Amphibolites, structures and metamorphism on Flekkerøy, south Norway

Abstract: The island of Flekkerøy outside Kristiansand, southern Norway, consists of meta-greywacke, amphibolitic gneiss, granitic gneiss, tonalitic gneiss, hornblenditic mafite, pegmatite and metadolerite. The oldest rocks were probably formed around 1.5 Ga in an arc environment. During the Sveconorwegian orogeny, 1.14-0.90 Ga, the rocks were folded, recrystallized and metasomatosed. The rocks are therefore highly deformed and banded. The large scale structures are dominated by two synforms with fold axes running northwest and one inclined antiform with a fold axis approximately perpendicular to the two former. Lineations run northwest in general, but with a spread describing a great-circle in an equal area diagram. The structures indicate at least two stress directions. Metadolerites show alteration but are not foliated and have probably intruded after the deformation of the Sveconorwegian orogeny but before the end of the metamorphism. The different amphibolitic gneisses show variations in appearance and chemistry and most likely have different origins, either as basalt, dolerites, gabbro or calcareous clay. The trace element distribution is not similar to basites but rather to bulk continental crust. This indicates contamination of the magmas or metasomatism during the metamorphic event. The absence of good equilibrium textures makes PT-estimates uncertain. Approximate results are within the amphibolite facies.