Mineral Characteristics and Metamorphic Evolution of Pelitic Gneiss in Wulashan Area, Central Inner Mongolia

Wulashan Neoarchean peliticgneiss is exposed in the eclogite gneiss formation of Wulashan group in central Inner Mongolia. It is the most widely distributed plutonic metamorphic rock in Wulashan formation. The main lithology is biotite garnet monzonite gneiss. Through detailed petrological observation, this paper makes a systematic mineral chemical analysis of typical metamorphic minerals, and studies their metamorphic evolution characteristics and tectonic significance. The results show that the pelitic gneiss in the eclogite gneiss formation of Wulashan group has experienced four metamorphic evolution stages, and its metamorphic conditions are limited by metamorphic mineral thermobarometer inversion: early progressive metamorphic stage (M₁), its mineral assemblage is characterized by core garnet + biotite + plagioclase + quartz ± sillimanite; The mineral assemblage of the peak metamorphic stage (M₂) is characterized by garnet + sillimanite + biotite + plagioclase + quartz + potassium feldspar. The temperature and pressure conditions of this stage are T= 771~870℃, p = 7.5~11.2 Kb; The post peak near isothermal decompression stage (M₃) is characterized by the coronal structure of cordierite at the edge of garnet. Its mineral assemblage is garnet + cordierite + biotite + plagioclase + quartz ± sillimanite. The temperature and pressure conditions of this stage are T= 750~800℃, p = 6.1~7.0 Kb; The mineral assemblage in the late cooling stage (M₄) is characterized by garnet + biotite + plagioclase + quartz + magnetite ± K-feldspar. The temperature and pressure conditions in this stage are T= 650~659℃, p= 4.3~4.8 Kb. The metamorphic evolution P-T track of argillaceous gneiss has a clockwise pattern of nearly isothermal depressurization. It shows that the Wulashan group in the late Neoarchean in Wulashan area experienced the dynamic process of crustal subduction and thickening followed by exhumation and uplift.