U-Pb-Hf-REE-Ti zircon and REE garnet geochemistry of the Cambrian Attunga eclogite, New England Orogen, Australia: Implications for continental growth along eastern Gondwana

Abstract

The timing and location of eclogite metamorphism is central to understanding subduction events responsible for the assembly of eastern Gondwana. The Attunga eclogite is one of only six eclogites in Australia and occurs as small blocks within a schistose serpentinite mélange known as the Weraerai terrane, along the Peel Fault of the southern New England Orogen. Our zircon data reveal the presence of high Th/U oscillatory zoned magmatic zircon with a weighted mean 206Pb/238U age of 534 ± 14 Ma and recrystallized metamorphic domains with an age of 490 ± 14 Ma. The latter have lower Th/U ratios, mostly no Eu anomalies and heavy rare earth element (HREE)‐depleted patterns. Garnet rims demonstrate that the final stages of garnet growth occurred in a HREE‐depleted environment, due to coeval formation with metamorphic zircon. Direct application of the Ti‐in‐zircon thermometer to metamorphic zircon yields temperatures of 770–610°C. Hf isotopic analyses of the zircons have an average εHf(t) of +13, indicating a juvenile crustal signature. We interpret the Attunga eclogite to be an indicator of Late Cambrian subduction beneath an oceanic suprasubduction zone prior to accretion against eastern Gondwanan in the latest Devonian. Phillips et al. (2015) suggest two metamorphic age populations within the Attunga eclogite, based on U‐Pb zircon and 40Ar/39Ar phengite data. These are ~515 Ma and ~480 Ma. We confirm these data, but our zircon trace element chemistry data indicate that the Early Cambrian age (530 Ma) represents igneous protolith formation rather than eclogite metamorphism. Show more