Pressure-temperature Evolution of the Qingshuiquan Mafic Granulite: Implications for Proto-Tethys Subduction in the East Kunlun Orogenic Belt, Northern Tibetan Plateau

Granulite is in general a key metamorphic rock that can be used to understand the tec-tonic architecture and evolutionary history of an orogenic belt. The Qingshuiquan mafic granulite in the East Kunlun orogenic belt, northern Tibetan Plateau, occurs as tectonic boudins together with lower-grade ophiolitic melange assemblages within an amphibolite-facies crystalline basement. In this study, we investigated the geochemistry, geochronol-ogy, mineralogy, and phase modeling of the Qingshuiquan mafic granulite. Based on mineralogical observations and microstruc-tures, three mineral assemblage generations were distinguished: an assemblage found as inclusions within garnet and amphibole com-prising clinopyroxene + plagioclase + am-phibole + quartz + ilmenite + rutile (M1); an inferred peak assemblage of gar-net + clinopyroxene + plagioclase + amphi-bole + quartz + ilmenite +/- orthopyroxene (M2) in the matrix; and a retrograde assem-blage of amphibole and biotite coronae (M3) around clinopyroxene or orthopyroxene. Thermobarometric calculations and phase equilibrium modeling constrained a clockwise pressure-temperature (P-T) path for the Qin-gshuiquan mafic granulite with peak T con-ditions of 830-860 degrees C at 8.0-9.5 kbar. Prior to the peak T conditions, a pressure maximum of-11 kbar at-800 degrees C was recorded by ru-tile, ilmenite, and clinopyroxene inclusions in garnet and amphibole. The retrograde path was defined by a decompression segment fol-lowed by final cooling. The whole-rock geo-chemical results indicated that the protolith of the Qingshuiquan mafic granulite was similar to present-day enriched mid-ocean -ridge basalt (E-MORB) displaying low total rare earth element (REE) concentrations and a slight enrichment of light REEs, as well as flat high field strength element patterns in the primitive mantle- normalized trace-element diagram. Geochronologic results revealed that the protolith crystallization age of the mafic granulite is 507 +/- 3 Ma, and the timing of granulite-facies metamorphic overprint is 457-455 Ma. This evidence, taken together with results from previous studies, indicates that the protolith of the Qingshuiquan mafic granulite can be interpreted as basaltic rocks of Proto-Tethys oceanic crust that experi-enced a first high-pressure granulite-facies imprint followed by subsequent decompres-sion and granulite-facies overprint at slightly lower P and slightly higher T. This granulite-facies metamorphism can be attributed to the subduction of Proto-Tethys oceanic crust, which also generated numerous contempora-neous subduction-related magmatic rocks in the East Kunlun orogenic belt.