辽西朝阳地区张家口组火山岩地球化学特征及构造背景

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

朝阳地区张家口火山旋回可分为3个亚旋回,火山岩主要为流纹岩类、粗面岩类和粗安岩类.地球化学特征表明,张家口旋回火山岩属高钾钙碱性岩石系列.火山岩具有高SiO2,Al2O3,K2O,Na2O和低MgO,TiO2的特征,里特曼指数均值为2.59.火山岩富集大离子亲石元素K,Rb,Sr,Zr,Ba,Pb,Li等,相容元素Ti,Co,Ni,Cr,V亏损.火山岩为轻稀土富集型,总体显示弱负铕异常、弱负铈异常.综合研究认为,张家口组火山岩成岩物质主要来源于地壳,火山岩的成因以部分熔融为主.火山岩形成于挤压造山与伸展构造的过渡环境,具有大陆板内造山带火山岩的基本特征.

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, A1203, K20 and Na20, and comparatively low MgO and TiO2 components. The average value of cr 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.