Klotdioriten vid Slättemossa, centrala Småland : mineralkemi och genes

At Slattemossa, Hultsfreds kommun, Kalmar lan, Sweden, two outcrops of orbicular rock occur. Both are smaller than 20 m2. The rock is classified as a monzodiorite to diorite; it contains closely spread orbicules in a matrix. The orbicules are made up of a medium-grained core surrounded by three fine-grained shells: an inner dark shell, a light shell and an outer dark shell. The matrix is medium-grained and similar to the core in composition and in appearance. Feldspars in the orbicular rock, especially in the light shell, are altered to phyllosilicates. There are several models to explain how orbicular rocks form; the models are both magmatic and metamorphic. The results of this study show that the Slattemossa rock is magmatic in origin. Orbicular rocks are rare and always occur in small volumes, a few 100 m3, thus orbicular rocks are local phenomena and the processes leading to them are rare. Differences in grain size of the shells compared to that of cores and matrix indicate a change in crystallization condition during the formation of the rock. In the shells, there are signs of supercooling indicating a change of pressure. A supercooling linked to water release from the magma explains the alterations in the orbicular rock. A decrease in pressure at a volcano eruption or caused by a “caldera subcidence” would be a possible model for the origin of this orbicular rock.