Guibin Zhang

UHP metamorphic evolution of coesite-bearing eclogite from the Yuka terrane, North Qaidam UHPM belt, NW China

Coesite, recognized petrographically and confirmed by Raman spectroscopy, is reported from a coarse-grained eclogite from the Yuka terrane, North Qaidam UHP metamorphic belt. This represents the first record of UHP metamorphism in the western part of this belt. In addition, inclusions of Ab + Qtz assemblage in gamet with strong evidence for volume expansion may have derived from a Jd + Coe precursor. Two types of eclogite, coarse-grained and fine-grained, are exposed in the Yuka terrane. The eclogites experienced three distinguishable metamorphic stages: (1) a pre-peak metamorphic stage recorded by the mineral assemblage of Amp + PI + Qtz in the garnet core at P = 0.8-1.0 GPa, T = 450-560 °C for the coarse-grained eclogite and P = 1.0-1.2 GPa, T = 450-510 °C for the fine-grained eclogite, and by the mineral assemblage of Cpx + Phn in the garnet mantle at P = 2.25-2.65 GPa, T = 550-610 °C for the coarse-grained eclogite; (2) the peak stage characterized by the mineral assemblage of Grt + Omp + Phn + Qtz/Coe + Rt at P = 3.0 GPa. T = 652 °C for the coarsegrained eclogite, and Grt + Omp + Phn + Rt + Qtz at P = 2.9-3.2 GPa, T = 566-613 °C for the fine-grained eclogite; and (3) a retrograde stage recorded by the garnet reaction rims, the breakdown of omphacite, and the pervasive retrograde mineral assemblage of Grt + Amp + Pl + Qtz at P = 0.7-1.1 GPa, T = 550-590 °C for the coarse-grained eclogite. The coesite inclusion and the calculated P-T paths suggest that the Yuka UHPM terrane experienced fast and deep subduction (∼100 km) under a cool geothermal gradient (6-7 °C/km) followed by rapid exhumation.

Ultra-deep subduction of Yematan eclogite in the North Qaidam UHP belt, NW China: Evidence from phengite exsolution in omphacite

Abstract Phengite exsolution in omphacite from the Yematan eclogite, North Qaidam UHP belt, NW China, is described. Mineralogical investigations show that the precursor omphacite in the Yematan eclogite contains up to 1.16 wt% K2O and ~10 000 ppm H2O. Experimental studies document this omphacite to be formed at pressures higher than 6 GPa (at 900 °C). The pressure-temperature conditions of 3.68 GPa and 892 °C for phengite exsolution in omphacite associated with garnet during the exhumation were obtained by using Grt-Omp-Ph geothermobarometer. We conclude that omphacitic-clinopyroxene in subducted eclogites may act as a robust medium to transport H2O and potassium deep into the interior of the Earth. This study suggests that the Yematan eclogite in North Qaidam UHP metamorphic belt, NW China, may have been subducted and exhumed from depths of more than 200 km.