Dating explosive volcanism perforating the Precambrian basement in southern Norway.

Abstract

K-Ar and Rb-Sr dating have been performed on various hypabyssal diatreme-facies volcanic rocks perforating the Sveconorwegian zone of the Precambrian basement in southern Norway. The dated rocks were taken from the Fen peralkaline-carbonitic area, the Gardnos and Hjølmodalen explosion breccias, the marginal breccia of the Fjone calcite-syenite plug, damtjernites from Gulbrandstjern, Presøya and Brånan, and the carbonatized damtjernitic explosion breccia with abundant xenoliths from Tveitan and Hønstjern in the Bambøe region. The results reveal two episodes of explosive volcanic activity prior to Permian magmatism, one about 600 Ma ago (latest Precambrian) and another probably about 350-300 Ma ago (Carboniferous). These episodes of volcanic activity possibly reflect precursor phases of the Caledonian orogeny and the Permian epeirogeny, respectively. A dolerite dike in the Fen area may be of Permian age.