Table Cape vent xenolith suite, northwest Tasmania: Mineralogy and implications for crust-mantle lithology and Miocene geotherms in Tasmania

Abstract

The Miocene Table Cape vent erupted a diverse mantle-crust xenolith suite within its fractionated nephelinitic matrix. Assemblages include mantle metaperidotites, garnet-metawebsterites and rarer garnet-metadinopyroxenitcs, garnct-mctawehrlites, metawebsterites and crusta] two-pyroxene granulites. Most metapyroxenires and granulites represent the Ti-Al-bearing augite suite and their bulk geochemistry indicates transitional olivine basalt magmatic affinities. Metasomatised, hydrous lithologies are only rarely present. Co-existing pyroxenes in the xenoliths provide re-equilibration temperature estimates from 860-1 0750C (for the whole suite) and temperature-pressure estimates for the garnet metawebsterires from 1055-1 070°C and 1.2-1.4 CPa, This gives a Miocene mantle geotherm gradient at least 80--130°C higher than the Southeast Australian (SEA) western Victorian geotherms. However, considerations of Moho from new seismic surveys below Table Cape (~.32 km) suggest that the indicated georherm is more strongly perturbed in its lower levels than at the mantle-crust transition. This localised perturbation is attributed to magma chamber in the mantle (Boat Harbour just prior to Table Cape vent activity. Tasmanian Miocene geotherms Table Cape, Bow Hill) achieve relatively high gradients and reinforce suggestions of local variation in East Australian geothermal gradients, They illustrate the potential complexities in com paring xenolith• derived geotherms from different areas in general, both from thermometer/barometer selection and from associated magmatic heat inputs.