UHP metamorphism recorded by coesite-bearing metapelite in the East Kunlun Orogen (NW China)

摘要

The East Kunlun Orogen (EKO) is the NW part of the Central China Orogenic Belt, which records the evolutionary history of the Proto- and Palaeo-Tethys Oceans from the Cambrian to the Triassic. An Early Palaeozoic eclogite belt has been recognized in recent years, which extends dis-continuously for similar to 500 km as three eclogite-bearing terranes. In this study, we report an integrated study of zircon grains from mica-schists accompanying the eclogites, in terms of mineral inclusions, U-Pb age systematics and P-T conditions. The presence of coesite is identified, as inclusions within the metamorphic domain of zircons, which provides unambiguous evidence for subducted terrigenous clastic rocks of the Proto-Tethys Ocean exhumed from coesite-forming depths. U-Pb dating of the metamorphic zircons yields a concordia age of 426.5 +/- 0.88 Ma, which is likely to be the time of ultrahigh-pressure metamorphism in the Kehete terrane. P-T calculations suggest that metapelite may have experienced a clockwise P-T path with peak P/T conditions of 685 +/- 41 degrees C and >28 kbar, and equilibrated at 482-566 degrees C and 5.6-8.9 kbar during subsequent exhumation. The high-pressure - ultrahigh-pressure (HP-UHP) metamorphic belt within the EKO may have formed by collision between the Qaidam Block and the South Kunlun Block, as a consequence of the closure of the Proto-Tethys Ocean.