A post-Caledonian, NW-SE trending, dolerite dyke on Magerøy has a geochemical signature comparable to that of continental tholeiites. Clinopyroxenes from three samples of the dolerite gave K-Ar ages of ca. 312, ca. 302 and ca. 266 Ma, suggesting a Perrno-Carboniferous age. In view of low K2O contents of the pyroxenes and the possible presence of excess argon, it is argued that the most reliable estimate of the emplacement age of the dyke is provided by the unweighted mean value, namely 293 ± 22 Ma. This Late Carboniferous age coincides with the later stages of a major phase of rifting and crustal extension in adjacent offshore areas. The dyke is emplaced along a NW-SE trending fault. Parallel or subparallel faults on Magerøy and in other parts of northem Finnmark may thus carry a component of Carboniferous crustal extension in their polyphase movement histories.
David Roberts, Geological Survey of Norway, Post Box 3006-Lade, N-7002 Trondheim, Norway;
John G. Mitchell, Department of Physics, The University, Newcastle upon Tyne NE1 7RU, England;
Torgeir B. Andersen, Institutt for geologi, Post Box 1047, University of Oslo, N-41316 Oslo 3, Norway.