Geochemical And U-Pb Geochronology Of K-bentonites From Pingliang Formation Of Ordovician,Gansu,China

With the establishment of the international Earthtime organization,the high-precision isotopic geochronology has rapidly developed.Furthermore,the advance of the CA-ID-TIMS U-Pb experiment method provides a great chance for the perfection of the high-precision geologic time scale.Volcanic ash,as a special event and temporal key layer,playes an important role in ancient environmental,palaeogeographic and palaeotectonic analysis.In this study,we conduct the geochemistry and high precision U-Pb radioactive isotope geochronology on K-bentonites from the Pingliang Formation at Yindongguangzhuang section in Gansu,North China.The K-bentonites show high Si O₂.Al2O3/Ti O₂,Zr/Hf and Ti/Th ratios suggest that they were derived from the acidic volcanic rocks.Rare earth elements?REE?distribution patterns are enrichment of light rare earth element,the Eu negative anomaly and no Ce anomaly,which are similar to the REE compositions of the contemporary K-bentonitens.Trace elements spider diagram displays the Sr and Ti negative anomaly but positive Pb anomaly.Zr/Ti O₂-Nb/Y diagram indicates possible genetic relations with the intermediate-acid magma.Th/Yb-Nb/Yb and Th-Hf-Ta diagrams provide consistent results.Together with characteristics of trace elements,we suggest that the original magma formed in the volcanic arc environment.The Pingliang Formation covers the conodonts of late Ordovician conodont zones of Baltonniodus variabilis and Baltonniodus gerdea.The identified short eccentricity cycles in the section indicate a duration of 2.862 Myr for the interval of two K-bentonite layers?Fang et al.,2016?.The bottom and the top K-bentonite has a zircon SIMS U-Pb age of 457.6±3.8 Ma and 454.8±3.8 Ma,respectively,which demonstates a sandbian age of Ordovician Period correlated to biostratigraphic framework.The calculated duration of two K-bentonite layers is 2.8 Myr.Within the error range,therefore,ages from GTS2012,U-Pb zircon dating and astronomical time scale share a good agreement,which achieves the inter-correction of different dating methods.The data provides a new evidence for absolute age for the Sandbian of late Ordovician.It is important to note that the two concordant ages have the same time with the Kinnekulle bentonite from Europe Baltoscandia and Deicke bentonite from North American,respectively,but the stepwise discriminant analysis on chemical composition indicate that they are not homogenous.According to the regional tectonic background,the development of stratigraphy,and the constraint of magmatic events,we conclude the K-bentonites mainly came from the eruption of North Qilian-North Qinling arc volcanic.