Geochemistry and REE minerals of nepheline syenites from the Motzfeldt Centre, South Greenland

Whole-rock analyses including the REE are given for nineteen samples from four of the six overlapping nepheline syenite intrusions which constitute the Proterozoic (∼ 1310 ma) Motzfeldt Centre. Larvikite margins of one unit (SM5) are apparently related to the nepheline syenites by early fractionation of Ol + Pl followed by fractionation of Ol + Pl + Cpx ± Amph at current levels of exposure. Positive Eu-anomalies in the larvikites might be explained by large-scale incorporation of earlier Pl-rich cumulates. Low initial ⁸⁷Sr/⁸⁶Sr ratios imply a mantle source, and the most basic magma was a late alkali gabbro giant dike with plagioclase megacrysts. REE- and incompatible element-rich peralkaline lujavrites with large negative Eu-anomalies are late stage products of extreme fractionation. Electron probe analyses of REE in eudialyte confirm this mineral as the major host to Zr and REE in the lujavrites. Other REE-bearing minerals in the nepheline syenites are Ca–Na–Ce–Ti–Zr–Nb-silicates of the rinkite–møsandrite and låvenite series, with up to 22 wt.% RE-oxides; some may be related to metasomatic fluids which had geochemical signatures similar to the lujavrites.