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) inc ludes carbonatite, lamprophyre, basalt, trachybasalt, phonolite and rh yolite 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, geochemic al 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. delta(13)C and delta(18)O ratios show that the is otope evolution of the dykes is consistent with closely related carbon atite and ultramafic lamprophyre magma genesis, prior to the productio n of more voluminous alkaline lamprophyre and basalt magma during repe ated rifting episodes. The associated phonolite melts, produced by fra ctional crystallisation of basic magmas late in this evolution, had le ngthy residence times in the crust during which they exchanged C, O an d Sr isotopes with crustal rocks. Despite geochemical evidence suggest ing a genetic link between phonolites and carbonatites, the C and O is otopic evidence shows that the phonolites are not related directly to the carbonatites by liquid immiscibility, whilst combined field, geoch emical and isotopic data strongly point to an origin of carbonatites b y liquid immiscibility from ultramafic lamprophyres.