Study On Metamorphism Of The Kuanping Group, Northern Tongbai

The Tongbai area is situated at the junction between the Paleozoic North Qinling orogen and the Mesozoic Dabie-Sulu orogen. The most comprehensive information of the area is the preservation of two rock suites of different age and metamorphic style. Paleozoic complex shows low to high metamorphic grade from greenschist faceis, amphibolite faceis to granulite faceis, whereas Mesozoic complex is characterized by high-pressure metamorphism from blueschist facies, greenschist facies to eclogite facies. Therefore, The Tongbai area holds key to the understanding of orogenesis between the North and South China Blocks. The Kuanping Group in northern Tongbai occurred on the southern margin of the North China Block. Metamorphic rocks in this group consist of amphibolites, metapelites, marbles and intercalated quartzites, and can be divided into the Huilongsi and Zuolaozhuang Formations from the north to the south. This rock association is comparable with that of the Kuanping Group in Qinling. Petrological study indicates that the Kuanping Group was metamorphosed at the middle-pressure amphiblite facies conditions. Using conventional geothermo-barometery, P-T conditons were estimated to be 590-610℃and 0.95-1.14 GPa for the Huilongsi Formation and 630-650℃and 0.65-0.89 GPa for the Zuolaozhuang Formation, in agreement with lower and upper amphibolite facies, respectively. This implies a progressive metamprhism occurring in the Kuanping Group. In addition, a clockwise P-T path involving a decompression accompanying a thermal relaxation is deduced by the transformation of kyanite to sillimanite in metapelites. SHRIMP U-Pb zircon dating reveals a metamorphic age of 442±6 Ma and a reworking age of 415±5 Ma for a garnet amphibolite. While zircons from a syn-deformed garnet-bearing pegmatite yield an emplacement age of 439±24 Ma for it. It is obvious that the amphibolite facies metamorphism and deformation of the Kuanping Group took place during the early Silurian (ca.440 Ma). The close association of lower to upper amphibolite facied Kuanping Group, greenachist to lower amphibolite facies Erlangping Group and granulite facies Qingling Group of simlar metamorphic ages (440-400 Ma) suggest collisional orogensesis occurred between intraoceanic arc, microcontinent and southern margin of the North China Block during the Silurian to ealry Devonian.