New, high-precision, ⁴⁰Ar/³⁹Ar age determination on a plagioclase separate and whole rock samples from a post-Caledonian dolerite dyke from southem Magerøy gives ages of 337.3 ± 0.4 Ma and 340 ± 4 Ma respectively. A conventional K/Ar whole rock analysis of the same sample gives 301 ± 8 Ma. The ⁴⁰Ar/³⁹Ar ages are Early Carboniferous (Visean) and indicate an age older than previously published K/ Ar results from a comparable dyke. The dykes were intruded parallel to a set of NW -SE trending normal faults associated with the Late Palaeozoic rifting in the Barents Sea area. The new age determination on the dykes places constraints on the timing of Late Palaeozoic rifting in the southern part of the Barents Sea.

S. J. Lippard & T. Prestvik, Department of Geology and Mineral Resources Engineering, Norwegian University of Science and Technology (NTNU), N-7034 Trondheim, Norway.