A study was made of the structural position of 266 small, tabularly shaped pegmatites in an amphibolite in the pre-Cambrian Kongsberg-Bamble formation on the island Rytterholmen near Kragerø, southern Norway.
The pegmatites and the surrounding amphibolite are briefly described petrographically. Special attention is paid to the analysis of the structural position of the pegmatites in terms of shear-stress zones and tension directions which developed during the dynamo-thermal metamorphism of the area. The lang axes of the pegmatites show a pronounced directional orientation in each of the five small areas examined in detail. This preferred orientation is related to zones of shear which are about perpendicular to the tension directions. The ratio length/width of the pegmatites is found to be roughly inversely correlated to the basicity of the surrounding amphibolite. The theory of origin by metamorphic differentiation is discussed and found to be the most satisfactory explanation of the genesis of the pegmatites. Basified zones in the amphibolite adjacent to the pegmatites and the difference between the An % in the surrounding amphibolite and the pegmatites substantiate this conclusion.