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

The Grownnedal-Ika 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 alnoite, and then extensively faulted. The nepheline syenite magmas produced by fractional crystallisation of basic magmas, show a range in delta(13)C (-3.86 to -7.57 parts per thousand) and delta(18)O (8.27 to 15.12 parts per thousand), distinctly different to the carbonatites which form a tight group with average delta(13)C = -4.31 +/- 0.22 parts per thousand, (1 s.d.) and average delta(18)O = 7.18 +/- 0.41 parts per thousand (1 s.d.). Initial Sr-87/Sr-86 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 (delta(13)C -3.86 to -7.86 parts per thousand and delta(18)O 9.12 to 10.81 parts per thousand) form a coherent group whose stable isotope compositions overlap part of the syenite field, and again are distinctly different from the carbonatites. A single alnoite has delta(13)C = -3.32 parts per thousand and delta(18)O = 12.34 parts per thousand. 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 Gronnedal-Ika and from the Gardar Igaliko Dyke Swarm. The possible role of F in controlling delta(13)C and delta(18)O during crystallisation of calcite from carbonatite magmas is discussed.