碱性岩浆-热液演化与稀土成矿作用的关系：赛马和庙垭碱性岩体矿物学对比研究

Alkaline rock is one of major host rocks for rare earth minerals. Magmatic fractionation and fluid activity play important roles in REE mineralization of alkaline rocks, and specific factors affecting the REE mineralization need to be further constrained. REE mineralization in the Miaoya and Saima alkaline complex mainly results from magmatic fractionation and fluid activity respectively, and rare earth minerals in the Saima peralkaline pegmatite is characterized by obvious hydrothermal alteration and REE remobilization. The comparative study of these two complexes plays an important role in deciphering the relationship between alkaline magmatic-hydrothermal evolution and REE mineralization.. This project will study the syenite in the Miaoya complex and peralkaline pegmatite in the Saima complex by the combined research of petrography, mineralogy, fluid inclusion and isotope geochemistry, to confirm their mineral constitution, especially rare earth mineral species and their secondary mineral assemblages, to define the physicochemical information for rare earth mineral crystallization, and to discuss the nature and origin of hydrothermal fluid. Based on these steps, it is expected to understand the effect of magmatic fractionation and fluid activity on REE mineralization, to unveil the role of physicochemical condition and fluid nature differences during the alkaline magmatic-hydrothermal evolution in REE enrichment, and for providing new idea for the exploration of the alkaline rock related REE deposits.

碱性岩是稀土矿物的主要母岩之一，岩浆分异和流体作用在稀土成矿过程中发挥着重要作用，两类地质过程中稀土控矿因素需要进一步探讨。湖北庙垭和辽宁赛马碱性岩体稀土成矿作用分别代表了岩浆分异和流体作用的结果，并且赛马碱性伟晶岩中发生了明显的稀土矿物蚀变和稀土元素再迁移。两者对比研究在探究碱性岩浆-热液演化与稀土成矿作用关系方面具有重要的研究意义。. 本项目拟通过对湖北庙垭正长岩和辽宁赛马岩体碱性伟晶岩进行一系列岩相学、矿物学、流体包裹体和同位素地球化学精细研究，确定矿物组成特征、重点查明稀土矿物类型和次生蚀变矿物组合，探究稀土成矿过程中物理化学条件、引起稀土矿物蚀变和稀土元素再迁移的热液流体性质、来源等。在此基础上比较岩浆分异和流体作用在稀土成矿过程中发挥的作用，总结岩浆-热液演化阶段物理化学条件和流体差异性对稀土矿化的影响，为碱性岩型稀土成矿机制提供新的认识。

碱性岩-碳酸岩型稀土矿床成矿过程与碱性岩-碳酸岩岩浆演化及热液活动密切相关。辽宁赛马碱性岩体和湖北庙垭碱性岩-碳酸岩体分别形成于板内拉张和陆内造山环境，并普遍经历热液交代，具有不同程度的稀土矿化。据此，本项目对两者的：（1）岩浆过程及稀土赋存状态；（2）热液活动及稀土迁移行为展开了精细的矿物学对比研究，获得了以下结果和认识：. 辽宁赛马碱性岩体稀土矿化晚于锆矿化，但总体稀土含量不高。随着碱性岩浆结晶分异，代表性稀土矿物——层硅铈钛矿（rinkite-(Ce)）集中于岩浆演化晚期异霞正长岩和霞石正长伟晶岩阶段（220-230 Ma）。从早期到晚期，层硅铈钛矿REE含量（ƩREE2O3 12.47→23.47 wt.%）和HREE/REE比值（0.086→0.470）不断升高。伟晶岩中层硅铈钛矿常与方解石共生，表明碱性硅酸盐熔体晚期分异出富稀土硅酸盐相和碳酸盐相。晚期Na-CO2-F自交代流体交代层硅铈钛矿，使得REE发生迁移、含量降低，Ca、Sr和Na含量升高，最终原地形成富Ca菱锶矿+铈硅磷灰石+钠沸石+萤石+方解石假晶。. 庙垭碱性岩-碳酸岩稀土矿床是岩浆-流体叠加作用的结果。硅酸盐熔体与碳酸盐熔体的结晶分异或不混溶作用（420-440 Ma），使得晚期碳酸岩中稀土富集于方解石、氟磷灰石、独居石和氟碳铈矿中，铌赋存于含铌金红石、铌铁矿和烧绿石中。后期流体对稀土再富集起到重要作用，体现为流体将稀土从方解石和氟磷灰石中“萃取”出来，以热液独居石和氟碳铈矿的形式沉淀，同时造成了烧绿石的蚀变。烧绿石蚀变矿物组合和U-Th-Pb定年结果显示，流体具有高温、富Fe-Si-S-CO2的特征，作用时间为235.4 ± 4.1 Ma，与前人热液独居石、氟碳铈矿和铌铁矿年龄（206-234 Ma）较为一致，指示了该流体不太可能来自于庙垭碱性岩-碳酸岩岩浆系统，可能与后期勉略洋闭合的构造-热液改造相关。. 相比赛马岩体，庙垭岩体后期热液活动更为强烈，稀土矿化也更加显著，进一步证实了热液流体对碱性岩-碳酸岩型稀土成矿至关重要。