The petrogenesis and platinum-group element geochemistry of the Newer Volcanic Province, Victoria, Australia

The Newer Volcanic Province (NVP) of Victoria is an extensive lava field constituting the largest contiguous area of Late Tertiary to Quaternary basalt in Australia. New major, trace, rare-earth, and platinum-group element data are presented for the NVP, and these are collectively used to determine its petrogenetic evolution and provide mantle source constraints. P rocks vary widely in chemical composition and can be categorized into three distinct basalt suites: (1) a tholeiitic plains suite; (2) a transitional plains suite; and (3) an alkalic cones suite. Geochemical data are consistent with derivation of the transitional and alkalic suites by decreasing degrees of partial melting of a similar garnet-bearing mantle source material. There appears to be no simple cogenetic relationship between these basalt suites and the tholeiitic suite, although the presence of garnet in its source material is also suspected given the compatible behaviour of Y during partial melting. E geochemistry of the NVP basalts indicate that all magma suites underwent S saturation prior to eruption. The very low Pd abundances in some basalt samples closely approximate the predicted Pd concentration in a silicate melt that has fully equilibrated with a fractionated immiscible sulfide melt. This observation allows a determination and comparison of the relative sulfide melt/silicate melt Nernst partition coefficients (D) for Pd and Pt, and it is deduced that DPt may be up to a magnitude less than DPd. Using this information, it is postulated that the weaker chalcophile behaviour exhibited by Pt relative to Pd will be more strongly emphasized when sulfide precipitation in a magma is low compared to when it is higher. Conditions of low sulfide precipitation may occur in a magma if the S concentration cannot be maintained at or above its S capacity during magmatic evolution. , and Au appear to exhibit weaker chalcophile behaviour in the transitional magmas compared to the tholeiitic magmas, and this is believed to reflect relative differences in the amount of sulfide precipitation within the magmas. It is argued that this difference is a function of the mantle source composition, the transitional suite being derived from a source material with a lower relative S concentration. This suggests that the transitional basalt suite (and by inference, the alkalic suite) is probably derived from a refractory mantle source material, whereas the tholeiitic basalt suite evolved from a non-melt-depleted mantle source material. P rocks exhibit a typical “within-plate” basalt signature with enrichments in LILE, LREE, and HFSE relative to MORB. The NVP basalt suites are best interpreted as the partial melting product of different compositional components of: (1) a previously subducted, eclogitized oceanic slab, or (2) a basaltic melt-enriched lithospheric upper mantle.