LA-MC-ICPMS Zircon U-Pb Dating and Geochemical Characteristics of Late Ordovician Volcanic Rocks in Southern Qilian Mountains, Qinghai Province

Duosuoqu Formation in the Southern Qilian Mountains is composed of pyroclastic rocks, volcanic lava and sedimentary rocks, and its rock types vary from basalt to dacite, belonging to Calc-alkaline series combination.Using LA-MC-ICPMS zircon U-Pb dating, the authors obtained the ²⁰⁶Pb/²³⁸U weighted average age of (449.9 ±1.5)Ma and (449.5 ±1.8)Ma for the first time in the Duoosuo Formation volcanic rocks, which proves that these volcanic rocks were formed in late Ordovician.The results of geochemical analysis show that these volcanic rocks have high Al₂O₃ and low TiO₂, and they are enriched in LILE element (Sr, Rb, Ba), incompatible high field strength elements (U, Th)and LREE (La, Ce), but depleted in Ta, Nb, P, Ti and strong compatible elements (Cr, Ni).These volcanic rocks have relatively high ΣREE, with enrichment LREE, weak positive Eu anomalies to weak negative Eu anomalies.Comprehensive analysis shows that these volcanic rocks were formed in an activity continental margin/island arc environment.