桂北龙胜地区基性超基性岩的变形分解构造及其大地构造意义

THE DEFORMATION PARTITIONING STRUCTURE OF BASIC-ULTRABASIC ROCKS IN LONGSHENG AREA OF NORTH GUANGXI AND ITS GEOTECTONIC SIGNIFICANCE

桂北龙胜地区早期顺层侵位的基性超基性岩遭受了后期强烈变形分解作用的改造,在岩体的边部形成了一系列强剪切应变域——边缘韧性剪切带。带内发育典型的基性超基性糜棱岩,而弱应变或无应变的块状基性超基性岩构成一系列呈NNE向展布的透镜体,透镜体与边缘韧性剪切带共同构成一种透镜体型变形分解构造体系。龙胜地区基性超基性岩中的透镜体型变形分解构造与丹洲群浅变质岩中的纵向构造置换作用密切相关,形成于统一的近东西向挤压的地壳收缩构造体制,这是一种有别于前人认为龙胜基性超基性岩属元古代“岛弧型蛇绿岩”,“洋壳碎块蛇绿混杂岩”或中生代“碰撞造山带上蛇绿混杂岩推覆体残留”的观点。

The basic-ultrabasic rocks in Longsheng area of North Guangxi were suffered from intense deformation partitioning. In the intense shear strain field, a series of marginal ductile shear zones, in which typical basic-ultrabasic mylonites were developed, were formed. But in the less intense or no strain field, large numbers of lensoids in NNE direction are distributed. Lensoids and marginal ductile shear zones combined to form lensoid type deformation partitioning structural system. This system is closely related to longitudinal transpositional structure in low grade metamorphic rocks of Danzhou Group. They all were formed by crustal compression in east-west direction. The new idea is different from two previous conclusions, of which one is that the basic-ultrabasic rocks belong to late Proterozoic arc type ophiolite or melange, another is that the basic-ultrabasic rocks are the remains of ophiolitc melange nappe in collision orogens. Also, the new conclusion is significant to further study on the earlier geotectonic evolution in South China.