GEOCHEMICAL RESEARCH INTO ORIGINS OF TWO TYPES OF MESOZOIC RHYOLITES IN DAXING'ANLING
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摘要: 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.Abstract: 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 typesof rhyolitesis related to the extension of overlying lithosphere resulting from the upwelling of mantle-plume.
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Key words:
- rhyolites /
- geochemistry /
- petrogenesis /
- Daxing'anling
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图 1 大兴安岭中生代火山岩分布
Fig. 1. Sketch map showing distribution of Mesozoic volcanic rocks in Daxing'anling
图 3 大兴安岭流纹岩类REE分布曲线
a.Ⅰ型流纹岩; b.Ⅱ型流纹岩
Fig. 3. REE patterns of Mesozoic rhyolites in Daxing'anling
图 4 大兴安岭流纹岩不相容元素的原始地幔标准化
a.Ⅰ型流纹岩; b.Ⅱ型流纹岩
Fig. 4. Primitive mantle＿normalized incompatible elements spidergram for rhyolites in Daxing'anling
表 1 大兴安岭中生代流纹岩类主量元素和微量元素分析结果
Table 1. Major and trace element compositions of Mesozoic rhyolites in Daxing'anling
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