developed over the anhydrous,

qtz (c, Fig. 2). These anhydrous, HPG cores are

sult of deep burial during an earlier crustal thick-

unfoliated and importantly are non-lineated as

ening event.

pyroxene-bearing assemblage indicates that the HPG metamorphism occurred prior to D2 as a re-

they do not carry either the D2 or D3 fabrics of their host. Early hydration is indicated by the as-

Discussion

semblage brown hornblende + plag + grt in equi-

The structural and textural relationships indicate

librium in veins within the anhydrous rocks (d, Fig.

that burial forming the HPG assemblage pre-

2). Decompression reactions (f, Fig. 2) are found

ceded the main late Ordovician - early Silurian

between grt-cpx-plag forming occasional corona

phase of Caledonian deformation (D2) and am-

structures. The cores show various stages of

phibolite facies metamorphism. Direct dating of

later hydration and retrogression as anasta-

the amphibolite precursors or the HPG minerals

mosing, dark green, amphibole-bearing veins

would best constrain the age. However, poly-

penetrate through the pyroxene-bearing assem-

phase metamorphism and the very fine-grained

blage. The margins of the cores are similarly al-

complex textures prevent this. Single zircon U-

tered and are transitional with the D3 assem-

Pb ages indicate deposition after -1000 Ma

blage. They show a progressive hydrous altera-

(Kinny et al., 1999); a grouping of such Grenville-

tion, interpreted to pass through a D2 assem-

aged grains indicates there was an appreciable

blage of hbl + grt + plag ± tit ± qtz to the stable

input from such a source. Therefore, the basic

assemblage under D3 conditions of hbl + plag +

sheets in which the HPG assemblages are pre-

tit ± clinozoisite ± qtz (y, Fig 2). These two stages

served must also be post-Grenvillian and the

of green hornblende growth both clearly post-date

HPG assemblage cannot be correlated with the c.

Tectonics Special Research Centre, School of Applied Geology, Curtin University of Technology, September 1999

1100 Ma eclogitic rocks at Glenelg (Sanders et

al., 1984). This means that a separate,

younger,

nation for the generation of the HPG assemblage is that it developed in a subducted volcano-

high-pressure event has been identified in this

sedimentary arc and represents the climax of

part of the NW Scotland Caledonides. No record

crustal thickening. The Dewey & Ryan (1990)

of -870 Ma metamorphism (Friend et al., 1997)

model has the volcanic arc in the hanging wall

has been found in zircons from either the Naver

which is with respect to the subduction-related

or the structurally higher Kirtomy nappe (Kinny et

structure and not the presently recognised

al, 1999). The underlying psammites are migma-

thrusts, which are associated with the Caledonian

titic, anatexis occurring on a wet melting curve

NW-directed transport. Following thickening,

because muscovite is stable and alumino-silicate

early partial hydration formed brown amphibole,

is absent in both psammitic and pelitic compo-

which can be equated to crystallisation of the

nents. This melting is pre-D2 as the granite veins

melts and release of water on recrossing the melt

and sheets are deformed by the NW-directed

curve to the low temperature side. Following the

transport and is interpreted to record formation of

decompression and second hydration the exact

the HPG assemblages as this is the only place

trajectory is lost. This early high pressure, cold

the solidus is crossed (b, Fig. 2). A U-Pb age of

trajectory is quite distinct from the path expected

467 ±10 Ma, obtained from newly grown zircon as

with the D2 NW-directed transport.

rims on detrital grains, is interpreted to date this

This latter path is characterised by the essentially

anatexis (Kinny eta/., 1999).

prograde reactions observed associated with D2

From the available metamorphic and geochro-

fabrics. For the majority of rocks these culminate

nological data a partial P-T-f-afloop has been

short of the wet melting curve as no melts were

constructed for this evolution (Fig. 2). The pre-

generated. Muscovite is stable in most rocks so

served HPG assemblages require burial to a

conditions were on the low- T side of muscovite +

minimum of -30 km which is simply constrained

quartz breakdown. Sillimanite-bearing assem-

along a geotherm of ~20°C/km. The mafic rocks

blages are found in only a few rocks of appropri-

preserve relic igneous plagioclase (An37) indicat-

ate composition. Because the rocks have not

ing they were not subject to peak conditions long

melted, the maximum P-Tconditions

enough for textural or chemical equilibrium. This

D2 phase of deformation within the Naver nappe

for the main

is also backed up by the small amount of melt

can be constrained to be within the relatively

produced in the psammites. Because muscovite

small, sillimanite-stable area below the melt

appears stable the reactions are constrained by

curve. Garnet-biotite thermometry indicates tem-

the wet melting curve and cannot have reached

peratures about 636-659°C at - 5 kbar for syn-D2

much higher temperatures, the melting probably

assemblages and the age for this stacking is

being limited by the elimination of a phase, in this

thought to be -440 Ma (Barr eta/., 1986). The

case K-feldspar. This -467 Ma burial event is in-

formation temperature of the D3 garnet-biotite as-

terpreted to represent the early stages of the con-

semblages was determined at between 508-558°

struction of the orogenic belt resulting from arc-

C at 3kbar. The final stages of recovery from this

continent collision (c.f. Dewey & Ryan, 1990) and

position are constrained using Ar/Ar cooling ages

is considered to be Taconic. The simplest expla-

from hornblende and muscovite (y, z, Fig. 2).

Two Billion Years of Tectonics & Mineralisation

These new age and metamorphic constraints per-

these complexes have probably been obliterated

mit an early Ordovician age to be deduced for the

by high Caledonian strain, or they have been re-

HPG assemblage. If these deductions are cor-

worked. Alternatively, because there are many

rect, this high-pressure metamorphism corre-

anastamosing thrusts it is possible some have

sponds to collisional events described in the

been misidentified. As to how the HPG rocks

Taconic evolution of Ireland, an event not previ-

were exhumed, the easiest mechanism is to have

ously been recorded in Scotland. This interpreta-

a series of early thrusts, the direction of which is

tion requires that the overall evolution of this por-

uncertain, that thickened the crust rapidly and

tion of the Caledonian orogenic belt be rethought.

then this thickened pile is dissected by a series of

Fitting the exhumation of these rocks into the

later NW-directed thrusts - those which carry the

foreland propagating, thin-skinned model, which

Moine nappes.

excavates rocks from only -12km (e.g. Butler

The structural and metamorphic evidence now

1986), is difficult because the thrusts do not

available suggests that the Caledonian orogenic

penetrate to the depths required to satisfy the

belt in NW Scotland comprises two distinct parts

mineral assemblages preserved in the HPG

which require different structural and metamor-

rocks. If this model is correct these data suggest

phic models to explain them. The allochthonous

that either the sequence of thrusts is not a simple

nappe pile above the Moine Thrust was con-

foreland propagating one, or the assumptions

structed initially by arc accretion that took place at

used for the model are wrong. Exhumation using

c. 467 Ma, producing the HPG assemblages.

the Soper & Barber (1982) thick-skinned model

This is presently taken to indicate the maximum

with a steeper trajectory presents two main prob-

development of the orogenic belt before general

lems. First, the granulites they reasonably sug-

collapse commenced. Subsequent excavation of

gested were present at depth were not excavated

these rocks from the deep crust is best explained

to the surface and second, the model does not

using a thick-skinned model. However, a thin-

satisfy the pre-D2 467 Ma melting.

skinned model is still the best explanation of the

The new data suggest that there was an early

later, c. 435-408 Ma, brittle portion in the MTZ,

high-pressure metamorphic event, now preserved

which is adjacent to and involves all the compo-

in only a few, rare localities. This assemblage re-

nents of the foreland (e.g. Butler & Coward 1984).

cords early crustal thickening that occurred prior to c. 467 Ma. Therefore, the overall evolution of the Caledonides is this region is not as simple as the currently proposed models for the northern part of the British Caledonides have indicated (e.g. Butler & Coward, 1986). The only events in this part of Laurentia occurring prior to c. 467 Ma are those associated with the Taconic. A series of subduction-related high-pressure complexes could be extended from Ireland northeastwards into Scotland. The original thrusts related to

Tectonics Special Research Centre, School of Applied Geology, Curtin University of Technology, September 1999

Folding as a result of lower crustal flow during rifting: A new technique of centrifuge modelling applied to the study of high-grade gneiss terrains. 1

L.B. HARRIS & 2 H.A. KOYI

TSRC, Department of Geology & Geophysics, UWA, Nedlands 6907, Australia. Hans Ramberg Tectonic Laboratory, Institute of Earth Sciences, Uppsala University, Uppsala S - 752 36, Sweden.

Regional, recumbent, fold-nappe structures in high-grade gneisses have traditionally been inter-

Villavagen 16,

in the otherwise ductile crust on folding during extension.

preted as forming within a convergent-margin tec-

Models were constructed of materials whose den-

tonic setting. Such structures are thought to form

sity and viscosity scale to replicate a simplified

contemporaneously with thrusts during bulk con-

lithospheric strength profile when accelerated at

traction across the orogen. Subsequent refolding

900 to 1000 g in the large centrifuge in the Hans

by upright folds is generally interpreted as occur-

Ramberg Tectonic Laboratory at Uppsala Univer-

ring during continued contraction (eg. locking of

sity. The deformation rig used necessitated use of

displacement on thrusts and/or the formation of

a cohesive upper brittle layer. Canderel™ (a com-

thrust ramps). Any normal shear zones and ex-

mercial fine-grained maltodextrin and aspartame

tensional kinematic indicators, where recognized,

powder) mixed with Plastilene modeling putty (the

are generally attributed to late orogenic collapse.

Swedish equivalent to Plasticine) to bind it, repre-

Alternatively, recumbent folds in high-grade

sented the upper, brittle crust. Layers of a mix of

gneisses have been interpreted in terms of

acetate and Vaseline and/or glycerine and plaster

spreading associated with diapiric structures

of Paris (both of which readily fractured) were ap-

where deformation is purely gravity-driven. The

plied at successive stages of rifting in down-

study of metamorphic core complexes has shown

thrown areas of models to represent syn-rift sedi-

that folding and doming may also take place dur-

ments. The extreme reflectivity and often ob-

ing lithospheric-scale extension and that high-

served laminated structure of the lower crust in

grade rocks may be exhumed in this process. A new technique and materials for centrifuge modelling of the progressive development of structures during asymmetric rifting have been developed to examine: (i) Structures formed during ductile lower-crustal

deep seismic profiles is best explained if it is compositionally heterogeneous. In our experiments, Dow Corning silicone putty layers (whose density was increased to 1.2 gem"3 by adding BaS0 4 , and coloured using powdered pigment) simulate weaker zones in which ductile flow can occur in the middle to lower crust. Thin layers of

flow during rifting;

Plastilene represent more competent layers within

(ii) Mechanisms for the formation and progressive

the ductile matrix. Except for cases of extreme

development of folds in the middle to lower crust;

surface heat flow, the lithospheric mantle repre-

and

sents a strong, load-bearing layer for continental

(iii) The effects of boudinage of competent layers

Two Billion Years of Tectonics & Mineralisation

lithosphere. Lithospheric mantle was treated as

a competent, brittle-ductile layer and was modelled using plastiline mixed with Canderel™ . The intention of this study was to study structures within the ductile crust in an asymmetric rift system. In order to initiate this style of deformation, an inclined cut was made through the plastiline Canderel layer to represent a pre-existing shear zone cutting the lithospheric mantle. A ductile basal layer was used to represent mantle asthenosphere. This was modelled using Rhodosil Gum silicone putty mixed with magnetite and plastilene 3

to increase its density to 1.42 gem' , and acid oil to decrease viscosity. Thin layers of clear PDMS (a low viscosity silicone gel) were applied to the end and side boundaries of the model to reduce friction and to minimise 'drag' during the collapse of the model.

fused together. In these experiments, although the geometry of structures produced is asymmetric, deformation has not been concentrated into a discrete shear zone that transects the lithosphere, as portrayed in simple shear rift models. Instead, strain has been partitioned into deformation along discrete structures in the upper brittle crust and mantle asthenosphere, and to within a broad zone of ductile flow in the middle- and lower crust. A significant feature of these models is that the upper brittle crust may deform in a simple manner (producing a wide sag basin in which the position of active rifting varies with time) whilst the middle and lower ductile crust may undergo complex deformation, including formation of refolded recumbent folds. Different structural features are developed

Whilst the models are by necessity extreme sim-

at different levels in the lithosphere due to differ-

plifications of the continental lithosphere (e.g. all

ing rheology and proximity to other deforming lay-

boundaries are sharp instead of transitional as

ers. If a shear zone develops early in the ductile

some are expected to be in nature), they repre-

crustal layers, then deformation will remain con-

sent simplified dynamic simulations that can pro-

centrated within it. Minor conjugate normal shear

vide valuable insights in possible deformation

zones develop in the ductile crust, on the bound-

styles and processes that may occur in nature.

ary of the uplifted mantle lithosphere footwall

The models were extended parallel to the initial

block, and on the flank of the mantle astheno-

compositional layering as a response to the ap-

sphere ridge. Uplift of 'mantle lithosphere' results

plied 900 to 1000 g acceleration acting normal to

in the flattening in the dip of early-formed ductile

the layers. Experiments thus represent a passive

shear zones.

style of rifting, with thinning of model layers representing continental lithosphere and passive upwelling of the basal material representing mantle asthenosphere. Typical experiments consisted of 7 to 12 runs, each of 2 to 10 minutes duration. Removing spacers between a movable 'backstop' and the end wall of the rig controlled increments of extension. Models were partially frozen, sliced and photographed at different stages of deformation to record the progressive development of structures. After slicing, the models were reassembled in the deformation rig and left to return to room temperature upon which silicone layers

Upright folds develop due to flow of ductile layers into boudin necks. Some folds may initiate as upright structures, especially above the triangular tip of the uplifted mantle lithosphere footwall, but these are progressively rotated to a recumbent attitude with progressive deformation and become more drawn out in the shear zone. Open folds of competent layers in the ductile crust have also formed due to rollover on listric normal shear zones. Folds developed within the shear zone may form in an original horizontal orientation, due to high shear strains in normal shear zone above the shallowly dipping rotated footwall to the pre:

Tectonics Special Research Centre, School of Applied Geology, Curtin University of Technology, September 1999

existing fault in the mantle lithosphere, and due to irregularities in the footwall surface of the mantle lithosphere. Structures produced result in stratigraphic repetition and complex fold-nappe geometries that resemble structures produced in a thrust-convergent margin tectonic setting or folds formed by gravitational spreading at an upper structural level, except for the lack of deformation in the overlying ductile and brittle crust. Early-formed recumbent folds may be refolded due to either vertical movements (doming) of the mantle asthenosphere, or due to interference by later upright folds formed at a higher level in the crust between boudinaged competent layers. Uplift of the mantle asthenosphere may also produce isoclinal folds in the lower ductile crust within normal shear zones on the flanks of the asthenosphere dome when the mantle lithosphere has separated. The results of this study show a strong agreement with field data from extensional high-grade terrains (eg. West Greenland, the Archaean Napier Complex of East Antarctica and the Ruby Mountains of Nevada) and deep-crustal seismic data of rifts (eg. Rhine Graben, Red Sea). This new modelling technique and the newly developed materials used have illustrated possibilities for complex deformation during flow in the ductile crust during extension of continental lithosphere. The resulting structures may have previously been interpreted as forming in a convergent tectonic setting. This study therefore highlights the need for extreme care to be taken in basing interpretations of tectonic setting in high-grade gneiss terrains on fold style and geometry. It may therefore lead to the reexamination of the tectonic interpretation of many high-grade metamorphic terrains.

Two Billion Years of Tectonics & Mineralisation

Landsat thematic mapper as a regional mapping and exploration tool in the Late Archaean to Palaeoproterozoic Hamersley Province, WA. 1

DAVE HOLLINGSWORTH, 2BOB HICKEY & 1 PETER CAWOOD

TSRC, School of Applied Geology, Curtin University of Technology, GPO Box U1987, Perth WA 6845. School of Spatial Science, Curtin University of Technology, GPO Box U1987, Perth WA 6845.

Landsat Thematic Mapper (TM) imagery is a valuable tool for the exploration or mapping geologist. It allows a quick and relatively

c inexpensive view of any area (worldwide) prior to

fieldwork or geophysical surveys. In the Hamersley Province of Western Australia, it has been successfully used as a regional mapping

Landsat TM is able to minimize the effects of the discolouration and reliably distinguish between rock types. As Landsat TM data is readily available for the entire area of WA, it gives geologists the ability to quickly see relationships between regional fold belts, shear zones, rock types, and local features.

tool. In addition to rock types, drainage,

The versatility of digital Landsat TM data lies in

structure, and vegetation have been defined from

three aspects of the data. Firstly, the data can be

the imagery.

displayed in either "true colour" or colour infrared - looking very similar to aerial photographs (colour or colour infrared,

Introduction.

respectively). The second feature of the data is

Rapid advances in technology over the past 10

that it extends beyond the visible and near-

years have seen the use of digital remote sensing

infrared (NIR) into the MIR region - making it

data move from the domain of the large

possible to highlight some unseen (in the visible

workstation computer to the average desktop PC.

to NIR) lithological variations. The third, and

Where once gravity and magnetic surveys were

most important feature, is that the data can be

the most common digital data sets, Landsat TM

tailored to suit individual needs by looking at

data is now used widely in exploration. Landsat

combinations or ratios of different wavelength

TM offers many advantages over the aerial

bands. When generating an image using

photographs that are widely used in the

Landsat TM data, it is necessary to assign a band

construction of interpretive maps during the early

(or mathematical combination of bands) to a

stages of exploration. The biggest advantage

colour (red, green or blue). Using the ratio of two

that Landsat TM holds is that two of the bands

bands as opposed to a single band often enables

are within the mid-infrared (MIR) region of the

features to be highlighted. By using band ratios,

spectrum. In an area such as the Hamersley

it is possible to highlight such things as variations

Province of Western Australia (WA), aerial

in vegetation, water, drainage systems, rock type,

photographs tend to show the ubiquitous iron-rich

and structure. Regional-scale mapping can be

red dust that discolours the rocks. By imaging

undertaken rapidly if the stratigraphy can be

beyond the visible light region of the spectrum,

correlated with other mapping in the area, or if

Tectonics Special Research Centre, School of Applied Geology, Curtin University of Technology, September 1999

"ground truthing" is carried out within the area of interest so that lithology can be matched with the Landsat TM image. The combination of Landsat TM with other data sources such as gravity or aeromagnetic data is easily achieved using modern software packages. The ability to quickly overlay data from various sources allows better recognition of possible target sites.

