Toward age determination of the M0r (Barremian–Aptian boundary) of the Early Cretaceous

We carried out integrated paleomagnetic and geochronologic investigations on Cretaceous lava flows at the Mashenmiao-Zhuanchengzi (MZ) section in Yixian, Liaoning Province, northeast China in seeking to understand the onset of the magnetic polarity chron M0r and the associated Barremian–Aptian boundary (BAB), which has been reported to be 125.0 ± 1.0 Ma [Ogg, J.G., Agternerg, F.P., Gradstein, F.M., 2004. The Cretaceous period. In: Gradstein, F.M., Ogg, J.G., Smith, A.G. (Eds.). A Geologic Time Scale. Cambridge University Press, UK, pp. 344–383]. Stepwise thermal or alternating field demagnetization indicates that all lava flows in the studied section were reversely magnetized. 40Ar/39Ar ages obtained from three lava flows are 121.2 ± 1.3 Ma, 120.2 ± 1.5 Ma and 122.0 ± 1.3 Ma, respectively, with a weighted mean age of 121.2 ± 0.5 Ma (2σ). In combination with previous studies, we argue that these lavas are M0r in age, which therefore is probably some 4 Myr younger than estimated in the 125 Ma of the most recent geologic time scale.