Petrogenesis and tectonic implications of blueschist-facies garnet-mica schist in the Dabie orogenic belt, central China

Abstract

The Dabie orogenic belt (DOB) in central China is renowned for studies of ultrahigh-pressure metamorphism due to deep continental subduction. However, relatively high-pressure and low-temperature (HP/LT) metamorphism in the DOB is poorly understood. To provide constraints on such metamorphism, we carried out a detailed petrological investigation on a garnet-mica schist from the Central Dabie metamorphic zone. Thermodynamic calculation, based on petrographic and mineral chemical data, revealed three distinct metamorphic episodes (i.e., M1, M2, and M3) with pressures and temperatures of 1.98 ± 0.01 GPa/457 ± 3°C, 1.93 ± 0.06 GPa/515 ± 19°C, and 1.51 ± 0.19 GPa/525 ± 14°C, respectively, which are generally under the HP/LT conditions of blueschist-facies metamorphism. The resulting clockwise P–T path reflects isobaric heating (M1 → M2) and isothermal decompression (M2 → M3), suggesting termination of the slab subduction and subsequent rapid uplift, respectively. Furthermore, zircon analysis yielded two distinct groups. Zrn-I represents the core of the zircons and exhibits an inherited magmatic origin, with a Neoproterozoic age range of 827–682 Ma; Zrn-II corresponds to the rim of the zircons and has a metamorphic origin, with an age of 197 ± 5 Ma, correlating with the M3 episode. These results, combined with previous studies, suggest that the transition from shallow warm subduction to deep cold subduction in the DOB may occur at a depths of 35–65 km in a low geothermal gradient environment, and the final exhumation of the orogenic belt to the surface may originate from the large-scale extension and deformation in eastern China.