A SIMS zircon age for a biostratigraphically dated Upper Vis,an (Asbian) bentonite in the Central-European Variscides (Bardo Unit, Polish Sudetes)

A Lower Carboniferous platform sedimentary sequence (the Paprotnia Beds) in the Bardo Unit of the central Sudetes (NE part of the Bohemian Massif, SW Poland) is biostratigraphically well dated, based on rich macro- and micro-fossil evidence, as Late Vis,an (late Asbian, crenistria, Go III alpha zone). The beds contain several bentonite layers, one of which was dated using the U-Pb SHRIMP method on volcanic zircons and yielded an age of 334 +/- A 3 Ma. This date fits well to the recently established chronostratigraphic limits of the Vis,an, and is consistent with the newest isotopic age constraints of 336.5-332 Ma for the Asbian boundaries.