古亚洲洋东段闭合时限的古地磁学研究

The final closure timing of the Paleo-Asian Ocean (PAO) is critical to the paleogeographic reconstruction of the East Asia. Two popular but contradictory models were proposed regarding the closure of the eastern PAO: (i) the Early-Closure model, arguing for a Late-Devonian closure followed by Carboniferous extension with the second closure terminating by the Late Permian; (ii) the Late-Closure model, insisting that the subduction was persistent before the final closure of the PAO by the Permo-Triassic boundary. The focus of the above argument lies in the Carboniferous paleogeography of the PAO, while the translational history of the block to the north of the suture zone has not been well constrained during this period. The South Mongolia-Xing’an Belt (SMXB), situated at the southern margin of the Mongolia block during the Carboniferous, is one of the most ideal area to study the above puzzle. Several suites of Carboniferous sedimentary rocks, volcanics and dykes are exposed in the southeast part of South Gobi province, Mongolia. Parts of their counterparts in China side expose as the Late Carboniferous to Early Permian volcanic-sedimentary rocks near Sonid Left banner and East Ujimqin banner areas. Ideal results are prospective to be obtained based on the previous field investigation along with preliminary demagnetization results. In this study, we plan to perform a combined rock-magnetic, paleomagnetic and geochronologic study on those strata. Vital constraints on the paleopostional variation of the SMXB during the Carboniferous will be provided. Key information will also be yielded to identify whether the PAO was closed during that period. Spatial and temporal framework of the eastern Central Asian Orogenic Belt will be built to quantitatively describe the amalgamation history of blocks therein.

古亚洲洋的闭合时限是东亚古地理重建中的关键科学问题，前期研究中存在着早闭合（晚泥盆世闭合-晚石炭世伸展-晚二叠世再闭合）及晚闭合（晚二叠世-早三叠世）模式的争论，而石炭纪的古地理格局正是争论的焦点所在，尤其在该时期古亚洲洋北侧块体的古地理位置一直未能得到可靠约束。南蒙古-兴安构造带在石炭纪处于蒙古板块最南缘，是研究这一问题的最理想区域之一，蒙古南戈壁省东南地区出露有石炭纪的多套沉积岩、火山岩及岩墙等岩石单元，与之对应的中国境内苏左旗和东乌旗也保留有晚石炭世至早二叠世的沉积岩浆记录，是记录古地磁信息的良好载体，前期研究结果显示很可能在上述地层获得理想数据。本项目拟对上述地层开展系统的岩石磁学、古地磁学及同位素年代学研究，以精确限定南蒙古-兴安构造带在石炭纪的古地理位置及变迁过程，为查明古亚洲洋在石炭纪是否闭合提供关键信息，并对中亚造山带东段块体的汇聚过程提供定量约束。

古亚洲洋构造域是Pangea超大陆重建中最具有不确定的地区之一，其晚古生代的古地理格局仍存在以下问题亟待解决：（1）古亚洲洋的闭合时限是晚泥盆世还是早三叠世，还是其他时限？（2）在两种主流争议的中间时段（早石炭世-晚二叠世）期间，古亚洲洋是否存在，范围有多大？（3）东北亚微陆块群何时聚合成统一的Amuria复合陆块？（4）古亚洲洋周边块体何时成为Pangea超大陆的一部分？（5）蒙古板块在石炭纪的时空运动学格架如何？为了解决上述争议，本项目在南蒙-兴安构造带的乌里雅斯太陆缘、Gurvansayhan地体以及松辽地块西缘针对石炭-二叠纪的火山沉积地层和侵入地层的中基性岩墙群开展了系统的古地磁学和年代学工作，获得了5个可靠的古地磁结果，并取得以下主要认识：（1）论证了华北-Amuria联合陆块的形成时间，即古亚洲洋的最终闭合时限为~250 Ma；（2）重建出晚石炭世至早二叠世古亚洲洋呈喇叭状向东开口的古地理格局，定量估算出其南北向宽度在西段和中段分别为1300 km和2700 km，论证出古亚洲洋呈剪刀式闭合，并将构造古地理重建与生物古地理的响应相耦合；（3）提出东北亚微陆块群聚合成为Amuria复合陆块的时间为~300 Ma；（4）提出了古亚洲洋周边块体在晚石炭世至早二叠世期间已经是Pangea超大陆的一部分；（5）建立了蒙古板块石炭纪期间的古纬度变化曲线，提出了蒙古板块的视静止效应是由于蒙古-鄂霍茨克洋后撤式俯冲导致的蒙古板块岩石圈北向移动，与石炭纪真极移作用的叠加效应。本项研究为重建Pangea超大陆古亚洲洋构造域晚古生代的构造演化提供了定量的古地磁制约。共计发表第一资助SCI论文4篇，第二资助SCI论文1篇，包含2篇Nature Index论文和2篇一区Top论文。项目负责人参加国内外会议5次，包含4次口头报告，1次专题特邀报告。