HP-UHT granulites in the East Kunlun Orogen, NW China: Constraints on the transition from compression to extension in an arc setting of the Proto-Tethys Ocean.

The East Kunlun Orogen (EKO) of the northwest part of the Central China Orogenic belt records long-lived orogenic events from the Cambrian to the Triassic, involving the tectonic evolution from Proto-Tethys to Palaeo-Tethys. Although high-and ultra high pressure (HP-UHP) metamorphism has been recognized in the EKO, other metamorphic events associated with the orogenic history are little known. The metamorphic history of high-and ultra high temperature granulites and their pressure-temperature-time (P-T-t) path is keys to reconstructing tectonic events associated with a subduction-to-collision transition. In this contribution, we present P-T-t results for the mafic and pelitic granulite facies metamorphic rocks in the Qingshuiquan region of the EKO, determined using phase equilibrium modelling and zircon geochronology. The Jingshuikou granulite terrane records a clockwise P-T path with high-P and ultra high temperature (HP-UHT) conditions at Pmax ~900 to 950°C at 13 kbar, at Tmax of ~950°C at 9-12 kbar, and a final stage of 800-895° C at 6-10 kbar. LA-ICP-MS U-Pb dating of zircons provides systemic constraints on a long-lived metamorphic age span of 530-465 Ma, with a peak of c. 503 Ma. The HP-UHT metamorphism of the Qingshuiquan granulite terrane reveals a complete process from arc compression at the early stage of subduction to extension on the active continental margin related to rollback of the subducted slab in the Early Palaeozoic, driven by heat transfer from upwelling of the sub-arc hot asthenosphere. The timing and P-T conditions of the high geothermal gradient metamorphism can be used to understand the subduction and collision process of the Proto-Tethys Ocean, including the relationship between high-P and high-to ultra-temperature metamorphism in the East Kunlun and other orogenic belts. [ABSTRACT FROM AUTHOR]