New high-precision U-Pb zircon age constraints on the Sandbian stage of the Ordovician System from the Guanzhuang section in North China

The Middle–Late Ordovician transition is a crucial period in geological history, marked by a series of paleoceanographic, paleoclimatic, and biotic events. However, radioisotopic age constraints for this period remain limited. The Guanzhuang section in North China Craton has robust biostratigraphic control and was a candidate for the GSSP for the Middle–Upper Ordovician boundary. In this study, we present integrated SIMS and CA-ID-TIMS Usingle bondPb zircon dates from the Late Ordovician Pingliang Formation at the Guanzhuang Section. Three ash beds, from the bottom upwards, in the Pingliang Formation were dated by SIMS at 457.5 ± 3.3 Ma, 454.2 ± 3.7 Ma, 454.5 ± 8.6 Ma, respectively. The stratigraphically lowest and highest samples were further dated by CA-ID-TIMS, yielding refined results of 457.29 ± 0.23 Ma and 454.23 ± 0.21 Ma. Based on these results, a Bayesian age-depth model yields an estimated age of 457.97 + 1.76/−0.39 Ma for the base of the Sandbian Stage. The interval from the base of the conodont Pygodus serra biozone to the base of the Baltoniodus gerdae biozone is further constrained to approximately 6.14 Myr, allowing the calculation of an average cooling rate of ∼2 °C Myr−1. This suggests that the gradual temperature decline facilitated biotic adaptation and sustained the Great Ordovician Biodiversification Event.