Multi-method radiometric dating of volcano-sedimentary layers from northern Italy: Age and duration of the Priabonian stage
Résumé
A comparison between conventional K-Ar (biotite) ages and fission track (zircon and apatite) and U-Pb (zircon) ages obtained from stratigraphically well-constrained Priabonian (Late Eocene) volcano-sedimentary deposits of northern Italy is presented. Two sections at Priabona (one level) and Possagno (two levels) were dated. The application of fission track dating appears fruitful for obtaining reasonably precise ($\pm$ 4 to 5% 2$\sigma$ errors) ages useful for time-scale calibration. The concordancy of apatite and zircon fission track ages, and the reproducibility of results provide the time of volcanic eruption and deposition. The U-Pb analysis of the zircons has not been unsuccessful, but discordancy does not permit accurate dating. Significant dates obtained from Possagno are: K-Ar method, 35.0 $\pm$ 0.5 Ma (duplicate analysis on K-rich biotite from the same level); fission track dating method, 35.8 $\pm$1.4 Ma (weighted mean age on 2 apatite and 3 zircon separates from the same level); U-Pb method, 36.7 $\pm$ 1.0 Ma (maximum age of discordant zircons from the same level). The comparison between the present results and recent multi-method and multi-laboratory results obtained from time equivalent Priabonian (Late Eocene) biotite-rich layers from the Apennines shows perfect agreement and supports the location of a Priabonian stage between about 37.5 Ma and about 33.7 ($\pm$0.5) Ma; the alternative ages preferred by the Decade of North American Geology convention should be abandoned and a large portion of this scale revised accordingly.