Geochemical Characteristics and Tectonic Setting for Volcanic Rocks of Zhangjiakou Formation in the Chaoyang Area,Western Liaoning

Zhangjiakou volcanic cycle in Chaoyang area may be divided into three sub-cycles.The volcanic rocks of Zhangjiakou formation mainly consist of rhyolites,trachyte and trachyandesites.The geochemical characteristics indicate that the volcanic rocks belong to the high-K calc-alkaline magmatic serious.The volcanic rocks are characterized by high SiO2,Al2O3,K2O and Na2O,and comparatively low MgO and TiO2 components.The average value of σ is 2.59.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 are riched in LREE.There is a slightly low Eu negative abnormality and low Ce negative abnormality.Combined to the study of regional structures and geochemistry of volcanic rocks,it is considered that the volcanic rocks are mainly originated from the crust materials and the lithogenesis take partial melting as a major part.The volcanic rocks of Zhangjiakou volcanic cycle are formed in the transitional environment of compressional orogeny and extensional structure.They have the basic characteristics of continental intra-plate volcanic rocks.