Abstract: The Bikou terrane lies on the northwestern margin of the Yangtze Block. It comprises pelitic rocks in the north and maficintermediate volcanic rocks in the south. Rocks in the north metamorphosed into phyllite and mica schist, while rocks in the south experienced greenschistblueschist facies metamorphism. Presently, there are no detailed analyses on the deformation sequence and regional metamorphism of the Bikou terrane; geochronological constraints for regional metamorphism are also lacking. As a result, nature of the Bikou terrane and age of regional metamorphism and tectonic settings are not known, causing controversy over the orogenies of West Qinling and Longmen Mountain. Here, after detailed field study and structural analysis, we divided the Bikou terrane deformation sequence into three stages, D1, D2 and D3. D1 is marked by tight fold, foliation S1 and mineral lineation L1; D2 is normal ductile shear dipping to the south, transforming thrust faults and resulting in formation of Jurassic halfgraben basins; and D3, formed in the sinistral strikeslip thrust in the Himalayan, is characterized by brittle deformation. In this study, we obtained UPb zircon age of 227.2±6.2 Ma from deformed felsic veins in the Bikou terrane. Since the undeformed intrusive plutons after D1 deformation yielded age of 226215 Ma in previous studies, the age of regional metamorphism in the Bikou terrane is constrained at ca. 220 Ma. Therefore we infer that, D1 formed during subduction and collision between the North and South China blocks in the Late Triassic, when regional metamorphism of the Bikou terrane occurred, and D2 and D3 were related to eastward expansion of the Tibetan Plateau.

Key words:  Bikou terrane, regional metamorphism, Yangtze Block, Triassic