Issuu

Geological Society of Australia

ABSTRACTS Number

Evolution of Metamorphic Belts La Trobe University 16-17th December 1996

Specialist Group in |Geochem5stry,f/1ineralog and Petrology

Specialist Group in Geochemistry, Mineralogy, and Petrology

ISSN 0729 011 X

National Library of Australia Cataloguing-in-Publication data

Ian S. Buick and Ian Cartwright Geological Society of Australia, Abstracts Number 42 Evolution of Metamorphic Belts (Specialist Group in Geochemistry, Mineralogy, and Petrology Conference; LaTrobe University, Vic; 16-18th December, 1996)

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Mrs M. Frankel Business Manager Geological Society of Australia 1203 Wynyard House 301 George St Sydney, NSW 2000

16th-i7th December, 1996 Glenn College, La Trobe University A Meeting Organised by

SGG Specialist Group in Mineralogy, Geochemistry and Petrology

Abstract Volume No. 42 Meeting Convened by Dr. Ian 8. Buick (School of Earth Sciences, La Trobe University; geoisb@lure.latrobe.edu.au) and Dr. Ian Cartwright (Department of Earth Sciences, Monash University; icart@artemis.earth.monash.edu.au)

Program Monday 08.00-08.45 09.05-09.20

December, 1996

Registration, Glenn College Airport Lounge Welcome

Session 1: The Geochronology of High-Grade Metamorphic Belts (Chairs: Ian Cartwright and Bas Hensen) 09.20-09.40

T.C.W. Fitzsimons. P.D. Kinny, & S.L. Harley Timescales of Pan-African metamorphism, melting and exhumation at Brattstrand Bluffs, east Antarctica.

09.40-10.00

I.S. Buick. I.S. Williams, R. Frei & I. Cartwright. The duration of LP/HT metamorphism in the Reynolds Range, central Australia: constraints from U-Pb SHRIMP and stepped leaching Pb geochronology.

10.0-10.20

S. Costa & P. Rey Constraining the evolution of an orogen from continental collision to post-thickening collapse: How geochronology can help.

10.20-10.40

G. Fraser. D.J. Ellis & S. Eggins Zircon ages from high-grade metamorphic rocks: Progress towards a more rigorous interpretation.

10.40-11.20

Morning Tea & Coffee/Posters

11.20-11.40

S. Bodorkos & N.H.S. Oliver A syntectonic granitoid in the Halls Creek Orogen, W.A.: Proterozoic tectonics, magmatic processes and geochronology.

11.40-12.00

A. Moller. P. Appel, K. Mezger & V. Schenk High-pressure granulite metamorphism prior to continental collision: P-T-t history of the Pan-African Belt in eastern Tanzania.

12.00-12.20

B. Goscombe, R. Armstrong & J.M. Barton Tectonothermal evolution of high-grade basement incorporated within metamorphic belts: Proterozoic Zambezi Mobile Belt and the eastern Himalaya modern analog.

12.20-12.30 12.30-14.00

Discussion Lunch/Fosters

Session 2: Processes in High-Pressure Terranes (Chair: Mike Sandiford) 14.00-14.20

G. L. Clarke. J. C. Aitchison & D. Cluzel. Eclogites and blueschists of the Pam Peninsula, NE New Caledonia: A reappraisal.

14.20-14.40

J. Arnold The blueschist-greenschist transition in the North D'Aguilar Block, southeastern Queensland .

14.40-15.00

M.A. Forster & G.S. Lister Separate episodes of eclogite and blueschist metamorphism in the los Metamorphic Complex, Cyclades, Greece.

15.00-15.20

J. McL. Miller , D.R. Gray & R.T. Gregory Refuting evidence for late stage extension in an exhumed High P/ Low T terrain, northeastern Saih Hatat, Oman.

15.20-15.40

Afternoon Tea & Coffee/Posters

Session 3: Low-Temperature Terranes (Chair Sessions 3 and 4: Ron Vernon) 15.40-16.00

D. A. Foster, D.R. Gray & M. Bucher Diachronous metamorphism and deformation in the western Lachlan Fold Belt.

16.00-16.20

R. Offlen S. McKnight & V. Morand Tectono-thermal history of the western Lachlan Fold Belt - Insights from white mica studies.

Session 4: Textural studies of high-grade rocks 16.20-16.40

T.H. Bell & K.A.Hickey A new method for distinguishing multiple generations of garnet formed during progressive metamorphism: Implications for integrating P-T-t and structural paths during orogenesis.

16.40-17.00

K.A. Hickev & T.H. Bell Spiral and staircase inclusion trail axes within garnet and staurolite porphyroblasts from the Bolton Syncline, Connecticut: Timing of porphyroblast growth and the effects of fold development.

17.00-18.30 19.00

Drinks/Posters Conference Dinner, La Trobe University Union

Tuesday 17th. December, 1996 Session 5: Australian Regional Metamorphism (Chairs: Dave Foster & Geoff Clarke) 09.00-09.20

M. Ballevre, B. J. Hensen & B. Reynard Orthopyroxene - andalusite - quartz symplectites after cordierite, a new twist to the P-T path of granulites in the Strangways Range, Arunta Block.

09.20-09.40

M. Hand Does a transient view help or hinder the interpretation of reaction textures in LP/HT rocks ?

09.40-10.00

J.J. Vassallo & R.H. Vernon Megacrystic felsic gneisses at Broken Hill: Pre to syn-tectonic granites ?

10.00-10.20

M. Sandiford

10.20-11.00

Morning Tea/Coffee

11.00-11.20

M. J. Rubenach & D. R. W. Foster Interrelationships between multiple metamorphic episodes, high strain zones, granites and metasomatism, Mount Isa Inlier. C. Venn. K. Ehlers & A. Nutman. Re-evaluation of the tectonic and thermal history of the Mt Robe region. Broken Hill

The problem with Mt Painter.

11.20-11.40

11.40-12.00

R. Pagan Regional metamorphic conditions in the Eastern Goldfields of W.A.

12.00-12.20

J.A.C. Anderson. I.S. Williams, R.C. Price & P.D. Fleming U-Pb zircon ages from the Koetong Adamellite; implications for granite genesis and the local basement in NE Victoria.

m 12.20-12.40

S. Wetherlev. J. Ridley & Kurt Stuwe Determination of the timing and P-T conditions of metamorphism using spectacular textural relationships and pseudosections of the Mount Barren Group, Western Australia .

12.40-12.50 12.50-14.00

Discussion Lunch/Posters

Session 5: High-grade Processes (Chairs: Mike Rubenach and Ian Buick) 14.00-14.20

I. Scrimgeour & D. Close Regional subeclogite facies metamorphism in the Mann Ranges, Musgrave Block, Northern Territory.

14.20-14.40

L.M. Kriegsman & B.J. Hensen High-grade metamorphism in a strike-slip setting: the Pinjarra Orogen, Western Australia.

14.40-15.00

J. Teasdale & J. Dougherty-Page Very high grade metamorphism in the western Gawler Craton.

15.00-15.20

D.J. Dunklev. G.L. Clarke & S. L. Harley Diffusion metasomatism in aluminous sapphirine-bearing granulite from Rumdoodle Peak, Framnes Mountains, east Antarctica.

15.20-16.00

Afternoon Tea/coffee and Posters

16.0-16.20

G.S. Lister. K. McPhee, T. Barr & K. Stuwe HEATTHINK and the geodynamics of metamorphism.

16.20-16.40

R.W. White & G.L. Clarke The role of deformation in aiding recrystallization: an example from a high-pressure shear zone, central Australia.

16.40-17.00

K. Stuwe Bulk composition changes during cooling

17.00-17.30 17.30-18.30

Discussion and farewell Cheese and Wine

CONTENTS Pages Conference Program

i-iii

Contents

iv-vi

Participants

vii -x

Oral Presentations JA.C. Anderson, I.S. Williams, R.C. Price & P.D. Fleming: U-Pb zircon ages from the Koetong Adamellite; implications for granite genesis and the local basement in NE Victoria.

J. Arnold: The blueschist-greenschist transition in the North D'Aguilar Block, southeastern Queensland.

M. Ballevre, B. 7. Hensen B. Reynard: Orthopyroxene - andalusite - quartz symplectites after cordierite, a new twist to the P-T path of granulites in the Strangways Range, Arunta Block.

T.H. Bell & K.A. Hickey: A new method for distinguishing multiple generations of garnet formed during progressive metamorphism: Implications for integrating P-T-t and structural paths during orogenesis.

S. Bodorkos & N.H.S. Oliver: A syntectonic granitoid in the Halls Creek Orogen, W.A.: Proterozoic tectonics, magmatic processes and geochronology.

I.S. Buick, I.S.Williams, R. Frei iSc I. Cartwright: The duration of LP/HT metamorphism in the Reynolds Range, central Australia: Constraints from U-Pb SHRIMP and stepped leaching Pb geochronology.

G. L. Clarke, J. C. Aitchison & D. Cluzel: Eclogites and blueschists of the Pam Peninsula, NE New Caledonia: A reappraisal.

S. Costa & P. Rey: Constraining the evolution of an orogen from continental collision to post-thickening collapse: How geochronology can help.

D.J. Dunkley, G.L Clarke & S. L Harley: Diffusion metasomatism in aluminous sapphirinebearing granulite from Rumdoodle Peak, Framnes Mountains, east Antarctica.

R. Fagan: Regional metamorphic conditions in the Eastern Goldfields of W.A..

LC.W. Fitzsimons, P.D. Kinny, & S.L. Harley: Timescales of Pan-African metamorphism, melting and exhumation at Brattstrand Bluffs, east Antarctica.

M.A. Forster & G.S. Lister: Separate Episodes of eclogite and blueschist metamorphism in the los Metamorphic Complex, Cyclades, Greece.

D. A. Foster, D.R. Gray &. M. Bucher: Diachronous metamorphism and deformation in the western Lachlan Fold Belt.

G.Fraser, D.J.Ellis & S. Eggins: Zircon ages from high-grade metamorphic rocks-Progress towards a more rigorous interpretation .

B. Goscombe, R. Armstrong & J.M. Barton: Tectonothermal evolution of high-grade basement incorporated within metamorphic belts: Proterozoic Zambezi Mobile Belt and the eastern Himalaya modem analog.

M. Hand: Does a transient view help or hinder the interpretation of reaction textures in LP/HT rocks ?

K.A. Hickey & T.H. Bell: Spiral and staircase inclusion trail axes within garnet and staurolite porphyroblasts from the Bolton Syncline, Connecticut: Timing of porphyroblast growth and the effects of fold development.

L.M Kriegsman & B.J. Hensen: High-grade metamorphism in a strike-slip setting: the Pinjarra Orogen, Western Australia.

G.S. Lister, K. McPhee, T. Barr & K. Stuwe: HEATTHINK and the geodynamics of metamorphism.

J. McL. Miller, D.R. Gray & R.T. Gregory: Refuting evidence for late stage extension in an exhumed high P/ low T Terrain, northeastern Saih Hatat, Oman.

A. Mdller, P. Appel, K. Mezger & V. Schenk: High-pressure granulite metamorphism prior to continental collision: P-T-t history of the Pan-African Belt in eastern Tanzania.

R. Offler, S. McKnight Sc Vince Morand: Tectono-thermal history of the western Lachlan Fold Belt - Insights from white mica studies.

M. J. Rubenach & D. R. W. Foster: Interrelationships between multiple metamorphic episodes, high strain zones, granites and metasomatism. Mount Isa Inlier.

M. Sandiford: The problem with Mt Painter.

I. Scrimgeour & D. Close: Regional subeclogite facies metamorphism in the Mann Ranges, Musgrave Block, Northern Territory.

44 46

K. Stuwe: Bulk Composition changes during cooling. J. Teasdale & J. Dougherty-Page: Very high grade metamorphism in the western Gawler Craton.

J.J. Vassallo & R.H. Vernon: Megacrystic felsic gneisses at Broken Hill: Pre to syn-tectonic granites ?

C. Venn, K. Ehlers & A. Nutman: Re-evaluation of the tectonic and thermal history of the Mt Robe Rregion, Broken Hill.

S. Wetherley, J. Ridley & Kurt Stiiwe: Determination of the timing and P-T conditions of metamorphism using spectacular textural relationships and pseudosections of the Mount Barren Group, Western Australia.

R.W. White & G.L. Clarke: The role of deformation in aiding recrystallization: an example from a high-pressure shear zone, central Australia.

Poster Presentations Ian S. Buick & Ian Cartwright: Fluid budgets during high-temperature retrogression of granulites: Evidence from the Reynolds Range, central Australia.

DJ. Clark, B. J. Hensen & LM. Kriegsman: Geology and evolution of a Mid-Proterozoic continental margin, southern Fraser Orogen, Western Australia.

C. Davids, ANU: New age information from southern Fiordland, New Zealand: Constraints on existing tectonic models.

D. A. Foster, D.R. Gray & M. Bucher: Orogenic concepts: Accommodation mechanisms for deformation in an oceanic setting, Lachlan Fold Belt, Australia.

G. Fraser, L McDougall, DJ. Ellis & I.S. Williams: The metamorphic history of Rundvagshetta, east Antarctica: Time constraints on a "clockwise" P-T-t path.

M. Hand & P. Kinny: Bay, east Antarctica.

SHRIMP constraints on Palaeozoic high-T exhumation in SW Prydz

S. Hapugoda, J. Arnold & A. Takasu: Petrography and mineral chemistry of garnet-bearing metapelites, chamockitic gneisses and mafic granulites from the Central Granulite Belt in Sri Lanka: Implications for their P-T evolution.

MA. Hendrickx, A.P. Magart, C.E. Williams & A.H.M. Vandenberg: The Kuark Metamorphic Complex, a low pressure, high temperature metamorphic belt in eastern Victoria.

L.M. Kriegsman, J. Van Gool, M. Marker, & G.T. Nichols: A structural and petrological Ttraverse through the Palaeoproterozoic Nagssugtoqidian Orogen, western Greenland: P-T-D evolution of a transpressional orogen.

S. McLaren, M. Sandiford, M. Hand, N. Neumann & N. Wall: Heat production distributions in Australian Proterozoic terranes: Implications for metamorphic thermal energy budgets and field gradients.

B. Mocek: Geodynamic model for the protoliths' evolution from the Blueschist Unit of Siphnos (Cycladic Islands, Greece): Geochemical evidence.

M. Mohajjel: Structure and metamorphism of the June area, Sanandaj-Sirjan Zone, Iran.

R. L. Oliver & C. M. Fanning: "Mawson Block" metamorphism: a preliminary synthesis.

G. Xu: Fluid inclusions in crack-seal veins at Dugald River, Mount Isa Inlier: Implications for palaeostress states and deformation conditions during orogenesis.

Meeting Participants James Anderson, School of Earth Sciences, La Trobe University, Bundoora, Vic. 3083. E-mail: geojaa@lure.latrobe.edu.au Jo Arnold, Department of Earth Sciences, University of Queensland, St. Lucia, Qld. 4072. E-mail: jamold@earthsciences.uq.edu.au Tim Bell, Department of Earth Sciences, James Cook University, Townsville, QLD 4 8 n . E-mail: Tim.Bell@jcu.edu.au David Belton, School of Earth Sciences, La Trobe University, Bundoora, Vic. 3083. E-mail: geodxb@lure.latrobe.edu.au Simon Bodorkos, School of Applied Geology, Curtin University, GPO Box U1987, Perth, WA 600L E-mail: ebodorkos@ALPHA2.CURTIN.EDU.AU Ian Buick, School of Earth Sciences, La Trobe University, Bundoora, Vic. 3083. E-mail: geoisb @ lure.latrobe .edu .au Gary Burton, Dept. Mineral Resources, NSW Geological Survey, 32 Sulphide St., Broken Hill, NSW 2880. Ian Cartwright, Department of Earth Sciences, Monash University, Clayton, Vic. 3168. E-mail: icart@earth.monash.edu.au Dan Clark, Department of Applied Geology, University of NSW, Kensington, Sydney, NSW. E-mail: d.clark@unsw.edu.au Geoff Clarke, Department of Geology and Geophysics, University of Sydney, NSW 2006. E-mail: geoffc@extro.ucc.su.oz.au Dot Close, Northern Territory Geological Survey, Alice Springs, NT 087L E-mail: dclose@dme.nt.gov.au Alfredo Comacho, Australian Geological Survey Organisation, GPO Box 378, Canberra, ACT 260L Sylvie Costa, 43 Beard Street, Eltham VIC 3095 . E-mail: scosta@latrobe.edu.au Peter Crowhurst, School of Earth Sciences, La Trobe University, Bundoora, Vic. 3083. E-mail: geopuc@luge.latrobe.edu.au Corine Davids, Research School of Earth Sciences, Australian National University, Acton, ACT 0200. E-mail: Corine.Davids@anu.edu.au Brett Davies, Normandy Exploration, PO Box 1143, West Perth, WA 6872. Tony Donaghy, Department of Earth Sciences, Monash University, Clayton, Vic. 3168. Nigel Duncan, Northern Territory Geological Survey, Alice Springs, NT 0871. E-mail: duncann@dme.nt.gov.au Dan Dunkley, Department of Geology and Geophysics, University of Sydney, NSW 2006. E-mail: danield@extro.ucc.su.OZ.AU Christine Edgoose, Northern Territory Geological Survey, Alice Springs, NT 0871. E-mail: edgoose @ dme.nt.gov.au Karin Ehlers, Department of Earth Sciences, Monash University, Clayton, Vic. 3168. E-mail: kehlers@earth.monash.edu.au Robert Fagan, Western Australian School of Mines, PO Box 597 Kalgoorlie, WA 6430. Ian Fitzsimons, Department of Earth Sciences, Monash University, Clayton, Vic. 3168. E-mail: fitz@earth.monash.edu.au

vm Peter Fleming, School of Earth Sciences, La Trobe University, Bundoora, Vic. 3083. E-mail: pfleming@mojave.geol.latrobe.edu.au Marnie Forster, Department of Earth Sciences, Monash University, Clayton, Vic. 3168. E-mail: mforster@earth.monash.edu.au Damien Foster, Department of Earth Sciences, James Cook University, Townsville, QLD 4811. E-mail: Damien.Foster@jcu.edu.au David Foster, School of Earth Sciences, La Trobe University, Bundoora, Vic. 3083. E-mail: dfoster@mojave.geol.latrobe.edu.au Geoff Eraser, Research School of Earth Sciences, Australian National University, Acton, ACT 0200. E-mail: Geoff.Fraser@anu.edu.au Andrew Gleadow, School of Earth Sciences, La Trobe University, Bundoora, Vic. 3083. E-mail: agleadow@mojave.geol.latrobe.edu.au Ben Goscombe, Tasmanian Geological Survey, PO Box 56, Rosny Park, TAS 7018. E-mail: bgoscombe @ mrt. tas. go v. au David Gray, Department of Earth Sciences, Monash University, Clayton, Vic. 3168. E-mail: dgray@earth.monash.edu.au Martin Hand, Department of Geology and Geophysics, University of Adelaide, Adelaide, SA 5001. E-mail: mhand@geology.adelaide.edu.au Sarath Hapugoda, Department of Earth Sciences, University of Queensland, St. Lucia, Qld. 4072. E-mail: hapugoda@earthsciences.uq.edu.au Douglas Haynes, Exploration Division, WMC Resources, GPO Box 860 K, Melbourne, Vic. 3001. E-mail: douglas.haynes@wmc.com.au Nick Hayward, WMC Resources Ltd, PO Box 91, Belmont, WA 6104. E-mail: nicholas.hayward@wmc.com.au Marc Hendrickx, Geological Survey of Victoria, PO Box 2145, Fitzroy, Vic. 3065. E-mail: hendrickxm@wizza.agvic.gov.au Bas Hensen, Department of Applied Geology, University of NSW, Kensington, Sydney, NSW. E-mail: B.Hensen@unsw.edu.au Ken Hickey, Department of Earth Sciences, James Cook University, Townsville, QLD 4811. E-mail: ken.hickey@jcu.edu.au Mary Jane, Department of Earth Sciences, Monash University, Clayton, Vic. 3168.E-mail: mary_jane@earth.monash.edu.au Barry Kohn, School of Earth Sciences, La Trobe University, Bundoora, Vic. 3083. E-mail: bkohn@mojave.geol.latrobe.edu.au Sue Keay, Research School of Earth Sciences, Australian National University, Acton, ACT 0200. E-mail: sue.keay@anu.edu.au Tony Kemp, Department of Geology, Australian National University, Acton, ACT 0200 Leo Kriegsman, Department of Applied Geology, University of NSW, Kensington, Sydney, NSW. E-mail: L.Kriegsman @ unsw.EDU. AU Hyun Lee, Department of Earth Sciences, James Cook University, Townsville, QLD 4811. E-mail: Hyun.Lee@jcu.edu.au

Gordon Lister, Department of Earth Sciences, Monash University, Clayton, Vic. 3168. E-mail: gordon@earth.monash.edu.au Patrick Lyons, Australian Geological Survey Organisation, GPO Box 378, Canberra, ACT 2601. E-mail: plyons @ agso.gov.au Tyler MacCready, Department of Earth Sciences, Monash University, Clayton, Vic. 3168. E-mail: tylermac@earth.monash.edu.au Sandra McLaren, Department of Geology and Geophysics, University of Adelaide, Adelaide, SA 5001 Andrew Magart, Geological Survey of Victoria, PO Box 2145, Fitzroy, Vic. 3065. E-mail: magarta@wizza.agvic.gov.au Jo Mawby, Department of Geology and Geophysics, University of Adelaide, Adelaide, SA 5001. E-mail: mawby@geology.adelaide.edu.au Jodie Miller, Department of Earth Sciences, Monash University, Clayton, Vic. 3168. E-mail: jmiller@earth.monash.edu.au Beate Mocek, Geomar, Research Center of Marine Geosciences Kiel, Germany. E-mail: bmocek@geomar.de Mohammed Mohajjel, School of Geosciences, University of Wollongong, Wollongong, NSW, 2522. Andreas Moller, Department of Earth Sciences, Monash University, Clayton, Vic. 3168. E-mail: a.moeller@unsw.edu.au Yoichi Motoyoshi, Department of Applied Geology, University of NSW, Kensington, Sydney, NSW. E-mail: y.motoyoshi@unsw.edu.au Narelle Neumann, Department of Geology and Geophysics, University of Adelaide, Adelaide, SA 5001 Sandra Occhipinti, Geological Survey of Western Australia, 100 Plain St., East Perth, WA 6004. E-mail: s.occhipinti@dme.wa.gov.au Robin Offler, Department of Geology, University of Newcastle, Newcastle, NSW 2308. E-mail: ROFFLER@GEOLOGY.NEWCASTLE.EDU.AU Robin Oliver, Department of Geology and Geophysics, University of Adelaide, Adelaide, SA 5001. E-mail: roliver@geology.adelaide.edu.au Nick Post, Department of Applied Geology, University of NSW, Kensington, Sydney, NSW. E-mail: N.Post@unsw.edu.au Roger Powell, School of Earth Sciences, University of Melbourne, Parkville, Vic. 3052. E-mail: RP@earth_sciences.unimelb.edu.au Mandy Raouzaios, Department of Earth Sciences, Monash University, Clayton, Vic. 3168. E-mail: mandyr@artemis.earth.monash.edu.au Tim Rawling, Australian Geodynamics CRC, Department of Earth Sciences, Monash University, Clayton, Vic. 3168. E-mail: timr@earth.monash.edu.au Patrice Rey, Department of Earth Sciences, Monash University, Clayton, Vic. 3168. E-mail: prey@earth.monash.edu.au Mike Rubenach, Department of Earth Sciences, James Cook University, Townsville, QLD 4811. E-mail: Michael.Rubenach@jcu.edu.au Mike Sandiford, Department of Geology and Geophysics, University of Adelaide, Adelaide, SA 5001. E-mail: msandifo@geology.adelaide.edu.au

Ian Scrimgeour, Northern Territory Geological Survey, Alice Springs, NT 0871. E-mail: iscrimge@dme.nt.gov.au Roric Smith, Normandy Exploration, PO Box 1143, West Perth, WA 6872. Catherine Spaggliari, Department of Earth Sciences, Monash University, Clayton, Vic. 3168. Adrian Spencer, School of Earth Sciences, La Trobe University, Bundoora, Vic. 3083 David Steele, School of Earth Sciences, University of Melbourne, Parkville, Vic. 3052. E-mail: david_steele@muwayf.unimelb.edu.au Kurt Stuwe, Department of Earth Sciences, Monash University, Clayton, Vic. 3168. E-mail: kstuwe@earth.monash.edu.au Jon Teasdale, Department of Geology and Geophysics, University of Adelaide, Adelaide, SA 5001. E-mail: jteasdal @geology.adelaide.edu.au Ulrike Troitzsch, Department of Geology, Australian National University, Acton, ACT 0200. E-mail: ulrike @ geology.anu.edu.au Pons Vandenberg, Geological Survey of Victoria, PO Box 2145, Fitzroy, Vic. 3065. E-mail: vandenbergf@wizza.agvic.gov.au Leon Vandenberg, Department of Earth Sciences, Monash University, Clayton, Vic. 3168. E-mail: leon@earth.monash.edu.au Caroline Venn, Department of Earth Sciences, Monash University, Clayton, Vic. 3168. E-mail: cstreets@earth.monash.edu.au Ron Vernon, School of Earth Sciences, Macquarie University, NSW 2109. E-mail: rhvemon@laurel.ocs.mq.edu.au Simon Wetherley, Dept. of Geology and Geophysics, University of Western australia, Nedlands, WA 6907. Email: swetherl@uniwa.uwa.edu.au Richard White, School of Earth Sciences, Macquarie University, NSW 2109. E-mail: dick@es.su.oz.au, Brenton Worley, School of Earth Sciences, University of Melbourne, Parkville, Vic. 3052. E-mail: b.worley@earth_sciences.unimelb.edu.au Guojian Xu, Department of Earth Sciences, James Cook University, Townsville, QLD 4811. E-mail: guojian@jcu.edu.au David Young, Northern Territory Geological Survey, Darwin, NT 0801. E-mail: young@dme.nt.gov.au Sylvia Zakowski, Department of Earth Sciences, Monash University, Clayton, Vic. 3168.

