Metamorphic P–T–water conditions of the Yushugou granulites from the southeastern Tianshan orogen: Implications for Paleozoic accretionary orogeny

Abstract

The South Tianshan Accretionary Complex (STAC), forming the southern segment of the Central Asian Orogenic Belt, underwent a long-lived and subduction-related accretionary orogenic process. The high-pressure (HP) and ultrahigh-pressure (UHP) metamorphic rocks within this complex are traditionally considered to be metamorphic ophiolitic slices. In this paper, we report a detailed study of petrology and water content of nominally anhydrous minerals (NAMs) of granulites from the Yushugou HP massif occurring as a fault-bounded tectonic slab in the Paleozoic accretionary complex. The studied granulites consist of garnet, orthopyroxene, plagioclase, K-feldspar, quartz, biotite, ilmenite and rutile and show distinct mylonitic foliation. The augen garnet is dominated by almandine and pyrope components, and has compositional zoning with increasing grossular content from the core to rim of the grain. The augen orthopyroxene has high Al2O3 content (up to 7.91wt.%), and shows compositional zoning characterized by a decreasing Al2O3 content from core to rim. Phase equilibria modeling indicates that the granulite underwent ultrahigh-temperature (UHT) (>930°C) and HP (10.5–14.5kbar) metamorphism and partial melting under a high geothermal gradient of ca. 24°C/km, and a possible prograde process characterized by heating and burial. Analyses of Fourier transform infrared spectroscopy indicate that hydrogen was incorporated in all NAMs of the granulites in the manner structural OH and sub-microscopic fluid inclusions and that the average water content (H2O weight) is in the range of 63–215ppm in garnet, 1–54ppm in orthopyroxene, 172–533ppm in feldspar and 34–66ppm in quartz. The present results show that the Yushugou massif probably derived from the deep root of hot continental magmatic arc. The trace amounts of water in NAMs obviously affected ductile deformation of the near-dry granulites. This study indicates that the thickened lower crust of the Paleozoic Tianshan accretionary orogen is characterized by high-thermal flow, UHT granulite-facies metamorphism, anatexis, ductile deformation and coeval magmatism and crustal growth.