Application to the Hamersley Province, Western Australia The project's goal was to test the capability of Landsat TM to differentiate rock types (major and minor) within the Hamersley Province of Western Australia (Figure 1). This area is ideal because there are large lithological/mineralogical

Figure 2. A regional geological map of the test area generated using Landsat TM.

variations in the stratigraphy which is exposed over a large region. The stratigraphy includes the

area and consists of Hamersley and upper

Fortescue (clastic sediments, mafic volcanics,

Fortescue lithologies. ER Mapper 5.5 was the

and shales), Hamersley (BIF, shale, minor

software used for this project.

dolomite, and volcanics), Turee Creek (shale, sandstone, volcanics), and Wyloo (sandstone, conglomerate, mafic volcanics) Groups. The relief in the Hamersley Province also helps to expose some of the recessive rock types. The test region (Figures 1 and 2) is about 12x14 km in

To test the capabilities of Landsat TM, a number of three band combinations and ratios were visually tested. The best combination involved Band 7 (MIR), Band 5 (MIR), and Band 1 (visible - blue). This outperformed even the "standard" 7/5, 3/1, and 5/4 ratio combination for lithology delineation. Once the testing was complete, rock types and structures were digitised (on-screen) directly on top of the 7,5,1 display. These data were then exported to Corel Draw 8 for final cartographic presentation (Figure 2). Should it be required in the future, additional information can easily be derived frdrti the existing TM data, including vegetation, drainage, and roads.

Hgrdy Syncjinj • 23 '~—j7~-MfTTurnerl .: Syncline—

The most notable response to stratigraphic variation occurs in the Hamersley Group, where stratigraphy comprises alternating Banded Iron Formation (BIF) and shale units. Shale units that

Figure 1. Map showing the location of the Hamersley Province.

Two Billion Years of Tectonics & Mineralisation

are about 50 m thick can be recognised from the Landsat images. The stratigraphy of the

Hamersley Group also includes dolomite, rhyolite

zones can be seen in the images.

and dolerite sills, all of which can be distinguished. Conclusions Landsat TM imagery was successfully used to define the lithology and structure in the Hamersley Province of Western Australia. This was tested by comparing the maps generated to the 1:100,000 GSWA geologic maps and a few field surveys. Predictably, there were differences which require fieldwork to resolve - particularly in the mapping of BIF. The Marra Mamba Iron Formation, at the base of the Hamersley Group, contains more chert and less iron oxides than the Brockman Iron Formation. On the imagery, the two BIF units appear significantly and consistently different. However, the geologic maps which were (most probably) compiled from black and white aerial photographs which rely heavily on outcrop pattern, have not made the distinction. Thus, the Landsat images have provided information, particularly in the MIR, that was not available to the initial mapping work. In addition to the scientific benefits of Landsat TM imagery, it is cheap - the images used in this project cost - $8/km2 (educational price). Mapping of large areas can also be completed far quicker than either via field mapping or aerial photograph interpretation. It is important to note that regional mapping using Landsat TM imagery may not always be as successful as in the Hamersley Province, which features a wide range of fundamentally different lithologies and bulk-rock chemical composition. In areas which contain rocks that are not as different, for example an area comprising primarily different sandstone units, the delineation of rock types from Landsat TM imagery is often more difficult. However, in nearly all terrains, structural features such as faults, folds, and shear

Tectonics Special Research Centre, School of Applied Geology, Curtin University of Technology, September 1999

Palaeomagnetic Evidence for an United North and West Australian Cratons by ca. 1.7 Ga. z.x. LI TSRC, Department of Geology and Geophysics, The University of Western Australia, Nedlands, WA, 6907; Australia Email: zli@tsrc.uwa.edu.au. A palaeomagnetic study of the Elgee Formation redbeds in the Paleoproterozoic Kimberley Basin, northwestern Australia, revealed an extremely stable magnetic remanence carried by hematite from seven sampling sites. The age of the formation is confined by precise SHRIMP U-Pb xenotime ages for rocks both above and below the formation (McNaughton et. a/., 1999) to be 1704 +7/-14 Ma. The primary origin for the remanence is supported by a positive fold test, the lack of regional overprint in the 1790 Ma Hart Dolerite (R.W. Page, in Blake, 1997) just north of the study region, the dissimilarity of the remanent direction from expected younger palaeomagnetic directions, and the extreme stability of the remanence. The mean direction of D = 92.2°, I = 14.9°, a95 = 6.4° gives a 1704 +7/-14 Ma palaeopole at (4.4°S, 210.0°E) with dp = 3.3°, dm = 6.5°. This pole, together with a previously reported palaeopole from the Hart Dolerite, agrees with palaeopoles of similar ages from the McArthur Basin of northern Australia. (Idnurm et al., 1995), therefore supporting the geological interpretation that the Kimberley craton and the Lucas Craton to its east joined together to form the North Australian Craton by -1.8 Ga (e.g., Tyler and Page, 1996; Myers etal., 1996). Ca. 1700 magnetic overprint poles related to the Ashburton Orogeny (between 1.8 and 1.65 Ga) along the southern Pilbara Craton (e.g., Schmidt and Embleton, 1985; Li etal., 1993; Schmidt and Clark, 1994; Li etal., in prep.) also agree with palaeopoles from the McArthur Basin. There is thus no need by palaeomagnetic results to have any ocean between the North and West Two Billion Years of Tectonics & Mineralisation

Australian Cratons after ca. 1700 Ma. This interpretation differs from the model postulated by Myers etal. (1996) which suggests a suturing time of 1300-1000 Ma between the two cratons. There is yet no 1800-1700 Ma pole from the South Australia Craton to constrain its relative position to the rest of the Australian craton at that time. References.

Blake, D.H., 1997. Proterozoic thrusting in the Osmand Range area of the East Kimberley, Western Australia. AGSO Research Newsletter, No. 26, 4-5. Idnurm, M., Giddings, J.W., and Plumb, K.A., 1995. Apparent polar wander and reversal stratigraphy of the PalaeoMesoproterozoic southeastern McArthur Basin, Australia. Precamb. Res., 72, 1-41. Li, Z.X., Powell, C.M., and Bowman, R., 1993. Timing and genesis of the Hamersley iron-ore deposits. Expl. Geophys., 24, 631-636. McNaughton, N.J., Rasmussen, B., and Fletcher, I.R., 1999. SHRIMP uranium-lead dating of diagenetic xenotime in siliciclastic sedimentary rocks. Science, 285, 78-80. Myers, J.S., Shaw, R.D., and Tyler, I.M., 1996. Tectonic evolution of Proterozoic Australia, Tectonics, 15,14311446. Schmidt, W., and Clark, D.A., 1994. Palaeomagnetism and magnetic anisotropy of Proterozoic banded-iron formations and iron ores of the Hamersley Basin, Western Australia, Precamb. Res., 69,133-155. Schmidt, W., and Embleton, B.J.J., 1985. Prefolding and overprint magnetic signatures in Precambrian (-2.9-2.7 Ga) igneous rocks from the Pilbara Craton and Hamersley Basin, NW Australia, J. Geophys. Res, 2967-2984. Tyler, I.M., and Page, R.W., 1996. Palaeoproterozoic deformation, metamorphism and igneous intrusion in the central zone of the Lamboo Complex, Halls Creek Orogen, Geol. Soc. Aust. Abstract 41, 450. 25

Deformation and metamorphism during the c.2000 Ma Glenburgh Orogeny and c.1800Ma Capricorn Orogeny. S. A. OCCHIPINTI, S. SHEPPARD, I. TYLER & D. NELSON Geological Survey of Western Australia, 100 Plain Street, East Perth, WA 6004. In the southern Capricorn Orogen, particularly in

intrusive rocks of the Bryah Group. This succes-

the Gascoyne Complex, there is evidence for two

sion is overlain by the Padbury Group (Padbury

main orogenic events. The 2000-1960 Ma Glen-

Basin), consisting of iron-formation, siliciclastic

burgh Orogeny has recently been recognised

and carbonate sedimentary rocks. The Bryah and

(Occhipinti etal., 1999) and is thought to reflect

Padbury Groups are faulted against the Yarlar-

northwest-southeast accretion of a late Archaean

weelor gneiss complex and poly-deformed and

to Palaeoproterozoic microcontinent onto the Ar-

metamorphosed in the greenschist facies.

chaean Yilgarn Craton. The 1830-1780 Ma Capricorn Orogeny reflects the oblique north-south

Along the northwestern margin of the Yilgarn Cra-

collision between the Archaean Yilgarn and Pil-

ton, and in the southernmost part of the

bara Cratons (Tyler and Thome, 1990), and was

Gascoyne Complex, monzogranite plutons of the

associated with widespread felsic magmatism

Wooramel suite were emplaced at ca. 1960 Ma

(Occhipinti etal., 1998; Krapez and McNaughton,

toward the end of the Glenburgh Orogeny. At

1999).

1820-1800 Ma sheets and dykes of monzogranite and syenogranite of the Moorarie Supersuite

Different geological domains in the south Capri-

were intruded into the Yarlarweelor gneiss com-

corn Orogen include the 3300-1800 Ma Yarlar-

plex. The sheets and dykes range from sub-

weelor gneiss complex, the 2550-1790 Ma

parallel to highly discordant to gneissic layering in

Gascoyne Complex, and the ca. 2000 Ma Bryah

Archaean granitic gneiss, and range from well fo-

and Padbury Basins (Fig. 1). The Yarlarweelor

liated to massive and undeformed. Local melt

gneiss complex is a part of the Archaean Narryer

patches associated with granitic gneiss adjacent

Terrane (Yilgarn Craton) that was metamor-

to sheets of ca. 1812 Ma granite, together with

phosed, deformed and intruded by voluminous

amoeboid and polygonal granoblastic texture in

felsic magma at ca. 1800 Ma. The Errabiddy

the granite sheets and veins, suggest that they

Shear Zone separates the Yilgarn Craton and the

were associated with a syn- to post-tectonic me-

Yarlarweelor gneiss complex from the Gascoyne

dium- to high-grade metamorphic event. Younger

Complex. The Gascoyne Complex consists of

granite at ca. 1800 Ma intruded into the Yarlar-

metamorphosed 2550-1970 Ma tonalite,

weelor gneiss complex contains greenschist fa-

trondhjemite, granodiorite and monzogranite, and

cies assemblages, with sericite after biotite and

amphibolite, mafic granulite, pelitic schist, calc-

feldspar, and textures that are dominantly mas-

silicate gneiss and dolomitic marble intruded by

sive igneous textures, but are locally foliated and

granites at ca. 1800 Ma. The Bryah Basin con-

technically banded. Some 140° and 170° trend-

tains siliciclastic and chemical sedimentary rocks

ing sigmoidal dykes suggest emplacement during

and mafic to ultramafic volcanic and associated

dextral strike-slip faulting. This is also consistent

Tectonics Special Research Centre, School of Applied Geology, Curtin University of Technology, September 1999

Fault Geological boundary Road

Alluvium, colluvium +

Camel Hills is r Metamorphics ICS L

Mine Nardoo Granite (c. 1975 Ma) Foliated and gneissic granite (2000-1985 Ma) Metasedimentary rock Gneissic granite and granitic gneiss (2550-71985 Ma)

Moorarie r n. 7] Supersuite [_ I + I Granite (1830-1780 Ma) Wooramel suite

Homestead

p T Dalgaringa Supersuite

I Discretion Granite (c. 1620 Ma) Mount James Formation

•

Yarlarweelor gneiss complex (3300-1800 Ma)

Granite (c. 1960 Ma)

Figure 1. The components of the Archaean Yilgarn Craton, Yarlarweelor gneiss complex, South Gascoyne Complex, and Errabiddy Shear Zone. Inset: Geological domains within the southern Capricorn Orogen. with dextral shear sense indicators on many 080° and 110° faults.

schist and gneiss, pelitic schist, calc-silicate gneiss, and dolomitic marble, form distinct bands within the granitic gneiss units of the Gascoyne

Within the Gascoyne Complex at ca. 2550 Ma to-

Complex.

nalite, trondhjemite, monzogranite and granodiorite intruded into c. 2550 Ma granodiorite, tonalite

U-Pb SHRIMP zircon dating of calc-silicate and

and monzogranite. Tonalite and granodiorite of

pelitic schists and gneisses of the Camel Hills

the Nardoo Granite intruded these rocks at ca.

Metamorphics in the Errabiddy Shear Zone, indi-

1975 Ma. The 2000-1975 Ma granites form the

cates that they have maximum protolith ages of

Dalgaringa Supersuite, and they have been het-

ca. 1960 Ma. The pelitic schists are locally mig-

erogeneously deformed, locally forming granitic

matised and intruded by ca. 1960 Ma trondhje-

gneisses. Supracrustal rocks including mafic

mite indicating that deposition, metamorphism

Two Billion Years of Tectonics & Mineralisation

and intrusion of trondhjemite may have occurred

hornblende, and locally hypersthene and garnet

over only a few million years. Detrital zircons from

in mafic gneiss; biotite, quartz, cordierite, and silli-

within the pelitic schists are dominated by ca.

manite in pelite. These mineral assemblages are

2000 Ma to c. 2250 Ma ages, whereas within the

locally overprinted by lower temperature M2 as-

calc-silicate gneisses, detrital zircons are mostly

semblages; for example, in pelitic rock staurolite

ca. 2600 Ma to ca. 2700 Ma in age. Therefore,

and kyanite grow at the expense of cordierite and

the sedimentary protoliths to the calc-silicate

sillimanite. The assemblages that formed coinci-

gneisses were probably sourced from the Yilgarn

dent with the schistosity in the Nardoo granite

Craton, whereas precursors to the pelitic schists

consist of biotite, oligoclase-andesine, and epi-

may have been sourced from the Gascoyne

dote, indicating that these rocks were metamor-

Complex.

phosed at epidote-amphibolite grade during M2.

Four deformation events (D1-D4) are recognised

Granitic gneisses of the Dalgaringa Supersuite,

in the Gascoyne Complex. D1 and D2 are attrib-

mafic to ultramafic gneisses, calc-silicate gneis-

uted to the Glenburgh Orogeny, and D3 and D4

ses, dolomitic marbles and pelitic schists are

to the Capricorn Orogeny. D1 is associated with

folded into close to tight upright, shallow to mod-

M1, during which a high-grade (amphibolite to

erately plunging east-southeast, east and east-

granulite facies) foliation developed in the ca.

northeast trending F3 folds. These rocks are cut

2550 Ma and ca. 2000 Ma granitic rocks, mafic

by sheets and veins of 1830-1795 Ma Moorarie

gneisses and pelitic schists. Locally metamor-

Supersuite rocks that trend subparallel to the fold

phism outlasted deformation and granoblastic

axial surfaces. The Moorarie Supersuite includes

textures are strongly developed throughout the

the ca. 1810 Ma Dumbie Granodiorite, which is

rocks. Some of the granitic gneisses contain iso-

massive to well foliated and contain a strong L-

clinally folded granite slivers with F2 fold axial

tectonite fabric. Narrow shear zones that cut

surfaces subparallel to S1. Well-foliated mon-

these rocks are veined by ca. 1795 Ma biotite

zogranite, 5km south of Glenburgh homestead,

monzogranite, and contain clusters of tourmaline.

intrudes folded pegmatite and monzogranite

Changes in fold orientation are due to refolding

banded tonalite gneiss as sheets, trending sub-

about open upright north-northeast trending F4

parallel to the F2 fold axial surfaces. This ca.

folds that may have developed as late as ca.

1987 Ma granite contains biotite clots, developed

1000 Ma or younger. Discrete faults and shear

after garnet, indicating it was metamorphosed at

zones trend east-southeast in the Dalgaringa Su-

high-grade (amphibolite or higher), and then ret-

persuite rocks adjacent to the Errabiddy Shear

rogressed. These rocks were intruded by the ca.

Zone. These faults ductiley deformed the Dalgar-

1975 Ma Nardoo Granite, which was metamor-

inga Supersuite, and probably developed at ca.

phosed at medium grade (M2) at ca. 1960 Ma.

1800 Ma.

M1 mineral assemblages include: plagioclase,

Outcrops of low-grade metamorphosed sedimen-

quartz, biotite, K-feldspar, minor muscovite, allan-

tary rocks of the Mount James Formation form

ite, epidote, and garnet in meta-tonalite; plagio-

elongate lenses in high-strain zones within the

clase, clinopyroxene, biotite, orthopyroxene in

south Capricorn Orogen. The formation is uncon-

meta-quartz diorite; clinopyroxene, plagioclase,

formable on, and faulted against Gascoyne Com-

Tectonics Special Research Centre, School of Applied Geology, Curtin University of Technology, September 1999

plex rocks. West of the area shown on Figure 1, a

the Glenburgh Orogeny, including the Errabiddy

matrix supported quartz pebble to boulder con-

Shear Zone, were re-oriented during this event.

glomerate contains clasts probably derived from

Granites of the Moorarie Supersuite that intruded

basement amphibolite. The Mount James Forma-

throughout the Gascoyne and Yarlarweelor

tion is isoclinally folded and contains a well-

gneiss complexes do not intrude into the Bryah

developed greenschist facies foliation.

and Padbury Groups. This suggests either that the Bryah and Padbury basins were not in contact

The Bryah and Padbury Groups have a maximum

with the Yarlarweelor gneiss complex during the

age of ca. 2000 Ma and contacts between these

ca. 1800 Ma magmatic event, or that part of the

groups are unconformable (Martin, 1994) and/or

Bryah and/or Padbury Basins have been subse-

faulted. The Bryah Basin developed as a back-

quently removed by faulting. Deposition, deforma-

arc rift, whereas the Padbury Basin evolved as a

tion and metamorphism of the Mount James For-

retro-arc foreland basin (Martin, 1994), possibly

mation probably took place during the waning

during the early stages of the Capricorn Orogeny.

stages of the Capricorn Orogeny.

Principal structures include, subhorizontal D1 folds and foliation, which are refolded into upright,

References

tight to isoclinal north or east-trending structures.

