中条山北缘韧性剪切带变形特征、形成时代及构造意义

The North China Craton (NCC) is one of the oldest cratons in the world. The tectonic evolution processes of the NCC have been debated for decades. Recently, we found that NE-NEE trending extensional ductile shear zones developed in the Paleoproterozoic granitic gneiss (1.95 Ga) and Shushui Complex in the northern margin of Zhongtiaoshan mountains, the middle part of the NCC. The ductile shear zone was unconformity covered by the Changcheng System, which indicate that the deformation time was late Paleoproterozoic. The extensional ductile shear zones developed in the Northern margin of Zhongtiaoshan mountains is contradictive with the point of the compression environment in 1.85 Ga resulted from the collision of the western block and the eastern block, while it is conformed to the point of the extensional environment resulted from the upwelling asthenospheric material. In this study, detailed structural analysis and microstructure analysis of granite gneiss different parts of the northern margin shear zone of the Zhongtiaoshan mountains will be used to define the deformation characteristics. Isotopic dating of different deformed minerals will be used to define the precise deformation time of this ductile shear zone. Combined with the tectono-magmatic events of in the NCC at the late Paleoproterozoic proposed by previous researchers, we will discuss the tectonic significance of the extensional ductile shear zones in the northern margin of the Zhongtiaoshan mountains.

华北克拉通是世界上最古老的大陆之一，其构造发展阶段与演化过程一直存在争议。最近，申请人等的研究发现，华北克拉通中部中条山北缘古元古代花岗片麻岩（1.95 Ga）及涑水杂岩中发育的北东-北东东向韧性剪切带为倾向北西-北北西的中低角度伸展型韧性剪切带。由于该剪切带被长城系浅变质碎屑岩不整合覆盖，因此其变形时代应该为古元古代晚期。中条山北缘韧性剪切带的伸展特征与前人认为的1.85 Ga左右华北克拉通处于东、西陆块碰撞所造成的挤压构造背景相矛盾，而符合这个时期岩石圈物质上涌导致的伸展背景。本项目拟对中条山北缘韧性剪切带不同构造部位进行详细的构造解剖及显微构造变形分析，研究确定其构造几何学特征、运动学指向；选取同变形矿物进行同位素定年，确定其变形时代；结合前人关于华北克拉通古元古代末构造热事件的研究成果，探讨中条山北缘伸展型韧性剪切带的构造意义。

华北克拉通是世界上最古老的大陆之一，其构造演化发展阶段与演化过程存在争议，而古元古代末是华北克拉通演化的关键时限。本次研究以中条山北缘发育的韧性剪切带为研究对象，通过详细的野外地质调查、室内分析及同位素测年，确定了韧性剪切带的几何学、运动学及年代学特征,，结合前人研究进展探讨其构造意义。该剪切带发育在古元古代片麻岩中，总体走向NE，剪切面理以倾向NW为主，拉伸线理向NW倾伏，发育旋转碎斑、S-C组构、云母鱼等，显示伸展剪切的特点。对变形矿物-白云母进行40Ar-39Ar年代学测定，为1820.41±4.8 Ma；被辉绿岩脉切穿，辉绿岩锆石U-Pb年龄为1751±16 Ma；同构造花岗岩锆石U-Pb年龄为1824±3 Ma，综合考虑认为其形成时代为古元古代末。通过详细的构造学及构造年代学分析认为：中条山北缘在古元古代时发生了两期构造变形事件：（1）古元古代末发育一系列的伸展型韧性剪切带（D1）；（2）呈NE向的伸展的基性岩脉（D2）。结合区域地质背景，中条山北缘伸展型韧性剪切带代表了华北克拉通最初裂解的时代始于古元古代末。