The origins of carbonatites and related rocks from the Grønnedal-Íka Nepheline Syenite complex, South Greenland: C-O-Sr isotope evidence

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 dolentes, 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 δ ¹³C (-3.86 to -7.57‰) and δ ¹⁸O (8.27 to 15.12‰), distinctly different to the carbonatites which form a tight group with average δ ¹³C= -4.31 ± 0.22 ‰, (1 s.d.) and average δ ¹⁸O= 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 (δ ¹³C -3.86 to -7.86‰ and δ ¹⁸O 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 δ ¹³C = -3.32‰ and δ ¹⁸C= 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 δ ¹³C and δ ¹⁸O during crystallisation of calcite from carbonatite magmas is discussed.