First direct age determinationfor the Kelseaa Dolerite Dyke, Bornholm, Denmark

Abstract

The Danish island Bornholm on the southwestern margin of the Baltic Shield was subject to dyke injection during the Proterozoic. The dykes probably result from several magmatic events. We present U-Pb geochronological data for the largest of the dykes, the tholeiitic Kelseaa dyke. The resulting age, 1326 ±10 (2σ) Ma, places the dyke significantly earlier in the Proterozoic than previously assumed. No other dykes of this age have been reported from the western part of the Baltic Shield. The NE–SW strike of the Kelseaa dyke is evidence for extension oblique to the border of the Baltic Shield.The Kelseaa dyke is the first evidence for this event that was subsequent to the emplacement of theBornholm and Karlshamn (SE Sweden) granites and prior to the intrusion of the Central Scandinavian Dolerite Group, and possibly also the majority of mafic dykes on Bornholm.