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碧口群火山岩系中部的基性熔岩以拉斑系列为主 ,基性火山岩的Sr同位素初始比值和Nd同位素初始比值较低 ,分别为 0 .70 12 4 8～ 0 .70 4 4 13和 0 .5 110 80～ 0 .5 12 341,大部分样品的εNd(t) >0 ,表明母岩浆主要来自地幔源区。岩石显示明显的富2 0 7Pb及2 0 8Pb特征。在以部分熔融作用为主的演化过程中岩浆发生了一定程度的分异 ,LREE、大离子亲石元素等表现了从亏损到富集的变化特点。基性熔岩的稀土与微量元素表现出与俯冲碰撞型的弧区玄武岩相同的特点。分析表明 :碧口群火山岩系形成于岛弧环境 ,它与横丹群浊积岩系在扬子板块西北缘构筑了一个弧盆系统 ,其时代为新元古代。
The basic lavas in the central part of the Bikou Group volcanic series include the Lama series, and the initial ratios of Sr isotopes and Nd isotopes in the basic volcanic rocks are relatively low, ranging from 0.70 12 4 8 to 0. 70 4 4 13 and 0 respectively .5 110 80 ~ 0 .5 12 341, εNd (t)> 0 for most of the samples, indicating that the parent magma mainly comes from the mantle source. Rocks show pronounced rich 207Pb and 208Pb features. The magma has some degree of differentiation during the partial melting process. LREE, large ion lithophile and other elements show the change from depletion to enrichment. The rare earths and trace elements of basic lavas show the same characteristics as the subduction collision arc zone basalts. The analysis shows that the Bikou Group volcanic rocks formed in the island arc environment and it formed an arc basin system with the Yidan Group turbidite system in the northwestern margin of the Yangtze Plate.