Geochemical overview of the Ilimaussaq alkaline complex, South Greenland

The Ilimaussaq alkaline complex is one of the latest members of the Mesoproteorozoic Gardar igneous province. It was emplaced in three phases, 1) augite syenite, 2) alkali acid rocks and 3) the dominant agpaitic nepheline syenites. The immediate parent for the nepheline syenites is the augite syenite whose geochemical feature, such as the low Zr/Nb ratio (3.7), match benmoreites from a nearby alkali basalt series. The agpaitic nepheline syenites are characterised by exceptionally high contents of Zr, Hf, Nb, Ta, REE, Th, U, Sn, Li, Be, Rb, Zn, Pb, Sb, W, Mo, As and Ba, and the volatile elements F, Cl, Br, I and S but exceptionally low levels of Ba, Sr, Co, Cu, Ni, Sc, V and Cr. Fractionation of cumulus phases with a distinctive geochemistry, such as sodalite (rich in Cl, Br, I, B), eudialyte (Zr, Hf, Nb, Ta, W, As) and arfvedsonite (Co, Sc, V, Cr), caused exhaustion of these elements in the residual magmas. The agpaitic magmas underwent extreme fractionation with the final lujavrite forming after 99% crystallisation of the augite syenite. Zirconium was apparently soluble in Ilimaussaq melts up to a concentration of c. 9000 ppm. Whole-rock analyses define a number of discrete Zr-U arrays which are restricted to certain intervals of the cumulate stratigraphy and are taken as evidence for liquid layering in the Ilimaussaq magma chamber. The distribution of the less common trace elements (Bi, Cd, Co, Ge, In, I, Hg, Sc, Se, V, W) and of the more abundant trace elements which failed to form minerals (Br, Cs, Ga, Hf, Rb, Sr) is summarised. (au)