THREE KINDS OF DUCTILE SHEAR ZONES WITH DIFFERENT TECTONIC LEVELS IN THE HELANSHAN GROUP
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摘要: 本文根据对贺兰山群的变形构造与变质岩组构特征，以及它们形成序次的综合分析，提出该群存在以下三种不同时期和不同层次韧性剪切带：（１）前变质（２１亿年以前）中浅层次（绿片岩相）韧性剪切－变余鞘褶带；（２）同变质（２１～１８亿年）中深层欢（高角闪岩相）韧性剪切－强面理带；（３）后变质（１８亿年以后）中浅层次（低角闪岩－绿片岩相）韧性剪切－糜棱岩带。以上三种剪切带揭示了华北地台西缘古老基底，在元古宙时期构造演化的基本特点。Abstract: On the basis of an integrated study of the features of the deformation and metamorphic struc-tures and fabrics fo the Helanshan Group and the order of their formation, it is suggested thatthere exist three kinds of ductile shear znes formed in different phases and at different tectoniclevels: （1） pre-metamorphic （prior to 2100 Ma B. P.） middle shallow-level ductile shear-meta-morphosed sheath folds congruous reclinal folds and flow marble lenses and occurrence of schist-facies metamorphism; （2） syn-metamorphism （2100-1800 Ma B. P.） middle-deep-level ductileshear-strong foliation zones, which are characterized by occurrence of high amphilolite-faciesmetamorphism with T≈558-618℃ and p=458Pa and formation of plane strain fabrics（v=-0.03～0.35） in the stong foliation zones; （3） post-metamorphism （after 1800 Ma B. P.）middle-shallow-level ductile shear-mylonite zones, which show low amphibolite-greenschist faciesmetamorphism with T≈450-305℃ and are characterized by formation of thrust-type myloniteby means of compresso-shear mech anism （v=0.3～0.6）. The three above-mentioned kined ofshear zone reveal the basic features of tectonic evolution of the old basement on the western mar-gin of the North China platform during the Proterozoic
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