Age and origin of sillimanite schist from the Chinese Altai metamorphic belt: implications for late Palaeozoic tectonic evolution of the Central Asian Orogenic Belt

Reconstructing late Palaeozoic metamorphism of the Central Asian Orogenic Belt (CAOB) can provide a better understanding of how the CAOB formed. The petrology of sillimanite-bearing metapelitic schists from high-grade portions of the Permian Chinese Altai metamorphic belt (andalusite-type) reflects the effects of poorly understood high-T, low-P metamorphism. Phase equilibria modelling in the Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2-TiO2-O (NCKFMASHTO) system restricts P-T conditions of the sillimanite schists to approximately 635-670 degrees C at approximately 5.8-6.8 kbar. SHRIMP U-Pb analyses of zircon from the rocks yield a concordant age of 299.2 +/- 3.4 Ma. Combined with the slightly younger (292.8 +/- 2.3 Ma) areally restricted pelitic granulite with peak P-T conditions of approximately 780-800 degrees C at approximately 5-6 kbar and high-T granulite with P-T conditions of approximately 860 degrees C at approximately 6 kbar, these metamorphic rocks reflect prograde heating at relatively low pressure in early Permian time. Together with contemporary and widespread magmatic activities, they are best explained in the context of a post-orogenic extensional environment related to a mantle plume.