西天山伊宁县北琼阿希河谷火山岩地球化学特征及构造背景探讨

对西天山伊宁县北琼阿希河谷出露的火山岩进行了地球化学特征研究,结果表明,该地区火山岩属于钙碱性火山岩系列,LREE强烈富集(La/Yb)N介于2.76~7.03之间],具有弱的Eu负异常,样品的强不相容元素(Cs、Rb、Ba、Th、U)LILE均明显富集,总体在原始地幔的10倍以上,显示了岛弧火山岩的Nb-Ta负异常特征。但是,其(Th/Nb)N、(La/Nb)N的比值远远大于1,Ba/Nb、La/Nb的变化范围较大,Zr/Nb、Zr/Hf接近N-MORB的比值,Ta/Nb却接近上地壳的比值,暗示该地区的火山岩岩浆在上升过程中受到一定程度的地壳混染,所有样品均显示为中钾-高钾系列,这种钾质类型的优势地位也反映了陆壳基质在岩浆活动中占有重要的地位。样品的Zr均大于300、Zr/Y均大于4,显示了板内玄武岩的成分特征,在Zr-Zr/Y判别图解中,大多数样品投到板内玄武岩的构造环境区域内。结合区域上的地质特征,认为该地区在泥盆世一直处于抬升阶段,在石炭纪时碰撞结束并进入碰撞后伸展阶段,局部地区产生一定的裂谷化特征。琼阿希河谷的火山岩的Nb-Ta负异常特征应为地壳混染引起,火山岩的形成环境为碰撞后伸展的构造环境。

An investigation of geochemistry and tectonic setting of Qiongaxi Valley volcanic rocks in northern Yining County, western Tianshan

Based on studying the volcanic rocks in Qiongaxi Valley of northern Yining County,the authors found that the volcanic rocks in thia area belong to calc-alkaline volcanic rocks strongly enriched in LREE and weakly depleted in Eu. All the samples are enriched in large-ion-lithophile elements(LILE) such as Cs,Rb,Ba,Th and U,which are ten times higher than their values in the primitive mantle.In the primitive mantle-normalized trace elements diagram,the sample shows characteristics of the island-arc volcanic rock,such as distinct depletion of Nb-Ta.However,its(Th/Nb)N>>1 and(La/Nb)N>>1,the ratios of Ba/Nb and La/Nb vary in a large range,Zr/Nb and Zr/Hf ratios are similar to ratios of N-MORB,and Ta/Nb ratios are similar to ratios of the upper crust,which suggests that the volcanic rocks were mixed by the lithosphere during the upward migration of the magma.All the samples point to the intermediate potassium-high potassium series,and the dominance of such a potassium type also implies that the continental crust matrix must have played an important role in the magmatic activity.The features of Zr>300 and Zr/Y>4 of all the samples show characteristics of intraplate volcanic rocks.In the diagram of Zr-Zr/Y,most samples fall into the intralate basalt region.Combined with the geological features of the study area,the authors hold that the area experienced uplifting in Devonian,and in Carboniferous the collision terminated and the evolution entered into the post-collision extension stage.Some local places produced certain rifting characteristics.Therefore,the distinct depletion of Nb and Ta might have been caused by the mixing of the lithosphere,and the environment of the Qiongaxi Valley volcanic rocks should belong to the post-collision extension phase.