Belmore Volcanic Province, northeastern New South Wales, and some implications for plume variations along Cenozoic migratory trails *

Tables 7−12 (indicated by an asterisk in the text and listed at the end of the paper) are Supplementary Papers; copies may be obtained from the Geological Society of Australia's website (www.gsa.org.au) or from the National Library of Australia's Pandora archive (http://nla.gov.au/nla.arc-25194).

Abstract

The little known Belmore Volcanic Province, near Baryugil, northeast New South Wales, is a half-eroded structure, 21 – 19.5 Ma in age. The remnants occupy 140 km² and suggest an original volcanic field ∼14 km³ in volume. The eruptive sequence, fed by dykes, plugs and compound centres ranges up to 350 m thick. Sodic, metaluminous trachytes predominate and include ne and qtz normative types, with Na₂O/K₂O 1.0 – 1.3 and mol. Al₂O₃/(Na₂O + K₂O) 1.4 – 1.5. Minor evolved basalts (mugearite and ne hawaiite) and low Si-rhyolites form <5% of the province. The trachytes can be derived by fractionation modelling from parental mugearite, through crystallisation of limited olivine, moderate amounts of clinopyroxene, plagioclase, anorthoclase, and minor spinel, ilmenite and apatite. More extensive feldspathic fractionation with the addition of sanidine can produce the rhyolites. Ne hawaiite forms the last eruptive stage of the volcano. Some trachytes include crustal feldspar ‘xenocrysts’ and have high ⁸⁶Sr/⁸⁷Sr (0.7126) suggesting significant crustal assimilation. The Belmore volcanic complex is unusual among eastern Australian felsic-bearing provinces in lacking prominent basalts, but petrologically resembles the smaller Mt Macedon–Trentham felsic province in central Victoria. Belmore illustrates the extreme variations that occur in eastern Australian plume lines, both in volume and petrological products (14 km³ alkaline melts, compared to 300 km³ tholeiitic derivatives at Ebor–Dorrigo and 3000 km³ alkaline, tholeiitic and silicic derivatives at Tweed volcano). Isotopic trends (Sr, Nd, Pb) suggest Belmore basaltic melts may be linked to similar asthenospheric sources, as in the neighbouring Tweed and Ebor–Dorrigo fields. The Belmore plume source extended into the shallow asthenosphere, judging from present southeast Australian plume seismic profiles.

key words:

• alkaline magmatism
• assimilation
• Belmore Volcanic Province
• fractional crystallisation
• plume volcanism
• trachytes

Acknowledgements

We would like to thank Gayle Webb (Australian Museum, Sydney) for help with some of the fieldwork. Thin-sections of samples were made through Rod Hungerford (University of Technology, Sydney) and preparation of the manuscript was assisted by Sue Folwell, Roger Springthorpe, Natasha Galea, Jasmine Crea, and Jack Tarlinton (Australian Museum). Funding for the project was provided by the Australian Museum Trust. The manuscript was read by Larry Barron. Constructive reviews of the manuscript were made by Ian Nicholls and Paul Carr.

Notes

Tables 7−12 (indicated by an asterisk in the text and listed at the end of the paper) are Supplementary Papers; copies may be obtained from the Geological Society of Australia's website (www.gsa.org.au) or from the National Library of Australia's Pandora archive (http://nla.gov.au/nla.arc-25194).