Trace-element correlation of Carboniferous ignimbrites, southern New England Orogen, Australia, by factor and discrimination analysis*

Abstract

Factor and discrimination analysis of trace-element abundances in Carboniferous (Visean to possibly Westphalian) ignimbrites and minor lavas from a forearc basin succession of the southern New England Orogen, are examined to test the validity of proposed stratigraphic correlations between ignimbrites in three adjacent thrust sheets in the Rocky Creek region of the northern Tamworth Belt, northwestern New South Wales. Analyses of the ignimbrites within individual fault blocks indicate that each has a signature that can be distinguished with a high level of statistical confidence from those of the other ignimbrites. Elements that are generally the most distinctive are Nb, Y, Zr and V. A comparison of units from the Boomi and Kathrose blocks and the southern part of the Rocky Creek Syncline indicates that an ignimbrite in the lower Clifden Formation, previously informally named the ‘Eastons Arm Rhyolite Member’, belongs to the Peri Rhyolite Member. The Peri Rhyolite Member marks the base of the Kiaman reversal. In the Kathrose block, an ignimbrite, previously mapped as Peri Rhyolite Member, is removed from the latter unit and assigned to unnamed dacitic ignimbrite Cl6. In terms of trace-element content, all of the sampled ignimbrites from the Rocky Creek Conglomerate have individual signatures and are unlikely to belong to the same unit. These include: the Plagyan Ignimbrite Member from the Boomi block, Rocky Creek Syncline and southern Nandewar region, the Mt Hook Rhyolite, Darthula Rhyodacite and Pound Rock Rhyodacite Members from the Darthula block, and unnamed red ignimbrite members (including R8 and R12) from the southern part of the Maules Creek Anticline. The Wanganui Andesite Member and unnamed dacitic ignimbrite Cl13 in the lowermost Clifden Formation, and the Ermelo Pyroclastics are distinctive in terms of trace-element signatures. The Wanganui Andesite Member is the youngest, normally polarised ignimbrite beneath the base of the Kiaman reversal.

Keywords:

• Carboniferous
• discrimination analysis
• factor analysis
• ignimbrite
• New England Orogen
• trace elements

ACKNOWLEDGEMENTS

Field work and laboratory analyses were financed by ARC Large Grant A39533029 to J. Roberts. R. Offler kindly reviewed the manuscript and offered useful advice. G. Caprarelli and M. E. Perkins are thanked for their critical review that significantly modified the early version of this paper. and G Caprarelli is also thanked for kindly providing detailed comments and suggestions on the final version of this paper.

Notes

*Appendix 1 [indicated by an asterisk (*) in the text and listed at the end of the paper] is a Supplementary Paper; 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).

Additional information

Notes on contributors

X Wang

*Appendix 1 [indicated by an asterisk (*) in the text and listed at the end of the paper] is a Supplementary Paper; 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).