U-Pb zircon ages from the Koetong Adamellite; Implications for granite genesis and the local basement in NE Victoria J. A. C. Anderson^. 1. S. Williams^, R. C. Priced & P. D Fleming! ^ School of Earth Sciences, La Trobe University, Bundoora, Victoria 3083 Research School of Earth Sciences, Australian National University, Acton, ACT 0200, Australia. The possible petrogenetic link between the Koetong Adamellite, its entrained high grade metasedimentary enclaves (represented by a quartzofeldspathic gneiss and a refractory biotitesillimanite-cordierite type) and the local high grade metasedimentary country rocks (the Gundowring terrane) was examined by comparing the U-Pb ages of inherited zircon in the adamellite with the ages of detrital zircons in the enclaves and country rock. The age measurements were made using the ANU SHRIMP II. The Koetong granite is a muscovite-biotite "S-type" (Price, 1983), which, like S-type granites in the eastern Snowy Mountains region (Williams, 1992), is rich in inherited zircon. The inherited zircon occurs as variably rounded cores, within younger, euhedral, melt-precipitated overgrowths. The weighted mean Pb/U age of the overgrowths (the emplacement age of the adamellite) is 419±6 Ma (95% confidence limits), consistent with the biotite-whole rock Rb-Sr age, 418±4 Ma (2a), of this body. The ages of 35 inherited cores cluster mainly into groups at 470-700 Ma and 950-1100 Ma, both of which comprise several subgroups. Mixture modelling of the younger group (18 grains) reveals the presence of principal subgroups at 482±8 Ma (7 grains) and 544±17 Ma (5 grains). The zircons in the metasedimentary enclaves are dominantly detrital, the younger overgrowths being <10 |im thick.. 45 zircons from each enclave were analysed. The ages of the zircon cores cluster at 450-700 Ma and 800-1200 Ma. The 450-700 Ma group in both enclaves is composite; mixture modelling shows the quartzofeldspathic gneiss to have principal subpopulations at 471 ±7 Ma (7 of 21 grains) and 491±10 Ma (7 of 21 grains), and the refractory enclave to have subpopulations at 475±17 (4 of 22 grains) and 496±8 Ma (9 of 22 grains). The zircon in a quartzofeldspathic gneiss from the Gundowring terrane is wholly detrital and free of overgrowths. Analyses of 71 grains revealed a major 450-600 Ma age group, with lesser, more dispersed groups between 925-1325 Ma and >1500 Ma. Mixture modelling of the 450-600 Ma age group (32 grains) reveals subgroups at 467±19 Ma (4 grains) and 493±8 Ma (16 grains).

The enclave and sediment samples contain Ordovician zircons that mixture modelling consistently resolves into subgroups at -470 Ma and -490 Ma. The adamellite shows a single population at -480 Ma, probably an unresolvable mixture of the two. Further, the Early Cambrian inherited zircon population in the adamellite has more subdued expression in the enclaves and the Gundowring terrane samples. These differences are within the fluctuations expected for small sample statistics; the main feature of the data is the close similarity between the zircon age spectra from the adamellite, enclaves and host metasediment, and in turn their similarity to the age spectra for zircons from the Lachlan Fold Belt Late Ordovician flysch (Williams etal,

1994).

The presence of an Ordovician inherited zircon component in the Koetong Adamellite indicates that a Middle Ordovician sedimentary precursor was involved in producing the granite; it does not represent a partial melt of Proterozoic basement. The similarity of the inherited and detrital age spectra suggests in turn that the Koetong Adamellite and the metasediments of the Gundowring terrane are closely linked petrogenetically. The Ordovician component also constrains the deposition age of the Gundowring terrane, formerly believed to be Cambrian in age (Fleming et aL, 1985). If high grade metamorphism of the terrane occurred at -460 Ma as proposed by Steele (1993), then migmatite-grade metamorphism possibly followed sedimentation by only about 10 Ma. The presence of the Ordovician zircon component in both metasedimentary enclaves indicates that they also have an Ordovician, not Proterozoic, precursor.

Garnet-aluminosilicate-

plagioclase-quartz geobarometry constrains the maximum pressures of enclave formation to <500 MPa, indicating derivation of the metasedimentary enclaves from around 15 km. This implies that at the time of emplacement of the Koetong Adamellite (Late Silurian), the middle crust in the Koetong area was occupied by Middle Ordovician metasedimentary rocks.

References Fleming, P. D., Steele, D. A. & Camacho, A. 1985. Abstracts of the Victorian Lithosphere Symposium, pp. 14-15. Price, R. C. 1983. Geochim Cosmochim Acta, 47; pp. 31-42. Steele, D. A. 1993. Petrological studies on gneisses and granites of the Tallangatta region, NE Victoria. PhD thesis. La Trobe University, Melbourne (unpubl.). Williams, 1. S. 1992. Some observations on the use of zircon U-Pb geochronology in the study of granitic rocks. Trans, Roy. Soc. Edinburgh Earth Sci., 83; pp. 447-458. Williams, I. S., Chappell, B. W., Crook, K. A. W. & Nicholl, R. S. 1994. GeoL Soc. Aust., Abstracts, 37; p. 464.

The blueschist-greenschist transition in the North D'Aguilar Block, southeastern Queensland Jo Arnold Department of Earth Sciences, The University of Queensland, Queensland, 4072 The New England Orogen of eastern Australia provides a rare opportunity to investigate the effects of subduction and lateral accretion on the Australian continent. This study deals with the metamorphic evolution of the subduction zone complex exposed in the North D'Aguilar Block in southeastern Queensland and the influence of compositional, deformational and fluid heterogeneity on the metamorphic mineralogy. The North D'Aguilar Block in the northern New England Foldbelt exposes an array of blueschist to greenschist facies assemblages in the mafic rocks of the Rocksberg Greenstone and Mount Mia serpentinite matrix melange. These units comprise part of an ocean basin sequence of mafic igneous rocks, clastics, cherts, marbles and calc-silicates which experienced the high P/T conditions associated with subduction zone metamorphism. They display a blueschist facies foliation which is axial planar to regional scale north-trending, west-dipping mid-Carboniferous folds and is characterised by a steep (MO mineral lineation (Little et al., 1993). This fabric is variably overprinted by a near-horizontal greenschist facies foliation (M2) which developed in response to tensional forces in the late Carboniferous (Holcombe & Little, 1994). In many high pressure terranes, the re-equilibration of high pressure-low temperature (e.g. blueschist facies) assemblages to their lower-pressure counterparts (e.g. greenschist facies) is fortunately incomplete. As a result the rocks retain information about their early accretionary history as well as their exhumation. In the North D'Aguilar Block many of the rocks of high pressure origin are volatile-rich and multiply deformed. In all likelihood, they experienced prograde metamorphism subsequent to their formation and during exhumation. Despite this, the high pressure assemblages are only partially overprinted by higher temperature assemblages. The factors which control this re-equilibration may include partitioning of deformation, heterogeneous fluid flow, grain size and variation in mineralogical or lithological composition. This study attempts to constrain the importance of these factors in controlling the blueschist greenschist transition in the North D'Aguilar Block. Variable development of the Mi assemblages in the North D'Aguilar Block has complicated the development of the M2 blueschist-greenschist transition in the area. Low-strain zones within

the Rocksberg Greenstone retain relics of coarse grained primary minerals, with evidence of dominantly re-equilibrated blueschist facies metamorphism and pervasive M2 greenschist mineral development. Higher strain zones appear to have developed more pervasive blueschist facies metamorphic assemblages, implying that the high strain associated with Mi metamorphism was an important factor in controlling mineral equilibration.

Subsequent

greenschist facies re-equilibration is also variably developed, resulting in actinolite-chlorite assemblages which are interlayered with relict blueschist assemblages in the (Mj) higher strain zones. Preliminary data suggest that the effects of compositional variation between the units and subunits of the sequence are important in re-equilibration of the high pressure assemblages.

References Little TA, Holcombe RJ, Sliwa R, 1993. Structural evidence for extensional exhumation of blueschist-bearing serpentinite matrix melange, New England Orogen, southeastern Queensland. Tectonics, 12: 536-549. Holcombe RJ & Little TA, 1994. Blueschists of the New England Orogen: Structural development of the Rocksberg Greenstone and associated units near Mt. Mee, southeastern Queensland. Australian Journal of Earth Sciences, 41: 115-130.

Orthopyroxene - andalusite - quartz symplectites after cordierite, a new twist to the P-T Path of granulites in the Strangways Range, Arunta Block. Michel Ballevrel, Bastiaan J. Hensen^ and Bruno Reynard 1 ^Geosciences Rennes, UPR CNRS4661y Universite Rennes-l 35042 Rennes Cedex, France ^Department of Applied Geology, University of New South Wales, Sydney NSW 2052, Australia The granulite facies gneisses of the Arunta Block are dissected by numerous retrograde shearzones at all scales from kilometres to metres, and even down to centimetres. Early shearing took place under amphibolite facies conditions with hydration of Mg -rich metapelites to firstly gedrite-kyanite and thereafter staurolite-sillimanite. The coarse grained orthopyroxene-bearing granulite domains, away from shear zones, have remained relatively unaffected by the retrograde event. However, in the granulites fine- to very fine-grained minerals develop at grain boundaries, mainly at the expense of cordierite (Crd). Symplectites with combinations of either orthopyroxene (Opx), gedrite (Ged), anthophyllite or biotite (Bt) with either sillimanite (Sil) or kyanite (Ky) have been described by earlier workers and have been interpreted as the result of reaction during isobaric cooling, or during a separate event at higher pressure and temperature. We have re-examined similar symplectites from the Edward's Creek area because their extremely fine-grained nature, and the scarcity of evidence for strain during their formation, are not consistent with growth during a prograde tectonothermal event at elevated temperature. The very fine grained aluminosilicate polymorphs have been identified by Laser Raman Spectroscopy. Three main stages of metamorphism are distinguished in the Edward's Creek area. Early, Ml, assemblages in Qtz-bearing rocks are relatively coarse-grained and consist of garnet (Grt)-CrdSil, Grt-Opx-Crd and Opx-Crd-Bt. These assemblages are variably overprinted by finergrained aggregates (M2) containing Bt and prismatic Sil, which define the dominant foliation, parallel to the lithological layering and to the axial plane of isoclinal folds. The latest transformations consist of very fine grained symplectites (M3) replacing Crd. These include early minor Grt overgrowths, more magnesian (Mg37) than the Ml Grt (Mg28), coexisting with fine blades of kyanite. Later, even more fine grained symplectites include three types of orthopyroxene-bearing reaction textures: 1) Adjacent Opx-andalusite (And) and And-Qtz symplectites in cordierite, growing inwards from Crd-Qtz or Crd-Crd grain boundaries. These can be described by: Crd = Opx + And + Qtz + H2O. 2) Opx-And symplectites on Crd - Bt grain boundaries. This variety appears to corrode both cordierite and biotite and can be described by: Crd + Bt = Opx + And + K+ + H2O.

6 3) Fine granular orthopyroxene rims on biotite and minor replacement along cleavages interpreted as: Bt = Opx + Qtz + K+ + H2O. No K-feldspar is observed in either of the latter two types. All three reactions involve dehydration and consequently move to the right with decreasing a(H20). The reactions involving biotite breakdown are also favoured by low aCK"*"). The extrapolated stability limit for pure Mg-cordierite does not intersect the And-Sil phase boundary. However, Fe-substitution in cordierite (Mg80) will reduce this limit, potentially to pressures below the aluminosilicate invariant point, and thus allow the stable co-existence of Opx (Mg60) and And under extremely low a(H20) conditions (<0.1-0.2). The existence of the Opx-And-Qtz assemblage in Qtz-rich rocks can be taken as evidence that this can be a stable assemblage, for unusually low H2O activities at a pressure of about 4 kbar and a temperature of c. 500°C. The fine-grained nature of the reaction products, their local development and the fact that they are confined to grain boundaries indicate formation at low temperature by fluid infiltration. Complex symplectites of And-Qtz and Bt-Qtz occur in close proximity, and even in contact with Opx-bearing intergrowths. These symplectites which can be described by the reaction: Crd + H2O + K+ = Bt + And + Qtz require relatively high activities of H2O and K"'". It has not been possible to establish, on textural criteria, the order in which the apparently contradictory Opxand Bt-bearing assemblages were formed. Either there are considerable variations in a(H20) and a(K+) on a micrometric scale or there have been, possibly transient, changes in the fluid activities during exhumation. The fact that breakdown of the Opx-And intergrowths to biotite has not been observed suggests that the Opx-forming reactions may have been the last ones to take place.

Implications for the P-T path After the metamorphic peak the coarse grained rocks have undergone a variable amount of deformation associated with the formation of relatively coarse grained Bt-Sil (or Ged in Al-poor rocks) assemblages, at the expense of cordierite. Subsequently minor garnet (Mg37) mostly in the form of overgrowths on earlier grains (Mg28) forms with kyanite and quartz. The final further partial replacement of cordierite by fine-grained symplectites, consisting of orthopyroxene and andalusite and quartz, or biotite, andalusite and quartz, is interpreted to have taken place at < 4 kbar, 500±25°C during exhumation of the granulite terrane. The low pressures prevailing during the formation of the finest symplectites suggest that they were formed in response to minor deformation and fluid influx during the exhumation of the terrane, as part of the Palaeozoic Alice Springs Orogeny. We propose a simple two stage history, with essentially isobaric cooling from peak conditions, forming sillimanite-bearing assemblages in the Proterozoic, followed by reactivation, with crustal thickening and subsequent exhumation of the terrane in the Palaeozoic, forming first kyanite, and then andalusite-bearing assemblages, under fluid-present conditions. Locally, and possibly transient, very low water activity conditions prevailed during this evolution.

A new method for distinguishing multiple generations of garnet formed during progressive metamorphism: Implications for integrating P-T-t and structural paths during orogenesis T.H. Bell & KA.Hickey Department of Earth Science, James Cook University, Townsville, Qld. 4810, Australia Multiple generations of garnet can be distinguished in rocks that appear to have just one or two stages of growth using variation in the orientation of apparent rotation axes of sigmoidal, staircase and spiral-shaped inclusion trails from the core to rims of porphyroblasts. This can be accomplished in rocks that have been multiply deformed after porphyroblast growth: these axes maintain consistent orientations around fold structures that postdate their formation. Where several sets of axes are present, the sequence of gamet growth can be established by finding sufficient rocks where differing but consistent changes in orientation of the axis from core to rim are preserved to cover the range of trends preserved. This provides a new method for distinguishing and correlating phases of metamorphism along and across orogens that was previously only possible in a very limited manner through direct dating of porphyroblasts. It enables changes in inclusion trail chemistry versus gamet composition to be correlated from rock to rock as well as with the direction of bulk motion within the rock as well as with the shear senses defined by the inclusion trail asymmetries. Thus a detailed metamorphic P-T-t path can be determined and correlated with the structural motion of rocks through an orogen in a manner not previously conceived. Consequently, a much larger span of orogenic history can now be examined than was previously possible, in rocks where porphyroblast inclusion trails need not bear any relationship to the matrix.

A syntectonic granitoid in the Halls Creek Orogen, W,A.: Proterozoic tectonics, magmatic processes and geochronology S. Bodorkos & N.H.S. Oliver School of Applied Geology, Curtin University, GPO Box U1987, Perth WA. 6001 The Early Proterozoic Lamboo Complex of the Halls Creek Orogen is of considerable interest since it contains widespread intermediate (granodioritic-tonalitic) intrusives analogous to those found in many modem collisional settings, where their presence is commonly interpreted as evidence for contamination of melt source rocks by subducted slab material. Recent regionalscale studies conducted by GSWA and AGSO have revealed differences in the nature and timing of processes across the Lamboo Complex, with -1850 Ma felsic magmatism in the Western zone contemporaneous with high-grade metamorphism in the Central zone and turbiditic sedimentation in the Eastern zone (Tyler et al 1994, Sheppard et al 1995). A collisional plate-margin model could account for these disparities, contrary to the earlier model of Etheridge et al (1987), which invoked intracratonic tectonics for the Halls Creek Orogen and similar Early Proterozoic terrains in northem Australia. The focus of this study is the Mabel Downs Tonalite, one of the largest plutons (--1000 km^) with intermediate composition in the Lamboo Complex. It has an igneous crystallisation age of 1832±3 Ma (SHRIMP U-Pb zircon; Page et al 1995) and the tonalite suite is found in all three zones, so this data provides a minimum age for juxtaposition of these terranes. The aim of the study is to examine field relations, petrology, geochemistry and radiogenic isotope systematics of the pluton in order to determine its relationships to metamorphism and deformation. Petrogenetic models in a collisional plate-margin setting will also be assessed. The pluton is sill-like with respect to external markers and the regional metamorphic fabric; however, defining the boundary is difficult due to interfingering with the wallrocks over metreto >100 metre scales. In many places, the intrusion displays marked heterogeneity, ranging from monzogranitic to trondhjemitic and gabbroic compositions. Well-exposed examples of magma mingling and syntectonic melt emplacement are present. Several outcrops contain evidence that intrusion was synchronous with at least one major deformation event (S3 in regional terminology, possibly comprising two or three short-lived phases). For example, a distinctive trondhjemitic phase is observed as S3a layer-parallel segregations cut by dextral S3b shears, as lubricating magma on S3b shear surfaces and also intruded along cross-cutting sinistral S4 shears. It is further suggested that the unusual

composition of this melt precludes its formation in a wide range of chemical differentiative settings and thus generation from a specific source under restricted conditions is favoured. In addition, mutual cross-cutting relationships indicate that emplacement of all these different phases took place over a relatively short time. U-Pb SHRIMP zircon determinations from separate granodiorite and tonalite samples (both of which contained an S3a foliation) gave indistinguishable ages at 1833±3 Ma and 1828±4 Ma respectively. Cross-cutting the granodiorite is a felsic dyke containing only a magmatic fabric, and although imprecisely dated at 1819±8 Ma, it nevertheless provides a useful constraint on the timing of the S3a foliation. Prior to the intrusion of the Mabel Downs Tonalite, high-temperature low-pressure metamorphism affected metasediments of the Central zone, producing gamet-sillimanite-biotite pelitic gneisses and a variety high-grade assemblages in metacarbonates including diopsidewollastonite marbles and gamet-clinopyroxene skam. This event has been dated at 1850±4 Ma by SHRIMP determination on zircon from the leucosome of a pelitic migmatite. The southwestern margin of the Mabel Downs pluton is separated from these older metasediments by a large dextral S3b shear zone. On the eastern side of the pluton, however, direct contact relations are well preserved. Tonalite has migmatised nearby pelites, garnet is occasionally entrained by trondhjemite melts and localised patches of diatexite are produced. This second phase of anatexis at --1830 Ma implies that the temperature of the wallrocks was at least 400°C at the time of intrusion, which may reflect slow cooling from the 1850 Ma metamorphism, or more widespread re-heating prior to syntectonic granitoid emplacement at 1830 Ma. References Etheridge M.A., Rutland R.W.R. & Wyborn L.A.I. 1987. Orogenesis and tectonic processes in the Early to Middle Proterozoic of northern Australia. American Geophysical Union Geodynamics Series 17, 131-147. Page R.W., Tyler LM & Blake D.H. 1995. Geochronology of magmatism and high-grade metamorphism, Kimberley region, W.A. Australian Conference on Geochronology Abstracts, 25. Sheppard S., Griffin T.J. & Tyler I.M. 1995. Geochemistry of felsic igneous rocks from the southern Halls Creek Orogen. Geological Survey of Western Australia Record 1995/4, 81pp. Tyler I.M., Griffin T.J., Page R.W. & Shaw R.D. 1994. Are there terranes within the Lamboo Complex of the Halls Creek Orogen? Geological Survey of Western Australia Annual Review 1994, 1-9.

The duration of LP/HT metamorphism in the Reynolds Range, central Australia: Constraints from U-Pb SHRIMP and stepped leaching Pb geochronology JanBuickl, Ian Williams^, Robert Frei3 & Ian Cartwright^ School of Earth Sciences, La Trobe University, Bundoora, Vic. 3068, Australia. Research School of Earth Sciences, Australian National University, Acton, ACT 0200, Australia. Gruppe Isotopengeologie, Mineralogisch-Petrographisches Institut, Universitat Bern, Erlachstrasse 9a, CH-3012 Bern, Switzerland. Dept. of Earth Sciences, Monash University, Clayton, Vic. 3168, Australia. Studies of regionally-extensive low-pressure/high-temperature (LP/HT) metamorphism commonly indicate a link between local metamorphic highs and close proximity to intrusive rocks of broadly granitic composition. This may indicate that batholithic granites provide the heat source and therefore drive short-lived episodes of regional LP/HT metamorphism and deformation. In the Reynolds Range (central Australia), metasediments of the Reynolds Range Group were intruded by voluminous sheet-like --1.78 Ga granites (-60% of the terrain) and subsequently regionally metamorphosed (M2-D2) from greenschist (-400 °C) to granulite (-750-800 °C) grade at -4-5 kbar. At the highest M2 grades, granulite-facies metacarbonates, metapelites and metapsammites were partially retrogressed to amphibolite grade in narrow, kilometre-scale strike-parallel zones after the M2 peak. The zones of retrogression were channelways for water-rich fluids that were probably exsolved from crystallising partial melts in the granuUtefacies metapelites at, or near, the granite solidus (-650 °C). Zircons from little-segregated partial melt in unretrogressed, granulite-facies metapelite contain inherited cores with SHRIMP U-Th-Pb ages > -1.8 Ga, and -30 |im-thick, newly-formed low-Th/U overgrowths that grew during granulite-facies metamorphism at 1594 ± 6 Ma. Monazite in the same rock cooled to its blocking temperature at 1576 ± 8 Ma. Within the retrograde zones, locally abundant high-temperature quartz veins contain much greater abundances of newly-formed U-rich zircon that yield SHRIMP ages between 1586 ± 5 Ma (early, semi-concordant quartz-cordierite veins) and 1568 ± 4 Ma (late, discordant quartzsillimanite veins).These data suggest that: a) regional metamorphism occurred -200 Ma later than, and was therefore unrelated to, voluminous granite emplacement; b) rather than being a very short-lived LP/HT event, the high grade rocks remained at or above the solidus for a protracted period of at least 26 Ma; and c) after the M2 peak the terrain cooled slowly (-26°C/Ma) to the solidus.

Discordant grossular-andradite garnet- and epidote-bearing quartz stockworks commonly occur at the lower contacts of retrogressed calc-silicate rocks where segregated partial melts sourced from the underlying high-grade metagranites ponded, crystallised and exsolved a water-rich fluid (Hand & Dirks, 1991). Mineral assemblages in the quartz veins suggest that they developed at -600 °C (Cartwright et al, 1996). Pb stepwise leaching (PbSL) experiments on garnet from two discordant quartz veins in the stockworks yield comparable single mineral isochrons of -1570-1580 Ma, which is interpreted as the age of garnet growth in the veins. These ages are in excellent agreement with SHRIMP U-Th-Pb age determinations of singlepopulation, newly-formed zircon from retrograde quartz-vein systems elsewhere in the area (Williams et aL, 1996) and the age of the last-crystallisated zircon population in the segregated melts that pooled below the quartz stockworks (Hand et aL, 1995). The isochrons are not likely to date fortuitously inclusions of zircon within the garnet because any zircon inherited from the metagranites is likely to contain several components that range in age between >2.5 Ga and --1.58 Ga (Collins & Williams, 1995; Hand et aL, 1995). A PbSL experiment on epidote intergrown with garnet in one of the veins yielded an isochron of --1480 Ma. Such an age has not previously been obtained from any accessory phase from the Reynolds Range, suggesting that the PbSl age dates epidote, rather than other mineral inclusions. The younger age of the epidote compared with the garnet suggests that the its closure temperature to lead diffusion was considerably less than 600 'C during retrogression in the Reynolds Range. The epidote PbSL age could record either further slow cooling after the regional peak to temperatures significantly lower than the granite solidus (-550-500 °C), or partial resetting during a subsequent reheating event, such as the --400 Alice Springs Orogeny, which locally reached mid-amphibolite-facies conditions in retrograde shear zones that transect the area (Cartwright & Buick, unpublished data).

References Cartwright, I., 1996. Precamb. Res., 77, 211-219 Collins, W. & Williams, I.,1995. Precamb. Res., 71, 69-89 Hand, M. & Dirks, P.H.G.M., 1991. J. Struc.Geol., 14, 591-604. Hand, M. et aL, 1995. Geol. Soc. Aust. Abs., 40, 60-61 Williams, I. et aL, 1996, J. Met. Geol., 14, 29-48 .

Eclogites and blueschists of the Pam Peninsula, NE New Caledonia: A reappraisal G. L. Clarke^ J. C. Aitchison^ & D. Cluzel^ ^ Department of Geology & Geophysics, University of Sydney, Sydney, NSW 2006, Australia; email - geoffc@ucc.su.ozMu 2 Department of Earth Sciences, University of Hong Kong, Polrfulam Road, Hong Kong. ^ Laboratoire de Geologie, Universite Frangaise du Pacifique, BP 4477, Noumea, Nouvelle Caledonie High-P rocks of the Pam Peninsula, NE New Caledonia are divided into three zones: (1) an uppermost ferroglaucophane-lawsonite zone of Cretaceous to Eocene metasediments and metavolcanics of the Diahot terrane that experienced peak conditions involving P=7-9 kbar and T=400±58 °C; (2) albite-epidote-omphacite zone Diahot terrane rocks that experienced blueschist facies conditions of P=14.5±2.8 kbar and T=580±86 °C; (3) lowermost metabasic eclogites of uncertain age that form the Pouebo terrane, which experienced high-P conditions of P=21-24 kbar and T= 475-580 Eclogite occurs as metre to kilometre-scale pods in coarsegrained hydrous mineral-rich "glaucophanite" formed during hydration and decompression of the Pouebo terrane. Metamorphism and deformation were consequent to Eocene convergence, when sedimentary and ophiolitic nappes were thrust over the eclogites in a SW direction. Large steps in metamorphic grade are coincident with SW-dipping and NE-dipping faults that separate the three zones and were formed during two stages: (1) comparatively slow uplift of the Pouebo terrane that juxtaposed it with the Diahot terrane at P«14 kbar; (2) comparatively rapid uplift of both the Diahot and Pouebo terranes to form a domal core of eclogite flanked by significantly lower grade rocks to the SW and NE.

