湖北庙垭杂岩体富硅碳酸岩成因及其对稀土成矿的指示意义

湖北庙垭正长岩-碳酸岩杂岩体赋存我国重要的碳酸岩型稀土矿床，查明不同于主体碳酸岩（w（SiO₂） < 10%），富硅碳酸岩（w（SiO₂）>30%）的岩石学、矿物学特征和成因机制对认识碳酸岩型稀土成矿作用具有重要意义。选取庙垭富硅碳酸岩样品为研究对象，利用电子探针（EPMA）、X射线荧光光谱仪（XRF）、激光剥蚀-电感耦合等离子体质谱（LA-ICP-MS）等分析方法，获得其岩石学和矿物学信息，并与前人研究进行对比，以期查明庙垭杂岩体富硅碳酸岩的成因和稀土成矿差异性。结果表明:①富硅碳酸岩中主要硅酸盐矿物为钾长石、钠长石和石英，与围岩梅子垭群绢云母片岩中硅酸盐矿物不同，而与共生正长岩中矿物组成相似，指示这些硅酸盐矿物可能为正长岩捕掳晶；②钾长石不规则环带特征（成分差异）和全岩高87Sr/86Sr值的特征，可能指示了成岩后的流体作用；③热液独居石年龄（（235.1±1.5）Ma）与华北古板块和扬子古地块沿勉略断裂碰撞引起的勉略洋闭合（246~221 Ma）相关的区域变质事件同步，进一步限定了热液作用年龄和地质事件。与勉略洋壳的俯冲闭合相关的富Sr，Ba构造-热液流体交代富硅碳酸岩，促使Si和REE分别从钾长石等硅酸盐捕掳晶和磷灰石等原生矿物中活化-迁移进入流体，Si的加入可能进一步加强了流体对REE的搬运能力，使得富硅碳酸岩全岩稀土含量低于主体碳酸岩。 

Genesis of silica-rich carbonatite in the Miaoya complex,Hubei Province and its implications for REE mineralization

The Miaoya complex in Hubei Province hosts one of the most important REE deposit in China. This complex is composed of syenite, carbonatite (SiO₂ < 10%), and a suite of silica-rich carbonatite (SiO₂>30%). The petrology and mineralogy of silica-rich carbonatite were investigated by the electron probe microanalyzer (EPMA), X-ray fluorescence spectrometer (XRF), laser ablation inductively coupled plasma mass spectrometer (LA-ICP-MS) and other analytical methods, aiming to decipher the genesis and potential REE mineralization of silica-rich carbonatite from the REE ore bodies in carbonatite. First, silicate minerals of silica-rich carbonatite are dominated by K-feldspar, albite and quartz, which is similar with associated syenite, rather than adjacent Silurian Meiziya Group schist. This suggests that these silicate minerals are probably xenocrystals from syenite. Second, the captured K-feldspar in silica-rich carbonatite has irregular compositional zoning and higher Sr isotopes compared to marine sediments, implying the presence of postmagmatic fluid activity. In addition, the Th-Pb age of hydrothermal monazite associated with silicate minerals in silica-rich carbonatite yields (235.1±1.5) Ma, suggesting that this fluid activity is probably related to the regional metamorphic event related to the closure of the Mianlue Ocean (246-221 Ma) during the collision between the North China Craton and the Yangzi Craton. During the postmagmatic stage, Si and REEs are remobilized from silicate xenocrystals and fluorapatite into fluids, respectively. Moreover, Si in fluids could promote REE transport by these Sr-Ba-Si-rich fluids and ultimately lead to relatively low REE contents in these silica-rich carbonatites.