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

Published online by Cambridge University Press:  05 July 2018

N. J. G. Pearce ,
M. J. Leng ,
C. H. Emeleus  and
C. M. Bedford
N. J. G. Pearce
Affiliation:
Institute of Earth Studies, University of Wales, Aberystwyth, Ceredigion, SY23 3DB, UK
M. J. Leng
Affiliation:
NERC Isotope Geosciences Laboratory, Keyworth, Nottingham, NGI2 5GG, UK
C. H. Emeleus
Affiliation:
Department of Geological Sciences, University of Durham, Durham, DH1 3LE, UK
C. M. Bedford
Affiliation:
Department of Geological Sciences, University of Durham, Durham, DH1 3LE, UK Present address: Radley College, Abingdon, Oxford, OX14 2HR, UK
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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 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

carbonatitenepheline syeniteGrønnedal-Íka complexGardarGreenland
Type
Intraplate Alkaline Magmatism
Information
Mineralogical Magazine , Volume 61 , Issue 407 , August 1997 , pp. 515 - 529
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1997

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