Nepheline sovites as parental magmas in carbonatite complexes: Evidence from Dicker Willem, southwest Namibia

Abstract

The 5 km2 Dicker Willem complex, of Eocene age (49 Ma), is a subvolcanic intrusion consisting almost exclusively of carbonatite. Rock types range from early nepheline sovites through sovite and dolomite alvikite to late-stage ferroan carbonatites. Xenoliths within the sovite include fenite, nepheline sovite and calcite-bearing ijolite. Although sovites and ijolite have very similar Sr and Nd isotope ratios (143Nd/144Nd 0.51263-0.51272, 87Sr/86Sr 0·70325-0·70353) and contain minerals with overlapping composition ranges, they do not represent conjugate immiscible liquids. Nepheline sovites, essentially silicocarbonatites with between 10 and 20% SiO2, contain layers and lenses rich in silicate minerals, interpreted as cumulates. According to phase relationships in the pseudoquaternary system CaO-(Na2O + K2O)-(MgO + FeO)-(SiO2 + TiO2 + Al2O3), neither calcitic ijolites nor the vast majority of calciocarbonatites can represent primary melts. Only the nepheline sovites, which plot on the silicate-carbonate liquidus boundary, are capable of coprecipitating calcite and silicate phases, which by gravity settling, may yield complementary sovite and ijolite cumulates. Nepheline sovites are identified as parental magmas, which through fractionation of Sr-calcite, sodic diopside, Zr-Ti-rich melanite, nepheline, magnetite, apatite and pyrochlore may yield the lineage of SiO2-depleted alvikite and late-stage ferroan dolomitic carbonatites.