Clinopyroxenes and coexisting mafic minerals from the alkaline Ilímaussaq intrusion, South Greenland

The Ilímaussaq intrusion consists of an augite syenite shell and a central cumulitic series of layered peralkaline (agpaitic) nepheline syenites with both roof and bottom cumulates. Microprobe analyses of the strongly zoned mafics show that the pyroxenes range from ferro-salite through hedenbergite to aegirine, the amphiboles from hastingsite through katophorite to arfvedsonite, and the olivines from Fo_16.8 to F_O0.₂. Aenigmatite analyses are also presented. The crystal chemistry of the pyroxenes is discussed in detail. The behaviour of the various elements is discussed in relation to the conditions in the coexisting magma. The relation between oxygen fugacity and silica activity and the stability of fayalite and Na-poor pyroxenes in over-and undersaturated magmas is considered. The persistent stability of these minerals in the strongly undersaturated Ilimaussaq magma shows that the oxygen fugacities in Ilímaussaq were lower than in any other known alkaline suites. Based on the compositional development in the mafic minerals it is inferred that the intrusion formed from at least three separate pulses of successively more differentiated magma: (1) an augite syenite magma congealed inwards from the sides, (2) a peralkaline undersaturated magma gave rise to the roof cumulates which congealed successively downwards. The differentiating magma was water-undersaturated and underwent depletion in Mg and enrichment in Zr and Na. The contemporaneous bottom cumulates are hidden beneath the now visible bottom cumulates which possess a still more differentiated liquidus mineral assemblage and are ascribed to a third separate magma pulse.