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

The 5 km(2) Dicker Willem complex, of Eocene age (49 Ma), is a subvolcanic intrusion consisting almost exclusively of carbonatite. Rock types range fr om early nepheline sovites through sovite and dolomite alvikite to late-sta ge ferroan carbonatites. Xenoliths within the sovite include fenite, nephel ine sovite and calcite-bearing ijolite. Although sovites and ijolite have v ery similar Sr and Nd isotope ratios (Nd-143/Nd-144 0.51263-0.51272, Sr-87/ Sr-86 0.70325-0.70353) and contain minerals with overlapping composition ra nges, they do not represent conjugate immiscible liquids. Nepheline sovites , essentially silicocarbonatites with between 10 and 20% SiO2, contain laye rs and lenses rich in silicate minerals, interpreted as cumulates. Accordin g to phase relationships in the pseudoquaternary system CaO-(Na2O + K2O)-(M gO + FeO)-(SiO2 + TiO2 + Al2O3), neither calcitic ijolites nor the vast maj ority of calciocarbonatites can represent primary melts. Only the nepheline sovites, which plot on the silicate-carbonate liquidus boundary, are capab le of coprecipitating calcite and silicate phases, which by gravity settlin g, may yield complementary sovite and ijolite cumulates. Nepheline sovites are identified as parental magmas, which through fractionation of Sr-calcit e, sodic diopside, Zr-Ti-rich melanite, nepheline, magnetite, apatite and p yrochlore may yield the lineage of SiO2-depleted alvikite and late-stage fe rroan dolomitic carbonatites.