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

N. J. G. Pearce, M. J. Leng, C. H. Emeleus, C. M. Bedford

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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 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.

Original languageEnglish
Pages (from-to)515-529
Number of pages15
JournalMineralogical Magazine
Volume61
Issue number4
DOIs
Publication statusPublished - Aug 1997
EventConference on Intraplate Alkaline Magmatism - BIRMINGHAM
Duration: 11 Apr 199612 Apr 1996

Keywords

  • carbonatite
  • nepheline syenite
  • Gronnedal-Ika complex
  • Gardar
  • Greenland
  • NEUTRON-ACTIVATION ANALYSIS
  • RB-SR
  • OXYGEN
  • EVOLUTION
  • CANADA
  • Nepheline syenite
  • Grønnedal-Íka complex
  • Carbonatite

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