Early Paleozoic HP/HT metamorphism on the northern Qilian block: insight on thermal evolution of the overrding plate in convergent margins

HP (High-pressure)/HT( high temperature) granulites can be considered as part of the thickened lower crust in overrding plate and may provide an important information for thermal evolutions of convergent plate margins. The Qilian orogenic belt is a typical early Paleozoic accretionary orogenic belt. The Qilian block, which is mainly composed of Precambrian basement, is located in the southern side of the North Qilian suture. In recent year, HP/HT granulites, subjects of this contribution, are identified on the northern margin of the Qilian block. Petrographic observations identified three-stage metamorphic assemblages for Grt–Cpx granulites: the first generation (M1) is Grt + Amp + Pl + Kfs + Ttn + Bt + Qz + Liq; the second generation (M2) is Grt + Pl + Amp + Cpx + Ttn + Bt + Qz + Liq; the final assemblage (M3) is Grt + Pl + Amp + Cpx + Ilm + Bt + Qz + Liq. Four stage metamorphic assemblages are identified from pelitic granulite: the first generation (M1) is Grt + St + Pl + Rt + Bt + Qz + Liq; the second generation (M2) is Grt + Ky + Pl + Bt + Qz + Liq; the third generation (M3) is represented by the occurrence of silimanite, rutile and ilmenite within plagioclase in matrix and the Bt + Qz + Sil + Crd + Pl + Ilm symplectites around the rim of garnet;the final generation (M4) is Grt + Crd + Bt + Pl + Qz + Ilm + Liq.    

Zr-in-rutile thermometry, Zr-in-titanite thermometry and phase equilibria are applied for evaluating the P–T history and in situ U-Pb datings are used to define the timings of different metamorphic stages. The results indicate that the P_max condition of Grt–Cpx granulite is at 12.8-13.7 kbar/ 735-760 °C and T_max condition at 7.5-9.6 kbar/770-845 °C. The P_max condition of pelitic granulite is at ~12 kbar/ 750-800 °C and T_max condition at 5.2-6.8 kbar/780-840 °C. Both rocks show a similar decompressional heating P-T path. In-situ U-Pb datings (rutile, monanite, titanite, and zircon) indicate that the timing of prograde stage is at 500~470 Ma and decompressional heating stage is at 460~450 Ma. Our new data combined with the regional geological data show the early Paleozoic HP/HT metamorphism is related to terrane accretion and the southward subduction of Paleo-Qilian Ocean (Proto-Tethyan Ocean) during the early Paleozoic era.