An erratic boulder of orbicular diorite occurs near Dyranut-Tråastølen, Hardangervidda, in an area underlain by Precambrian rocks. The orbicules are both mono- and multi-shelled, with the darker shells relatively enriched in biotite and magnetite. A radial alignment is defined by plagioclase at places, while biotite is tangentially oriented. Primary textures are masked, however, by strong secondary mineralization. An igneous origin by comb-layering around early-precipitated mineral aggregates (autoliths) is postulated for formation of the orbicules.
Orbicular diorite from Hardangervidda, south Norway