南迦巴瓦地区麻粒岩变质作用PTt轨迹的相平衡研究

The Namche Barwa complex is located in the core of the Himalayan Orogeny. This complex contains typical high-pressure granulites that have experienced migmatism during the deep continent subduction of the Indian Plate, which provides a precious opportuntiy to investigate the collision-orogeny process and geodynamics. However, there are many controversies of the PTt paths and the petrogenesis of these granulites. This project will investigate deeply the metamorphic evolution of the metapelitic and metabasic granulites from the Namche Barwa complex based on the methods of metamorphic petrology, phase equilibrium, zircon U-Pb dating and in-situ mineral trace elements. The PT paths of the two types of granulites will be firstly constrained and then discuss the metamorphic reactions, the mineral growth and evolution as well as the growth and decomposition of zircon during the metamorphic processes. Combined with the trace elements characters of the metamorphic minerals and zircons, we will determine the geological significance of the zircon dating results. Finally, the PTt paths of the high-pressure granulites will be constructed based on the comparsion of the metapelitic and metabasic granulites, which will provide the petrologic evidence for discussing the evolution and collision-orogeny process of the Himalayan orogen.

南迦巴瓦地区位于喜马拉雅造山带核部，该地区发育了印度大陆深俯冲过程中形成的典型的混合岩化高压麻粒岩，是研究碰撞造山及动力学的理想对象，但是这些麻粒岩变质作用PTt轨迹及成因还存在很多争议。本项目拟利用变质岩石学、相平衡模拟、锆石U–Pb定年及矿物微量元素分析等研究方法，系统研究南迦巴瓦地区泥质和基性麻粒岩变质演化过程，限定其各期变质作用的PT条件，探讨两类麻粒岩变质演化过程中发生的变质 (深熔) 反应、矿物组合的形成与演化以及锆石的生长与分解行为，进而准确限定锆石定年结果的地质意义，并通过两类麻粒岩的对比研究，完整地再造高压麻粒岩变质作用PTt轨迹，为建立喜马拉雅造山带的形成与演化历史和大陆碰撞造山过程提供岩石学证据。

南迦巴瓦杂岩带位于喜马拉雅造山带最东端的核部，发育了印度大陆深俯冲过程中形成的典型高压麻粒岩，是研究喜马拉雅造山带变质演化历史的的理想对象。但是，这些高压麻粒岩的变质作用PTt轨迹及其构造意义还存在很多争议。本项目通过详细的野外地质调查和系统采样，利用变质岩石学、矿物主量和微量元素分析、相平衡模拟、锆石岩石年代学等研究方法，系统研究了南迦巴瓦杂岩带高压变泥质、变基性和变杂砂质麻粒岩的变质作用，确定了南迦巴瓦高压麻粒岩经历了顺指针的PT演化轨迹，峰期均达到了高压麻粒岩相，并确定了南迦巴瓦杂岩带核部经历了较高的峰期变质条件。通过相平衡的定量模拟，确定了南迦巴瓦高压麻粒岩的深熔过程：部分熔融主要发生在进变质演化过程的升温阶段，杂砂质麻粒岩在早期的抬升阶段也产生了少量的熔体，部分熔体脱离岩石，形成淡色斑块、脉体或淡色花岗岩；残留的熔体在退变质演化过程的降温降压阶段结晶。根据锆石形态、阴极发光图像和稀土元素特征以及相平衡模拟结果，确定了麻粒岩中的变质锆石主要形成在岩石降温减压的熔体结晶阶段，少量变质锆石形成在进变质阶段，进而确定了南迦巴瓦地区高压麻粒岩的P–T–t演化轨迹，并据此完善了东喜马拉雅构造结在印度–亚洲大陆碰撞过程中的构造–变质演化历史。本项目共撰写了学术论文4篇，其中2篇已发表在国际SCI检索期刊，1篇发表在中文SCI检索期刊，1篇已投稿到国际SCI检索期刊。