U–Pb and 40Ar/39Ar dating of the Miocene fossil track site at Ipolytarnóc (Hungary) and its implications

Abstract Abundant Early Miocene mammal and bird tracks and a rich plant assemblage is preserved by the emplacement of an ignimbrite sheet of the Gyulakeszi Rhyolite Tuff Formation (GRTF) near Ipolytarnoc in northern Hungary. The tuff that overlies the track-bearing sandstone yielded a single-crystal zircon U–Pb total isochron age of 17.42 ± 0.04 Ma and a single-crystal laser-fusion plagioclase 40 Ar/ 39 Ar age of 17.02 ± 0.14 Ma (uncertainties are quoted at the 2 σ level). An additional 40 Ar/ 39 Ar age of 16.99 ± 0.16 Ma was obtained from the equivalent rhyolite tuff near Nemti, where the underlying terrestrial clay yielded early proboscidean remains assigned to the MN4 mammal zone. The new, high-precision dates allow revision of the numeric age and correlation of the Ipolytarnoc fossil site and the GRTF, previously based on an average K–Ar age of 19.6 ± 1.4 Ma. The difference of 0.40 ± 0.15 Ma between the U–Pb and 40 Ar/ 39 Ar ages support the growing evidence for a systematic bias between the two isotopic systems due to the inaccurately known 40 K decay constant but likely also includes an undetermined pre-eruptive residence time of zircon. Published biostratigraphic data from under- and overlying marine strata establish correlation with the NN3 nannoplankton zone and, together with the new radioisotopic ages, suggest assignment of the fossils and the tuff to the Ottnangian regional stage of the Central Paratethys. A global correlation lends support to the recently suggested astronomical calibration of the Early Miocene time scale that revised the previous scales towards younger ages. The 40 Ar/ 39 Ar age from Nemti provides a reliable correlation of the MN4 mammal zone in Central Europe with the numeric time scale and places a minimum constraint on the age of the regional Proboscidean Datum, the migration event of proboscideans from Africa to Europe through the emerging “ Gomphotherium landbridge”. Contrary to suggestions for a significantly earlier European Proboscidean Datum, it appears that the originally suggested age of c. 17.5 Ma is realistic but it is significantly younger than the South Asian Proboscidean Datum.