Geological Society of London
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Gondwana margin evolution from zircon REE, O and Hf signatures of Western Province gneisses, Zealandia

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journal contribution
posted on 2016-06-21, 11:39 authored by Joe Hiess, Keewook Yi, Jon Woodhead, Trevor Ireland, Mark Rattenbury

U–Th–Pb dated zircons from Western Province paragneisses and orthogneisses were analysed for rare earth element (REE) concentrations, as well as oxygen and hafnium isotopic compositions. Experiments performed in situ using a sensitive high-resolution ion microprobe (SHRIMP) and laser ablation multicollection inductively coupled plasma mass spectrometer (LA-MC-ICPMS) allow better understanding of crustal growth on the Zealandia margin of Gondwana from the micron scale. Paragneiss zircons were probably derived from similar sources to those that supplied the regional Ordovician Greenland Group and correlative southern Australian and Antarctic meta-sedimentary rocks. Detrital zircon grains record variable REE patterns relating to magmatic and metamorphic crystallization processes operating prior to and following Ordovician deposition. δ18O and εHf(T) values trace major phases of juvenile crust formation and subsequent reworking in provenance sources, signifying an increase in the recycling of compositionally diverse, evolved crustal materials through time. Orthogneiss zircons relate to two episodes of magmatism that record similar REE concentration patterns. Devonian zircons have elevated δ18O and un-radiogenic εHf(T); Cretaceous zircons record more primitive δ18O and radiogenic εHf(T). Both orthogneiss suites require thorough mixing of mantle-derived magmas with a component of Greenland Group rocks. The relative proportion of this crustal contamination is c. 20–50% for the Devonian orthogneisses and c. 10–40% for the Cretaceous orthogneisses. Orthogneiss protolith materials were largely hybridized prior to and during zircon crystallization, suggesting that plutonic assembly occurred over restricted structural levels. These results demonstrate the ability of zircon to retain detailed petrogenetic information through amphibolite-facies metamorphism with excellent fidelity.