A HUGE UHP METAMORPHIC BELT OF OCEANIC-TYPE IN SW TIANSHAN, CHINA: PT PATH, METAMORPHIC AGES AND COEVAL ARC ROCKS

Recently, a huge UHP metamorphic belt of oceanic-type with more than 80 km long has been recognized in SW Tianshan, China. It is the largest oceanic-type UHP metamorphic belt reported so far. It has formed due to northward subduction of the Tianshan Paleo-Ocean. Petrological studies show that the UHP metamorphic rocks include three groups: eclogite and blueschist, garnet phengite schist and serpentinites. The well preserved coesite inclusions have been found in lawsonite-bearing eclogite and garnet phengite schist. Ti-clinohumite and Ti-chondrodite have identified in serpentinite. Therefore, the three main rock types of eclogite, garnet phengite schist and serpentinite have undergone UHP metamorphism. The PT peudosection calculations show that both eclogites and garnet phengite schists have the similar PT path characterized by the peak PT conditions of 450-500 C and 2.6-2.8 GPa and then heating to the maximum temperature of 550 C during the decompression, which is quite different from other oceanic subduction zone metamorphism. The U-Pb dating of metamorphic rims of zircons from a coesite-bearing garnet-phengite schist yields a peak UHP metamorphic ages of 320±3.7 Ma. Meanwhile, the syn-subduction granitic rocks have been identified in the field of low-P metamorphic bet composed a paired metamorphic belt with UHP belt together, which have SHRIMP zircon U-Pb ages of 333±3 Ma and 326±3 Ma and characterized by the arc-like geochemical affinity with enrichment of LILE (K, Rb, Sr and Ba) and depletion of HFSE (Nb, Ta and Ti) and P, and moderate initial 87Sr/86Sr ratios of 0.7062-0.7071 and neutral εNd(t) values of -1.5 to 1.6. These suggest that the syn-subduction granitic rocks would be formed by partial melting of possible juvenile arc-derived rocks with minor involvement of ancient continental rocks during the subduction zone HP-UHP metamorphism. Conseqently, the dehydration of UHP metamorphic rocks takes an important role in the formation of coeval arc magmatic rocks.