Crystal mat-formation as an igneous layering-forming process: Textural and geochemical evidence from the ‘lower layered’ nepheline syenite sequence of the Ilímaussaq complex, South Greenland

• Detailed study on layered nepheline syenites of Ilímaussaq intrusion.
• Layered units have sharp contacts against each other but each is modally graded.
• Composition of cumulus eudialyte shows systematic and repetitive changes.
• Composition of cumulus amphibole shows restricted variation only.
• We propose crystal mat-formation and gravitational sorting as for the layering.

The lower layered nepheline syenite sequence (kakortokites) of the Mesoproterozoic alkaline to peralkaline Ilímaussaq complex, South Greenland shows spectacular rhythmic meter-scale igneous layering. The 29 exposed units have sharp contacts against each other and each of these units consists of three modally graded layers dominated by arfvedsonitic amphibole, eudialyte-group minerals, and alkali feldspar, respectively.This study uses field observations on changes in mineral orientation, recurrent mineral textures, compositional data from eudialyte-group minerals and amphibole, and settling rate calculations based on a modified Stokes' equation to explain the igneous layering of the kakortokites. We propose that the three major cumulus minerals (amphibole, eudialyte s.l., and alkali feldspar) were separated from each other by density contrasts, resulting in modally graded layers within each unit. The densest of these three minerals (amphibole) formed crystal mats within the cooling magma body. These crystal mats acted as barriers that inhibited large-scale vertical migration of melts and crystals with increasing effectiveness over time. The sub-volumes of magma captured in between the crystal mats evolved largely as geochemically independent sub-systems, as indicated by the observed trends in mineral composition.