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

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.