The Geochemistry and Experimental Petrology of Sodic Alkaline Basalts from Oatlands, Tasmania

Sodic basalts of Oligocene-Early Micene age occur within anEarly Tertiary graben in the Oatlands district of Tasmania.They include olivine tholeiites, alkali olivine basalts, basanites,transitional nephelinites, nepheline hawaiites, and nephelinemugearites. They have compositional characteristics in commonwith 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 consistentwith their derivation by polybaric partial melting of a garnetlherzolite source enriched in P₂O₅, light rare earths, Nb, andother incompatible trace elements. Experimental data for a primitivenepheline basanite from the Oatlands district indicate thatconcentrations of H₂O+CO₂ need not have been more than 6?5 wt.%for the original basanite magma to have derived from an amphibole-bearinggarnet lherzolite source. In the case of more SiO₂-undersaturatedolivine melilitites from the neighbouring Central Plateau, theexperimental evidence is consistent with either higher concentrationsof H₂O+CO₂ (approaching 14 wt.%), or higher pressures of origin(>35 kb). Petrographic and geochemical evidence suggeststhat the latter is the more probable of the alternatives.