Krapez, B., and McNaughton, N. J., 1999, SHRIMP zir-

East-trending folds are locally deformed about

con U-Pb age and tectonic significance of the

north-trending open, upright folds. Most rocks

Palaeoproterozoic Boolaloo Granodiorite in the

were metamorphosed at greenschist facies

Ashburton Province, Western Australia: Austra-

(during D1 and D2), but a sliver of Padbury Group faulted against the Yarlarweelor gneiss complex was metamorphosed at upper greenschist to amphibolite facies. The southern part of the Gascoyne Complex is a latest Archaean to Palaeoproterozoic microconti-

lian Journal of Earth Sciences, v. 46, p. 283-287. Martin, D. McB., 1994, Sedimentology, sequence stratigraphy, and tectonic setting of a Palaeoproterozoic turbidite complex, Lower Padbury Group, Western Australia: Unpubl. PhD thesis, The University of Western Australia, 194p. Occhipinti, S. A., Sheppard, S., and Tyler, I. M., 1999, The Palaeoproterozoic tectonic evolution of the

nent that accreted onto the Yilgarn Craton during

southern margin of the Capricorn Orogen, West-

the ca. 2000 Ma Glenburgh Orogeny. The Er-

ern Australia: Geological Society of Australia,

rabiddy Shear Zone marks the boundary between the south Gascoyne Complex, and the Yilgarn Craton including the Yarlarweelor gneiss complex. Accretion of the south Gascoyne Complex onto the Yilgarn Craton was probably completed by ca. 1960 Ma, the time at which granites intruded into the southern margin of the Gascoyne Complex and northernmost part of the Yilgarn Craton (Wooramel suite). The Capricorn Orogeny reflects the prolonged (1830-1780 Ma) oblique

Abstract Series 53, p. 173-174. Occhipinti S. A., Sheppard, S., Nelson, D. R, Myers, J. S., and Tyler, I. M., 1998, Syntectonic granite in the southern margin of the Palaeoproterozoic Capricorn Orogen, Western Australia: Australian Journal of Earth Sciences, v. 45, p. 509-512. Tyler, I. M., and Thome, A. M., 1990, The northern margin of the Capricorn Orogen, Western Australia - an example of an early Proterozoic collision zone: Journal of Structural Geology, v. 12, p. 685-701.

convergence between the Archaean Yilgarn and Pilbara Cratons. Structures that developed during

Two Billion Years of Tectonics & Mineralisation

Geology, mineralisation and geodynamic evolution of the Palaeoproterozoic Yerrida and Earaheedy Basins, W.A. FRANCO PI RAJ NO Geological Survey of Western Australia, 100 Plain Street, East Perth, IM4 6004. The volcano-sedimentary Yerrida and sedimentary Earaheedy Basins are Paleoproterozoic rift-related basins along the eastern part of the Proterozoic Capricorn Orogen. Gravity and magnetic data indicate both basins are floored by Archaean granite-greenstone rocks. The basins contain several granitegreenstone inliers (Marymia, Goodin, Malmac), indicating fragmentation of the adjacent northeastern Yilgarn Craton. The ages of both basins are poorly constrained: the Earaheedy Basin is unconformable on the Yerrida Basin (Fig. 1), and best estimates from sparse Pb-Pb model ages, U-Pb zircon analyses, and field relationships, suggest ages of 2.2 - 1.9 Ga for the Yerrida Basin, and 1.8 - 1.65 Ga for the Earaheedy Basin. Unraveling the history of these basins contributes to understanding the overall tectonic evolution and geological processes during the Capricorn Orogen. Yerrida Basin.

The Yerrida Basin covers approximately 10,000 km2 (Fig. 1), and contains the Windplain and Mooloogool Subgroups within the Yerrida Group (Pirajno et al., 1998). The Yerrida Basin is in fault contact with the Bryah Basin in the west, which is characterized by rocks of oceanic affinity (Pirajno et al., 1998). The lower unit of the Windplain Subgroup, the Juderina Formation, is dominated by siliciclastic rocks and stromatolitic carbonate, with local evaporitic rocks. The overlying Johnson

Cairn Formation is a shaly sequence, reflecting relatively low sediment influx and basin deepening. The rocks were deposited in mature fluvial and tidal environments, within a sag basin. The Mooloogool Subgroup marks a change to a rift-fill setting with abundant volcanism. The Thaduna and Doolgunna Formations, at the base of the Mooloogool Subgroup, consist of conglomerates and turbidites. The former contains lithic fragments mostly from the Marymia Inlier, and megabreccias in the latter were sourced from rocks that overlie the Goodin Inlier (Juderina Formation). Both formations interdigitate with volcanic rocks of the Killara Formation, so volcanism and clastic sedimentation were contemporaneous. The Killara Formation is a 1000 m-thick succession of tholeiitic sills and subaqueous to subaerial lavas, emplaced during a rifting event. Thick successions of lava flows with no intervening sedimentary material indicate a high rate of eruption. Tholeiites in the formation have affinities with continental flood basalts (Pirajno et al., 1998). In the southeast of the Yerrida Basin, the tholeiites are capped by volcaniclastic, evaporitic and chemical sedimentary rocks of the Bartle Member (Killara Formation), deposited during a final and quiescent stage of the Killara volcanic activity, in an environment of salt lakes and localized hot springs (Pirajno and Grey, 1997 and in prep.). The lithology and sedimentary structures of sedimentary units in the Mooloogool Subgroup are in-

Tectonics Special Research Centre, School of Applied Geology, Curtin University of Technology, September 1999

dicative of lacustrine deposition. The stratigraphic

the northwest. Above the Chiall Formation, the

relationships between rock units suggest that the

shaly Wongawol Formation and overlying

Yerrida lake was a dynamic depositional system,

nearshore stromatolitic Kulele Limestone reflect

in which turbidites and tholeiitic lavas were

cessation of tectonism and gradual sediment

broadly contemporaneous, and the lake ranged

starvation. The Mulgarra Sandstone, at the

from deep to shallow and evaporitic. The end of

preserved top of the Earaheedy Basin

the lake's life is marked by its infilling with sul-

succession, may indicate renewed tectonism and

phidic shales of the Maryloou Formation.

progradation.

Earaheedy Basin.

The northern margin of the Earaheedy Basin is

The Earaheedy Basin covers approximately 2

40,000 km (Fig. 1), and contains the Earaheedy Group, which is subdivided into Tooloo and Miningarra Subgroups. The Tooloo Subgroup contains three formations. The basal Yelma Formation consists of siliciclastic rocks and stromatolitic carbonate. The overlying Frere Formation is dominated by granular iron formation with intercalated Fe-rich siltstone and black shale units. Its shale component increases eastward, at the expense of iron content. The source of the Fe could have been volcanic activity in the west or northwest. The Frere Formation is overlain by the Windidda Formation, which consists of rhythmically laminated shale, carbonate, and minor sandstone. It accumulated in a low-energy, shallow-water, ?lacustrine setting. The Miningarra Subgroup, above, contains the Chiall and Wongawol Formations, Kulele Limestone and Mulgarra Sandstone. The Chiall Formation is disconformable on the Windidda Formation, and comprises the Wandiwarra (dominantly shelfal) and Princess Ranges (dominantly coastal to fluvial) Members. It is interpreted as reflecting overall northwestwards deltaic progradation in a northwestwards-deepening basin. Tectonism on cross-basin faults probably caused both terrigenous influx and accelerated subsidence in

Two Billion Years of Tectonics & Mineralisation

characterized by intense deformation, manifested by tight folds with sheared limbs, locally with thrusts and mylonite zones. Along much of this margin the rocks have a strong penetrative fabric and have been grouped into the Troy Creek Schist. This comprises phyllite, quartz-sericite schist, quartz-chlorite schist, and shale, all metamorphosed to greenschist facies, and is considered to be dynamically metamorphosed Earaheedy Group. Geodynamic evolution and mineralisation Inter-cratonic collisional tectonics are proposed for the western parts of the Capricorn Orogen during the Glenburgh and Capricorn Orogenies by Occhipinti et al. (1999) and Tyler et al. (1998). In the eastern Capricorn Orogen, the Yerrida and Earaheedy Basins are floored by, and contain inliers of, Archaean granite-greenstone rocks, and probably developed by intracratonic processes, at different times. The Yerrida Basin developed by sagging and subsequent rifting of continental crust, prior to collision of the Yilgarn Craton with cratonic blocks to the north or west. Strike-slip faulting led to a pull-apart structure, the geometry of which exerted an influence on rift-fill sedimentation. The basin was subsequently deformed during the Capricorn Orogeny. The Earaheedy Basin only developed after deformation of the Yerrida Basin, as undeformed Yelma Formation

Marymia 4lnlier. +

Shoemaker ^Structure ^

:Goodin :: Inlier: WILUNA

MEEKATHARRA

Phanerozoic cover O

-r o ^ N 06 2 o® go

Officer Basin Bangemall Basin Scorpion Group

I Troy Creek Schist Miningarra Subgroup Wongawol Subgroup

2(5 (0

Granites and Greenstone belts

X ' / / ^ Narryer Gneiss Terrane

Fold-and thrust belt

Padbury Group Bryah Group Yerrida Group

crosses the suture that juxtaposes the Bryah

episodes resulted first in the Yerrida fold-and-

Group against the Yerrida Group (Fig. 1). The ab-

thrust belt, along the Goodin Fault on the western

sence of volcanism in the Earaheedy Basin, un-

side of the basin during the Glenburgh Orogeny

like the Yerrida Basin, suggests rifting was insig-

and perhaps the Capricorn Orogeny, and later in

nificant in the basin's history. Instead, the Eara-

the fold-and-thrust belt along the northern margin

heedy Basin formed by sagging of continental

of the Earaheedy Basin (Troy Creek Schist; Fig.

crust and progressive subsidence from west to

1), at the end of the Capricorn Orogeny or even

east, with initial sedimentation of carbonate,

later (but prior to development of the Bangemall

shale, and sandstone followed by major clastic Fe

Basin).

oxides deposition (granular iron-formation). This

Mineralisation in these basins is the result of their

was followed in turn by mixed carbonate-clastic

geodynamic evolution. Thus, there are two types

deposition of the Windidda Formation, in a major

of mineral deposits: 1) basin-fill related and pre-

lake or shallow sea. The Chiall Formation was de-

orogenic; and 2) hydrothermal syn-post orogenic.

posited by deltaic and fluvial progradation from a

To the first category belong: a) black shale-

technically rejuvenated basin-hinterland.

hosted Cu-Zn-Ba-Pd sulphide occurrences pres-

Tectonic movements along the northeastern mar-

ent in the Yerrida Basin; b) carbonate-hosted Pb-

gins of the Yilgarn Craton resulted in reworking

Zn-Cu and Pb mineralisation (MVT style) in rocks

and uplift of basement units and deformation of

of the Yelma Formation; and c) the huge Fe ox-

the basin successions. Successive deformation

ides accumulation of the Frere Formation. Sev-

Tectonics Special Research Centre, School of Applied Geology, Curtin University of Technology, September 1999

eral carbonate-hosted Pb-Zn-Cu prospects are

resource of about 17 0001 of Cu metal at an

present to the west of the Shoemaker Impact

average grade of 3.4% (Pirajno and Preston,

Structure. The role of this structure in the redistri-

1998). In places, Au mineralisation is associated

bution and possible upgrade of this mineralisation

with thrusts and mylonite zones in the Troy Creek

is currently being assessed. The Magellan Pb de-

Schist.

posit, situated in the southeast of the Yerrida Ba-

References

sin, is hosted in an outlier of the Yelma Formation of the Earaheedy Group, but the mineralisation

Occhipinti, S. A., Sheppard, S., & Tyler, I. M.,

transgresses the unconformity and is also found

1999. The Palaeoproterozoic tectonic

within the upper units of the Yerrida Group. The

evolution of the southern margin of the

discovery of the Magellan deposit was announced

Capricorn Orogen, Western Australia.

by Renison Goldfields Consolidated in 1993, with

Geological Society of Australia Abstracts

resources estimated at approximately 220 Mt at

Series 53, p. 173-174.

2.2% Pb. The mineralization consists of cerussite,

Pirajno, F. and Grey, K., 1997, A

plattnerite (PbO) and pyromorphite. The deposit

Palaeoproterozoic hot spring environment for

may be the result of palaeoweathering processes,

the Bartle Member cherts of the Yerrida Basin,

under physico-chemical conditions which were

Western Australia. Western Australia

conducive to the oxidation and subsequent mobi-

Geological Survey Annual Review 1996-97,

lization of the Pb metal. The Pb was possibly sourced from weathered basement rocks. This is supported by Pb isotopic values (206Pb/204Pb of 15.97153 and 15.96831; 207Pb/204Pb of 15.48658 and 15.48611;

208

204

Pb/ Pb of 35.35561 and

35.35841). These data provided a Pb-Pb model age of the carbonate ore material of 1.65 Ga. Both MVT and stratiform/stratabound Pb mineralisation probably originated from the expulsion of basinal brines. Lateral and vertical tectonic com-

p.116-121. Pirajno, F., and Preston, W. A., 1998, Mineral deposits of the Padbury, Bryah and Yerrida Basins: In Geology of Australian and Papua New Guinean Mineral Deposits, The Australasian Institute of Mining and Metallurgy, Melbourne, (edited by D. A. Berkman and D. H. Mackenzie), p. 63-70. Pirajno, F., Occhipinti, S., A. and Swager, C. P.

pression of these sedimentary successions re-

1998, Geology and tectonic evolution of the

sults in the flow of fluids along permeable hori-

Palaeoproterozoic Bryah, Padbury and Yerrida

zons, from areas of intense deformation towards

basins (formerly Glengarry Basin), Western

tectonically quiet zones. The tectonically induced

Australia: Precambrian Research, v.90, p.

flow may have also played an important role in

119-140.

the genesis of these base metal deposits. Epigenetic hydrothermal syn-post orogenic mineralisation includes shear zone-hosted Cu and Au mineralisation in both the Yerrida Basin. The Thaduna Cu deposits produced a total of

Tyler,!., Pirajno, F., Bagas, L., Myers, J. S., and Preston, W. A., 1998, Geology and mineral deposits of the Proterozoic of Western Australia: AGSO Journal of Geology and Geophysics, v. 17, p. 223-244.

28231 of Cu metal and have a total remaining

Two Billion Years of Tectonics & Mineralisation

Rock magnetic and palaeomagnetic results from high-grade metamorphic and intrusive rocks: Determination of magnetic anisotropy and a 1.2 Ga palaeomagnetic pole from the Bremer Bay area, Albany Mobile Belt, Western Australia. SERGEI PISAREVSKY & LYALL HARRIS TSRC, Dept Of Geology & Geophysics, UWA, NEDLANDS WA 6907. Palaeomagnetic studies are generally carried out on intrusive igneous rocks (such as mafic dykes) and low-grade sedimentary rocks. High-grade metamorphic rocks, however, may also be carriers of palaeomagnetic information that corre-

Belt separates the Yilgarn Craton from the East Antarctic Shield (Harris, 1995). The belt is correlated with the basement gneiss complex in Meghalaya in northeastern India, from where it is thought to continue across India along the Central

sponds to the time of uplift and cooling (Dunlop

Indian Tectonic Zone (Harris, 1993).

and Ozdemir, 1997). This study was aimed at de-

Granulite facies tonalitic and mafic gneisses of

termining whether meaningful paleomagnetic re-

the Central Domain are best exposed in the

sults could be obtained from high-grade rocks

Bremer Bay region in the eastern part of the belt,

within a Mesoproterozoic orogenic belt. The

where it trends northeasterly. This study sought to

Bremer Bay region of the Albany Mobile Belt,

determine a palaeomagnetic pole for these high-

Western Australia, was chosen as the deforma-

grade rocks and late dykes for comparison to the

tion history is well known, and constrained by U -

Australian Apparent Polar Wander Path

Pb SHRIMP and Rb-Sr geochronology.

(AAPWP). The AAPWP for the Mesoproterozoic

In the Albany Mobile Belt along the southern and southeastern margins of the Archaean Yilgarn Craton in Western Australia, Archaean and Paleoproterozoic granitoids, gneisses, metasediments and mafic dykes have been reworked within a dextral transpressional regime at approximately 1.2 Ga (Black et. al., 1992). A Central Do-

(Idnurm etal., 1995) is presently constrained by a small number of sites, the majority of which have been obtained from northern Australia. Magnetic anisotropy was also measured in order to relate magnetic fabric with regional strain, and to thus indirectly establish the timing for acquisition of the magnetic fabrics.

main of granulite fades gneiss has been obliquely

The presence of 3.1 Ga zircon cores with 1.2 Ga

thrust onto amphibolite fades gneiss of the North-

zircon over-growths and Sm-Nd model ages of

ern Domain (Beeson eta!., 1988). A transitional

-2.1 Ga (Black et. al., 1992) in tonalitic gneisses

contact is seen with the Southern Domain, which

indicates Archaean and Paleoproterozoic precur-

is comprised of dominantly amphibolite fades

sors. High-grade metamorphism in the Central

gneiss and syn- to post-tectonic granitoids.

Domain granulites and intrusion of late- to post-

Granulite facies assemblages are locally pre-

tectonic granitoids occurred at ~1.2 Ga based on

served in the Southern Domain, especially in

U-Pb SHRIMP geochronology, whereas the Rb-

cores of regional anticlines. In reconstructions of

Sr system (which has a lower closure tempera-

the Rodinia super-continent, the Albany Mobile

ture than the Curie temperature) has closed at

Tectonics Special Research Centre, School of Applied Geology, Curtin University of Technology, September 1999

-1.1 Ga. (Black et. al., 1992). A southwards pro-

show no penetrative foliation. A Rb-Sr date of

gressive overprinting of Proterozoic fabrics and

~1.1 Ga for these dykes may not be a true intru-

increase in metamorphic grade across the North-

sive or metamorphic age due to their coarse-grain

ern Domain (Beeson etal., 1988) indicates that

size (Black et. al., 1992).

mafic granulite layers that are generally concordant with the regional gneissosity represent metamorphosed equivalents of dolerite dykes of the southern Yilgarn Craton.

Initial measurements of Natural Remanent Magnetisation (NRM) and magnetic susceptibility have shown a large variation. Only dolerites define a compact group with relatively high values of

A complex deformation history during high-grade

the Koenigsberger's Q ratio between 1.94 and

metamorphism is recorded in the Bremer Bay

8.02. Such values indicate that these rocks have

area. NW-verging folds and thrusts, along with

stable remanences (Piper, 1987). The same is

minor SE-verging 'back-thrusts', indicate sub-

true for most of the studied gneisses; however,

horizontal NW-SE contraction during conver-

three samples carry smaller values of Q. Mag-

gence between East Antarctica and the Yilgarn

netic properties of mafic granulites vary widely,

Craton. NE-striking normal shear zones and per-

and seven samples show Q > 0.5. Granites are

vasive boudinage of mafic layers indicate inter-

relatively weakly magnetised and their Q values

vening episodes of horizontal NW-SE extension

are low.

resulting from orogenic collapse and/or delamination of a lithospheric root. Quartz, plagioclase, Kfeldspar, orthopyroxene and clinopyroxene ± magnetite ± biotite pegmatites which intrude boudin necks and normal shear zones gave a Rb-Sr age of 1140±40 Ma. Dolerite dykes that intrude during these extensional stages along NEstriking normal shear zones and as discrete crosscutting bodies have also been sampled for palaeomagnetic study for comparison with older

The low field susceptibility versus temperature curves for specimens from the various rock types all point to a narrow Curie temperature range close to that of pure magnetite. Curves do not show any significant Hopkinson's peaks. Minor features on heating curves at about 350°C are probably caused by the oxidation of some small amount of maghemite. The fact that cooling curves show some decrease of magnetic susceptibility after heating supports this suggestion.

mafic granulite and host gneiss. A sub-vertical NE-striking spaced foliation within pegmatites and

Saturation IRM (SIRM) curves show similar coer-

mafic dykes indicates a return to NW-SE contrac-

civity properties for all studied rock types. Despite

tion after their emplacement. Mafic boudins with

the dominance of multi-domain (MD) magnetite,

NW-trending necklines, NW-striking normal duc-

IRM also increases between ca. 100 and 500 mT.

tile shear zones and local development of a NE-

This may be caused by the presence of single-

SW oriented mineral lineation indicate that the

domain (SD) and/or pseudo-single-domain (PSD)

maximum extension direction was horizontal and

magnetite (Lowrie, 1990; Dunlop and Ozdemir,

oriented NE-SW.

