The petrology of high pressure xenoliths and associated Cenozoic basalts from Northeastern Tasmania

Abstract

Abundant mantle xenoliths are found in widespread undersaturated Cenozoic basaltic rocks in Northeastern Tasmania and comprise lavas, dykes, plugs and diatremes. The basanites and nephelinites, include primitive magmas (11–14 wt.% MgO) with OIB-like geochemical features. Trace element and Pb– and Sr–Nd isotope data suggest that they were generated by mixing of melts derived from low degree (< 5%) melting of both garnet- (∼ 90%) and spinel lherzolite (∼ 10%) facies mantle sources with HIMU and EMII characteristics. The associated xenolith suite consists mainly of spinel lherzolite and rare spinel pyroxenite with predominantly granoblastic textures. Calculated oxygen fugacities indicate equilibration of the xenoliths at 0.81 to 2.65 log units below the fayalite–magnetite–quartz (FMQ) buffer. Mantle xenolith equilibration temperatures range from 890–1050 ± 50 °C at weakly constrained pressures between 0.8 and 11.5 GPa. A hot xenolith's geotherm is indicated and attributed to tectonothermal events associated with the break-up of Gondwanaland and/or the opening of the Tasman Sea.