Cenozoic multi -metamorphism, shear deformation and geological significance of Ailaoshan high-grade metamorphic complex, western Yunnan, China

Precise determination of deformation and metamorphic events of the continental-scale strike-slip fault zone in the orogenic belt is of great significance for the study of the evolution of the orogenic belt. Ailaoshan-Red River ( ASRR) strike-slip fault is an important tectonic belt located on the western margin of the South China block and the southeastern margin of the Tibet Plateau, which has been widely advocated to be the result of the long-term interaction between the Indochina block and the South China block. There are four isolated narrow massifs exposed along the ASRR strike-slip fault, which recorded the Mesozoic collision-subduction event between the Indochina block and the South China block and deformation-metamorphism evolution of the Indian-Eurasian plate collisionsubduction-exhumation since Cenozoic. Garnet gneisses widely exposed in the Ailaoshan high-grade metamorphic complex are studied, combined with microstructural observations, cathodoluminescence, petrographic analysis, geothermobarometer, thermodynamic phase equilibrium calculations and zircon LA-ICP-MS U-Pb dating. The results show that the Ailaoshan metamorphic complex has undergone at least three stages of deformation-metamorphism evolution, which are the early prograde metamorphic stage ( M1), peak granulite facies metamorphic stage ( M2) and post-peak near isothermal decompression stage ( M3) with strong plastic left-lateral shear. The relic mineral assemblage preserved as inclusions in garnet porphyroblasts belongs to prograde stage ( M1) and records P-T conditions of low amphibolite facies. Dehydration reactions of muscovite, biotite and mineral assemblages of sillimanite + K-feldspar formed at the peak granulite facies metamorphic stage ( M2) yield P-T conditions of 780 840 C, 0. 95GPa. Post-peak near isothermal decompression stage ( M3) is characterized by the white-eye fabric which shows P-T conditions of low-amphibolite facies. The lattice preffered orientation analysis of the deformed minerals by EBSD suggests that the post-peak near-isothermal decompression stage ( M3) is consistent with the left-lateral shear deformation stage. The zircon rims of gneisses were dated indicating metamorphic age at 35. 2 0. 9Ma 33. 87 0. 8Ma. All these data also argue that the Ailaoshan high-grade metamorphic complex has undergone at least three stages of continuous metamorphism and sequential deformation through a clockwise P-T-t path.