湘中紫云山岩体暗色微粒包体的成因:岩相学、全岩及矿物地球化学证据

暗色微粒包体广泛分布于湘中紫云山岩体中的似斑状角闪石黑云母花岗闪长岩中，但其研究程度较低.对具有火成结构的暗色微粒包体及其寄主岩进行了岩相学、全岩及长石、辉石、黑云母的矿物地球化学研究，探讨其岩石成因及构造意义.寄主岩的全岩主量、微粒元素较为均一，而暗色微粒包体变化较大，且后者相对贫SiO₂而富Na₂O，但总体上二者均具有准铝质、钙碱性、镁质的特征，均富集轻稀土和大离子亲石元素，而亏损重稀土和高场强元素.寄主岩和暗色微粒包体的斜长石、辉石和黑云母均分别属于中长石、次透辉石－低铁次透辉石和铁质黑云母的范畴，显示相似的矿物地球化学特征.详尽的岩相学和地球化学特征表明，寄主岩属于I型和ACG型花岗岩，具有明显壳幔混合的特点；而暗色微粒包体形成时处于液态并具有流动性，与寄主岩间存在明显的机械和化学混合作用，并具有早期为骤冷快速结晶、晚期缓慢结晶这两期过程.因此，紫云山岩体中出现大量暗色微粒包体，是印支晚期湘中地区在强烈挤压之后的松弛阶段，由于软流圈物质上涌，并与其诱发的壳源酸性岩浆混合作用的产物. 

Petrogenesis of Mafic Microgranular Enclaves: Evidence from Petrography,Whole-Rock and Mineral Chemistry of Ziyunshan Pluton,Central Hunan

The porphyraceous hornblende biotite granodiorite is generated in the Ziyunshan pluton, Central Hunan Province, which hosts many mafic microgranular enclaves (MMEs). This paper presents petrography, major and trace elements of whole-rock, and mineral chemistry of feldspar, pyroxene, as well as biotite of the igneous-texture mafic microgranular enclaves and host rocks, their genetic mechanism and geotectonic significance were investigated. The whole-rock geochemistry of host rocks is more stable than those of mafic microgranular enclaves, and the latter were characterized by relatively depleted SiO₂ and enriched Na₂O. Both of them are represented by metaluminous, calc-alkaline and magnesian, and enrichment of LREE and LILE and depletion of HREE and HFSE. The plagioclase, pyroxene and biotite of host rocks and mafic microgranular enclaves belong to andesine, ferrosalite-salite and ferribiotite, respectively, showing similar characteristics of mineral chemistry. According to a comprehensive analysis of petrographical characteristics and geochemical data, it is suggested that (1) the host rock belongs to I-type and ACG-type granite, showing the characteristics of crust-mantle mixing; (2) the mafic microgranular enclaves were formed in liquid state, having liquidity and featuring both mechanical and chemical interactions with host rocks; (3) the crystallization of the mafic microgranular enclaves can be classified into two stages: the initial one is undercooling and rapid crystallization stage, and the latter is slow crystallization stage. To sum up, the mafic microgranular enclaves in the Ziyunshan pluton were generated by mixing of mafic magmas (sub-alkaline tholeiite which formed from mantle) and its induced crustal felsic magma (products of partial melting of arenaceous rock in crust) in the relaxation period after the compressive stress period in the late Indo-Chinese epoch.