Chemical and physical evolution of the ‘lower layered sequence’ from the nepheline syenitic Ilímaussaq intrusion, South Greenland: Implications for the origin of magmatic layering in peralkaline felsic liquids

Abstract

The Mid-Proterozoic composite Ilímaussaq complex, South Greenland, is a classic locality to study magmatic layering in evolved peralkaline magmas. Most of the rock units show magmatic layering to differing extents, but ‘kakortokites’ – generally medium-grained agpaitic nepheline syenites – show a spectacular recurrence of black, red and white layers, which is due to regular changes in the modal contents of arfvedsonitic amphibole, eudialyte (sensu lato), alkali feldspar and nepheline, respectively. These three-layer units are found in the lower part of the intrusion and recur 29 times before grading into the overlying lujavrites (melanocratic agpaitic nepheline syenites), which are generally fine-grained and fissile with less-developed layering. The compositional trends observed in amphibole and eudialyte throughout the stratigraphic sequence reflect various processes including the chemical evolution of the melt by crystal fractionation, changes in the crystallising mineral assemblage and sub-solidus alteration. Eudialyte is the first mineral to crystallise in the investigated sequence and is therefore appropriate for recording evolution trends within the melt. Amphibole, on the other hand, always crystallises later and is therefore affected by other crystallising minerals. A detailed microprobe study of both minerals through the whole kakortokite stratigraphy displays surprisingly little change in mineral compositions within the kakortokites, but strong fractionation trends in the overlying lujavrites. Although various models have been proposed to explain the recurrence of the 29 rhythmic units, the origin of this prominent magmatic layering in the kakortokites and the lack of mineralogical and strong mineral chemical changes has not been quantitatively explained. We propose, that in the kakortokites, minerals were probably separated from each other as a result of their different densities. The interior cooled, resulting in crystallization but only a very small proportion of crystals (0.1–0.3% for each of the four minerals) could remain suspended in the melt before gravity forced them to settle down in a stagnant layer of reduced convection. A combination of volatile pressure variations caused by eruptive activity and repeated replenishment can explain the oscillating liquidus temperature, the small changes in mineral compositions and such a process would produce enough crystals to form the 29 layers.