拉萨地块北缘中生界物源分析及对拉萨-羌塘初始碰撞时限的约束

Placing precise constraints on the timing of initial Lhasa-Qiangtang collision is essential to deeply understand the assembly process and uplifting mechanism of the Tibetan Plateau before the India-Asia continental collision. Along the over 3000-km-long Bangong-Nujiang suture zone, the timing of initial collision remains in great dispute, ranging from Middle Jurassic to Late Cretaceous, because of different methods applied at different locations. The first development of a peripheral foreland basin on the northern margin of the Lhasa block (the first arrival of sedimentary material from the Qiangtang block onto the Lhasa continental margin) is one of the most direct and most effective methods to constrain the timing of initial collision. This project will focus on the Mesozoic strata exposed along the northern margin of the Lhasa block in the Dingqing area, including the Xihu, Lagongtang and Duoni formations. Detailed field observation and section measurement will be performed to analyze the depositional environments of these units and further to recognize the basin types. A comprehensive study of sandstone petrology, detrital Cr-spinel geochemical analysis, detrital zircon U-Pb dating and Hf isotope analysis will be carried out in order to track the provenance of the Mesozoic clastic rocks. A combined study of biostratigraphy and isotope geochronology will be implemented to date the provenance change from Lhasa block to Qiangtang block, and further to constrain the timing of initial Lhasa-Qiangtang collision. This project may provide critical timing constraints for reconstructing the closing processes of the Bangong-Nujiang Ocean.

精确约束拉萨-羌塘初始碰撞时限对于深入理解青藏高原早期（印度-欧亚大陆碰撞前）的形成过程与隆升机制至关重要。在长达三千多公里的班公湖-怒江缝合带上，由于研究地点和研究方法的不同，获得的初始碰撞时限差异很大：从中侏罗世到晚白垩世。利用拉萨地块北缘周缘前陆盆地的启动时间（源自羌塘地块的碎屑物质最早到达拉萨地块北缘的时间）来约束初始碰撞时限是最直观和最有效的方法之一。本项目拟以丁青地区出露于拉萨地块北缘的中生界希湖组、拉贡塘组和多尼组为研究对象，通过详细的野外考察和实测剖面，判别各组地层的沉积环境，确定盆地的性质；运用砂岩岩相分析、碎屑铬尖晶石地球化学分析、碎屑锆石U-Pb年龄和Hf同位素分析等多种方法，追踪各组碎屑岩的物源区；综合生物地层学和同位素年代学，限定碎屑物源由拉萨物源向羌塘物源转变的时间，精确约束拉萨-羌塘初始碰撞时限。本项研究可以为重建班公湖-怒江洋的汇聚消亡历史提供关键时间约束。

班公湖-怒江缝合带呈近东西向横贯青藏高原中部，是分割拉萨地块与羌塘地块的构造界限，代表了班公湖-怒江洋（班怒洋）残留的遗迹，是重建这条古大洋“威尔逊旋回”演化史的理想研究载体。目前，关于班怒洋的初始裂解时间、俯冲极性、蛇绿岩侵位机制以及最终闭合时限都争议颇大。本次研究综合新的物源分析数据和前人发表的多学科研究资料，旨在重建班怒洋的“威尔逊旋回”演化史。概述如下：（a）基于最古老的MORB型蛇绿岩年龄和变质洋壳的原岩年龄，班怒洋的初始裂解时间应不晚于晚二叠世（~260 Ma）。（b）班怒洋岩石圈经历了南、北双向俯冲，其中北向俯冲广为认可，羌塘地块南缘发育有完整的与俯冲相关的沟-弧-盆体系，限定初始俯冲时间为早侏罗世（~195 Ma）；南向俯冲目前证据较少，前人报道了拉萨地块北缘发育的与南向俯冲相关的（班戈-盐湖）岩浆弧，然而仍缺失相应的弧前盆地和增生楔等，初始俯冲时间尚不清楚。（c）缝合带内沉积记录可以复原羌塘南缘岩浆弧演化历史。当色哇组弧前盆地，沙木罗组残留海盆地和竟柱山组磨拉石盆地沉积时，碎屑中侏罗纪-白垩纪锆石主要源自羌塘南缘岩浆弧。碎屑锆石年龄分布指示南羌塘岩浆弧主要包括三期岩浆活动，即~186-170 Ma，~169-146 Ma和~129-100 Ma。其中早侏罗世这期岩浆活动在现今改则地区并未出露，区域上只在安多地区被发现，说明在羌塘南缘早侏罗世岩浆活动曾经广泛分布，并不局限于安多地区。（d）基于锆石年代学和铬尖晶石地球化学数据，提出班公湖-怒江蛇绿岩的侵位发生于大洋北向俯冲过程中，其侵位方式属于科迪勒拉型。藏北班公湖-怒江蛇绿岩和藏南雅鲁藏布江蛇绿岩分别代表了SSZ型蛇绿岩的两种典型侵位方式，丰富了SSZ型蛇绿岩侵位理论。（e）基于改则和丁青地区前陆盆地碎屑物源转变时间，班怒洋的最终闭合时限被限定为晚侏罗世-早白垩世（~150-130 Ma）。