Mantle heterogeneity beneath the Cenozoic volcanic provinces of central Victoria inferred from trace-element and Sr, Nd, Pb and Hf isotope data

Abstract

Volcanic rocks in central Victoria play a key role in unravelling magmatic relationships within an extensive zone of Late Mesozoic to Quaternary volcanism in southeastern Australia. In this study, new bulk-rock major, trace-element and Sr, Nd, Hf and Pb isotope data are presented for 20 samples of Late Cenozoic mafic volcanics from the central Victorian region. Mineral chemistry and bulk-rock analyses confirm that all samples can be treated as liquid compositions and that some were probably primary magmas. On the basis of incompatible trace-element and isotope ratios, processes such as magma mixing and crustal assimilation can be largely ruled out, and significant mantle-source heterogeneity beneath the area is proposed. If this heterogeneity extends to other regions of the post-10 Ma Victorian volcanics, the influence of the Mortlake Discontinuity and level of involvement from the lithospheric mantle in the genesis of these rocks, may need to be re-evaluated. Finally, it is argued that there is no clear evidence to suggest the New South Wales leucitite suite extends into Victoria; rather the Cosgrove leucitite flow represents part of the post-10 Ma Victorian volcanics.