华北克拉通北缘喀喇沁变质核杂岩早白垩世构造演化过程与形成模式

摘要

The structural process and formation model of the Kalaqin metamorphic core complex (KMCC) in the Early Cretaceous remain controversial. In this paper, the tectonic evolution of the KMCC in the Early Cretaceous is constrained and its formation model is proposed on the basis of a detailed field investigation and stress field inversion from slip-data in combination with the previous geochronological data. The results demonstrated that the KMCC, originating in the Late Jurassic, was subjected to intense extension and magmatism in the Early Cretaceous (141-100 Ma). The NE-striking Louzidian-Balihan and Shangdian-Dongfeng brittle normal faults with opposite dips and developed along the edges of the core complex during the extensional activity controlled the formation of the Xiaoniu and Pingzhuang half-graben-type basins. Normal faulting along the two major faults and the subsequent isostatic rising resulted in the exhumation of the core complex and the development of the earlier NE-NNE striking extensional ductile deformation belts and the later brittle normal faults with similar orientation. The ductile and brittle extensional structures suggest a consistent NW-SE extension in this period. Comprehensive analyses demonstrated that the KMCC experienced the initial extension with syn-tectonic granitic intrusion during 141-134 Ma, intense faulting along the boundary faults and uplifting during 133-126 Ma, and isostatic rising of 125-100 Ma. It is, therefore, suggested that the KMCC changed into a horst-type, extensional dome during the Early Cretaceous extension. The extension, which took place during the peak destruction of the North China Craton and under the dynamic setting of far-field back-arc extension, is suggested to be related to the subduction of the paleo-Pacific Plate.%对于喀喇沁变质核杂岩早白垩世构造过程与形成模式长期以来存在着不同的认识.通过详细的野外构造观察及擦痕应力场反演, 并结合前人年代学数据, 有效地制约了喀喇沁变质核杂岩早白垩世的构造演化, 并对其形成模式进行了分析.结果表明, 起源于晚侏罗世的喀喇沁变质核杂岩, 在早白垩世 (141～100 Ma) 再次经历了强烈的伸展与岩浆活动.在此伸展活动中, 沿着核部杂岩两侧分别发育了NE走向、倾向相反的楼子店-八里罕和上店-东风大型正断层, 进而控制两侧半地堑式的小牛与平庄盆地的发育.在这两条边界断层的伸展运动及随后的均衡隆升中, 核部杂岩不断抬升与剥露, 先后发育了NE-NNE向的伸展韧性变形带与脆性正断层.这些早白垩世韧性和脆性伸展构造一致指示拉张方向为NW-SE向.综合分析表明, 区内早白垩世伸展活动经历了141～134 Ma的初始伸展与同构造岩体侵位阶段、133～126 Ma的边界断层强烈活动与核部快速抬升阶段以及125～100Ma的均衡隆升阶段.喀喇沁变质核杂岩在早白垩世的伸展活动中转变为地垒式伸展穹窿, 其强烈伸展活动出现在华北克拉通破坏峰期, 动力学背景是古太平洋板块俯冲导致的远场弧后拉张.