The origins of carbonatites and related rocks from the Gronnedal-Ika nepheline syenite complex, South Greenland; C-O-Sr isotope evidence

University of Wales, Institute of Earth Studies, Aberystwyth, United Kingdom

The Gronnedal-Ika ring complex (1299+ or -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 -7.5 per mil) and delta ¹⁸ O (8.27 to 15.12 per mil), distinctly different to the carbonatites which form a tight group with average delta ¹³ C = -4.31+ or -0.22 per mil, (1 s.d.) and average delta ¹⁸ O = 7.18+ or -0.41 per mil (1 s.d.). Initial ⁸⁷ Sr/ ⁸⁶ Sr 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 per mil and delta ¹⁸ O 9.12 to 10.81 per mil) 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 per mil and delta ¹⁸ O = 12.34 per mil. 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.

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