Structural style and metamorphic conditions of the Jinshajiang metamorphic belt: Nature of the Paleo-Jinshajiang orogenic belt in the eastern Tibetan Plateau

The Jinshajiang metamorphic belt is a Barrovian sequence distributed within the Jinshajiang Suture Zone (JSZ), which was formed in the thickened Paleo-Jinshajiang orogenic belt after closure of the Jinshajiang Ocean. We have identified three metamorphic phases, M-1, M-2 and M-3, corresponding to deformation stages D-1, D-2 and D-3, in the Barrovian sequences. The metamorphic belt exhibits a metamorphic field gradient from.chlorite to biotite, garnet, staurolite-kyanite and sillimanite grades. Inclusions in garnet and staurolite (chlorite, mica, quartz, feldspar, ilmenite and graphite) indicate that M-1 reflects greenschist facies metamorphism. Pervasive My metamorphism formed a dominant S-1 schistosity within the Barrovian sequence. Peak metamorphic conditions for metapelites of the garnet-staurolite and staurolite-Icyanite grade were similar to 580 degrees C and similar to 0.65 GPa according to petrogenetic grids. Peak metamorphism was in conditions of similar to 635 degrees C and 0.50 GPa for the metapelites and similar to 650 degrees C and 0.61 GPa for amphibolites in sillimanite grade. Greenschist fades retrograde metamorphism, M-3, followed D-2 deformation, a top-down-to-southeast shear in the JSZ. The D-3 deformation is characterized by well-developed brittle faults with fault gouge and breccia. Zircon grains from an amphibolite sample have cores with igneous oscillatory zoning and metamorphic rims. However, the metamorphic rims are too narrow to analyze. Laser ablation-inductively coupled plasma mass-spectrometry (LA-ICPMS) analyses of the igneous cores of zircons yielded a crystallization age of 242 Ma. 40Ar/39Ar dating of white mica from a garnet-schist gave a plateau age of 224 Ma. The peak metamorphism is thus limited to be between 242 and 224 Ma. We thus suggest a tectonic shift from collision to extension of for the Permian to Triassic PaleoJinshajiang orogenic belt. (C) 2015 Elsevier Ltd. All rights reserved.