拉萨地块“竟柱山”时期 (~90Ma) 的沉积-岩浆事件：构造体制转变？

Effective constraints of tectonic regime shift time, is the key to understanding for the subduction-collision process in the orogenic belt. The jingzhushan formation uncomfortably laid on the underlying strata in the middle-northern Lhasa terrane, combining with the obviously different geochemistry of intrusions developed at ~90Ma (such as high Sr and low Y and HREE concentrations) from those rocks occurred in Early Cretaceous, indicates that an important tectonic regime transformation happened in the Lhasa terrane during Late Cretaceous. However, their formation time and material source have not been well constrained. Based on the intensively analysis for the previous researches about the Late Cretaceous sedimentary-magmatic event developed during subduction- and collision-process, this project is planning to conduct systematical geochronological, petrologcial, elemental, and isotopic geochemical studies on the sedimentary and volcanic rocks occurred within the Jingzhushan formation and contemporaneous igneous (~90Ma) in the middle-northern Lhasa terrane, which can constrain the spatial and temporal distribution. These new data, combined with the previous results, will help us to explore the sedimentary-magmatic event and corresponding deep transformation process during “Jingzhushan” period (~90Ma). This project is expected to provide not only constraints on the deep dynamic mechanism developed beneath the middle-northern Lhasa terrane during Late Cretaceous, but also solid evidence whether the crustal thickening and uplift of the Lhasa terrane occurred prior to India-Asia collision.

有效约束构造体制转变发生时限，是研究造山带俯冲-碰撞过程的关键之一。拉萨地块中北部晚白垩世竟柱山组区域上角度不整合超覆于下伏地层，同时发育有明显不同于早白垩世的高Sr低Y(HREE)的岩浆活动(~90Ma)，指示出拉萨地块在晚白垩世时期发生了一次重要的构造体制转换事件, 然而它们的形成时限、物质源区和构造意义尚未得到很好的约束。在充分分析晚白垩世时期拉萨地块中北部沉积-岩浆事件研究现状的基础上，本项目拟以通过对拉萨地块中北部典型竟柱山组剖面中的沉积岩和火山岩以及~90Ma岩浆岩进行系统的年代学、岩石学、元素和同位素地球化学研究，有效的约束其在时空上的分布规律，分析“竟柱山”时期(~90Ma)沉积-岩浆活动事件及深部构造转变机制。本项目不仅可以为拉萨地块中北部晚白垩世时期的构造演化过程提供关键约束，而且还可为青藏高原南部在印度与欧亚大陆碰撞之前是否已经发生抬升和加厚提供可靠的证据。

有效约束构造体制转变发生时限，是研究造山带俯冲-碰撞过程的关键之一。拉萨地块中北部晚白垩世竟柱山组区域上角度不整合超覆于下伏地层，同时发育有明显不同于早白垩世的高Sr低Y(HREE)的岩浆活动(~90Ma)，指示出拉萨地块在白垩纪时期发生了一次重要的构造体制转换事件。通过对拉萨地块西北部白垩纪沉积物和岩浆作用的综合分析，初步认为拉萨地块西北部地壳在该时期存在两个阶段的地壳增厚过程：即早白垩世早期（140~110 Ma）地壳从较薄的厚度增生到正常的平均大陆地壳厚度（33 Km）以及随后的早白垩世晚期到晚白垩世早期（110~90 Ma）地壳从平均大陆地壳厚度（33 Km）加厚到约为50 Km（平均值）。结合研究区中生代晚期的构造演化，我们初步认为研究区早期（140~110 Ma）的地壳增长与底侵的幔源镁铁质物质有关，而晚期(110~90 Ma）的地壳增厚则很可能与班公湖-怒江洋的闭合及随后的拉萨地块与羌塘地块的碰撞挤压有关。