郯庐断裂带南段张八岭群变质岩的原岩时代及其构造意义

大别造山带东缘郯庐断裂带上分布着绿片岩相变质的张八岭群。对于它们的原岩时代长期没有同位素年代学数据,而其变形与变质原因也一直没有明确的认识。本次工作中选择了该带上8处张八岭群变火山岩进行了锆石LA-ICP-MS U-Pb定年。结果表明,它们的原岩时代为748~750 Ma,属于新元古代中期的南华纪,为扬子板块下部盖层而非前人认为的变质基底。结合张八岭群的变形与变质特征及前人白云母40Ar/39Ar定年结果,并与大别造山带进行对比,本文认为大别造山带东南缘张八岭群的变形与变质是造山带内俯冲与折返的结果,而其东缘郯庐断裂带内张八岭群的变形与变质是碰撞造山期该断裂带左行走滑活动所致。这些认识再次为郯庐断裂带起源于华北与扬子板块的碰撞过程中提供了重要的证据,也支持其造山期起源于陆内转换断层或斜向汇聚边界。

Protolith Ages of Metamorphic Rocks of the Zhangbaling Group along the Southern Segment of the Tan Lu Fault Zone and Their Tectonic Implications

The greenschist facies Zhangbaling Group is exposed along the Tan Lu Fault Zone at the eastern margin of the Dabie orogenic belt. Its protolith ages and reason for the deformation and metamorphism remain unknown. Eight meta volcanic rock samples were collected for LA-ICP-MS U-Pb dating. The results indicate their protolith ages of 748~750 Ma, i.e. the Nanhua period of the middle Neo proterozoic. They belong to the lower cover of the Yangtze Plate rather than the basement rocks as proposed before. On the bases of their deformation and metamorphism, previous muscovite 40Ar/39Ar dating results as well as comparison with the Dabie orogenic belt, it is proposed that the deformation and metamorphism of the Zhangbaling Group in the southwestern margin of the orogenic resulted from the orogenic crust subduction and exhumation, and those along the Tan Lu Fault Zone at the eastern margin were caused by sinistral shearing of the fault zone. The understanding offers important evidence for the origin of the Tan Lu Fault Zone during collision between the North China and Yangtze plates, and support the collision related origin mdoel of intralplate transform faulting or an oblique collision boundary.