秦岭商丹糜棱岩带石英组构特征变化及成因分析

商丹糜棱岩带岩湾、沙沟和商南等3个区段石英C-轴组构和显微构造特征表明,该带自西向东石英C-轴组构形式由单一环带型转为Y-轴方向的点极密型,然后变为Ⅰ型交叉环带型,构造变形环境从低绿片岩相过渡到中-高绿片岩相,然后变为高绿片岩相-低角闪岩相。构造变形环境差异是造成石英组构形式变化的主要因素。随着温压条件的升高,石英滑移系从以底面〈a〉滑移系和柱面〈a〉滑移系的共同作用为主转向以单一柱面〈a〉滑移系为主,进而底面〈a〉滑移系和柱面〈a〉滑移系又重新变得活跃,且菱面滑移系的作用也变得十分重要。

THE VARIATION OF QUARTZ FABRICS IN THE SHANGDAN MYLONITE ZONE OF QINLING BELT AND ITS ORIGINATION ANALYSIS

Quartz lattice fabrics are important indicators of the environments, strain characters and mechanisms of structural deformation. The Shangdan mylonite zone is a large mylonite zone distributed along the Shangdan suture of the Qinling Belt. Systematic studies of the zone indicate that different sections along this mylonite zone are of distinctly different quartz fabrics and microstructures. From the quartz C fabrics and microstructures of three sections(Yanwan, Shagou and Shangnan)in this mylonite zone, the following conclusions were obtained. 1. From the west to the east along this mylonite zone, the pattern of quartz C fabrics changes from single girdle to maximum around the Y-axis, and then becomes Type I cross girdle. The quartz C fabrics of single girdle pattern in the Yanwan section are related to the cooperation of basal 〈a〉 slip systems and prism 〈a〉 slip systems and those of maximum around the Y-axis pattern in the Shagou section are caused mainly by the movement of prism 〈a〉 slip systems. While the quartz C fabrics of Type I cross girdle pattern in the Shangnan section are the results of cooperation of basal 〈a〉 slip systems, prism 〈a〉 slip systems and rhomb slip systems. 2. From the west to the east along this mylonite zone, the environments of structural deformation vary from low greenschist grade in the Yanwan section to mid upper greenschist grade in the Shagou section, and then to high greenshist low amphibolite grade. The variations of quartz fabric pattern in this mylonite zone are mainly caused by difference in the environments of structural deformation. 3. Along with the increase in temperature and pressure conditions, the major active slip systems in quartz change from cooperation of basal 〈a〉 slip systems and prism 〈a〉 slip systems to single prism 〈a〉 slip systems. As the temperature and pressure increase further, basal 〈a〉 slip systems and prism 〈a〉 slip systems become active again and the rhomb slip systems become very important.