Petrotectonic assemblages and LA-ICP-MS zircon U-Pb age of Early Paleozoic volcanic rocks in Qimantag area, Tibetan Plateau

A study of the Early Paleozoic volcanic rocks in Qimantag area reveals that they were formed in the Middle-Late Ordovician period. The volcanic rocks can be assigned to four volcanic rock belts, i.e., Gonglugou-luanshigou belt, Kuangou-Xiaolangyashan belt, Bayan Gol-Suhait belt and Naling Gol belt. The first volcanic rock belt belongs to a combination of Nb-enriched basalts characterized by high content of Nb (7.38×10-6～10.1×10-6), TiO2 (1.56%～2.24%) and P2O5 (0.24%～0.34%). The combinations of the volcanic rocks in the second belt are complicated，the SiO2 mass fractions of these rocks are concentrated on two end members, and these rocks can be further divided into two sub-combinations of basalt+basatic andesite and dacite+lithoidite. U-Pb LA-ICP-MS analyses of zircons in basalt and lithoidite yielded the data of 440.2±2.4Ma and 450.3±1.2Ma, respectively. The content of TiO2 (1.1%) and K2O(0.14%) is low while that of Na2O(2.17%) is high for the rocks of the tholeiite combinations in the third belt. Although the content of TiO2 (1.64%) is relatively high, the tholeiite combinations in the fourth belt are very similar to those in the third belt.