Metamorphic records for subduction erosion and subsequent underplating processes revealed by garnet-staurolite-muscovite schists in central Qiangtang, Tibet

Subduction erosion is confirmed as a crucial geodynamic process of crustal recycling based on geological, geochemical, and geophysical observations at modern convergent plate margins. So far, not a single metamorphic record has been used for constraining a general tectonic evolution for subduction erosion. Here we first revealed metamorphic records for a subduction erosion process based on our study of the Late Paleozoic garnet-staurolite-muscovite schists in the central Qiangtang block, Tibet. Provenance analyses suggest that the protoliths of garnet-staurolite-muscovite schists have the Northern Qiangtang-affinity and were deposited in an active continental margin setting. Mineral inclusion data show that the early metamorphic stage (M1) recorded blueschist facies pressure-temperature (P-T) conditions of 0.8–1.1 GPa and 402–441°C, indicating that a part of the material from the overriding plate had been abraded into the subduction channel and undergone high-pressure/low-temperature metamorphism. The peak metamorphic stage (M2) recorded amphibolite facies P-T conditions of 0.3–0.5 GPa and 470–520°C. The 40Ar/39Ar cooling ages (263–259 Ma) yielded from muscovite suggest the amphibolite facies metamorphism (>263 Ma) occurred at oceanic subduction stage. The distinctly staged metamorphism defines a clockwise and warming decompression P-T-t path which reveals an underplating process following the early subduction erosion. During the tectonic process, the eroded low-density material escaped from the cold subduction channel and rise upward into the warm middle-lower crust of the upper plate, undergoing amphibolite facies metamorphism. Our new results revealed a complete evolutional process from the early subduction erosion to the subsequent underplating during the northward subduction of the Paleo-Tethys Ocean.