1997). The magnetic susceptibility versus SIRM diagram proposed by Thompson and Oldfield

The last samples come from granitic and pegma-

(1986) shows that MD magnetite dominates in

tite dykes striking 135° that bisect late conjugate

many cases; however, at the estimated grain

faults and brittle-ductile shear zones. These

sizes of dolerites, some gneisses and some mafic

dykes intruded during NW-SE contraction / NE-

granulites are inside or close to the PSD range.

SW extension at a shallower crustal level, and

Two Billion Years of Tectonics & Mineralisation

Thompson and Oldfield (1986) also proposed to use a plot of SIRM versus remanence coercivity [(BO)CR] to determine the structural state in magnetic mixtures. All treated dolerites and one sample of mafic granulite fall into the PSD area. The version (Dunlop and Ozdemir, 1997) of the Lowrie-Fuller test (Lowrie and Fuller, 1971) in most cases gave a zero result. There were only two exceptions that indicate the SD behaviour. All these experiments indicate that the studied dolerites, mafic granulites and gneisses are suitable for palaeomagnetic analysis as they contain a sufficient amount of magnetite with MD, PSD and SD structure. Further demagnetisations also indicated the presence of haematite in at least some of these rocks. It seems to be unlikely that the granites studied contain some stable remanence. Anisotropy of magnetic susceptibility (AMS) was measured in all studied specimens. The degree of anisotropy is high enough in the great majority of cases. The parameter Pj (Tarling and Hrouda, 1993) ranges from 1.1 to 12.0 with a median value of 1.301. All rocks except granites seem to have a similar magnetic fabric, and no correlation between rock types and directions of principal axes of anisotropy was found. Agreement of magnetic fabric determinations for rocks with different structural and intrusive relationships suggests that no pre-metamorphic magnetic fabric survived during high-grade metamorphism. Well-grouped directions of the maximum axes define the fabric lineation (Borradaile and Henry, 1997; Tarling and Hrouda, 1993) with mean direction of 217° decl. and 19° incl. This lineation parallels the NE-SW maximum extention direction determined for the last penetrative ductile deformation event. The directions of intermediate and minimum axes form a girdle which is characteristic for the prolate ellipsoid of the magnetic fabric. However the concentration of a big group of minimum axes around the direction of 316° decl. and 32° incl. indicates

a minor foliation perpendicular to this direction. Such a foliation is observed regionally, associated with late-stage ductile deformation where structures confirm NW-SE contraction (its inclination may reflect either the original orientation of the principal strain axes, or minor post-deformational tilting). Three specimens from granite dykes show quite different directions of AMS axes. This may be due to the younger age of these dykes, which postdate penetrative deformation. More samples must, however, be studied to confirm this suggestion. Thermal and AF demagnetisations revealed two characteristic components of NRM (ChRM) - BB1 and BB2. Not a single sample contains both of these components. Only in granites is a stable remanence absent. A BB1 bipolar component has been isolated in 24 samples of gneisses, dolerites and mafic granulites. Its mean direction is: D=176.9°,1=72.5°, k=14.0, a95=8.2fi. No correlation between polarity and rock type was found. Moreover, both polarities are present in the late, cross-cutting dolerite dyke. This one-component, bipolar stable remanence in all studied rock types of the area (except granites) was probably acquired in the process of slow cooling during the uplift at the last stage of the metamorphic event. A few samples from contacts of the dolerite dyke and older mafic bodies also have the BB1 direction. This also can be explained by a metamorphic overprint. The reversal test of McFadden and McElhinny (1990) gave a positive result for the BB1 component with classification C. The palaeomagnetic pole (66.6fiN, 303.7fiE) fits the ca.1.2 Ga part of AAPWP (Fig.1). A BB2 single-polarity component has been isolated in seven samples of mafic granulites. Its mean direction is: D=141.9fi, 1=3.6°, k=6.6, a95=25.5fi. No systematic differences in magnetic

Tectonics Special Research Centre, School of Applied Geology, Curtin University of Technology, September 1999

characteristics (NRM, magnetic susceptibility, Q

References

ratio, SIRM and (BO) C R) of these samples and

Beeson, J., Delor, C. P. and Harris, L. B. 1988. A

mafic granulites with BB1 direction have been

structural and metamorphic traverse across

found. The palaeomagnetic pole (41.82S, 243.7°E)

the Albany Mobile Belt, Western Australia.

falls at some distance from ca.0.8 Ga part of AAPWP (Fig.1).

Precambrian Res. 40/41,117-136. Black, L., Harris, L. B. and Delor, C. P. 1992. Reworking of Archaean and Early Proterozoic components during a progressive, Middle Proterozoic tectonothermal event in the Albany Mobile Belt, Western Australia. Precambrian. Res. 59, 95-123. Borradaile, G.J. & Henry, B., 1997. Tectonic applications of magnetic susceptibility and its anisotropy. Earth Sci. Rev., 42, 49-93. Dunlop, D.J. & Ozdemir, O., 1997. Rock magnetism. Fundamentals and Frontiers. Cambridge University Press, 573 pp. Harris, L.B. 1993. Correlations between the Central Indian Tectonic Zone and the Albany Mobile Belt of Western Australia: evidence for a continuous Proterozoic orogenic belt. In: Findlay, R.H, Unrug, R., Banks, M.R. and Veevers, J.J. (Eds). Gondwana 8: assembly,

Figure 1. Palaeomagnetic poles from Bremer Bay rocks. 1, the fragment of the Precambrian APWP for Australia (Idnurm etal., 1995); 2, fragments of the Phanerozoic APWP for Australia (Harris & Li, 1995). The consistency of the paleomagnetic pole from

evolution and dispersal. A.A. Balkema, Rotterdam. 165-180. Harris, L.B. 1995. Correlations between the Albany, Fraser and Darling mobile belts of Western Australia and Mirnyy to Windmill Is-

the Albany Mobile Belt with the AAPWP illustrates

lands in the East Antarctic Shield: implications

that high-grade rocks from within an orogenic belt

for Proterozoic Gondwanaland reconstruc-

with a complex deformation and intrusive history

tions. In Yoshida, M. and Santosh, M. (Eds),

may yield meaningful palaeomegnetic results.

India and Antarctica during the Precambrian.

The magnetic anisotropy is also in complete agreement with structural data. This study thus

Memoir Geological Society of India 34, 47-71. Harris, L.B. & Li., Z.,X., 1995. Palaeomagnetic

opens the way for more research in the Albany

dating and tectonic significance of dolerite in-

Mobile Belt and in similar orogenic belts (where

trusions in the Albany Mobile Belt, Western

palaeomagnetic work has not been previously at-

Australia. Earth Planet. Sci. Lett.,131,143-

tempted), on paleomagnetic determinations and the use of magnetic fabric for structural analysis.

Two Billion Years of Tectonics & Mineralisation

164. Idnurm, M., Giddings, J.W. & Plumb, K.A., 1995.

Apparent polar wander and reversal stratigraphy of the Palaeo-Mesoproterozoic southeastern McArthur Basin, Australia. Precambrian Res., 72, 1-41. Lowrie, W., 1990. Identification of ferromagnetic minerals in a rock by coercivity and unblocking temperature properties. Geophys.Res. Lett., 17, 159-162 Lowrie, W. & Fuller, M., 1971. On the alternating field demagnetisation characteristics of multidomain thermoremanent magnetization in magnetite. J.Geophys.Res76, 6339-6349. McFadden, P.L. & McElhinny, M.W., 1990. Classification of the reversal test in paleomagnetism. Geophys.J.lnt., 103, 725-729. Piper, J.D.A., 1987. Palaeomagnetism and the continental crust. Open Univ.Press, Milton Keynes, New York-Toronto, 434 pp. Tarling, D.H. & Hrouda, F., 1993. The magnetic anisotropy of rocks. Chapman and Hall, London, 217 pp. Thompson, R. & Oldfield, F., 1986. Environmental magnetism. Allen and Unwin, London, 227 pp.

Tectonics Special Research Centre, School of Applied Geology, Curtin University of Technology, September 1999

Ar/39Ar laserprobe dating: Applications to metamorphic and de-

formation histories. STEVE REDDY TSRC, School of Applied Geology, Curtin University of Technology, GPO Box U1987, Perth, WA 6845 40

Ar/39Ar dating is a widely used tech-

nique to constrain the thermal history of 14

rocks and thereby aids the quantification of T-t paths during cooling of metamor-

13 12 10

phic terranes. Usually, 40Ar/39Ar data is interpreted to correlate with the time at which a rock passes through its 'closure

EMC Phengite

990 80|

temperature' during cooling: the closure

H *

+ +

7 70

660

temperature for a particular isotopic system and a particular mineral being a mathematical simplification of the age distribution recorded across a mineral grain. 40Ar/39Ar laser-

500

1000

1500

2000

2500

3000

3500

Distance (|im)

Fig 1. Apparent ^Ar/^Ar ages across a single mica grain. The profiles were taken parallel to the mica cleavage.

probe dating is a high-spatial resolution technique that enables the intra (and inter) grain spatial variation of 40Ar/39Ar ages to be recognised. The technique's principal advantage is that it allows the 'in situ' analysis of grains from a single sample. Consequently it is ideal for the study of samples where an ability to relate preserved Ar ages to spatial variations in texture is essential. Such samples include those with a range of intimately mixed grain sizes, a range of different microstructural sites and grain-scale heterogeneous deformation. The technique is therefore a significant advance over traditional whole grain or multiple grain analyses. In this paper, three different applications of the 40Ar/39Ar laserprobe technique are outlined to show the type of data that can be obtained and the different ways that such data may be interpreted. It will be shown that the approach is excellent for establishing thermal histories and constraining the effects of processes such as deformation that are important in influencing Ar isotope diffusion in metamorphic rocks.

Two Billion Years of Tectonics & Mineralisation

The first example illustrates the distribution of Ar isotopes in a single deformed mica grain from the Eclogite Micaschist Complex (EMC) of the Italian Alps. The analysed grain records a range of ages from mid-Jurassic to Upper Cretaceous (Fig.1). These 'apparent age' variations are spatially related to both location within the grain and to intragrain microstructure. Stepped-heating analysis of micas from the same sample record no such age distribution but yield an average age suggesting the homogenisation of Ar isotopes during furnace heating. Modelling of the laser-derived data shows that the profile formed by the diffusion of an excess argon component into the grain, parallel to the mica cleavage. Profile asymmetry is explained by temporal variations in microstructural development enabling excess argon to enter the grain at different times in different places. The temperatures of the initiation of deformation and the possible time scales for the deformation can be calculated as a function of cooling rate (Fig.

cord a Permian to Upper Cretaceous age range Robbins (1972)

Eol Cooling rate (°C/Ma)

D dt/dT 0.00010 (cm 2 /°C)

® O

right hand margin

0 350

sizes yielding the youngest ages (Fig 3). This re-

left hand margin

/ /

0.00005

300

that correlates with grain size: the smallest grain

Hamilton el al. (1989) |o]

0.00015

lationship is best explained by a partial resetting

of biotite during an Alpine thermal event initiated

400

450

50(

Temperature (°C)

Fig 2. Diagram showing the temperature at which the diffusion profiles for the left- and right- hand boundaries of the grain shown in Fig. 1 initiated as a function of cooling rate and different values of DO and E. Integrated diffusion is calculated assuming that no diffusion occured below 250°C and also assumes linear cooling.

not more than 70 Ma ago. Modelling of these data suggest that the sample never exceeded 300°C during Alpine orogenesis (Fig. 3). This data supports post-metamorphic juxtaposition of the EMC and IIDK units by faulting. Although argon migration in minerals is strongly dependent on temperature, the above examples show a qualitative, but systematic, relationship between argon ages preserved in metamorphic

2). All estimates suggest deformation at greenschist-facies, in accord with the observed retrograde mineral assemblage. Absolute temperature estimates for deformation vary by less than 22°C

Age (Ma)

for cooling rates of 10 and 30°C/Ma. The duration of deformation was at least 2 Ma at 10°C/Ma or 0.7 Ma at 30°C/Ma. Integration of such data with other isotope/mineral systems would yield abso-

Grain Radius (pm)

lute ages for both the timing and duration of deformation. The second example comes from the uppermost structural levels of the Italian Alps and outlines the benefits of analysing different grain sizes from different microstructural sites. The IIDK unit of the Sesia Zone records amphibolite/granulite preAlpine mineral assemblages with only a local Al-

Fig 3.Age variation vs mean grain size based for IIDK biotites. Modelled argon loss for different grain sizes of biotites for a heating event of 10 Ma duration assuming that heating took place at 70 Ma ago and that at the time of heating different grain sizes had the same argon composition (equivalent to present day value of 260 Ma). Dotted lines represent different temperatures of heating (200, 250, 300 and 350°C).

pine, greenschist fades metamorphic overprint. Alpine geologists have longed argued over the Al-

minerals and their deformation history. The third

pine history of the IIDK. One group argues that

example outlines a combination of forescatter

the IIDK must have gone to eclogite fades condi-

scanning electron microscopy (SEM) and laser

tions, along with the EMC with which it is now in

contact, but did not undergo metamorphic reac-

of potassium feldspar. The forescatter SEM tech-

tions for kinetic reasons. A second group argues

nique provides an orientation contrast image of a

that the IIDK did not go to eclogite fades but was

mineral and thereby allows differences in crystal-

Ar/39Ar dating applied to a single deformed grain

juxtaposed with the EMC during or after EMC ex-

lography orientation of different parts of the grain

humation. Biotite grains from the IIDK sample re-

to be recognised. In this way, intragrain microstructure can be non-destructively qualified (and

Tectonics Special Research Centre, School of Applied Geology, Curtin University of Technology, September 1999

quantified) and the same area may be then analysed by the laserprobe. In this study, the form and distribution of deformation-induced microstructures has been mapped using intragrain variations in lattice orientation. These variations are related to thin section and regional structural data to provide a well-constrained deformation history for the feldspar grain. Thirty laser analyses show age variations that correlate with the internal microstructure. This approach allows the recognition of different Ar reservoirs within the deformed feldspar and identifies microstructural controls on Ar migration. Such a relationship has important implications for the understanding and interpretation of 40Ar/39Ar geochronological data and the subsequent use of such data to constrain the temperature - time evolution of deformed rocks. These three studies illustrate the complexities of Ar distribution in deformed metamorphic rocks and illustrate that detailed analysis of individual mineral grains can provide information that yields the complex thermal histories of rocks. Studies of the relationship between deformation and Ar isotope systematics are further providing new insights that may ultimately lead to better approaches to the absolute dating of deformation.

Two Billion Years of Tectonics & Mineralisation

Late Paleoproterozoic amalgamation of the north-eastern Siberian craton: ancient terranes, shear zones and granites melt out (a synthesis of Sm—Nd and U—Pb dating). 1O.M. ROSEN, 2M.K. SUKHANOV

, 2D.Z. ZHURAVLEV, 3E.V. BIBIKOVA & 1V.L. ZLOBIN

1nstitute of the Lithosphere of Marginal Seas, Russian Academy of Sciences, Staromonetny per. 22, Moscow, 109180, Russia. Institute of Geology of Ore Deposits, Petrography, Mineralogy, and Geochemistry (IGEM), Russian Academy of Sciences, Staromonetny per. 35, Moscow, 109017 Russia. 3 Vernadsky Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences, ul. Kosygina 19, Moscow, 117975, Russia.

The north-eastern Siberian craton was formed in the Late Paleoproterozoic at 2.0-1.8 Ga owing to accretion of the Archean microcontinents (granulite-gneiss and granite-greenstone terranes) juxtaposed along the collisional shear zones (Rosen etal., 1994). A resultant mountain range was eroded before 1.65 Ga, when Late Proterozoic clastic sediments fell on the peneplain revealing the middle level of the eroded continental crust. That level is now exposed and sampled in the Anabar Shield and Olenek uplift and also sampled from borehole cores and kimberlite pipe xenoliths where cratonic basement is overlapped by the Late Precambrian and Phanerozoic sediments. Juvenile crust of the granulite-gneiss (Magan and Daldyn) and granite-greenstone (Birekte and Markha) terranes was extracted from depleted mantle at 3.1 (whole rock isochron (Spiridinov et a!., 1994)) to 2.8 Ga (sNd(T) = + 3.1+/-1.5, T=3.1 Ga) and 2.5-2.4 Ga (eNd(T) = +4.5, T=2.5 Ga), respectively. Volcanogenic complexes developed on their basement were formed at 2.4 Ga (sNd = -2.61+/0.6, T=2.4 Ga, the Vyurbyur Group of the Magan granulites) and 2.1 Ga (8Nd(T) = + 1.56+/-0.49, T=2.1 Ga (Dhzuravlev & Rosen, 1991), the Khapschan granulite and Aekit greenschist fold belts of the Birekte terrane).

Collisional shear zones between terranes were formed in two stages similar to those known in Phanerozoic collisional systems (Himalayas, Caucasus and others). Based on U-Pb zircon dating of metamorphic rocks and autochthonous granites in the shear zones, ages of the first and second stages are respectively determined as 1.9 and 1.8 Ga and dolerite intrusions have taken place at T(Nd)DM = 2.01 Ga. Each stage of the local metamorphism and granite extraction within that collisional shear zones was accompanied by coeval regional granulite metamorphism in the adjacent ancient terranes (3 mineral isochrons in the Daldyn terrane, 1.9 Ga, eNd = -10.8 - -0.4 +/0.5-0.8, T=1.9 Ga; and 2 mineral isochrons in the Birekte terrane, 1.9 Ga, sNd = +0.3 - -0.6+/-0.10.3, T=1.9 Ga; and 1 mineral isochron in the Markha terrane 1.8 Ga (Neymark etal., 1992). That data indicate the development of the fairly homogeneous field of high temperatures and pressures within the crust thickened owing to terrane overthrusting during collision. Anorthosites localized inside the collisional shear zones were derived from mantle and contaminated by the lower crustal granulites at about 2.6 Ga (whole rock isochron, eNd = 2.60+/- 0.17, T=2.6), metamorphosed at 2.18 Ga (Sukhanov etal., 1990) (mineral isochron, sNd = -2.8, T= 2.60 Ga) and uplifted to the upper crustal levels during collisional overthrusting in the Late Paleoproterozoic.

