扬子地块北缘西乡群孙家河组火山岩形成时代及元素地球化学研究

西乡群孙家河组为一套低绿片岩相浅变质火山-沉积岩系,主要由基性-中基性-酸性火山岩和凝灰岩、沉凝灰岩、泥岩、硅质岩组成,火山岩岩石类型包括玄武岩、安山岩、英安岩和流纹岩.LA-ICPMS锆石U-Pb定年揭示流纹岩形成时代为832.9±4.9Ma,辉石玄武岩的形成时代为845.0±17Ma,两者在误差范围内一致,属新元古代同期岩浆作用产物.元素地球化学研究表明,孙家河组玄武岩属拉斑玄武岩系列,具有受地壳混染的板内玄武岩的地球化学特点.玄武岩-安山岩-英安岩主量元素成分投点呈规律性变化、REE球粒陨石标准化及微量元素原始地幔标准化分配型式具有一致性并相互重叠,不相容元素Th和相容元素Cr相关模拟图中沿分离结晶线分布,证明玄武岩-安山岩-英安岩为同一基性岩浆分离结晶的产物.REE和微量元素分配型式以及微量元素比值对的显著差异,暗示流纹岩与玄武岩-安山岩-英安岩来源于不同源区.Sr-Nd同位素研究表明,玄武岩-安山岩-英安岩样品的ε_(Nd)(t)值均大于0以及在ε_(Nd)(t)-(~(87)Sr/~(86)Sr)_t图解中位于OIB成分区,表明其源区应为与洋岛玄武岩类似的地幔源区;流纹岩样品具有可与基性熔岩相比拟的ε_(Nd)(t)值,暗示流纹岩最有可能是初生玄武质地壳部分熔融而成.本文所研究的原划孙家河组火山岩系列的形成时代、构造环境的确定以及扬子陆块乃至世界上同一时间内普遍发育大陆裂谷岩浆岩组合的地质事实,说明原划孙家河组以及西乡群中的确存在新元古代的组成部分,它们应是新元古代大陆裂谷的产物,它和扬子地块820M8后造山裂解环境花岗岩均是新元古代晚期Rodinia超大陆裂解作用的岩浆响应.

Zircon U-Pb dating and geochemical study of volcanic rocks from Sunjiahe Formation of Xixiang Group in northern margin of Yangtze Plate

The Sunjiahe Formation is a suit of low-metamorphosed volcano-sedimentary association, and is composed of basic, intermediate-basic and acid volcanic rocks with interbedding tuff, sediment-tuff, mudstone and siliceous rocks, the major rock types of volcanic rock are basalt, andesite, dactie and rhyolite. LA-ICPMS zircon U-Pb dating of rhyolite and basalt show us the ages of 832.9 ±4.9Ma and 845.0±17Ma respectively, implying rhyolites and basalts occurred at the same time in Neoproterozoic. Based on major, trace element, and Sr-Nd isotope data, the basalts can be classified into tholeiitic magma series and have the geochemistical characteristics of crust-assimilated continental within-plate basaltic. In addition, basalts, andesites and dacites exhibit the same REE chondrite-normalized and trace element primitive mantle normalized patterns with overlapping one another, and samples plot along crystallized curve in Th-Cr diagram can be explained by crystallization from a common basic parental magma. On the other hand, the remarkable difference in REE and trace element normalized patterns of basalt, andesite, dacite and rhyolite suggests the different source. All the ε_(Nd)(t) values of basalts, andesites and dacites greater than 0 and ploting in OIB region in ε_(Nd) (t) -(~(87)Sr/~(86)Sr)_t diagram indicate their source is similar to mantle source of OIB. Whereas, the rhyolites have the same ε_(Nd) (t) values as basalts, suggesting rhyolite is generated by partial melting of juvenile basaltic curst. The age, tectonic setting of Sunjiahe volcanic series in this article and the development of continental rift intrusions in Yangtze Plate and in other areas in the world at the same time, showing the existence of Neoproterozoic rocks in the Sunjiahe Formation, and the Sunjiahe Formation is the product of continental riftism in Neoproterozoic, which is the precursor of the Neoproterozoic break-up of Redinia supercontinent.