黑鳞云母花岗质岩浆的结晶分异及钠长花岗质岩浆的形成  

Fractional crystallization of a protolithionite granitic magma and the formation mechanism of albite granitic melt

作　　者：熊小林[1] 饶冰[2] 朱金初[2] 赵振华[1] 王贤觉[1] 

机构地区：[1]中国科学院广州地球化学研究所,广州510640 [2]南京大学成矿作用国家重点实验室,南京210093

出　　处：《岩石学报》2002年第2期223-230,共8页Acta Petrologica Sinica

基　　金：国家自然科学基金(49973022;49873017);中科院知识创新工程项目(KZCX2-102;KZCX2-SW-117);国家重点基础研究项目(G1999043200);南京大学成矿作用国家重点实验室国家教委访问学者项目

摘　　要：以含F、过铝质、K_2O>Na_2O为特征的浅色黑鳞云母花岗岩为初始物,在P=150MPa,T=850～650℃和H_2O不饱和条件下进行熔化-结晶分异实验,结果表明:首先晶出的是石英和富钾碱性长石;随结晶温度下降和结晶相含量的增加,残余熔体中挥发份F及Na_2O、Al_2O_3含量逐渐增加,SiO_2、K_2O、CaO含量减少,显示含F浅色花岗质熔体具有向贫硅富钠的富氟钠长花岗质熔体组成演化的趋势;700℃开始,残余熔体具有天然富氟钠长花岗岩的组成特征:富Na_2O(Na_2O>K_2O)、Al_2O_3(>16％)和F(>1.5％)。上述残余熔体组成的演化特征与自然界从含氟黑鳞云母花岗岩到富氟钠长花岗岩的组成变化规律是一致的,充分证明了在本文的实验条件下,含氟黑鳞云母花岗岩能够经结晶分异演化出富氟钠长花岗岩熔体。F-bearing leucogranitic rocks (Topaz granites) are a special type of felsic rocks enriched in fluorine and rare metal elements. They usually develop characteristic vertical zonation from a relatively potash and silica-high zone upward to a sodic and silica-low zone at the apical region of the plutons. In order to investigate their fractional crystallization process and the zonation mechanism, F-bearing protolithionite granite from the Xianghualing Pluton is used as the starting material to perform fractional crystallization experiments and to determine its crystallization sequence and melt differentiation trends at 150 MPa, 850similar to650degreesC and H2O-unsaturated conditions. The run products include quartz, alkali-feldspar, mica and quenched glass. Quartz and alkali-feldspar are the first crystallized phases and are sequentially followed by mica. There are systematic variations in the compositions of alkali feldspar and residual melt compositions with decreasing run temperature. The contents of Or (KAlSi3O8) increase and the contents of Ab (NaAlSi3O8) and An (CaAl2Si2O8) decrease. The most apparent fractionation trends in residual melt compositions are decreases in the contents of SiO2 and K2O, and increases in the contents of Na2O, Al2O3, F, and the ratios of A/NKC and Na2O/K2O. At Tless than or equal to 700 degreesC, the residual melt has the compositional characteristics of natural F-rich albite granite. These experimental results indicate that the low-silica and sodic granitic melt could be derived by a relatively high-silica and potash F-rich leucogranitic magma via fractinal crystallization, which is very important for the understanding of the fractional crystallization process and the zonation mechanism of F-bearing leucogranite plutons.

关 键 词：黑鳞云母花岗质岩浆 结晶分异 钠长花岗质岩浆 含氟浅色花岗岩 钠长花岗质熔体 

分 类 号：P588.121[天文地球—岩石学] P584[天文地球—地质学]