Year

2019

Degree Name

Doctor of Philosophy

Department

School of Earth, Atmospheric and Life Sciences

Abstract

Within the New England Orogen, NSW, Australia, lays a narrow vestige of enigmatic Cambrian-Devonian oceanic Palaeozoic rocks. Speculation on how they came to be located in the midst of a Carboniferious oceanic-continental convergent margin have been examined over the decades. These questions are the subject of this Ph.D. thesis. Two schools of thought dominate how Cambrian – Devonian oceanic material ended up in the middle of the Orogen; 1) the Tasmanides underwent back-arc opening and closing (Accordion tectonics) and, 2) exotic oceanic material was added progressively to the west-dipping convergent margin, in discrete episodes (quantum tectonics). Three published papers presented in this thesis argue the later scenario is most likely. The igneous and metamorphic rocks sampled on average had a low zircon yield, with SHRIMP being employed to determine the zircon U–Pb ages. Zircon Lu–Hf isotopic ratios acquired via LA– ICP–MS provided information on the tectonic setting (juvenile crust or reworked continental crust). A Cambrian protolith age for Weraerai terrane, is supported here by the oldest age presented in the paper, the Attunga eclogite (Weighted mean 206Pb/238U age of 534 ± 14 Ma) and two ages from Barry, one from the gabbro, 504.9 ± 3.5 Ma and the other from amphibole-bearing felsic dykes of 503.2 ± 5.7 Ma. This Cambrian oceanic crust formed in a supra-subduction zone. Zircons from the Attunga eclogite have an average εHf(t) of +13, and those from the Barry gabbros and felsic dykes contained within them have average εHf(t) of + 11.1 and + 11.6 respectively. Eclogite facies metamorphic zircons from the Attunga eclogite have a 206Pb/238U age of 490 ± 14 Ma. Ti-in-zircon provide temperature for zircon formation - 770–610°C. This is conformed by the depletion of heavy rare earth elements in the garnet rims. Trondhjemites from the Gamilaroi terrane, provided Early Devonian zircon U–Pb ages of 413 ± 8.7 Ma, with εHf(t) values of + 5.0 to + 2.9. This range is typical of island arc rocks, and is reinforced by enrichment in LREE and strong negative Nb anomalies. Rhyolites from a small transtensional basin at Echo Hills, have a SHRIMP zircon U–Pb age of 295.6 ± 4.6 Ma. Whole rock geochemistry indicates these are peraluminous felsic melts enriched in LREE and incompatible elements. Rock units located along the Peel Fault in the southern New England Orogen, formed remotely from the margin of Gondwana.

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