大山口剪切带大理岩糜棱岩的变形机制分析

大山口韧性剪切带是大别造山带"结晶轴带"内平行造山带延长方向的走滑型剪切带,其内发育的大理岩糜棱岩呈现了各种在绿片岩相变质条件下形成的显微构造,如碎斑构造、S-C组构、核幔构造等。糜棱岩化作用是以方解石颗粒粒度的减小为特征的,这一过程表现了大别造山带古老中地壳应变软化带和局部化带的岩石流变行为和物理状态。根据各种显微标志,并综合宏观特征,得出大山口剪切带总体为以右行剪切为主的走滑型剪切带,并被以正向滑动为主的近水平剪切带所改造。作者在综合分析宏观及微观地质资料的基础上,初步建立了广水地区主要地质及变形事件序列,并指出:大别造山带三维空间上的变形及结构图象,主要是印支-燕山期陆-陆俯冲、碰撞造山运动的变形及造山期后的揭顶、塌陷等作用的综合结果。

THE DEFORMATION MECHANISM OF MARBLE MYLONITESIN THE DASHANKOU SHEAR ZONE

The Dashankou Ductile Shear Zone (DDSZ) is a typical example of strike slip ductile shear zone with in the crystal line axis of the Dabei orogenic belt parallel to its extension.The marble mylonites of which show a variety of microstructures usually occurring under greenschist facies conditions, such as mortar structure, mica fish, subgrain, S-C structure, asymmetric strain shadow and mineral crystal preferred orientation.The mylonitization is characterized by decreasing grain-size of calcits.Based on a large number of microstructures along with macroscopic kinematic characteristics, It is shown that the DDSZ is a dextral strike-slip ductile shear zone, which was modified by a low-angle ductile shear zone.From the deformed mineral assemblages, microstructures and fluid inclusions thermometry, the DDSZ was determined to have formed at a temperature of 350-450℃, and a pressure of 364-468 MPa with a differential stress of 30-130 MPa, a shear strain of 6.06-7.02, and a strain rate of 2.87×10-14 1 29×10-10 s-1, and a shear strain rate of 1.17×10-13-2.23×10-10 S-1, and the corresponding displacement rate being 25 30 mm/a.From an analysis of macrostructural, microstructural and petrologic data, it is inferred that the Dashankou dextral strike-slip ductile shear zone may be the result of an oblique collision between the North China craton and Yangtze craton during the Indosinian-Yanshanian orogenies.