Alkali loss and retention in an iron-rich peralkaline phonolite dyke from the Gardar province, south Greenland

An unusually iron-rich, strongly peralkaline dyke occurring near the Ilimaussaq intrusion, south Greenland, is described. The dyke has developed two facies characterised respectively by the mafic assemblages pyroxene+amphibole+aemigmatite, and pyroxene+magnetite. The differences can be explained as an effect of loss of up to 4% Na₁O from the magnetite-bearing facies (relative to the magnetite-free facies). The effect is in accordance with the phase relations at 1 atm in the synthetic system Na₂OAl₂O₂Fe₂O₂SiO₂. Continued alkali loss and magnetic precipitation will change the differentiation trend from agpaitic with iron enrichment towards miaskitic with iron depletion. It is suggested that the extreme degree of iron enrichment in the dyke resulted from the coincidence of low oxygen fugacity and strong peralkalinity tending to stabilise both ferrous and ferric iron in the liquid.