An intrusive-like tectonic breccia in the archipelago of southwestern Finland

Abstract

On an islet in the archipelago of southwestern Finland with metamorphic rocks belonging to the Svecofennian zone it is possible to follow in exposure for a distance of some metres the transition from a layered paragneiss with amphibolite bands to a breccia with amphibolitic fragments in a strongly recrystallized gneiss with tonalitic composition. The ultrametamorphic parts of the gneiss lack schistosity. The absence of microcline in these shows that no metasomatic granitization has taken place. The occurrence shows an initial stage of the process which has developed the banded series, a strongly deformed gneiss-amphibolite mixture which forms a zone, about 170 kilometres long, through the archipelago.

Keywords

• tectonic breccia
• intrusive-like
• paragneiss
• recrystallization
• Finland