THREE KINDS OF DUCTILE SHEAR ZONES WITH DIFFERENT TECTONIC LEVELS IN THE HELANSHAN GROUP

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