内蒙古新巴尔虎右旗地区塔木兰沟组火山岩地球化学特征及其构造意义

Geochemical Characteristics and Tectonic Setting of Volcanic Rocks in Tamulangou Formation From Xinbaerhuyouqi Area,Inner Mongolia

新巴尔虎右旗地区塔木兰沟组火山岩为玄武岩-玄武粗安岩-玄武安山岩组合。该套火山岩属高硅钙碱性系列,偏铝质,镁指数小;富集大离子亲石元素(Rb、Ba、Sr)和轻稀土元素(LREE),亏损重稀土元素和高场强元素(Nb、Ta、Ti等),稀土元素配分模式呈左高右低缓倾斜分布,轻重稀土分馏较强,弱的Eu负异常(δEu=0.73~0.83)。综合前人及笔者同位素测年结果显示,塔木兰沟组火山岩形成于中侏罗世末期—晚侏罗世,成岩年龄为166~133 Ma。塔木兰沟组火山岩岩浆来源于受俯冲流体交代的大陆岩石圈地幔熔融,与地壳物质发生混染作用。结合区域构造演化,塔木兰沟组火山岩的形成与蒙古-鄂霍茨克洋闭合造山后岩石圈伸展有关。

The volcanic rocks of Tamulangou Formation in Xinbarhuyou Banner area are mainly composed of basalt,basaltic trachyandesite and trachybasalt.Based on the previous and this study,it was shown that the volcanic rocks of Tamulangou Formation were formed during the Middle and Late Jurassic.The main rocks fall into the high silicon and calc-alkaline series,which is aluminized and small in magnesium index,rich in macroion lithophile element(LIL)and LREE,depleted in high field strength elements(HFSE)and HREE and strong isotopic fractionation.Their REE patterns is left-leaning with enriched LREE,deficient HREE and weak negative Eu anomaly(δEu=0.73~0.83).This study displayed that the volcanic magma of Tamulangou Formation probably came from enriched mantle metasomatized by fluid in subduction belt which was intermixed source of continental crust.The volcanic rocks of the Tamulangou Formation were believed to be formed in lithosphere extension environment after Mongolia-Okhotsk Ocean closed.