Tectonics Special Research Centre, School of Applied Geology, Curtin University of Technology, September 1999

In the eastern Siberian craton the main part of juvenile mantle matter (about 60 % of crust) rose at 3.3-2.7 Ga (T(Nd)DM statistics on 101 analyses). It appears to be the first global episode of supercontinent formation resulting from wholemantle convection, and the juvenile matter supply occurred in the Late Archean (Condie, 1998). Geological boundaries and individual objects are not clearly known; however, relics of granulite grade metamorphism of 2.7 Ga (U-Pb dating on zircon relics) probably demonstrate crustal thickening caused by the first collision and supercontinent formation. Next the mantle convection changed to a layered mode resulting in supercontinent break-up to individual microcontinents (terranes) and supply of new juvenile matter. Presumably at that stage the Siberian terranes above were separated one from another and developed in different ways. Afterwards, at the second episode of wholemantle convection, accretion of sialic crustal masses recurred to form another supercontinent at the Late Paleoproterozoic. That second episode is the most important event to form the north eastern Siberian craton by collision of the ancient terranes at 1.9-1.8 Ga. The inflow of juvenile mantle matter connected with that process was mostly ceased (first percent of crust only). Granite magmas of this epoch have been derived from the ancient crustal source. They represent a product of partial melting and subsequent extraction of the granite melt upward from the lower crust, where complementary basificated residual granulites remained Tayler & McLennan, 1995) are now exposed in the Anabar shield. This study was supported by the Russian Basic Research Foundation, projects 97-05-64463, 9905-68642.

Two Billion Years of Tectonics & Mineralisation

References Condie, K. C., 1998. Episodic continental growth and supercontinents: a mantle avalanche connection? Earth Planet. Sci. Lett., v.163, p.97-108. Dhzuravlev, D.Z., Rosen O.M., 1991. Sm-Nd adge of the metasediments of the granulite complex of the Anabar Shield. Doklady AN USSR, v.317, No.1, p.189-193 (in Russian). Neymark, L.A., Nemchin A.A., Rosen O.M,, Serenko V.P., Spetsius Z.V. and Shuleshko I. K., 1992. Sm-Nd isotope systems in lower crust xenolithes of the Yakutian kimberlites.. Doklady AN USSR, v.327, No.3, p.374-378 (in Russian). Rosen, O.M., Condie, K.C., Natapov, L.M. and Nozhkin, A.D., 1994. Archean and early Proterozoic evolution of the Siberian craton: a preliminary assessment. K.C. Condie, ed., Archean crustal evolution, Elsevier, Amsterdam, p. 411-459 Spiridonov, V.G., Karpenko S.F., Lyalikov A.V., 1994. Sm-Nd ages and geochemistry of the granulites of the central Anabar Shield. Geochemistry International, No.5, p.35- 50. Sukhanov, M.K., Spiridonov, V.G. and Karpenko S.F., 1990. The first dating of the anorthosites of the Anabar Shield by the Sm-Nd isochron method. Doklady AN USSR, v.310, No,2, p.448-453 (in Russian). Taylor, S.R., McLennan S.M., 1995. The geochemical evolution of the continental crust. Reviews of Geophysics, v. 33, No.2, p. 241265.

Granites of the southern Capricorn Orogen, Western Australia S. SHEPPARD, S. A. OCCHIPINTI, D. R. NELSON, & I. M. TYLER Geological

Survey of Western Australia,

100 Plain Street, East Perth, WA 6004.

The Capricorn Orogen is a major Proterozoic tec-

granites intruded at 1830-1790 Ma may have had

tonic zone that initially developed in response to

a sedimentary component in their source region.

collision between the Yilgarn and Pilbara Cratons (Tyler and Thome, 1990). Recent SHRIMP U-Pb

Dalgaringa

zircon dating in the southern part of the Capricorn

The Dalgaringa Supersuite is restricted to the

Supersuite

(2005-1970

Ma)

Orogen (Nelson, 1998; Nelson, 1999) suggests

southern part of the Gascoyne Complex (Fig. 1).

that the collision occurred at c. 1820 Ma

The supersuite consists of regionally extensive,

(Occhipinti et al., 1998). The southern part of the

massive, foliated and gneissic quartz diorite, to-

Capricorn Orogen includes Palaeoproterozoic ig-

nalite, granodiorite, and monzogranite dated at

neous and metamorphic rocks of the Gascoyne

2005-1985 Ma, intruded by large coalesced plu-

Complex, and a number of Palaeoproterozoic

tons of tonalite and granodiorite at 1980-1970

sedimentary basins, as well as the deformed

Ma. Rocks of the supersuite plot in the calc-

northern margin of the Yilgarn Craton. The north-

alkaline field on AFM and Si02 vs K 2 0 plots. The

western margin of the Yilgarn Craton consists of

supersuite has a range in Si02 from 54 to 76 wt

early to late Archaean granitic gneisses and gran-

%, and is characterized by low K 2 0, K20/Na20,

ites of the Narryer Terrane. Part of the Narryer

and Rb/Sr, and high CaO and high K/Rb. Yttrium

Terrane was extensively deformed and metamor-

contents are variable, but many rocks contain low

phosed, and then intruded by granite sheets and

Y (<18 ppm) and high Sr/Y ratios (>40). The

dykes during the Capricorn Orogeny, and is re-

rocks have similar major and trace element abun-

ferred to as the Yarlarweelor gneiss complex

dances to Mesozoic subduction-related batholiths

(Occhipinti and Myers, 1999; Sheppard and Swa-

such as the Coastal Batholith of Peru, and the

ger, 1999).

granites of West Palmer Land in Antarctica.

Granite ages and compositions.

Wooramel

Granite plutons, sheets, dykes and veins were in-

Granite plutons of the Wooramel suite intruded

truded into the southern part of the Capricorn

the northwestern margin of the Yilgarn Craton

suite (c. 1960 Ma)

Orogen between c. 2000 Ma and c. 1620 Ma.

(Fig. 1). The plutons are found in both reworked

Granite intrusion was not continuous over this pe-

Archaean rocks of the Yarlarweelor gneiss com-

riod, but consists of several short-lived episodes.

plex, and in unreworked Narryer Terrane. The

Most of the granites were intruded either at 2005-

Wooramel suite comprises even-textured and

1970 Ma or 1830-1790 Ma, but smaller peaks of

porphyritic biotite monzogranite and granodiorite.

intrusive activity are recorded at c. 1960 Ma and

The rocks are weakly peraluminous l-types with

1620 Ma. The granites are all weakly peralumi-

Si02 contents of 69-73%, and K20/Na20 ratios of

nous (ASI <1.1) or metaluminous l-types. No

0.3-1.2. The suite contains low Rb (53-106 ppm)

strongly peraluminous (i.e. S-type) granites have

and Y (<16 ppm) and low to moderate Sr (133-

been identified, but some tourmaline-bearing

414 ppm) and Ba (310-860 ppm).

Tectonics Special Research Centre, School of Applied Geology, Curtin University of Technology, September 1999

117^17

SqiiNBOR^ 1 yo^ROaiNsgRA^ ^

(Mount Clere

yDalgety ,'.Downs.

>V</>/. d

——•—t——i—a BRYAH-PADBURY BASINS /

Errabiddy

YILGARN

CRATON

117° 00'

Yarlarweelor PILBARA CRATON

BRYAH-PADBURY BASINS MARYMIA INLIER

YARLARWEELOR GNEISS COMPLEX

Fault

Errabiddy Shear Zone

Geological boundary Road

'YERRIDA BASIN

YILGARN CRATON

Alluvium, colluvium

•

Homestead

Mine Nardoo Granite (c. 1975 Ma)

Dalgarincja Supersuite

Foliated and gneissic granite (2000-1985 Ma)

Discretion Granite (c. 1620 Ma) Mount James Formation Moorarie Supersuite W

mel

Mefa^orphtes [

| + +1 I + I Granite (1830-1780 Ma)

°su£e [ EZ3

Granite (a 1960 Ma)

•

Metasedimentary rock Gneissic granite and granitic gneiss (2550-71985 Ma) | I Yarlarweelor gneiss 1 complex (3300-1800 Ma)

Figure 1. Sketch map of the southern part of the Capricorn Orogen showing the main tectonic elements, and the distribution of the main age groupings of granites. Moorarie Supersuite (1830-1790 Ma)

otite monzogranite and pegmatite were intruded

The Moorarie Supersuite is both widespread and

during deformation and medium- to high-grade

voluminous. The supersuite comprises plutons,

metamorphism at c. 1812 Ma (Occhipinti et al.,

sheets, veins and dykes that extensively intruded the Gascoyne Complex and the Yarlarweelor gneiss complex, and to a lesser extent, the northwestern edge of the Yilgarn Craton. The Moorarie Supersuite does not intrude Palaeoproterozoic low-grade metasedimentary and metavolcanic rocks of the Padbury and Bryah Basins, which are in faulted contact with the Yarlarweelor gneiss complex. In the southern part of the Gascoyne Complex, the Moorarie Supersuite consists of dykes, veins and plugs of medium-grained, even-textured biotite-(tourmaline) and biotite-muscovite monzogranite and granodiorite, porphyritic granodiorite, and pegmatite. In the Yarlarweelor gneiss complex, sheets and veins of coarse-grained bi-

1998). This was followed by intrusion of eventextured biotite monzogranite and granodiorite dykes at c. 1795 Ma, during dextral strike-slip faulting (Sheppard and Swager, 1999). The medium-grained, even-textured granites of the Moorarie Supersuite are siliceous (69-76 wt% Si02), weakly peraluminous, and have high K 2 0+Na 2 0 contents, but with a large range in K 2 0/Na 2 0 ratios from 0.5 to 2. The granites show a wide range in Ba (400-2500 ppm) and Y (5-30 ppm) contents and K/Rb (50 to >500). Most of the rocks, including some of the tourmaline- and muscovite-bearing varieties, contain low Rb/Sr ratios (<1.5), and they are unlike melts derived from pelites or greywackes. The biotite- and biotitemuscovite monzogranite and granodiorite were probably derived from melting of meta-igneous

Two Billion Years of Tectonics & Mineralisation

rocks. The tourmaline granites may have been

southern margin of a south Gascoyne micro-

derived from melting of a mixed igneous-

continent. Granites of the c. 1960 Ma Wooramel

sedimentary source.

suite along the northwestern margin of the Yilgarn

The coarse biotite granite and pegmatite sheets

Craton may reflect subsequent collision of the

that intrude the Yarlarweelor gneiss complex are

craton with such a south Gascoyne micro-

very leucocratic and siliceous (>71 wt% Si02).

continent. Granites of the Moorarie Supersuite

They have high K 2 0+Na 2 0, but with moderate

were intruded following collision of the Yilgarn

CaO (0.9-3.3 wt%), moderate to high Ba (800-

and Pilbara Cratons. The tectonic setting and sig-

2600 ppm) and Sr (-200-600 ppm), and low Rb

nificance of the Discretion Granite is not yet un-

(mostly <100 ppm). They have low initial eNcj val-

derstood.

ues of -11.5 to -13.5 and were probably derived via melting of middle Archaean gneisses and late

References

Archaean granites of the Narryer Terrane.

Nelson, D.R., 1998, Compilation of SHRIMP UPb zircon geochronology data, 1997, Western

Discretion Granite (c. 1620 Ma) The only granite of c. 1620 Ma age known from the southern Capricorn Orogen is the Discretion Granite (Fig. 1). It covers about 300 km2 and con-

Australia Geological Survey, 242 p. Nelson, D.R., 1999, Compilation of geochronology data, 1998, Western Australia Geological Survey, 222 p.

sists of massive porphyritic l-type biotite mon-

Occhipinti, S.A., and Myers, J.S., 1999, Geology

zogranite. The Discretion Granite is in faulted

of the Moorarie 1:100 000 sheet, Volume

contact with sedimentary rocks of the Bangemall

1:100 000 Geological Series Explanatory

Basin. Analyses from the intrusion have 69-74 wt

Notes, Western Australia Geological Survey,

% Si0 2 , and are characterised by high K 2 0, LREE, Th, U, and Nb.

p. 20. Occhipinti, S.A., Sheppard, S., Nelson, D.R., Myers, J.S., and Tyler, I.M., 1998, Syntectonic granite in the southern margin of the Palaeo-

Conclusions The 2005-1970 Ma Dalgaringa Supersuite is sub-

proterozoic Capricorn Orogen, Western Aus-

stantially older than the 1880-1800 Ma potassic

tralia: Australian Journal of Earth Sciences, v.

granites that dominate the Palaeoproterozoic of

45, p. 509-512.

northern Australia. The supersuite may have

Sheppard, S., and Swager, C.P., 1999, Geology

formed in a tectonic setting similar to Mesozoic

of the Marquis 1:100 000 sheet, Volume

subduction-related batholiths from convergent

1:100 000 Geological Series Explanatory

continental margins. The supersuite provides

Notes, Western Australia Geological Survey,

some clues as to the nature of tectonic processes operating in the Palaeoproterozoic before wide-

p. 21. Tyler, I.M., and Thome, A.M., 1990, The northern

spread orogenic events recognised throughout

margin of the Capricorn Orogen, Western

the Australian Proterozoic between 1900 and

Australia - an example of an early Proterozoic

1800 Ma. Granites of the Dalgaringa Supersuite

collision zone: Journal of Structural Geology,

are absent from the northern margin of the Yil-

v. 12, p. 685-701.

garn Craton, and may have formed above a northward-dipping subduction zone along the

Tectonics Special Research Centre, School of Applied Geology, Curtin University of Technology, September 1999

Palaeoproterozoic orogeny in Western Australia IAN TYLER

Geological Survey of Western Australia, 100 Plain Street, East Perth, WA 6004. The geological evolution of the Palaeoproterozoic in Western Australia can be described in terms of continental break-up, terrane accretion, and plate aggregation (Myers et al., 1996; Tyler et al., 1998). Palaeoproterozoic rocks are exposed around the margins of the Archaean Pilbara and Yilgarn Cratons, and in the North Australian Craton. The main orogenic belts are the Capricorn Orogen, the Paterson and Musgrave Orogens, the King Leopold and Halls Creek Orogens, the Albany—Fraser Orogen, and the Arunta Orogen

histories. Capricorn Orogen Rifting, controlled by west-northwesterly trending faults in the southern Pilbara Craton, initiated the Hamersley Basin (Fortescue Group) in the late Archaean at c. 2770 Ma. The margins of the rift were buried beneath a breakup unconformity overlain by a 2690 Ma to 2470 Ma passive margin sequence (uppermost unit Fortescue Group and lower Hamersley Group). The upper part of the Hamersley Group may have

Kimberley Craton

Pilbara Craton Capricorn Orogen"

been deposited in a collisional setting between 2470 and 2440 Ma, although there is no evidence

King Leopold and Halls ureek Orogens

N0RTH

of compressive deformation at that time. The Tu-

.AUSTRALIA^

ree Creek Group and lower Wyloo Group were

SLCRATON CENTRAL. .AUSTRALIAN

Arunta Orogen

TERRANES AUSTRALIAN

Pinjarra Orogen

Paterson-Musgrave Orogen

CRATON

deposited in a foreland basin (the McGrath Trough) along the southwestern margin of the Hamersley Basin. The trough developed in front of a northward verging fold belt during an Ophthalmian orogenic event at c. 2200 Ma (Martin et al., 1998).

Yilgarn Craton Albany-Fraser Orogen

South Australian Craton

Proterozoic orogenic belts Archaean Craton beneath Proterozoic basins Archaean Craton

The c. 2100 Ma Windplain Subgroup of the Yerrida Group was deposited in a sag basin on Archaean crust in the northern part of the Yilgarn Craton. The overlying Mooloogool Subgroup marks an abrupt change to a rift-fill setting. Rifting may have taken place as a response to strike-slip faulting along the then margin of the Yilgarn Cra-

Figure 1. The main components of the Western Australian crust

(Fig. 1). Models are presented here for the Capricorn Orogen and the King Leopold and Halls Creek Orogens to illustrate the complex nature of these belts and the differences in their tectonic

Two Billion Years of Tectonics & Mineralisation

ton to the northwest. In the southern part of the Gascoyne Complex during the 2000 to 1960 Ma Glenburgh Orogeny, extensive 2000 to 1975 Ma felsic magmatism (Dalgaringa Supersuite) developed above a northward-dipping subduction zone along the southern margin of a late Archaean to Palaeopro-

terozoic micro-continent. Deformation, metamor-

and the adjacent Yilgarn Craton. Uplift provided

phism and further c. 1960 Ma magmatism

the source for the Padbury Group, deposited

(Wooramel Supersuite) accompanied southeast-

within a localized retro-arc foreland basin.

ward subduction and accretion of the microcontinent to the northwestern margin of the Yilgarn Craton. Medium- to high-grade metasedimentary rocks forming the Camel Hills Metamorphics initially had an Archaean source represented by the Yilgarn Craton, while later rocks had a Palaeoproterozoic source. The Bryah Group was deposited in a back-arc basin setting at this time.

Following deposition of the Capricorn Formation large-scale dextral strike-slip faulting developed along the southern margin of the Pilbara Craton after c. 1790 Ma. Further to the northwest the Mount Minnie Group was derived from an uplifted source area to the north and east. To the south the Mount James Formation overlies the Gascoyne Complex. The Earaheedy Group was deposited on the northeastern margin of the Yil-

The Capricorn Orogeny involved the collision and

garn Craton between 1790 and 1630 Ma as a

suturing of the geologically distinct Archaean Pil-

post-collisional sag basin, which deepened to the

bara and Yilgarn Cratons between 1840 and 1790

northwest. The Bresnahan Group represent a mo-

Ma. Closure of the ocean basin between the Pil-

lasse-style deposit derived from the uplifted

bara and Yilgarn Cratons was oblique with a

Gascoyne Complex and northwestern Ashburton

north-verging foreland fold and thrust belt devel-

Basin, and controlled by southeasterly to south-

oped in the southeastern Hamersley Basin. Depo-

erly dipping normal faults.

sition of the c. 1840 Ma upper Wyloo Group took place within the Ashburton Basin and represents the establishment of a foreland basin parallel to the craton margin with deep water submarine fans being derived from the Sylvania Inlier, from

The Troy Creek Schist represents the deformed northeastern margin of the Earaheedy Basin with southeasterly plunging, southwesterly verging tight folds and thrusts forming the Stanley Fold Belt.

the Gascoyne Complex and from Hamersley Basin rocks.

King Leopold and Halls Creek Orogens

North-verging recumbent folds developed in the

the Halls Creek Orogen, c.1910 Ma mafic and fel-

southern Ashburton Fold Belt, where metamor-

$ic volcanic rocks may represent a rift at the mar-

In the Eastern zone of the Lamboo Complex in

phic grade reached uppermost greenschist fa-

gin of predominantly Archaean continental crust

des, before intrusion of the c. 1790 Ma Boolaloo

to the east, and are unconformably overlain by

Granodiorite (Krapez and McNaughton, 1999).

the c. 1880 Ma lower Halls Creek Group.

