Nepheline Sövites as Parental Magmas in Carbonatite Complexes: Evidence from Dicker Willem, Southwest Namibia.

Cooper, Alan F.; Reid, David L.

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 sövites through sövite and dolomite alvikite to late-stage ferroan carbonatites. Xenoliths within the sövite include fenite, nepheline sövite and calcite-bearing ijolite. Although sövites 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 sövites, 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 sövites, which plot on the silicate–carbonate liquidus boundary, are capable of coprecipitating calcite and silicate phases, which by gravity settling, may yield complementary sövite and ijolite cumulates. Nepheline sövites 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.

NAMIBIA; CARBONATITES; DOLOMITE; NEPHELINE syenite; PETROGENESIS; EOCENE Epoch; IGNEOUS intrusions

Journal of Petrology, 1998, Vol 39, Issue 11/12, p2123

0022-3530

Academic Journal

10.1093/petroj/39.11-12.2123