A detailed study was made of the changes in the olivine gabbro surrounding three pegmatite veins at Risør, Norway. The pegmatite veins and the surrounding rocks are briefly describecl petrographically. From the modal analyses of twenty samples the changes in the chemical composition of the surrounding rock as a function of distance from the veins were calculated. These calculations show that near each of the veins there exist zones of markedly increased basicity. A cliscussion of the distribution of pressure near a break in a competent rock (e. g. olivine gabbro) surrounded by a rock flowing plastically (e. g. gneiss) during the dynamic regional metamorphism leads to the conclusion that pressure differences would be created such that thermodynamic gradients would necessitate the migration of material from zones of rclatively high pressure to zones of relatively low pressure, minerals of low mol volume being preferred in the zones of high pressure, while minerals of large mol volume would be preferentially formed in the zones of low pressure. The pegmatite veins and the nearby zones of increasecl basicity are therefore interpreted as the result of a process of metamorphic segregation during the dynamic metamorphism of the area.
Calculations of the density of cations per unit number of oxygen ions according to Barth's standard cell methocl (2, 3), and calculations of the changes in the volume occupied by a unit number of oxygen ions, based on a method taking into account the mol volumes and the modal percentages of the constituent minerals, verify the validity of the mode of formation proposed for the pegmatite veins and the nearby depleted zones of increased basicity.