Metamorphic Evolution During Deep Subduction and Exhumation of Continental Crust: Insights from Felsic Granulites in South Altyn Tagh, West China

The South Altyn Tagh continental high-pressure-ultra-high-pressure (HP-UHP) metamorphic belt in West China is claimed to host the most deeply subducted (> 300 km) part of Earth's continental crust, based on the presence of pseudomorphs and paramorphs after stishovite. Felsic granulites that occur as tectonic blocks in the HP-UHP belt have protoliths of high-(sample A761) and low-Ca (sample A1501) potassic granitoid. Both consist of garnet, kyanite, perthite, plagioclase, quartz, rutile, biotite and minor spinel and corundum. Four generations of mineral assemblages (I-IV) were distinguished for constraining the P-T evolution using pseudosection modelling calculated with THERMOCALC. The first generation of assemblages (I) represented by the remarkable chemical zoning in garnet core (I-a) and mantle (I-b) domains from high-Ca sample A761 is interpreted to record an early prograde evolution to the peak UHP eclogite facies stage, with P-T conditions of 37 GPa/700-1100 degrees C, where the fluid-phase is modelled to be absent. The peak UHP conditions are also supported by clinopyroxene exsolution in garnet and low Al2O3 in orthopyroxene in interlayered garnet clinopyroxenites and peridotites. The second generation of assemblages is characterized by the formation of coronate/atoll-like garnet around kyanite or perthite (II-a), and by the formation of ternary feldspar and plagioclase coronas (II-b). The former is attributed to decompression evolution in supra-solidus eclogite facies conditions from 2.8 to 2.4 GPa for a fixed temperature of 1000 degrees C. The latter is suggested to reflect decompression in high-pressure-ultrahigh- temperature (HP-UHT) granulite facies conditions (2.5-1.4 GPa/1000-1090 degrees C for A1501, 2.4-1.75 GPa/970-1040 degrees C for A761) on the basis of the anorthite content of rehomogenized perthite and plagioclase, and compositional zoning in garnet rim domains. The third generation of mineral assemblages is marked by the formation of micro-grained aggregates of spinel + corundum 6 plagioclase (III) between garnet and kyanite and is suggested to reflect a further rapid decompression to low pressure (LP) granulite facies conditions at similar to 0.3-0.4 GPa/900-1000 degrees C. The fourth generation of mineral assemblages characterized by the later growth of biotite, together with very fine grains of quartz and feldspar (IV), is interpreted to indicate an isobaric cooling evolution with residual melt crystallization towards the water-saturated solidus at similar to 680 degrees C and 0.3 GPa. LA-ICP-MS zircon U-Pb dating for sample A1501 yields two distinct age groups: c.900Ma magmatic protolith ages and c.500Ma metamorphic ages, where the metamorphic ages are interpreted to represent the late isobaric cooling stage. Geodynamically, it is possible that a continental slab represented by the felsic granulites may have subducted to depths of > 120-200 km, and was then exhumed rapidly to the base of thickened crust to form the typical HP-UHT granulite facies assemblages observed in the South Altyn Tagh.