五台山区韧性剪切带变形岩石特征

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

本文对五台山区早前寒武纪花岗—绿岩地体中发育的韧性剪切带变形岩石的构造特征及演化、岩石类型划分、形成机制与剪切方向判别、岩石成份变化、变质作用物化环境等作了系统描述和分析,揭示了韧性剪切带变形岩石的特征。

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,SiO_2 and Na_2O 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.