The granitic plutons forming the Minnie Creek batholith were intruded between 1840 and 1800 Ma and were probably generated by southward subduction of oceanic crust. To the north S- and l-type granitoids intruded migmatitic Morrissey Metamorphics, which pass into the upper Wyloo Group. To the south the granites of the 1830 to 1790 Ma Moorarie Supersuite were intruded synchronously with dextral strike-slip faulting and associated thrusting of Gascoyne Complex rocks

In the Hooper Complex of the King Leopold Orogen and the western zone of the Lamboo Complex the c. 1870 Ma Marboo Formation is unconformably overlain by the Whitewater Volcanics, which is comagmatic with voluminous granitic and gabbroic rocks of the 1865 to 1850 Ma Paperbark supersuite. Deposition and magmatism occurred in an extensional setting marginal to the predominantly Palaeoproterozoic Kimberley Craton. The

Tectonics Special Research Centre, School of Applied Geology, Curtin University of Technology, September 1999

Hooper Orogeny represents closure of the basin and uplift of the craton margin, possibly in response to a change in position of the subduction zone. Low- to high-grade mafic volcanic and metasedimentary rocks of the c. 1865 Ma Tickalara Metamorphics formed either as an island arc (subduction to the southeast) or an ensialic marginal basin (subduction to the northwest) in the Central zone of the Lamboo Complex. They were deformed and metamorphosed, and intruded by numerous felsic, and basic to intermediate sheetlike bodies between 1865 and 1850 that have geochemical signatures similar to tonalite and trondhjemite suites from Phanerozoic island arcs and convergent continental margins. Layered mafic-ultramafic intrusions occur in both the Western and Central zones of the Lamboo Complex at this time. Further deformation and peak metamorphism occurred between 1850 and 1845 Ma (Bodorkos et al., 1999), possibly during collision of the Central zone with the Western zone. Submarine alkaline volcanism in the Eastern zone of the Lamboo Complex may mark rifting parallel to a continental margin between 1870 and 1850 Ma. Turbiditic rocks of the upper Halls Creek Group were deposited by a submarine fan. Volcanic rocks of the Koongie Park Formation were erupted during rifting of the Central zone at c. 1840 Ma, accompanied by the intrusion of layered mafic-ultramafic intrusions. This was followed by continued subduction of oceanic crust to the northwest and collision and suturing of the Kimberley Craton with the rest of northern Australia by c. 1820 Ma during the Halls Creek Orogeny. During and immediately following the collision plutons of granite and gabbro were intruded to form the Sally Downs supersuite at the same time as the intrusion of large layered mafic-ultramafic bodies. Early tonalite plutons have compositions Two Billion Years of Tectonics & Mineralisation

similar to high-AI tonalite-trondhjemitegranodiorite suites, while later granites either have compositions similar to Phanerozoic cordilleran batholiths or to the Paperbark supersuite. Folding and thrusting accompanied metamorphism in the Central zone of the Lamboo Complex. The Speewah Group were deposited on the Kimberley Craton at c. 1835 Ma, at the same time as rocks of the Sally Downs supersuite were being intruded into the Lamboo Complex. The Kimberley Group oversteps the Speewah Group onto the Lamboo Complex, and both sedimentary groups were derived from the north. The intrusion of the Hart Dolerite at c. 1800 Ma may be related to the development of a mantle plume and associated continental break-up centred to the north, at the same time as a granite intruded the southern part of the Lamboo Complex. References Bodorkos, S., Oliver, N. H. S. and Cawood, P. A., 1999, Thermal evolution of the central Halls Creek Orogen, northern Australia: Australian Journal of Earth Sciences, v. 46, p. 453-465. Krapez, B. and McNaughton, N. J., 1999, SHRIMP zircon U-Pb age and tectonic significance of the Palaeoproterozoic Boolaloo Granodiorite in the Ashburton Province, Western Australia: Australian Journal of Earth Sciences, v. 46, p. 283-287. Myers, J. S., Shaw, R. D. and Tyler, I. M., 1996, Tectonic evolution of Proterozoic Australia: Tectonics, v. 15, p. 1431-1446. Martin, D. McB., Li, Z. X., Nemchin, A. A. and Powell, C. McA., 1998, A pre-2.2 Ga age for giant hematite ores of the Hamersley Province, Western Australia: Economic Geology v. 93, p. 1084-1090. Tyler, I. M., Pirajno, F., Bagas, L., Myers, J. S. and Preston, W. A., 1998, The geology and mineral deposits of the Proterozoic in Western Australia: AGSO Journal of Australian Geology and Geophysics, v. 17, p. 223-244. 49

The spatial distribution of mineralization. JULIAN VEARNCOMBE AND SUSAN VEARNCOMBE Veamcombe & Associates PTY LTD, 14A Barnett Street, Fremantle 6160, WA. Trends in the distribution of spatially-located point

effective alternative to directional variography

data are a critical component of geological

being independent of models and statistics. At

research, mineral exploration, mining, resource

prospect- to mine-scales successful drilling which

evaluation and grade control, and they are

intersects economic mineralization can be

commonly analysed by a combination of

analysed relative to the distribution of all drilling.

structural geology and geostatistical methods.

This eliminates the spatial bias inherent in

Here we present SpaDiS™, as a new and exciting

systematic grid drilling programs enabling the

geometric analysis of spatial distributions and a

determination of ore shoot direction, spacing,

complement to existing methods (Vearncombe

repetitions and any offsets. Grade control data

and Vearncombe, 1999). Point data are perhaps

can be examined to deduce directions of

the most commonly used data in geology. At the

continuity of mineralization within open pits and

regional scale spatial coordinated point data

underground. For due diligence SpaDiS™

include, deposit location and endowments,

analysis is a fast and effective method to test

regolith, stream and rock chip sampling data, and

relationships between geology and drilling results.

geophysical data including gravity. At prospect and mine scales, assay results, metal ratios,

Porphyry Copper and Vein-Replacement

other geochemical and some geophysical data

Deposits in Arizona, USA

including IP data can all be analysed. Similarly,

Late Cretaceous to Early Tertiary (Laramide)

there are applications in hydro-geology, isotope

mineral deposits of Arizona include porphyry

geology and petroleum exploration.

coppers and vein/replacement hydrothermal base

Point data on the spatial distribution of

and precious metals (Titley, 1986). They are

mineralization are analysed spatially in SpaDiS™

distributed through much of southern and western

by a plot on which distance and direction from

Arizona in domains with Laramide compression

each data point to each other data point are

followed by mid-Tertiary "Basin and Range"

recorded by a point at that distance and direction

extension. The deposits occur in distinct clusters,

from a common origin. The results are presented

with several areas comprising just one deposit

in translation plots and rose diagrams. For n

type (Fig. 1a). The translations plots and the

points there are n2-n spatial relationships and,

derivative rose diagrams show the join frequency

because of the square function the analysis is

versus direction to determine the similarities in

effective for small as well as large data sets. At

the distribution of these deposit types. The

terrane- to prospect-scales, deposits and old

translations for porphyry copper and vein/

workings ranked by endowment, tonnage or

replacement deposits (respectively Figs 1 b and

grade can be analysed to deduce the structural

1d) and rose diagrams (Figs 1c and 1e) are

directions controlling deposit size and to verify

remarkably similar. The only prominent difference

aeromagnetic and imagery interpretations.

being the 105°-trend that appears more important

The method is a geometric analysis and an

for the vein/replacement deposits but is still

Tectonics Special Research Centre, School of Applied Geology, Curtin University of Technology, September 1999

Vein/Replacement and Porphyry Copper Deposits

Porphyry Copper Deposits

80 km + Vein & Replacement Deposits 0 Porphyry Copper Deposits

FIG. 1. (a). Map of Laramide porphyry and vein/replacement (hydrothermal) deposits in Arizona (after Titiey, 1986). (b). Translations for porphyry copper deposits, (c). The derived rose diagram of the translations, showing a prominent ~1459-trend, and subsidiary trends to about 0109,050g, 080Q and 105Q. (d). Translations for the vein/replacement deposits, (e). Rose diagram of translations for the vein/replacement deposits showing prominent trends to about 1059 and 1459, and subsidiary trends at about 0109, 0509 and 0809. present in the porphyry analysis.

and effectiveness of exploration. The similarity

Local geological controls such as calderas, local

between the vein /replacement hydrothermal and

faults, and availability of ground water are

porphyry copper distributions suggests that both

inadequate in explaining the clustering and

reflect the same fundamental structural control, at

spatial distribution of hydrothermal ore deposits in

scales of tens to possibly hundreds of kilometres.

Arizona. Mineralization processes operate at

This control comprises ~1459-trending regional

scales other than those of a mining district. The

corridors which were extensional during the mid-

observed distributions of porphyry and vein/

Tertiary, and ~0109- and ~0509-trending

replacement hydrothermal deposits are a function

structures common in both Mesozoic and Tertiary

of the structural controls on mineralization,

strata, but which may be inherited from the

subsequent faulting and cover, and the extent

underlying Precambrian basement. The 050s-

Two Billion Years of Tectonics & Mineralisation

trending structural lineaments were probably

owner. For this study data are projected to the

transfer zones during the extensional

surface (map plane) with the drill hole

deformation, but their role in the Laramide

intersections and mineralization recorded as gold

deformation remains uncertain. The nature and

grade x thickness (g.m/t Au). The purpose of the

origin of the 105Q-trend is not obvious and

analysis here is to determine directions of ore

requires investigation.

shoots and controlling structures, and likely

Exploration Drilling Data

map projection of drill intercepts (Fig. 2a) shows

repetition and spacing of these ore shoots. The The spatial distribution of mineralization as

the spatial distribution of drilling at Canyon.

recorded by the location of successful drill holes

Translations of all drilling (Fig. 2b) and the rose

is constrained by the distribution of drilling. We

diagram (Fig. 2c) show the local grid bias in the

illustrate this with prospect-scale RC drilling

drilling. Note the east-west bias of the drilling grid

results from Canyon, based on a real example

and the overall NW-SE bias in the drilling pattern.

these data have been re-oriented and massaged

For Canyon drilling results of >50g.m./t show ore

to prevent identification at the request of the

shoot direction and spacing (Fig. 2d) oriented to

(a) Resource drilling

(b) Translations

Drilling, results in \>.ni/t

.-mi

»4mMt+fi/y* *,

(d) Translations >50g.m/t

(e) Absolute rose of translations >50g.m/t

(f) Relative rose of translations >50g.m/t

,V •

FIG. 2. (a) The map projection of drill intercepts shows the spatial distribution of drilling at Canyon, (b and c) Translations of all drilling and the rose diagram show the local grid bias in the drilling. Note the east-west bias of the drilling grid and the NW-SE bias in the drilling pattern, (d) For Canyon drilling results of >50g.m./t show ore shoot direction and spacing oriented to the NE with repetition of ore shoots spaced about 80m apart, (e) The absolute rose diagram for >50g.m/t shows the direction of mineralization to the NE as well as the bias in the drilling to the NW. (f) The relative rose diagram which is a plot of percentage success relative to total drilling for each 10° radial sector, shows clearly the direction of ore, with 30% success of translations towards ~050°. The relative rose diagram is contoured for percentage translations (at 20% intervals) with the outer circle at 100%.

Tectonics Special Research Centre, School of Applied Geology, Curtin University of Technology, September 1999

the NE with repetition of ore shoots spaced about 80m apart. The absolute rose diagram for >50g. m/t (Fig. 2e) shows the direction of mineralization to the NE as well as the bias in the drill pattern to the NW. The relative rose diagram (Fig. 2e), which is a plot of percentage success relative to total drilling for each 109 radial sector, shows clearly the direction of ore, with 30% success of translations towards ~0502, eliminating the bias in the drill pattern.

Summary Regional applications of SpaDiS™ analysis include assessing deposits and old workings to deduce structural directions controlling mineralization and to verify aeromagnetic interpretations. The assessment can be according to deposit size, to deduce variations in structural directions controlling mineralization according to deposit endowment. Mine- and prospect-scale applications are used to determine the distribution of successful drill holes relative to all drilling. This reduces the spatial bias inherent in the distribution of drilling to determine ore shoot directions. Grade control data can be examined to deduce directions of continuity of mineralization within open pits. References Titley, S.R., 1986, An overview of Laramide metallogenesis in Arizona: Arizona Geological Society Digest, v. 16, p. 84-88. Vearncombe, J. and Vearncombe, S. 1999. The spatial distribution of mineralization: applications of Fry analysis. Economic Geology, v. 94, p. 475-486.

Two Billion Years of Tectonics & Mineralisation

Early Neoproterozoic metamorphism in central East Greenlandimplications for Rodinian reconstructions. 1

GORDON WATT & 2KRISTINE THRANE

TSRC, School of Applied Geology, Curtin University of Technology, GPO Box U1987, Perth WA 6845 Geological Survey of Denmark & Greenland, Thoravej 8, 2400 Copenhagen NV, Denmark.

Correlation of the Grenville Orogen of North

major sediment deposition in the North Atlantic

America with the Sveconorwegian Orogen of

region. Erosion associated with post 1.1 Ga col-

Scandinavia is one of the keystones in recon-

lapse of the Grenville-Sunsas Orogeny is the

structions of the Rodinian supercontinent. The

most likely protolith for the majority of the detritus,

Grenville Orogeny resulted from the accretion of

since the corresponding Baltican margin contains

a series of Archaean, Palaeoproterozoic and

very little 1.4—1.1 Ga material (which makes up a

Mesoproterozoic terranes onto the eastern mar-

significant part of the zircon detritus in the

gin of Laurentia over the period 1.4—1.0 Ga.

Krummedal supracrustal sequence). We suggest

Remnants of the Grenville Orogen today extend

that the Krummedal supracrustal sequence was

from Texas to Labrador, with terranes in the por-

deposited south-west of its present location, and

tion of the belt between Texas and the Appalachi-

was emplaced on the parautochthonous Ar-

ans affected by a widespread major thermal event at around 1.1 Ga which has been used to define the Grenville Orogeny sensu stricto. This event is absent on the western margin of Baltica, and therefore the docking of Baltica and Laurentia cannot have occurred during the main phase of the Grenvillian Orogeny. New U—Pb zircon SHRIMP dating on Krummedal supracrustal se-

chaean foreland during the Caledonian orogeny. The early Neoproteropzoic events in central East Greenland can therefore therefore be considered as the metamorphism of a metasedimentary package of Laurentian-Amazonian affinity during the Sveconorwegian orogeny, as a result of collision of Baltica and Laurentia.

quence metasedimentary gneisses and associated granites has shown that metamorphism and intrusion in central East Greenland occurred around 0.95—0.93 Ga, 150 Ma after the main phase of orogenesis in the North American Grenville Province. These data are consistent with plate reconstructions that suggest Baltica occupid a more northerly position in Late Mesoproterozoic times and may have collided with Laurentia post 1.0 Ga during the Sveconorwegian orogeny. Detrital zircon SHRIMP U-Pb dating studies instead show that the Krummedal supracrustal sequence was deposited between ca. 1.05—1.035 Ga and no later than 0.95 Ga, during a time of

Tectonics Special Research Centre, School of Applied Geology, Curtin University of Technology, September 1999

SHRIMP U—Pb analysis of baddeleyite: crystal orientation effects and implications for geochronology. 1M.T.D. WINGATE & 2W. COMPSTON 1 2

TSRC, The University of Western Australia, Nedlands, WA 6907, Australia Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia

We discovered recently that 206Pb/238U ratios

along preferred directions, and/or differential ioni-

measured in baddeleyite by ion microprobe vary

sation of secondary species. No orientation-

significantly (up to ±10% or more) and systemati-

related differences in 206Pb/238U were detected

cally with the relative orientation of the baddeley-

during ion microprobe analysis of zircon or mona-

ite crystal structure and the primary ion beam.

zite.

Low dispersion between multiple analyses of single crystals demonstrates that it is possible to measure undispersed 206Pb/238U ratios in multigrain baddeleyite samples if all material is in a single orientation. Because of the small size of most crystals and their ubiquitous polysynthetic (100) twinning, however, these orientation effects constitute an intractable problem, and place unacceptable limits on the accuracy and precision that can be obtained for 206Pb/238U in baddeleyite by ion microprobe techniques. Comparisons with isotope dilution measurements indicate that any Pb isotope discrimination during analysis of baddeleyite by ion microprobe is negligible, and there is no evidence that Pb isotope ratios vary with crystal orientation. Although the sensitivity of 207pb/206pb r a t j o s a s a m e a s u r e 0 f a ge decreases for younger samples, rocks younger than 1 Ga can be dated precisely if the baddeleyite is enriched sufficiently in uranium and if enough analyses are performed. Baddeleyite 207Pb/206Pb ratios (and in some cases 206Pb/238U ratios, in a semiquantitative manner) can be used to distinguish a magmatic zircon population from zircons of different ages. We speculate that orientation effects in baddeleyite might involve channelling of primary ions into the crystal, emission of secondary ions

Two Billion Years of Tectonics & Mineralisation

High-pressure metamorphism of mafic granulites from the TransNorth China Orogen: Implications for Palaeoproterozoic amalgamation of the North China Craton. GUOCHUN ZHAO TSRC, School of Applied Geology, Curtin University of Technology, GPO Box U1987, Perth 6845, WA

Regional Setting

zoic khondalites, which mainly crop out along the boundary with the Trans-North China Orogen.

