Abstract
The Kattsund-Koster dyke swarm in the Sveconorwegian Province of the Baltic shield is a dense swarm of evolved tholeiites derived from N-MORB type parental magmas selectively contaminated with K, Rb and Ba. In the Koster archipelago the NNE-SSW trending dyke swarm enters a ductile sinistral shear zone in the margin of which the dolerites are partially recrystallised metadolerites. In the highly deformed interior of the shear zone the dykes are completely recrystallised to amphibolites. Rb-Sr isotope analyses have been carried out on samples of the dolerites and the partially recrystallised metadolerites. A profile through one dolerite yields a whole rock age of 1421 ± 25 Ma with a (87Srl"6S)0 ratio of 0. 7028 ± 0.0002. Samples of the dolerites and partially recrystallised metadolerites lie close to the isochron. The age of 1421 Ma indicates that the dyke swarm is the oldest member of a 1420-1300(?) Ma old bimodal suite of tholeiites and potassic granites, which were injected into the crust under tensional conditions.
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