浙西北大桥坞地区白垩纪火山岩熔体包裹体研究：对原始岩浆提供铀源潜力的探讨

Most of the Cretaceous volcanic rocks in Daqiaowu, NW Zhejiang Province are pyroclastic rocks，which is an important uranium-bearing rock type in China, but there is no studies involving the differences of uranium-bearing potentials of pyroclastic rocks from different formations, and it is also unclear whether mantle-derived components have been added into the primary magma of volcanic rocks. Since complex component sources, whole rocks analysis of these rocks has limited capacities to trace their magma, thus a robust method is needed to carry out detailed researches on these pyroclastic rocks. In this proposal, international advanced technique-in situ analysis LA-ICP-MS of melt inclusions-will be used to obtain the characteristics of primary magma of pyroclastic rocks (especially volatiles, U, Th contents); an integrated method of EPMA, Raman Spectrometry, SEM and EDS will be performed to identify chemical and physical properties of melt inclusions, including daughter minerals, gaseous components, element mapping, etc.; geochronological framework of volcanic strata will be established by zircon U-Pb geochronology; and zircon Hf isotope will also be used to trace the temporal evolution of the contribution of mantle components to volcanic petrogenesis. With the application of advanced analysis technologies, we attempt to: (1) identify the geochemical characteristics of primary magma of volcanic strata; (2) reveal the contribution of mantle components to their petrogenesis; (3) analyze differences of uranium-bearing potential between these volcanic strata. This study will put forward the studies of pyroclastic rocks and make a breakthrough to benefit greatly evaluation of uranium-bearing potential of pyroclastic rocks.

浙西北大桥坞地区白垩纪火山岩主体为火山碎屑岩，是我国重要的产铀岩石类型。然而各时期火山碎屑岩提供铀源的潜力是否存在差异性目前尚缺乏研究，成因上是否存在幔源组分的贡献也未明确。由于其成分来源复杂，全岩分析存在局限性，因此需要运用新方法对其开展精细研究。本申请采用国际前沿的熔体包裹体LA-ICP-MS原位分析技术获取火山碎屑岩的原始岩浆地球化学特征（特别是挥发份、U、Th含量）；综合运用电子探针、激光拉曼、扫描电镜和能谱等查明熔体包裹体的子晶矿物、气相组分、元素分布等物化性质；结合锆石U-Pb年代学和Hf同位素示踪火山岩成岩过程中新生幔源组分的贡献及时间演化。通过新方法的应用，查明火山碎屑岩的原始岩浆地球化学特征，揭示成岩过程中新生幔源组分的贡献，进而分析不同层位火山碎屑岩提供铀源潜力的差异性及原因。本研究有助于突破火山碎屑岩的研究瓶颈，为判定火山碎屑岩产铀的能力提供了新的思路。

浙西北大桥坞地区白垩纪火山岩是我国重要的产铀岩石类型，这些火山岩在成因上是否存在幔源组分的贡献一直受到学界的关注。各时期火山碎屑岩提供铀源的潜力是否存在差异性也缺乏相关研究，究其原因主要是火山碎屑岩成分复杂，全岩分析存在局限性。本项目对大桥坞地区的火山碎屑岩及其周边火山岩开展了岩石学和地球化学综合研究。岩相学和锆石年代学研究结果表明大桥坞地区的火山岩形成于早白垩世（135-123 Ma），经历了四个喷发阶段从而形成厚层的火山碎屑岩，其中黄尖组是发现大量熔体包裹体的主要层位。元素地球化学和Sr-Nd-Hf同位素研究在赣杭构造带识别出一条早白垩世A型火山-侵入岩带，并揭示了火山岩成岩过程中新生幔源组分的贡献随着时间变新而增强的规律。这一认识也得到同期形成的镁铁质岩石的验证（构造背景从早白垩的弧后环境转变为晚白垩软流圈地幔上涌为特征的板内拉张环境）。在壳幔岩浆混合增强的背景下，由于幔源岩浆较壳源岩浆贫U和Th，前者的加入会稀释壳源岩浆中U和Th的含量，进而降低围岩提供铀源的潜力。激光拉曼分析证实大桥坞的熔体包裹体中气泡为收缩气泡而无气相组分。运用电子探针结合激光原位分析，发现火山岩原始岩浆较全岩组成明显更富集U（最高25 ppm）和挥发份F（最高1.8%）、Cl（最高0.23%），暗示火山岩喷发过程中可能经历了显著的去气作用。