Zircon U-Pb dating and geological significance of the bentonites from the Upper Ordovician Wufeng Formation and Lower Silurian Longmaxi Formation in western Daba Mountains

Abundant bentonites are well developed in the Late Ordovician-Early Silurian strata in western Daba Mountains, northern Yangtze block. The bentonite samples for this study are collected from the Upper Ordovician Wufeng Formation and Lower Silurian Longmaxi Formation sections in Maliu, Ziyang, Shaanxi and Piwo, Wanyuan, Sichuan in western Daba Mountains, and give the zircon U-Pb ages of 445.1 ±3.5 Ma and 446.1 ±7.2 Ma, respectively. These ages represent those of the deposition of the Wufeng and Longmaxi Formations, and provide data support for volcanic eruption events and stratigraphic chronology between the Ordovician and Silurian boundary in these areas. They are also synchronous with the formation time of the Ordovician magmatic arcs in the Qinling-Dabieshan orogen, and slightly later than the ages (449.0-465.8 Ma)of the Ordovician bentonites from the southwestern margin of the North China craton. The volcanic activity in this period may be associated with the northward subduction of the Palaeo-Qinling oceanic crust, and the volcanic tuffs may be originated from the volcanic arc eruption along the northern margin of the Palaeo-Qinling Ocean. The multi-episodic high-frequency volcanic eruption during the later Middle Ordovician to the earlier Early Silurian would influence in that time on many aspects such as the changes of oceanic chemical conditions, oscillation of carbon cycling, climatic colling and pulsation of biological radiation, and resulted in the initiation of glaciers and biological massive extinction at the end of the Late Ordovician.