变质过程中的流体运移和稀土元素活动——庐山星子群变质脉体的微量元素地球化学

Fluid migration and REE mobility during regional metamorphism: Evidence from trace element geochemistry of metamorphic veins within Xingzi Group of Lushan, SE China

对庐山栖贤寺和黄岩寺两地星子群沉积变质岩及其中脉体的微量元素地球化学进行了研究。结果显示，栖贤寺变质石英脉继承了其围岩（变粒岩）的稀土元素地球化学特征，表明形成石英脉的流体来源于围岩；而黄岩寺以长英质脉为主的变质脉体与其围岩（主要是片岩）的稀土元素特征有较大的差异，显示了原岩组成对变质流体组成、性质及由流体导致的稀土元素活动性特点有明显的制约。此外，两地脉体的微量元素对Zr-Hf和U-Th均与变质岩不同，且有明显的分异，指示了脉体是流体作用的产物。变质流体通过侧向扩散作用形成了星子群变沉积岩中的变质脉体。

Trace element geochemical studies of the metamorphic veins and their host metasedimentary rocks of the Xingzi Group at two localities, Qixiansi and Huangyansi, in Lushan have been conducted in this paper. The results show that the metamorphic quartz veins at Qixiansi inherited the REE geochemical characteristics from their host rocks (granulitites), suggesting that the vein- forming fluids were from the host rocks. At Huangyansi, however, the obvious differences of REE geochemical characteristics exist between the metamorphic veins (mainly felsic ones) and their host rocks (mainly schists), which suggests that the composition of the protolith has strong constraints on the composition and nature of metamorphic fluids and the character of REE mobility caused by the fluids. Additionally, trace element pairs Zr- Hf, and U- Th of the veins at both two localities have remarkable fractionation being much different from those of their host rocks. This also indicates that the veins are the products of the fluids. The metamorphic veins within the Xingzi Group metasedimentary rocks were formed principally by lateral diffusion of the metamorphic fluids.