西藏班戈－那曲地区晚白垩世时期的深部过程与地表抬升

Constraining effectively the timing of deep process operating at collision orogenic belt is crucial for better understanding the crustal evolution and surface uplift in orogenic belt. Current situation-based intensive analysis for the Late Cretaceous magmatic rocks and surface uplift, this project is planning to conduct systematical petrological, geochronological, elemental, and isotopic geochemical studies on the Late Cretaceous magmatic rocks with distinct geochemical signatures from Baingoin and Nagqu located south side of the Lhasa-Qiangtang collision zone, and perform low-temperature thermochronology analysis (including zircon and apatite U-Th/He dating) for representative plutons emplaced by 100 Ma from both sides of the Lhasa-Qiangtang collision zone. This project will explore the origin and petrogenesis of the Late Cretaceous magmatic rocks, trace the deep process involved in the generation of the magmatic rocks, and reconstruct the cooling and exhumation histories of representative plutons emplaced by 100 Ma, with scientific objective for unraveling the coupling relationship between deep process and surface uplift occurred in the Lhasa-Qiangtang collision zone during the Late Cretaceous. This project is expected to provide not only crucial constraints on the deep process operating at the Lhasa-Qiangtang collision zone during the Late Cretaceous, but also solid lines of evidence for the uplift of the Tibetan Plateau at this time in terms of interplay of deep and surface processes.

可靠约束碰撞造山带深部过程发生的时限，是研究造山带大陆地壳演化和地表抬升过程的关键。在充分分析拉萨－羌塘碰撞带晚白垩世岩浆岩和地表抬升研究现状的基础上，本项目拟以拉萨－羌塘碰撞带南侧班戈－那曲地区以往研究程度极低的、具有独特地球化学特征的不同时期晚白垩世岩浆岩为对象开展系统的岩石学、年代学、元素和同位素地球化学研究，以拉萨－羌塘碰撞带南北两侧侵位于100 Ma以前的代表性岩体为对象开展锆石和磷灰石(U-Th)/He低温热年代学测试，分析晚白垩世岩浆活动的岩浆起源和岩石成因，论证其深部过程，查明代表性岩体的冷却剥露历史，建立拉萨－羌塘碰撞带晚白垩世深部过程与地表抬升之间的耦合关系。本项目不但可为拉萨－羌塘碰撞带晚白垩世时期的深部过程提供关键约束，而且还可为青藏高原是否在晚白垩世发生了隆升提供深部与浅部相结合的可靠证据。

精确刻画造山带深部过程是研究造山带大陆地壳演化和造山带地表抬升的关键。拉萨－羌塘碰撞带地壳增厚之后晚白垩世（< 95 Ma）时期发生的深部过程，及其空间迁移特点和触发机制，迄今仍然未能得到很好讨论。本项目以北部拉萨地体班戈地区晚白垩世岩浆岩为重点，在三次野外调研基础上，开展了系统的岩石学、元素和同位素地球化学以及高质量的锆石U-Pb 年代学和锆石Hf 同位素研究。利用新获得的岩石学、年代学和地球化学数据，主要获得以下新认识：（1）通过论证班公湖－怒江特提斯洋白垩纪时期的俯冲极性和关闭过程，指出晚白垩时期拉萨－羌塘碰撞带处于碰撞带后陆内环境。（2）北部拉萨地体班戈地区晚白垩世（约94 Ma）埃达克质英安玢岩和流纹斑岩具有亏损的同位素组成，高镁和低镁埃达克质英安玢岩可能指示了岩石圈拆沉作用，区域上晚白垩世94-80 Ma向西迁移的小规模高镁埃达克质岩浆活动可能形成于向西迁移的小规模岩石圈拆沉作用，由Rayleigh-Taylor 不稳所触发。上述新成果不但刻画了拉萨－羌塘碰撞带晚白垩世时期的深部过程，而且为研究其它古老造山带具有类似特征的后碰撞岩浆作用的驱动机制提供了研究实例。本项目发表国际SCI论文11篇，培养了2名已毕业博士研究生（王青、李世民）和1名在读博士研究生（易建康）。