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

Abstract

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, M1, M2 and M3, corresponding to deformation stages D1, D2 and D3, 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 M1 reflects greenschist facies metamorphism. Pervasive M2 metamorphism formed a dominant S1 schistosity within the Barrovian sequence. Peak metamorphic conditions for metapelites of the garnet–staurolite and staurolite–kyanite grade were ∼580°C and ∼0.65GPa according to petrogenetic grids. Peak metamorphism was in conditions of ∼635°C and 0.50GPa for the metapelites and ∼650°C and ∼0.61GPa for amphibolites in sillimanite grade. Greenschist facies retrograde metamorphism, M3, followed D2 deformation, a top-down-to-southeast shear in the JSZ. The D3 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 242Ma. 40Ar/39Ar dating of white mica from a garnet-schist gave a plateau age of 224Ma. The peak metamorphism is thus limited to be between 242 and 224Ma. We thus suggest a tectonic shift from collision to extension of for the Permian to Triassic Paleo-Jinshajiang orogenic belt.