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 i s 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) cin der cones, lava shields, and maars. Analyses of over four hundred and fifty lavas for major and trace element and strontium isotopic composi tion 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 type s 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, T i, Sr, and Nb are linearly correlated. Trace element and isotopic patt erns indicate that a component with a crustal geochemical signature (h igh Sr-87/Sr-86, high K/Nb, and high Pb/Ce) was involved in the genera tion and/or evolution of some basaltic icelandites. The possibility th at the crustal geochemical signature reflects assimilation/crystal fr actionation processes cannot be entirely precluded, but it is more lik ely that isotopic and trace element variation for the more magnesian l avas of the sub-province, including basaltic icelandites with relative ly high magnesium numbers, reflects heterogeneity in the mantle source s. Projected onto an east-west profile through the sub-province, the s trontium isotopic data reveal a north-south boundary (the Mortlake Dis continuity) separating eastern and western sectors with different mean strontium isotopic ratios. The basalts of the eastern sector show a h igher 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 0.704 2 and all but one sample in the range 0.7037-0.7046). The Mortlake dis continuity coincides with a major tectonic boundary separating the two principal Palaeozoic mobile belts of southeastern Australia. It is pr oposed that the tectonic boundary extends into the subcontinental lith osphere and that the isotopic and geochemical differences observed in the basalts of eastern vs. the western sectors arise because of differ ences in the geochemistry of heterogeneous lithospheric mantle on eith er side of this boundary. Strontium isotopic analyses in conjunction w ith other geochemical information, geomorphological and petrographic o bservations, and available geochronological information have also been used to define isotopic domains in the sub-province ranging in size f rom a few up to several hundred square kilometres. The isotopic domain s define individual flows or groups of flows representing individual m agma batches and it is postulated that the domains in part reflect sma ll scale geochemical heterogeneity in the lithospheric mantle. Copyrig ht (C) 1997 Elsevier Science Ltd