甘肃天祝地区乌鞘岭一带双峰式火山岩的发现及其构造环境

The discovery of bimodal volcanic rocks in Wuqiaoling area of Tianzhu County,Gansu Province,and its tectonic setting

北祁连乌鞘岭一带出露一套以玄武质岩石为主的双峰式火山岩组合,火山岩岩石类型以变质玄武岩为主,夹少量的流纹岩。玄武质岩石为蚀变玄武岩,属拉斑玄武岩系列,低SiO2、K2O,高Ti,富Na、Th、U等,轻稀土元素亏损—略富集型,无明显Eu异常,微量元素具Nb、Ta负异常。流纹质岩石属钙碱性系列,高SiO2,低K2O,为钠质系列,大离子亲石元素富集,具明显的Nb、P、Ti、Sr负异常和Th、U正异常,轻稀土元素富集,(La/Yb)N为2.98～11.46,Eu负异常明显,δEu介于0.07～0.60之间。结合区域资料分析,乌鞘岭双峰式火山岩可能形成于弧后盆地环境。

A suite of basaltic bimodal volcanic rocks is exposed in Wuqiaoling area,North Qilian Mountain.The volcanic rocks are mainly metamorphic basalts intercalated with small amounts of rhyolites.Basaltic rocks are usually metamorphic basalts of the tholeiite series,with low SiO2 and K2O,high Ti and rich Na,Th and U.The light rare earth elements of basaltic rocks are of the depletion-slight enrichment pattern with no obvious Eu anomaly but obvious Nb,Ta negative anomalies.The rhyolitic rocks are of the calc-alkaline series with high SiO2 and low K2O,belonging to the agpaitic type,enriched with large lithophile elements and showing obvious Nb,P,Ti and Sr negative anomalies and Th,U positive anomalies.Light rare earth elements are concentrated,with（La/Yb）N being 2.98-11.46.Eu has an obvious anomaly with δEu being 0.07-0.60.In combination with the regional data analysis,the authors hold that Wuqiaoling bimodal volcanic rocks might have been formed in a back arc setting.