Gaoligong Metamorphic Complex: Rock Formation And Tectonic Evolution, Western Yunnan

As the western boundary of Indochina’s southeastward escaping,the structural evolutions of the Gaoligong Shear (Fault) Zone and its accommodation on the India-Asia collision since the Cenozoic have been extensively studied. The Gaoligong Shear (Fault) Zone suffered two episodes of dextral strike-slip ductile shearing which are characterized by the shearing deformation of the granitic mylonite of Gaoligong Mountain.Based on macro-and micro-structural observations and crystallographic preferred orientation analysis of quartz, this paper holds the opinion that the rocks in Gaoligong Shear (Fault) Zone all underwent an early stage of high temperature coaxial deformation and were superposed a late stage of relative lower temperature shear deformation during the Cenozoic. As for the granitic mylonite near the Gaoligong shear zone generally experienced deformation at a higher temperatures (550-650℃, amphibolite facies)and superposed deformation at a lower temperature (400-550℃), but for those that are far away from the shear zone has deformed under a low temperature (<400℃) condition which revealed that deformation degree of the rock is subjected to the shearing and its metamorphic grade is mainly high greenschist to lower amphibolite facies.The Gaoligong Mountain metamorphic complex is mainly composed of biotite-plagioclase leptynite augen granitic mylonite, granitic mylonite, migmatitic granite, biotite-quartz schist, and sporadic outcrops of plagioclase amphibolite, Dalirock, quartzite. Systematic sampling and LA-ICP-MS zircon U-Pb isotopic dating of the Baodeng-Bende section indicate that the metamorphic complex is a suit of granite intruded in early Cenozoic (Himalayan) and a set of biotite riched granite intruded in Yanshan Period. Both of the two sets of rocks suffered shearing deformation during the active period of the Gaoligong Shear (Fault) Zone, the former formed granitic mylonite, while the latter formed biotite leptynite.The combining study of Zircon U-Pb, muscovite and biotite Ar-Ar geochronology has shown that the metamorphic complex cooled rapidly from 41 Ma. Then, from about 32Ma, the metamorphic complex stepped into the slow exhumation stage and at 18Ma, it started its fast exhumation process. According to the analysis of apatite fission track ages, the fast exhumation stage continued to 1Ma±.The early Contractional deformation under pure shearing, the later ductile deformation under late simple shearing, magmatic activity and the variation of stress field between the northern and southern part of Three river area in eastern Tibet since Pliocene occurred along the Gaoligong shear zone during Cenozoic are all controlled by the process of early forward collision, later lateral collision and oblique convergence of the Indian-Eurasian, the rotation of tectonic plates and crustal blocks and many other factors.