青藏高原南羌塘地块二叠纪纬度变化的古地磁约束

The time when the southern Qiangtang block rifted away from Gondwana is hotly debated, which hinders our understanding of the evolution of Longmu Co-Shuanghu ocean and the early evolution of the Tibetan plateau. We use paleomagnetism to constrain this debate. However, only few paleomagnetic results were reported from southern Qiangtang, and even no Permian paleomagnetic data was ever reported. Recently, our field observation confirmed volcanic layers have clear bedding orientations from Lower Permian Zhanjin and Middle Permian Tunlonggongba formations. Our preliminary paleomagnetic results from Tunlonggongba Lavas show that thermal demagnetization method can successfully isolated primary magnetizations. Thus, in this project, we will carry out further combined paleomagnetic and zircon U/Pb geochronological researches on Zhanjin and Tunlonggongba formation lavas to constrain the Permian latitudinal motions of the southern Qiangtang block and then clarify the time when the southern Qiangtang block rifted away from Gondwana, through combining with the apparent polar wander path of Indian Gondwana.

对南羌塘地块从冈瓦纳大陆裂离的时间和过程存在较大分歧，阻碍了对龙木错-双湖洋演化和青藏高原早期形成过程等方面的认识。本项目选择古地磁学方法对这一问题开展研究，然而南羌塘的古地磁数据较少，迄今尚无二叠纪的古地磁结果报道。近来，对西藏阿里地区的初步野外踏勘结果，证实下二叠统展金组和中二叠统吞龙共巴组中火山岩地层产状清晰，非常适合开展构造古地磁学研究，对吞龙共巴组火山岩的预研究结果表明热退磁能分离出原生剩磁。基于此，我们提出以现有研究为基础，对南羌塘地块展金组和吞龙共巴组火山岩，开展更进一步的构造古地磁学及U-Pb锆石年代学研究，重建南羌塘地块二叠纪时期的古地理位置及其古纬度变化，结合印度冈瓦纳大陆的视极移曲线，对南羌塘地块从冈瓦纳大陆的裂离时间和过程进行约束。

对南羌塘地块从冈瓦纳大陆裂离的时间和过程存在较大分歧，阻碍了对龙木错-双湖洋演化和青藏高原早期形成过程等方面的认识。通过对南羌塘地块二叠纪吞龙共巴组火山岩和沉积岩系统的古地磁学和年代学研究，揭示南羌塘地块在中二叠世时期位于南纬~20度，可能南羌塘地块在中二叠世时期已经在热带-亚热带地区从冈瓦纳大陆裂离出来。通过南羌塘南缘始新世火山岩的年代学和古地磁学研究，提出印欧碰撞早期（65-30 Ma）地壳缩短变形集中在中央谷地，缩短量有1100公里，与中央谷地的快速隆升期一致；碰撞后期（30-20 Ma）变形向北传递到羌塘腹地和西昆山地区，缩短量有700公里，大亚洲地壳在印欧碰撞中共缩短了1800公里。通过羌塘地块中三叠世火山岩的古地磁学研究，提出古特提斯洋最终消亡的时间为晚三叠世晚期，在完全闭合前约240 Ma，在古特提斯洋内发生了弧陆碰撞（北羌塘地块与义敦岛弧或古特提斯洋洋岛弧）。