Constraining the evolution of an orogen from continental collision to post-thickening collapse. How geochronology can help. Svlvie Costal & Patrice F. Rey2

143 Beard Street, Eltham VIC 3095 2 Earth Sciences, Monash University, Clayton VIC 3168 Introduction: Measuring time accurately is an important factor for understanding the complex processes involved in the evolution of metamorphic belts. There exists a large range of isotopic methods that can provide accurate and reliable chronologic and thermochronologic information for various mineral and rock types involved in different geological processes. Using appropriate dating techniques vi^ithin a well-constrained, structural, petrological and geophysical frame may allov^' to decipher important milestones of the evolution of an orogen. Through the example of the Variscan Belt of Europe, and more specifically of the Massif Central in France, it is emphasised that application of complementary dating tools (Ar-Ar, UPb, Sm-Nd) to appropriate geological targets can give insights into the evolution of the orogen over a period of 100 Ma, from continental collision to post-thickening collapse. The geochronological data also show that the lower continental crust acquired its main features (seismic fabric, intrusion of mantle-derived magmas, granulite-grade metamorphism) when the upper continental crust underwent post-thickening extension through gravitational collapse. Geological constraints: The Variscan belt of Europe resulted from closure of oceanic domains in the Silurian and accretion of two main continental blocks in the DevonianCarboniferous (1). During the latter time-span the accreted continents experienced thickening processes and subsequent extensional collapse (2, 3). In the Massif Central, which is part of the internal thickened zone of the belt, the upper continental crust kept the memory of the complete geological history from oceanic subduction to post-thickening collapse. The impact of these orogenic events on the lower continental crust can be assessed with the help of deep seismic profiles and by studying lower crustal xenoliths scavenged by volcanism. The upper half of the crust consists of high-grade metamorphic nappes, containing relics of high-P rocks included in amphibolite facies gneisses (4). The nappes represent the previously subducted continental margins, that were thrust onto parautochthonous domains during continental collision. During post-thickening collapse, metamorphic core complexes developed in the parautochthonous domains. The complexes are usually characterised by a large-scale extensional shear-zone, that controlled deposition of a continental basin on the hanging wall and exhumation of a regional anatectic dome in the footwall (5,6). Seismic data show that the lower half of the crust is characterised by near horizontal low and high velocity laminae, resulting from tectonic transposition of contrasting lithologies (7). This peculiar feature is a resuh of gravitational collapse of previously thickened crust (8). The lower crustal xenoliths consist of metasediments, and felsic and mafic meta-igneous rocks, that^all show primary assemblages in equilibrium at granulite-facies conditions (4-9 kbar; 800-900°C) (9). The felsic granulites represent anatectic melts of the metasediments and the mafic granulites represent mantle-derived magmas emplaced into the lower crust (9). This mafic/felsic lithological contrast may be the cause of the alternation of low and high velocity laminae observed in the deep seismic profiles (8,10). Geochronological data: In the Massif Central, the use of a combination of isotopic techniques on carefully selected field targets has allowed to gain important chronologic information: * Ar-Ar ages of syn-metamorphic minerals associated with thrust deformation indicate that thrusting and cooling of the high-grade nappes took place 360-340 Ma ago (11).

14 * Ar-Ar age determinations on synkinematic minerals sampled throughout the extensional shear zones indicate that extension occurred at 320-290 Ma (12). * Granitic melts from the anatectic dome emplaced between 300 and 280 Ma as indicated by U-Pb concordia-intercept ages (10). . * U-Pb ages have been previously measured on zircons from metasedimentary and felsic granulitic xenoliths (9). The zircons form discordia yielding lower-intercept ages clustering at 300-280 Ma, which reflects the age of the granulite-grade metamorphism. Zircons from basic granulitic xenoliths have also been analysed. They show very weak discordance and are grouped close to 310-290 Ma, which may represent the crystallization time of the mafic magmas under granulite-facies conditions (10). * A Sm-Nd gamet/whole-rock isochron of 220 + 1 Ma was also obtained on the basic granulites whose zircons have been dated (13). This age is in agreement with Sm-Nd dates previously obtained on some metasedimentary granuHtes (9). Discussion: Together the data collected in the Massif Central indicate that: * CoUision-related tectonism and metamorphism occurred prior to 340 Ma during the Lower Carboniferous. * Intrusion of mantle-derived magmas into the base of the crust, granulite-grade metamorphism, lower crustal seismic layering, and extraction of granites from the melted deep crust, occurred simultaneously in the Late Carboniferous (320-280 Ma). From the data obtained in the upper crust, we know that this is the time when gravitational collapse of the thickened Variscan crust took place. Therefore this may suggest that the Variscan lower crust acquired its main feamres during this same geodynamic process. * Consequently to gravitational collapse, the crust recovered a normal thickness and relaxation of the associated thermal perturbation took place. The younger Sm-Nd values of 220 Ma, compared to the U-Pb zircon ages, may reflect isobaric cooling of the lower crust (below 600°C) after it recovered a normal thickness. Incorporation of mantle-derived magmas into the lower crust as a result of extensional collapse has strong implications for the evolution of continental lithosphere (10). Indeed, addition of mafic magmas to the pre-existing continental crust contributes to its rejuvenation and growth. Because collapse makes the extraction of granitic melts from the deep crust easier, it facilitates the differentiation of the continental crust into a granitic upper part and a granulitic lower part. In collapsed orogens, the lower crust apparently loses the memory of many of the events that preceded collapse, whereas the upper crust keeps a more complete record of most orogenic events (10). Through this example, we show that the use of appropriate radiometric techniques on carefully selected field targets within a reasonably well-constrained geological frame is a powerful mean for a better understanding of crustal evolution during orogenic processes. This also allows to track crustal evolution at different structural levels. References (1) Matte, 1991: Tectonophysics, 196, 209-337. (2) Burg, Van Den Driessche & Brun, 1994: Geol. France, 3, 33-51. (3) Rey, Burg & Casey (in press): Geol. Soc. Sp. Publ., 121. (4) Dufour, 1985: Lithos, 18,97-113. (5) Malavieille, Guihot, Costa, Lardeaux & Gardien, 1990: Tectonophysics, 177, 139-149. (6) Rey, Burg & Caron, 1992: Geodinamica Acta, 5, 17-36. (7) Bois & ECORS, 1990: Tectonophysics, 173, 397-410. (8) Rey, 1993: Tectonics, 12, 580590. (9) Downes, Kempton, Briot, Harmon & Leyreloup, 1991: EPSL, 102, 342-357. (10) Costa & Rey, 1995: Geology, 23, 905-908. (11) Costa, Maluski & Lardeaux, 1993: Chem. Geol., 105, 339-359. (12) Costa, 1990: PhD, Montpellier, France. (13) Costa, Rey, Todt & Goldstein, 1994: Mineral. Mag., 58A, 197-198.

Diffusion metasomatism in aluminous sapphirine-bearing granulite from Rumdoodle Peak, Framnes Mountains, east Antarctica Daniel. J. Dunkley^. Geoffrey Clarke 1 & Simon Harley^ ^Department of Geology and Geophysics F05y University of Sydney, NSW 2006, Australia ^Department of Geology and Geophysics, University of Edinburgh, Scotland UK EH9 3JW

At Rumdoodle Peak, east Antarctica, a xenolith of metasedimentary granulite is enclosed by a stock of Mawson Chamockite, an extensive c. 960 Ma intrusion. The xenolith contains a disrupted layer of spinel-orthopyroxene-phlogopite gneiss that is truncated by quartz-rich pegmatite. On the contact of these two units, reaction bands millimetres to centimetres thick have developed, comprising a sequence of mineral zones containing spinel, orthopyroxene, sapphirine, cordierite and plagioclase. A typical sequence of zones can be summarised as: Qtz [ Opx [ Pl+Opx [ Crd+Opx [ Opx+Crd+Spr+Spl [ Opx+Spr+Spl [ Opx+Spl (Mineral abbreviations as per Kretz, 1983.) The minerals in each zone are listed in order of abundance, and the boundaries between the zones (marked by

) are sharply defined. The

zone sequence approximately matches that predicted by a model of closed system diffusion metasomatism in the Si-Al-Mg-Fe-0 system: Qtz [ Opx [ Pl+Opx [ Crd+Opx [ Opx+Spr [ Opx+Spl The diffusive exchange of silica for iron and magnesium controls the sequence of zones observed. The sequence differs from the model by the number of minerals in each zone, and by the presence of disequilibrium textures, including coronas of cordierite around sapphirine and sapphirine around spinel grains. Despite disequilibrium on a nrni scale, mass transfer on a cm scale has occurred. The growth of the reaction band was controlled by diffusion along intergranular pathways, with local equilibrium having been maintained within grain boundary space and inside grain edges. The development of corona textures was the result of slow reaction rates, which were controlled by the limited diffusive exchange of Si and A1 within mineral grains. Reference Kretz, R., 1983. Symbols for rock-forming minerals. American Mineralogist, 68, 277-279.

Regional metamorphic conditions in the Eastern Goldfields of W.A. Robert Fagan Department of Mining Geology, Western Australian School of Mines, P.O. Box 597, Kalgoorlie, WA. 6430 Regional metamorphosed greenstones and associated metasediments of Archaean age comprise some 20-25% of the Yilgam Block of Western Australia, immersed in a mass of granite and granite gneiss. Up to five metamorphic zones are recognised across a range of parental rock types including: banded iron formations, felsic-to-intermediate volcanics and volcanogenic sediments; both mafic and ultramafic lavas and intrusives. The following metamorphic zones and facies are noted in the Norseman-Wiluna belt. 1. PREHNITE-PUMPELLYITE FACIES 2. PUMPELLYITE-ACTINOLITE FACIES 3. CHLORITE ZONE OF THE GREENSCHIST FACIES 4. BIOTITE ZONE OF THE GREENSCHIST FACIES 5. CORDIERITE ZONE OF THE AMPHIBOLITE FACIES 6. ANDALUSITE STAUROLITE ZONE OF THE AMPHIBOLITE FACIES 7. SILLIMANITE-K-FELDSPAR ZONE OF THE AMPHIBOLITE FACIES

} } VERY LOW GRADE } LOW GRADE MEDIUM GRADE HIGH GRADE ZONE VERY HIGH GRADE

A P-T diagram compiled for some of the equilibrium reactions in evidence in the Eastern Goldfields defines a shallow curve outlining a geothermal gradient of approximately 50°C/km. The peak metamorphic conditions and the derived geothermal gradient indicate that partial melting should have been initiated within lithologies containing granite minimum melt compositions (quartz-orthoclase-albitic plagioclase and water). The felsic volcanic units and the highest grade pelitic lithologies would have possessed the appropriate mineralogy and should have been sufficiently wet from numerous dehydration reactions for some melting to have conmienced. There are no S-type granites, significant migmatites, and little other evidence that melting in felsic units within the greenstone belts has occurred. The three dimensional geometry of various metamorphic zones as they may have appeared before the intrusion of numerous granitoids that now dominate the surface exposures have been reconstructed. The pattern depicted is one of broad domal high-grade zones trending NNWSSE with intervening lower-grade trough regions. If the present distribution pattern for the granitoids is superimposed on the metamorphic zones the majority of the granitoids are found to occupy the core regions of metamorphic domes. This implies a causal relationship between metamorphism and granite emplacement. The absence of migmatites and S-type granites suggests that the granitoids acted as both a source of heat and hydrothermal solutions for the metamorphic episode and were not themselves the product of the metamorphic event. The distribution of metamorphic zonal boundaries appears to represent largely primary, slightly undulating, isothermal surfaces superimposed on largely subhorizontal isobaric surfaces. The two sets of surfaces do not appear to have been significantly deformed by any major tectonic event although they may have been steepened against intruding granitoids adjacent to the highest grade zones. This pattern appears to post-date any major folding event and reflect crustal temperature variations with depth at the time of peak metamorphism. The general increase in metamorphic grade towards the edges of the greenstone belts appears to reflect increased heatflow to higher crustal levels along what are currently the exposed margins of the metamorphic belts. The highest grade metamorphic rocks occur in the narrow keel regions of greenstones wedged between granites in the southern portion of the Southern Cross belt and reflect higher temperature conditions. This may be in response to deeper crustal exposures through increased levels of unroofing in these regions or may reflect a higher geothermal

gradient in response to hotter, drier magmas. Hot dry magmas would result in higher metamorphic grades forming over narrower intervals at higher crustal levels. The voluminous granitoids with solidus temperatures of 650°C, saturated water contents of around 8 wt. %, minimum melt levels of at least 90%, and occupying in excess of 75% of the Yilgam Block appear more than capable of providing the heat and hydrothermal fluids to account for the metamorphism. The granulite facies lithologies in the Southern Cross belt would require significantly hotter and drier melts in order to form. This implies deeper conditions and heating from drier magmas derived from relatively dry melting source regions beneath this zone. Isotopic data from a number of sources indicate air-fall tuffs and interflow sediments within the greenstones of the Kalgoorlie terrain being deposited around 2.7 Ga. Late stage gold deposition in the low-metamorphic grade Golden Mile deposits gives an age of 2673 Ma. This age represents the earliest decline in metamorphism in low grade regions and should be indicative of the initiation of the decline in peak metamorphic conditions. Younger ages on biotites at Kambalda (2,555±10 Ma), record somewhat longer cooling events over higher grade regions. Radiometric dating indicates that the entire period involving the cessation of sedimentation, the initiation of significant deformation, intrusion, regional metamorphism and the subsequent decline of the metamorphic event due to uplift and erosion, may have been of very short duration lasting a maximum of 20 million years. The unroofing of the metamorphic and plutonic complexes to the present level of erosion indicates the removal of some 13 km of crust. The age data for the low metamorphic grade Golden Mile deposits indicate declining peak temperatures and the first 7-8 km of unroofing were achieved over no more than 20 million years. Higher grade regions cooled more slowly from higher temperatures. The remaining 6-7 km of unroofing to the present level of exposure occurred largely prior to the intrusion of late small granitic and syenitic stocks and dykes dated around 2.5 Ga. The present thickness of the greenstone belts derived from seismic reflection profiling is about 5-6 km. A further thickness of 10-12 km may be added estimated from peak metamorphic pressures around 3.5 kbar giving a total original thickness of around 15-18 km. It appears that only the bottom 25% was made up of mafic to ultramafic units with the remainder composed largely of felsic and clastic sediments.

Timescales of Pan-African metamorphism, melting and exhumation at Brattstrand Bluffs, east Antarctica. I.C.W. Fitzsimonsl P.D. Kinny^ & S.L. Harley^ ^Department of Earth Sciences, Monash University, Clayton, Victoria 3168, Australia '^Department of Applied Physics, Curtin University of Technology, Perth 6001, Australia ^Department of Geology & Geophysics, University of Edinburgh, Edinburgh EH9 3JW, UK Metapelitic migmatites at Brattstrand Bluffs, East Antarctica, have a complex geological history (Fitzsimons, 1996). Granitic leucogneiss comprises 25% of exposed rocks, and represents crystallised partial melt derived from the metapelite. Melting reactions consumed biotite, and stabilised anhydrous assemblages in the metapelite while H2O was concentrated in the melt. Mineral reactions indicate 11 km of exhumation after peak metamorphism, achieved at least in part by tectonic unroofing along ductile shear zones that locally overprint peak mineral assemblages and fabrics. These shear zones are associated with local rehydration of hightemperature mineral assemblages by hydrous fluids released from residual partial melts and late felsic plutons as temperatures approached the granite solidus. We used the WA SHRIMP at Curtin University for in situ U-Pb analysis of zircons and monazites in thin sections of two leucogneiss samples from Brattstrand Bluffs, to constrain the timing of partial melting and peak metamorphism. These samples (88/105 and 88/122) are from two localities spaced 20 km apart, and both preserve a planar fabric associated with the late shear zones. Zircons in both rocks have regular shapes and igneous-style zonation. No structural cores were observed, although some grains have fine rims visible under cathodoluminescence but too narrow to analyse. 9 analyses of 7 zircons in 88/105 and 3 analyses of 1 zircon in 88/122 define a single concordant population (Fig. 1), with mean 206pb/238u ages of 535±13 Ma for 88/105 and 536±35 Ma for 88/122. These are taken as the initial crystallization age of the leucogneiss. There is no indication of older inherited material, 0.10

Brattstrand Bluffs zircons 0.09 238 U

although relatively few grains were studied.

Vz/zzzyvT^^ cnn ^^^ OUU

0.08 S h r i m p

0.07 .., 1 , 1 _i 0.62

data

(535±13Ma) • 88/105 E 88/122

1—1— 0.70 207pb/235U

0.78

Fig. 1. A conventional concordia plot for SHRIMP U-Pb analyses of zircon in the Brattstrand Bluffs leucogneiss samples. All error boxes and uncertainties here and elsewhere in the text are given as ±2a. ^^'^Pb was used to correct for common Pb, and Pb/U ratios were calibrated with the CZ3 zircon standard (564 Ma).

High-contrast back-scattered electron images show that many of the monazites in both samples have dark irregular cores (lower-Th)

and bright rims (higher-Th) although most of the studied grains in 88/105 are composed entirely of the Th-rich phase. 16 analyses of 8 monazites in 88/105 and 14 analyses of 5

19 monazites in 88/122 were made in total. Lower-Th cores in both samples gave indistinguishable mean 206pb/238u ages (528±4 Ma in 88/105 and 527±11 Ma in 88/122), which are within error of the zircon ages, implying that the monazite cores crystallised from the melt. The slightly younger mean ages for monazite may reflect a lower effective blocking temperature than for zircon. The Th-rich monazite rims and grains gave mean 206pb/238u ages of 518±3 Ma in 88/105 and 512±14 Ma in 88/122, which are within error of each other but sUghtly younger than the Th-poor monazite cores, although individual analyses overlap in their uncertainties (Fig. 2). This age difference corresponds to a clear petrographic feature in the monazite grains, and we interpret Th-rich monazite growth as a discrete event some 20 Ma after the initial crystallization of the leucogneiss samples. 550

88/105 monazite data

<d540

<G)

V Pp 9D

?530

£520 510 500

Low-Ih cores •

Zircon age 535±13Ma

M 96

00 00

s>6

Fig. 2. SHRIMP 206pb/238u monazite ages for sample 88/105. Analyses of low-Th cores are distinguished from high-Th rims, and both are presented in order of decreasing age. Pb/U ratios were corrected for common Pb using ^^^Pb and normalised to the MAD monazite standard (2<>6pb/238u _ 0.083). The U-Pb zircon age for the same sample (solid line) with ±2G error limits (dashed lines) is shown for comparison.

High-Th rims o

The 535 Ma zircon ages represent the initial stages of leucogneiss crystallization, and give a good estimate for the age of peak metamorphism and partial melting. We attribute the 515 Ma high-Th monazite ages to fluid flow along the ductile shear fabric preserved in both leucogneiss samples, which thus constrains the timing of final melt crystallization and local rehydration of the country gneiss. These age data can be combined with the established pressure-temperature path for Brattstrand Bluffs (Fig. 3) to define retrograde cooling and exhumation rates of 8°C/Ma and 0.5 nmi/year between initial melt crystallization close to the metamorphic peak and the final stages of melt crystallization at the wet granite solidus.

6 -

)hism and Peak metamorphism intitial stages es of melt crystallization

CQC M O ^^^ ^ ^

Fig. 3. Pressure-temperature path for Brattstrand Bluffs (Fitzsimons, 1996), highlighting the P-T conditions of the metamorphic peak and the retrograde hydrous overprint, which are constrained by this SHRIMP U-Pb study to have ages of about 535 Ma and 515 Ma respectively.

515Ma |V^

Q. 2

400

500

Crystallization of residual melts and release of hydrous fluids along shear zones \ I I

600 700 TfC)

800

900

Reference: Fitzsimons, I.C.W., 1996. Petrol 37, 395-414.

20 Separate episodes of eclogite and blueschist metamorphism in the los Metamorphic Core Complex, Cyclades, Greece M.A. Forster and G.S. Lister

VIEPS Department of Earth Sciences, Monash University, Melbourne 3168, Victoria Previous workers on the Aegean metamorphic core complex of los, Cyclades, Greece, have recognised three distinct episodes of metamorphism (Van der Maar, 1 9 8 0 ; Van der Maar and Jansen, 1983). MQ is described as a Hercynian amphibolite facies metamorphism that affected the rocks of the "basement" in the lower plate of the los core complex. Mi is described as an Eocene high pressure-low temperature blueschist facies metamorphism that affected both "basement" and the overlying upper plate "series", although in the "basement" much of the evidence for this event has been obliterated by later metamorphism. M2 is an Oligo-Miocene greenschist facies metamorphism that overprints the Mi mineral assemblages, to a variable degree in the upper plate of the los core complex, but dominates the petrology of the lower plate. In the upper plate of the los core complex there are eclogite boudins preserved amongst the surrounding blueschists. These have previously been interpreted to have formed during a single episode of (transitional) eclogite-blueschist facies metamorphism. This study has identified several distinct episodes of metamorphic mineral growth in what has been previously grouped as one event: • Mia involved growth of glaucophane, white mica, and epidote, but these are now seen only as inclusion trails in the cores of garnets in the cores of the eclogitic boudins. Mia most likely represents the blueschist facies assemblage which formed as the rocks were being buried, before they reached peak metamorphic grade eclogite facies conditions. • Mlb is the peak (eclogite facies) metamorphic event. It represents the maximum conditions of pressure and temperature attained by the rocks on los. Minerals grown in this metamorphic event are preserved only in the omphacite-gamet core of boudins surrounded by glaucophanitic schists. • Mic involved a retrograde (?) hydration phase during which pervasive porphyroblastic growth of sodic amphibole, white mica, garnet and clinozoisite took place. The Mic metamorphic mineral assemblages dominate the upper plate of los. The Mic blueschist facies overprint occurred on the exhumation path, as did the later greenschist facies overprint (M2). The above sequence of metamorphic growth episodes can be discerned within small boudins, or lenses, of eclogite facies rocks within glaucophanitic or retrogressed glaucophanitic schists in the los upper plate. The eclogite boudins are not only surrounded by the Mic blueschist mineral assemblages but are overprinted by them. The boudins consist of assemblages containing omphacite and garnet overprinted by (i) glaucophane bearing assemblages, and (ii) the later greenschist facies overprint. This is an observation based on microstructural evidence, independent from effects of compositional variation between the boudin and surrounding blueschist assemblages, and variation from the core to the rim of the boudins. No apparent particular structural feature determines the location of the eclogitic lenses and boudins. They appear to be randomly distributed within the schist packages. The Varvara boudin is exceptional in the degree of preservation of the early eclogite facies assemblages, and has been studied in detail. The preserved mafic boudins display a broad zonation from core to rim. The core of the boudin consists of a matrix of clinopyroxene (omphacite/aegerine-augite) and garnet porphyroblasts, with lesser amounts of later (?) sphene, and even later actinolite. The clinopyroxene and garnet assemblage is overprinted by stilpnomelane and riebeckite bearing assemblages and these also increase in abundance away from the boudin core. The boudins are mantled by a rim comprising coarse grained Na-amphibole and (Mic) garnet. The intensity of D2 deformation increases markedly towards the outside of the mantling blueschist rim, and progressive alignment of originally randomly oriented Na-amphibole prisms can be observed. The trend of the resultant lineations are in a NW direction, which is identical to the orientation of L2 lineations recognised in surrounding schists.

21 The garnets within the boudin core preserve evidence of a more complex history than does the omphacite matrix, and two generations of growth can be identified within these garnet porphyroblasts. There is a core zone (diameter nmi) mantled by a rim zone (thickness mm). The core of the garnet has abundant inclusions, but the mineralogy of the inclusions is difficult to decipher due to their small size. The more distinct crystals have been identified as glaucophane, epidote, and quartz. The rim of the garnets contain some inclusions (epidote, rutile, and opaques), but the orientation of the inclusions in the rim zone appears to be random. The boundary between the rim and the core is occasionally defined by elongate glaucophane needles that appear to wrap the rim. Several centimetres from the boundary of the outer rim, large (<2-3nmi) (and spectacular) stilpnomelane crystals radiate from the garnet porphyroblasts. The garnets at these locations are partially replaced (sometimes almost completely) by albite ± epidote ± blue amphibole. Veins of this mineral paragenesis also exist. Towards the outer rim an increased number of large Naamphibole prisms, overprint the omphacite-gamet paragenesis. These are poorly aligned with the omphacite foliation, with up to ±40° variation in orientation. At the very outer rim, there are no clear textural relationships, with the large Na-amphiboles being the dominant feature, and the stilpnomelane only minor. The observations suggest that the period of porphyroblastic blue amphibole growth (Mic) may have occurred under albite-epidote blueschist (transitional greenschist) facies conditions. The Na-amphiboles are Mg-rich in the core with Fe-rich rims, classified as crossite or magnesioriebeckite. They are idioblastic, mostly prismatic, and can be up to 4 nrni in length (generally are 1-2 mm). Some display a blue core and pale green rim. Some are completely pale green but most have darkish lavender-blue cores and darker blue pleochroic to pale green rims. The eclogite boudins are cross-cut by veins of both epidote and Na-amphibole (glaucophaneriebeckite). The epidote-bearing veins are younger, as they cut the Na-amphibole bearing veins. The results of the detailed study of the Varvara boudin (above) suggest that there is a period of eclogite facies metamorphism (Mib) that is distinct from a period of regional blueschist facies metamorphism (Mic). This raises a significant issue, because it is argued by several authors that this is the same metamorphic event. Glaucophane-rich assemblages, characteristic of the blueschist (Mic) metamorphic event, may have attained the same PT conditions as the eclogites, and that the variation in mineral paragenesis is due to different bulk compositions. However, on los, microstructural analysis shows that the glaucophane-rich assemblages postdate the earlier Mib eclogite facies metamorphic event. Thus the blueschist metamorphic event is a separate event to the eclogite metamorphic event. Similar observations can be repeated in other localities throughout the Cyclades where eclogite facies and blueschist facies assemblages have been reported (e.g., on Sifnos or Syros). It has been generally suggested that the variation in metamorphic mineral paragenesis is due to variation in original bulk composition. These results suggest quite the opposite, and imply substantially more complex metamorphic evolution than has been hitherto been reported for these classic high pressure metamorphic terrains. References: Van der Maar, P.A. 1980. The geology and petrology of los, Cyclades Greece. Ann, Geol Pays. Hellenique. 30, 206-224. Van der Maar, P.A. and Jansen, J.B.H. 1983. The geology of the polymetamorphic complex on los, Cyclades, Greece and its significance for the Cycladic massif. Geol Rund. 72, 283-299.

