湘南多金属矿集区燕山期成矿花岗岩的造岩矿物特征及其成因意义
Features of rock-forming minerals of Yanshanian mineralized granite in the polymetallic deposit concentration zone in southern Hunan and their implications for granite generation
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摘要: 提要：湘南地区燕山期成矿花岗岩有3种类型，其主要造岩矿物化学成分和种属明显不同；矿物的化学成分变异特征均显示出壳幔岩浆混合成因的特点：①角闪石均属于钙质角闪石亚类。②MC型早期次花岗岩中的黑云母多属镁质黑云母；CM型晚期次花岗岩中的黑云母主要为铁质黑云母；C型花岗岩中的黑云母主要为铁叶云母-铁黑云母，且多为铁锂黑云母。3类型花岗岩中黑云母的成分变异呈线型关系，暗示有成因联系。③3类型花岗岩中斜长石有明显区别，MC型多为中长石，CM的多为更长石，C型多为钠长石。斜长石环带构造发育程度不同，CM型早期次单元花岗岩中的斜长石环带最发育。④CM型花岗岩及其暗色微粒包体中的碱性长石主要属于相对富钾的正长石，包体中的个别属歪长石，表明其形成温度较高；C型花岗岩中碱性长石为相对贫钾的钠正长石及微斜长石。⑤造岩矿物特征和成分变异显示了成矿花岗岩的形成与壳幔岩浆混合作用有关，形成MC型和CM型早期次单元花岗岩的岩浆演化主要是岩浆混合作用，而CM型花岗岩晚期次的花岗岩类和C型花岗岩类的岩浆演化可能还存在分离结晶作用。Abstract: Abstract：There are three types of Yanshanian mineralized granite in southern Hunan, and the chemical compositions and species of their ore-forming minerals are notably different. The characteristics of variations in chemical compositions of the minerals all indicate a mantle-crust magma mixing origin. Systematic analyses show the following：(1)Hornblendes belong to Ca-hornblende.(2)Biotites in most early-stage MC-type granite are Mg-biotite; those in late-stage CM-type granite are mainly meroxene; and biotites in C-type granite are mainly sideophyllite and meroxene, most of which zinnwaldite. The variation in biotite composition of the three types of granite shows a linear relation, implying a genetic relation.(3)Plagioclases in the three types of granite are significantly distinguished. Plagioclases are mainly andesine in MC-type granite, oligoclase in CM-type granite and albite in C-type granite. The zoning structures of plagioclases are developed to different degrees, with the zoning of plagioclases in the early-stage CM-type granite best developed.(4)Alkali plagioclases in CM-type granite and its dark micro-inclusions are mainly K-rich orthoclase, and a few grains of plagioclase in inclusions are anorthoclase, indicating higher temperatures of plagioclase crystallization. Alkali plagioclases in C-type granite are relatively K-poor microcline and Na orthoclase.(5)The characteristics of rock-forming minerals and their compositional variations indicate that the formation of mineralized granites is related to crust-mantle magma mixing. The magmatic evolution that generated MC-and CM-type early-stage granites was mainly magma mixing, while the magmatic evolution that formed CM-type late-stage granitoids and C-type granitoids might be fractional crystallization in addition magma mixing.
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Keywords:
- mineral composition /
- rock-forming mineral /
- Sn-polymetallic deposit /
- granite /
- southern Hunan
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