Basic plutonic intrusions of the Risør-Søndeled area, South Norway: The original lithologies and their metamorphism

The emplacement of Pre-Cambrian basic intrusions is shown to have been a polyphase process and to have occurred largely between two major periods of metamorphism. The original igneous lithologies are discussed and a regional differentiation series is demonstrated, with changes in rock-type accompanied by some cryptic variation in the component minerals. Metamorphism after consolidation converted the intrusives to coronites and eaused partial amphibolitisation. Corona growths are described and attributed to essentially isochemical recrystallisations in the solid-state, promoted by the diffusion of ions in intergranular fluids. The amphibolitisation of the bodies is briefly outlined, and it is concluded that the formation of amphibolite involved the introduction of considerable amounts of extraneous water. The criterion for demonstrating the onset of amphibolitisation is thought to be the intemal replacement of pyroxene by hornblende, contrasting with the replacement of plagioclase by various amphiboles during corona growth. Both amphibolites and coronites have been scapolitised, and this may have been an extended process which certainly continued after amphibolitisation. Regional implications are considered and it is suggested that a large mass of differentiating magma may have underlain the whole region in Pre-Cambrian times.