Diachronous metamorphism and deformation in the western Lachlan Fold Belt: David A. Foster David. R. Gray^, Martin Bucherl Âustralian Geodynamics Cooperative Research Center, School of Earth Sciences, La Trobe University, Bundoora, Victoria 3083 Âustralian Geodynamics Cooperative Research Department University, Clayton,Center, Victoria 3168 of Earth Sciences, Monash Extensive areas of the western subprovince of Lachlan Fold Belt are dominated by Lower Paleozoic chevron-folded, sandstone-mudstone submarine fan deposits overlying Cambrian oceanic crust. Regional metamorphism in the belt is sub-greenschist to greenschist facies with metamorphic grade tending to increase near large thrust faults, where deeper levels of the sedimentary pile are exposed. Higher grades of metamorphism are restricted to the contact zones of large granitic plutons. Mica growth occurs late in folding and is probably coincident with attainment of maximum structural thickening and the thermal maximum. Dating of mica growth during penetrative deformation, at low to medium metamorphic grades, can constrain the timing and patterns of deformation during tectonic evolution of orogenic belts. This is particularly useful when white mica grows at or below its closure temperature to argon loss, followed by rapid cooling. In such cases precise determinations of the timing of cleavage formation are possible by ^Âr/^Âr dating. In the western subprovince, "^Âr/^Âr dating of strongly-cleaved slates and phyllites, with metamorphic micas being the only K-bearing phase, shows that cleavage development in the west began in Early Silurian time. ÔAr/^Âr data from four major sheet bounding (structural zone) thrust faults indicate an eastward progression of imbrication and unroofing from a deep level, basal detachment. Polydeformed and strongly cleaved slates and phyllites from the Landsborough, Avoca, Heathcote, and Mount Wellington Faults give well constrained plateau ages of 451 ± 2, 440 ± 2, 426 ± 3, and 411-400 Ma, respectively. Similar ages are obtained from sericite grains from syntectonic quartz veins within the thrust sheets. These results reveal a smooth progression of the main deformation front from west to east across the western Lachlan Fold Belt starting in Early through Late Silurian time. Rocks from intra-zone faults, such as the Whitelaw Fault, give ages similar to those from the sheet-bounding faults. Reactivation occurred within the sheets and along the bounding faults, in the west, during periods of imbrication and unroofing, in the east. For example, reactivation in the Stawell and Bendigo Ballarat zones occurred at -- 420 and 410 Ma, when deformation took place within the Heathcote and Mount Wellington fault zones. Apparent ages up to 10-15 Ma older than those from the respective fault zones, are given by metamorphic mica structurally higher in the thrust sheets. These presumably reflect metamorphism during shortening via chevron folding and

23 thickening of the pile, prior to imbrication in the major thrust zones. Detrital mica, preserved in the less deformed areas and in sandstones, give ages between 490 and 520 Ma (Delamerian) in the Stawell and Bendigo-Ballarat zones, and --SOO and --450-430 Ma in the Melbourne Zone. The results require that the orogenic framework of the Lachlan Fold Belt be redefined as continuous from Silurian through to Late Devonian times, with the end Ordovician-Silurian event being the most significant and most widespread orogenic event to affect the Lachlan Fold Belt. The Middle Devonian event (-380-370 Ma), formerly considered to be the paroxysmal cataclysmic orogenic event for the Lachlan Fold Belt, now has a markedly reduced significance and only represents limited deformation due to amalgamation of the western and central/eastern subprovinces of the fold-belt and attainment of freeboard.

Zircon ages from high-grade metamorphic rocks: Progress towards a more rigorous interpretation G.Fraserl D.J.Ellis^ and S. Eggins^ ^Research School of Earth Sciences, Australian National University, A.C.T,, 0200 ^Geology Department, Australian National University, A,C.T., 0200 Geochronological constraints on granulite-facies metamorphism are most commonly obtained from zircon U/Pb ages, because other commonly used isotopic systems record ages from only the retrograde history. Zircon ages from high-grade rocks are generally interpreted as approximating the time of peak metamorphism. We argue that zircon ages do not necessarily represent the time of peak metamophism, and that better understanding of the controls on zircon growth during metamorphism will allow for improved interpretations. We investigate the possibility that zircon growth in high-grade metamorphic rocks may be triggered by net-transfer reactions involving the breakdown of a zirconium (Zr) bearing phase. The concentration of Zr has been measured in the major minerals in granulite-facies rocks of various bulk composition. Both garnet and hornblende contain tens of parts per million Zr, and no other major phase contains significant Zr. Simple calculations show that breakdown of either garnet of hornblende to non-Zr-bearing phases will release sufficient Zr to account for at least some new zircon growth. U/Pb ages from metamorphic zircon resulting from either hornblende or garnet breakdown are not expected to record the time of peak metamorphism, but rather will record the time of particular metamorphic reactions, allowing direct correlation of zircon ages with petrologically derived P-T-t paths. We discuss chemical tracers which may allow the identification of such new-grown zircon amongst multiple zircon overgrowths. This approach offers the potential for more rigorous interpretation of the metamorphic significance of zircon ages than has previously been possible.

Tectonothermal evolution of high-grade basement incorporated within metamorphic belts: Proterozoic Zambezi Mobile Belt and the Eastern Himalaya modern analog. Ben Goscombe^, R. Armstrong^ & J.M. Barton^. ^Geological Survey of Tasmania, P.O. Box 56, Rosny PL Tasmania, ^Research School of Earth Science, ANU, Canberra. ^Geology Dept. Rand Afrikaans University, Johannesburg, RSA. Detailed mapping of the Chewore Inliers in the Zambezi Mobile Belt (ZMB) in Zimbabwe, has recognised three tectonically distinct terranes (Goscombe et al. 1994). The Granulite Terrane (GT) is a thrust bound block of highly deformed gneisses enclosed within the amphibolite facies gneisses of the ZMB, represented by the Zambezi Terrane (ZT) and shghtly higher grade QT. The GT involved two phases of isoclinal folding, with S to N transport, accompanying high-grade metamorphism (Mgi). The granulites have well annealed granoblastic textures, no strong planar foliation is developed but two mineral lineations are recognised. The GT is possibly Axchean because it is intruded by a ultra-mafic complex that is tentatively correlated with the 2461±16 Ma great dyke in the Zimbabwe Craton. The GT has been obliquely thrust to the SW, onto the QT, along a wide, steep mylonite zone with amphibolite facies assemblages (Mg2). The QT is thrust onto the ZT and both the GT and QT have been tilted into steep orientations during this over-thrusting. Over-thrusting is correlated with the latter stages (Dzs) of the Zambezi/Mozambique Orogeny (Mzi). The QT and ZT are lithologically different; the QT being dominated almost entirely by quartzite and quartz-pelites and the ZT by a wide variety of QFG with metapelites, calcsilicates, amphibolites and aluminous schists. Gneissic layering, partial melting and crystallisation of porphyroblastic and polygonal granoblastic peak metamorphic assemblages occurred during prograde (Dzi-Mzi) metamorphism of the ZMB. Both the QT and ZT display both a strong layer-parallel Sz2-Lz2 fabric and two phases of tight to isoclinal folding (DzsDz4), that accompany the peak of the major metamorphic event (Mzi). Dz2-Dz4 deformation accompanied SW-NE directed shortening and is compatible with the Zambezi/Mozambique Orogeny (Mzi) of 830±30 Ma. Dzi-Dz4 structural elements in the QT have been rotated into steep orientations during over-thrusting of this terrane onto the ZT during Dzs- Both the ZT and QT developed conjugate crenulation cleavages accompanying NE-SW shortening (Dzs). All terranes have N-S trending open warps and N-S tending dolerite dykes. Late-stage SE-NW trending pegmatites and 407-588 Ma Rb-Sr cooling ages (Barton pers. comm.) accompany the region^ly pervasive Pan-African thermal over-print. Igneous events, ductile deformation and approximately 18-30 km of denudation of the Chewore Inliers ceased before Karoo times (150285 Ma). A protracted period of faulting and uplift of the inliers accompanied N-S directed crustal extension and rifting during and after Karoo times resulting in the Chewore Inhers being a significant topographic high, at the triple junction, within the Zambezi/Luangwa Rift Valleys. Diagnostic Mzi mineral parageneses in the ZT differ between the northern inliers and southern, lower grade, inliers. Mzi assemblages in the northern inliers are migmatised gnbi±mu QFG, gn-hn-pl±cpx±bi mafics and ky-gn-bi-pl-kf-q metapelites with secondary fine sillimanite on primary kyanite. In the southern inliers, Mzi assemblages are bi-mu QFG, hnpl±bi±ep/clz±sph mafics and ky-st±gn-phl-q schists with chloritoid inclusions in garnet. Average PT estimates suggest peak Mzi conditions from cores of 570-675 ^C and 8.5-10.4 kbar, and rims equilibrated at 535-640 ^C and 5.0-7.9 kbar. Assemblages in the southern inliers equilibrated at lower temperatures. Diagnostic Mzi assemblages in the QT are ky-sill-gnbi-q±pl±kf metapelites and gn-hn-pl±bi±cpx±sph mafics in the south and cpx±opx-hn-pl mafics in the north. Peak metamorphic kyanite and sillimanite are in equilibrium and so indicate that the highest peak-Mzi temperatures were attained in the QT. PT estimates from the QT have larger errors, but are suggestive of equilibration at 725 ^C and 9.4 kbar. Mzi metamorphism

involved a clockwise PT path, from early low-T chloritoid-bearing parageneses with decompression accompanying peak metamorphism as evidenced by fine sillimanite overgrowths on kyanite and in Sz2-Lz2 fabrics. Diagnostic Mgi mineral parageneses in the GT are opx-cpx-hn-pl mafics with garnet coronas, dry gn±opx±sill±bi QFG with hercynite and sillimanite inclusions in garnet and migmatised sill±ky-gn-bi-pl-kf-q metapelite and quartz-pelite also with hercynite inclusions. Cores equilibrated at 680-810 ^C and 3.6-6.0 kbar and rims typically 50 ^C and <1.3 kbar lower. Spinel inclusions suggest early high-T, low-P metamorphic conditions and a possible anticlockwise PT path during Mgi. Mg2 mineral parageneses in the southern bounding mylonite zone include gn-bi QFG and gn-cpx-hn-pl-bi mafics and were equilibrated at 540-650 ^C and 7.1-7.8 kbar. Mg2 conditions coincide with peak Mzi conditions in the ZT, thus these mylonites are thought to have equilibrated during over-trusting of the GT during the Zambezi/Mozambique Orogeny (Mzi). Samples in the GT closest to the bounding thrust have been partially re-equilibrated during Mzi metamorphism at conditions of 540-615 ^C and 5.66.5 kbar. The resulting spread in calculated PT conditions from GT samples, gives an "apparent" isobaric cooUng path. Otherwise, Mgi assemblages in the GT are apparently not reequilibrated during Mzi metamorphism and preserve evidence of the earlier high-T, low-P, Mgi metamorphism. Mzi is the major pervasive tectonothermal event of the ZMB, thus it has been shown that thrust bound blocks of dry granulite basement can be incorporated within a metamorphic belt and preserve the older mineral parageneses, with partial re-equilibration of only some samples, during tectonothermal events of the superimposed metamorphic belt. Such partial re-equilibration by later tectonothermal events may be responsible for many of the apparently isobaric cooling paths in metamorphic belts worldwide. This same scenario is further illustrated by comparison to a modem metamorphic belt, the Eastern Himalaya. A high-grade, thrust bound unit of Proterozoic basement, the Barun Gneisses, are incorporated within the Himalayan Metamorphics. Rocks crystallised during the Himalayan Orogeny are characterised by gn-ky-bi-mu-q±pl and gn-st±ky-bi-mu-q±pl±kf schists, gn-cpx-hn-ep-sph-pl-q calc-silicate and gn-hn-pl-scap mafics in the underlying schists and bi-mu-pl±kf-q schists overlying the Barun Gneisses. The underlying schists preserve a clockwise P-T path from cores equilibrated at 500-555 ^C and 9.5-10.5 kbar and peak-T conditions, of 610-650 ^C and 6.9-8.0 kbar, preserved in rims and further decompression with cooling to 570 ^C and 5.9 kbar recorded in the rims of some samples. The overlying schists similarly record decompression with minor cooling but at lower PT conditions; from 570 ^C and 6.5 kbar to 530 ^C and 4.8 kbar. Diagnostic assemblages in the Barun Gneisses are gn±cdsill-bi-pl-kf-q-ilm-rut metapelites and QFG with cordierite over-grown by fine sillimanite, gnhn-pl-cpx-bi-q-ilm mafics and cpx-scap-pl-q-sph-phl-ilm±cc calc-silicate. Peak conditions in the Barun Gneisses was 775-830 ^C and 5.8-6.9 kbar. Rims in some aluminous samples and all mafic samples in total, were re-equilibrated at temperatures (460-670 ^C) equivalent to the Himalayan Metamorphism peak-T, but still preserve Barun Gneiss Metamorphism pressures of 4.5-6.0 kbar. Thus the apparent isobaric cooling path in Barun Gneisses is due, in part, to partial re-equilibration of some samples during Himdayan Metamorphism. Goscombe B., Fey P. & Both F. 1994. 7. African Earth ScL 19, 199-224. Powell R. & Holland T.J.B. 1988. /. Meta. GeoL 6, 173-204.

Does a transient view help or hinder the interpretation of reaction textures in LP/HT rocks? Martin Hand Geology and Geophysics, Adelaide University, Adelaide 5.A. 5005 The interpretation of reaction textures in LP/HT terrains can be strongly influenced by our perceptions of the processes that lead to LP/HT metamorphism. In convergent settings, the existing paradigm states that LP/HT metamorphism is driven by advective processes, and is therefore likely to be transient (<1-10 Ma). Within this framework, reaction textures are likely to form during rapid (10-100°C/Ma) near isobaric heating, and subsequent cooling. Where reaction textures imply a non-isobaric evolution, a logical interpretation is that successive advective heating events are separated by intervals of low-T exhumation or burial that are not recorded by the rocks (e.g. Vernon, 1996). In the Anmatjira Range (northern Arunta Inlier) there is a broad spatial association between regional LP/HT metamorphism and voluminous granitic magmatism, presenting a prima facie case for a link between the two. In the SE Anmatjira Range, the regional foliation formed during compressional deformation associated with 1580 Ma LP/HT metamorphism ('-760'^C, 500 MPa) (Hand et al, 1995). This foliation is defined by sillimanite-cordierite-garnet-Kfeldspar-ilmenite (± biotite, quartz and spinel). This assemblage has been overprinted by finegrained late kinematic reaction textures formed at the expense of gamet + sillimanite. Several textures can be recognised: (1) gamet + sillimanite = cordierite + spinel ± (quartz); (2) gamet + sillimanite + ilmenite + K-spar = cordierite + spinel + biotite; (3) gamet + sillimanite + Ti- rich biotite = cordierite + spinel + ilmenite + Ti-poor biotite; (4) gamet + sillimanite + melt? = spinel + biotite + K-spar + quartz + plagioclase; (5) gamet + sillimanite + K-spar (or melt?) = spinel 4- biotite. In some instances, fine grained cordierite-spinel symplectites are replaced by a biotite-spinel association. Occasionally, fine grained K-spar + plagioclase + quartz intergrowths are enclosed within the spinel + biotite textures, suggesting that development of the textures may have been accompanied by melt crystallisation. Late micro-shears that tmncate the regional foliation have minor growth of new fine-grained gamet together with fibrolite and biotite. Fibrolite locally overgrows spinel produced in the above reactions. Within the conventional view of LP/HT metamorphism, the interpretation of the textures described above is relatively straightforward, i.e. they would probably reflect thermal pulses intersecting a generally low-T exhumation history (e.g. Vernon, 1996). Attendant to this view is the requirement that metamorphism is coeval with magmatism, and ideally, repeated magmatic pulses should be identifiable. The relatively late kinematic growth of the spinelbearing reaction textures indicates that associated magmatism must post-date the bulk of the regional deformation, and therefore be easily recognisable in the field or in magnetic data sets. Furthermore, if repeated magmatic pulses had occurred, there should be some evidence that

28 low-T hydrous assemblages formed during cooling from one thermal pulse were overprinted by new higher-T assemblages associated with the next thermal pulse. If successive thermal pluses resulted in granulite grade assemblages, there should also be evidence of multiple partial melting events. One difficulty with the application of advective models to account for the metamorphism in the Anmatjira and adjacent Reynolds Range is the increasing recognition that regional geochronological relationships point to a large temporal discrepancy between magmatism and metamorphism. Regional magmatism occurred in the interval -- 1820-1780 Ma (Collins & Williams, 1995; Vry et al, 1996) and predates regional LP/HT metamorphism and deformation by -- 200 Ma (e.g Williams et al, 1996; Vry et al, 1996; Hand et al , 1995). This age discrepancy is consistent with field evidence which indicates the granitic bodies in the highest grade regions are all strongly deformed and metamorphosed (Dirks & Wilson 1990; Hand et al, 1992, 1993; Buick et al, 1994). In addition there are two further features that are difficult to explain if advective heating was the controlling factor during the metamorphism: (1) the duration of high-T conditions at c. 1580 Ma in the Reynolds Range is at least > 26±3 Ma (Williams et al, 1996); (2) a later (1500-1400 Ma or 300-400 Ma)? higher-P lower-T event has a similar isograd geometry to the 1580 Ma LP/HT event (Dirks et al, 1991), implying a similarly distributed, but less intense heat source. An alternative interpretation for the reaction textures in the Anmatjira Range metapelites is formation during decompressional cooling in the latter stages of a single LP/HT event. This interpretation is more consistent with the regional geochronology. Indeed the apparent duration of high-T conditions makes it likely that even relatively small amounts (< 4 km) of exhumation will be recorded. The absence of late magmatic rocks, the simple distribution of LP/HT isograds and the spatial association of a single episode of retrogression with the highest grade rocks (e.g Williams et al, 1996) also provide evidence for a relatively simple thermal history. Furthermore decompressional cooling in the Anmatjira Range is consistent with the decompressional evolution for the Reynolds Range (Vry & Cartwright, (1994), CMP, 116, 7891). It seems clear the available data from the Anmatjira and Reynolds Range region requires a reassessment of the thermal processes that can lead to LP/HT metamorphism in convergent settings. For LP/HT metamorphism in the northern Arunta Inlier, the essential features that must be accounted for are (1) some exhumation at relatively high-T, (2) repeated metamorphism with a similarly distributed heat source, (3) an apparently peripheral role for advective processes.

References:

Buick, I. et al, 1994, JMG, 12, 789-810. Hand, M. et al, 1995, G Soc. Aust. Abs. 40, 60-61 Collins, W. & V^illiams, L, 1995, Prec. Res. 71, 69-89 Vernon, R., 1996, JMG 14, 143-153. Dirks, P. & W^ilson, C., 1990, JSG, 12, 651-665 Vry, J. & Cartwright, L, 1994, CMP, 116, 78-91. Dirks, P. et al, 1991, JMG, 9, 641-661 Vry, J. et al, 1996, JMG, 14, 335-350 Hand, M. et al, 1992, 1993 Geol. 20,649-652, 21, 954-955 Williams, I. et al, 1996, JMG, 14, 29-48 .

Spiral and staircase inclusion trail axes within garnet and staurolite porphyroblasts from the Bolton Syncline, Connecticut: Timing of porphyroblast growth and the effects of fold development K.A. Hickev & T.H. Bell Department of Earth Sciences, James Cook University, Townsville, Q. 4811, Australia

Three generations of garnet porphyroblasts from the Littleton Formation grew before formation of the macroscale Bolton Syncline in Connecticut, USA. Inclusion trails of foliations overgrown by these porphyroblasts are always truncated by the matrix foliation suggesting they predate it. However, staurolite porphyroblasts, which are also present in some of these rocks, grew at the time this macroscale fold formed. Local rim overgrowths of the matrix foliation suggest some staurolite continued to grow after the fold formed where it was overprinted by younger weak crenulations. The axes of curvature or intersection of fohations preserved as inclusion trails inside the garnet porphyroblasts (FIA) lie oblique to the axial plane of the Bolton Syncline but do not change orientation across it. This strongly suggests they were not rotated during the younger deformations associated with fold development. Similar axes for staurolite porphyroblasts lie close in orientation to the axial plane of the Bolton Syncline and do not change in orientation across it either. A younger set of staurolite FIA present in two samples are associated with younger deformations that postdate formation of the macroscale fold. Correlation of this FIA data with that from Vermont allows the metamorphic effects of the overprinting Alleghenian Orogeny to be distinguished from those of the Acadian. All garnet growth in these rocks occurred during the Acadian and all staurolite porphyroblasts grew in the Alleghenian.

High-grade metamorphism in a strike-slip setting: the Pinjarra Orogen, Western Australia

Kriegsman, Leo M.. Hensen, Bas J. Dept. of Applied Geology, University of New South Wales, Sydney NSW 2052, Australia Three high-grade gneiss complexes crop out on the western side of the N-S trending Darling Fault in Western Australia: the Leeuwin, Northampton and Mullingarra Complexes. These basement blocks have been grouped together as the Pinjarra Orogen (Myers, 1990) and their tectonic setting has been described in terms of alternating dextral and sinistral strike-slip movements along the Darling Fault (Byrne & Harris, 1993; Harris, 1994). The petrological evolution of these basement rocks may therefore shed light on metamorphic processes in strikeslip settings. The Northampton Complex consists primarily of metasediments, interlayered with granitic bands derived from partial melting, and intrusive bodies of porphyritic granite along its eastern margin. The metapelites display a sequence of dehydration and melting reactions, followed by rehydration. Minerals involved are Muscovite (Mu), Biotite (Bt), K-feldspar (Kf), Sillimanite (Sil), Quartz (Q), Garnet (Gt), Cordierite (Cd) and Spinel (Sp). Large Kf-Sil intergrowths pseudomorphing Mu suggest prograde breakdown of Mu + Q, either producing vapour or melt. Most samples contain elongated patches of Cd with either Gt or Sp, alternating with layers rich in Kf. Bt is absent or has a low modal abundance. We propose that the KFMASH univariant boundaries Bt + Sil + Q = melt + Kf + Gt + Crd and Bt + Sil + Gt = melt + Kf + Crd + Sp have both been crossed along the prograde path. The growth of Sil on Crd-Sp contacts and of Gt on some Sp-rims suggests operation of the divariant FMASH reaction Crd + Sp = Gt + Sil + vapour along an up-pressure path. Other rocks show Crd overgrowing an earlier Gt-Sil-Q assemblage, pointing to decompression and minor rehydration. Extensive rehydration occurred near later intrusions, causing breakdown of Gt, Cd and Kf to produce a Bt-Musc-Sil assemblage. The succession of prograde reactions involving dehydration and melting and one up-pressure reaction, followed by decompression and rehydration suggests a clockwise P-T path. Combining high-T pelite grids for vapourpresent and vapour-absent reactions with or without melt, we conclude that peak P-T conditions in the Northampton Complex were 850-900 ^C at about 6-7 kbar. Thermometry on Gt-Bt, Gt-Opx and Gt-Crd pairs in various rock types, however, yields temperatures below 750 ^C only. These low temperatures may be attributed to pervasive retrograde Fe-Mg exchange, caused by a combination of post-peak deformation, local fluid influx and slow cooling. The discrepancy between grid-derived temperatures and results from thermometric techniques demonstrate that petrogenetic grids are invaluable tools not only to derive P-T paths, but also to constrain peak temperatures.

31 The Leeuwin Complex consists mainly of oithogneisses ranging from granitic to anorthositic and gabbroic composition. A boudinaged mafic layer enclosed by anorthosite shows symplectitic intergrowths of clinopyroxene (Cpx) + ilmenite + Pig (90% anorthite) pseudomorphing titanite (sphene), rimmed by coarser Cpx-Plg symplectites against Gt. Gt-Cpx thermometry indicates temperatures of -700 ^C, while pressure estimates are in the order of 6-7 kbar. We interpret these textures in terms of the prograde breakdown of a Gt-titanite assemblage as a response to anorthosite intrusion at mid-crustal levels. Breakdown of Gt + Q to Opx + Pig is recorded in a metabasite cross-cutting granitic orthogneisses and may reflect decompression. P-T estimates on this assemblage are 650-700 ^C, 4.8-5.5 kbar. The first phases of deformation affecting both areas were accompanied by high-grade metamorphism and resulted in a subhorizontal gneissic layering. Published geochronological data show that this event is of Grenvillean age (-900-1100 Ma) in the Northampton Complex, but of Panafrican age (--700-500 Ma) in the Leeuwin Complex. The resulting fabrics were subsequently refolded into upright folds with NNW-SSE trending axial planes and transected by retrograde shear zones in the Northampton and high-grade shear zones in the Leeuwin Complex. The similarity in style and orientation of upright folds in both complexes suggests that they either formed simultaneously in relation to Pan-African dextral shear along the Darling Fault, or that the Pan-African and Grenvillean tectonic settings were very similar. Geochronological work is in progress to determine which hypothesis is correct. In both complexes, high-grade metamorphism predates compressional tectonics. Peak temperatures were attained during pervasive horizontal stretching, probably in a transtensional setting. The Northampton case suggests that such a setting can lead to a clockwise P-T path.

References Byrne, D.R., Harris, L.B.: Ore Geology Reviews fi, 89-115 (1993) Harris, L.B.: J. Geol. Soc. London, 111, 901-904 (1994) Myers, J.S.: Geological Survey of Western Australia, Memoir 2, 264-274 (1990)

HEATTHINK and the geodynamics of metamorphism

G.S, Lister. K. McPhee, T. Barr and K. Stiiwe VIEPS Department of Earth Sciences^ Monash University, Melbourne 3168, Victoria There have now been many attempts to provide a physical science based explanation for the nature and origin of region^ metamorphism. Severd theoretical models are now available, and these approaches provide quite specific predictions as to the pressure-temperature-time path that a rock at a particular spatial location in an orogenic belt should follow. In addition, there is now an established "observational" dataset that provides data for the timing and duration of metamorphic mineral growth events, information as to their number and relative sequence, their periodicity (or otherwise), and their relative age in relation to major deformational events. This data provides (albeit more qualitative) constraints for geodynamic models as to the nature and origin of regional metamorphism, because this data (if it is correctly interpreted) can be used to "reality test" existing geodynamic models, or to provide the basic building blocks for a new or improved model. If we need to develop new models because the existing models fail in some fundamental way, we need a tool with which we can test our hypotheses, in particular in relation to the types of temperature-time curves that can be expected. There are many different scenarios that have been proposed in relation to the transfer of heat in and out of a particular rock volume. We have quantified these, so that each different scenario can be examined individually, or in combination. Predictions can be made in relation to the timing and duration of heating events, and as to the nature of the P-T-t path that will be followed by a particular rock mass. This program (which is still in development) is called HEATTHINK. The problem of predicting the types of P-T-t paths that will be followed by a particular rock volume can be addressed on both the crustal and/or Uthospheric scale, assuming that orogenic belts can be described essentially as semi-continuous media. We have developed 1-D approximations to heat flow in a vertical section through the Earth's crust that allow the inclusion of multiple faults and shear zones, erosional and tectonic denudation, uniform stretching or thickening of the crust (differently across a detachment), and (time variant) thickening or thinning of the continental lithosphere. We have included different ways in which heat can be generated and/or absorbed, for example including the advective transfer of heat by fluids and/or magmas. These can be introduced pervasively, or at a particular crustal level. We have included shear heating, and linked this to crustal thickening or crustal extension, either as the result of uniform pure shear, and/or as the result of pervasive simple shear in a shear zone, or on a fault. The capability also exists that would allow the effect of (localised or pervasive) exothermic hydration reactions, or endothermic prograde dehydration reactions to be simulated. By creating a numerical tool with sufficient flexibility to be used at a variety of different scales, it is anticipated that HeatThink will be extremely useful in allowing us to test different hypotheses in relation to the transfer of heat in and out of a particular rock volume (at a variety of different length scales). In this way we intend to calculate the P-T-t paths predicted by different theoretical models for the nature and origin of regional metamorphism, and to compare and contrast these with our observations. The dominant view in the literature is that the most of the heat required for regional metamorphism is supplied as the result of conduction through the lithosphere, from the convecting asthenospheric mantle. It is argued, for example, according to the England and Thomson model, that collisional orogeny produces a sharply lowered geotherm. Thereafter, as geodynamic processes such as erosional denudation take place, the Uthospheric geotherm returns towards its original value. A material point deep within the orogen will advect through a geotherm that changes as the result of radioactive heating, as well as due to the thermal relaxation attendant in the above model. As a result, rocks deep within the roots of the mountain belt would be subject to a regime of decreasing pressure while temperature steadily increased. These predictions have greatly influenced many current models for the nature and origin of regional metamorphism. Accordingly, the England-Thompson model has been built into HEATTHINK and predictions based on this model can be examined in combination with other factors.

