Isotopic age determinations in South Norway: I. The Skar volcanic breccia, Greipstad, Vestagder

Abstract

Two samples of the fine-grained acidic matrix of the Skår volcanic breccia about 16 km WNW of Kristiansand in Vest Agder, southwestern Norway were investigated by whole-rock K-Ar. The breccia is situated on the intersection of a WNW and a NNE trending tectonic line, associated with the NE trending 'Great Breccia', a major tectonic feature of southern Norway. The samples yielded Late Permian ages of 248 ± 15Ma and 233 ± 15Ma. This indicates that the explosive Skår volcanism was related to the period of epeirogenesis that in southern Norway gave rise to the formation of the Oslo-graben and Hardangerfjord-Graben. Possibly there is a genetic connection between the acid volcanism at Skår and periodic alkaline mafic magmatism in the whole of northern Europe and parts of northern North America during the late Palaeozoic and the Mesozoic. The alkaline magmatism in the South Scandinavian Shield during this prolonged period might be an expression of a fundamental, mainly tensional 'Basin and Range-like'tectonic regime which prevailed in the northern hemisphere after the Caledonian orogeny and which lead to the opening of the Atlantic Ocean. The acid volcanic breccia erupted at Skår could be a product of contact-anatexis of crustal material generated by intruding alkaline basic magma.