BENTONITES IN THE BOTTOM OF LOWER SILURIAN LONGMAXI FORMATION ON THE WESTERN MARGIN OF YANGTZE PLATE: Zircon U-Pb Age, Trace Element Characteristics and Geological Implication

Bentonites are widely distributed and developed in several layers in the Ordovician-Silurian boundary, which is of great significance for the study of stratigraphic chronology, stratigraphic correlation, event stratigraphy and mass extinction in the period. The zircon U-Pb dating of bentonites in the bottom of Upper Ordovician-Lower Silurian Longmaxi Formation in Yongshan area on the western margin of Yangtze Plate yields the weighted average age of 443.5±1.7 Ma, which defines the sedimentary age of the strata and provides a reliable basis for the study of Ordovician-Silurian volcanic eruption event and stratigraphic chronology on the western margin of Yangtze Plate, as well as basic information for the establishment of Ordovician-Silurian high-resolution stratigraphic framework on the periphery of Yangtze Plate. The trace element characteristics of zircon show the features of I-type granite formed in magmatic arc or orogenic belt. The multi-stage and high-frequency volcanic activity in the Late Ordovician-Early Silurian is possibly the dominant factor causing the mass extinction.