阿尔山地区银江沟火山岩岩石学和地球化学特征

Characteristics of Petrology and Geochemistry of Volcanic Rocks in the Yinjianggou,Aershan Area

通过银江沟上侏罗统火山岩典型剖面的研究,认为该地区为一套中酸性、酸性火山岩及火山碎屑岩的岩相组合,中酸性火山岩与酸性火山岩成分交替韵律变化非常清楚,显示共有两次爆发-溢流相旋回。地球化学测试表明,SiO2含量为68.14%～75.40%,富碱,高铝,高钾,低镁,为高钾钙碱性火山岩;稀土总量(∑REE)及LREE富集,LREE和HREE分馏较强,强烈负Eu异常以及弱负Ce异常,富集Ba、Rb、Sr等大离子亲石元素(LILE),亏损Pb、Ti等高场强元素(HFSE)。中酸性火山岩均反映了岛弧火山岩和大陆弧火山岩的一些特征。

The study of a typical profile of the Upper Jurassic volcanic rock in the Yinjianggou area suggests that this region has a lithological combination of a suit of medium-acid and acid volcanic rocks and volcanic clastic rocks. The rock types include sakalavite,rhyolite,crystal tuff,and tuff breccia. The alternating rhythmic changes between medium-acid and acid volcanic rocks in composition is very clear,indicating two outbreak-overflow phase cycles. Geochemical analysis of the samples shows that the content of SiO2 is 68.14~75.40%,rich in alkali,as high-alumina,high-potassium,and low-magnesium,implying a high-potassium calcalkalic volcanic rock. It demonstrates total REE and LREE enrichment with sharp negative Eu anomalies（δEu=0.398~0.611）,weak negative Ce anomalies（δCe=0.901~0.982）and intense fractionation between LREE and HREE. The primitive mantle-normalized spidergrams are characterized by LILE enrichment and HFSE depletion,for a significantly negative Th-Pb-Ti anomaly and an intense enrichment Ba-Rb-Sr anomaly. These medium-acid and acid volcanic rocks reflect some features of island-arc and continental arc settings.