Growth and evolution of NE Australian continental crust interpreted from complex melting-hybridization histories of northern Queensland granulite xenoliths

Abstract

During active tectonic processes the lower continental crust plays a major role in the development and evolution of the overlying crustal column. The lower crust is heavily influenced by influx of mafic magma from the mantle and extraction of more felsic magma to the shallower crust. Evidence of such interactions are preserved in residues after melting that are rarely brought to the surface as suites of granulite-facies, lower-crustal xenoliths.Here we investigate the petrology, geochemistry and petrochronology of a set of granulite xenoliths from the Pleistocene Hill 32 volcanic cone that has intruded through the geologically complex Georgetown Inlier, north Queensland, which has experienced multiple magmatic-tectonic events since the Paleoproterozoic. Petrochronology constrains the temperature in the Pleistocene lower crust to above the closure temperature of U-Pb in apatite (350–550°C) and rutile (∼400–500°C), which define an eruption age for Hill 32 at 1.6±0.1 Ma, and below the closure temperature of titanite (up to 800°C), which gives an age of 219±4 Ma. The samples contain petrographic and petrochronological evidence of recrystallisation and melt extraction at >900°C, conditions able to reset U-Pb systematics in zircon. The zircons give U-Pb ages consistent with the timing of the Permo-Triassic New England orogeny, indicating zircon recrystallisation/regrowth coeval with major felsic magmatism in the upper crust at that time. U-Pb in zircon records minor preservation of earlier events. While, εHf indicates mixing between Proterozoic to Archean precursor crust with juvenile mantle-derived magma, which provided both heat and radiogenic Hf. The samples are interpreted to be the product of melt-precursor rock hybridisation followed by melt extraction in the lower crust that represents a critical and rarely observed component of crust formation processes.