The compositional relationships of biotite and garnet in the Göteborg area of south-western Sweden and their thermo-metric implications

Abstract

A micro-probe study of six pairs of co-existing garnet and biotite samples from a polymetamorphic migmatite in the Göteborg area of Sweden demonstrated similar mineral compositions in the neosome and paleosome. Distribution equilibrium was closely approached, the distribution coefficient K_DFe ^Ca-Hi having a mean value of 4.5 (one deviating value excluded). This indicated the occurrence of intermediate-to-high-grade regional metamorphism. Application of the distribution thermometer, as developed by Perchuk, yielded a temperature of 580±20°C. Comparatively high contents of Ca and Mn in the garnets suggested that this was a minimum estimate. The thermometrical estimates were in accordance with the general impression of low almandine—amphibolite-facies metamorphism, which was derived from the overall rock paragenesis in the region.