伟晶岩与细晶岩的成因--以广西栗木稀有金属花岗岩地区为例

Genesis of pegmatite and aplite: rare metal granite study of Limu in Guangxi

伟晶岩与稀有金属等诸多矿床关系密切,但伟晶岩的成因问题长期存在争议。广西栗木花岗岩是华南地区具有代表性的含锡、钨、钽、铌矿的稀有金属花岗岩,发育有3种不同的伟晶岩-细晶岩组合,分别位于花岗岩体的顶上带、岩体顶部带及岩体内中-上部,岩石中还发育一些特殊的共生变异结构构造。本文将伟晶岩-细晶岩与共生的花岗岩统一作为研究对象,在岩相学的基础上,运用热动力学对岩石中的共生变异结构构造进行成因分析,得出的结论构成了岩浆演化的动态证据链并相互印证,且与热动力学方程检验结论相吻合。研究表明:伟晶岩-细晶岩的成因以及稀有金属花岗岩的岩浆演化、成岩作用都与岩浆的气-液分异作用有关;伟晶岩是富水花岗岩岩浆经过二次气-液分异形成的残余富气流体相缓慢结晶的结果,细晶岩是二次气-液分异形成的残余熔体相就地过冷却结晶的结果;而与伟晶岩-细晶岩共生的花岗岩是富水岩浆在第一次气-液分异形成的新熔体相过冷却结晶后、上部又遭受二次气-液分异形成的残余富气流体交代的结果。新认识可以解释以往伟晶岩成因观点中无法解释的各种地质现象,对花岗岩岩浆演化及成岩成矿作用认识具有重要的启示意义。

There is a close correlation between pegmatite and rare metal deposits. However,the genesis of pegmatite has long been controversial. The Limu granite in Guangxi,rich in tin-tungsten deposits and tantalum-niobium deposits,is typical rare metal granite in south China. Three types of pegmatite-aplite assemblages occur in different spaces,all above the granite body,or on the top of the granite body,and in the middle-upper parts of the rock mass,respectively. Some special symbiotic variability structures are found in the rocks. Taking the pegmatite-aplite and their symbiotic granite as a unified research object,on the basis of petrography study,thermodynamics genesis analysis of these symbiotic variability structures in the rocks is made in this paper. The results constitute the chain of evidence of magmatic evolution,verify with each other and correspond to the test results of thermodynamic equation. The result of this paper shows that the genesis of pegmatite-aplite,magmatic evolution and petrogenesis of the granite were all closely related to the liquid-gas fractionation. The pegmatite was the result of slow crystallization of residual vapor phase,the aplite was the result of quick crystallization of residual liquid phase,the residual vapor phase and residual liquid phase were all produced by secondary liquidgas fractionation from water-rich granitic magma. The symbiotic granite was produced by quick crystallization of new residual liquid phase formed by the first liquid-gas fractionation of water-rich granitic magma,and the upper part of the granite then suffered a metasomatism by residual vapor phase. New understanding of various geological phenomena,which could not be explained in previous views on the genesis of pegmatite,is of great significance to explain the magmatic evolution and mineralization of granite.