蒙古国南部富REE碳酸岩成岩成矿作用研究：以Lugiin Gol和Ulgii Khiid碳酸岩为例

Mantle-derived carbonatite plays important role in studying composition and evolution of the mantle. The carbonatite is also rich in rare earth and rare metal mineral resources, render carbonatites always as research focus worldwide. Most of the presently known carbonatite intrusions generally occur in anorogenic settings related to intracontinental rifting. Whereas more and more carbonatites emplaced in orogenic setting have been discovered in recent years, with most of them host large REE deposits. For example, a number of Mesozoic carbonatite intrusions have been discovered in Southern Mongolia where the crust has experienced intensive and episodic tectonic and magmatic processes as a result of convergent orogenies. REE mineralization was closely associated with these carbonatite complexes. However, there is lack of systematic and detailed studies of petrogenesis and metallogenesis for these carbonatites. In this proposal, we choose the representative Lugiin Gol and Ulgii Khiid REE-rich carbonatite intrusions for a systematic petrological, mineralogical, geochemical, fluid-inclusion and geochronological study. We plan to investigate mineralogical assemblage, crystallization sequence and geochemical composition of major minerals, determine rock- and ore-forming ages of the carbonatite intrusions and related REE deposits with high-precision radiometric methods, and understand migration and enrichment mechanisms of the REEs during the evolution of carbonatitic magmas. We also propose to use C-O-Sr-Nd isotopic tracers to analyze the carbonatites and to constrain characteristics of their mantle source. A major objective is to determine the tectonic background of petrogenesis and metallogenesis of carbonatites. We believe the proposed study would enhance our understanding of petrogenesis and REE metallogenesis of carbonatites and constrain the geotectonic evolution in the region.

碳酸岩是研究地幔组成和深部演化的重要岩石类型之一，与稀土和稀有金属成矿关系密切，一直是国际地学研究的热点。碳酸岩常产于裂谷环境，近年来越来越多的碳酸岩在造山带被发现，并常伴生（超）大型的稀土矿床。蒙古国南部碰撞造山作用广泛发育，区域内出露大量中生代碳酸岩，其中大部分碳酸岩与稀土成矿密切相关，但关于碳酸岩及相关稀土矿床的成因仍缺乏系统研究。本申请选择蒙古国南部Lugiin Gol和Ulgii Khiid两处富REE碳酸岩为研究对象，通过系统的矿物学、岩石学、年代学、流体-熔融包裹体、地球化学和矿床地质研究工作，查明碳酸岩及相关稀土矿体中矿物的共生组合，结晶顺序及成分；厘定碳酸岩的成岩成矿年代；探究REE在碳酸岩演化过程中迁移和富集成矿作用；揭示碳酸岩母源岩浆性质及地幔源区特征，以及碳酸岩成岩成矿作用的大地构造背景，为区域构造演化提供约束，以期完善碳酸岩成岩成矿理论。

碳酸岩是典型的幔源岩石，在所有火成岩中具有最高的稀土元素含量，但仅有少数碳酸岩蕴藏具有经济价值的稀土矿床。因此，关于碳酸岩中稀土元素“源”“运”“聚”等科学问题的研究，是我们解释稀土元素在碳酸岩中超常富集的关键。蒙古国南部构造作用复杂，区域内出露大量晚中生代-新生代碳酸岩-碱性岩杂岩体，其中大部分碳酸岩杂岩体与稀土成矿作用密切相关。Ulgii Khiid和Lugiin Gol碳酸岩-碱性岩杂岩体位于蒙古东南部，是区域内两处典型的富稀土碳酸岩-碱性岩杂岩体。同位素定年显示，Ulgii Khiid 碳酸岩中磷灰石U-Pb年龄为 165±10Ma，Lugiin Gol碳酸岩全岩Rb-Sr 年龄为244.9 ± 22.4 Ma，两处碳酸岩-碱性岩杂岩体均形成于华北-塔里木板块和西伯利亚板块俯冲碰撞后的板内裂谷环境中。然而，两处岩体却显示出截然不同的稀土成矿特征。. 同位素研究显示，Ulgii Khiid碳酸岩及正长岩围岩Sr-Nd同位素具有明显亏损的特点，亏损的地幔源区限制了REE在碳酸岩初始熔体中的富集。Ulgii Khiid碳酸岩磷灰石内含有大量随机分布的熔融包裹体，其中含有包括稀土矿物在内的多种矿物子晶及磷酸盐熔体相，且高温淬火后可观察到多相熔盐不混溶现象。地球化学分析表明，P和REE在不混溶过程中优先进入碳酸盐熔体并分离形成富REE的磷酸盐熔体，说明熔体的不混溶作用可以促使REE在碳酸岩中的富集。但Ulgii Khiid碳酸岩中仅发育少量原生稀土矿物，并未形成矿床。这可能是由于岩浆体系中P元素过饱和，REE优先进入磷酸盐熔体及磷灰石矿物中，稀土矿物无法大量结晶沉淀。. Lugiin Gol碳酸岩及正长岩围岩Sr-Nd同位素指示其地幔源区具有EMII富集端元的特点，结合其成岩年龄，俯冲所携带的壳源物质为可能为地幔源区的富集提供了物质来源，为REE初始富集提供了物质基础。与Ulgii Khiid碳酸岩不同，Lugiin Gol碳酸岩仅含少量磷灰石，且岩体遭受了不同程度的热液蚀变，并发育有大量热液成因稀土矿物（独居石和氟碳钙铈矿）。由此可见，多期热液改造作用对碳酸岩矿床中稀土元素的活化、再富集以及工业成矿具有重要意义。