Geochemical Characteristics and Genesis of Volcanic Rocks from Zhangjiakou Formation in the Chaoyang Basin, Western Liaoning

摘要

The Zhangjiakou Formation of Late Jurassic is mainly distributed in the Chaoyang basin in Western Liaoning. Zhangjiakou volcanic cycle may be divided into three subcycles. The volcanic rocks of Zhangjiakou Formation mainly consist of rhyolites, trachyte and trachyandesites. The volcanic rocks are characterized by high SiO_2, Al_2O_3, K_2O and Na_2O, and comparatively low MgO and TiO_2 components. σ=1.76-4.34, the average value is 2.59. Chemical composition of volcanic rocks indicates that they belong to the high-K calc-alkaline magmatic serious. The volcanic rocks are obviously enriched in incompatible elements (K, Rb, Sr, Zr, Ba, Pb, Li), and depleted in compatible elements (Ti, Co, Ni, Cr, V).The volcanic rocks is rich in LREE. The spider patterns are the right-incline. There is a slight low Eu negative abnormality and low Ce negative abnormality. The geochemistry characteristic shows that volcanic rocks of Zhangjiakou volcanic cycle were formed in the transitional environment of compressional orogeny and extensional structure. They have the basic characteristics of continental intra-plate volcanic rocks. The volcanic rocks were mainly derived from the crust materials. The lithogenesis take partial melting as a major part.