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Precambrian Geology of the North Mawson Escarpment Area, Prince Charles Mountains, Antarctica

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Precambrian Geology of the North Mawson Escarpment Area, Prince Charles Mountains, Antarctica Precambrian geology of the North Mawson Escarpment area, Prince Charles Mountains, Antarctica Adrian F. Corvino Submitted in total fulfilment of the requirements of the degree of Doctor of Philosophy May 2009 School of Earth Sciences The University of Melbourne ii Declaration This is to certify that (i) the thesis comprises only my original work towards the PhD; (ii) due acknowledgement has been made in the text to all other material used, and; (iii) the thesis is less than 100,000 words in length, exclusive of tables, maps, bibliogra- phies and appendices. Signed, Adrian F. Corvino iii iv “In the exploration of a continent the mountainous areas are generally the last strongholds of mystery...” — Eric Shipton v vi Abstract Part of East Antarctica’s shield collided with Greater India during assembly of the Rodinia supercontinent, causing these landmasses to become welded together by about 900 million years ago. The pre-Rodinia continental margin of the Antarctic landmass is represented by the North Mawson Escarpment area of Antarctica’s Prince Charles Mountains. The size of this postulated Antarctic landmass is still unknown, though it might make up a considerably vast segment of the presently subglacial East Antarctic shield. Only with the assembly of Gondwana about 500 million years ago was the entire shield fully consolidated. The above conclusion is supported by the findings presented in this thesis, which is the first detailed examination of the Precambrian geology of the North Mawson Escarpment area incorporating the Lawrence Hills and Clemence Massif. The North Mawson Escarpment, 72◦30’–73◦10’S, 68◦E, consists of grey gneisses of granitic to granodioritic composition, including adamellitic augen-gneiss types, and meta- supracrustal successions; the latter contain multilayered calcsilicate gneiss, metacarbonate, metapelitic gneiss and schist, quartzite and amphibolite. Radiometric dating results pre- sented in this work demonstrate conclusively that the grey gneisses are of Palaeoproterozoic age. These grey gneisses originated from large magmatic bodies that were episodically em- placed and crystallised in the period 2490–2420 Ma. Other granitic protoliths, of lesser volume, were emplaced in the interval 2180–2080 Ma at which time the older types were at least thermally overprinted if not substantially deformed. The exact age and origin of the metasupracrustal rocks is unknown, though their precursors were for the most part laid down after 2450 Ma and before 900 Ma. They are interpreted as having been deposited during periods of tectonic extension operating throughout the Mesoproterozoic in a pas- sive margin setting; that is, on the converging Antarctic landmass prior to its collision with India in Rodinia times. Bordering directly on the northern margin of the escarpment are the low-lying Lawrence Hills, 72◦30’S, 68◦43’E, where Palaeoproterozoic and Late Mesoproterozoic crustal com- ponents are juxtaposed. Further north at Clemence Massif, 72◦12’S, 68◦40’E, only high- vii grade metamorphic rocks of Late Mesoproterozoic age are exposed. Thus, a southward progression from Late Mesoproterozoic to Palaeoproterozoic to Archaean crust is clearly recognised for over 100 km along the study area. Having examined the structure, it is my conclusion that the rock masses were stacked by oblique northward overthrusting motions while they were deforming pervasively in the infrastructure zone of a convergent orogen, i.e. when this part of pre-Rodinia Antarctica collided with India. The older over-riding Palaeoproterozoic basement of the North Mawson Escarpment was at this time exhumed to shallower, but still deep crustal levels, resulting in a Barrovian-type metamorphism that evolved to high–T, low–P conditions. This transitional amphibolite to granulite fa- cies regional metamorphism outlasted most of the ductile deformation with temperatures reaching 750◦C, though potentially greater, under pressure conditions of about 5 kbar. Whereas, the underthrust Late Mesoproterozoic rocks of Clemence Massif were meta- morphosed to even higher granulite facies temperatures followed by an isobaric cooling history. It is also found that the areas dominated by Late Mesoproterozoic crust, i.e. from Lawrence Hills northward, correspond closely to a prominent series of linear NE–SW trending positive magnetic anomalies. Such anomalies do not appear across the North Mawson Escarpment, though these older rocks nevertheless share the same trend in their basement structure. There is little doubt that the trend of the magnetic anomalies is directly related to the deformation that produced the Rayner belt, which is the zone of