柴达木盆地东南缘晚三叠世火山岩地球化学特征及构造环境分析

Geochemical Characteristics and Tectonic Environmental Analysis of Volcanic Rocks from Late Triassic in Southeastern Margin of the Qaidam Basin

柴达木盆地东南缘晚三叠世鄂拉山组火山岩主要为一套中—酸性火山岩组合,以流纹岩、英安岩为主,其次为安山岩。主量元素、稀土元素及微量元素的分析结果表明:其样品的里特曼指数δ小于4,属钙碱性岩石系列;SiO2的平均值为71.95%,大部分样品具低铝特征,且碱质含量较高,属于亚碱性火山岩。轻重稀土比值较大且负铕异常明显,具典型的岩浆分异晚期稀土分馏特征。从总体上说,样品显示为具有轻稀土富集特征。构造环境判别图显示鄂拉山火山岩的构造环境为岛弧带-板内造山带火山岩区。这种特征可能正是反映了该期火山活动是在西秦岭造山带向柴达木区块俯冲的构造环境下进行的。

Elashan volcanic rocks from late Triassic Elashan Group in southeastern margin of the Qaidam Basin are mainly consisted of a set of medium and acid volcanics combination,including rhyolite and dacite primarily,followed by andesites.The results of the analyses on principal elements,rare-earth elements and trace elements show that Rittman index（δ） of the samples are all below 4,and belongs to calc-alkaline volcanic rocks;the average of SiO2 is 71.95%,most of the samples have a characteristic of low aluminum,alkali content is higher,and that belongs to the sub-alkaline volcanic rocks.The ratios between LREE and HREE are large and negative Eu anomalies are obvious,so they have the typical late rare earth fractionation characteristics of magmatic differentiation.The samples display a characteristic of LREE enrichment.The discriminant diagram of tectonic environment shows its environment is volcanic arc-within plate volcanic region.This feature may reflect this volcanic activity happened in the tectonic environment of Qinling orogenic belt underthrusting to Qaidam block.