THE PETROGENESIS AND PLATINUM-GROUP ELEMENT GEOCHEMISTRY OF THE NEWERVOLCANIC PROVINCE, VICTORIA, AUSTRALIA

The Newer Volcanic Province (NVP) of Victoria is an extensive lava fie ld constituting the largest contiguous area of Late Tertiary to Quater nary basalt in Australia. New major, trace, rare-earth, and platinum-g roup element data are presented for the NVP, and these are collectivel y used to determine its petrogenetic evolution and provide mantle sour ce constraints. The NVP 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 con es suite, Geochemical data are consistent with derivation of the trans itional 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 t holeiitic suite, although the presence of garnet in its source materia l is also suspected given the compatible behaviour of Y during partial melting. The PGE geochemistry of the NVP basalts indicate that all ma gma suites underwent S saturation prior to eruption. The very low Pd a bundances in some basalt samples closely approximate the predicted Pd concentration in a silicate melt that has fully equilibrated with a fr actionated immiscible sulfide melt. This observation allows a determin ation and comparison of the relative sulfide melt/silicate melt Nernst partition coefficients (D) for Pd and Pt, and it is deduced that D-Pt may be up to a magnitude less than D-Pd. Using this information, it i s postulated that the weaker chalcophile behaviour exhibited by Pt rel ative to Pd will be more strongly emphasized when sulfide precipitatio n in a magma is low compared to when it is higher, Conditions of low s ulfide precipitation map occur in a magma if the S concentration canno t be maintained at or above its S capacity during magmatic evolution. Pt, Ni, and Au appear to exhibit weaker chalcophile behaviour in the t ransitional magmas compared to the tholeiitic magmas, and this is beli eved to reflect relative differences in the amount of sulfide precipit ation within the magmas, It is argued that this difference is a functi on of the mantle source composition, the transitional suite being deri ved from a source material with a lower relative S concentration. This suggests that the transitional basalt suite (and by inference, the al kalic suite) is probably derived from a refractory mantle source mater ial, whereas the tholeiitic basalt suite evolved from a non-melt-deple ted mantle source material, All NVP rocks exhibit a typical ''within-p late'' basalt signature with enrichments in LILE, LREE, and HFSE relat ive to MORB. The NVP basalt suites are best interpreted as the partial melting product of different compositional components of: (1)a previo usly subducted, eclogitized oceanic slab, or (2) a basaltic melt-enric hed lithiospheric upper mantle.