Journal of Petrology Advance Access originally published online on September 24, 2004
Journal of Petrology 2005 46(1):191-217; doi:10.1093/petrology/egh069
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Journal of Petrology vol. 46 issue 1 © Oxford University Press 2004; all rights reserved
The Grønnedal-Ika Carbonatite–Syenite Complex, South Greenland: Carbonatite Formation by Liquid Immiscibility
1 INSTITUT FÜR GEOWISSENSCHAFTEN, EBERHARD-KARLS-UNIVERSITÄT TÜBINGEN, WILHELMSTRASSE 56, D-72074,TÜBINGEN, GERMANY
2 INSTITUT DE MINÉRALOGIE ET GÉOCHIMIE, UNIVERSITÉ DE LAUSANNE, UNIL-BFSH2, CH-1015 LAUSANNE, SWITZERLAND
The Grønnedal-Ika complex is dominated by layered nepheline syenites which were intruded by a xenolithic syenite and a central plug of calcite to calcite–siderite carbonatite. Aegirine–augite, alkali feldspar and nepheline are the major mineral phases in the syenites, along with rare calcite. Temperatures of 680–910°C and silica activities of 0·28–0·43 were determined for the crystallization of the syenites on the basis of mineral equilibria. Oxygen fugacities, estimated using titanomagnetite compositions, were between 2 and 5 log units above the fayalite–magnetite–quartz buffer during the magmatic stage. Chondrite-normalized REE patterns of magmatic calcite in both carbonatites and syenites are characterized by REE enrichment (LaCN–YbCN = 10–70). Calcite from the carbonatites has higher Ba (
5490 ppm) and lower HREE concentrations than calcite from the syenites (54–106 ppm Ba). This is consistent with the behavior of these elements during separation of immiscible silicate–carbonate liquid pairs. 
Nd(T = 1·30 Ga) values of clinopyroxenes from the syenites vary between +1·8 and +2·8, and Nd(T) values of whole-rock carbonatites range from +2·4 to +2·8. Calcite from the carbonatites has 
18O values of 7·8 to 8·6
and 13C values of –3·9 to –4·6. 18O values of clinopyroxene separates from the nepheline syenites range between 4·2 and 4·9. The average oxygen isotopic composition of the nepheline syenitic melt was calculated based on known rock–water and mineral–water isotope fractionation to be 5·7 ± 0·4. Nd and C–O isotope compositions are typical for mantle-derived rocks and do not indicate significant crustal assimilation for either syenite or carbonatite magmas. The difference in 18O between calculated syenitic melts and carbonatites, and the overlap in Nd values between carbonatites and syenites, are consistent with derivation of the carbonatites from the syenites via liquid immiscibility.
KEY WORDS: alkaline magmatism; carbonatite; Gardar Province; liquid immiscibility; nepheline syenite
Corresponding author. Telephone (+49) 7071/2972930. Fax: (+49) 7071/293060. E-mail: markl{at}uni-tuebingen.de
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