The Grønnedal-Íka ring complex (1299 ± 17 Ma) in the Gardar province, South Greenland is composed of a range of layered nepheline syenites which were intruded at a late stage by xenolithic syenite and a plug of carbonatite. The complex was subsequently intruded by a variety of basic dykes, including olivine dolerites, kersantites, vogesites, spessartites, camptonites and an alnöite, and then extensively faulted. The nepheline syenite magmas, produced by fractional crystallisation of basic magmas, show a range in δ¹³C (−3.86 to −7.57‰) and δ¹⁸O (8.27 to 15.12‰), distinctly different to the carbonatites which form a tight group with average δ¹³C = −4.31 + 0.22 ‰, (1 s.d.) and average δ¹⁸O = 7.18 ± 0.41‰ (1 s.d.). Initial ⁸⁷Sr/⁸⁶Sr isotope ratios (typically 0.703) suggest the syenites and carbonatites have not assimilated crustal rocks, and therefore the C and O isotope variation within each group is a result of isotopic evolution during fractional crystallisation. A suite of lamprophyre dykes (δ¹³C −3.86 to −7.86‰ and δ¹⁸O 9.12 to 10.81‰) form a coherent group whose stable isotope compositions overlap part of the syenite field, and again are distinctly different from the carbonatites. A single alnöite has δ¹³C = −3.32‰ and δ¹⁸O = 12.34‰ C and O isotope ratios are consistent with origins of syenitic and lamprophyric magmas from a similar source. Despite geochemical evidence which suggests a genetic link between nepheline syenites and carbonatites, C and O isotopic evidence shows that they are not related directly by liquid immiscibility. Comparisons are made between similar rock types from Grønnedal-Íka and from the Gardar Igaliko Dyke Swarm. The possible role of F in controlling δ¹³C and δ¹⁸O during crystallisation of calcite from carbonatite magmas is discussed.