FEATURES OF DEFORMED ROCKS IN THE DUCTILE SHEAR ZONE IN THE WUTAI MOUNTAIN AREA

Deformed rocks in the early Precambrian ductile shear zone in the Wutai Mountain area is characterized by well-developed nodular,oval,podiform, pseudorhyolitic,rod-shaped,chrysanthemum stamen-shaped and highly schist- ose structures,which are the basis for recognizing the shear zone,providing information of rock formation and distinguishing the types of deformed rocks. Through measurements of finite strain and observation of shear direction,the author has determined the mechanism for the formation of deformed rocks—a simple shear mechanism—and the shear direction. The formation of deformed rocks in the ductile shear zone progressed through four phases:brittle-ductile deformation——strain softening（dynamic recrystallization and formation of hydrated schistose minerals）——plastic deformation——static recrystallization.In the wake of the deformation,SiO₂ and Na₂O in rocks increased and melanocratic components such as iron and magnesium lose and furthermore silicification,sodification and bleaching occur- red accordingly.The tectonogeochemical processes of the ductile shear zone proceeded in an open system.Metamorphism reached the greenschist facies grade with a metamorphic temperature of 300°—400℃ and a pressure of 4 kb.