早前寒武纪多期变质变形区主期韧性剪切带

DUCTILE SHEAR ZONES IN EARLY PRECAMBRIAN POLYPHASE DEFORMATION AND METAMORPHISM AREAS

早前寒武纪多期变质变形区的韧性剪切带，根据其与变形变质的关系可分为（１）早期主变形变质作用形成的韧性剪切带，简称主期韧性剪切带；（２）晚期叠加变形变质作用形成的韧性剪切带，简称叠加韧性剪切带。主期韧性剪切带不发育糜棱岩、退变质和新生面理等标志，其高应变特征主要通过岩石中片理和线理发育程度、矿物定向程度、矿物粒度及应变大小表现出来。由于主期韧性剪切带内的片理和线理及矿物组合与围岩中的相同，因此相同岩石的应变强度比较是识别它的有效途径。比较拟引入反映岩石变形强度的物理参数ｎ（ＸＺ面上平行Ｘ和垂直Ｘ两个方向上单位长度矿物颗粒数之比）、Ｓ（单位体积内矿物颗粒表面积）和Ｒ＿ｓ（矿物形态应变）作为比较参数。

Early Precambrian polyphase deformation and metamorphism zones may be classified accord-ing to their relations with deformation and metamorphism into: (1) ductile shear zones formed byprincipal deformation and metamorphism at the early stage, called principal-stage ductile shearzones for short, and (2) ductile shear zones formed by superimposed deformation and metamor-phism at the late stage, called superimposed ductile shear zones for short. No mylonite retrogres-sive metamorphism and neofliation are developed in the principal stage ductile shear zones andtheir high-strain features are mainly manifested by the degree of developmnent of schisosity andlineation in rocks, orientation of minerals, grain size of minerals and strain magnitude. As theschistosity lineation and mineral assemblage of the rocks in the principal-stage ductile shear zonesare the same as those of their surrounding rocks, the comparison of the same strain intensity ofrocks is an effective means for recognizing them. In comparison, the physical parameters that canreflect the intensity of rock deformation such as n (the specific ratio of mineral grains in the direc-tions parallel and vertical to X on the XZ plane), S (the surface area of mineral grains in unit vol-ume) and Rs (strain of mineral forms) are used.