松辽盆地三台地区营城组珍珠岩地球化学特征及地质意义

松辽盆地东南缘的三台地区营城组火山岩分布广泛,其中一部分为珍珠岩。通过岩石薄片鉴定,挑选了三台地区北山上的6块珍珠岩样品,进行了主量元素、微量及稀土元素分析测试。珍珠岩的主量元素组成表明,它们与流纹岩成分相当,具有高SiO_2、较高(NaO_2+K_2O)、低TiO_2和低CaO含量,属于铝饱和-弱过饱和的钙碱性系列。稀土元素配分模式以轻稀土富集、Eu负异常强烈为特征。在不相容元素蛛网图上,珍珠岩相对于洋脊花岗岩总体上富集大离子亲石元素和轻稀土元素,而明显亏损高场强元素Sr、Nb、Y。在Nb-Y和Rb-(Y+Nb)判别图上,它们分别落入板内和火山弧区。这些结果显示,所研究的珍珠岩应为类似弧后盆地的引张环境。

Geochemistry of perlite of the Yingcheng Formation in Santai region, Songliao basin and its tectonic implications

The volcanic rock of the Yingcheng Formatiom distributes widely in Santai region, Songliao basin. The characteristics of lithofacies shows that perlite develops in the Yingcheng Formation. The major elements, trace elements and REE of 6 samples from Beishan are analysed, which they are selected by examination of thin sections. Its major element compositions indicate that the perlite is similar to rhyolite. Perlite is characterized by high SiO₂, high (Na₂O+K₂O), low TiO₂ and low CaO contents, symbolizing that it belongs to the aluminous saturated-weak supersaturated calc alkaline series. All the samples are plotted in the sub-aluminous field in the ACNK-ANK diagram. The chondrite-normalized REE patterns for these rocks display LREE enrichment and strongly negative Eu anomaly. Compared to ocean ridge granite, the rocks are generally enriched in LILE and LREE, but depleted in HFSE including Sr, Nb, Y as indicated in the spider diagram. In addition, the samples are plotted in the fields defined by volcanic arc and intraplate granite in the Nb-Y and Rb-(Y+Nb) discrimination diagrams. All these characteristics suggest that the perlite formed in an extension which is similar to back-arc basin.