Metamorphism and Zircon Geochronological Studies of Metagabbro Vein in the Yushugou Granulite-Peridotite Complex from South Tianshan, China

Yushugou granulite-peridotite complex, located at the east part of the northern margin of South Tianshan, may represent an ophiolitic slice subducted to 40–50 km depth with high-pressure granulite facies metamorphism. Although a lot of studies have been conducted on rocks in this belt, the rock association and tectonic background of the ophiolitic slice are still in dispute. A detailed study on petrology, phase equilibrium modeling and U-Pb zircon ages have been performed on the metagabbro vein in peridotite unit to constrain the tectonic evolution of the Yushugou granulite-peridotite complex. Three stages of mineral assemblage in the metagabbro were defined as stage I: Cpx A +Opx A +Pl A , which represents the original minerals of the metagabbro vein; stage II: Cpx B +Opx B +Pl B +Spl, which represents the mineral assemblage of granulite facies metamorphism with peak P-T conditions of 4.2–6.9 kbar and 940–1 070 °C; stage III is characterized by the existence of prehnite, thomsonite and amphibole in the matrix, indicating that the metagabbro vein may be influenced by fluids during retrograde metamorphism. Combined with the crosscut relationship, it can be deduced that the metagabbro vein, together with the peridotite in Yushugou granulite-peridotite complex has experienced similar high-temperature granulite facies metamorphism. The zircon chronological data shows that the protolith age of the metagabbro vein is 400.5±6.2 Ma, reflecting Devonian magmatism event and the granulite facies metamorphism occurred at ~270 Ma which may be related to the post-collisional magmatism.