Crystallization of orbicular rocks exemplified by the Slättemossa occurrence, southeastern Sweden

Abstract

The orbicular rock at Slättemossa, southeastern Sweden, has a quartz monzodioritic composition. The cores of the orbicules crystallized directly from the orbicule-forming magma; cores made up of xenoliths have not been observed. Outside the core follow first an inner mafic, a felsic and then an outer mafic shell. The orbicules occur in a matrix, which is similar to the core. They grew simultaneously and show an almost perfect parallelism in evolution. After initial ‘normal’ crystall-ization of the magma, superheating probably triggered by a sudden addition of volatiles destroyed earlier formed nuclei and also affected the already crystallized part of the rock. Cooling caused heterogeneous nucleation and rapid crystallization, which formed the inner mafic shell. This is enriched in mafic minerals, especially biotite, compared to the core. At the same time the grain size becomes significantly smaller. Depletion in mafic components, possibly intensified by a sudden change in physical conditions, destabilized biotite and amphibole crystallization, causing oversaturation in plagioclase components, forming a felsic shell having a sharp boundary with the mafic shell. Plagioclase is extremely altered. Mafic minerals were then stabilized, probably due to depletion of plagioclase components, and an outer mafic shell formed. With the return to homogeneous nucleation, matrix formation concluded the crystallization. Orbicules might have moved in the magma causing some squeezing of magma surrounding the orbicules, but major movements involved the settling of the whole package of orbicules and matrix in the surrounding non-orbicular magma.