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

Thursday, 18 December 2014
Anouk Margaretha Borst1,2, Matthijs A Smit3, Tod E Waight2, Troels F D Nielsen1 and Henrik Friis4, (1)Geological Survey of Denmark and Greenland, Copenhagen, Denmark, (2)University of Copenhagen, Copenhagen K, Denmark, (3)University of Copenhagen, Copenhagen, Denmark, (4)Natural History Museum Oslo, Oslo, Norway
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.