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 nephclinitic matrix. Assemblages include mantle metaperidotites, garnet-metawebsterites and rarer garnet-metadinopyroxcnites, garnet-metawehrlires, metawcbstetites and crustal two-pyroxene granulites. Most metapyroxenites 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 860-1075°C (for the whole suite) and temperature-pressure estimates for the garnet metawebsterites from 1055-1070°C and 1.2-1.4 GPa. This gives a Miocene mantle geotherm gradient at least 80-l 30°C higher than the Southeast Australian (SEA) western Victorian geotherms. However, considerations of Moho depths from new seismic surveys below Table Cape (�32 km) suggest that the indicated geotherm is more strongly pcrrurbed in its lower levels than at the mantle-crust transition. This localised perturbation is attributed to adjacent magma chamber activity in the mantle (Boat Harbour basalts) just prior to Table Cape vent activity. Tasmanian Miocene geotherms (Table Cape, Bow Hill) achieve relatively high gradients and reinforce sugges tions of local variation in East Australian geothermal gradients. They illustrate the potential complexities in comparing xenolith-derived geotherms from diff erent areas in general, both from thermometer/barometer selection and from associated magmatic heat inputs.

Contrasts in gem corundum characteristics, eastern Australian basaltic fields: trace elements, fluid/melt inclusions and oxygen isotopes

“…These temperatures approach those expected for crust-mantle boundary conditions here (O'Neil et al, 2003), based on the known Moho depth (>37 km) and its likely intersection with southeastern Australian palaeogeotherms (>930ºC; Sutherland et al, 2005). Barrington rubies, however, do not need mantle depths to account for their O isotope ranges, as ultramafic bodies thrust into the fold belt here provide a suitable higher-level alternative.…”

“…The mantle lies at~30 km depth under Weldborough (Rawlinson et al, 2001), which equates to~0.9À1.0 GPa and 900À950ºC for the Tasmanian palaeogeotherm (Sutherland et al, 2005). The Weldborough basalts include mantlederived spinel lherzolite xenoliths, which enhance the potential for corundum transport from such levels.…”

Corundum (sapphire) and zircon relationships, Lava Plains gem fields, NE Australia: Integrated mineralogy, geochemistry, age determination, genesis and geographical typing

“…Lava Plains aluminosilicate-sapphire phase relationships for meta-xenolith D53799, across a metastable corundum-quartz-sillimanite stability field (c-q), after Harlov et al (2008). The northeast Queensland Moho and East Australian geotherm profiles (EMAC, SEA, TAS) represent data from Pearson et al (1991), O'Reilly et al (1997 and Sutherland et al (2005). A general retrogressive trend across the stability field (thick dashed arrow) and a potential cooling path from maximum PT for corundum-quartz-stability towards the SEA geotherm (line joining filled circles) are indicated.…”

Unusual pegmatoid crystallisations in a nephelinite plug, near Round Lagoon, eastern Central Plateau, Tasmania

“…The lack of garnet or its replacements in Round Lagoon inclusions suggests the magma stalled and fractionated at mantle levels shallower than the spinel-garnet lherzolite transition, i.e., <1.7-1.8 GPa and 1100-1200°C for Tasmanian Miocene geotherms (Sutherland et al 2005). The W()-n"'ilrnXf>flf> temperatures for Round Lagoon spinel meta-Ihpt"'7r'\11TP reach 950°C (table Based on the Tasmaniañ ""''-J'''~~''''''~~~.~, this to fractionate at around 1.0-1.1 GPa at mantle depths shallower than SI )JII~II-V~.Jrll~1 pyroxenite transition.…”