Modally-Graded Rhythmic Layering in the Skaergaard Intrusion

Abstract

The origin of igneous layering in the Skaergaard and other intrusions is the subject of much debate. Currently held theories for its origin include gravitational sorting, rhythmic nucleation, and chemical diffusion models. This paper presents data on small-scale, modally-graded, rhythmic layers, one of several distinct types of layering found in the Skaergaard. These layers occur only in the Layered Series of the intrusion, in which they are a widespread, almost ubiquitous feature. They are characterized by strong variations in modal composition, with magnetite, ilmenite, and olivine concentrated in the lower part of the layers, pyroxene in the middle of the layers, and plagioclase in the upper part of the layers. In addition, the layers are size graded, with the largest grain sizes of the different minerals generally occurring where the mineral is most abundant. The chemical composition of both plagioclase and Ca-rich pyroxene do not vary in any regular manner within the layers studied. The forsterite composition of olivine is consistently higher in the lower part of the layers. This trend, however, correlates very strongly with the modal variations of olivine and can be explained by minor amounts of subsolidus reequilibration between cumulus and adcumulus olivine, suggesting that the original composition of cumulus olivine within individual layers was constant. These results are consistent with layers formed by some gravitational sorting process and, in several cases, are at odds with other theories that have been proposed for the origin of igneous layers. Based on these data and the widespread evidence for current interaction (crossbeds, trough-shaped layers, and internally graded layers), we propose that these layers result from periodic disruption of a static zone of crystal growth on the floor of the magma chamber by convection or density currents. This mechanism, however, is almost certainly not responsible for the formation of all of the types of layering found in the intrusion.