碧口群火山旋回及其地质构造意义

Volcanic cycles of the Bikou Group and their tectonic implications

通过详细的野外剖面观察和室内岩石地球化学研究,将碧口群火山岩划分为3个火山旋回,每个旋回下部为基性火山岩(细碧岩及细碧质凝灰岩),上部为酸性火山岩(石英角斑岩或石英角斑质凝灰岩)。基性火山岩富集LREE及LIL。第一旋回基性火山岩属碱性系列,第二、三旋回基性火山岩属拉斑玄武岩系列。基性火山岩浆形成于与洋岛玄武岩类似的地幔柱源,其中第三旋回基性火山岩浆晚期受岩石圈地幔混染;酸性火山岩为地壳重熔作用的产物。碧口群火山岩属大陆裂谷双峰式火山岩系,是中—新元古代扬子地块北缘大陆拉张作用的产物。这种拉张作用是秦岭造山带从震旦纪开始的大规模扩张作用的先兆。

The study of volcanic rocks of the Bikou Group concerns the problem of the Meso-and Neo-proterozoic tectonic system on the north margin of the Yangtze block.The volcanic rocks of the Bikou Group were previously considered as a component part of an ophiolite suite.Based on detailed field obser-vations and indoor petrological and geochemical studies,the volcanic rocks of the Bikou Group are divided into three volcanic cycles,and each cycle consists of basic volcanic rocks(spilite or spilitic tuff)in the lower part and acid volcanic rocks(quartz keratophyre or quartz keratophyric tuff)in the upper part.The basic volcanic rocks are rich in LREE and LIL.Two magma series can be identified,namely,the alkali basalt magma series and the tholeiitic magma series.The basic volcanic rocks of the first cycle belong to the alkali basalt magma series and those of the second and the third cycles the tholeiitic magma series.The basic volcanic magma was derived from the mantle plume source similar to the OIB source and that of the third cycle underwent strong contamination with the lithospheric mantle;the acid volcanic rocks are the product of the anatexis of the crust.The volcanic rocks of the Bikou Group are of the typical bimodal volcanic rocks formed in a continental rift and is the product of continental extension on the north margin of the Meso-to Neoproterozoic Yangtze block.This ex tension was the precursor of large-scale spreading of the Qinling orogenic belt that was initiated in the Sinian.