Origin and Evolution of Rhyolitic Intraplate Magmatism: A Case Study from the Peak Ranges Volcanics, Central Queensland, Australia

Abstract The Peak Ranges Volcanics represent one of the most extensive and compositionally diverse Cenozoic volcanic centres of eastern Australia, and hence can provide crucial insights into the evolution of continental intraplate magmatic systems. Trachytic to rhyolitic volcanic suites are well preserved as a series of eroded plugs, domes and lava flow stacks, and can be divided into three zones based on spatial and temporal associations, mineralogy and geochemistry. The Southern Volcanic Zone features peralkaline silicic volcanic rocks with highly enriched incompatible element (e.g. REE, Zr, Nb, Ta) contents and isotopic compositions (ɛNdi ~ +3 to +4; 87Sr/86Sri ~ 0.704 to 0.7045) that overlap with the mafic volcanic rocks (ɛNdi ~ +3.5 to +6; 87Sr/86Sri ~ 0.703 to 0.7045) that dominate Peak Ranges. The Northern Volcanic Zone largely comprises peraluminous rhyolites, with relatively unradiogenic Nd isotope (ɛNdi ~ −1 to +1) and radiogenic 87Sr/86Sri (~0.7045 to 0.7065) compositions. The Central Volcanic Zone has chemical and isotopic affinities that are intermediate between the Northern and Southern Volcanic Zones. We interpret the rhyolites of the Northern Volcanic Zone (and most of the Central Volcanic Zone) to represent erupted products of highly fractionated mantle-derived magmas that had experienced approximately 10 to 20% crustal assimilation at lower crustal depths. This magmatic evolution was favoured by an overall N-S compressional regime at this time (ca. 30 to 32 Ma). Subsequent relaxation of compressional stress by ca. 28 Ma allowed effective tapping of alkaline, mantle-derived magmas to upper crustal chambers, where they underwent extensive crystal fractionation, largely or completely free of crustal contamination, to produce the enriched peralkaline southern rhyolites. Despite the dominant mantle origin for Peak Ranges magmatism, our study highlights the major control that the physical state and structure of the overlying continental lithosphere has on the compositional evolution of silicic volcanic rocks, and in turn their potential to host critical metal mineralization. Lithospheric geodynamics is, therefore, considered integral to understanding continental intraplate magma evolution.