Ductile Shear Zone in High-Grade Metamorphic Rocks and Its Rheomorphic Mechanism in the Daqing Mountain Area, Inner Mongolia

The high-grade metamorphic rocks in Daqing Mountain area not only record the early formation and evolution of the North China craton, but also retain rheomorphic characteristics in the middle-lower crust. These rocks undergwent metamorphism, deformation and anatexis of high amphibolite facies-granulite facies in the lower crust, forming the complex structure styles and association of structural elements. As the main structural feature in the high-grade metamorphic rocks, the ductile shear zones controlled the major tectonic framework of Pre-Cambrian high-grade metamorphic rocks in this area. According to the superimposed reformation between the geological occurrence and deformed deformation characteristics with the structural elements, the ductile shear zones are divided into early subhorizontal bedding extension type and later steep type. The subhorizontal bedding extension ductile shear zone remained at the locations that experienced weak-deformation in later stage. They formed along interfaces of different geological bodies and lithologies under an extensional deformation system. The later steep ductile shear zone was distributed in near E-W direction with a larger scale. It formed in the later orogenic compressional environment and reformed the early structural features. These two types of ductile shear zone were both formed under the high amphibolite facies-granulite facies metamorphism in the middle-lower crust. And, the mian deformation is the melt-enhanced grain side diffusion and the grain flow, which caused a large scale plastic flow of the rocks. Some macrostructural elemenrs, such as asymmetry flowing fabric, perthitic and band structural, melting lineation and intraformational flowing fold were formed. However, the microfabrics features of the rocks are equilibrium textures of triple sides, similar to the metafabrics of rocks by static crystalline metamorphism; the deforrnation of mineral grain is not obvious and the intracrystalline deformation textures did not develop:the metamorphic tectonite of lower crustal tectonic 1evel, the tectonic gneiss, was formed.