Metamorphic evolution of the coesite-bearing ultrahigh-pressure terrane in the North Qaidam, Northern Tibet, NW China

Widespread evidence for ultrahigh-pressure (UHP) metamorphism is reported in the Dulan eclogite-bearing terrane, the North Qaidam-Altun HP-UHP belt, northern Tibet. This includes: (1) coesite and associated UHP mineral inclusions in zircon separates from paragneiss and eclogite (identified by laser Raman spectroscopy); (2) inclusions of quartz pseudomorphs after coesite and polycrystalline K-feldspar + quartz in eclogitic garnet and omphacite; and (3) densely oriented SiO2 lamellae in omphacitic clinopyroxene. These lines of evidence demonstrate that the Dulan region is a UHP metamorphic terrane. In the North Dulan Belt (NDB), eclogites are characterized by the peak assemblage Grt + Omp + Rt + Phn + Coe (pseudomorph) and retrograde symplectites of Cpx + Ab and Hbl + Pl. The peak conditions of the NDB eclogites are P = 2.9-3.2 GPa, and T = 631-687 degreesC; the eclogite shows a near-isothermal decompression P-T path suggesting a fast exhumation. In the South Dulan Belt (SDB), three metamorphic stages are recognized in eclogites: (1) a peak eclogite facies stage with the assemblage Grt + Omp + Ky + Rt + Phn at P = 2.9-3.3 GPa and T = 729-746 degreesC; (2) a high-pressure granulite facies stage with Grt + Cpx (Jd < 30) + Pl (An24-29) + Scp at P = 1.9-2.0 GPa, T = 873-948 °C; and (3) an amphibolite facies stage with the assemblage Hbl + Pl + Ep/Czo at P = 0.7-0.9 GPa and T = 660-695 °C. The clockwise P-T path of the SDB eclogites is different from the near-isothermal decompression P-T path from the NDB eclogites, which suggests that the SDB was exhumed to a stable crustal depth at a slower rate. In essence these two sub-belts formed in different tectonic settings; they both subducted to mantle depths of around 100 km, but were exhumed to the Earth's surface separately along different paths. This UHP terrane plays an important role in understanding continental collision in north-western China.