The migmatite complex of the Risør area, Aust-Agder, Norway

The Pre-Cambrian basement of the Risör area consists of a migmatite complex composed of metamorphic rocks intimately mixed with 'granitic' gneisses (sensu lato). The metamorphites occur as bands, folded layers, !enses, and 'ghost' relics within the 'granitic' rocks. A number of tectonic and metamorphic phases have been recognized and it seems probable that more than one period of Pre-Cambrian orogeny has affected the area. The main granitisation was accompanied by upper amphibolite facies metamorphism but subsequent metamorphism of similar grade locally mobilised 'granitic' pegmatites formed extensively. Petrographical investigations have revealed a series of recrystallisations and replacements within the rocks and have shown evidence of extensive potash-metasomatism and microclinisation, probably preceded by a phase of soda-metasomatism. It is suggested that the solutions eausing granitisation were very similar to those which formed the pegmatites and both appear to have been derived from depth.