In the Precambrian gneisses adjacent to the Fen circular complex of per-alkaline and carbonatite rocks, dike-rocks of tinguaitic composition occur in great quantity. The dikes are often composite, with a thin rim on each side, consisting mainly of K-feldspar. The tinguaites have textures and compositions similar to the rocks from the type locality Tingua in Brasil. Dike-rocks of this type are observed only outside the actual Fen complex, but are present in greatest amount nearest to its borders. This has led to the conclusion that the intrusion of tinguaites is an early event in the evolution of the carbonatite and per-alkaline complex. Hypotheses for the origin of the tinguaite magma are discussed. Dikes of carbonatite, damkjernite phonolite, and diabase occur in minor amounts in the same area.
Tinguaite dikes adjacent to the Fen alkaline complex in Telemark, Norway