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

摘要

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 Na_2O-CaO-K_2O-FeO-MgO-Al_2O_3-SiO_2-TiO_2-O (NCKFMASHTO) system restricts P-T conditions of the sillimanite schists to approximately 635-670°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°C at approximately 5-6 kbar and high-r granulite with P-T conditions of approximately 860°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.