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

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.