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.

KEY POINTS

1. Diatremes at Bokhara River are basanitic and erupted during the mid-Jurassic; they are not leucitites of the Cosgrove Track.

2. Entrained ultramafic xenoliths are fertile mantle spinel lherzolites, which experienced cryptic metasomatism by a silicate melt.

3. This is evidence for the oldest mantle metasomatism event in the eastern Australian orogenic lithospheric mantle.

Keywords:

• intraplate volcanism
• Bokhara River
• xenoliths
• Jurassic
• basanite
• mantle metasomatism

Acknowledgements

We would like to thank Isra Ezad for the useful discussion on annealed kelyphite. We would also like to thank Stan Szczepanski for his expertise in the AuScope-supported ⁴⁰Ar/³⁹Ar dating laboratory at The University of Melbourne. We also acknowledge the facilities of the Centre for Advanced Microscopy at the Australian National University, Canberra.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Data availability statement

All data reported in this manuscript are reported in Shea et al. (2024), in the supplemental data and at https://zenodo.org/records/10450582.

Additional information

Funding

This manuscript forms a chapter in the PhD thesis of JJS, which was funded by ARC grant FL180100134. SFF and AWL are funded by ARC grant FL180100134. AWL is currently funded by a Deutsche Akademische Austauschdienst (German Research Exchange Service) Research Grant 57507869. We would like the thank AuScope for funding analysis for Ar–Ar ages though the National Argon Map initiative led by ANU.