北京四海地区髫髻山组—后城组火山岩岩石化学特征

根据TAS图解，北京北部四海地区髫髻山组—后城组火山岩属于粗安岩 (粗面岩 流纹岩)组合。该套火山岩既不属于钾玄岩系列，也不是典型的碱性橄榄玄武岩系列，在总体上为高钾钙碱性系列、富钾质拉斑系列和过渡型高钾钙碱性系列三种岩石组合的混合。该套火山岩轻稀土元素明显富集。粗安岩类岩石没有负销异常或负铕异常不明显，而流纹岩和粗面岩类岩石存在负铕异常。四海地区髫髻山组—后城组火山岩形成于燕山运动早期或中期阶段的造山变形事件趋于结束时的构造环境中。

A Preliminary Discussion on Petrochemical Characteristics of the Volcanic Rocks of Tiaojishan-Houcheng Formation in Sihai Area, Beijing Municipality

According to the total alkaline vs. silica (TAS) diagram, the volcanic rocks of the Tiaojishan and Houcheng Formation exposed in Sihai area, northern part of Beijing Municipality, belong to the trachyandesite-trachyte-rhyolite association. These volcanic rocks cannot be classified as Shoshonite series or typical alkaline olivine basalt series. We proposed they are the collage of typical high-K calc-alkaline, K-rich tholeiitic and transitional high-K calc-alkaline series rocks. All samples of the volcanic rocks exhibit significant enrichment of light rare earth elements (LREE), and small or no negative Eu anomaly in trachyandesitic rocks, but negative Eu anomaly in rhyolite and trachytoid rocks. The volcanic rocks erupted in late stage of the early or middle Yanshanian deformation episode.