Characteristics of volcanic rocks in Songsainong of Zhidoi area，North Qiangtang Basin，Qinghai-Tibet Plateau, and their tectonic significance

Abstract：A set of Late Triassic acid-intermediate-basic volcanic rocks in Songsainong area is composed of basalt, andesite, dacite, rhyolite and a small amount of pyroclastic rocks as well as some other components. In the volcanic rocks, SiO2 values are in the range of 48.89-88.15% (67.44% on average), TiO2 vary between 0.22 and 1.85% (0.63% on average), and MgO range from 0.23 to 7.81%, similar to things of island arc calc-alkaline volcanic rocks. High field intensity elements such as Nb, Ta, Zr, Hf and Ti are depleted, large-ion lithophile elements like Rb, Sr and Ba are relatively concentrated, (La/Sm) n are 1.79-5.19, and LREE/HREE ratios vary mostly in the range of 4.08-6.63, suggesting a light REE enrichment type.δEu values are 0.47-0.99, Eu assumes negative anomalies, La/Nb ratios are mostly higher than 2.00, Nb/Zr> 0.04, Th/Ta> 4.41, and Th × Ta/Hf 2> 0.042, implying a margin of the volcanic arc. The above characteristics show that Late Triassic volcanic rocks in Songsainong of Zhidoi area were mostly produced in an environment where mainland collision and epicontinental arc are coexistent.