岩浆-变质成因独居石矿物学研究：以缅甸抹古泥质麻粒岩和淡色花岗岩为例

As one of the most important geological chronometers via U-Th-Pb isotope system, monazite usually has complex internal structures. As high-precision and high-spatial resolution dating technique advances, the coupling between identification of the genesis and U-Th-Pb isotope system behavior requires further exploration. This study intends to develop a scanning electron microscope-CL imaging technique for monazite with complex zonal features based on detailed charaterization of monazite grains within leucogranites and high-grade metamorphic pelitic granulite in the Mogok metamorphic belt in Myanmar. Internal structures of monazite revealed by CL combined with mineral assemblage, BSE, X-ray mapping, composition of the inclusions, detailed chemical composition, and geochronology are used to distinguish monazites of igneous and metamorphic origin, establish strategy on selecting optimal target position or area for U-Th-Pb in situ dating of the complex origin monazite and facilitate the interpretation of obtained ages. Combined with previous studies, ours results will provide more accurate mineralogy and chronological constraints for the metamorphic and tectonic evolutions of the Mogok metamorphic belt in Myanmar.

作为最重要的地质年代计之一，独居石常具有复杂的内部结构。随着高精度高空间分辨率微区定年技术发展，独居石成因类型的鉴别以及同U-Th-Pb同位素体系之间的相互联系亟需清晰的认识。本项目拟以缅甸抹古变质带内淡色花岗岩和高级变质的泥质麻粒岩为研究对象，研发复杂环带特征独居石的扫描电镜-阴极发光成像技术；在此基础上，通过独居石矿物共生组合、CL图像、BSE图像、电子探针X-ray Mapping元素面分布图、包裹体成分、元素组成成分及年代学的综合研究，归纳岩浆-变质成因类型独居石的鉴别标志，进而为复杂成因独居石U-Th-Pb原位微区定年工作提供分析点选点依据及其年龄结果给予合理的解释；同时结合前人研究，为缅甸抹古变质带经历的变质-构造演化过程提供更加精准的矿物学、年代学制约。

在具多阶段演化的岩浆岩（如淡色花岗岩）和变质岩（如变泥质岩）中，独居石常呈现复杂的年代学信息，为其地质含义的解译带来不确定性。其根源在于对独居石内部结构缺乏精细分析，矿物成因仍缺乏深入研究。因此，独居石成因类型的鉴别以及同U-Th-Pb同位素体系之间的相互联系亟需厘清。本研究通过优化场发射扫描电镜的工作距离、加速电压和束流大小等关键参数，研发了独居石阴极发光（CL）成像技术，清晰呈现矿物内部结构。在此基础上，对喀麦隆以及缅甸抹古变质带内淡色花岗岩和泥质麻粒岩中的独居石进行了矿物共生组合、CL图像、BSE图像、电子探针X-ray Mapping元素面分布图和元素组成的综合研究，归纳了岩浆和变质成因类型独居石的鉴别标志。进一步地，我们对缅甸抹古变质带淡色花岗岩中独居石开展了年代学分析。结果显示，淡色花岗岩独居石为典型的岩浆独居石，年龄分布在~21-17 Ma之间，指示了抹古淡色花岗岩经历了连续~4Ma的结晶历程，这为缅甸抹古变质带岩浆和热演化过程提供了更加精准的矿物学和年代学的证据。