Tectonic processes during metamorphism have been addressed also from a global viewpoint. It is assumed for example that orogenesis is the result of crustal accretion during ongoing convergence, or that the switch from compressional orogenesis to extensional tectonism is induced solely as the result of change in overall gravitational potential energy due to periodic thinning of the thickened mantle lithosphere and the consequent uplift that is generated (e.g., the Piatt and England model). Models such as that proposed by Piatt and England suppose that periodic thinning of an overthickened mantle lithosphere as the result of convective processes can result in an increase in the heat flow, and this can cause metamorphism under conditions of an increased geotherm. HEATTHINK has been given the capability to provide quantitative predictions as to the P-T-t path that would be followed by specific rock volumes, based on this model. It is important to realise that models such as those which are briefly described above are characterised by the longevity of metamo^hic and/or deformational events that they predict. Moreover, prograde metamorphic reactions are envisaged to somehow take place in a continuous fashion, so that "peak metamorphism" captures the conditions of maximum entropy (essentially the maximum temperature). An important driving force for this research has been that quite contrasting information has been provided by the study of fabrics and microstmctures produced during metamorphism, in particular when studies of geochronology have been linked to studies at the scale of the thin section. This data suggests that episodes of metamorphic mineral growth may be of quite short duration (e.g., <lMyr). Pervasive and/or localised retrograde reactions (which generally involve hydration) take place after "peak metamo^hism" dependent upon the ingress of fluids, and/or the creation of microdilation sites in which new minerals can grow. Microstmctures produced during metamorphic mineral growth also tell quite a different tale in respect to the concept that there is a single prograde equilibration of the "peak metamorphic" assemblage. There is little evidence for "equilibrium" or for the operation of "equilibrium processes. Even "single metamorphic events" in classic metamorphic terrains appear to involve sequences of quite distinct episodes of metamorphic mineral growth (e.g., see Forster and Lister, this volume) when examined from the point of view of the evolution of their fabrics and microstmctures. In terranes subject to low pressure-high temperature metamorphism there is substantial circumstantial evidence that points to the role of mafic and/or granitic magmas in terms of the necessary heat transport. Metamorphism involves sequences of thermal pulses, of short duration, and these can be followed by substantive deformational events. So-called "clockwise" P-T-t paths (which supposedly involved isobaric cooling over substantive time scales) now appear to involve sequences of (even tectonically unrelated) thermal pulses. Isobaric cooling may have taken place during the brief "instant" of geological time that is required for cooling to the "ambient" temperature. The apparent isobaric cooling path may be more related to the collage of P-T-t determinations that would result from the sequence of thermal events. It might not come as a surprise that "transient" metamorphism might characterise low pressure high temperature terranes. It is somewhat unexpected to find that similar evidence might be amassed in respect to medium pressure metamorphism (i.e., Harrovian facies conditions), or under conditions appropriate to high pressure - low temperature metamorphism, such as occurs throughout the Aegean domain. It is rather more difficult to explain the existence of thermal pulses at such great depths in the Earth's cmst. The data has to be carefully analysed in terms of what exactly it implies. For example, rapid cooling after a significant episode of porphyroblastic mineral growth may be the result of proximity to a major thmst surface. Altematively, rapid cooling might result because ambient temperatures were originally very low (much lower than would have been predicted by current geodynamic models). The question then that must be answered is how to create a pervasive, short-lived thermal pulse at such great depth in the Earth's cmst, and how to achieve rapid rates of cooling thereafter. We believe that HEATTHINK has considerable potential in enabling us to quantitatively test various hypotheses that can be made to explain these phenomena, and thus to constrain the nature of the geodynamic models that need to be constmcted before we have a satisfactory framework in which to describe the nature and origin of regional metamorphism.

Refuting evidence for late stage extension in an exhumed high P/ low T Terrain, northeastern Saih Hatat, Oman. J. McL. Miller i, D.R. Gray i & R.T. Gregory 2 ^ VIEPS, Department of Earth Sciences, Monash University, Australia. ^Southern Methodist University, Dallas, U.S.A. Continental shelf sequences in the Saih Hatat window, southwestern Oman mountains, have been subjected to blueschist to eclogite facies metamorphism. Major metamorphic omissions corresponding to at least 15 km of missing section, have been documented with a marked increase in metamorphic grade down the section (Michard et al 1994). In the past, this has been attributed to major, late stage extension resulting in the exhumation of the high pressure units (eg. Michard et al. 1994; Searle et al. 1994). Identification of a major detachment fault within this structural pile simplifies the interpretation of this terrain into upper and lower plate units. Exposures of the lower plate (As Sifah, Wadi Meeh and Wadi Huwl) are controlled by late dome and basin interference folds. Eclogite facies metabasalts (minimum pressure 12 kb, temperature 480 'C - 540 °C; El-Shazly et al. 1990) are preserved in megaboudins exposed in the eastern most exposure of the lower plate at As Sifah. The internal structure of these boudins is complicated with high pressure assemblages associated with varying senses of shear. Internal boudin necks within the megaboudins contain fibrous omphacite indicating the disruption of the mafic units into megaboudins initiated at high pressures. These earlier structures and peak metamorphic assemblages are almost completely overprinted outside the boudins by intense deformation associated with a marked north-south stretching lineation. Remnant jadeitic pyroxenes in mafic schists affected by this deformation indicate that the entire lower plate at As Sifah was metamorphosed at similar pressures (>12 kb) prior to this tectonometamorphic event. This north-south stretching lineation is initially defined in mafic lithologies by garnet, phengite, sodic amphiboles and clinozoisite but no pyroxene. Later fabrics are defined by gamet, phengite, sodic/calcic amphiboles, epidote, clinozoisite and albite. Finally, at the lowest grade the stretching lineation is defined by epidote, sodic/calcic amphiboles, albite, actinolite, hematite and chlorite. The lowest grade assemblages are frequently associated with sodic/calcic amphibole and albite bearing veins that are orthogonal to the stretching lineation. This stretching lineation is consistently associated with a south over north sense of shear irrespective of the metamorphic grade. Significantly, the lowest grade assemblages are synchronous with the formation of regional scale sheath-like nappes in the As Sifah region. The earlier higher grade assemblages are folded around these nappes. In marked contrast, the lower plate at Wadi Meeh and Wadi Huwl does not contain gamet and pyroxene. Thus an "apparent" metamorphic omission appears to exist in the lower plate, given the present 4 km separation between the exposed sections. Mafic schist within the lower plate at Wadi Meeh and Wadi Huwl also contains a strong stretching lineation defined by epidote, sodic/calcic

35 amphiboles, albite, actinolite, hematite and chlorite. Like the lower plate at As Sifah this stretching lineation is also associated with the formation of regional nappes with sheath-like form. Higher pressure sodic amphiboles are preserved as inclusions in epidote porphyroblasts, but defining the value of this earlier pressure in the lower plate at Wadi Meeh and Wadi Huwl is problematical. Rare occurrences of pumpellyite (El-Shazly et al 1990) suggest there may have been a thermal gradient between the two sections of the lower plate. Geobarometers of Brown (1977) and Maruyama et al (1986) yield similar pressures for the formation of the regional nappes in the lower plate at As Sifah, Wadi Meeh and Wadi Huwl (4.5 - 6 kb). Significantly these nappes are truncated by the detachment that delineates the upper-lower plate discontinuity, indicating these nappes predate the formation of this detachment. The lower plate appears to have been in the same low grade metamorphic field during nappe formation, and the eclogites where thus "exhumed" prior to the formation of the detachment. The "apparent" problem of a massive pressure increase over a small distance in current exposure (4 km) is negated if the affect of the deformation associated with the regional nappes is removed. Ascertaining the exact distance between the two exposed regions of the lower plate prior to nappe formation is difficult to quantify. There is no need (or evidence) for massive late normal faults juxtaposing rocks of different grade. In the case of Oman, any "problem" in metamorphic grade in the lower plate, if one exists, predates deformation associated with the fold nappes and the formation of the upper and lower plate discontinuity. The last major tectonometamorphic event in the lower plate in Oman is associated with a low grade of metamorphism that has previously been mistakenly associated with late stage extension, rather than regional nappe formation that effectively "telescoped" the terrain. Demonstrating large "apparent" down-section increases in metamorphic grade is not sufficient proof of late stage extension if macroscale structure cannot be accurately constrained.

References Brown E.H.1977. J. Petrol, 18, 53-72. El-Shazly A.K., Coleman R.G. & Liou J.G. 1990. Petrol, 31, 629-666. Maruyama S., Cho M. & Liou J.G. 1986. In: Evans B.W. & Brown E.H. (eds), Blueschists and eclogites, Geol Soc. Am. Mem,, 164, 1-16. Michard A., Goffe B., Saddiqi O., Oberhansli R. & Wendt A.S. 1994. Terra Nova, 6, 404-413. Searle M.P., Waters D.J., Martin H.N. & Rex D.C. 1994. J. Geol Soc. Lon., 151, 555-576.

High-pressure granulite metamorphism prior to continental collision: P-T-t history of the Pan-African Belt in eastern Tanzania Moller. Appel, P.l, Mezger, K 2 & Schenk, V.l ^ Mineralogisch-Petrographisches Institut und Museum, Universitdt Kiel, 0-24098 Kiel, Germany 2 Max-Planck Institut f . Chemie, Abt, Geochemie, Postfach 3060, D-55022 Mainz, Germany * present address: Dept. of Applied Geology, Univ. of NSW, Sydney, 2052 NSW, Australia The Mozambique Belt of East Africa has been regarded as a prime example for a high-grade gneiss belt which formed by collision of large continental blocks, in this case East- and WestGondwana during the end of the Precambrian. The high-pressure granulites of eastern Tanzania have thus been interpreted as the product of tectonic crustal thickening during the collision process (Shackleton, 1986) by a metamorphic overprint on predominantly Archaean crustal material. The age of granulite-facies metamorphism, loosely defined by U-Pb dating of zircon between 645 Ma and 715 Ma (Coolen et al, 1982; Maboko et aL, 1985) has been thought to date the time of collision. It has recently been shown that the western part of the Mozambique Belt in Tanzania as defined by Holmes (1951) is in fact part of the UsagaranUbendian orogen which experienced its main metamorphic event at 2 Ga (Moller et aL, 1995). The data-sets from our integrated petrologic and isotopic study allow to evaluate and revise the collision model for the granulite-facies rocks of eastern Tanzania. Metamorphic petrology provides detailed information on the pressure-temperature evolution and peak-metamorphic conditions of the granulites, whereas U-Pb dating of zircon and metamorphic minerals (monazite, sphene, rutile) yields information on the age of metamorphism and the early cooling history of the different granulite complexes within the Pan-African belt. The main study areas are the Pare- and Usambara Mountains in the northeast of Tanzania close to the Kenyan border, and the Uluguru Mountains 400 km to the south. Additional samples were collected in the lowlands around these mountain areas and in the Umba Steppe, east of the Pare- and Usambara Mountains. In all these areas, mafic rock types contain the high-pressure assemblage gamet-clinopyroxene-quartz, and cordierite is absent in metapelitic assemblages. The peak metamorphic conditions are estimated at 9.5 to 11 kbar and 810±40 near-identical to the estimates on the Furua granulite complex about 300 km south of the Uluguru mountains (Coolen, 1980; Berman, 1991). It can thus be argued that the conditions of granulite-facies metamorphism in eastern Tanzania were largely uniform over a very extensive area (approx. 700 X 200 km). Inclusion and reaction textures from mafic and metapelitic rocks in our study areas show that the granulites followed an anti-clockwise P-T path. Rare early kyanite and kyanite-staurolite inclusions in garnet point to an early evolution in the kyanite stability field, whereas abundant sillimanite as folded inclusions in garnet and in the matrix point to most of

the garnet growth taking place in the sillimanite stability field. Garnet breaks down to kyanite bearing assemblages, is interpreted as a result of retrograde cooling. This interpretation is consistent with coronas of late garnet, cUnopyroxene and quartz around orthopyroxene found in mafic samples , which yield lower temperatures, but only slightly lower pressures from thermobarometric calculations. The age of metamorphism has been determined by U-Pb dating of monazite from 9 metapelite locations covering the whole study area. Monazite and one zircon fraction from a meta-quartzdiorite of the western Uluguru Mts. and monazite from a metagranitoid in the central Usambara Mts. have been analysed for comparison with the metasedimentary samples. The results are partially consistent with the lower age limit of the previously attained zircon data of 640 to 650 Ma. However, monazite and zircon from the northwestern Uluguru Mts. and all monazites from the Usambara Mts. were dated at 625 Ma. Monazite from the Umba Steppe yielded an age as young as 615 Ma. These apparent differences in the age of metamorphism are supported by the results from sphene and rutile, minerals with lower closure temperatures. The postmetamorphic cooling-rates calculated are low at about 3-5°C/Ma for all areas and thus support the interpretation of tectonically undisturbed near-isobaric cooling after the peak of metamorphism. New paleomagnetic data show that the collision of East and West Gondwana did not occur before 550 Ma (Meert et al, 1995). This is consistent with the interpretation that the highpressure granulites of eastern Tanzania were not formed by the collision, because they predate the collision event by at least 60 million years, but were possibly formed in a convergent margin setting, while the 'Mozambique Ocean' was being subducted. Such a scenario is consistent with the type of P-T-t path found, which we interpret as being caused by underplating and intraplating of igneous material into the lower crust.

References Berman, R. G., 1991, Canadian Mineralogist, v. 29, p. 838-855. Coolen, J. J. M. M. M., 1980, V. 13, p. 258. „ x, uCoolen, J. J. M. M. M., Priem, H. N. A., Verdumen, E. A. T. and Verschure, R. H., 1982, Precambrian Research, v. 17, p. 31-40. Holmes, A., 1951, International Geological Congress. 18th,: London 1948, Proceedings, 14, p. 254-269. Maboko, M. A. H., Boelrijk, N. A. I. M., Priem, H. N. A. and Verdurmen, E. A. T., 1985, 1995,Precambnan Research, v. 74, p. 225-244. Moller, A., Appel, P., Mezger, K. and Schenk, V., 1995, Geology, v. 23, p. 1067-1070. Shackleton, R. M., 1986,m Coward, M.P., Ries, A. C., ed.. Collision tectonics: p. 329-349.

Tectono-thermal history of the western Lachlan Fold Belt Insights from white mica studies. Robin Offlerl, Stafford McKnight^ and Vince Morand^ 1 Geology Department^ University of Newcastle^ NSW 2308, 2 Geology Department. University of Ballarat, VIC 3353. Detailed bo cell parameter and crystallinity ("Illite Crystallinity"-IC) studies of K-white micas in slates from the Stawell and Ballarat-Bendigo Zones, reveals a more complex metamorphic pattern than proposed by previous authors. The IC data indicate that rocks regionally metamorphosed prior to the intrusion of the Early and Late Devonian granitoids, vary in grade from epizonal (greenschist facies) to diagenetic (zeolite facies) and that most are of epizonal to upper anchizonal (upper prehnite-pumpellyite facies) grade. In the Stawell Zone, most rocks have attained epizonal to upper anchizonal grade but the metamorphic pattern is not well defined owing to paucity of outcrop. By contrast in the Ballarat-Bendigo Zone, a clear decrease in grade occurs with epizonal conditions recorded in the west which change transitionally to anchizonal and then to diagenetic conditions to the east adjacent to the Melbourne Zone. Across fault zones, IC values change slightly indicating that substantial displacements have not taken place on them. This is accord with the thin-skinned tectonic model proposed for the Western Lachlan Fold Belt. bo cell parameters obtained from white micas in slates ( x = 9.024; n =118) indicate baric conditions intermediate between those recorded in rocks from the New Hampshire (x = 9.003, P = Al2Si05 Triple point) and Otago (x = 9.039; Intermediate P; Guidotti & Sassi, 1986). Thus a moderately low geothermal gradient existed at the time these rocks were formed which is contrary to the high gradient proposed by other authors for this part of the Lachlan Fold Belt. The bo values obtained from the higher grade (epizonal/upper anchizonal) rocks further suggest P of -- 4 kbar (assuming T-350^ C) indicating that the crust has been thickened to -15 km as a result of the Late Ordovician-Early Silurian deformation (Gray et al. 1996) which agrees with values determined from structural data. In some parts of the WLFB, particularly in the St. Arnaud area, where Early Devonian granitoids associated with schist bearing aureoles are abundant, low bg values (x = 9.002; n = 27) are recorded by white micas in slates. They reflect the high T-low P conditions brought about by the emplacement of granitoids into the crust at this time and indicate that shallower crustal conditions prevailed during this metamorphic event than during the earlier 430Ma event responsible for the deformation of the Cambro-Ordovician sediments. Pressures indicated by the bo values are in the order of 2-2.5 kbar which are in accord with those obtained from the andalusite-cordierite assemblages in the high grade schists from the aureoles. This implies that

39 --6.5-8 km of meta-sediment in the BBZ have been removed prior to the emplacement of the Early Devonian granitoids.

References Gray, D.R., Foster, D.A. & Bucher, M. 1996. Revised Lachlan Fold Belt Orogenic Patterns based on a new "^^Ar/^^Ar dataset. Geological Society of Australia, Abstracts No. 41. 13th Australian Geological Convention, Canberra, February 1996. Guidotti, C.V. & Sassi, F.P. 1986. Classification and correlation of Metamorphic Facies Series by means of Muscovite bQ data from low grade meta-pelites. Neues Jahrbuch fur Mineralogie, Abhandlungen, 153, 363-380.

Interrelationships between multiple metamorphic episodes, high strain zones, granites and metasomatism, Mount Isa Inlier M. J. Rubenach & D. R. W. Foster Earth Sciences, James Cook University, Townsville 4811, Queensland Metamorphism in the Mount Isa Inlier is low-P/high-T, is characterised by anticlockwise P-T-t paths, and peaked during the high-strain D2 event (Reinhardt 1992; Rubenach 1992). However, recent work in the Snake Creek area (situated 30 km SE of Cloncurry; Rubenach & Barker in press) and west of Mount Isa has indicated multiple porphyroblast growth episodes. In the Snake Creek area, andalusite and staurolite are interpreted as having grown pre-/early-D2, late-Di/early-D2, syn-D3, and syn-D4, the latter two events being manifested microstructurally by crenulations with respectively shallowlydipping axial planes and steep northerly-oriented axial planes. On the basis of syn-D2 kyanite and replacement of early andalusite by staurolite, the temperature is interpreted as having fallen after Di, rising to its peak late in D2. Although the ultimate source of the thermal anomalies in the Mount Isa Inlier was likely to be intrusion of mafic magmas into the lower crust and/or lithospheric mantle delamination (Loosveld & Etheridge 1990; Oliver et al 1991), the heat was probably transferred into the middle crust by granites. With the exception of the syn-D2 Mica Creek Pegmatites (Rubenach 1992; Connors & Page 1995), which probably represent the top of a large pluton under the amphibolite facies rocks exposed west of Mount Isa, evidence of such granites appeared to be lacing until recent age dating (Page & Sun in press) indicated overlap between earlier plutons of the Williams Batholith and the D2 event. Figure 1 shows a simpUfied isograd map of the southern part of the inlier. The diopside zone in calcsilicate rocks corresponds roughly to the sillimanite and sillimanite/K-feldspar zones. Several narrow NS-oriented amphibolite facies belts are evident, and include one running west of Mount Isa, the Wonga Belt Shinfield Zone through Mary Kathleen, and the Eastern Fold Belt south of Cloncurry. These belts correspond to D2-related high strain zones and show abundant evidence of syn- and post-D2 metasomatism. Besides the sodic-calcic alteration in mainly calcsilicate rocks in the Wonga Belt/Shinfield Zone (Oliver 1995) and adjacent to the Cloncurry Fault (de Jong & Williams 1995), a variety of metasomatic lithologies also occur in metapelite-dominant belts. Syn-D2 metasomatic cordierite rocks (+ Qtz ± And ± Sill) occur along at least the northern half of the 170 km long Mount Isa belt and syn-D2 greisenization and albitization of the Sybella granite is prevalent NW of Mount Isa. Syn- to lateD2 skams and tremolite ± albite alteration is conmion west of Mount Isa. In the Snake Creek area, syn-D2 quartz veins have selvages of staurolite, kyanite and andalusite, and syn-D2 albitization (Ab ± Anth ± St ± Grt) of metasedimentary rocks is locally abundant. The infiltrating fluids are interpreted as being a combination of magmatic fluids derived from underlying granites and fluids derived from metamorphic de volatilisation reactions. Heat transfer via such infiltrating fluids is believed to be important in the overall thermal budget in many of the high strain/high grade belts in the Mount Isa Inlier.

References Connors K. A. & Page R. W. 1995. Precambrian Research 71, 131-153. de Jong G. & Williams P. J. 1995. Australian J, Earth Sciences 42, 281-90. Loosveld R. J. H. & Etheridge M. A. 1990. J. Metamorphic Geology 8, 257-167. Oliver N. H. S. 1995. Australian J. Earth Sciences 8, 267-279. Oliver N. H. S. et al. 1991. Austalian J. Earth Sciences 38, 425-456. Page R. W. & Sun S-s in press. Australian J. Earth Sciences. Reinhardt J. \992. Geological Magazine 129,41-53. Rubenach M. J. 1992. J. Metamorphic Geology 10, 333-346. Rubenach M. J. & Barker A. J. in press. Australian J. Earth Sciences. Rubenach M. J. & Foster 1996. Unpubhshed report, Crustal Geodynamics CRC.

• Ernest Henry

141'OO' SillimBnit«/K-f<!»ldspar zone

Biobte zone. calcsHicatee

Silimanite zor>«

Clinopyroxene zone, calcsilicales

StauroIKe «id/or andakjsi(« zone

Pre and poet-meUwnorphic granites

Cordierite zone

Basement. Sillimanite zone overprinted by biotte zor>e

Undiffefenbated greenschist/amphibolite (actes

Major fault

Greenschist facies

Isograd

#Cloocurry

Site of major mine or prospect

Figure 1: Distribution of m e t a m o r p h i c z o n e s a n d isograds a n d major mineral deposits or prospects In t h e southern portion of t h e M o u n t Isa Inlier. Modified from R u b e n a c h a n d Foster ( 1 9 9 6 ) .

The problem with Mt Painter Mike Sandiford, Department of Geology and Geophysics, Adelaide University, SA.

The heat production of exposed Mesoproterozoic basement gneisses in the Mt Painter region of the northern Flinders Ranges averages about 12-15 |LIW m"^ with significant volumes of rock having heat production rates in excess of 40 |LIW m"^. Such high heat production rates (averaging about 5 times typical upper crustal average) adequately account for the recorded present day heat flow of 126 mW m'^ (a heat flow measurement previously regarded with considerable suspicion). For typical crystalline basement thermal conductivities of around 3 W m'^ the modern day heat flow implies a near surface geothermal gradient of about 40°C km\ Since wet sediments generally have a much lower thermal conductivity than crystalline rocks, the burial of this basement sequence beneath the -10 km of Neoproterozoic to Cambrian cover succession during the formation of the 'Adelaide Geosyncline' is likely to have been sufficient for the development of the spectacular unconformity-related contact metamorphic aureole observed in the lower 2-3 km of the cover. The local development of diopside and cordierite-anthophyllite within this aureole, within 100 m of the basement unconformity, suggests maximum temperatures of about 550°C and implies average thermal gradients of around 55°C/km in the cover succession. Local intrusion of per-aluminous granites at about the time of metamorphism suggests that crustal temperatures only locally exceeded the solidus at levels at or below the current exposures, with the limited melt generation raising the intriguing problem of why the lower crust did not experience pervasive melting during this metamorphic event. While the modem heat flow in the northern Flinders Ranges and in the contiguous parts of the Stuart Shelf is anomalously high (averaging 90 mWm"^, Cull, 1982) reflecting anomalous regional crustal heat production associated with Mesoproterozoic magmatic rocks, several lines of suggest that the total crustal heat production in this region shows very significant lateral variations on the 10-100 km scale. These include: • the spectacular metamorphic effects observed around the Mount Painter Inlier are not observed in an adjacent structural culmination some 30 km further west (the Mt Burr diapir) which exposes very low grade Burra Group sediments, • heat flow measurements in the vicinity of the Roxby Downs Cu-U-Au-REE deposit shows variations from 70-120 mWm'^ on the 10-20 km scale (Houseman et al., 1989), • spectral analysis of gravity and topography from the northern Flinders Ranges shows complex coherence at the 200-600 km scale suggesting very variable flexural properties, and hence thermal regimes, at length scales of less than 200 km (Meyer & Sandiford, work in prep.). Thermal models which incorporate spatially variable heat production distributions appropriate to the northern Flinders Ranges and Stuart Shelf (see accompanying figure) show that the extremely anomalous upper crustal thermal regimes appropriate to metamorphism at Mount Painter need not translate to extraordinary deep crustal temperatures providing:

43 • the anomalous heat production is located in the upper crust, and that it has a horizontal lengthscale no more than 1 - 2 times the thickness of the crust, and • heat flows from the mantle are low (as may be appropriate following extended sagging of a sedimentary basin). 1 r1 200

-5 300

Model parameters

-10

-20

Region

400

-15

^^500

srllim^nite

-30

K (W/m/°K)

qm (mw/m2)

-25

\700

A B C

H (mW/m3)

100 The analysis of Mount Painter presented here provides some important insights which should be of general relevance to the understanding of low-P metamorphism in the Australian context. Firstly, the Australian Proterozoic crust contains some extremely radioactive granites. Indeed, the degree of enrichment is so extraordinary as to render useless any attempt at understanding the metamorphic evolution of Australian Proterozoic low-P terranes as representing transient perturbations from generally accepted notions of 'normal' crustal geothermal regimes. Secondly, the analysis highlights the sensitivity of thermal regimes in metamorphic piles to the conductivity of thermally insulating sedimentary blankets, with one important impUcation being that the deposition of an insulating blanket above a radiogenic basement (or granite) probably plays a fundamental role in thermally priming many low-P, intermediate- to high-T metamorphic piles. Conversely, the removal of the insulating blanket or an increase in its thermal conductivity through diagenetic/metamorphic processes may lead to significant cooling of any deeper metamorphic pile. Finally, the role of the mantle heat flow during the generation of steep upper crustal geotherms appropriate to low-P metamorphism now seems fundamental, in a way which is quite unexpected (especially to those of us who have argued for elevated mantle heat flows as a primary thermal energy source for high-T crustal metamorphism). The problem with Mt Painter, and many other low-P metamorphic behs, of why there is so little crustal melting during metamorphism, implies low mantle heat flows. Only with low mantle heat flows is it possible to prevent the deep crust from undergoing pervasive melting while simultaneously developing steep upper crustal geotherms appropriate to the observed low-P metamorphism! -35

Cull, J.P., 1982., BMR Journal of Australian Geology and Geophysics, 7, 11-21. Houseman, G.A., Cull, J.P., Muir, P.M. & Patterson, H.L., 1989, Geophysics, 54, 158-170.

