Eclogite and blueschist in the Southern New England Fold Belt: P–T–t conditions and long-lived subduction on the Gondwanan eastern margin Thesis submitted in accordance with the requirements of the University of Adelaide for an Honours Degree in Geology Renée Jade Tamblyn November 2016 Renée Jade Tamblyn Eclogite and blueschist in eastern Gondwana ECLOGITE AND BLUESCHIST IN THE SOUTHERN NEW ENGLAND FOLD BELT: P–T–T CONDITIONS AND LONG-LIVED SUBDUCTION ON THE GONDWANAN EASTERN MARGIN RUNNING TITLE: Eclogite and blueschist in eastern Gondwana ABSTRACT Eclogite and blueschist in the Tasminides represent clear evidence for subduction- related metamorphism on the Gondwanan eastern margin during the Palaeozoic. These eclogites and blueschists are located in the serpentinite-bearing Peel Manning Fault System in the Southern New England Fold Belt (SNEFB) of eastern Australia. U–Pb zircon and Lu–Hf and Sm–Nd multimineral geochronology give ages of ca. 490 Ma for lawsonite-bearing eclogite and ca. 470 Ma for garnet-bearing blueschist at Port Macquarie in the SNEFB, in agreement with Cambro-Ordovician dates for eclogite metamorphism across the eastern Gondwanan margin. In combination with this, Ar–Ar data dates exhumation and cooling in the subduction channel at ca. 460 Ma, suggesting that high-pressure metamorphism at Port Macquarie was continuously active for upwards of 40 Ma. This is supported by mineral equilibria forward modeling, which demonstrates that 24–27 kbar eclogite from Port Macquarie and Pigna Barney in the SNEFB experienced high-pressure but low-temperature retrograde paths, consistent with their residence in the subduction channel. Geochemical and isotopic results suggest that MORB as well as oceanic arc-related material was subducted and metamorphosed in a westwards dipping subduction system on the Australian cratonic margin during the ca. 515–490 Ma Delamerian Orogeny, and subsequently entrapped in the subduction channel during rollback. This rollback resulted in the development of a large backarc system on the upper plate in which the protoliths to the Lachlan Orogen accumulated, as well as on-going blueschist-facies metamorphism in the subduction channel. Ultimately, rollback led to translocation of subduction products and their exhumation over 2000 km eastwards to their current position in the New England orogen. In contrast to this, further south in Tasmania and Antarctica, the subduction of continental material led to 2 Renée Jade Tamblyn Eclogite and blueschist in eastern Gondwana rapid burial and exhumation of eclogite, representing differing styles of Cambro- Ordovician high-pressure metamorphism on the eastern Gondwanan margin. KEYWORDS Eclogite, blueschist, subduction, high-pressure, metamorphism, Gondwana, Southern New England Fold Belt, pseudosection 3 Renée Jade Tamblyn Eclogite and blueschist in eastern Gondwana TABLE OF CONTENTS Abstract.................................................................................................................................................... 2 Keywords ................................................................................................................................................ 3 Table of Contents ................................................................................................................................. 4 List of Figures and tables .................................................................................................................. 6 Introduction ........................................................................................................................................... 7 Geological background ................................................................................................................... 10 The Southern New England Fold Belt .................................................................................... 10 High-pressure localities .............................................................................................................. 13 Analytical methods ........................................................................................................................... 17 Zircon U–Pb Geochronology ...................................................................................................... 17 Lu–Hf and Sm–Nd Geochronology .......................................................................................... 17 Rare earth element mapping in garnet ................................................................................. 17 Whole rock Sm–Nd ........................................................................................................................ 18 Electron Probe Micro Analyses ................................................................................................ 18 Phase Equilibria Forward Modelling ..................................................................................... 18 Phase Abundance Contours ....................................................................................................... 19 Results ................................................................................................................................................... 20 Samples .............................................................................................................................................. 20 Petrography ..................................................................................................................................... 20 Mineral chemistry ......................................................................................................................... 23 Laser ablation ICP–MS maps ..................................................................................................... 25 U–Pb Zircon geochronology ...................................................................................................... 27 Lu–Hf and Sm–Nd geochronology ........................................................................................... 28 Whole rock Sm–Nd isotopes ...................................................................................................... 31 Phase equilibria modelling ........................................................................................................ 31 Discussion ............................................................................................................................................ 39 Timing of high pressure metamorphism in the SNEFB .................................................. 39 Pressure-temperature conditions during metamorphism ............................................ 43 Duration of physical conditions ............................................................................................... 44 Nd and protoliths to the high-P rocks ................................................................................. 48 Tectonic implications and development of the southern Tasminides ...................... 50 4 Renée Jade Tamblyn Eclogite and blueschist in eastern Gondwana Differing styles of high-pressure metamorphism on the eastern Gondwanan margin ................................................................................................................................................... 55 Conclusions ......................................................................................................................................... 56 Acknowledgements .......................................................................................................................... 57 References ........................................................................................................................................... 58 Appendix 1a: Extended geochronology analytical methods ............................................ 63 Appendix 1b: Extended electron probe micro analyses methods ................................. 63 Appendix 1c: Extended phase equilibria forawrd modelling methods ....................... 64 Appendix 2a: Extended electron probe micro analyses .................................................... 64 Appendix 2b: Extended electron probe elemental maps .................................................. 67 Appendix 3: U-Pb geochronology data ..................................................................................... 70 Appendix 4: Whole rock geochemistry .................................................................................... 77 Appendix 5: Geochemical plots and graphs ............................................................................ 78 Appendix 6: Detrital zircon data from Rocky Beach and Town Beach, Port Macquarie ............................................................................................................................................ 79 5 Renée Jade Tamblyn Eclogite and blueschist in eastern Gondwana LIST OF FIGURES AND TABLES Fig. 1: Relative position of Australia, Tasmania and Antarctica on the Gondwanan eastern margin ............................................................................................................................ 8 Fig. 2: Geological map of the Southern New England Fold Belt. ..................................... 11 Fig. 3: Field photographs of high-pressure rocks ................................................................. 14 Fig. 4: Photomicrographs of eclogite and blueschist. ......................................................... 22 Fig. 5: EPMA Elemental