Emplacement and geochemical evolution of highly evolved syenites investigated by a combined structural and geochemical field study: The lujavrites of the Ilímaussaq complex, SW Greenland

• A combined structural and petrological approach to understand the petrogenesis of an igneous rock suite
• Magma recharge processes are recorded by eudialyte group mineral chemistry
• The lujavrites were emplaced by roof uplift between older rock suites
• Magmatic to high temperature solid-state deformation textures record the movement of magma in the lujavrite chamber

Structural mapping and the combined study of magmatic to solid-state deformation textures and mineral compositions in highly evolved nepheline syenites (lujavrites) of the alkaline to peralkaline Ilímaussaq complex (South Greenland) reveal detailed insight into the emplacement and geochemical evolution of the melts they crystallized from.Based on magmatic to solid-state flow textures such as foliations and lineations, we propose that the investigated rock sequence forms a sill-like structure with a steep feeder zone that flattens out over a short distance and intrudes into less evolved overlying units as sub-horizontal sheets by roof uplift. Systematic compositional variation of early-magmatic eudialyte-group minerals (EGM) in the investigated rock sequence monitors the geochemical evolution of the lujavrite-forming melt(s). The chlorine contents of EGM decrease successively upwards within the rock sequence, which probably indicates a successive increase of water activity during differentiation, consistent with a change from sodic pyroxene (aegirine) to sodic amphibole (arfvedsonite) in the mineral assemblage. Both REE contents and Fe/Mn ratios of EGM are promising differentiation indicators, which increase and decrease, respectively, upwards within the sequence due to fractional crystallization. This closed-system evolution is interrupted by a shift towards less evolved melt compositions in one lujavrite unit, for which we assume magma recharge.Our study demonstrates the strength of a combined structural and petrological approach to understand the petrogenesis of an igneous body in more detail and highlights their close connection.