The Gronnedal-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 680-910degreesC 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
(La-CN-Yb-CN = 10-70). Calcite from the carbonatites has higher Ba
(similar to5490 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. epsilon(Nd)(T = 1.30 Ga) values of clinopyroxenes from the
syenites vary between +1.8 and +2.8, and epsilon(Nd)(T) values of
whole-rock carbonatites range from +2.4 to +2.8. Calcite from the
carbonatites has delta(18)O values of 7.8 to 8.6parts per thousand and
delta(13)C values of -3.9 to -4.6parts per thousand. delta(18)O values
of clinopyroxene separates from the nepheline syenites range between 4.2
and 4.9parts per thousand. 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 +/- 0.4parts per thousand.
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 delta(18)O between calculated
syenitic melts and carbonatites, and the overlap in epsilon(Nd) values
between carbonatites and syenites, are consistent with derivation of the
carbonatites from the syenites via liquid immiscibility.