The Oslo Rift: new palaeomagnetic and Ar-40/Ar-39 age constraints

Ar-40/Ar-39 whole-rock and alkali feldspar ages demonstrate that dioritic t o monzonitic dykes from Boverbru and Lunner belong to the youngest recorded magmatic activity in the Oslo Rift region, southeast Norway. These dykes r epresent the terminal phase of rift and magmatic activity in the Oslo Grabe n, at the dawn of the Triassic (246-238 Ma). The Boverbru and Lunner dyke ages are statistically concordant. However, th e palaeomagnetic signature of the Boverbru dyke is complex, and directions from the margins and the interior of the dyke differ in polarity. Therefore , the new Early Triassic palaeomagnetic pole for Baltica (Eurasia) is exclu sively based on the less complex Lunner dykes and contacts (palaeomagnetic pole: latitude = 52.9 degrees N, longitude = 164.4 degrees E, dp/dm = 4.5 d egrees/7.3 degrees). The early Triassic palaeomagnetic pole [mean age: 243 +/- 5 Ma (2 sigma)] is slightly different from the Upper Carboniferous-Perm ian (294-274 Ma) and Kiaman-aged poles from the Oslo Rift.