腾冲火山岩的熔体包裹体研究: 对其源区性质的直接制约

Tengchong Volcanic Field (TVF), located in the western Yunan province, China, and on the southeastern edge of Tibetan Plateau as well as at the boundary of India-Eurasia collision belt, is the only one volcano geothermal region of Himalayan geothermal belt. In the past decades, considerable petrological, mineralogical, geochemical, tectonical studies have been conducted on these rocks, but there are still large divergences on the magma source due to the lack of the evidences that directly reflect the nature of magma source. It's worth noting that the analysis and study on melt inclusion (MI) entrapped in the minerals of Tengchong volcanic rocks probably is the key point to solve this question. Therefore, in this study, the volcanic rocks (mainly for basalts) from the TVF and their entrapped MI are mainly chosen as the research objects. On the one hand, the advanced experimental technique of laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) and the computational method that the applicants improved for determination of melt compositions of MI will be used to systematically determine the compositions of the entrapped MI in order to trace the origin and evolution of magmas. On the other hand, the analyses of microthermometric and fluid compositions of gas/liquid phases as well as the calculation of the MI entrapment pressure will be conducted to constrain the physical and chemical environment of magma source. In addition, the X-ray fluorescene spectrometry (XRF) and ICP-MS will be adopted to analyze the major and trace elements of whole rocks, combined with the determination of the mineral compositions using the electron microprobe. All these results will be combined to investigate the formation and evolution of magmas, and thus provide direct and comprehensive constraints on the origin and magma source of these rocks. This research will have important implications for the evolution processes, geodynamic mechanism and tectonic background of the regions (including Tengchong and Tibetan Plateau regions) located at the boundary of India-Eurasia collison belt.

云南腾冲火山区，是我国喜马拉雅地热带唯一的火山地热区域。虽然前人对腾冲火山岩开展了许多的相关研究, 但对其源区性质仍存在较大的分歧, 主要是缘于缺乏直接的证据。而对腾冲火山岩矿物中所捕获的熔体包裹体的研究很可能是解决这一问题的关键。因此，本研究拟以腾冲玄武岩为主要研究对象，通过运用先进的LA-ICP-MS实验技术和申请人所改进的熔体包裹体成分计算分析方法对火山岩矿物中所捕获的熔体包裹体进行系统的成分分析和计算，示踪腾冲火山岩岩浆来源和演化，辅以包裹体显微测温、气相成分分析和捕获压力计算, 确定岩浆源区的物理化学环境, 并运用X射线荧光光谱仪 (XRF)和ICP-MS分析火山岩全岩主量和微量成分，结合电子探针获取火山岩矿物主量元素组成，从而直接和全面制约腾冲火山岩的源区属性特征。该研究将对探讨印度-亚欧板块碰撞边界地区（包括腾冲和青藏高原）岩石圈演化、动力学机制和构造背景具有重要意义。

腾冲火山区位于青藏高原东南缘，是沿印支-亚洲缝合带由西藏西南部延伸至云南西部的喜马拉雅地热带附近唯一的新生代火山地热区。虽然前人对腾冲火山岩开展了众多岩石学、地球化学和年代学研究, 但对其源区性质仍存在较大的分歧, 主要是缘于缺乏直接的证据。本研究首次对腾冲不同火山岩及其捕获的熔融包裹体进行了系统性的研究，以期对腾冲岩浆源区和火山岩成因有更深入的了解。主要研究内容和结果包括：（1）腾冲火山岩的岩石学、矿物学和地球化学特征研究。清凉山火山岩主要为玄武岩，斑晶主要为橄榄石、单斜辉石和斜长石。黑空山火山岩主要为玄武质粗安岩和粗安岩，其斑晶主要包括斜方辉石、单斜辉石和斜长石。打鹰山火山岩主要为粗安岩，斑晶以斜长石为主，其次为斜方辉石。腾冲火山岩富集稀土元素和大离子亲石元素，而相对亏损高场强元素（2）腾冲火山岩中熔体包裹体成分（包括液相和气相成分）及其物理属性（如形成捕获温度、压力等）系统性的研究。腾冲火山岩玄武岩和安山岩中的橄榄石、单斜辉石、斜方辉石和斜长石中含有大量的熔融包裹体。这些熔体包裹体气相成分主要为CO2，捕获温度和压力约为1506 K 和17.9 kbar(约59.6km)，对应于上地幔深度；富含SiO2和碱，富集轻稀土元素(LREE)和大型离子亲石元素(LILES)（如Rb、Th、U、K、Pb），但缺乏高场强元素(HFSEs)，（如Nb、Ta、Ti、P），具有岛弧特征。熔融包裹体的地球化学特征表明，TVF火山岩可能同源，但从清凉山到打鹰山的岩浆演化程度逐渐增加。（3）腾冲火山岩源区性质、成因及动力学机制的研究。熔体包裹体的熔体和微量元素模拟表明，腾冲岩浆可能起源于石榴石稳定区域中LREE富集的上地幔不同程度的部分熔融，其中印度洋俯冲洋壳贡献显著。岩浆在上升过程中发生分异结晶以及受到大陆地壳的混染。火山活动受晚新生代右旋走滑断裂的控制，与印度板块的地球动力学演化有关。本研究将对探讨印度-亚欧板块碰撞边界地区（包括腾冲和青藏高原）岩石圈演化、动力学机制和构造背景具有重要意义。