WHAT IS THE TECTONIC SETTING OF LONG-LIVED GRENVILLIAN-AGED ULTRAHIGH TEMPERATURE, HIGH GEOTHERMAL GRADIENT METAMORPHISM IN THE MUSGRAVE PROVINCE, CENTRAL AUSTRALIA?

The east-west trending Musgrave Province is a major Grenvillian-aged belt in central Australia. It covers an area of some 120000 km² and is the least studied, and thus least understood, Proterozoic province within central Australia. The Musgrave Province occurs at a ‘triple junction’ between three older and thicker cratonic blocks. Understanding the tectonic (and temporal) evolution of the Musgrave Province is of importance for reconstruction models of Australia and for further understanding the global Grenvillian system.

The most pervasive and long-lived tectonic event to have affected the Musgrave Province is the Musgrave Orogeny, which, based on available (including new) U-Pb SHRIMP geochronological data, lasted for a interval approaching 100 My, from ca. 1220 Ma to ca. 1140-1120 Ma (e.g. Smithies et al 2010, Geol Survey West Australia Report 106). Voluminous and long-lived 1220-1120 Ma felsic magmatism of the Pitjantjatjara Supersuite involved high-Ti, high-Phosphorus ‘charnockite series’ melts of >1000 °C (Smithies et al 2010, Geol Survey West Australia Report 106). There is an almost complete paucity of mafic magmatism of Musgrave age.

Despite this detailed work, the tectonic setting of the Musgrave Orogeny remains unclear. The metamorphic record is used in this study to make inferences about potential tectonic environments based on apparent geothermal gradients, coupled with the results of igneous geochemistry above. The peak mineral assemblage at each of the (rare metapelitic) localities across the entire 500km strike of the province is garnet + sillimanite + spinel + quartz. Post-peak cordierite + spinel ± garnet coronae and symplectites have developed in all the samples. Calculated phase equilibria suggest that peak P-T uniformly reached 7—8 kbar and ≥1000 ˚C, equating to a steep regional apparent geothermal gradient of >1000 ˚C/GPa (or >>30-40 ˚C/km) in the lower crust. This very steep apparent geothermal gradient requires that the heat source is not purely conductive and also long-lived. SHRIMP monazite ages from the studied metapelitic rocks range between about 1230 Ma and 1180 Ma, and thus record early Musgrave Orogeny ages. We suggest that the thermally extreme Musgrave Orogeny may have been characterized by a restricted extensional/upwelling zone within an intracontinental setting.