The Origins of Carbonatites and Related Rocks from the Grønnedal-Íka Nepheline Syenite Complex, South Greenland: C-O-Sr Isotope Evidence

N. J. G. Pearce1, M. J. Leng2, C. H. Emeleus3 and C. M. Bedford3
1 Institute of Earth Studies, University of Wales, Aberystwyth, Ceredigion, SY23 3DB, UK
2 NERC Isotope Geosciences Laboratory, Keyworth, Nottingham, NG12 5GG, UK
3 Department of Geological Sciences, University of Durham, Durham, DH1 3LE, UK
4 Present address: Radley College, Abingdon, Oxford, OX14 2HR, UK

Abstract: 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.

Keywords: carbonatite • nepheline syenite • Grønnedal-Íka complex • Gardar • Greenland

Mineralogical Magazine; August 1997 v. 61; no. 407; p. 515-529; DOI: 10.1180/minmag.1997.061.407.04
© 1997, The Mineralogical Society
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