青海省同仁印支期火山岩地球化学特征与构造环境分析

Geochemical Features of the Indosinan Volcanic Rocks and Tectonic Environment in the Tongren,Qinghai Province

青海省同仁地区位于秦岭、祁连山结合部,构造演化历史悠久。该区广泛分布一套印支期火山岩,火山岩的岩石组合主要为安山岩、玄武岩、玄武安山岩等。作者对出露于青海同仁瓜什则乡拉则那合当火山岩地球化学特征进行了探索性研究,结果表明,该区火山岩Na2O明显偏高,而K2O偏低,为中钾钙碱性系列。火山岩稀土元素总量较高,轻重稀土分馏明显,其稀土配分曲线表现为轻稀土富集而重稀土较为平坦的右倾型;具有弱负铕异常,负铈异常,大离子亲石元素(Rb、Ba、Th)等强烈富集,Pb、Sr富集,而高场强元素Ta、Nb、Ce、Pr、Nd等亏损。综合地质、地球化学特征认为,该区火山岩形成环境为洋壳向下俯冲的岛弧环境。

Tongren area in Qinghai Province is located in the conjunction of the Qinling Mountains and the Qilian Mountains.There is a long history of tectonic evolution.The volcanic rocks of Indo-Chinese epoch is widely distributed in this area.The volcanic rock assemblage is mainly composed of basalt,andesite and basaltic andesite.The authors studied the geochemistry characteristics of the exposed volcanic rocks in the area,obtained a lot of valuable geological information.The results of the research indicate that the Na2O is significantly higher concentration in the volcanic rocks,and the K2O is slightly low,they belong to medium potassium calc-alkaline series.The content of rare earth elements is high with obvious fractionation between the heavy and light rare elements,and their REE curve appears right-inclined type for the LREE is enrichment and the HREE relatively flat.Large ion lithophile elements（Rb、Ba、Th）is extremely high.Pb、Sr are enrichment,and Nb、Ce、Pr、Nd are depletion.Tectonic setting distinction reveals that the volcanic rocks of this area formed in an island arc tectonic setting.