The Gronnedal-Ika Carbonatite-Syenite Complex, South Greenland: Carbonatite Formation by Liquid Immiscibility |
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Authors: | HALAMA RALF; VENNEMANN TORSTEN; SIEBEL WOLFGANG; MARKL GREGOR |
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Institution: | 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 |
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Abstract: | The Grønnedal-Ika complex is dominated by layered nephelinesyenites which were intruded by a xenolithic syenite and a centralplug of calcite to calcitesiderite carbonatite. Aegirineaugite,alkali feldspar and nepheline are the major mineral phases inthe syenites, along with rare calcite. Temperatures of 680910°Cand silica activities of 0·280·43 weredetermined for the crystallization of the syenites on the basisof mineral equilibria. Oxygen fugacities, estimated using titanomagnetitecompositions, were between 2 and 5 log units above the fayalitemagnetitequartzbuffer during the magmatic stage. Chondrite-normalized REE patternsof magmatic calcite in both carbonatites and syenites are characterizedby REE enrichment (LaCNYbCN = 1070). Calcite fromthe carbonatites has higher Ba (5490 ppm) and lower HREE concentrationsthan calcite from the syenites (54106 ppm Ba). This isconsistent with the behavior of these elements during separationof immiscible silicatecarbonate liquid pairs. Nd(T =1·30 Ga) values of clinopyroxenes from the syenites varybetween +1·8 and +2·8, and Nd(T) values of whole-rockcarbonatites range from +2·4 to +2·8. Calcitefrom the carbonatites has 18O values of 7·8 to 8·6and 13C values of 3·9 to 4·6. 18Ovalues of clinopyroxene separates from the nepheline syenitesrange between 4·2 and 4·9. The average oxygenisotopic composition of the nepheline syenitic melt was calculatedbased on known rockwater and mineralwater isotopefractionation to be 5·7 ± 0·4. Nd and COisotope compositions are typical for mantle-derived rocks anddo not indicate significant crustal assimilation for eithersyenite or carbonatite magmas. The difference in 18O betweencalculated syenitic melts and carbonatites, and the overlapin Nd values between carbonatites and syenites, are consistentwith derivation of the carbonatites from the syenites via liquidimmiscibility. KEY WORDS: alkaline magmatism; carbonatite; Gardar Province; liquid immiscibility; nepheline syenite |
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Keywords: | : alkaline magmatism carbonatite Gardar Province liquid immiscibility nepheline syenite |
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