Geochronological Constraint On Bikou Blueschist Metamorphism, Northwestern Margin Of Yangtze Block (China)

The Bikou terrane is located on the northwestern margin of the Yangtze Block in South China. The terrane comprises of turbidite sequence in the north and mafic-intermediate volcanic rocks in the south. The rocks in the north were metamorphosed in to slate and phyllite, while rocks in the south experienced greenschist to blueschist facies metamorphism. Blueschist and eclogite facies indicate lithospheric plate subduction, since they require unusually cold upper mantle geotherms as those in present day subduction zones. The blueschist facies rocks exposed along the Qingchuan-Pingwu fault/ductile shear zone consist of strongly foliated psammitic schist and mafic schist. Metamorphic minerals in the blueschist include Na-amphibole, epidote, phengite, albite and quartz. Mineral paragenesis and Si content in phengite suggest the P-T condition of 300-400℃ and 0.8 Gpa of blueschist metamorphism. The widespread Neoproterozoic magmatic complexes developed on the western margin of the Yangtze Block and their Neoproterozoic setting is controversial. The Bikou blueschist has been inferred as evidence in debating Neoproterozoic setting of the western margin of Yangtze Block. However, there is still no geochronological study of the blueschist. The distribution of the blueschist is still unknown; hence its geological meaning is very equivocal. In our study we carefully investigated the distribution of the blueschist and its P-T condition. We present the first geochronological evidence for Bikou blueschist metamorphism. U-Pb zircon dating of deformed intrusive veins along with geology evidences indicates blueschist metamorphism at ca. 220 Ma. The blueschist formed during the amalgamation of North and South China Blocks. Bikou blueschist and Mianlue suture zone are both indicators of Triassic subduction and collision in the South Qinling.