The North China Craton is not well-constrained in terms of its tectonic evolution. Traditionally, it has been considered to be composed of a uniform Archean to Paleoproterozoic basement, overlain by younger cover, and its tectonic history was explained using a pre-plate tectonic model. Terrane

The Archean TTG gneisses and supracrustal rocks experienced metamorphism at about 2500 Ma, with anticlockwise P-T-t paths, whereas the Paleoproterozoic khondalites underwent metamorphism at -1800 Ma, with clockwise P-T-t paths (Zhao etal., in press).

accretion and collision models have only been applied recently, including recognition of a Paleopro-

The Eastern Block is composed predominantly of

terozoic orogen - the Trans-North China Orogen -

the middle to late Archean TTG gneisses and

which separates the craton into eastern and

syn-tectonic granitoids, with minor rafts or sheets

western blocks (Fig. 1; Zhao etal., 1999a, b).

of the early to middle Archean supracrustal rocks including ultramafic to felsic volcanic rocks and metasediments. The TTG gneisses make up over 80% of the basement and the structural style is dominated by ovoid domes, separated by linear belts. All these rocks underwent regional metamorphism at -2500 Ma, with anticlockwise P-T-t paths (Zhao etai, 1998). Intervening between the Western and Eastern blocks is the Trans-North China Orogen. The orogen consists of a series of low-grade and high-

Fig. 1. Spatial distribution of the eastern and western blocks separated by the Trans-North China Orogen. WB-Western block; EB-Eastern block; TNCO-TransNorth China Orogen.

grade belts containing the re-worked Archean components and juvenile Paleoproterozoic igneous and sedimentary rocks. Geochemical and geochronological studies show that these rocks developed in magmatic arc and intra-arc basin

The Western Block has a double-layered base-

environments, and experienced regional meta-

ment, with the late Archean TTG gneisses and

morphism at about 1900-1800 Ma.

supracrustal rocks overlain by the Paleoprotero-

Tectonics Special Research Centre, School of Applied Geology, Curtin University of Technology, September 1999

Mafic granulite is one of the major lithologies in

symplectic texture. The similar texture has been

high-grade belts of the Trans-North China Orogen

reported from other high-pressure granulite ter-

and preserves a variety of mineral assemblages

rains in the world (Heinrich, 1982; Smelov &

and textural relations. We have discussed their

Beryozkin, 1993), and its origin has been consid-

tectonic implications in our recent papers (Zhao

ered to be related to retrograde metamorphism of

etal.,

1999a, b). In this extended abstract, we

present some new petrographic and thermobarometric evidence for the high-pressure meta-

eclogite into high-pressure granulites through the breakdown of omphiolite: omph + qz = cpx + Na-rich plg(1)

morphism of these rocks. It gives further constraints on the evolution of the North China Cra-

M2 is represented by orthopyroxene + Ca-rich

ton.

plagioclase symplectites on garnet or clinopyroxene + orthopyroxene + Ca-rich plagioclase coro-

Spatial Distribution. The high-grade belts containing high-pressure mafic granulites are mainly exposed in the central parts of the Trans-North China Orogen. The north and the south parts of the orogen are occupied by greenschist to amphibolite facies terranes. In

nas surrounding garnet grains. Similar symplectites or coronas have been found in many other granulite-facies terrains and have been related to near-isothermal decompression following the peak metamorphism through the following generalized reactions:

field, high-pressure mafic granulites occur as enclaves, boudins and sheets within TTG gneisses.

gt + qz = opx + Ca-rich pig (2)

In most cases, the foliation within high-pressure

gt + hbl = opx + Ca-rich pig + H 2 0 (3)

mafic granulites is parallel to the external foliation

gt + qz = cpx + opx + Ca-rich plagioclase (4).

of TTG gneisses, but in some places, foliations between mafic granulites and TTG gneisses are

M3 is represented by hornblende + Ca-rich pla-

not consistent.

gioclase ± magnetite symplectite on garnet, or by retrograde hornblende rim surrounding clinopy-

Petrography & Metamorphic Stages. The high-pressure mafic granulites from the Trans-North China Orogen are composed mainly

roxene or orthopyroxene grains. These symplectites and retrograde rims may have developed through the following hydration reactions:

of clinopyroxene, plagioclase, garnet, quartz ± orthopyroxene ± hornblende; rutile, magnetite and

gt + cpx + qz + H 2 0 = hbl + Ca-rich pig (5)

ilmenite are present as accesory phases. Based

gt + plg1 + qz + H 2 0 = gt2 + plg2 + hbl (6)

on reaction textural relations, three separate metamorphic stages (M1 to M3) have been rec-

Thermobarometry.

ognized from the high-pressure mafic granulites. M1 represents the formation and growth of garnet porphyroblasts and clinopyroxene + Na-rich plagioclase + quartz ± hornblende ± rutile in the matrix. In this assemblage, clinopyroxene and Narich plagioclase (An10-20) occur as graphic or

Two Billion Years of Tectonics & Mineralisation

The P-T conditions of three metamorphic stages were estimated using the conventional geothermobarometry combined with the TWEEQU program. The temperature-pressure conditions of the M1

assemblages were estimated at 700-750°C /15-

dalites gave the U-Pb zircon age of 1892±23 Ma

17 kbar based on the core compositions of gar-

(Guo etal1996).

net, clinopyroxene and Na-rich plagioclase using

represent the age of the high-pressure metamor-

garnet-clinopyroxene thermometer and garnet-

phic event.

These data are considered to

clinopyroxene-plagioclase-quartz barometer. The rim compositions of garnet, clinopyroxene and

Tectonic Scenario.

Na-rich plagioclase yielded 700-750°C /12-13

Based on this study and our previous work, we

kbar, which may reflect resetting conditions. The

propose the following tectonic scenario for the

P-T conditions of M2 were estimated at 700-750°

evolution of the North China Craton:

C / 6.0-7.5 kbar using garnet-orthopyroxene/ clinopyroxene thermometers and garnetorthopyroxene/clinopyroxene-plagioclase-quartz barometers, based on the compositions of garnet rim and symplectic or coronitic plagioclase, clinopyroxene and orthopyroxene. The conditions of

1. In the late Archean to Paleoproterozoic, the North China Craton did not have a uniform basement, but consisted of two separate continental blocks - the Western and Eastern blocks. The Eastern block had an active-type continental margin on which continental arcs and intra-arc basins

hornblende + plagioclase symplectite were estimated at 600-650°C/5.0-6.0 kbar using garnethornblende termometer and garnet-hornblendeplagioclase-quartz barometer, based on the compositions of garnet rim and symplectic or coronitic hornblende and plagioclase. P-T Path The combination of textural relations and thermobarometric estimation for the high-pressure mafic granulites from the Trans-North China Orogen defines a nearly isothermal decompressional (ITD) clockwise P-T path, which suggests a tectonic process initiated by crustal thickening (M1), and followed by rapid exhumation and coolingretrogression. The ITD-type P-T path is generally thought to be related to continental collisional environments (England & Thompson, 1984; Brown, 1993).

Temperature (°C) Fig. 2. Metamorphic P-T path of high-pressure mafic granulites from the Trans-North China Orogen.

Timing of high pressure metamorphism. A high-pressure mafic granulite sample gave the garnet-pyroxene-whole rock Sm-Nd isochron age of 1824±18 Ma; the U-Pb zircon age of the same sample is 1833±23 Ma; and the associated khon-

developed, whereas the Western block had a passive-type continental margin on which stable continental marginal sediments were deposited, forming the protoliths of khondalitic rocks. Separating the two blocks was an old ocean. The oce-

Tectonics Special Research Centre, School of Applied Geology, Curtin University of Technology, September 1999

anic floor was subducted beneath the margin of the eastern continental block. 2. At around 1900-1800 Ma, the old ocean between the two blocks completely disappeared by subduction and the collision between the Eastern and Western blocks occurred. The collision caused crustal folding, thrusting and thickening, and resulted in high-pressure metamorphism (M1). 3. Following the high-pressure metamorphism, the thickened crust underwent exhumation, resulting in decompression metamorphism (M2). 4. Finally, retrogressive metamorphism (M3) took place when the crust was exhumed to the shallow levels. These tectonic processes led to the final assembly of the North China Craton at about 1800 Ma. References Brown, M., 1993, P-T-t evolution of orogenic belts and the causes of regional metamorphism. Journal of Geology Society, London, 150, 227241. England, P. C., and Thompson, A. B., 1984, Pressure-temperature-time paths of regional metamorphism, I. Heat transfer during the evolution of regions of thickened continental crust. Journal of Petrology, 25, 894-928. Guo, J. H., Bian, A. G. & Shi, X., 1996. Highpressure granulites, retrograde eclogites and granites from the Early Precambrian Sanggan structural belt. In Granulites and lower continental crust in North China craton (ed M. G. Zhai), pp. 21-54. Beijing: Seismological Press.

Heinrich, C. A., 1982. Kyanite-eclogite to amphibolite facies evolution of hydrous mafic and pelitic rocks, Adula Nappe, central Alps. Contributions to Mineralogy and Petrology, 81, 3038. Smelov, A. P., and Beryozkins, V. I., 1993. Retrograded eclogites in the Olekma granitegreenstone region, Aldan Shield, Siberia. Precambrian Research, 62, 419-430. Zhao, G. C., Wilde, S. A., Cawood, P. A., and Lu, L. Z., 1998, Thermal evolution of Archean basement rocks from the eastern part of the North China craton and its bearing on tectonic setting. International Geology Review, 40, 706721. Zhao, G. C., Cawood, P. A., and Lu, L. Z., 1999a, Petrology and P-T history of the Wutai amphibolites: implications for tectonic evolution of the Wutai Complex, China. Precambrian Research, 93,181-199. Zhao, G. C., Wilde, S. A., Cawood, P. A., and Lu, L. Z., 1999b. Thermal evolution of two types of mafic granulites from the North China craton: evidence for both mantle plume and collisional tectonics. Geological Magazine, 136, 223-140. Zhao, G. C., Wilde, S. A., Cawood, P. A., and Lu, L. Z., in press. Tectonothermal history of the basement rocks in the western zone of the North China craton and its tectonic implications. Tectonophysics.

Harley, S. L., 1989. The origins of granulites: a metamorphic perspective. Geological Magazine, 126,215-247. Two Billion Years of Tectonics & Mineralisation

Reconstructions of global 2.1—1.8 Ga collisional orogens and associated cratons: implications for two pre-Rodinia supercontinents? GUOCHUN ZHAO, PETER A. CAWOOD & SIMON A. WILDE TSRC, School of Applied Geology, Curtin University of Technology, GPO Box U1987, WA 6845. Introduction Reconstructions of Rodinia and Gondwana have revealed that

North Atlcntic Supercontinent

global collisional orogenies (e.g. Grenville, Pan-African) resulted in the amalgamation of ancient continental blocks to form supercontinents. Therefore, the correlation of

South Atlcntic Supercontinent

Precambrian collisional orogenic belts that were once continuous but are truncated at modern or ancient continental margins provides a means of establishing former linkages between separated continents. In this paper, we review the major 2.1-1.8 Ga collisional orogens around the world and propose the existence of two pre-Rodinia supercontinents: one comprising the major Archean-Paleoproterozoic cratons in circum-South Atlantic provinces and possibly Western Australia and India shields, referred to herein as South Atlantic (SA) Su-

Fig. 1. Reconstructions of the proposed two pre-Rodinia supercontinents. Abbreviations: A-Aldan Shield; AZAmazon Craton; B-Baltic Shield; CA-Central Australia; CC-Congo-Chailu Craton; EA-East Antarctica; l-lndian Shield; K-Kaapvaal Craton; NA-North America Shield; NC-North China Craton; P-Pilbara Craton; R-Rio de la Plata Craton; S-S&o Luis Craton; SF-Sao Fransisco Craton; SG-South Greenland Craton; WA-West African Craton; Y-Yilgarn Craton. 1-Trans-Hudson Orogen; 2Penokean Orogen; 3-Taltson-Thelon Orogen; 4-Wopmay Orogen; 5-Cape Smith-New Quebec Orogen; 6-Torngat Orogen; 7-Foxe Orogen; 8-Makkovik Orogen; 9-Ketilidian Orogen; 10-Nagssugtoqidian Orogen; 11-Kola-Karelian Orogen; 12-Svecofennian Orogen; 13-Trans-North China Orogen; 14-Aldan Orogen; 15-Transantarctic Orogen; 16Barramundi Orogen; 17-Transamazonian Orogen; 18Eburnian Orogen; 19-Limpopo Orogen; 20-Capricorn Orogen.

percontinent (Fig. 1), and the other consisting mostly of the major ArcheanPaleoproterozoic cratons in circum-North Atlantic

Reconstructions of circum-South Atlantic

provinces and possibly Siberia, Central Australia,

provinces.

Antarctic and North China cratons, referred to herein as North Atlantic (NA) Supercontinent (Fig. 1).

In circum-South Atlantic provinces, the 2.1-1.8 Ga orogens are represented by the Transamazonian belt in South America and the Elburnean belt in

Tectonics Special Research Centre, School of Applied Geology, Curtin University of Technology, September 1999

West Africa. They involve the accretion and colli-

they show similar structural and metamorphic

sion of the Archean-Paleoproterozoic blocks

evolution. These formations either rest directly

(Ledru etal., 1994).

upon Archean basement, as is the case for the

On the map of the classical Bullard (1965) fit of Africa and South America, the structural trend of the Transamazonian Orogen along the eastern margin of the Sao Fransisco Craton is consistent with that of the Eburnean Orogen along the west-

Francevillian Unit in Gabon and the Jacobina Unit in Brazil, or they overlie the upper part of the Paleoproterozoic, as with the Tarkwaian in Ghana and in French Guiana. They were deposited in foreland basins which formed during the 2.0 Ga

ern margin of the Congo-Chaillu Craton, both are

collisional orogeny. The tectonic style of these

south-north trending. These two orogens also

formations indicates a frontal collision between

show similar lithologies and deformational and

the Congo-Chaillu and Sao Fransisco cratons

metamorphic evolutionary processes which in-

(Ledru etal., 1994).

volved an initial phase of crustal thrusting and

Reconstructions of circum-North Atlantic

thickening, followed by exhumation and final cool-

provinces.

ing (Ledru et aI., 1994). These imply that they may belong to the same orogen joining the Sao Fransisco and Congo-Chaillu cratons, prior to the Pan-African orogeny, and their structural and metamorphic features reflect the amalgamation of the two cratons. Recent research shows that the Transamazonian and Eburnean orogens are also exposed in the northeast Brazil and in Cameroon, respectively (Ledru etal., 1994). Therefore, the south-north trending Transamazonian-Eburnean orogen intervening between the Sao Fransisco and Congo-Chaillu cratons can be connected to the orogenic belts along the eastern margin of West African Craton, and as Bertrand and Jardim

The Archean to Paleoproterozoic cratons in circum-North Atlantic provinces include the North American, Greenland, Baltic and Scotland shields, each of which can be subdivided into a number of tectonic provinces. These provinces were welded together by the 2.1-1.8 Ga orogenic belts which are represented by the TransHudson, Penokean, Taltson-Thelon, Wopmay, New Quebec, Torngat, Cape Smith and Foxe orogens in the North American Shield, the Nagssugtoqidian and Ketilidian orogens in the Greenland Shield, and the Svecofennian and Kola-Karelian orogens in the Baltic Shield.

de Sa (1990) suggested, they may be the same

Lithological, structural, metamorphic and geo-

Paleoproterozoic transcontinental collisional su-

chronological correlations support links among

perbelt suturing the South American and West Af-

the Archean provinces in the North American and

rican shields.

Greenland shields, the Lewisian Complex in northern Scotland and the South Lapland-Karelia

These fits are further constrained by the similarity or parallelism of major structural features such as volcanic and orogenic belts or strike-slip faults on each side of the Atlantic Ocean (Bertrand and Jardim de Sa, 1990; Hoffman, 1991), and by gravity data (Lesquer etal., 1984). Ledru

etal.

(1994) noticed that the Paleoproterozoic fluviodeltaic formations are exposed in nearly every

province in the Baltic shield (Gower, 1985; Hoffman, 1989; Park, 1992; Ah^ll and Connelly, 1998). These Archean provinces are considered to constitute a large continental block referred to as the Nain Craton (Hoffman, 1989). The 2.0-1.8 Ga orogens surrounding these Archean provinces also show consistent connections. The Torngat collisional belt in southeastern Rae Province has

craton in circum-South Atlantic provinces and

Two Billion Years of Tectonics & Mineralisation

been linked to the Nagssugtoqidian belt in south-

vaal and Zimbabwe cratons in South Africa, has

ern Greenland, which has also been linked with

been recently proved to be a 2.05-1.95 Ga colli-

the Kola-Karelian collisional belt in Baltica (Park,

sional belt (Holzer ef a/., 1998), not an Archean

1992; Ah&ll and Connelly, 1998), and they formed

mobile belt as previously suggested. Thus, nearly

during the 1.9-1.8 Ga collision between the Nain

all Archean blocks are welded by 2.1-1.8 Ga colli-

and southeastern Rae cratons (Hoffman, 1989).

sional belts. On the basis of tectonostratigraphic,

The Makkovik-Ketilidian belt in northeastern

structural, metamorphic, geochronological and

North America is correlated with the Ketilidian belt

paleomagnetic data, some connections have

in southern Greenland and the Svecofennian belt

been proposed between Siberia and northwestern

in southern Baltica (Hoffman, 1989); they are all

North America (Hoffman, 1991; Condie and

accretionary orogens and may represent a large

Rosen, 1994; Frost et al., 1998), central Australia

subduction-related magmatic arc along the mar-

and northwestern North America (Moores, 1991;

gin of the Nain Craton (Hoffman, 1989; Park,

Dalziel, 1991; Hoffman, 1991; Borg and DePaolo,

1992).

1994), East Antarctica and southwesten North

Paleomagnetic data indicate that over the period from about 1.8 to 1.3 Ga ago most cratons in North America, Greenland and Baltic shields show an apparent polar wandering path restricted to a ±30° paleolatitude range, suggesting the ex-

America (Dalziel, 1991; Moores, 1991; Borg and DePaolo, 1994), North China and Baltica (Qian et al., 1997), and Western Australia and Southern Africa (Cheney, 1996). These connections support the existence of pre-Rodinia supercontinents.

istence at this period of the North Atlantic super-

Evidence from magmatic and sedimentary re-

continent. New paleomagnetic investigations also

cords.

suggest estimates of the 'north-south' width of about 4000 km for the Manikewan Ocean between the Superior province and Hearne and Rae provinces before 1.85.Ga (Dunsmore and Symons, 1990). This implies that these Archean blocks did not amalgamate until 1.85 Ga.

