白乃庙逆冲推覆构造的地质特征及其对华北克拉通北缘晚古生代造山作用的制约

Bainaimiao thrust is located at the contact of northern margin of North China Craton and Bainaimiao island arc in the south of Xing’an-Mongolian Orogenic Belt, and the Baiyun Obo Group has thrust and covered on the Paleozoic strata in Bainaimiao island arc. This thrust system is the intuitionistic response of the closure of the Paleo-Asian Ocean and the collisional orogeny. We confirm that the system was formed at Carboniferous or Permian from the tectonic contact relationship of the geologic bodies. We plan to finish the following researches in this project. The geometric, kinematic and dynamic characteristics will be confirmed by the detailed geological structural analysis. Based on identification of fossil specimens and the isotopic dating of the zircons and syn-tectonic minerals in the plutons and strata, the thrust timing should be made sure. The macro- and micro-tectonics of rock and minerals distributed along the fault plane can indicate the environment of the deformation. To combine these studies with the geochemical results inside and around the research area, we’ll ascertain the relationship between the thrust system and the Late Paleozoic orogenesis at the contact of northern margin of North China Craton and Bainaimiao island arc, and the mode and process of the Late Paleozoic orogenesis.

白乃庙逆冲推覆构造发育在兴蒙造山带内的白乃庙岛弧岩带和华北克拉通北缘的拼合部位，将白云鄂博群推覆至白乃庙岛弧岩带的古生代地层之上。该逆冲推覆构造是古亚洲洋闭合及碰撞造山作用形成的最直观的构造现象，根据野外对各地质体的构造接触关系可以大致判断该逆冲推覆构造形成于石炭纪至二叠纪。本项目拟通过详细的野外地质构造解析确定该逆冲推覆构造几何结构、运动学和动力学特征；通过相关岩体和地层中锆石的U-Pb测年、同构造矿物40Ar/39Ar测年和化石鉴定，确定逆冲推覆构造的确切形成时间；通过上、下盘岩石和矿物的宏观、显微构造特点分析构造变形的形成环境。并结合区内和邻区的岩石地球化学研究成果，研究逆冲推覆构造与晚古生代造山作用的关系，并分析晚古生代白乃庙岛弧带与华北克拉通北缘碰撞造山的方式和过程。

白乃庙逆冲推覆构造发育在温都尔庙-白乃庙地区，属于华北克拉通与兴蒙造山带交接部位。该逆冲推覆构造西起徐尼乌苏经由博日和、章毛乌苏向东延伸至华德,延伸长度大于150km；由北至南可分为前锋带、中带和跟带，南北向跨度约50km. 逆冲方向NW-NNW，将白云鄂博群推覆至早古生代的岛弧和弧后盆地沉积地层以及晚古生代的海相、陆相地层之上，形成了一系列的构造窗、飞来峰和逆冲断层。锆石测年结果显示下盘火山岩主要形成于499.4±1.8Ma~478.1±1.6Ma，440.9±1.8Ma，333 ± 4Ma， 267.0±2Ma~279.0±2.1Ma等四个阶段，采集到可鉴定化石的层位分别为顶志留统和石炭-二叠系。而截切逆冲推覆构造下盘原地岩石和上盘推腹体的花岗岩体年龄为269.3Ma。由上可知，白乃庙逆冲推覆构造形成于二叠纪末期至早三叠世。.根据年代学、地球化学、层序学特征可知，奥陶纪至早-中志留世期间，温都尔庙地区发生俯冲-消减事件。德言其庙斜长角闪岩形成于490.3±4.6Ma，显示出岛弧环境的地球化学特征，表明研究区在此时除了形成白乃庙岛弧外可能还存在一个洋内岛弧。徐尼乌苏组的岩石组合、年代学、岩石地球化学等资料显示白乃庙弧后盆地可划分为三个演化阶段：早期快速堆积阶段（452.3～440.9Ma）；中期火山喷发阶段（440.9～440.1Ｍａ）；晚期稳定沉积阶段（＜440.1Ma）。.温都尔庙附近的晚古生代基性火山岩表现出类似洋中脊玄武岩或岛弧玄武岩的地球化学特征，而白乃庙附近及南侧的火山岩表现出伸展环境的双峰式火山岩的特征。上述特征表明逆冲推覆作用之前研究区处于伸展背景下的洋盆或滨浅海环境。逆冲推覆作用后，研究区全部转入陆相环境，指示逆冲推覆构造是晚古生代俯冲碰撞、大洋闭合事件的产物。.