A new U-Pb zircon age is reported for the Bathonian - Callovian (late Middle Jurassic) boundary. The age was obtained by dating single, annealed and leached zircons (Mattinson, 2005; Mundil et al., 2004) from a volcanic ash layer within the Chacay Melehue Formation, in the Chacay Melehue section, Neuquen province, Argentina. The ash bed represents the only dated horizon that is directly related to a well-documented ammonite faunal succession at the Bathonian-Callovian boundary. The ash layer occurs above the last record of ammonites defining the regional Standard Zone of Lilloettia steinmanni and below the first of those of the Eurycephalites vergarensis Standard Zone, which have been referred to, respectively, as the uppermost Bathonian and the lowermost Callovian. The age is older than the previously reported U-Pb zircon age of 160.5 ± 0.3 (Odin et al., 1992) for the same bed. It is in fact closer to an estimation of 164.7 Ma for the Bathonian-Callovian boundary, based on scaling durations of ammonite zones to their subzones in the subboreal standard zonation and more in agreement with the age model for the Oxfordian through Bathonian M-sequence magnetic anomalies in the Pacific (see Ogg, 2004).
Mattinson, J.M., 2005. Zircon U-Pb chemical abrasion ("CA-TIMS") method: combined annealing and multi-step partial dissolution analysis for improved precision and accuracy of zircon ages. Chemical Geology 220, 47-66.
Mundil, R., Ludwig, K.R., Metcalfe, I., and Renne, P.R., 2004. Age and timing of the Permian mass extinctions:U/Pb dating of closed-system zircons. Science 305, 1760-1763.
Odin, G.S., Baadsgard, H., Hurford, A.J., and Riccardi, A.C., 1992. U-Pb and fission track geochronology of Bathonian-Callovian “tuffs” from Argentina. In: Odin, G.S., ed., Phanerozoic Time Scale, Bulletin de Liaison et Information 11: 11-17.
Ogg, J.G., 2004. The Jurassic Period. In: Gradstein, F., Ogg, J., and Smith, A., eds., A Geologic Time Scale 2004, pp. 307-343.