南秦岭杀熊洞和庙垭碳酸岩杂岩体成岩成矿过程探究

Carbonatites, due to their unique physical and chemical propertieis, play a significant role in tracking the upper mantle geochemistry and the deep global carbon cycle. They are also important ore deposits (e.g., REEs, Nb). Currently, several issues related to carbonatites are still under discussion: 1) their mantle sources (especially the depth of their generation and how subducted sediments involved in the generation of carbonatites?) 2) the genetic relationship between carbonatites and their associated silicate rocks (e.g., geochronologically consistent?) 3) the enrichment mechanism for REEs. Majority of carbonatites worldwide are considered to be cumulates, hence, the mutli-micro-analysis of the constituent minerals from both carbonatite and silicate rocks are more representative compared to the whole rock data to track their generation and evolution. Shaxiongdong and Miaoya carbonatites are among the few carbonatite complexes located in an orogenic setting, which are important REEs (and Nb) deposits. This study carries out a combined in-situ chemical, isotopic and geochronological investigation of important constituent minerals from both carbonatite and syenite within these two carbonatite complexes. This comparative investigation will provide great insights into the mantle sources, petrogenesis, and metallogenesis for carbonatites. In addition, it will further elaborate on the significance of multi- micro-analysis on minerals from alkaline rocks for deciphering the genesis and evolution of carbonatites.

碳酸岩因其特殊的物理化学性质，可指示上地幔地球化学特征，与碳循环密切相关，是重要的稀土矿床。目前，碳酸岩地幔源区特征（如形成深度及俯冲沉积物的贡献）、与伴生硅酸盐岩成因关系（如其年代学特征）及稀土元素富集机制仍是国际上关注的重要科学问题。大部分碳酸岩被认为是堆晶形成的，其组成矿物微区分析较之全岩更能代表其岩浆的形成与演化。南秦岭杀熊洞和庙垭碳酸岩是两个重要的稀土（和铌）矿床，是世界上为数不多位于造山带内的碳酸岩杂岩体。本项目拟利用原位微区的分析手段，开展南秦岭两处碳酸岩杂岩体碳酸岩和硅酸盐岩中重要组成矿物全面微区地球化学特征分析，包括主微量元素含量、C-O-S-Sr-Nd-Pb-Hf同位素组成和U-(Th)-Pb年龄，来探究碳酸岩的地幔源区特征、成岩成矿年代、与伴生硅酸盐岩成因关系及稀土富集机制等科学问题，期望进一步展示单矿物全面微区地球化学特征研究的重要性。

本项目通过对南秦岭两处碳酸岩杂岩体碳酸岩和硅酸盐岩中重要组成矿物全面的微区地球化学特征分析，包括主微量元素含量、C-O-B-Sr-Nd-Pb-Hf同位素组成和U-Th-Pb年龄，来探究碳酸岩的地幔源区特征、成岩成矿年代、与伴生硅酸岩成因关系及稀土富集机制等科学问题，期望进一步展示单矿物全面微区地球化学特征研究的重要性。本项目取得的重要结果包括：1）通过详尽的岩相学观察和多种副矿物（锆石、磷灰石、独居石和铌铁矿）的微区原位U-Th-Pb年代学研究确定了庙垭和杀熊洞碳酸岩杂岩体的侵位时代约为440-430Ma，其中碳酸岩侵位稍晚于正长岩(约10Ma)，同属于板内裂谷环境下的南秦岭早古生代碱性岩岩浆活动；庙垭独居石和铌铁矿的U-Th-Pb年龄表明稀土和铌矿化均受到三叠纪热液交代作用的影响，稀土矿化主要在三叠纪成矿（230Ma）。2)南秦岭碳酸岩与正长岩的C-O-B及Sr-Nd-Pb同位素组成一致，指示两者来自同一类似于PREMA或HIMU的地幔源区；值得一提的是，微区原位同位素数据表明后期热液交代过程改变了矿物Sr-Pb同位素组成，仅最为亏损的Sr和Pb同位素才可以被用来讨论碳酸岩的地幔源区。3)庙垭碳酸岩杂岩体遭受了三叠纪强烈的热液交代作用，导致岩浆方解石和磷灰石的REE被溶解-迁移-再沉淀形成独立的稀土矿物相（独居石和氟碳铈矿），表明后期热液交代导致了稀土进一步矿化；而杀熊洞碳酸岩杂岩体受交代影响较弱，其方解石和磷灰石稀土元素分布变化多样可能是矿物结晶分异的产物，也可能是三叠纪交代作用的结果。本项目通过矿物微区原位元素-同位素地球化学特征和年代学特征的研究，精细刻画了碳酸岩来源与演化及成岩成矿过程，展示了微区分析在碳酸岩成岩成矿过程示踪上的重要性。同时，该项目成果也深入探讨了碳酸岩的岩浆侵位与后期热液交代过程对稀土富集成矿的重要性。