对青藏高原拉萨及羌塘地块三叠纪古纬度的定量约束

The formation of the Himalayan-Tibetan orogeny and the uplift the Tibetan Plateau is the most prominent geologic event occurred during the Phanerozoic. The accurate paleopositional determinations of the Lhasa block and the Qiangtang block prior to Indo-Asia collision have placed key restrictions on constraining the evolution of the Tibetan Plateau as a whole during Cenozoic time. Therefore, it has become an urgent and extremely important scientific question of accurately determining the paleolatitudes of the Lhasa block and the Qiangtang block respectively during the Triassic era. Well-exposed Gajia Formation of volcanic-sedimentary rock suites with Triassic age in middle part of the Lhasa block, the tilting bedding of the gabbro dykes interbedded with diabase dykes,and overlain by red quartz-rich silicolite with Triassic age also observed within the Bangong-Nujiang suture zone; volcanic-sedimentary rock suites of the Jipala Formation in middle part of the Qiangtang block, all of these rocks of being igneous origin recorded the paleomagnetic direction during the Triassic. Therefore, these well-exposed igneous rocks-sedimentary sequences have become the naturally favorable materials to decipher the scientific question by revealing the paleodirections recorded in these rocks. We attempt to provid precise and quantitative constraints for accurately determining the paleolatitudes of the Lhasa block and the Qiangtang block during the Triassic era, and also for tectonic evolution of the Tibetan Plateau as a whole during Cenozoic by using paleomagnetic methods based on primarily acquired U-Pb dating results on selected zircons from volcanic rocks of the Gajia Formation volcanic sequence of Triassic age, the tilted bedding of the gabbro dykes interbedded with diabase dykes, and overlain by red quartz-rich silicolite with Triassic age were also observed within the Bangong-Nujiang suture zone, and volcanic rocks of the Jiapila Formation in middle part of the Qiangtang block.

印度/欧亚大陆碰撞导致的喜马拉雅-青藏高原造山带的隆升是显生宙最为重要的地质事件。两大陆碰撞前组成青藏高原拉萨、羌塘两地块在早中生代的古地理位置的精确限定对青藏高原新生代的整体演化具有重要的制约作用。因而，精确限定拉萨、羌塘两地块在早中生代（尤其是三叠纪）时期的古纬度就成为当前亟待解决的重要科学问题。拉萨地块中部广泛出露的三叠纪嘎加组火山-沉积岩序列、羌塘地块中部出露的三叠纪甲丕拉组火山岩-沉积岩及班公-怒江缝合带内出露的辉长、辉绿岩墙及上覆含石英硅质岩序列很好地记录了两地块位于不同纬度时的古地磁场方向信息，因而这些出露良好的岩石露头就成为解决该科学问题的重要研究对象。 本项目拟在对上述出露岩石单元开展精细的锆石U-Pb年代学研究的基础上，利用古地磁学研究手段获得两地块在早中生代时期的古纬度，从而为两地块在三叠纪时期的古地理重建以及青藏高原碰撞后整体演化提供重要定量约束。

对青藏高原拉萨、羌塘地块开展的年代学及古地磁研究表明，拉萨地块在180 Ma位于南半球3.7°±3.4°S，表明拉萨地块很可能更晚从Gondwana大陆裂解出来（早三叠世），之后持续向北漂移，并在180 Ma到达南半球赤道附近。在120 Ma，拉萨地块南缘已经到达北半球13.1°±2.7°N中纬度地区（参考点，30.0°N, 91.1°E），结合前人在拉萨地块开展的研究，拉萨地块自120 Ma直到印度-欧亚大陆碰撞前（65-50 Ma）并未发生明显的纬向运动（3.2°±6.2°），这很可能与拉萨地块在120 Ma时已经完成与北方欧亚大陆的拼贴紧密相关，该结果也暗示了印度-欧亚大陆初始碰撞时限不晚于59.3 Ma，且自120 Ma起，由于印度-欧亚大陆碰撞引起的在亚洲大陆一侧（雅鲁藏布缝合带以北）所产生的构造缩短量估计为1450±400 km。. 本项目资助下对北羌塘地块开展的研究表明，北羌塘地块更早从Gondwana大陆北缘裂解出来，且在297 Ma时位于南半球21.9°±4.7°S中低纬度地区，结合前人碎屑锆石年龄对比等分析，可以确定北羌塘地块来自Gondwana大陆而非北方的Laurasia或者Cathyasia。之后，北羌塘地块持续向北漂移，在213-204 Ma到达位于北半球中高纬度地区（31.7°±6.6°N）（参考点，34.1°N, 92.4°E），表明此时北羌塘地块古纬度在95%置信区间内与现今古纬度（34.1°）一致，结合前人在北羌塘地块开展的早三叠世古纬度重建研究以及在华北板块开展的早中生代古纬度重建研究表明，早三叠世起，北羌塘地块开始发生快速北向漂移，至晚三叠世（213-204 Ma），此时北羌塘地块古纬度已经与华北板块古纬度重合，表明此时北羌塘地块与欧亚大陆拼贴已经完成，二者在运动学意义上成为一体。. 本项目所完成的研究结合前人结果，重建了晚古生代-中生代-现今时期拉萨、羌塘地块运动学演化全过程，为东特提斯域构造演化提供了关键证据支持。