The kakortokites of the Mesoproterozoic Ilímaussaq complex, South Greenland, comprise a rhythmically layered series of agpaitic nepheline syenites that crystallized at the base of a shallow crustal magma chamber. They host eudialyte-group minerals (EGM), i.e. structurally complex Nazirconosilicates, as a major cumulate phase, and have attracted considerable interest as a potential resource for rare earth elements (REE), Zr, Nb, Hf and Ta. The origin of the macrorhythmic (c. 8m) layering has been the subject of much debate, and both open system processes including nucleation cycles induced by periodic replenishment of the magma chamber, and closed system mechanisms involving gravitational sorting and crystal mat formation, have recently been hypothesized. Here we present new compositional data on eudialyte cores and overgrowths from the full layered series and part of the overlying lujavrites to reflect on the proposed models for the kakortokite layering and overall evolution of the complex. Based on these data we argue for continuous bulk liquid fractionation and in situ fractionation in macrorhythmic compartments of kakortokite mush gradually building up from the floor of the magma chamber. Eudialyte in the kakortokites displays complex magmatic zoning patterns, typically comprising a sector- and oscillatoryzoned core with subhedral concentric overgrowths. Sector-zoned eudialyte cores reveal stratigraphical fractionation trends of decreasing Ca/(REE+Y), Fe/Mn, Ti, Nb and Cl contents upwards through the layered series. These are interpreted to reflect continuous differentiation of a single agpaitic bulk melt and support models for closed system evolution of the kakortokites. Upward trends become more pronounced in the overlying lujavrites (decreasing Ca/(REE+Y), Fe/Mn, Zr/Hf and Cl), while others remain constant (Ti), or are even reversed (Nb). Eudialyte overgrowths have compositions that diverge from the overall fractionation trends recorded in the cores, and also vary systematically across the sequence. These overgrowths are interpreted to reflect in situ fractionation trends of intercumulus mush melts that were chemically isolated from the bulk magma following compartmentalization of the crystal mush. As such, EGM overgrowths are interpreted to record changing layering dynamics as well as varying co-crystallizing intercumulus phase assemblages at the kakortokite-lujavrite transition. The data provide new insights into the geochemical evolution of the Ilímaussaq complex, with broader implications on the emplacement and layering mechanisms operating in peralkaline systems, and demonstrate the importance of detailed petrographic and in situ mineral chemical analyses where zoned minerals record contrasting evolution of bulk and mush melts. Deciphering such records is fundamental to understanding the full complexity of magma chamber processes.
- Programområde 4: Mineralske råstoffer