Mid-Jurassic volcanism at Bokhara River and insights into metasomatism in the lithospheric mantle of the Thomson Orogen, eastern Australia

Abstract

Eastern Australia is covered by extensive, thick regolith, which obfuscates much of its basement geology, making geological sampling difficult. The Bokhara River diatremes erupted through the Thomson Orogen in eastern Australia and are covered by ∼300 m of Cretaceous regolith cover. Hitherto, these diatremes have only been characterised by magnetic anomaly surveys and private exploration drilling. The melts were assumed to be leucititic, owing to their proximity to Miocene leucitite centres to the south, while their position along the Cosgrove hotspot track led to the assumption that the diatremes erupted at ca 20 Ma. However, whole-rock chemistry shows that the diatremes are basanites and, according to 40Ar/39Ar dating, erupted during the mid-Jurassic (ca 180 Ma; estimated by reproducible slightly discordant ages), substantially older than the assumed age and coincident with widespread Mesozoic flood basalts across Gondwana. The basanites entrained abundant mantle material during accent, which contaminated the basanite with ∼12.8 wt% xenocrystic material. The xenoliths are all fertile spinel lherzolites with Mg# 87–89, CaO 2.64–5.90 wt% and Al2O3 2.87–3.83 wt% that have been cryptically metasomatised by a mafic silicate melt, which resulted in chromatographic rare-earth element patterns. This demonstrates that mantle metasomatism occurred during the Mesozoic in this part of eastern Australia, revealing another expression of intraplate volcanism prior to Australia rifting away from Gondwana. The identification and characterisation of this Jurassic volcanism hosting evidence for mantle metasomatism suggest that intraplate volcanism and mantle metasomatism are synonymous within orogenic eastern Australia, and not restricted to the Cenozoic.