The origin of carbonatites and related rocks from the Igaliko Dyke Swarm, Gardar Province, South Greenland: field, geochemical and C-O-Sr-Nd isotope evidence

The Igaliko Nepheline-Syenite Complex of Gardar age (1300-1120 Ma) includes carbonatite, lamprophyre, basalt, trachybasalt, phonolite and rhyolite dykes. The relationship of carbonatite magma to the other magma types in this area is not immediately obvious. Field, geochemical and petrological studies suggest three equally acceptable models for the evolution of the carbonatites. Here we present petrological, geochemical and isotopic (C, O, Sr and Nd) data for a series of dykes from the Igaliko Nepheline-Syenite Complex which are pertinent to the origin of these carbonatites. δ ¹³C and δ ¹⁸O ratios show that the isotope evolution of the dykes is consistent with closely related carbonatite and ultramafic lamprophyre magma genesis, prior to the production of more voluminous alkaline lamprophyre and basalt magma during repeated rifting episodes. The associated phonolite melts, produced by fractional crystallisation of basic magmas late in this evolution, had lengthy residence times in the crust during which they exchanged C, O and Sr isotopes with crustal rocks. Despite geochemical evidence suggesting a genetic link between phonolites and carbonatites, the C and O isotopic evidence shows that the phonolites are not related directly to the carbonatites by liquid immiscibility, whilst combined field, geochemical and isotopic data strongly point to an origin of carbonatites by liquid immiscibility from ultramafic lamprophyres.