Geochemical Characteristics and Tectonic Environment of the Continental Facies Volcanic Rocks in Elashan Area, Qinghai Province

Late Triassic volcanic rocks with the combination of neutral-acidic volcanic rock of magmatic belt in Elashan area,composed of the basaltic andesite,andesite,dacite,rhyolite and a little of pyroclastic rock etc.The volcanic rock altered strongly,layered poorly,columnar joints developed,which showed the typical characteristics of terrestrial eruptions.Volcanic rocks are Al saturation type,and Rittmann index(δ) and petrologic features display a calc-alkaline,(FeO/MgO) and K₂O/Na₂O show that the volcanic rock may have the characteristics of continental margin island arc environment.It has high degree of LREE fractionation and enrichment,δEu is less than 1,with a weak negative anomalies.It has similar curves of REE distribution patterns and island-arc curve REE patterns.The trace elements such as Rb,Ba,Th and other elements significantly enriched while Ti,Y,Yb,Sc,Cr and other elements are poor.The characteristics of Nb/Zr,La/Nb,Th/Ta,Th×Ta/Hf₂ and other elements reflected that the volcanic rock in Elashan Formation was formed in the environment of continental margin volcanic arc.According to these characteristics of the regional geological setting and geochemical,structural environment of the volcanic rock,we have summarized this conclusion that late Triassic volcanic rocks in Elashan area were produced in the environment of the coexistence of East Kunlun block and West Qinling block mutually collision with continental margin arc environment.