On the age and tectonic significance of Permo-Triassic dikes in the Bergen-Sunnhordland region, southwestern Norway

New Ar-40/Ar-39 step heating data for amphiboles from three N-S to NNW-SSE- trending alkaline dikes in western Norway yield remarkably consistent plate au ages at about 221 Ma. Together with most previously published amphibole and whole-rock K/Ar ages from Sunnhordland, which cluster around 220 Ma, th e new data provide convincing evidence for a Late Triassic main pulse of di ke intrusion. We conclude that most (but not all) of the K-Ar data are reli able and that Permian K-Ar and fission-track ages from four dikes in the re gion suggest an earlier pulse of intrusion at around 250-260 Ma. However, i t is argued that the previously published Jurassic whore-rock age of one of the dikes is caused by hydrothermal alteration of the groundmass. A simila r to 220 Ma(40) Ar/Ar-39 plateau (and isochron) age of unaltered hornblende is believed to reflect more closely the age of intrusion, in agreement wit h other ages of late dikes in the region. The Permian (250-260 Ma) and Tria ssic (220-230 Ma) purses of dike injection are correlated with Permian and Triassic phases of extension in the North Sea basin to the west. The strong influence of Permo-Triassic stretching at the eastern side of the Viking G raben and only mild rejuvenation of Permo-Triassic structures in the late J urassic help to explain the apparent absence of Jurassic magmatism in south western Norway.