A geochemical and U-Pb isotope study of lower crustal xenoliths from the Monaro Volcanic Province, NSW: implications for the deep crustal evolution of eastern Australia

Granulite xenoliths are fragments of the Earth’s lower continental crust. These samples are transported to the Earth’s surface by younger, and explosive volcanic pipes, providing a unique insight into the composition of rock material at inaccessible crustal depths. This research builds on a detailed petrological study of granulite xenoliths collected from alkali basalt pipes in the Monaro Volcanic Province and breccia pipes near the town of Delegate, both within New South Wales, Australia. Geochemical signatures indicate that these samples are basaltic cumulates, formed by the accumulation of plagioclase during basaltic underplating that subsequently underwent granulite facies metamorphism. Whole rock geochemistry further indicates a magmatic provenance similar to arc basalts, suggesting that a subduction component has influenced the composition of the lower crust beneath eastern Australia. Samples are also depleted in heat producing elements (Th, U), characteristic of a residual lower crust, with some showing a strong REE enrichment and negative Eu anomaly. These compositions are suggestive of fractionated melt, perhaps complementary to the depleted granulites.
Ion microprobe analysis of zircon from two Monaro Volcanic Province samples (MVP10436 and MVP10437), yields a U-Pb concordia age of 419.4 ± 6.6 Ma and 402.5 ± 6.3 Ma respectively. The 402.5 ± 6.3 Ma age is derived from a loose array along concordia and its significance remains ambiguous. The 419.4 ± 6.6 Ma age is defined by a tighter cluster of concordant data and is shown to be contemporaneous with the emplacement of the Late Silurian I-type granites within the eastern Lachlan Fold Belt. This age relationship provides new evidence for a genetic link between the mafic and granulitic lower crust, and the more felsic upper crustal granites in eastern Australia, signifying that arc magmatism and fractionation magma differentiation are key processes in the growth and evolution of the continental crust during the Palaeozoic.