The Fen circular complex is composed of carbonatites and alkaline silicate rocks. It is temporally associated with breccias, explosion pipes, and kimberlitic-alkalibasaltic dikes occurring from Jonnbu in Lifjell in the northeast to Greipstad near Kristiansand in the southeast over a distance of about 300 km, but greatly concentrated in the near vicinity of Fen. It is assumed that a majority of these breccias, etc., are genetically related to the Fen volcanism. Isotope ratios indicate an age range at the Precambrian-Cambrian transition.
Eocambrian volcanism in Southern Norway