Regional subeclogite facies metamorphism in the Mann Ranges, Musgrave Block, Northern Territory

Ian Scrimgeour and Dorothy Close Northern Territory Geological Survey, P.O. Box 2655 Alice Springs, Northern Territory

Most metamorphic terrains in Australia are characterised by relatively low pressure metamorphism, with few recognised occurrences of regionally exposed deep crustal rocks. Recent studies in the Musgrave Block (Ellis and Maboko, 1992; Clarke et al, 1995; Camacho et al., in press) have identified the existence of subeclogite facies metamorphic assemblages in shear zones overprinting Mesoproterozoic granulite facies gneisses in the Musgrave Ranges and Tomkinson Ranges in South Australia and Western Australia. Attempts to constrain the regional extent and significance of this high pressure overprint have previously been hampered by the lack of data from the Mann Ranges, which are central to this terrain. In the central Musgrave Block, gneisses which were metamorphosed to granulite facies at 1200-1180 Ma are extensively intruded by voluminous 1190-1150 Ma granites, 1070 Ma layered mafic intrusions and dykes and 1080-1050 Ma granites (Sun et al, 1996). Within the Mann Ranges, a variable but pervasive mylonitic fabric overprints all of these lithologies, and locally intensifies into major south dipping shear zones, with a south plunging lineation. Mafic dykes containing the mylonitic fabric have re-equilibrated at 12-13 kbars and 700-750°C to an assemblage of garnet, clinopyroxene, hornblende, rutile, Na-rich plagioclase and quartz. In many mafic dykes and granites, plagioclase has partially reacted to form kyanite and clinozoisite. Unlike elsewhere in the Musgrave Block, this high pressure overprinting occurs almost pervasively, and represents an extensive zone of deep crustal high strain. Where the mylonitic fabric is less intense, garnet and clinopyroxene bearing corona textures and symplectites occur. Recent geochronological data from elsewhere in the Musgrave Block has established that the timing of this deformation was during the Cambrian Petermann Orogeny (Camacho et al., in press; Clarke et al., 1995) Within the Mann Ranges, two distinct types of lower crustal shear zones occur. One generation represents intensification of the pervasive high pressure fabric, and is characterised by relatively anhydrous garnet-clinopyroxene-rutile assemblages. These mylonites are analogous to the subeclogite shear zones described elsewhere in the Musgrave Block. A second generation of high pressure mylonites contains abundant hornblende-bearing melts, and a coarse garnethornblende bearing assemblage. This is the first recorded occurrence of partial melting associated with the Petermann Orogeny, and is restricted to hydrous shear zones which presumably acted as conduits for fluids during metamorphism. These hydrous shear zones only occur in the large granite bodies which did not previously undergo granulite metamorphism. The lower crustal shear zones are overprinted by mid to upper crustal biotite-

45 hornblende, biotite-gamet and biotite-muscovite shear zones reflecting the rapid exhumation and cooUng of the terrain along major south dipping thrusts. The major feature along which the lower crustal terrain was exhumed, the Woodroffe Thrust, does not outcrop in this region. Scattered outcrops of granite, extending 30-35 km north of the Mann Ranges, contain mylonitic high pressure garnet amphibolite to subeclogite facies assemblages and therefore define the approximate position of the thrust to be further north than was previously recognised. This confirms the regionally extensive nature of the high pressure metamorphism. The widespread occurrence of a pervasive high pressure mylonitic fabric south of the Woodroffe Thrust, and the abundance of partial melting within high strain zones suggests that the scale and intensity of the Petermann Orogeny is far greater than was previously recognised.

References: Camacho A. et al (in press); Journal of Metamorphic Geology Clarke G.L. etal (1995); AGSO Journal of Australian Geology and Geophysics, 16, 127-146 Ellis D.J. & Maboko, M.A.H. (1992); Precambrian Research, 55, 491-506. Sun S-s. et al. (1996); AGSO Research Newsletter, 24, 13-15

Bulk composition changes during cooling

Kurt Stiiwe Department of Earth Science, Monash University, Clayton Vic 3168, Australia email: kstuwe @artemis, earth monash, edu. au Metamorphic assemblage development is strongly controlled by the bulk composition of a rock. Thus, proper interpretation of reaction textures in a rock that has experienced a given PT path must be based on detailed knowledge of the bulk composition of the rock, for example when pseudosections or T-X sections are used. However, the definition of the appropriate bulk composition is plagued a range of problems. For example, on the scale of a thin section the bulk composition of a rock may be homogeneous, but on a sub-grain scale the bulk composition across the boundary between minerals A and B varies from 100% A to 100% B. Clearly, specification of the "appropriate" bulk composition - the effective bulk composition is difficult. Indeed, minerals may develop retrograde zoning profiles during cooling by successively "closing" larger and larger parts of the grain to subsequent equilibration. This removes these mineral grains (or parts of grains) effectively from the reacting rock volume. Thus, the effective bulk composition of a rock containing zoned minerals changes during cooling. Interpretation of reaction textures on a phase diagram constructed for a single bulk composition may therefore be limited, if not misleading. Curved paths on T-X sections may be an appropriate way to illustrate and interpret the effect (Fig. 1). It is suggested here, that the conmion observation of apparently contradicting reaction textures, for example the occurrence of different retrograde minerals between the same reactants may be explained with this model.

o 0

Q. E

Proportion of garnet in bulk composition

Figure 1.: Schematic TX section illustrating possible reaction paths on hand of a pelitic rock (of overall bulk composition and peak temperature at the black dot) in which garnet is the only mineral developing a retrograde zoning profile during cooling. Different reaction fields on the TX section are omitted for clarity. The horizontal (compositional) axis of the diagram is the proportion of garnet in the effective bulk rock. Path (a) is traditionally used to interpret reaction textures in the rock that may develop during cooling. This path is appropriate only if all minerals stay in equilibrium down to temperature Tl. Path (b) is for a fine-grained rock in which garnet begins to develop zoning at temperature T2. Path (c) is for a coarsegrained rock in which initial closure of garnet starts at a much higher temperature T3. Note that the three paths are all for rocks of the same overall bulk composition (differing merely in grain size of the garnets), but potentially terminate in different reaction fields.

Very high grade metamorphism in the western Gawler Craton /-Page Department of Geology & Geophysics, University of Adelaide, SA 5005

Aluminous pelites and ironstones from Ooldea, in the westem Gawler Craton, preserve mineral assemblages indicating peak metamorphic conditions in excess of 950 and 9kb. The inferred peak mineral assemblages in the pelites are (i) quartz-perthite-sillimanitemagnetite±sapphirine±hypersthene±osumilite, and (ii) quartz-perthite-garnet-sapphirinesillimanite-magnetite±corundum. Ironstones preserve sapphirine-quartz, spinel-quartz, and the extremely rare corundum-quartz parageneses. Texturally, peak medium-coarse grained, granular mineral associations have been overprinted by finer grained reaction/replacement textures associated with the development of a pervasive, high strain, mylonitic fabric. The stability of the assemblage hypersthene-sillimanite-quartz within the mylonitic fabric indicates deformation occurred above 8kb and 900 Late cordierite-biotite growth within the mylonitic fabric in both the hypersthene and garnet bearing pelites indicates decompressive cooling occurred during deformation. Regional aeromagnetic data, acquired by Mines and Energy, South Australia, under the South Australian Exploration Initiative, indicates that the Ooldea 2 drill hole intersected part of a regionally extensive magnetic trend which is truncated to the east by the Karari Fault. Magnetic profile modelling indicates that the high strain foliation observed in these rocks is sub-vertical. Kinematic observations of the Ooldea 2 drill core indicate a strong down-dip lineation, and north-block-up vergence. Hence high grade metamorphism observed at Ooldea was probably coeval with compressional deformation. Analysis of assumed metamorphic zircons from Ooldea 2 using the Kober zircon evaporation technique yielded an age of 1689±6Ma, synchronous with major I-type, mid-upper crustal plutonism on the eastern side of the Karari Fault (1683±4 Ma). This data suggests an important genetic link between I-type plutonism, extremely high grade metamorphism and compressional deformation in the westem Gawler Craton.

Megacrystic felsic gneisses at Broken Hill: Pre to syn-tectonic granites? JJ. Vassallo and R.H. Vernon School of Earth Sciences, Macquarie University, Sydney, NSW 2109,

Australia

Felsic gneisses at Broken Hill have been interpreted as metasedimentary (arkoses, volcaniclastic or granitized sediments), volcanic flows, and metagranitoids. Many of them may be metagranitoids, especially those with K-feldspar megacrysts (augen), as suggested by Andrews (1922), Browne (1922), Stillwell (1922), Vernon (1969) and others. They were intruded in the form of sheets, and were emplaced just prior to, or during the early stages of the Olarian orogeny. The sheet-like shapes of many of them may suggest a volcanic or volcaniclastic origin, but many Proterozoic granitoids are of this form (e.g., in the Arunta Block) and deformation could also be a contributing factor. Three megacrystic felsic gneisses (augen gneisses or granite gneisses) surrounding the Broken Hill Line of Lode are inferred by us to be pre- to syntectonic granitoids that are now represented as deformed orthogneisses. The Feral gneiss is interpreted as a pre-Dl intrusion, as it contains a pre-Sl gneissic foliation that is folded into mesoscopic sinusoidal folds with axial planes approximately parallel to SI. The Alma Gneiss and Rasp Ridge Gneiss, both containing SI as the earliest foliation, transect and intricately intrude the Feral gneiss, and are interpreted by us as pre- to syn-Dl granitoids. Evidence suggesting that the megacrystic felsic gneisses were originally granitoids includes: intrusive contacts that cut earlier-formed leucosomes, intrusive contacts between the Feral gneiss and the other two gneisses, euhedral to subhedral K-feldspar phenocrysts, microgranitoid enclaves (especially where megacrystic and relatively large), aplite dykes (restricted to plutonic rocks and therefore very reliable indicators), metasedimentary xenohths, and magmatic foliations overprinted by tectonic foliations. Despite the high mobility of light-ion-lithophile elements during metamorphism, the megacrystic gneisses show chemical affinities with Proterozoic granitoids in the Arunta and Mt Isa Inliers, which is additional evidence for a granitic parent. K-feldspar megacrysts are much more common in granites than in volcanic rocks, and survive relatively well, even in strongly deformed rocks. Features indicating a magmatic origin for K-feldspar (Vernon, 1986; Vernon & Williams (1988) include: (1) euhedral shape; (2) simple twinning, (3) oscillatory compositional (especially Ba) zoning, (4) zonally arranged inclusions (not inclusion trails), (5) euhedral plagioclase inclusions, (6) alignment of megacrysts in a magmatic foliation or lineation. In contrast, metamorphic K-feldspar is characterised by: (1) irregularly shaped porphyroblasts, (2) general (universal?) absence of simple twinning, (3) rounded inclusions

49 (e.g., Vemon, 1968), (4) lack of oscillatory zoning, and (5) inclusion trails that reflect pre-existing foliations, not zonally (concentrically) arranged. The full extent of granitoids in the Broken Hill Block is as yet unknown, but felsic gneisses, many of which are megacrystic, appear to be most abundant in the highest-grade metamorphic zone (Brown et al. 1983, fig. 1). A close relationship between high-grade metamorphism and granites is typical of low-pressure/high-temperature (LP/HT) regional metamorphic terranes, which are common world-wide and are typical of the Australian Proterozoic. Furthermore, many of the megacrystic granites at Broken Hill show the effects of the first tectonic foUation (Vassallo, 1995), as do markedly transgressive leucogneisses in the north of the Broken Hill and in the Euriowie Block (Stevens et al, 1988, p. 309), indicating that granites were being intruded at the earliest stages of the metamorphic/deformation history. Other evidence also supports the idea of early heating in Australian LP/HT terranes (Vernon et al, 1993). Therefore, granites may have contributed to the metamorphic heat, at least locally (e.g., Collins & Vemon, 1991, 1992; Collins et al, 1991; Vemon et al, 1993), although the three felsic gneisses investigated by us appear to have been intmded early, and so may have contributed to the earliest part of the thermal history. If volcanic or subvolcanic activity was involved in the development of the Broken Hill Lode, early granites may have contributed fluids or helped to mobilize extemal fluids for ore deposition and/or modification. Thus, a plutonic-volcanic origin for the orebody may be appropriate. Megacrystic felsic gneisses are conmionly used as stratigraphic markers in the Broken Hill Block. However, if our inference that at least some of the them are metagranites is correct, they should not be used for stratigraphic correlation, unless they can definitely determined to be of volcanic flow or tuffaceous origin.

References Andrews, E.G. 1922: Mem. Geol Surv. NSW, 8. Brown, R.E. et al. 1983: Rec. Geol. Surv. NSW, 21, 127-226. Browne, W.R. 1922: Mem.Geol.Surv. NSW, 8, 295-353. Collins, WJ & Vernon, R.H. 1991: Geology, 19, 835-838. Collins, W.J. & Vernon, R.H. 1992: Tectonophysics, 214, 381-400. Collins, W.J. et al., 1991:/. Struct. Geol., 13, 1157-1171. Stevens, B.P.J., et al. 1988: Precamb. Res. 40/41, 297-327. StillweU, F.L. 1922: Mem. Geol. Surv. NSW, 8, 354-396. Vassallo, J.J. 1995: Unpubl. BSc thesis, Macquarie University, Sydney, 129 pp. Vernon, R.H. 1968: J. Petrology, 9, 1-22. Vernon, R.H. 1969: J. Geol. Soc. Aust., 16, 20-55. Vemon, R.H. 1986: Earth-Sci. Rev., 23, 1-63. Vemon, R.H., et al, 1993: Tectonophysics, 219, 241-256. Vemon, R.H. & Williams, P.P. 1988: Aust. J. Earth ScL, 35, 379-388.

Re-evaluation of the tectonic and thermal tistory of the Mt Robe region, Broken Hill Caroline Venn^, Karin Ehlersl, Allen Nutman^ ^Department of Earth Sciences, Monash University Clayton VIC 3168 ^RSES The Australian National University, Canberra ACT 0200 An integrated study of structure, metamorphism and geochronology has resulted in a new structural model for the region. The identification of an early tectonic fabric and D3 folding event, not identified by previous workers, does not support the established and widely accepted Willyama Supergroup stratigraphy. The entire region has been highly strained original sediments have been attenuated and gross reorientations of lithological units have taken place. Moreover, multiple episodes of thermal activity have been identified and are relatively timed with respect to discrete episodes of deformation. Determination of the absolute ages of these events are presently being targeted using U-Pb zircon geochronology and will place constraints on the timing of deformation, metamorphism and magmatic activity throughout the region. Mount Robe is located approximately thirty-five kilometres northwest of Broken Hill in the Barrier Range. The region lies west of a major fault zone (the Mount Franks Fault and the Apollyon Valley Schist Zone) and east of a younger escarpment, the Mundi, Mundi Fault. The rock types in the area consist of a diverse array of meta-sediments consisting of biotitesillimanite and andalusite schists and meta-quartzites. U-Pb zircon geochronology from a metaquartzite yielded a maximum deposition age of 1740 Ma. Other rock types consist of amphibolites, pegmatites and granites. Previous interpretations (Willis 1989, Hobbs et al 1984) for the Mount Robe region are based on the presence of a continuous stratigraphic sequence which has been deformed into a major south plunging synformal anticline. According to this stratigraphy, the stratigraphically lower, Thackaringa Group occupies the core of the synform and is underlain by younger Broken Hill Group rocks. Interpretations of the amphibolites (Often 1983, Reynolds 1975) suggest that they are volcanic in origin, hence representing a distinct stratigraphic horizon in the upper Broken Hill Group. However, the amphibolites locally preserve relict meta-gabbroic and metadoleritic igneous textures and yield igneous zircons which give a SHRIMP 207pb/206pb age of 1673 ±23 Ma. This is interpreted as the magmatic intrusion age of the amphibolite. These observations suggest that the amphibolite unit is a highly deformed intrusive rock rather than a stratigraphic horizon. The region has undergone at least four episodes of intense deformation and four thermal events. The earliest tectonic fabric Si (Di) is identified by mesoscopic rootless, isoclinal fold hinges.

51 Microstructurally, this fabric is identified in S2 microlithons as aligned sillimanite, decussate biotite and garnet porphyroblasts which grew during M i , upper amphibolite metamorphism. Intense deformation during D2 produced the dominant fabric throughout the region (S2), and is characterised by a high temperature (M2) mylonite fabric defined by biotite and coarse blades of sillimanite. F 2 folds often transpose lithological layering and melt veins parallel and subparallel with S 2 .

The melt veins are thought to post-date D i and are pre-syn D 2 , i.e. possibly

associated with M2.

Regional pegmatite intrusions characterise the third thermal event and are

located within the cores of regional F4 folds. The pegmatites cross cut the S 2 fabric and predate younger deformations, i.e. post-S2-pre-D3. Locally, an S3 and S4 crenulation cleavage is observed, which crenulates bedding, S 2 and relict S i foliations. The last thermal event is characterised by remelting of the pegmatite and is identified by zones of coarsely recrystallised quartz, feldspar and white mica within the pegmatite and by quartzo-feldspathic veins which cross cut the S2 fabric and suspected F3 axial zones. This thermal event may also be associated with static metamorphism (M4), during which garnet grew and white mica and quartz were recrystallised.

Present work focuses on S H R I M P U-Pb dating of metamorphic zircons and magmatic zircons grown during melting and pegmatite intrusions. Dating these events will place constraints on the timing of deformation and metamorphism.

The large scale Mount Robe structure consists of complex, fold interference patterns with the prominent geometry representing a series of F4 upright folds. Earlier folds are detected which have implications for the accepted stratigraphic model throughout the Broken Hill region. Younging changes observed within the limbs of large scale D4 structures, suggest that older, F3 fold closures exist.

Further evidence for F 3 folding is suggested by a zone of strongly

crenulated S2, in an F3 axial zone near the peak of Mount Robe.

This study does not support the previous interpretations of an overturned, continuous stratigraphy from upper Thackaringa Group to Broken Hill Group, but suggests that the rock package today has been grossly modified from its original state due to attenuation, reorientation, metamorphism and partial melting.

References: Hobbs, et al. 1984. In Precamhrian tectonics illustrated Kroner A. and Ceiling R eds (Stuttgart: E Schweiz Verlag), 353-368. Often, M. 1983, Geological Survey of New South Wales Department of Mineral Resources. Reynolds, G.D. 1975. Monash University (Melbourne) BSc Hons. Thesis (unpubl.) Willis, I.L 1989, Broken Hill Stratigraphic Map. New South Wales Geological Survey Sydney.

Determination of the timing and P-T conditions of metamorphism using spectacular textural relationships and pseudosections of the Mount Barren Group, Western Australia. Simon Wetherley^, John Ridley ^ and Kurt Stiiwe^ ^ Department of Geology and Geophysics, The University of Western Australia, Nedlands, 6907. 2 Department of Earth Sciences, Monash University, Clayton, Victoria, 3168 The Mount Barren Group (MBG) is located on the south coast of Western Australia in the Proterozoic Albany-Fraser Province. It consists of a succession of interbedded quartzites, pelites and conglomerates which have been intruded by a series of tholeiitic dolerite sills. The MBG has undergone two deformation events during which three main phases of folding developed. The first deformation event comprises two major fold phases. The first major phase is a large scale isoclinal folding event with an axial plane dipping to the south. A well developed spaced cleavage is associated with this folding phase. The second major phase of folding is upright, close to tight with an axial surface dipping moderately to the south east. Fold axes generally trend south west. A well developed crenulation and spaced cleavage is associated with this second phase of folding. The second deformation event contains a widespread, but uncommon folding phase which occurs as kinks and crenulations. A poorly developed zonal crenulation cleavage is associated with this folding phase. During the first deformation event, the rocks reached peak metamorphic conditions in the mid-amphibolite facies with the growth of garnet, kyanite, staurolite and biotite porphyroblasts in pelitic bulk compositions. Pseudosections have been constructed to determine the P-T conditions of formation of the porphyroblasts.

These indicate that for the peak of

metamorphism, the maximum temperature is up to 660 °C and a maximum pressure of 9.3 kbars and a minimum temperature of 590

and minimum pressure of 5 kbars.

The timing of porphyroblast growth with respect to the well-developed cleavages can be established from the spectacular textural relationships seen in thin section. Garnet has grown early in the second phase of folding, overgrowing the early stages of crenulation cleavage development. Garnet generally contains quartz inclusions with minor biotite, muscovite and chlorite inclusions. Garnet inclusions have been identified in staurolite, kyanite and biotite indicating that all of these minerals post-date garnet. Staurolite and kyanite overgrow a more advanced stage of the crenulation cleavage development as well as a spaced cleavage. These minerals have also grown randomly in the foliation plane. These relationships indicate both kyanite and staurolite grew late in the second major folding phase and post-dating it. Biotite grows in the axial plane of the crenulation cleavage and larger porphyroblasts overgrow the crenulations. This indicates that biotite was stable during the second major folding phase and post-dating it. The peak of metamorphism can be inferred to be late in and post-dating the second major folding phase.

The Role of deformation in aiding recrystallization: An example from a high-pressure shear zone, central Australia

R.W. White^ and G.L. Clarke^

^ School of Earth Sciences, Macquarie University, Sydney, NSW 2109, Australia ^ Department of Geology & Geophysics, University of Sydney, Sydney, NSW 2006, Australia

A dolerite body in the western Musgrave Block was variably recrystallised in a 0.5-1 mwide, sharply defined, high-grade shear zone at P=12-14 kbar and T~750

All samples

within and adjacent to the shear zone have a common protolith; they are divided into unfoliated group 1 samples outside the shear zone and foliated group 2 samples within the shear zone. Group 1 samples show a progressive increase in the degree of recrystallization as the shear zone is approached. Systematic changes in modal proportions and mineral composition reflect increased reaction rates due to increasing strain and, to a smaller extent, variations in aH20Group 1 samples are dominated by garnet-bearing coronas that separate orthopyroxene, clinopyroxene and ilmenite from plagioclase. Low strain in the group 1 samples allowed the preservation of comparatively Al-rich, Fe+Mg-poor compositional domains replacing igneous plagioclase and Fe+Mg-rich Al-poor domains replacing igneous pyroxene. Group 2 samples are more thoroughly recrystallized, do not contain coronas and have more constant mineral compositions reflecting higher diffusion rates. The wide range in mineral composition and textures and their relationship to strain intensity within the gabbro body emphasises the important role that deformation on all scales plays in the development of metamorphic textures.

Fluid budgets during high-temperature retrogression of granulites: Evidence from the Reynolds Range, central Australia. Ian S. Buickl & Ian Cartwright2 ISchool of Earth Sciences, La Trobe University, Bundoora, Vic. 3083, Australia. ^Department of Earth Sciences, Monash University, Clayton, Vic, 3168, Australia. Granulite-facies marbles and calcsilicate rocks of the Reynolds Range Group (northern Arunta Block) were metamorphosed to -4-5 kbar and - 750-800°^ at ~ 1594 ± 6 Ma. The granulitefacies rocks were partially retrogressed in narrow (lO's-lOO's x lOOO's metres) zones. Outside these zones the marbles were internally buffered to high Xc02 ^^ the M2 peak. The retrograde zones, which are characterised by resetting of mineralogy and oxygen isotope ratios, major element metasomatism, and quartz veining, were channelways for high-temperature (650-700 ^C) water-rich fluids (XcO2<0-2-0.3). These fluids were derived from crystallising melts sourced from underlying granulite-facies metapelites and metagranites on 10s to 100s of metre lengthscales. In the retrograde zones the following rock-types, which were affected by progressively increasing time-integrated fluid fluxes, occur: i) carbonate-bearing marl layers; ii) wollastonite- or clinohumite-bearing marbles; iii) almost monomineralic lenses of coarsegrained clinopyroxene, grandite-rich garnet, clinohumite, or wollastonite; and iv) hightemperature hydrothermal quartz vein systems. The marl layers appear to have unreset mineralogy and stable isotope compositions and were probably not infiltrated during retrogression. The wollastonite- or clinohumite-bearing marbles are the mineralogically-reset equivalents of peak-M2 calcite+quartz- and dolomite+forsteritebearing marbles, respectively. In the case of the wollastonite-bearing marbles, wollastonite (<10%) was probably formed by the reaction: Calcite + Quartz = Wollastonite + CO2 (1). The fluid flux necessary to form the wollastonite can be calculated if it is assumed that the infiltrating fluids flowed vertically through the marble unit after crystallising pelite-derived melts ponded at its base. For likely vertical flow paths of 10 to 200m through the marble, mineralogical resetting requires time-integrated fluid fluxes of 1.6-33 m^/m^. The 6l8o(carb) values of these altered marbles have also been lowered by as much as 4%c from typical unreset values. Marbles with the most reset oxygen isotope values (6l8o(carb) = 10-1 l%o) are in approximate equilibrium with the hydrothermal quartz vein sets and partially melted metapelites. Oxygen isotope resetting over the same vertical distances yield time-integrated fluid fluxes of 18-360 m3/m2. The high-variance lenses were probably formed through major element metasomatism. In the case of the wollastonite-rich lenses there is up to 60 vol.% more wollastonite than can be accounted for by reaction (1). It is likely that the additional wollastonite formed through silica metasomatism, which over a 10 to 200m path length requires minimum

55 time-integrated fluid fluxes of --2x103-4x104 m^/m^. Estimates of time-integrated fluid fluxes necessary to precipitate the quartz veins based on silica deposition for fluids flowing down temperature gradients of 100-25°C/km at 4 kbar are --5x105-2x106 m^/m^. Therefore, within the retrograde zones, time-integrated fluid fluxes in different layers or structures varied by 5 to 6 orders of magnitude. Fluid flow in the retrograde zones occurred for at least 18 Ma (Williams^/ al ,1996). If fluid flowed through each rocktype for 18 Ma then the time-integrated fluid fluxes correspond to the following Darcy fluid fluxes: unaltered marl layers (~0 m mineralogically reset marbles (2.8x10-15-5.8x10-14 m^/m^/s); the most isotopically reset marbles (3.2x10-14-6.3x10-13 m3/m2/s); metasomatised marbles (3.5x10-12-7.0x10-11 m^/m^/s); and hydrothermal quartz veins (8.8x10-1^-3.5x10-9 m^/m^/s). For vertical flow, the differences in Darcy fluid flux recorded in the marbles could have been accompUshed by differences in intrinsic permeability from 1.5x10-23-3.1x10-22 m2 (mineralogically reset marbles) to 1.8x1020-3.6x10-19 m2 (metasomatised marbles). These permeabilities are comparable with, or smaller than, those estimated in other terrains or experimentally determined (typically 10-21 to 10-16 m2). In the retrograde zones, the unaltered marls, altered but not metasomatised marbles, and the metasomatic calcsilicate rocks are interlayered on a centimetre to metre scale parallel to lithological layering and major structures, and record large variations in fluid flux and intrinsic permeability. The large differences in intrinsic permeability may reflect microscale, deformation-controlled variations in microfracture density in the interlayered rocks. However, the largest fluid fluxes occurred through the hydrothermal quartz vein systems that make up only a small proportion («5%) of the exposed rocks.