The effect of a supercontinent should be reflected in global magmatic and sedimentary records. In the South Atlantic Supercontinent, as discussed previously, the late Paleoproterozoic fluvio-deltaic intracontinental basins developed within all cratonic blocks. The basins show nearly similar com-

Reconstructions of other major continental

positional features, being composed mainly of

blocks.

conglomerate and sandstone and ranging from

Other major continental blocks are also characterized by Archean continental nuclei welded by 2.1-1.8 Ga linear mobile belts, e.g. the Amga belt separating the eastern and western Aldan shields of the Siberian platform, the Capricorn belt separating the Yilgarn and Pilbara cratons in Western Australia, the Transantarctic Mountains Orogen separating East Antarctica from unknown continental blocks, and the Trans-North China Orogen separating the western and eastern North China cratons. The Limpopo belt, separating the Kaap-

fluviatile, locally with debris flows, to deltaic (Ledru et al., 1994). The sedimentary formations include the Francevillian Unit in the Congo, the Tarkwaian Formation in West Africa, the Rosebel Formation in Guiana, the Jacobina Unit in Sao Fransisco, and they either rest directly upon Archean basement or overlie the upper part of the Paleoproterozoic sequence. These sedimentary rocks were deposited after the initial stage of the collision between the Congo and Sao Fransisco cratons and between the West African and Amazonian cratons, during the assembly of the South

Tectonics Special Research Centre, School of Applied Geology, Curtin University of Technology, September 1999

Atlantic Supercontinent. In the North Atlantic Su-

addition, 1.4-1.2 Ga mafic dyke swarms have

percontinent, a 1.80 to 1.30 Ga subduction-

been widely reported from North America, Green-

related magmatic belt extends from Arizona

land, Baltica, North China, East Antarctica and

through Colorado, Michigan, South Greenland,

Central-South Australia. These mafic dyke

Sweden, Finland to western Russia, bordering the

swarms constitute a plate-wide extensional epi-

present southern margin of the North American,

sode that may mark the youngest piercing points

Greenland and Baltic shields (Gower, 1985;

at which these cratonic blocks in the North Atlan-

Hoffman, 1989; Park, 1992; Ahall and Connelly,

tic Supercontinent can be paleomagnetically and

1998). Coeval magmatic rocks are also present in

geologically linked (Park, 1992). This episode of

southwestern Scotland, central Australia, and

extension is regarded as having signalled the

North China, now completing the link to all cra-

commencement of the rifting and breakup of the

tonic blocks in the North Atlantic Supercontinent. The occurrence of temporally and petrologically similar rocks across a distance of thousands of kilometres between these continents is impressive and supports the existence of the North Atlantic Supercontinent (Ah&ll and Connelly, 1998). Nd isotopic studies indicate that this large magmatic belt consists of volcanic and plutonic rocks resembling those of present-day island arcs and continental margins (Patchett and Arndt, 1986), and they may represent a major subductionrelated mantle-to-crust differentiation along the margin of the North Atlantic Supercontinent. Throughout much of the North Atlantic Supercon-

North Atlantic Supercontinent (Park, 1992). The Mesoproterozoic anorogenic igneous activity in the South Atlantic Supercontinent is characterized by alkaline ultrabasic rocks represented by kimberlites, lamproites and carbonatites. Kimberlite activity on a global scale coincided with periods of crustal spreading. Few Mesoproterozoic kimberlites have been reported from the cratonic blocks in the North Atlantic Supercontinent (Dawson, 1989), but the emplacement of Mesoproterozoic kimberlites and lamproites took place at numerous points within the cratonic blocks of the South Atlantic Supercontinent, beginning with the 1.6 Ga intrusion in Kuruman, situated on the

tinent, especially in North America, Greenland,

margin of the Kaapvaal Craton. Middle Mesopro-

Baltica and North China, the 1.6 to 1.2 Ga age

terozoic (~1.4 Ga) kimberlites were reported from

range is characterized by a wide spectrum of ano-

Gabon, on the west margin of the Congo Craton,

rogenic igneous activity including emplacement of

and from Liberia, on the southwest margin of

anorthosite massifs, charnockite intrusions,

West African Craton. Particularly extensive kim-

batholiths of potassium rapakivi granite, and car-

berlite-lamproite magmatic activity appears to

bonatite and alkaline intrusive bodies. A Meso-

have taken place in the late Mesoproterozoic

proterozoic anorthosite-charnockite-rapakivi gran-

(-1.2 Ga), widely distributed in the Western Aus-

ite suite is exposed in a huge belt that trends

tralian Shield, West African Craton, Indian Shield

across North America and southern Greenland

(e.g. Majhgawan), and Kaapvaal Craton (e.g. Pre-

into the Baltic region of northern Europe to as far

mier). This plate-scale kimberlite activity may rep-

east as the Ukraine Ural Mountains and North

resent an important mantle upwelling (mantle

China. The anorogenic igneous activity was

plume) episode that resulted in the final breakup

probably related to an extensive underplating

of the South Atlantic Supercontinent in the late

mechanism which preceded the dispersion of the

Mesoproterozic.

fragments of the North Atlantic Supercontinent. In

Two Billion Years of Tectonics & Mineralisation

References

metamorphic minerals. Precambrian Res. 87, 87-115.

Ahall, A.L., Connelly, J., 1998. Intermittent 1.531.13 Ga magmatism in western Baltica: age constraints and correlations within a postulated supercontinent. Precambrian Res. 92,1 20.

Bertrand, J.M., Jardim, D.S., 1990. Where are the Eburnean-Transamazonian collisional belts? Can. J. Earth Sci. 27, 1382-1393. Borg, S.G., DePalo, D.J., 1994. Laurentia, Australia, and Antarctica as a late Proterozoic supercontinent: constraints from isotopic mapping. Geology 22, 307-310. Bullard, E., Everett, J.E., Smith, A.G., 1965. The fit of the continents around Atlantic. In: Symposium on continental drift. Trans. R. Soc. London, Ser., A, 258, 41-51. Cheney, E.S., 1996. Sequence stratigraphy and plate tectonic significance of the Transvaal succession of southern Africa and its equivalent in Western Australia. Precambrian Res. 79, 3-24. Condie, K.C., Rosen, O.M., 1994. LaurentiaSiberia connection revisited. Geology 22, 168-170. Dalziel, I.W.D., 1991. Pacific margins of Laurentia and East Antarctic-Australia as a conjugate rift pair: Evidence and implications for an Eocambrian supercontinent. Geology 19, 598601.

Hoffman, F.P., 1989. Precambrian geology and tectonic history of North America. In: Bayy, A. W., Palmer, A.R. (Eds), The Geology of North America - an Overview, Vol. A, The geology of North America. Geol. Soc. Am. 447-511. Hoffman, F.P., 1991. Did breakout of Laurentia turn Gondwana inside-out? Science 252, 1409-1411. Ledru, P., Johan, V., Mil* si, J.P., Tegyey, M., 1994. Makers of the last stage of the Paleoproterozoic collision: evidence for 2 Ga continent involving circum-South Atlantic provinces. Precambrian Res. 69,169-191. Moores, E.M., 1991. Southwest U.S.- East Antarctic (SWEAT) connection: A hypothesis. Geology 19, 425-428. Park, R.G., 1992. Plate kinematic history of Baltic during the Middle to Late Proterozoic: A model. Geology 20, 725-728. Patchett, P.J., and Arndt, N.T., 1986. Nd isotopes and tectonics of 1.9-1.7 Ga crustal genesis. Earth Planet. Sci. Lett., 78: 329-338. Qian, X.L., 1997. Tectonic correlations of the Precambrian Evolution of the North China Craton with the Baltic Shield. In: Qian, X.L., You, Z. D., Halls, H.C. (eds), Precambrian Geology and Metamorphic Petrology. Utrcht, the Netherlands, pp. 43-58.

Dawson, J.B., 1989. Geographic and time distribution of kimberlites and lamproites: relationships to tectonic processes. Geol. Soc. Aust. Spec. Publ., 14: 323-342. Dunsmores, D.J., Symons, D.T.A., 1990. Paleomagnetism of the Lynn Lake gabbros in the Trans-Hudson Orogen and closure of Superior and Slave craton. Geol. Assoc. Can. Spec. Pap. 37, 215-218. Frost, B.R., Avechenko, O.V., Chamberlain, K.R., Frost, C.D., 1998. Evidence for extensive Proterozoic remobilization of the Aldan shield and implications for Proterozoic plate tectonic reconstructions of Siberia and Laurentia. Precambrian Res. 89, 1-23. Gower, C.F., 1985. Correlations between the Grenville Province and Sveconorwegian orogenic belt. In: Tobi, A.C., Touret, J.R.L.(Eds.), The deep Proterozoic crust in the North Atlantic provinces, Dordrecht, Netherlands, Riedel, pp. 247-257. Holzer, L., Frei, R., Baeton, J.M. & Kramers, J.D., 1998. Unraveling the record of successive high grade events in the Central Zone of the Limpopo Belt using Pb single phase dating of

Tectonics Special Research Centre, School of Applied Geology, Curtin University of Technology, September 1999

Paleoproterozoic khondalites in the western block of the North China Craton: P-T-D path and tectonic implications GUOCHUN ZHAO, SIMON A. WILDE & PETER A. CAWOOD TSRC, School of Applied Geology, Curtin University of Technology, GPO Box U1987, Perth 6845, WA Craton can be divided into the Archean eastern

Introduction.

and western blocks, separated by the PaleoproKhondalites, defined as a suite of granulite-facies

terozoic Trans-North China Orogen (Fig. 1; Zhao

supracrustal rocks including chiefly sillimanite-

et al., 1998,1999a). The blocks and the orogen

garnet gneisses and associated garnet-bearing

are bounded to each other by major faults (Fig.

felsic paragneisses (leptynites), quartzites, calc-

1). The western block has a two-layered base-

silicate rocks and marbles (Walker, 1902), have a

ment, with the late Archean tonalitic-

widespread distribution in the North China Craton,

trondhjemitic-granodioritic (TTG) gneisses and

and the construction and interpretation of their metamorphic pressuretemperature-deformation history are critical to understanding the tectonic setting and evolution of the craton. In this paper, we examine metamorphic mineral assemblages and reaction textural relations of the khondalites to establish the evolutionary stages of metamorphism, and apply the conventional thermobarometry combined with the TWEEQU program to estimate metamor-

Fig.1. Spatial distribution of the eastern and western blocks separated by the Trans-North China Orogen. WB-Western block; EBEastern block; TNCO-Trans-North China Orogen.

phic P-T conditions and define P-T-D paths, which in combination with structural and geochronological data place important constraints on the tectonic evolution of the North China Craton. Regional Setting. The Precambrian basement of the North China

supracrustal rocks overlain by Paleoproterozoic khondalite series metasediments. The structural style is characterized by Archean TTG gneiss domes in the northwest, flanked to the southeast by linear khondalite belts. The eastern block is composed predominantly of TTG gneisses and syn-tectonic granitoids, with

Two Billion Years of Tectonics & Mineralisation

minor rafts and sheets of supracrustal rocks in-

Deformation.

cluding ultramafic to felsic volcanic rocks and metasediments, metamorphosed from greenschist to granulite fades with anticlockwise P-T-t paths (Zhao et al., 1998). The TTG gneisses make up over 80% of the basement and the structural style is dominated by ovoid domes, separated by linear belts. Basement rocks have protolithic ages of 3.85 to 2.50 Ga and underwent regional metamorphism at 2.6-2.5 Ga.

Overprinting relationships allow the recognition of three main deformation episodes (D1, D2 and D3) from the khondalites. The D1 deformation fabrics are poorly preserved due to overprinting and transposition by subsequent deformation episodes. Evidence for this deformation phase includes small rootless intrafolial folds (F1) and an associated early foliation (S1) within garnet porphyroblasts. D2 is the dominant deformation

Intervening between the eastern and western

phase, represented by ubiquitous isoclinal folds

blocks is the Trans-North China Orogen which ex-

(F2) on various scales and a penetrative foliation

tends as a north-south trending belt through the

(S2) and sillimanite mineral lineation (L2). Also

provinces of Shanxi, Hebei and Inner Mongolia.

associated with D2 is a series of regional-scale

The orogen consists of a series of low- and high-

thrust structures and ductile shear zones. D3 re-

grade terrains containing reworked Archean com-

sulted in the asymmetric upright folding or crenu-

ponents and the juvenile Paleoproterozoic igne-

lation (F3) of the regional foliation (S2) and the

ous and sedimentary rocks metamorphosed at

development of a weakly spaced foliation (S3).

greenschist to granulite facies at ~1.8 Ga (Zhao et al., 1999a, b, in review). Geological Occurrence of Khondalites.

Metamorphic Stages. Garnet-sillimanite gneiss of the khondalites exhibits four metamorphic stages (M1 to M4). The M1

The spatial distribution of khondalites within the

assemblage is preserved as mineral inclusions

North China Craton is restricted to the western

within garnet grains. In most cases, the M1 as-

block in which the khondalites are mainly ex-

semblage comprises plagioclase, biotite and

posed in the areas adjacent to the boundary with

quartz, but in a few cases, staurolite, kyanite, ru-

the Trans-North China Orogen. In the field, khon-

tile and ilmenite appear as inclusions. Therefore,

dalite units are up to 100 m thick and are com-

the most complete assemblage that can be identi-

monly layered on a scale of a few centimeters,

fied for the M1 stage is plagioclase + biotite +

and can be traced along strike for several kilome-

quartz + staurolite + kyanite ± rutile ± ilmenite

ters, interlayered with mafic granulites or TTG

minerals. The M1 stage occurred simultaneously

gneisses. The relationships between khondalites

with the deformation episode D1. The M2 stage

and the Archean TTG gneisses and mafic granu-

represents the growth of garnet porphyroblasts

lites are controversial. Most workers believe that

and matrix plagioclase + biotite + quartz + silli-

the protoliths of khondalites were deposited on

manite ± spinel ± ilmenite, which developed con-

the TTG gneisses and mafic granulites, and inter-

temporaneously with D2. The M3 stage is repre-

pret the contact as an unconformity (Qian et al.,

sented by cordierite + sillimanite symplectic coro-

1987). However, because of poor exposures and

nas around garnet grains, or by cordierite coronas

complex deformation it could as well be inter-

around spinel. The M3 stage postdated D2 but

preted as a structural contact (Condie et al.,

predated D3. The M4 stage is defined by andalu-

1992).

site and muscovite porphyroblasts which overprint

Tectonics Special Research Centre, School of Applied Geology, Curtin University of Technology, September 1999

regional foliations (S2). The muscovite crystals do not show any preferred orientation, suggesting

that the M3 stage postdated the last deformation

episode (D3).

P-T Conditions and P-T path. 2

The P-T conditions of the M1, M2 and M3 stages 0

can be quantitatively estimated using the program

400 500

600

700

800

900

Temperature (°Q

TWEEQU (Berman, 1991). By using the TWEEQU program, eight possible equilibria (three independent) among the end-member sys-

Fig. 2. P-T-D path of the khondalites from the western block of the North China Craton.

tem quartz - anorthite - almandine - pyrope - grossular - annite - phlogopite - ilmenite - rutile - kyan-

5.5-6.5 kbar and 700-750QC for the cordierite +

ite give P-T estimates of 7.0-7.5 kbar and 600-

sillimanite symplectic assemblage (M3). The P-T

6502C for the M1 assemblage. The estimated P-

conditions of the M4 stage cannot be estimated

T conditions are considered to represent the

using the TWEEQU program because of the lack

maximum pressures and temperatures of the M1

of three or more independent reactions, but can

assemblage because the possible equilibria used

be qualitatively constrained by the appearance of

for the TWEEQU calculations do not include

andalusite. Taken together, these textural rela-

paragonite and margarite, which should be stable

tions and their P-T estimates define a clockwise

under these conditions, though they are not ob-

P-T-D path for the khondalites (Fig. 2), which

served in M1. Eight possible equilibria (three in-

suggests a tectonic process initiated by crustal

dependent) among the end-member system

thickening (M1 and M2), and followed by exhuma-

quartz - anorthite - almandine - pyrope - grossu-

tion (M3) and cooling-retrogression (M4).

lar - annite - phologopite - sillimanite - Kfeldspar - H20 yield P-T estimates of 7.5-8.0 kbar 9

Geochronology

and 750-800 C for the peak assemblage (M2).

Table 1 lists the available U-Pb isotopic ages for

Seven equilibria (three independent) among the

the khondalites in the western block of the North

end-member system anorthite - almandine - gros-

China Craton. Two age groups can be recognized

sular - pyrope - quartz - sillimanite - Mg-

from these data: 2.1-2.3 Ga and 1.8-1.9 Ga,

cordierite - Fe-cordierite yield P-T estimates of

which are interpreted as the protolithic (or inherInterpretation

References

Single-grain zircon Single-grain zircon

Inherited age Inherited age

Wang et al. (1995)

2114

Single-grain zircon

Inherited age

Wang et al. (1995)

Luoguanyao

1962

Multigrain zircon

Metamorphic age

Shen et al. (1987)

Sil-Grt-Kf gneiss

Luoguanyao

1821

Multigrain zircon

Metamorphic age

Shen et al. (1987)

Sil-Grt-Kf gneiss

Xiaobeizhi

1892

Single-grain zircon

Metamorphic age

Zhao et al. (1993)

Sil-Grt leptynite

Huangtuzui

1916

Single-grain zircon

Metamorphic age

Wang et al. (1995)

Bt-Hbl-PI gneiss

Huangtuzui

1858

Single-grain zircon

Metamorphic age

Wang et al. (1995)

Bt-Kf gneiss

Hadamengou

1868

Single-grain zircon

Metamorphic age

Wang et al. (1995)

Area

Age (Ma) Method

Bt-Kf gneiss

Hadamengou

Bt-Kf gneiss

Hadamengou

2345 2242

Bt-Kf gneiss

Hadamengou

Sil-Grt-Kf gneiss

Wang et al. (1995)

Table 1. The available U-Pb isotopic ages for the khondalites in the western block of the North China Craton.

Two Billion Years of Tectonics & Mineralisation

ited) and metamorphic ages, respectively.

References.

Tectonic Implications.

Berman, R. G., 1991. Thermobarometry using

The near-isothermal decompressional clockwise P-T-D path established for the khondalites in the western block is very similar to those P-T-D paths defined for the mafic and pelitic granulites from

multi-equilibrium calculations: A new technique, with petrological applications. Canadian Mineralogist, 29, 833-855. Brown, M., 1993, P-T-t evolution of orogenic belts

the Trans-North China Orogen (Zhao et al., in re-

and the causes of regional metamorphism.

view) and reflects continental collisional environ-

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1993). Considering the spatial and timing relationships between these rocks and the eastern and western continental blocks, this continental collision may be related to the amalgamation of the eastern and western continental blocks along the Trans-North China Orogen at about 1.8 Ga. In the Trans-North China Orogen, a large number of ages of c. 1800 Ma have been obtained by using

Condie, K. C., Boryta, M. D., Liu, J. Z., and Qian, X. L., 1992. The origin of khondalites: geochemical evidence from the Archean to Early Proterozoic granulite belt in the North China Craton. Precambrian Research, 59, 207-223. England, P. C., and Thompson, A. B., 1984,

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894-928. Guo, J. H., Bian, A. G. & Shi, X., 1996. Highpressure granulites, retrograde eclogites and granites from the Early Precambrian Sanggan structural belt. In Granulites and lower continental crust in North China Craton (ed M. G. Zhai), pp. 21-54. Beijing: Seismological Press.

by the mechanism of crustal isostatic compensa-

Qian, X. L., Chen, Y. P., and Liu, J. Z., 1987. Ar-

tion and formed widespread decompression tex-

chean crustal evolution of the northern

tures in the rocks. Finally, retrogressive cooling

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occurred after exhumation ceased. The mineral

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reaction textural relations and P-Tpaths of the

land, July 13-16, 1987. pp. 76-78.

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the basement rocks in the western zone of

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the North China craton and its tectonic implications. Tectonophysics. Zhao, G. C., Cawood, P. A., and Lu, L. Z., in re-

phic rocks in the central Inner Mongolia.

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tai amphibolites: implications for tectonic

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Wilde, S. A., Cawood, P. A., Wang, K. Y., and Nemchin, A., 1998. SHRIMP U-Pb zircon

Submitted to Precambrian Research. Zhao, Z. P., 1993. Precambrian crustal evolution

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the timing of crustal growth in the North China Craton. Chinese Bulletin of Science, 43, 144-145. Xu, R. H., Zhu, M., Chen, F. K., and Guo, J. H., 1995. A geochronological study of the Longquanguan ductile shear zone. Quaternary Sciences, 4, 332-342. Zhao, G. C., Wilde, S. A., Cawood, P. A., and Lu, L. Z., 1998, Thermal evolution of Archean basement rocks from the eastern part of the North China craton and its bearing on tectonic setting. International Geology Review, 40, 706-721. Zhao, G. C., Cawood, P. A., and Lu, L. Z., 1999a, Petrology and P-T history of the Wutai amphibolites: implications for tectonic evolution of the Wutai Complex, China. Precambrian Research, 93,181199. Zhao, G. C., Wilde, S. A., Cawood, P. A., and Lu, L. Z., 1999b. Thermal evolution of two types of mafic granulites from the North China craton: evidence for both mantle plume and collisional tectonics. Geological Magazine, 136, 223-140.

Two Billion Years of Tectonics & Mineralisation