Pressure-temperature evolution of the Qingshuiquan mafic granulite: Implications for Proto-Tethys subduction in the East Kunlun orogenic belt, northern Tibetan Plateau

Abstract

AbstractGranulite is in general a key metamorphic rock that can be used to understand the tectonic 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 mélange assemblages within an amphibolite-facies crystalline basement. In this study, we investigated the geochemistry, geochronology, mineralogy, and phase modeling of the Qingshuiquan mafic granulite. Based on mineralogical observations and microstructures, three mineral assemblage generations were distinguished: an assemblage found as inclusions within garnet and amphibole comprising clinopyroxene + plagioclase + amphibole + quartz + ilmenite + rutile (M1); an inferred peak assemblage of garnet + clinopyroxene + plagioclase + amphibole + quartz + ilmenite ± orthopyroxene (M2) in the matrix; and a retrograde assemblage 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 Qingshuiquan mafic granulite with peak T conditions of 830–860 °C at 8.0–9.5 kbar. Prior to the peak T conditions, a pressure maximum of ~11 kbar at ~800 °C was recorded by rutile, ilmenite, and clinopyroxene inclusions in garnet and amphibole. The retrograde path was defined by a decompression segment followed by final cooling. The whole-rock geochemical 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 experienced a first high-pressure granulite-facies imprint followed by subsequent decompression and granulite-facies overprint at slightly lower P and slightly higher T. This granulitefacies metamorphism can be attributed to the subduction of Proto-Tethys oceanic crust, which also generated numerous contemporaneous subduction-related magmatic rocks in the East Kunlun orogenic belt.