A new precise date for the Tolmie Igneous Complex in northeastern Victoria

Abstract

Among the Devonian igneous complexes of central and northeastern Victoria, the S-type Tolmie Igneous Complex (TIC) occupies a unique spatial position, astride the Governor Fault and the Mount Wellington Greenstone Belt, a probable terrane boundary, and also bridges the boundary between the Strathbogie and Tabberabbera granite provinces. This stitching position makes it important to determine the nature of the crustal source involved in its genesis. Here we present a new U–Pb zircon age for a sample of the Toombullup Ignimbrite, which forms a large part of the TIC. This age (377 ± 2 Ma) allows us to comment far more reliably on the initial Sr and Nd isotope ratios of the TIC magmas. The corrected data (initial ⁸⁷Sr/⁸⁶Sr = 0.70804 to 0.71085, εNd_t = –0.08 to 0.44) confirm a heterogeneous sediment-dominated protolith, but unlike those of other S-type granitic and felsic volcanic rocks of the region, one composed of materials that had not long previously been extracted from the mantle. This finding has importance in the context of future models for the geological and tectonic development of this part of southeastern Australia.

KEY WORDS:

• U–Pb dating
• zircon
• Late Devonian
• Tolmie Igneous Complex
• Toombullup Ignimbrite
• rhyolite
• Victoria
• Australia

ACKNOWLEDGEMENTS

JC acknowledges The University of Stellenbosch, Faculty of Science travel funds and the South African NRF Incentive Funds for Rated Researchers, both of which facilitated travel to Australia and Austria (as part of wider research visits) and also paid for the U–Pb dating. David Schiller (University of Salzburg) is thanked for assistance in preparation of the zircon mounts. The constructive comments and suggestions of two anonymous reviewers were much appreciated.

SUPPLEMENTARY PAPERS

Table A1 LA-SF-ICP-MS U–Pb dating methodology CAF, Stellenbosch University.

Table A2 U–Pb age dating results for reference materials Plešovice and M127 obtained during this study by LA-SF-ICP-MS.

Table A3 U–Pb age dating results for sample Toom-1 obtained by LA-SF-ICP-MS.