New precise zircon U-Pb ages are proposed for the Triassic-Jurassic
(Rhetian-Hettangian) and the Hettangian-Sinemurian boundaries, The ages
were obtained by ID-TIMS dating of single chemical-abraded zircons from
volcanic ash layers within the Pucara Group, Aramachay Formation in the
Utcubamba valley, northern Peru. Ash layers situated between last and
first occurrences of boundary-defining ammonites yielded Pb-206/U-238
ages of 201.58 +/- 0.17/0.28 Ma (95% c.l., uncertainties without/with
decay constant errors, respectively) for the Triassic-Jurassic and of
199.53 +/- 0.19/0.29 Ma for the Hettangian-Sinemurian boundaries. The
former is established on a tuff located 1 m above the last local
occurrence of the topmost Triassic genus Choristoceras, and 5 m below
the Hettangian genus Psiloceras. The latter sample was obtained from a
tuff collected within the Badouxia canadensis beds. Our new ages
document total duration of the Hettagian of no more than c. 2 m.y.,
which has fundamental implications for the interpretation and
significance of the ammonite recovery after the topmost Triassic
extinction.
The U-Pb age is about 0.8 +/- 0.5% older than Ar-40-Ar-39 dates
determined on flood basalts of the Central Atlantic Magmatic Province
(CAMP). Given the widely accepted hypothesis that inaccuracies in the
K-40 decay constants or physical constants create a similar bias between
the two dating methods, our new U-Pb zircon age determination for the
T/J boundary corroborates the hypothesis that the CAMP was emplaced at
the same time and may be responsible for a major climatic turnover and
mass extinction. The zircon Pb-206/U-238 age for the T/J boundary is
marginally older than the North Mountain Basalt (Newark Supergroup, Nova
Scotia, Canada), which has been dated at 201.27 +/- 0.06 Ma [Schoene
et al., 2006. Geochim. Cosmochim. Acta 70, 426-445]. It will be
important to look for older eruptions of the CAMP and date them
precisely by U-Pb techniques while addressing all sources of systematic
uncertainty to further test the hypothesis of volcanic induced climate
change leading to extinction. Such high-precision, high-accuracy data
will be instrumental for constraining the contemporaneity of geological
events at a 100 kyr level. (C) 2007 Elsevier B.V. All rights reserved.