The formation of sheaf-like aggregates of hornblende in garbenschiefer from the central Scandinavian caledonides

The lower parts of the Köli-sequence of the Seve-Köli Nappe Complex in the central Scandinavian Caledonides are characterized by the occurence of spectacular sheaf-like aggregates of hornblende. Optical microscope investigations indicate that several processes are responsible for the formation of these aggregates. (1) Plastic deformation of hornblende porphyroblasts followed by a stage of subgrain formation as the results of subgrain boundary adjustment and a subsequent stage of growth of the differently oriented subgrains to form the radiating aggregates. (2) Brittle deformation of hornblende porphyroblasts followed by a stage of growth. (3) Coincidental contact growth of randomly nucleated hornblendes to form irregular aggregates. Evidence for (2) and (3) comes from the Klimpfjäll area in the southern part of the province of Västerbotten in Sweden. Evidence for (1) comes from various parts of the Swedish and Norwegian Caledonides.