大兴安岭中生代两类流纹岩成因的地球化学研究

大兴安岭地区晚侏罗世—早白垩世流纹岩类广泛分布.根据岩石学和微量元素地球化学特征将其划分为两类, 分别称其为Ⅰ型流纹岩和Ⅱ型流纹岩.稀土和微量元素分析结果显示: (1) Ⅰ型流纹岩呈右倾的稀土分布曲线, 不相容元素以强烈富集Rb, Ba, Th, K和亏损Sr, Ti, P, Nb为特征, 其形成与钙碱性系列玄武岩浆的结晶分异作用有关; (2) Ⅱ型流纹岩具有与大陆裂谷流纹岩一致的稀土和不相容元素分布模式, 以Ba, Sr的强烈亏损与I型流纹岩相区别, 与碱性系列玄武岩类构成双峰式火山岩组合, 其成因与地壳岩石的非理想熔融作用相联系.两类流纹岩的形成与地幔柱上涌导致上覆岩石圈伸展作用有关. 

GEOCHEMICAL RESEARCH INTO ORIGINS OF TWO TYPES OF MESOZOIC RHYOLITES IN DAXING'ANLING

Widely exposed in Daxing'anling region are Late Jurassic and Early Cretaceous rhyolites that are classified as two major types in terms of petrology and trace_element geochemistry: Type_I rhyolite and Type_II rhyolite. The following conclusions can be drawn from the results in rare earth and trace elements: (1) Type_I rhyolite is characterized by the right_sloped REE distribution pattern. The incompatible elements in this type are featured by both the strong enrichment of Rb, Ba, Th, and K and the depletion of Sr, Ti, P, Nb. These features are related to the crystal differentiation of the calc_alkaline basaltic magma. (2) Type_II rhyolite is characterized by the REE and incompatible_element distribution pattern. The strong depletion of Ba and Sr in this type, a distinctive feature from Type_I rhyolite, established a bimodal volcanic association with alkaline basalts, whose origins are associated with the non_ideal melting of crustal rock. In addition, the formation of these two types of rhyolites is related to the extension of overlying lithosphere resulting from the upwelling of mantle_plume.