昂仁-萨嘎地区雅鲁藏布缝合带晚白垩世深水沉积与海沟盆地研究

The Neo-Tethyan subduction system along the south margin of the Lhasa block recorded important information on the oceanic subuduction. In the past decades, our knowledge about the subduction process has been greatly improved by investigating these units. However, most previous studies focused on the Gangdese magmatic arc, the Xigaze forearc basin and the Yarlung-Zangbo ophiolite, with limited attention given to the trench basin. It’s been widely documented that trench fills preserve significant information on uplift and erosion of the active plate, and indicate an erosional or accretionary active margin. Studies on the trench basin in the Neo-Tethyan subduction system are necessary to depict uplift-erosion pattern of the Lhasa block, and to discuss whether the active margin is accretionary or erosional..It’s reported that the Late Cretaceous trench fills were outcropped in the western Xigaze area (from Ngamring to Saga), including the Rongmawa Formation and the Luogangcuo Conglomerate. In this program, an integrated studies of stratigraphic, sedimentologic, petrologic and geochemical will be carried out to identify the sedimentary environments, depositional ages and provenances. Based on the geological data and regional comparison, we will discuss the characteristic and evolution of the sedimentary basin and provide more details for the Neo-Tehtyan subduction and uplift-erosion pattern of the Lhasa block. We believe that this program will improve our knowledge about the geology in the Xigaze area, and will also help us in understanding the Himalayan orogen.

拉萨地体南缘的新特提斯洋俯冲体系记录了大洋俯冲的重要信息。在过去几十年中，对这一体系的研究取得了大量的成果。然而，前人的研究主要集中于冈底斯岩浆弧、日喀则弧前盆地及雅鲁藏布蛇绿岩套，对海沟盆地的研究较为缺乏。海沟盆地的沉积记录可以反演活动大陆的抬升剥蚀历史，指示活动大陆边缘为增生楔或侵蚀型。因此，对新特提斯洋俯冲体系中海沟盆地进行研究，是探讨拉萨地体构造演化及活动大陆边缘属性的重要途径，同时也为其他地区海沟盆地的识别、研究提供参考信息。.区域地质调查发现，日喀则地区昂仁-萨嘎一带发育晚白垩世海沟沉积物，包括荣马瓦组和罗岗错砾岩。本项目拟以这套记录为研究对象，运用地层学、沉积学、岩石学和地球化学等手段进行深入研究，查清该单元的沉积环境、形成时代、物源区特征。在此基础上，探讨沉积盆地性质及其演化，以深入认识新特提斯洋的俯冲，为拉萨地体的构造演化及活动大陆边缘的属性提供约束。

本项目通过地层学、沉积学、岩石学、同位素年代学等多种方法系统地对藏南雅鲁藏布江缝合带内晚白垩世深水沉积记录（罗岗错组和甲查拉组）进行了综合研究。在此基础上，确定了罗岗错组和甲查拉组的形成时代、沉积环境、物质来源及盆地类型并初步讨论了新特提斯洋俯冲体系内的沉积物搬运过程。.三年期间研究人员在藏南江孜-萨嘎地区开展了两次野外工作，累计时间24天，实测剖面3条，完成砾石成分统计5处，采集样品200余件。磨制并鉴定岩石薄片82片，完成23件砂岩定量碎屑颗粒统计，完成15件砂岩样品的碎屑锆石U-Pb定年和Hf同位素测试。.罗岗错组沉积时代为~88-81 Ma，以深水滑塌沉积形成的砾岩和浊流沉积形成的浊积岩为主，砾石主要来自临近的俯冲增生楔，而砂岩碎屑则来自北侧的冈底斯岩浆弧和中拉萨地体。甲查拉组形成时代为~88-84 Ma，以浊积岩为主，物源同样来自冈底斯岩浆弧和中拉萨地体。基于二者均出露于代表俯冲增生楔的混杂岩南缘、沉积于新特提斯洋俯冲期间、碎屑物质来自亚洲板块，因而认为均代表了新特提斯洋俯冲的海沟沉积，表明晚白垩世亚洲南缘的碎屑物质可越过基本充填的弧前盆地到达海沟位置。