REFERENCES Williams, I.S., Buick, I.S., & Cartwright, I, 1996. An extended episode of Mesoproterozoic metamorphic fluid flow, Reynolds Range, central Australia. Journal of Metamorphic Geology 14 (in press).

Geology and evolution of a mid-Proterozoic continental margin, southern Fraser Orogen, Western Australia D.J. Clark, B. J. Hensen & L.M. Kriegsman. Department

of Applied Geology,

University of NSW, Sydney, NSW, 2052.

The Fraser Mobile Belt (FMB) forms the eastern part of the Albany-Eraser Orogen, a midProterozoic mobile belt along the southeastern margin of the Archaean Yilgam Craton. The FMB is -200km wide and several hundred kilometres long, extending from the south near Esperance, toward the northeast where it continues subsurface beneath the sedimentary cover of the Officer and Eucla Basins. The EMB has been divided into the northwestern Biranup Complex and the southeastern Nornalup Complex, based upon lithological and structural criteria (Myers, 1995). Although there is evidence for magmatism as early as 1600-1800 Ma within the Biranup Complex, the main tectono-thermal activity occurred from c. 1300-1100 Ma (Nelson et al., 1995). The EMB occupies a key position for correlations between Australia and East Antarctica. Detailed study of the tectono-thermal history of the EMB may provide insight into the relative movements between Australia and East Antarctic cratons during this period of global Grenvillean tectonism. The oldest rocks in the Nornalup Complex, the Malcolm Gneiss, comprises mature metasediments and intercalated metamorphosed mafic, intermediate and felsic igneous rocks. These rocks were metamorphosed c.1300 Ma (Nelson et al., 1995) under mid to upper amphibolite facies conditions, synchronous with the development of a pervasive subvertical northeast-trending fabric (SI). Migmatites occur locally in some mafic and pelitic lithologies. Voluminous intrusion of granitic and monzogranitic magma accompanied this event (Myers, 1995). Peak metamorphic conditions are estimated at T = -650 °C and P = 4 kbar. Shortly after the peak of metamorphism, continued tectonism (Dlb) deformed the SI fabric, producing open to tight, upright to northwest-verging folds with moderately southwest-plunging fold axes. Sib fabric elements, though poorly developed, are locally defined by muscovite, suggesting the rocks had cooled through the K-feldspar + sillimanite isograd prior to this deformation event. The metamorphism outlasted both phases of deformation, resulting in annealed textures. The Mt Ragged Metasediments, thirty kilometres northwest of the coastal outcrops, consist of dominant grey quartzites and muscovite quartzites including infrequent, thin pelitic layers. The rocks appear to have been affected by at least one phase of deformation, probably corresponding to Dlb in the Malcolm Gneiss. Toward the north of the belt rare kyanite-bearing mineral assemblages in pelitic lithologies locally overprint andalusite-bearing parageneses.

57 whereas to the south siUimanite is the stable aluminosiUcate. Peak metamorphic conditions of T = -550 °C and P = 4-5 kbar post-date the formation of upright to northwest verging folds. Within the context of the evolving orogen, the Mt Ragged metasediments may have been deposited in a local transtensional basin during the first phase of D1 deformation. This basin subsequently closed and was buried as a consequence of the second deformation (Dlb). Two hundred kilometres to the west, along the Heywood Thrust (Myers, 1995), igneous rocks of the Coramup Gneiss were strongly deformed and recrystallised under granulite facies conditions c.1300 Ma (Nelson et al., 1995). Metamorphic minerals are aligned within a shallowly southeast-dipping to subvertical, well developed S1 layering. Gametiferous twopyroxene assemblages including quartz were developed in mafic rocks while orthopyroxene or homblende+gamet-bearing assemblages are common in granitic lithologies. Garnet and clinopyroxene are often separated by plagioclase and orthopyroxene coronitic textures, suggesting decompression from peak conditions of T>700 °C and P = ~6 kbar, similar to textures described further northeast along the Heywood Thrust in the Fraser Complex (Clarke etal, 1995). A second tectono-thermal cycle (M2/D2), c.l 100-1200 Ma (the D3 of Myers, 1995), locally reactivated D1 structures in large northeast striking, subvertical dextral retrograde shear zones. Within the Malcolm Gneiss D2 shear zones are recrystallised in the greenschist facies, while rocks of the Coramup Gneiss experienced amphibolite facies conditions synchronous with D2 deformation. Strong shearing and drag folding of SI surfaces and boudinage of post-Dl pegmatite dykes is common near shear zones. The intrusion of widely scattered monzogranitic plutons and sills accompanied and slightly post-dated this tectonism. Throughout the FMB sinistral mylonites and cataclasites cut SI and D2 shear surfaces at a small counterclockwise angle. This deformation (D3) ranges from ductile to brittle in nature and suggests a change in the regional maximum shortening direction from oblique east-west convergence to a NNW-SSE attitude.

References Clarke, G. L. Sun, S. -S. & White, R. W. (1995). Grenville-age belts and associated older terranes in Australia and Antarctica. AGSO Journal of Australian Geology and Geophysics, 16(1/2), 25-39. Myers, J. S. (1995). Geology of the Esperance 1:1 000 000 sheet. Western Australian Geological Survey, 1:1 000 000 Geological Series Explanatory Notes, lOp. Nelson, D. R., Myers, J. S., & Nutman, A. P. (1995). Chronology and evolution of the middle Proterozoic Albany-Fraser Orogen, Western Australia. Australian Journal of Earth Sciences, 42,481-495.

New age information from southern Fiordland, New Zealand: Constraints on existing tectonic models? Corine Davids Research School of Earth Sciences, Australian National University, Canberra ACT 0200 New Zealand represents the former Pacific margin of the Gondwana continent, which rifted away from Australia and Antarctica in the late Mesozoic. The south island of New Zealand can be divided into a Western and an Eastern Province, which are separated by the Median Tectonic Zone (Kimbrough et al., 1994; Coombs, 1985). The Western Province represents the former continental margin, whereas the Eastern Province consists of Paleozoic to Mesozoic arc and accretionary wedge assemblages. Fiordland, in the south western part of New Zealand and east of the Alpine Fault, is part of the Western Province. The area can be divided into a high pressure (up to 12 kbar), two pyroxene- and gamet-granulite terrain, the N-S trending Western Fiordland Belt, which is overlain by amphibolite facies metasediments, the N-S trending Central Fiordland Belt. The Western and Central Fiordland Belts are separated from the south western Fiordland Block, which consists of unmetamorphosed to amphibolite facies metasediments intruded by large granitic plutons, by the proposed E-W trending Dusky Fault. The area has undergone at least two major orogenic events before the final uplift. The first, mid-Paleozoic, event is recorded in the metasediments and took place when Fiordland was situated on the eastern margin of the super continent Gondwana, where at that time oceanic crust was subducted westwards under the continental crust. It is characterised by high temperature, andalusite-sillimanite bearing, mineral assemblages and tight to isoclinal recumbent folds with a well developed axial plane cleavage. The second, late-Mesozoic, event is characterised by the intrusion of the granulite facies Western Fiordland Orthogneiss, an incomplete high pressure overprint in the county rock and the formation of large scale shear zones. Final exhumation took place in the Tertiary due to a change in motion along the Alpine Fault from strike-slip to reverse slip. Several tectonic models have been proposed to explain the formation and evolution of the large granulitic orthogneiss body and its relation to the mid-Paleozoic metasediments. The most recent model is the magma loading model by E.H. Brown (1996). In this model the orthogneiss body is intruded as horizontal sheets into the country rock causing vertical displacement of the country rock under the pluton and a high pressure overprint. Two other tectonic models favour either an extensional or a compressional regime. Gibson et al. (1988) propose synkinematic intrusion of a granulitic protolith during the initial stages of continental extension and breakup of Gondwana. Further extension along deep crustal shear zones juxtaposed the granulitic orthogneiss and the mid-Paleozoic amphibolitic metasediments. Bradshaw (1985) suggested synkinematic intrusion of a granitic protolith in the mid-Paleozoic metasediments and a subsequent increase in pressure, which is recorded in both the granulitic orthogneiss and the metasediments, due to crustal thickening by overthrusting. In this model the metamorphism and strucmres in Fiordland predate the extensional regime. New K-Ar and structural data from north and south of the Dusky Fault in southern Fiordland will provide constraints on these tectonic models.

Orogenic concepts: Accommodation mechanisms for deformation in an oceanic setting, Lachlan Fold Belt, Australia David A. Foster^. David. R. Gray^, Martin Bucher^ Âustralian Geodynamics Cooperative Research Centre^ School of Earth Sciences, La Trobe University, Bundoora, Victoria 3083 Âustralian Geodynamics Cooperative Research Centre, Department of Earth Sciences, Monash University, Clayton, Victoria 3168 The presently accepted orogenic framework for the Lachlan Fold Belt was delineated on the basis of local and/or regional unconformities in the rock record with the assumption that these reflected orogenic belt-wide tectonic events. This traditional approach revealed four main events: (1) Benambran Event (Late Ordovician-Early Silurian: -440-435 Ma), (2) Bowning Event (Late Silurian-Early Devonian: -409-396 Ma)/ Bindian Event (Early Devonian: -390396 Ma), (3) Tabberabberan Event (Early Devonian: -380-375 Ma), and (4) Kanimblan Event (Late Devonian-Early Carboniferous: -360-340 Ma). These do not necessarily allow for either localized or wide ranging, diachronous deformations, and as in other orogenic belts this concept of orogeny is outmoded. Extensive areas of the Lachlan Fold Belt are dominated by Lower Paleozoic chevron-folded, sandstone-mudstone submarine fan deposits overlying Cambrian oceanic crust. Deformation is dominated by an east-vergent fold- and thrust system which developed diachronously from west to east by migration of a deformation front during Silurian time. Deformation within the structurally thickening wedge, recorded by chevron folding, cleavage development and imbricate thrusting, initiated in the west while sedimentation was still ongoing in the east. About 60% of the shortening took place via chevron folding with cleavage development late in the history. The belt is cut by four prominent, relatively deeprooted zones of intense deformation, the Landsbough, Avoca, Heathcote and Mount Wellington fault zones. Recent "^ÂrPÂr dating of metamorphic mica growth in cleaved slates within these major imbricate thrust zones gives ages of 451 ± 2,440 ± 2,426 ± 3, and 411-400 Ma. These data indicate a smooth progression of imbrication and unroofing from the basal detachment from west to east at a rate of approximately 8 nmi/year. Mica growth, marking diagenesis/ metamorphism at higher levels, in the respective thrust sheets is up to 10-15 Ma older than the timing of imbrication. These data require a redefinition of the orogenic framework of the Lachlan Fold Belt as continuous over the extent of the belt through Silurian and Devonian times. Local and regional scale episodic events that dominate the rock record are related to changes in accommodation mechanisms at an orogen-wide scale (e.g. subduction, strike slip faulting). Contraction simultaneous with sedimentation in the oceanic sediment wedge occurred above a decollement, linked in some way to subduction. This phase in the evolution ended via the docking of the western province with an island-arc/fore-arc system, analogous to the closing of a back-arc or marginal basin.

The metamorphic history of Rundvagshetta, east Antarctica: Time constraints on a "clockwise" P-T-t path G. Fraserl. 1. McDougalll, DJ. Ellis^ and I.S. Williamsl ^Research School of Earth Sciences, Australian National University, A.CT., 0200 ^Geology Department, Australian National University, A.C,T, 0200 Gneissic rocks from Rundvagshetta, East Antarctica, have experienced a "clockwise" P-T-t evolution, during which high-temperature granulite-facies conditions were followed by a period of relatively isothermal decompression. A similar P-T-t path has been documented from other parts of the East Antarctic Shield, and from many granulite terrains worldwide. Here we integrate field, petrologic and geochronological evidence which constrains the timing and duration of parts of a typical "clockwise" P-T-t path, with particular emphasis on the rate of cooling and exhumation subsequent to peak metamorphic conditions. Metamorphic basement at Rundvagshetta is composed of a layered sequence of predominantly paragneisses, which have experienced metamorphic conditions in excess of 9(X)°C at around 10 kbar. High-grade conditions were accompanied by intense ductile deformation, with at least two stages of isoclinal folding recognised in fold interference patterns, followed by upright, open folds. Cathodoluminescence and SHRIMP analysis of zircons from several rocks reveals multiple zircon growth episodes, with a range of U/Pb ages from -610 Ma to --'520 Ma. To what extent any of these ages approximate peak metamorphic conditions is not clear, although the youngest measured episode of zircon growth, with an age of -520 Ma, was synchronous with a stage of ductile deformation, and predates a period of relatively isothermal decompression. The isothermal decompression phase lasted for only a few miUion years, and was closely followed by relatively rapid cooling, as indicated by hornblende closure to argon diffusion by -500 Ma. ^^Aift^Ar diffusion experiments on K-feldspars allow reconstruction of the cooling history in the temperature range 300 - 150°C and suggest a period of relatively rapid cooling through this temperature interval at -400 Ma. These results indicate that granulitefacies temperatures were followed by initially rapid cooling at rates of -15°C/Ma, then by a period of around 100 Ma during which cooling was very slow at -2°C/Ma, in turn followed by an increase in cooling rate to -10°C/Ma. The results are consistent with a scenario in which a rapid and short-lived period of exhumation resulted in relatively isothermal decompression and advection of geotherms towards the surface. Rapid cooling inmiediately prior to 500 Ma records thermal re-equilibration subsequent to this exhumation phase. The long-lived period of slow cooling is suggestive of thermal and isostatic equilibrium, and low erosion rates. Increased cooling rates at around 400 Ma probably record an independent tectonic or erosional episode responsible for further exhumation of the terrain.

SHRIMP constraints on Palaeozoic high-T exhumation in SW Prydz Bay, east Antarctica. Martin Hand^, Pete Kinny^ ^Department of Geology & Geophysics, Adelaide University, Adelaide SA 5005 Australia ^Department of Applied Physics, Curtin University of Technology, Perth 6001, Australia In collisional orogens, the mechanisms by which deeply buried rocks are exhumed, and the rate at which exhumation proceeds is intimately linked to the large-scale dynamics of the orogenic system. However it is clear from orogens such as the Himalaya, no one part of an orogen contains all the information necessary to constrain the duration of a collision, or details relevant to its internal or external dynamics. For this reason, it is necessary document the duration of exhumation, and the associated structures, in many parts of ancient orogens in order to build up a coherent picture. In Prydz Bay in east Antarctica it is becoming increasingly recognised that the regional structural and metamorphic architecture formed during an Early Palaeozoic collisional event that may have been associated with final Gondwana amalgamation (Carson et al, 1996, AJES, in press\ Hensen & Zhou 1995, AJES, 42, 249-258; Fitzsimons et aL, 1996, this volume). The Early Palaeozoic granulites in Prydz Bay are dominated by post-peak mineral textures that reflect high-T (>700°C) decompression in the order of 300-500 MPa from a range of peak pressures (700-1000 MPa). Although the thermobarometric evolution of granulites in Prydz Bay is broadly similar, there are significant differences in the structural style accompanying exhumation in different areas. In the Rauer Group in NE Prydz Bay, exhumation occurred in a compressional environment, whereas 100 km to the SW in the Bolingen Islands and Larsemann Hills, exhumation was associated with high-T extensional fabrics (Dirks & Hand, 1995, AJES, 42, 157-172; Carson et ai, 1995, Geol. Mag. 132, 151-170). Compressional structures in the Rauer Group are characterised by a complex array of subvertical high strain zones with eastplunging lineations separating domains dominated by collinear fold interference patterns (Sims et al, 1994, Ant. Sci, 6, 379-394; Harley, 1987, AJES, 34, 175-207). High-T textures implying decompression formed late in the compressional evolution, but are clearly deformed and recrystallised in some high-strain zones. In the Bolingen Islands and Larsemann Hills, the peak S2 fabric (800°C-700 MPa) developed during transpression, and contains a lineation parallel to that in Rauer Group, providing a possible structural link between the two terrains. However in the Bolingen Islands (and to some extent the Larsemann Hills), S2 is overprinted by a complex progression of extensional structures. The most important of these are roughly E-W-trending D3 gneissic domains which in total form a zone > 8 km wide. S3 contains a moderately SW-plunging lineation and records SW-down normal movement. In both mafic, felsic and pelitic rocks, garnet-breakdown textures ('->750°C, 500-600 MPa), formed late in D3, and were subsequently overprinted by D4 pseudotachylite-bearing fault zones which also record SW-down movements along individual generation surfaces. The pseudotachylites were recrystallised under LP/HT granulite conditions and reactivated as granulite facies S-down

ductile shears and then overprinted by a new gneissic fabric (S5, --TOO^C, 450 MPa) which was associated with S-down normal movement. The above structures are transected by a series of N-S-trending subvertical pegmatites (D7) that have upper amphibolite LP/HT mineral assemblages (650°C, 300-400 MPa) developed along dextrally sheared margins. In order to appreciate the significance of the contrasting structural styles accompanying exhumation in adjacent parts of the orogen, the relative timing and duration of each needs to be established. Unfortunately the existing data base constraining individual high-T structural events in Prydz Bay is relatively small. In the Larsemann Hills, late D2 granite was emplaced at 515±7 Ma (Carson et aL, 1996), and in the Brattstrand Bluffs, peak pressures (D2?) are inferred to have occurred around 535 Ma, with high-T decompression by 515 Ma (Fitzsimons et al, 1996). In the Rauer Group, high-T fabrics and assemblages were crosscut by pegmatite at 500±12 Ma (Kinny et aL, 1993, Ant. Sci. 5, 193-206). Using SHRIMP-II at Curtin University we have dated zircons from syn-kinematic pegmatite in a D5 high strain zone, and a D7 pegmatite in the Bolingen Islands to establish the duration of high-T exhumation in the Bolingen Island region. Euhedral overgrowths with moderate (0.7-1.0) Th/U ratios from D5 pegmatite give a 206pi3/238u age of 527±14 Ma (n = 11) which is interpreted to be the crystallisation age of the pegmatite. Narrow euhedral, moderate Th/U overgrowths on inherited zircons in D7 pegmatite give 487±17 Ma (n = 7). At the 95% confidence level, the timing of zircon growth over the interval D2-D5 is indistinguishable, suggesting that - 1 0 km of high-T exhumation occurred relatively rapidly. Given the moderate amount of decompression reflected by the late-D3 - D5 mineral textures, and scale of the D3 domains, extensional strains were apparently relatively small, and it remains to be determined to what degree extensional structures in SW Prydz Bay are coeval with compressional structures in the Rauer Group 100 km to the north. In fact there is no compelling reason why extensional and compressional deformation should not have occurred simultaneously. Prior to D4, lateral variations in metamorphic grade in Prydz Bay were apparently quite small, implying a relatively uniform high-T thermal structure. Furthermore, the regional development of post-peak decompressional textures throughout Prydz Bay also suggests a relatively uniform thermal regime which in all probability was a continuation of the primary thermal structure. While rapid exhumation of high-T material provides a plausible mechanism for creating LPHT conditions during exhumation, the existence of pseudotachylite-bearing structures (suggesting relatively low-T conditions) bracketed by granulite grade extensional fabrics in the Bolingen Islands is somewhat problematic if heat transfer was primarily controlled by exhumation, since there would be no inherent reason for significant temperature fluctuations. A plausible alternative is that several thermal pulses occurred in the latter stages of the exhumation history (including D7). Indeed, the presence of post-D3 granites in SW Prydz Bay supports this hypothesis, and suggests increased heat input from mantle sources toward the end of the Palaeozoic collisional event in Prydz Bay.

Petrography and mineral chemistry of garnet-bearing metapelites, charnocliitic gneisses and mafic granulites from the Central Granulite Belt in Sri Lanka: Implications for their P-T evolution Sarath Hapugoda^, Jo Amold^ and Akira Takasu^ ^ Department of Earth Sciences, University of Queensland, Queensland, 4072; and. 2 Department of Geology, Shimane University, Matsue 690, Japan. The Central Granulite Belt of Sri Lanka is exposed over 25000 km^ and consists of metasediments and intercalated meta-igneous rocks of the Highland Complex (HC) and Southwestern Complex (SWC). Lithologies represented include; metasediments, marbles, dolomites, metapelites, metapsanmiites and quartzites and meta-igneous rocks; chamockitic gneisses (meta-felsites) and mafic granulites (metabasites). Metapelites: metapelites display garnet, sillimanite, K-feldspar, plagioclase(An2o.57)» spinel, quartz and biotite assemblages. Inclusions of siUimanite, quartz and plagioclase are common in the cores of garnet porphyroblasts within HC metapelites whereas staurolite, kyanite, spinel (hercynite), sapphirine, biotite (Ti02% up to 7.5), rutile and quartz inclusions are common toward the rim regions. Coarser sillimanite defines inclusion trails in the outer rims of garnet. The presence of spinel, staurolite, kyanite and quartz inclusions in garnet indicates the prograde reaction staurolite+quartz->kyanite+spinel+garnet. In HC metapelites, evidence for the prograde reaction, biotite+sillimanite+quartz-> garnet+Kfs+H20 is also observed. The inclusions of aluminosilicates in poikiloblastic garnets indicate continued growth of garnet as the prograde P-T path crossed from sillimanite into kyanite and re-entered the sillimanite stability field. P-T estimates on HC metapelitic rocks indicated pressures in the vicinity of 6.7-8.2 kbar and temperatures 700-750 'C for gamet-sillimanite-biotite-plagioclase bearing assemblages. Petrological evidence in SWC metapehtes indicate formation of cordierite and garnet according to

the

prograde

reaction:

biotite+sillimanite+quartz->garnet+cordierite+K-

feldspar+mag+ilm+H20. Cordierite, wollastonite and andalusite have been observed in metapelites of the SWC. Sapphirine inclusions were also observed in garnet porphyroblasts in metapelites of SWC. Intensive development of cordierite-quartz, cordierite-quartz-biotite and cordierite-spinel symplectites around larger gamet porphyroblasts in SWC metapelites supports the breakdown of gamet according to the reactions: gamet+sillimanite+quartz—>cordierite and gamet+K-feldspar+H20~>biotite+Cordierite+Quartz. X-ray maps of the gamet porphyroblasts involved in the above reactions show homogeneous cores whereas the rim regions are zoned. Geo-thermobarometry indicates a range of temperatures (650-750 °C) and low pressures (< 5 kbar) for the gamet-cordierite assemblage and together with reaction textures are consistent with rapid uplift and slow cooling (isothermal decompression).

64 Chamockitic Gneisses(HC): Pleochroic orthopyroxene (enstatite-ferrosilite), garnet, plagioclase (antiperthite, An2o-3o). potassium feldspar, clinopyroxene, biotite, quartz and hornblende are the principal mineral constituents in chamockitic gneisses. P-T estimates based on core compositions of large garnet, orthopyroxene porphyroblasts and matrix plagioclase compositions indicate temperatures up to 790°C and pressures in the vicinity of 7.2-7.6 kbar. Gamet-clinopyroxene-quartz symplectites have been observed around larger orthopyroxene and garnet porphyroblasts. (Opx+plagioclase-->garnet+cpx+quartz). These symplectite mineral assemblage indicate temperatures of 700-715 and pressures around 7 kbar, implying the crust cooled without any significant change in pressure (near isobaric cooling). Mafic Granulites(HC): Assemblages including clinopyroxene (augite-ferroaugite), orthopyroxene (enstatite-ferrosilite), hornblende, garnet and plagioclase (An45.53) are common in mafic granulites. Temperature estimates based on intergrown Opx and Cpx indicate high temperatures in the range of 830-890 °C. Core mineral compositions of Grt-Cpx-Plg assemblages in mafic granulites indicate temperatures of 760-815 °C and pressures in the vicinity of 6.8-8.3 kbar. Orthopyroxene, plagioclase(An5o.63) coronas have been observed around garnet porphyroblasts within these rocks. These are result from the reaction gamet+cpx+quartz->opx+plg. Plagioclase in corona textures have higher anorthite contents than matrix plagioclase in these rocks. The corona assemblage indicates temperatures of 620710 and pressures of 5.0-6.2 kbar. As is the case in most metamorphic terranes aspects of the Central Granulite Belt's metamorphic evolution are constrained by a number of different rock types. Metapelites in SWC display evidence for a rapid decompression, gamet-clinopyroxene-quartz symplectites around orthopyroxene and gamet porphyroblasts within HC chamockitic gneisses indicates significant cooling (near isobaric cooling) of the crust before the uplift. Based on above observations a high temperature low pressure clockwise prograde P-T path is proposed for SWC metapelites. Metapelites in HC show evidence for a high P,T prograde P-T path. Chamockitic gneisses and mafic granulites in the HC show evidence for a retrograde P-T path. These observations help to define a complete clockwise prograde and retrograde P-T path for the HC rocks by combination of the evidence for the prograde P-T path from metapelites and retrograde P-T path from chamockitic gneisses and mafic granulites. Thermobarometry for the COB indicates a general metamorphic trend with P increasing from SWC to the central highlands then further toward the northeastem and eastem parts implying the erosion surface exposes gradually deeper cmstal levels, and thus that the terrane as a whole may have been tilted subsequent to cooling.

