黔桂交界龙胜地区玄武岩-流纹英安岩组合特征及其地质意义

Basalt-dacite assemblage in Longsheng district of the Guizhou-Guangxi border area and its geological implications

黔桂交界龙胜地区新元古界青白口系上部丹洲群三门街组玄武岩系内部产出条带状流纹英安岩,与玄武岩共同构成了双峰式火山岩组合。玄武岩、流纹英安岩具有过铝质、钙性的特点;稀土元素配分样式为轻稀土元素富集型,轻、重稀土元素分异明显,弱负Eu异常。玄武岩与流纹英安岩均具有富集型洋中脊玄武岩初始地幔标准化微量元素配分曲线的特点,指示其形成于伸展裂谷环境。双峰式火山岩组合的确立,结合已经发表的年代学数据,对正确进行黔桂地区地层对比、构建江南造山带西南段青白口纪构造具有重要意义。

In the Guizhou-Guangxi border area,several dacite layers were identified,which occur in interlayered form within the basalt of Sanmenjie Formation in upper Neoproterozoic Danzhou Group.The basalt and dacite compose a bi-model volcanic sequence.Both the basalt and the dacite are geo-chemically calcic and peraluminous,and display REE patterns with strong differentiation between LREE and HREE.Their primitive mantle normalized spider diagram is similar to that of enriched middle oceanic ridge basalt（E-MORB）,suggesting that they might have been formed in a continental rift.The identification of bi-model volcanism,along with previously published in-site zircon U-Pb results,provides positive constraints for the stratigraphic correlation of Guizhou and Guangxi and for the tectonic reconstruction of the southwestern segment of the huge Jiangnan orogenic belt.