碧口群火山岩性质及形成环境

The Nature and Formation Environment of Volcanic Rock Series of the Bikou Group

中元古代—新元古代碧口群火山岩系分布在扬子地块北缘、秦岭造山带南侧。碧口群火山岩系具大陆裂谷火山岩特点,形成于大陆裂谷作用构造环境。火山岩系特征为:①具双峰式火山岩组合,火山岩系由基性火山岩(细碧岩及细碧质凝灰岩)及酸性火山岩(石英角斑岩及石英角斑质凝灰岩)组成。②基性火山岩包括两个岩浆系列的岩石,即碱性玄武岩浆系列及拉斑玄武岩浆系列。③火山岩富集LREE及LIL。微量元素及同位素(Sm-Nd,Rb-Sr)研究表明,基性火山岩形成于相似于洋岛玄武岩源的地幔柱源,酸性火山岩为地壳重熔的产物。扬子地块北缘分布有碧口群、西乡群等中元古代—新元古代大陆火山岩系,表明在这一时期大陆裂谷拉张火山作用发育,成为秦岭地壳的主要增生成壳期。

The Mesoproterozoic—Neoproterozoic volcanic rock series of the Bikou Group is distributed along the northern margin of the Yangtze plate and the southern margin of the Qinling orogenic belt, occurring in a continental rift tensional environment and has the nature of continental rift volcanic rocks with the following three characteristics: ①the volcanic rocks are of bimodal assemblages and consist of basic volcanic rocks (spilite or spilitic tuff) and acid volcanic rocks (quartz-keratophyre or quartz-keratophyric tuff); ② two magma series can be identified in the basic volcanic rocks, namely the alkali basalt magma series and thetholeiitic magma series; ③ the volcanic rocks are rich in LREE and LIL. United studies on trace elements and isotope (including Sm-Nd and Rb-Sr) indicate that the basic volcanic rocks are derived from the mantle plume source similar to the OIB source and the acid volcanic rocks are the remelted product of the crust. The distribution of Mesoproterozoic—Neoproterozoic continental volcanic series of the Bikou and Xixiang Groups on the northern margin of the Yangtze plate suggests that continental rift tensional volcanism is developed in the Mesoproterozoic—Neoproterozoic, which is the major accretion period of the Qinling crust.