Age of volcanic rocks of the Waibizi Formation in the southern Zhangguangcailing and their Paleo-oceanic subduction geological records

Early Paleozoic volcano-sedimentary strata in Zhangguangcailing area are sporadic, it is an important material record of the evolution of the ancient ocean basin between the Jiamusi block and the Songnen block. In this paper, we study the petrology, geochemistry and zircon U-Pb chronology of the Upper Devonian Waibizi Formation rhyolitic tufflava divided by 1∶ 200 000 regional geological survey in Xiaobeihu area, southern Zhangguangcailing. The average weighted age of the LA-ICP-MS U-Pb zircon is 451.9±4.8Ma. The geochemical characteristics show that the content of SiO₂ and total alkali is higher, and the content of Al₂O₃ is higher, which belongs to the high potassium calc-alkaline peraluminous series; Light rare earth elements are enriched, light and heavy rare earth elements are fractionated obviously, with Eu negative anomaly; Rb, Th, U and other large ion lithophilic elements are enriched, while Nb, Ta, Ba, Sr and other high field strength elements are depleted. Zircon ε_Hf(t) values range from +0.35 to +3.52, the two-stage Hf isotopic model age t_DM2 is 1419-1222Ma. In this paper, the metamorphic rhyolitic tufflava of Waibizi Formation was formed in the Late Ordovician, it has the geochemical characteristics of arc magmatic rocks, the magma source area is the Mesoproterozoic accretive continental crust material,combined the diagenetic age of dacite is 453±2Ma, the diagenetic age of rhyolite is 451±2Ma and the geochemical characteristics, it is believed that the Waibizi Formation is formed in the Late Ordovician and is a set of arc volcanic rock assemblages, it was formed in the tectonic background of Palaeo-oceanic basin (Heilongjiang Ocean） subducted westward between Jiamusi block and Songnen block.