collision between the aforementioned Indian and Antarctic landmasses. It is my contention that the Late Mesoproterozoic crust and the magnetic anomalies correlate with arc-related crustal additions originally formed in a convergent margin setting leading up to this col- lision; that is, they lie within the intensely deformed orogen interior rather than on the continental basement southward. Structural and metamorphic interference between the Rayner belt and the younger Gondwanan Prydz belt, which is more NW–SE trending, becomes increasingly important southward of the study area. Much work still needs to be done to correctly determine the overprinting relationships between these two high-grade belts, a problem that is most likely to be resolved by more targeted analyses of rocks from where the North and South Mawson Escarpment overlap. viii ix x Acknowledgements First and foremost I thank Prof. Chris Wilson for his patience and help in supervising my research, and for granting me the greatest undertaking of my life to work in Antarctica. Secondly, I thank Steve Boger for sharing the fieldwork and providing endless hours of discussion on the geology of the Prince Charles Mountains, especially the Mawson Es- carpment. As far as I know, he remains the only person who has seen and traversed the entire length of this escarpment by foot! I owe a debt of gratitude to many other people. In no particular order, Glen Phillips and Mark McLean for their mateship, and who shared the journey of Ph.D. research on Antarctic geology; Friedhelm Henjes-Kunst for his collaboration on rock samples from the Lawrence Hills and Rofe Glacier; Ian Fitzsimons for his assistance with the U–Pb zircon dating and accommodating me on visits to Perth; Bill Baxter and Gary Kuehn for their invaluable field guidance; Richard White for his assistance with the computer program Thermocalc; Liaxi Tong and Cameron Quinn for discussions on the geology of the Prydz Bay area, and Jacqueline Halpin on the Stillwell Hills; Daniel Viete for the opportunity to assist with his fieldwork on the classic Barrovian series rocks in Scotland; Mark Quigley for the opportunity to collaborate on his rock samples from Tibet; Gordon Holm and Graham Hutchinson for their help with the thin section preparations and electron microprobe work, and; Richard Stanaway for lending his oblique aerial photos of the Mawson Escarpment. I thank them all for their friendship. I am also grateful to Ed Grew, Wilfred Bauer, Chris Carson, Geoff Clarke, Eugene Mikhalsky and Nigel Kelly for critically reviewing particular chapters published in the course of putting this thesis together. Special thanks to Ron Vernon and Ed Grew for their very generous thesis reviews. Thanks to Mum, Dennis, Kris, Jess and Dad for all their encouragement over the last six years. Lastly and most of all I thank Angela for her love and undying support of my endeavours. This thesis was compiled using the typesetting system LATEX 2ε. All errors are my own. xi xii Contents Abstract......................................... vii Acknowledgements ................................... xi Abbreviations...................................... xxiii Mineralsymbols ....................................xxv I Background Information 1 1 Introduction 3 1.1 Scientific relevance—understanding East Antarctica . ........... 4 1.2 Exploration in the Prince Charles Mountains . ....... 7 1.3 Fieldwork ..................................... 8 1.4 Thesis overview and publications . ..... 8 2 Physical geography 13 3 Tectonic subdivisions of the Prince Charles Mountains 17 II Clemence Massif and Lawrence Hills 21 4 General geology of Clemence Massif 23 4.1 Summary ..................................... 23 4.2 Introduction.................................... 24 4.3 Field relationships and petrography . ...... 27 4.4 Structure ..................................... 31 4.5 U–Pbzircondating................................ 35 4.6 Discussion..................................... 41 4.7 Concludingremarks ............................... 46 xiii 5 Metamorphism of a Clemence Massif metapelite 47 5.1 Summary ..................................... 47 5.2 Introduction.................................... 48 5.3 Petrography and mineral chemistry . ..... 48 5.4 Bulkchemicalcomposition. 52 5.5 Pseudosectionmodelling . 55 5.6 Temperature estimates using geothermometers . ........ 59 5.7 Discussion..................................... 59 6 2.5 and 1.1 billion year old crust in Lawrence Hills 63 6.1 Summary ..................................... 63 6.2 Introduction.................................... 64 6.3 Localgeologicsetting ............................ 65 6.4 Sample descriptions, geochemistry and radiometric dating results . 67 6.5 Evidence for Early Palaeoproterozoic crustal development .......... 80 6.6 Evidence for Late Mesoproterozoic crustal development ........... 84 6.7 Timing of reworking
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