Strontium isotopic and trace element heterogeneity in the plains basalts of the Newer Volcanic province, Victoria, Australia

The plains sub-province of the Newer Volcanics of Victoria Australia is a lava plain, ranging in age from 4.5 Ma to < 10 ka and covering an area of 15000 km(2), punctuated by generally younger (< 10-300 ka) cinder cones, lava shields, and maars. Analyses of over four hundred and fifty lavas for major and trace element and strontium isotopic composition show that the plains basalt suite is dominated by tholeiitic and transitional basalts with alkaclic rock types and basaltic icelandites being less common. There is a continuous transition from alkalic types through to quartz tholeiites and basaltic icelandites. SiO2 contents are positively correlated with Sr-87/Sr-86 and negatively correlated with K2O contents. Abundances of incompatible elements such as K, P, Ti, Sr, and Nb are linearly correlated. Trace element and isotopic patterns indicate that a component with a crustal geochemical signature (high Sr-87/Sr-86, high K/Nb, and high Pb/Ce) was involved in the generation and/or evolution of some basaltic icelandites. The possibility th at the crustal geochemical signature reflects assimilation/crystal fractionation processes cannot be entirely precluded, but it is more likely that isotopic and trace element variation for the more magnesian lavas of the sub-province, including basaltic icelandites with relatively high magnesium numbers, reflects heterogeneity in the mantle sources. Projected onto an east-west profile through the sub-province, the strontium isotopic data reveal a north-south boundary (the Mortlake Discontinuity) separating eastern and western sectors with different mean strontium isotopic ratios. The basalts of the eastern sector show a higher mean Sr-87/Sr-86 ratio (0.7047) and the ratios are more variable (0.7037-0.7058) than is the case in the western sector (mean is 0.7042 and all but one sample in the range 0.7037-0.7046). The Mortlake discontinuity coincides with a major tectonic boundary separating the two principal Palaeozoic mobile belts of southeastern Australia. It is proposed that the tectonic boundary extends into the subcontinental lithosphere and that the isotopic and geochemical differences observed in the basalts of eastern vs. the western sectors arise because of differences in the geochemistry of heterogeneous lithospheric mantle on either side of this boundary. Strontium isotopic analyses in conjunction with other geochemical information, geomorphological and petrographic observations, and available geochronological information have also been used to define isotopic domains in the sub-province ranging in size from a few up to several hundred square kilometres. The isotopic domains define individual flows or groups of flows representing individual magma batches and it is postulated that the domains in part reflect small scale geochemical heterogeneity in the lithospheric mantle. Copyright (C) 1997 Elsevier Science Ltd