热碰撞造山动力学：高喜马拉雅（尼泊尔中段）折返过程的三维构造、变质和深熔响应

The Himalayan belt, a type-example of collisional orogenic belt worldwide, consists of the Great Himalayan Crystalline Sequence (GHC) in the core as a result of “hot” collisional processes. GHC consists of high-grade (up to granulite facies) metamorphic rocks exhumed from middle to lower crustal levels. In addition, widespread ductile deformation and extensive anatectic processes are dominant components along the GHC. However, updated studies have casted doubts on these models and result in a number of competing models to account for the tectonic processes observed along the Himalayan orogen. .In this proposal, we will use the central Great Himalaya (Central Nepal) as a field base to investigate the nature and temporal sequence of structural deformation, metamorphic reactions, and partial melting processes along five major transects across the Main Central Thrust (MCT) and the Southern Tibet Detachment Systems (STDS). This study will focus on the exhumation mechanism of the GHCS under relatively hot collisional environment by integrating new structural, metamorphic, petrologic, and geochronologic data and techniques. These new data acquired from this project would potentially provide new constraints on (1) the spatial and temporal patterns of the tectonic processes in hot collisional belt, (2) mechanisms for the ductile extrusion of the GHCS, (3) factors that regulate the evolution of metamorphic reactions and partial melting processes; and (4) processes that affect the transition from early hot collision to later cold exhumation within the continental collisional belts. Ultimately, our primary objective of this project is to formulate a new dynamic model for the tectonic evolution of large collisional orogenic belts. Through collaboration with Nepal geoscientists, this project provides an ideal platform to investigate the exhumation mechanism of the GHC as a typical example of hot collisional belts worldwide.

喜马拉雅造山带是全球碰撞造山带的典型，作为其核心部位的高喜马拉雅结晶岩系, 由中-下地壳的高级变质岩组成，并有高温韧性变形和大量局部熔融的岩浆物质侵位，是热碰撞造山的产物。高喜马拉雅造山机制的探讨是青藏高原大陆动力学研究的重要窗口。本项目以大陆动力学理论为指导，热碰撞造山为核心，＂高喜马拉雅岩片折返机制＂为主题，变形/变质/深熔的记录为响应，热和流体为制约和驱动因素，以尼泊尔中段为主的高喜马拉雅为研究范围和切入点，对高喜马拉雅岩片内部及其构造边界--藏南拆离系（STD）和主中冲断层（MCT）的变形、变质和深熔作用进行多学科研究，揭示热碰撞造山的三维时空演化规律和全过程，探讨高喜马拉雅岩片的韧性挤出机制，变质演化、局部熔融作用以及从陆内热碰撞造山到冷造山折返过程的热－动力学的制约因素，力图建立新的三维动力学模式，为建立全球热碰撞造山带的典型提供科学的依据。

喜马拉雅造山带是世界上最雄伟、最典型的碰撞型造山带，大量研究认为喜马拉雅造山带的构造格架已经查明，24 Ma以来的构造演化和二维运动学已经被验证；以及高喜马拉雅岩片挤出的“地壳隧道流 ”成为主流观点。 但是高喜马拉雅岩片（GHC）的变形、变质和深熔的性质及相互作用与挤出机制的关系；碰撞以来喜马拉雅的早期演化（60-24 Ma）和三维运动学机制等关键问题并没有解决。本项目的科学目标是以大陆动力学理论为指导，通过追踪高喜马拉雅与特提斯-喜马拉雅的关系、厘定高喜马拉雅的结构以及探究变形、变质和深熔的关系，来重塑喜马拉雅构造格架，并以三维动力学为思路来揭示喜马拉雅形成演化的全过程。.本研究以尼泊尔-喜马拉雅中段为窗口，结合中国境内喜马拉雅中东段的辅助配合，通过野外科技地质长廊的调查、微观与宏观构造结合、 多学科互补、运用岩石地球化学和同位素年代学测年方法，达到预期科学目的。主要成果如下：. (1) 发现特提斯-喜马拉雅底部向南剪切的大型特提斯-喜马拉雅滑脱带(THD)，伴随大量局部熔融及高温变质，形成于~50Ma，终止于10 Ma，为特提斯-喜马拉雅与高喜马拉雅单元之间早期界限，是藏南拆离系（STD）的前身。.（2）查明高喜马拉雅的双层结构，由E-W向平行造山伸展拆离带组成的上构造层和自北向南逆冲断裂系组成的下构造层构成，确定双构造层形成于~40- 26 Ma， 24 Ma以后高喜马拉雅的地壳隧道流挤出。.（3）揭示高喜马拉雅岩片中淡色花岗岩体代表典型的地壳深熔岩浆系统，是深熔熔体汇聚、运移、侵位和最后岩浆过程的结果。高喜马拉雅变质岩的部分熔融是通过水致白云母脱水部分熔融产生淡色花岗质熔体。揭示长时期和多阶段的深熔作用可以导致深部的岩石单元在晚中新世甚至10 Ma左右最终发生部分熔融。. (4）根据喜马拉雅构造、岩浆、变质事件的时间表，建立喜马拉雅造山全过程以及三位运动学的构造模式。 . 上述创新性成果对于重新认识喜马拉雅格架、造山全过程、三维运动学以及热动力学演化具有重要科学意义！