New ⁴⁰Ar/⁹ Ar step heating data for amphiboles from three N-S to NNW -SSE-trending alkaline dikes in western Norway yield remarkably consistent plateau ages at about 221 Ma. Together with most previously published amphibole and whole-rock Kl Ar ages from Sunnhordland, which cluster around 220 Ma, the new data provide convincing evidence for a Late Triassic main pulse of dike intrusion. We conclude that most (but not all) of the K-Ar data are reliable and that Permian K-Ar and fission-track ages from four dikes in the region suggest an earlier pulse of intrusion at around 250-260 Ma. However, it is argued that the previously published Jurassic whole-rock age of one of the dikes is caused by hydrothermal alteration of the groundmass. A~220 Ma⁴⁰ Ar/³⁹ Ar plateau (and isochron) age of unaltered homblende is believed to refiect more closely the age of intrusion, in agreement with other ages of late dikes in the region. The Permian (250-260 Ma) and Triassic (220-230 Ma) pulses of dike injection are correlated with Permian and Triassic phases of extension in the North Sea basin to the west. The strong inftuence of Permo-Triassic stretching at the eastem side of the Viking Graben and only mild rejuvenation of Permo-Triassic structures in the late Jurassic help to explain the apparent absence of Jurassic magmatism in southwestem Norway.

Haakon Fossen, Department of Geology, University of Bergen, Allégt. 41, N-5007 Bergen. E-mail: This email address is being protected from spambots. You need JavaScript enabled to view it.
W. James Dunlap, Research School of Earth Sciences, The Australian National University, Canberra, ACT 0200, Australia. E-mail: This email address is being protected from spambots. You need JavaScript enabled to view it.