The Ilímaussaq alkaline complex, South Greenland, the type locality of agpaitic nepheline syenites
and of thirty minerals, among which arfvedsonite, eudialyte and sodalite, has been studied since
1806. The paper reviews the outcome of 200 years of geological investigations and presents an
overview and a synthesis of the petrology of the complex.
The site of the complex was invaded
successively by augite syenitic, alkali granitic and nepheline syenitic melts. Remnants of the augite
syenite and granite intrusions are found along the contacts of the nepheline syenites and as xenoliths
in these. The nepheline syenites, which occupy the major part of the complex, are divided into a
roof series, an intermediate series and a floor series. The roof series crystallized from the roof
downward in the order pulaskite, foyaite, and the agpaitic rocks sodalite foyaite and naujaite. The
floor series consists of cumulates: an inferred hidden part formed simultaneously with the roof
series and an exposed part of layered agpaitic nepheline syenites (kakortokites) which were formed
at least partly later than the roof series. The floor series passes gradually into the overlying
intermediate series consisting of the agpaitic rock type called lujavrites. The lujavrites enclose rafts
of naujaite and appear to have been emplaced by piecemeal stoping. The larger lujavrite masses are
floor cumulates, but lujavrite dykes and sheets occupy fractures in the roof series rocks.
According to one model for the evolution of the complex, the nepheline syenites formed by
consolidation of one magma batch in a closed system. The lujavrites were formed from the residual
melts left after the formation of the roof series and the floor series and were sandwiched between
these. A second model implies that the kakortokites and lujavrites formed from one or more
separate magma pulses which intruded the already consolidated roof series rocks. This model is
supported by new information on contact relations and especially on the petrology and
geochemistry of a marginal pegmatitic facies that forms a rim around the kakortokite-lower aegirine
lujavrite part of the complex. It consists of a massive-textured matrix intersected by pegmatites. The
matrix was the first rock to form in the lowermost exposed part of the complex and gives
information about the composition of the initial magma of the kakortokite-lujavrite sequence.
It is concluded that the agpaitic rocks of the complex were formed from at least two successive
magma injections, which formed respectively the roof series and the kakortokite-lujavrite sequence.