A new type of silicate liquid immiscibility in peralkaline nepheline syenites (lujavrites) of the Ilimaussaq complex, South Greenland

Abstract

The 1.15-Ga-old Ilimaussaq intrusive complex in South Greenland shows an extensive fractionation trend from alkaline augite syenite to various varieties of strongly peralkaline, agpaitic nepheline syenites ("lujavrites"). The lujavrites are rocks with highly fluidal textures and are interpreted to have compositions close to melt compositions with the possible exception of significant loss of volatiles. They crystallized between 800 and 450 °C at 1 kbar and the different textural and mineralogical varieties exhibit variable but extreme enrichment in incompatible elements, such as U, Th, Nb, Li, Be, REE, Zr or Rb. Dykes of one variety of these extreme agpaites exhibit spectacular features of macroscopically visible liquid immiscibility between an Fe–Mn–Nb–REE-rich and a Na–Al-rich melt. The latter forms light greyish pillow-like structures of up to some centimetres size – comparable to the ocelli known from lamprophyres – in a dark fine-grained matrix, which is one of the most Nb- and REE-rich melts known [up to 1.2 wt% Nb₂O₅ and up to 6.6 wt% (REE^tot)₂O₃]. Unmixed between 500 and 600 °C at 1 kbar, both former melt phases crystallized assemblages involving arfvedsonite, K-feldspar, steenstrupine, and a variety of rare trace minerals. Presumably, the Na–Al-rich melt additionally crystallized nepheline, which was completely converted to analcime in an auto-hydration process during cooling. Thisoccurrence offers the possibility to study the effect of compositional changes in melts and their bearing on immiscibility in great detail, because the two immiscible melt phases are macroscopically visible, can easily be separated from each other and a large part of the fractionation trend towards the solvus is recorded in the various lujavritic rock types of the Ilimaussaq complex.