Zircon U-Pb chronology, geochemistry of Mesozoic volcanic rocks from the Lingquan basin in Manzhouli area, and its tectonic implications.
满洲里地区灵泉盆地中生代火山岩的锆石U-Pb年代学、地球化学及其地质意义

LA-ICP-MS zircon U-Pb dating and geochemical data for the Mesozoic volcanic rocks from the Lingquan basin in Manzhouli area were presented in order to constrain their petrogenesis and the regional tectonic evolution. Zircons from nine representative volcanic rocks are euhedral-subhedral in shape and display striped absorption or oscillatory zoning in CL images as well as high Th/U ratios (0.25~7.78), implying their magmatic origin. The dating results indicate that the Mesozoic volcanic rocks in the region can be subdivided into three stages, i.e., the Middle Jurassic (about 166Ma), the early stage of the Early Cretaceous (about 142Ma), and the late stage of the Early Cretaceous (about 125Ma). The Middle Jurassic volcanic rocks, represented by the basaltic rocks from the Tamulangou Formation, are characterized by high alkali content (K2O>3%), high La/Yb, La/Nb, Hf/Sm, Nb/U and Ce/Pb ratios, enrichment in large ion lithophile elements (LILEs) and light rare earth elements (LREEs), relative depletion in Nb, indicating that its formation should be related to the lithospheric extension after the crustal thickening. The early stage of the Early Cretaceous volcanic rocks, represented by the rhyolites from the Jixiangfeng Formation, have high SiO2 and alkali contents, are rich in K, Rb, Th, and relatively poor in Al, Mg, Ca, Ni, Cr and Ti, similar chemically to A-type granites, suggesting that the volcanic rocks should form under an extensional setting. The late stage of the Early Cretaceous volcanic rocks displays a bimodal volcanic rock association, i.e., basaltic rocks from the Yiliekede Formation and the rhyolites from the Shangkuli Formation. The basaltic rocks are characterized by high alkali (especially K2O) contents, enrichment in LILEs and LREEs, belonging chemically to the shoshonitic series, whereas the rhyolitic rocks are similar to the A-type rhyolite. The bimodal volcanic rocks should form under an intense extensional setting. Combined with spacial distribution of the Mesozoic volcanic rocks, it is suggested that the formations of the Middle Jurassic and early stage of the Early Cretaceous volcanic rocks could be related to the evolution of the Mongol-Okhotsk orogen, whereas the formation of the latest Early Cretaceous bimodal volcanic rocks should be related to subduction of the Paleo-Pacific plate beneath the Eurasian continent.