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 +/- 13 Ma, respectively, with a weighted mean age of 121.2 +/- 0.5 Ma (2 sigma). 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. (C) 2008 Elsevier B.V. All rights reserved.