THE GEOCHEMISTRY AND EXPERIMENTAL PETROLOGY OF SODIC ALKALINE BASALTS FROM OATLANDS, TASMANIA

Sodic basalts of Oligocene-Early Miocene age occur within an Early Tertiary graben in the Oatlands district of Tasmania. They include olivine tholeiites, alkali olivine basalts, basanites, transitional nephelinites, nepheline hawaiites, and nepheline mugearites. They have compositional characteristics in common with sodic alkaline basalt suites from other parts of the world. With decreasing SiO2, concentrations of CaO, alkalis, P2O5, and incompatible trace elements increase. Compositional and experimental data for the basalts are consistent with their derivation by polybaric melting of a garnet lherzolite source enriched in P2O5, light rare earths, Nb, and other incompatible trace elements. Experimental data for a primitive nepheline basanite from the Oatlands district indicate that concentrations of H2O + CO2 need not have been more than 6.5 wt. % for the original basanite magma to have derived from an amphibole-bearing garnet lherzolite source. In the case of more SiO2-undersaturated olivine melilitites from the neighbouring Central Plateau, the experimental evidence is consistent with either higher concentrations of H2O + CO2 (approaching 14 wt. %), or higher pressures of origin (> 35 kb). Petrographic and geochemical evidence suggests that the latter is the more probable of the alternatives.