Pb-Pb zircon dating of tuff horizons inthe Cyrtograptus Shale (Wenlock, Silurian) of Bornholm

Waterlain fallout ashes are interbedded in the upper part of the Cyrtograptus Shale of Bornholm, theyoungest preserved member of the Lower Palaeozoic sequence at the southern coast of the island.Graptolite faunas indicate that these tuffaceous sediments belong to the Cyrtograptus lundgreni Zonedeposited during Late Wenlock. A 207Pb/206Pb mean age of 430 ± 1.9 Ma obtained by evaporation of idiomorphic single zircons from the tuff layers supports this observation. Geochemical studies of the pyroclastic rocks point to an explosive, calc-alkaline magmatic arc volcanism which probably occurred along or slightly south of the Tornquist-Teisseyre Lineament, and could have been induced by the collision of Avalonia with the southern margin of Baltica during the Silurian. This assumption is supported by the contemporaneous deposition of bentonites on the Swedish island of Gotland which might represent a distal facies of these fallouts. Further, the subduction-related volcanic activity is interpreted as a fingerprint for closing of the Tornquist Ocean during the Caledonian orogeny.