The Kuark Metamorphic Complex, a low pressure, high temperature metamorphic belt in eastern Victoria M.A. Hendrickx. A.P. Magart, C.E. Williams & A.H.M. Vandenberg Geological Survey of Victoria

The Kuark Metamorphic Complex comprises biotite phyllites and schists, andalusite and cordierite schists, K-feldspar and sillimanite schists and gneisses, migmatites and syntectonic granites. In MURRUNGOWAR these have been divided into a number of metamorphic zones defined by isograds based on the distribution of metamorphic index minerals, namely biotite, andalusite and cordierite and k-feldspar and sillimanite. The metamorphics are all derived from Lower Ordovician Pinnak Sandstone. The Upper Ordovician Bendoc group does not appear in the complex. The metamorphic isograds form a roughly symmetrical outcrop pattern, with the k-feldsparsillimanite zone at the centre. This pattern has been displaced by the Combeinbar Fault which trends ENE through the centre of the belt just north of Murrungowar. The wide distribution of higher grade (K-feldspar-sillimanite facies) rocks to the south of this fault indicates a southeast over northwest transport direction with minor dextral movement. These internal faults have been displaced by the Pheasant Fault with a west over east movement sense. Biotite Zone The gradational boundary between regionally metamorphosed lower greenschist facies Pinnak Sandstone and the Kuark Metamorphics, is only preserved in MURRUNGOWAR north of Morrison Track, on the east side of the Tooti Creek Fault. Elsewhere the boundaries are faulted; along the Combienbar Fault in the east and the Wrak thun barlluk Fault in the west. In the outer margins of the biotite zone, rocks generally show little change from their parent lithologies except that they are generally more strongly foliated, the metamorphic mineral assemblage for this part of the zone is chlorite+muscovite+green-brown biotite. Rocks comprise phyllitic mudstones and foliated sandstones (psammites) clearly derived from the Pinnak Sandstone. In the inner parts of the zone, metamorphic grade increases and a strong schistose fabric is present. The rocks are predominantly light grey-green to brown, fine grained biotite schist and slightly coarser biotite psammite. Bedding, where parallel to the schistosity, is generally well preserved as thin schistose pelitic layers and thicker psammitic layers; elsewhere it is overprinted by the schistosity and is obscured. The schistosity is defined by small flakes of muscovite, biotite and chlorite. The metamorphic mineral assemblage in the higher grade parts is characterised by muscovite+red-brown biotite±chlorite. Andalusite-Cordierite Zone The boundary between the biotite zone and the andalusite-cordierite zone is gradational, marked by the appearance of andalusite and/or cordierite, and by a textural change from biotite schist, in which bedding is preserved, to medium grained strongly differentiated schist in which bedding is rarely discernible. Thin sections show that fine andalusite and/or cordierite is generally present in these schists. Typical metamorphic mineral assemblages include quartz, plagioclase, biotite, muscovite ± cordierite ± andalusite. Chlorite is absent except in cordierite porphyroblasts altered to pinnite. K-feldspar-Sillimanite Zone The highest grade rocks form the core of the complex. The boundary with the andalusitecordierite zone is gradational, defined by the first occurrence of sillimanite and K-feldspar in addition to andalusite and cordierite. Rocks in the zone include differentiated biotite schist.

66 knotted porphyroblastic schist and gneiss, and minor migmatite. Differentiated biotite schist is the most common rock type. There is no change in the appearance of differentiated biotite schist, other than a coarser grain size. FibroUtic sillimanite is visible as colourless fibrous growths in some hand specimens. Knotted porphyroblastic schist is fine to medium grained and consists of fibrous sillimanite, biotite, orthoclase, retrogressed cordierite, minor andalusite and muscovite, with accessory tourmaline and zircon. The well developed schistosity is defined by aligned biotite, sillimanite, and elongate cordierite porphyroblasts. Gneiss is confined to a series of outcrops along the Towser Creek, and shows gneissic banding defined by alternating mica rich and quartzofeldspathic layers up to 5 mm thick. Biotite, blocky and fibrolitic sillimanite and muscovite dominate the mica rich zones, while quartz, microcline and orthoclase dominate the feldspathic layers. Migmatitic horizons occur throughout the inner parts of the K-feldsparsillimanite zone. They consist of thin veinlets and pods of granitic material up to 10 cm thick, generally lying parallel to the S2 foliation. A feature of the metamorphic belt is the presence of a number of syntectonic S-type intrusives in the andalusite-cordierite zone. They were probably derived from the localised melting of sediments within the Kuark Metamorphics. The metamorphic zonation in the Kuark Metamorphic belt is approximately symmetrical about a number of gabbroic bodies (Scrubby Flat and Arte Gabbro). From this relationship it is inferred that these high temperature mafic bodies were driving metamorphism in the belt. They have a combination of faulted and normal intrusive contacts and were probably emplaced in a number of stages in the history of the complex.

Keywords

Mallacoota 1:250,000; SJ 55-8; Murrungowar 1:100,000; 8622; Ordovician; Kuark Metamorphic Complex; metamorphic geology

References

Hendrickx, M.A., Willman, C.E., Magart, A.P.M., Rooney, S., Vandenberg, A.H.M., Oranskaia, A. & White, A.J.R., 1996. The geology and prospectivity of the Murrungowar 1: 100,000 map geological report. VIMP report 26.

A structural and petrological traverse through the Palaeoproterozoic Nagssugtoqidian Orogen, western Greenland: P-T-D evolution of a transpressional orogen

Kriegsman. Leo M. K Van Gool, Jeroen^, Marker, Mogens^, Nichols, Geoff T.^ ^Dept, of Applied Geology, University of New South Wales, Sydney NSW 2052, Australia ^Danish Lithosphere Centre, 0ster Voldgade 10, 1350 Copenhagen K, Denmark ^School of Earth Sciences, Macquarie University, New South Wales 2109, Australia

The Nagssugtoqidian Orogen in West Greenland is a --1.9 Ma old, E-W trending orogenic belt separating two Archean blocks. While the Nagssugtoqidian Orogen has been regarded as ensialic in the past, recent recognition of Palaeoproterozoic, juvenile intmsive rocks suggests major addition of new material to the crust, which is more typical for subduction-collision orogens. A traverse across the central part of this orogen was carried out in 1994 under the aegis of the Danish Lithosphere Centre. The southern part of the traverse is dominated by granitic gneisses with abundant mafic dykes, bound to the N by a major structural contact. Folded and metamorphosed, coarse-grained mafic dykes are restricted to the footwall of this contact and are correlated with the early Proterozoic Kangamiut dykes cross-cutting the southern Archean craton. A km-scale tectonic lens in the footwall is comprised of --50% mafic material and contains one lens of sapphirine-spinelbearing amphibolite. The stretching lineation is subhorizontal and parallel to the ENE-WSW long axis of both the amphibolite lens and the large-scale tectonic lens. Isoclinal, intrafolial folds occur locally and, together with the presence of boudins of garnet-rich layers, testify to high strains. The tectonostratigraphy N of the structural contact shows interleaved partially melted metapelites and orthogneisses. This sequence is repeated several times and is interpreted as a thrust stack with a southward displacement. Evidence for strike-slip movements along the same contact indicates a transpressional setting. Foliation dips in this part of the traverse vary from --NNW/SO in the S through NNW/70-80 in steep belts to subvertical in the N. Stretching lineations are particulai'ly well-developed in steep belts and have an average plunge of --SO^ towards WSW. Folds are mainly presei-ved outside the steep belts and have subhorizontal fold axes. Asymmetric fabrics in the steep belts generally indicate sinistral strike-slip in horizontal sections. The combination of sinistral strike-slip, WSW-plunging stretching lineations and upright folds is also suggestive of a transpressional tectonic setting.

The northernmost part of the traverse shows Archaean orthogneisses interleaved with a complex of Proterozoic supracrustal and intmsive rocks, which are in tectonic contact.

68 Ultramafic lenses in the Proterozoic complex, containing olivine and Cr-spinel, may represent peridotites and suggest the existence of a major tectonic boundary. P-T estimates on the early assemblage garnet + clinopyroxene + quartz + plagioclase + orthopyroxene (inclusions in quartz) in metamorphosed Kangamiut dykes from the southern part of the traverse indicate pressures of 13-14 kbar at temperatures of --700-800 ^C. Garnet breakdown to orthopyroxene + plagioclase occurred at -1-9 kbar and --650-700 which is similar to P-T estimates from metapelites N of the thrust contact. These results suggest that the Proterozoic Kangamiut dykes in this part of the orogen may have witnessed a high-P metamorphic event which does not seem to have been recorded in nearby metapelites and subsequently equilibrated at intermediate pressures. Strong, near-isothermal decompression recorded in these Kangamiut dykes may be interpreted in terms of extensional collapse of the orogen after the high-P stage. It is uncertain at present at which level the extensional strain was accommodated, but possible candidates include the structural boundary described above and the high-strain zone comprising the large-scale tectonic lens and adjacent gneisses. We propose that ENE-WSW stretching in this part of the orogen may be simultaneous with thrusting and transpression in the rest of the Nagssugtoqidian Orogen.

Heat production distributions in Australian Proterozoic terranes: Implications for metamorphic thermal energy budgets and field gradients Sandra McLaren, Mike Sandiford, Martin Hand, Narelle Neumann & Naomi Wall, Department of Geology & Geophysics, University of Adelaide, South Australia

Airborne radiometric survey data together with geochemical analyses of critical samples and modem heat flow data provide the basis for the assessment of the heat production distributions during metamorphism in three Australian Proterozoic provinces. This data provides an essential basis for evaluating the thermal energy budgets and style of metamorphism in these provinces. In the SW part of the Mt Isa Inlier, the Sybella Batholith is characterised by high heat production values* (8 |LiWm"^), while the surrounding metasediments average about 1.5 |LiWm' . Limited data suggest heat production of at least 3.5 |iWm"^ for the Big Toby Granite, while contemporary heat flow data of -80 mWm"^ imply that the crust to the east of the Sybella granite (which includes a considerable near-surface thickness of Big Toby Granite) averages 1.5-2 |LiWm"^. Thermal models show that the anomalous heat production associated with the laterally confined Sybella Batholith contributes up to 150''C at 15 km depth, while the anomalous heat production in the deeper crust (probably in the Big Toby Granite) contributes to an anomalously elevated background thermal regime in the upper crust. In the Mount Painter Inlier heat production values vary from 1 - 50 jiWm'^ and average between 12-15 |iWm'^ Low values are associated with quartzitic sequences that in part overlie a Palaeo-Mesoproterozoic basement comprising extraordinarily enriched granites, with the highest values recorded from the Yerila Granite and Paralana Gneisses. Th-U ratios of about four suggest the enrichment is a primary magmatic feature. As shown in a companion paper (Sandiford, this volume) the extreme heat production values associated with the basement are consistent with the measured modem day heat flow (126 mWm"^) as well as a unique style of unconformity-related contact metamorphism developed in Neoproterozoic sediments above the basement. Regional heat production values in the Eastem Arunta Inlier are typically in the range 13 |LiWm"^ and average about 1.7 |iWm"^ Contemporary heat flows elsewhere in the Arunta Inlier of --60 mWm suggest heat production in the deeper crust averages less than 1 jiWm"^ and thus the heat production-heat flow regimes for this terrane are more in keeping with ideas about typical crustal heat production distributions. The differences in heat production between the Eastem Arunta Inlier and the SW Mount Isa Inlier and Mt Painter Inlier are reflected in contrasting metamorphic styles. The former is characterised by regionally extensive sillimanite-kyanite grade metamorphism with comparatively little variation in peak temperatures across significant lateral and vertical sections of the cmst. Consequently, preserved metamorphic field gradients form steep arrays in PT space. In contrast metamorphism in the Mount Isa Inlier and Mt Painter Inlier is characterised by andalusite-sillimanite facies, with substantial lateral variations in metamorphic grade producing field gradients that form shallow arrays in PT space.

200

400

600

Temperature (°C)

Simple ID thermal models show that the steep field gradient arrays of the Eastern Arunta are consistent with metamorphism of a comparatively unradiogenic sequence beneath a much more radiogenic layer/body that has subsequently been removed during denudation. In contrast, the shallow field gradient arrays of the Mount Isa Inlier are consistent with metamorphism of sequences that contain or lie structurally above unusually radiogenic bodies. *heat production values are quoted for the relevant time of metamorphism

Geodynamic model for the protoliths' evolution from the Blueschist Unit of Siphnos (Cycladic Islands, Greece): Geochemical evidence. Mocek, B. Geomar, Research Center of Marine Geosciences Kiel Germany E-mail: bmocek@geomar.de Siphnos, one of the Cycladic islands in the Aegean Sea of Greece, is part of the lower unit of the Attic-Cycladic Crystalline Complex. During Eocene time (40 mill, b.p.) the unit was metamorphosed for the first time under eclogite-facies conditions. Later, at the transition of Oligocene to Miocene (14 mill, years ago) this unit was overprinted by a greenschist- to amphibolite facies event. The high-pressure metamorphism is related to collision and northward subduction of the Apulian Plate under the Eurasian Continent. The second overprint occurred with the NE directed subduction of the African Plate under the Apulian Plate. Siphnos itself can be subdivided into three main geological units, a blueschist unit in the northern part and a greenschist unit in the center and the eastern part. The greenschist unit is affected by both metamorphic events, while the blueschist unit is only affected by the first. Both units are divided by the main marble complex, which is exposed in the center and the far north of the island. The blueschist unit is characterized by different schists and gneisses with blueschists, eclogites, chlorite-actinolite rocks, jadeite gneisses as well as metasediments. In order to identify the protoliths of the blueschist unit and to reconstruct their geodynamic setting the chemical compositions of blueschists, eclogites and chlorite-actinohte gneisses were investigated. Trying to avoid low-temperature effects like weathering or sea-water alteration etc. the study is focused on immobile elements like Mg, Si, Al, Fe, Ti and the rather immobile REE. The analysed rock types show large geochemical variations. The protoliths of the blueschists have been tholeiitic and show affinities to tholeiitic andesites and basalts with high MgO- or FeO-content. The trace element distribution diagram as well as the REE-diagram show affinities to MORB-like and also volcanic-arc patterns, which are typical for protohths forming during the early stage of the evolution of back-arc basins. The eclogites can be described as calc-alkaline basalts and andesites, tholeiitic dacites and tholeiitic high FeO-basalts. The eclogites show a Nb-Ta trough, which indicates a subductionrelated formation of their protoliths. The negative Hf- and Ta-anomalies point towards an island-arc environment in which their protoliths intruded. The strong LREE-enrichment indicates that the mantle-reservoir was enriched in incompatible elements by subducted sediments and fluids, which play a significant role in the evolution of island-arcs. The protoliths of the chlorite-actinolite rocks have similarities to basaltic komatiites (AI2O3- and MgO-content). Based on their high Mg#-number (74-80) the chlorite-actinolite rocks' protoliths can be interpreted as primary and non-fractionated mantle melts. These magmatic rocks are not

72 typical for island-arcs. Their chemical composition is equivalent to picritic boninites, which are magmatic rocks with MgO-content higher than 9 wt.%, and also high in Cr and Ni. They also show a Ta-Nb trough. Apart from their incompatible elements this rock-type is more primitive than N-MORB. The mantle reservoir for their protoUths was enriched in incompatible elements by fluids. Considering the geochemical data the origin of the rocks can not be in a pure islandarc setting. I interpret them to be derived from the transition of an island-arc to spreading volcanism environment. Based on these geochemical data, I propose the following geodynamic model for the premetamorphic evolution of Siphnos and show how these different rock types can appear in such a restricted area. The evolution of the protoliths from the high-pressure metamorphic unit of Siphnos starts with the collision of different oceanic plates. When one plate is subducted beneath the other, the formation of an island-arc began. Basalts and andesites erupted, which are the protoliths of the eclogites. After the formation of the island-arc, asthenospheric material ascended in form of a mantle diapir. Based on the high heat flow an extensional region formed in the island-arc. During this stage the boninites (protoliths of the chlorite-actinolite rocks) were formed from the diapir. The ascending mantle diapir supported the extension, which initiated the evolution of a spreading center for formation of a back-arc basin. In the early stage of the formation of this basin, basalts and andesites formed the protoliths of the blueschists. Those magmatic rocks were later affected by metamorphism in a subduction zone during the Alpine Orogeny (see above). The geodynamic model, which I propose for the high-metamorphic unit of Siphnos is typical for the Mediterranean and can be observed from the Western-Mediterranean up to Turkey (Roeder & Scandone, 1992). Tectonic extension and compression processes play a significant role. Roeder and Scandone (1992) describe the evolution of the Alpine-Mediterranean orogenic belts by the following cycle: subduction, collision, topographic built up, extensional collapse, and back-arc spreading. This cycle is determined by the tectonics of the converging lithosphere and the flowing of the asthenosphere.

References: Roeder , D.A. & Scandone, P. (1992): Recent tectonics of the Mediterranean, In: D. Blundell, R. Freeman, S. Mueller (eds.): A Continent Revealed, The European Geotraverse, Cambridge, University Press, 202-214.

Structure and metamorphism of the June area, Sanandaj-Sirjan Zone, Iran M. Mohajjel School of Geosciences, University of Wollongong, Wollongong NSW 2522, Australia The Zagros orogenic belt of Iran includes a metamorphic belt (Sanandaj-Sirjan Zone) that extends for 1500 km from the northwest to southeast and is 100 km wide (Fig. 1). The study area covers 350 km^ of the Sanandaj-Sirjan Zone and is located 300 km southwest of Tehran. It contains Late Palaeozoic and Mesozoic intensely deformed and metamorphosed rocks. Three main stratigraphic units occur in the study area: (1) Permian crystalhne Umestone and dolomite, (2) Middle-Late Triassic metabasic and silicic volcanics, intrusive rocks and metasedimentary (limestone, dolomite and quartzite) components, and (3) Jurassic dark grey phyllite and slate with interbedded marble and greywacke. Four main deformations occurred with the most significant being D2. F j folds are tight to isoclinal with curved and angular hinges, axial planes are steeply dipping to the northeast and southeast (50^-800). p j plunge at 40^-60^ to the east and northeast. Si axial plane cleavage is associated with F j which are strongly overprinted by F2 with development of type 2 and 3 refold patterns at macroscopic and mesoscopic scales. Northeast trending, angular, similar (class 2 and 3) F2 are very tight to isoclinal, coaxial with F j , with axial plane cleavage (S2) which is strongly developed. Lithological layers in limb areas of many F2 folds are partially attenuated and/or dismembered, boudinaged and aligned parallel to S2. The large flattening component indicates high strain during D2. The third deformation consists of close to tight asymmetric folds moderately plunging to the east and axial plane steeply dipping to the northeast. S3 crenulation cleavage is well developed in phyllite and schist. The last deformation consists of open, upright folds with northeastsouthwest trending axial planes. Folds shallowly plunge both to northeast and southwest. A stretching lineation associated with D j is defined either by pressure fringes or mineral elongation and plunges gently to the northeast. Stretching lineations associated with D2 are sub-horizontal or plunge gently both to the northwest and southeast. Shear sense indicators show a dextral displacement during D2. The area is dominated by two northwest trending map-scale isoclinal overturned F2 antiform and a map-scale F2 synform that moderately plunge to the east (Fig. 2). All lithological layering and axial plane cleavage associated with this folds dip towards the northeast. A decrease in metamorphic grade occurs from south to the north grading from lower amphibolite-

greenschist facies to phyllites in the northeast of study area (Fig. 2). Plagioclase-homblendeepidote assemblages occur in lower amphibolite facies.

Plagioclase-garnet-biotite and

nuiscovitc -chloritc-biotite assemblages occur in schist. Marble and meta-dolomite exist with all greenschist facies. The southern limbs of the map-scale folds are attenuated and highly disrupted by north-dipping thrust faults with transport to the southwest.

The Middle Triassic-Jurassic metamorphic rocks were intruded by several plutons which are highly deformed, mylonitised and gneissic, and slightly deformed granite-granodiorites are intruded in phyllites with contact aureoles. Porphyroblast-matrix relationship indicates that peak temperatures were possibly pre or syn D i . The low-P metamorphism occurred synchronous with the compressional deformation events.

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'Mawson Block" metamorphism: A preliminary synthesis R. L. Oliver^ & C. M. Fanning^, ^Department of Geology & Geophysics, University of Adelaide, South Australia 2 PRISE, Australian National University A number of isolated meta-igneous and meta-sedimentary outcrops are scattered along a 300 km coastal section of Adelie Land and King George V Land, East Antarctica. Analysis of the terrain east of Commonwealth Bay is currently being pursued, but data as yet are few and interpretation is still much dependent on the observations of Stillwell. On the coast of the Cape Gray promontory, rock types can be grouped into: i) metasedimentary gamet-cordierite-sillimanite-biotite-feldspar-quartz gneisses, ii) igneous hypersthene-K feldspar-quartz-(gamet)-(biotite)-diorite (chamockite) with variable foliation, intrusive into the metasedimentary gneisses iii) mafic cpx-opx-plagioclase-hornblende-(garnet)-dykes and masses (metadolerite), intrusive into both the gneisses and the chamockitic diorite. It is suggested that the aluminous metasediments at the Cape Gray promontory are the equivalent of similar metasediments, 170 km to the west at Pointe Geologic, metamorphosed at 1.75-1.68 Ga. They are tentatively regarded as slices of cover to the older Cape Denison and Port Martin granitoids respectively, thrust laterally from a higher temperature environment. A possible correlative, here, is the garnet-bearing "phyllite' at Cape Hunter, on the west side of Commonwealth Bay, thought to be a volcanogenic sediment deposited in an extensional graben between 1.765 and L70 Ga . Metadolerites and metabasalts), now amphibolites, abundantly intrude the Cape Denison granodiorite. The chemical composition (major elements) of these (Sheraton et al), on average, resembles that of the mafic dyke series east of Commonwealth Bay (Stillwell); the abundance of pyroxene ± garnet in the latter, however, manifests a higher metamorphic grade than that of the Cape Denison dykes (cf Stuwe and Oliver). Structurally, also, there is a difference in that the Cape Denison dykes have a dominant verticality and north-north-west trend, parallel to the strike of a consistent foliation of the granodiorite, whereas the mafic dyke intrusions east of Commonwealth Bay tend to cut the foliation of the gneisses and have a more random orientation, though some of these dykes display a weak schistosity. It seems likely that the Cape Denison dykes were retrograded to a greater degreee than those east of Commonwealth Bay, the orientation of the former being more strongly influenced by the regional north-northwest-trending schistosity which is displayed variously by the metasediments and some of the meta-igneous rocks across the whole of Adelie Land and George V Land. The development of this schistosity in the Conmionwealth Bay area together with emplacement and metamorphism of the dykes, is mooted as manifesting a 1.7-1.5 Ga event (Stuwe and Oliver), roughly corresponding to the metamorphism and schistosity development in the gneisses at Point

Geologic (Peucat) and possibly, at a lower metamorphic grade, of the "phyllites" at Cape Hunter (Oliver and Fanning). Occurrences of hypersthene-alkali feldspar gneiss (chamockite) east of Commonwealth Bay, referred to above, include outcrop at Madigan Nunatak and Aurora Peak. Zircons from two specimens from Madigan Nunatak, with virtually identical mineralogy, analysed by SHRIMP microprobe, have very different U-Pb ages of 1.709±0.012 Ga and 2.35-2.70 Ga respectively. Generally speaking, the above described rocks from Adelie Land and George V Land appear to manifest events of two ages, ca 1.7 Ga and ca 2.4 Ga. These ages are recorded strongly, also, in southern Eyre Peninsula, and the range of rock types furthermore is similar. Thus the ca 2.4 Ga age and range of lithology of the Camot Gneisses of southern Eyre Peninsula (Drexel et al) resembles the age and lithologies of the Port Martin and Cape Denison granitoids. Aluminous, gamet-cordierite-sillimanite bearing gneisses, similar to those at Pointe Geologic and east of Commonwealth Bay, are derived from appropriate sediments of the middle Palaeoproterozoic Hutchison Group of southern Eyre Peninsula, during the late Palaeoproterozoic Kimban Orogeny (1.8-1.5 Ga). Also possibly related, here, are the phyUitic metasediments at Coffin Bay, southwestern Eyre Peninsula, suggested correlatives of those at Cape Hunter, Antarctica (Oliver and Fanning). Within the Lincoln Complex of southeastern Eyre Peninsula, hypersthene-alkali feldspar-quartz chamockites have intruded at ca 1.84 Ga supposedly into the Hutchison Group. Whether these are related to the lithologically similar, chronologically dissimilar, bodies in George V Land, is uncertain. Mafic dykes, however, metamorphosed variously to pyroxene gneiss and/or amphibolites in Adelie land and George V Land, are matched by similar meta-basalts or meta-dolerites intrusive into the Hutchison Group during the Kimban Orogeny or as part of the Lincoln Complex (Drexel et al).

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Fluid inclusions in crack-seal veins at Dugald River, Mount Isa Inlier: Implications for palaeostress states and deformation conditions during orogenesis. Guojian Xu

Earth Sciences, James Cook University, Townsville 4811, Queensland

An integrated geometrical investigation of fluid inclusion trails and microthermometric analysis were carried out for fluid inclusions in fibrous crack-seal veins at Dugald River, Mount Isa Inlier. Two general types of fluid inclusions were distinguished: one is C02±CH4-rich and the other one is H20-rich, and at least three stages of fluid percolation are interpreted to have occurred during vein formation. It is shown that there is a strong consistency between trails of pseudosecondary inclusions and the regional maximum principal stress (ai). The preferred orientation of these trails can be regarded as excellent structural markers. However, trails of secondary inclusions generally show a deviation from the bulk shortening direction and possibly reflect local stress reorientation as a result of structural heterogeneity associated with the formation of the veins. The evolution of deformation conditions during the development of crack-seal veins can be well constrained by the microthermometric data, together with knowledge of metamorphic reactions in this region. It is suggested that the P-T path of crack-seal deformation started at about 450°C and 2.7 kbar, then crossed 340°C and 1.2 kbar, and finally terminated around 130°C and near surface pressure, corresponding to the regional crustal uplift. This study shows the importance of combined geometrical and microthermometric studies of fluid inclusions for understanding the physical conditions of crack-seal deformation, a common phenomenon in low grade metamorphic terrains.