New precise zircon U{\textendash}Pb ages are proposed for the Triassic{\textendash}Jurassic (Rhetian{\textendash}Hettangian) and the Hettangian{\textendash}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 206Pb/238U ages of 201.58{\textpm}0.17/0.28~Ma (95\% c.l., uncertainties without/with decay constant errors, respectively) for the Triassic{\textendash}Jurassic and of 199.53{\textpm}0.19 / 0.29~Ma for the Hettangian{\textendash}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{\textendash}Pb age is about 0.8{\textpm}0.5\% older than 40Ar{\textendash}39Ar dates determined on flood basalts of the Central Atlantic Magmatic Province (CAMP). Given the widely accepted hypothesis that inaccuracies in the 40K decay constants or physical constants create a similar bias between the two dating methods, our new U{\textendash}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 206Pb/238U 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{\textpm}0.06~Ma [Schoene et al., 2006. Geochim. Cosmochim. Acta 70, 426{\textendash}445]. It will be important to look for older eruptions of the CAMP and date them precisely by U{\textendash}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.
}, year = {2008}, volume = {267}, pages = {266 - 275}, month = {2008/03/01/}, isbn = {0012-821X}, url = {http://www.sciencedirect.com/science/article/pii/S0012821X07007777}, language = {eng}, }