Geochemical characteristics and tectonic significance of Qixianian volcanic rocks in Zhigengaka area,Zhiduo County,eastern Qiangtang

Qixianian volcanic rocks in Zhigengaka area consist of pyroclastic rocks and lava.Geochemical researches on major elements,trace elements and Sr,Nd,Pb isotopes suggest that these volcanic rocks originated from the mantle in an extension or fresh rift basin environment on the continental margin and were induced by mantle plume upwelling.On the basis of major elements,the volcanic rocks can be divided into two types characterized respectively by high TiO2 and low TiO2.Chondrite normalized REE patterns show light REE enrichment,with MORB normalized trace elements like the pig back,indicating that these volcanic rocks were formed in an intra-continental rift environment.Sr,Nd,Pb isotopic geochemistry suggests that they were derived from a depleted mantle source.