Calibrating the Guadalupian Series (Middle Permian) of South China

The Guadalupian Epoch represents one of the most critical intervals during the Earth history which is characterized by a series of global geological and biological events. However, it is poorly constrained by radioisotopic ages in comparison with the high-precision dates from the Lopingian and lowest Triassic in South China and the Cisuralian in southern Urals, Russia. In this study, we report combined CA-ID-TIMS and SIMS U-Pb zircon geochronology for three ash beds from the base and lower part of the Kuhfeng Formation in the Chaohu area, southeastern China. Zircons from Ash Beds A, C and D were initially analyzed by SIMS and yielded weighted mean 206Pb/238U dates of 269.0 ± 1.4 Ma (MSWD = 1.03, n = 31), 266.3 ± 2.2 Ma (MSWD = 2.2, n = 29) and 262.2 ± 1.7 Ma (MSWD = 1.0, n = 20) in ascending order. However, high-precion U-Pb geochronology of the same ash beds by the CA-ID-TIMS method point to important inaccuracies in the SIMS results due to Pb-loss in zircons. Ash Beds A and C yielded two TIMS weighted mean 206Pb/238U dates of 272.95 ± 0.11 Ma (MSWD = 1.9, n = 7) and 271.038 ± 0.097 Ma (MSWD = 2.2, n = 8), respectively. The depositional age of 272.95 ± 0.11 Ma for Ash Bed A is postulated as the traditional Cisuralian-Guadalupian boundary (CGB) age in South China. Our new geochronology suggests that the Guadalupian Series in South China had a total duration of 13.85 ± 0.52 myr given a Guadalupian-Lopingian boundary (GLB) age of 259.1 ± 0.5 Ma. The new age calibration of the base of the Guadalupian Series is ca. 0.65 (± 0.51) myr older than – and improves upon – the previous estimates for the CGB age and the duration of the Guadalupian Epoch based on the 2016 International Chronostratigraphic Chart (ICC) and the 2012 Geologic Time Scale (GTS).