Zircon U-Pb age of bertonite of Upper Ordovician Wufeng Formation in Southeast Yangtze platform and its geological significance

A layer of bentonite is usually developed in upper Ordovician and lower Silurian strata of southeast Yangtze platform in Sinan area. Sampling was done in Wufeng Formation of upper Ordovician and Longmaxi Formation of lower Silurian in Luowantuo of Sinan, high precision U-Pb dating was conducted and as a result, the stratigraphic sedimentary age of Wufeng Formation and Longmaxi Formation in this area were constrained. The data obtained provide reliable evidence for volcanic eruption and stratigraphic chronology near Ordovician and Silurian boundary of southeast Yangtze platform in Sinan area. The age data are correlated well with the data obtained in Dabashan of northwest China, which is a little later than the age of Ordovician bentonite in southeast of the North China Craton (449.0~465.8Ma). The volcanic activity was probably related to the subduction of Cathaysian plate to Yangtze plate. During Middle-Late Ordovician and late Early Silurian, many volcanic eruptions influenced the chemical condition and ecological environment of the ocean in that period and caused the extinction and glacier initiation since middle-late Ordovician.