Geochemical Characteristics and Tectonic Settings of Volcanic Rocks from Queershan Group in Hongshishan Area, Gansu

The volcanic rocks of Queershan Group, located in Hongshishan area, north of Beishan, are mainly composed of intermediate-acid volcanics, andesites, dacite and rhyolite, which belong tocalc-alkaline series. Researches on the geochemistry suggest that, these volcanic rockshave high SiO₂ content(62.29%-75.74%, lower MgO content(<2.15%), relatively high alkali content(Na₂O+K₂O=6.24%-9.2%), and K₂O/Na₂O<1. These rocksare enrich in LREE relative to HREE((La/Yb)_N=2.94-6.36), with a strong fractionation in LREE((La/Sm)_N=1.94-3.33), presenting a strong negative anomaly of Eu(0.50-0.90). In the aspect of trace elements, these rocks are enrich in LILE more than HFSE, with relatively depletion of Nb, Ta, P and Ti, as well as the enrichment of Ba, Th, Zr, Ce and Sm. These geochemical characteristics approve that, these volcanic rocks formed in the island arc environment with the effect of oceanic crust subduction, which has a direct genetic relationship with the subduction of ancient oceanic crust between Kazakhstan plate and Tarim plate. During the process of subduction, the partial melting of the overlying mantle material was caused by the hydrous fluids of oceanic crust, and the product created by the rising magma has experienced a certain fractional crystallization and assimilation and contamination of upper crustal material.