大兴安岭中生代两类流纹岩成因的地球化学研究

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

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 .

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.