Geochemistry and Tectonic Setting of Marine Volcanic Rocks of Elashan Area in Qinghai

Elashan magmatic belt,an intersection location for East Kunlun,West Qinling and South Qilian orogenic belt,has complex internal structure and is an important junction in western China and ore concentration area in Qinghai.Marine volcanic widely outcrops in Elashan area and the rock types are ainly amygdaloidal basaltic andesite,amygdaloidal andesite,andesitic volcanic breccia,andesitic crystal-vitric tuff debris,andesitic breccia tuff lava,dacite and rhyolite.Marine volcanic rocks occurs by the volcanic strata,or laminated and lenticular shapes in the Suolaqu sandstone,Xingfucun fine-turbidite and Hekashan coarse-turbidite of the Hongshuichuan Formation,which is the product of a marine environment eruption.Chemical composition of volcanic rocks similar with the average chemical composition of island arc calc-alkaline andesite,The average δEu value is 0.75 indicating a weak negative anomalies,(La/Sm)_N,(Gd/Yb)_N values of the average were 5.09,1.76,and There are significantly fractionated between light and heavy rare earth elements and enrich in LREE.The trace elements such as Ba,Rb and Th are strongly enriched and Ti,Cr and other elements are depleted.The Th×Ta/Hf² value is between 0.029 to 0.162,shows volcanic arc character.From the above,volcanic rocks of the Hongchuichuan Formation produced in stretching after the collision environment.