Nepheline Sövites as Parental Magmas in Carbonatite Complexes: Evidence from

The 5 km2 Dicker Willem complex, of Eocene age (49 Ma), is a ite, as essential components. These two rock types are generally regarded as cogenetic, with the most popularsubvolcanic intrusion consisting almost exclusively of carbonatite. Rock types range from early nepheline sövites through sövite and hypothesis being that ijolite–carbonatite pairs represent conjugate liquids, derived by immiscible separation fromdolomite alvikite to late-stage ferroan carbonatites. Xenoliths within the sövite include fenite, nepheline sövite and calcite-bearing ijolite. a parental carbonated peridotite, nephelinite or melilitite (Le Bas 1977, 1987). Kjarsgaard & Hamilton (1988,Although sövites and ijolite have very similar Sr and Nd isotope ratios (143Nd/144Nd 0·51263–0·51272, 87Sr/86Sr 0·70325 – 1989) argued that crystal–liquid fractionation, coupled with liquid immiscibility, is capable of generating a spec-0·70353) and contain minerals with overlapping composition ranges, they do not represent conjugate immiscible liquids. Nepheline trum of silicate magmas, evolving from parental melilitite to derivative phonolite, which coexisted with immisciblesövites, essentially silicocarbonatites with between 10 and 20% SiO2, contain layers and lenses rich in silicate minerals, interpreted as carbonatite melts ranging from sövite to natrocarbonatite, respectively.cumulates. According to phase relationships in the pseudoquaternary system CaO–(Na2O + K2O)–(MgO + FeO)–(SiO2 + TiO2 In contrast to the immiscibility hypothesis, ijolites have also been interpreted as cumulates from nepheline syenite+ Al2O3), neither calcitic ijolites nor the vast majority of cal-ciocarbonatites can represent primary melts. Only the nepheline magmas (Beccaluva et al., 1992), as the crystalline product