Ultrahigh-temperature metamorphism of garnet-sillimanite-bearing pelitic granulites in Yushugou, Southern Tianshan, Xinjiang, and tectonic implications

The Tianshan orogen is an important part of the Central Asian Orogenic Belt, and peridotite-granulite terranes occur in the Southern Tianshan orogen in Xinjiang. The determination of the ultrahigh-temperature metamorphism and the construction of the P- T-t paths of the granulite in the Yushugou area, northern margin of Southern Tianshan orogen, provide an important basis for the study of the accretionary orogenic process in the Southern Tianshan orogen. This study mainly focuses on the three garnet-sillimanite-bearing pelitic granulites in Yushugou area. Based on detailed petrographic studies, P-T calculations, phase equilibrium modeling and zircon U-Pb age analyses were carried out. The results show that they generally retain the mineral assemblages of four metamorphic stages: (1) the pre-peak prograde stage (M1) is mainly composed of garnet core and inclusion minerals in the core, Grt1 + Btl + Pl1 + Ilml + Rtl +Qzl +/- Kspl; (2) The peak metamorphic stage (M2) consists of the garnet mantle and the matrix minerals, Grt2 + Sil2 + Ksp2 + Pl2 + Ilm2 + Rt2 +Qz2 + Liq2, with P-T conditions of 910-973 degrees C and 0.901.09GPa; (3) the high-pressure granulite facies stage (M3) mainly consists of the inner rim and inclusions of garnet rim, as well as the rims of minerals in the matrix, Grt3 + Bt3 + Sil3 + Ksp3 + Pl3 + Ilm3 + Rt3+Qz3 + Liq3, with P-T conditions of 845 similar to 869 degrees C and 1.01 similar to 1. 14GPa; and (4) the amphibolite facies stage (M4) represented by the outer rim of garnet and the minerals around the garnet, Grt4 + Bt4+ Sil4 + Ksp4 + Pl4 + Ilm4 + Qz4 +/- Rt4, with P-T conditions of 701-737 degrees C and 0.7 similar to 0.8GPa. The P-T results indicate that the pelitic granulites recorded anti-clockwise P-T paths. The zircon U-Pb age analysis results indicate that the sedimentary age for protolith was after 427 Ma. Ultrahigh temperature metamorphism occurred during 397 similar to 409 Ma. Then they experienced the peak pressure metamorphic event during 381-395 Ma. The time for retrograde metamorphism was 355-387Ma. The source of heat for ultrahigh temperature metamorphism may be the intrusion of Devonian arc magma, then the slab was involved in the subduction zone and underwent high-pressure metamorphism. Later, it was squeezed and uplifted so that it underwent retrograde metamorphism with decompression and cooling.