拉脊山蛇绿混杂带楔顶盆地的沉积学研究

The wedge-top basin is an important type of basin within the orogenic belt. The ophiolitic melange in the Lajishan Mountains of Qinghai province, which underlies the wedge-top basin with different sedimentary facies and environments, is the best area for studying the wedge-top basin. However, previous studies mainly focused on the ophiolitic geochemistry of this melange and the sedimentary rocks of the basin were overlooked. Using the methods of petrology, sedimentology, zircon U-Pb dating, geochemistry and Electron Microprobe, this project will focus on the field surveying and typical section to study the deformation patterns, lithologic assemblage, sedimentary environment and facies of the basin. Then, detrital model and source area of basin fills will be defined in this project, and the evolution of the basin and relationship between the basin and ophiolitic melange will also be studied. The aim of this project is: (1) to distinguish the rock association of the accretionary wedge and wedge-top basin and study their genesis and evolution seperately; (2) to provide some information for the place and form of accretionary wedge based on the sedimentary environments; (3) to accumulate new information and data for study the Paleozoic tectonic evolution and metallogenic geodynamics background of the Lajishan Mountains or Qilian orogenic belt.

增生楔楔顶盆地是造山带中重要的沉积盆地类型。青海拉脊山蛇绿混杂带中，分布着不同沉积环境与各种沉积相的楔顶盆地沉积，是研究楔顶盆地的最佳地区。前人对该混杂带的研究主要集中于蛇绿混杂岩地球化学特征而缺乏对楔顶盆地的研究。本项目将以野外地质调查为基础，结合区域地质资料，开展代表性剖面测量和综合研究，以岩石学、沉积学、锆石年代学、全岩地球化学和电子探针等为主要手段，查明拉脊山蛇绿混杂带楔顶盆地构造控制样式和岩石组合序列、沉积相和沉积环境，确定盆地充填物碎屑组成及物源区，剖析盆地形成及演化过程，探讨其与蛇绿混杂岩之间的相互关系。该项目的科学意义在于：（1）清楚地把增生楔形成与楔顶盆地沉积两个构造阶段岩石组合分开，分别进行成因与演化研究；（2）依据楔顶盆地沉积环境，提供增生楔就位环境和就位构成信息；（3）为探索拉脊山乃至整个祁连造山带古生代构造演化与成矿动力学背景积累新的信息和资料。

本项目按照计划书开展了系统野外地质调查和室内研究，完成了项目研究内容，共计发表论文13篇，会议摘要3篇，培养博士研究生2名，参加国内和国际学术会议各1次，实现了预期目标并取得了预期成果。研究表明，拉脊山蛇绿混杂带楔顶盆地基底是由蛇绿混杂岩和增生杂岩共同构成，盆地充填物是一套河流相、浅海相和深海相沉积组合，具有向南东方向水体加深的沉积古地理特征，碎屑组成主要来自于基底及寒武纪岛弧带岩石；化隆岩群/杂岩由940-850Ma和470-410Ma弧花岗岩以及新元古代和志留纪沉积岩共同构成，新元古代花岗岩和沉积岩所经历角闪岩相变质作用与其在古生代时期增生于原特提斯洋北缘过程中形成的弧岩浆作用密切相关；拉脊山南缘三叠纪火山-岩浆-沉积-成矿作用形成于活动大陆边缘，其岩浆源区为中元古代下地壳变基性岩，且岩浆可能受到地幔物质混染，由古特提斯洋向北俯冲形成；拉脊山南侧西倾山一带泥盆系为一套浅海相-河流三角洲相沉积组合，其碎屑沉积物来自于大陆边缘弧和化隆岩群以及扬子板块，形成于前陆盆地环境；南祁连构造带拉脊山寒武-奥陶纪火山-沉积组合构成了沟-弧-盆系，包括550-530Ma MOR-蛇绿岩、530-480Ma SSZ蛇绿岩、515Ma岛弧和增生楔以及460-440Ma大陆边缘弧，它们是寒武纪和奥陶纪时期原特提斯洋先后在化隆杂岩北侧和南侧发生向南和向北俯冲增生作用共同形成的，且该过程形成的南祁连大陆边缘弧在晚奥陶世晚期与中祁连发生碰撞，形成拉脊山及其南侧志留-泥盆纪前陆盆地沉积。