Clues to the chemical evolution of agpaitic magmas from compositional zoning in kakortokite-hosted eudialyte, Ilímaussaq Complex, South Greenland

Abstract:

This study addresses the petrological and geochemical variations of eudialyte and related phases in kakortokites from the ~1.16 Ga Ilímaussaq Complex, S Greenland. The kakortokites form a suite of rhythmically layered agpaitic nepheline syenites, highly enriched in REE, Zr, Nb, Ta (HFSE) and halogens (F, Cl). Instead of containing common HFSE-phases such as zircon or titanite, agpaitic rocks exhibit complex Na-Zr-Ti-silicates, e.g. eudialyte and rinkite. To gain further insight in the crystallisation parameters and layering forming processes in agpaitic systems, we performed major and trace-element analyses on cumulus eudialyte. The data reveal complex compositional zoning patterns, including hour-glass sector-zoning, sub-µm scale oscillatory zoning and core-rim relationships. The latter were explored to identify subtle trends in the chemical evolution and in-situ fractionation of the liquids from which they crystallised. Most crystals exhibit a general rimward increase in REE concentrations (1.0-1.3 wt% Ce₂0₃ and 0.5-0.7 wt% La₂O₃), which is consistent with REE enrichment in the fractionating bulk magma. Similarly, Ba contents significantly increase towards the rims (300-900 ppm), while Cl (1.0-1.6 wt%), Rb (7-20 ppm) and Pb (60-160 ppm) decrease. In few cases, crystals exhibit partial rims with relatively low REE contents (~0.5 wt% RE₂O₃), which is in contrast with the general core-rim trend in the euhedral cores. These particular eudialyte segments are tentatively interpreted as crystallites from inter-cumulate melt, which underwent a chemical evolution distinctly different from that of the bulk magma. The decrease in REE contents may reflect the introduction of new REE-phases at the inter-cumulus stage, such as rinkite-group minerals, which contain ~20 wt% RE₂O₃ and occur interstitial to the major cumulus minerals feldspar, nepheline, arfvedsonite and eudialyte. Hour-glass sector zoning in the euhedral eudialyte cores involves small variations (< 0.1 wt%) in Nb, Ce and La concentrations between coevally grown crystal sectors. These compositional heterogeneities are related to preferential incorporation of Nb and REE on a specific set of growth surfaces and could provide important clues to the diffusion mechanisms in agpaitic magmas.