甘肃省阿克塞县当金山花岗岩地球化学特征及构造意义

GEOCHEMICAL FEATURE OF DANGJINSHAN GRANITOID IN AKESAI COUNTY OF GANSU PROVINCE AND ITS SIGNIFICANCE

当金山黑云母二长花岗岩位于阿尔金大断裂南侧与南祁连构造带西部的交汇部位，岩石为高钾钙碱性一钙碱性，SiO2含量68．69％-72．53％，具相对富钠贫钾Na2O=3．97％～4．7％，K2O=3．28％。3．66％，Na2O／K2O=1．1-1．38，富镁（MgO=0．98％-2．14％）、低钛（TiO2=0.3％-0．5％）、低铝（Al2O3=13．08％-15．04％），准铝质（A／NKC=0．98-1）的特点，在CIPW标准矿物计算中有极少量的刚玉出现。REE分配曲线呈右倾的特，最，轻稀土相对富集，亏损重稀土，具弱NEu负异常。研究结果表明当金山黑云母二长花岗岩为I型花岗岩，推测其是在加里东晚期研究区从挤压体制向拉张体制转变时，地壳减薄，下地壳基性程度偏高的火成岩、变火成岩受到底侵玄武岩浆的加热部分熔融后，经过分离结晶作用形成的。这个结论为研究阿尔金地区的岩浆活动及构造演化提供了有力的证据。

The Dangjinshan monzonitic granites, located tectonically in the south margin of Alty and west of South Qilian tectonic belt, are characterized by high-K and tale-alkaline composition with SiO2 （68.69% 72.53% ）, high Na （Na2O=3.97%-4.7%）,Mg （MgO=0.98%-2.14%） and Na2O/K2O （1.1-1.38）, coupled with low Ti,A1, and metaluminous （A/NKC=0.98-1）. There is a small amount of corundum appear in the CIPW standard mineral calculation. The characteristics of REE relatively enrich LREE, deplete HREE, a weakly negative Eu anomaly. Our study results indicate that Dangjinshan monzonitic granite is I type, which was formed by basic igneous rock or metaigneous rock partial melting after intense fractional crystallization and differentiation, which was used to be heated by basaltic magma, when the study area changed from the compression to the tension system and then the crust thinning in the late Caledonian. This conclusion provides the powerful evidence for the study of magmatism and tectonic evolution in Ahy region.