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

Abstract

The 5 km 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 popular subvolcanic intrusion consisting almost exclusively of carbonatite. Rock types range from early nepheline sovites through sovite and hypothesis being that ijolite–carbonatite pairs represent conjugate liquids, derived by immiscible separation from dolomite alvikite to late-stage ferroan carbonatites. Xenoliths within the sovite include fenite, nepheline sovite and calcite-bearing ijolite. a parental carbonated peridotite, nephelinite or melilitite (Le Bas 1977, 1987). Kjarsgaard & Hamilton (1988, Although sovites and ijolite have very similar Sr and Nd isotope ratios (Nd/Nd 0·51263–0·51272, Sr/Sr 0·70325– 1989) argued that crystal–liquid fractionation, coupled with liquid immiscibility, is capable of generating a spec0·70353) 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 immiscible sovites, essentially silicocarbonatites with between 10 and 20% SiO2, contain layers and lenses rich in silicate minerals, interpreted as carbonatite melts ranging from sovite 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 calciocarbonatites can represent primary melts. Only the nepheline magmas (Beccaluva et al., 1992), as the crystalline products of liquids generated by melting of fenites (Kramm, 1994), sovites, which plot on the silicate–carbonate liquidus boundary, are capable of coprecipitating calcite and silicate phases, which by and as the products of nephelinization of pyroxenites, or gravity settling, may yield complementary sovite and ijolite cumulates. metasomatism of country rock gabbros (Harmer, 1992). Nepheline sovites are identified as parental magmas, which through At Spitzkop, using evidence from Sr and Nd isotope fractionation of Sr-calcite, sodic diopside, Zr–Ti-rich melanite, ratios, Harmer (1999) showed that the carbonatites are nepheline, magnetite, apatite and pyrochlore may yield the lineage of not fractionates from silicate magmas, nor are the silicate SiO2-depleted alvikite and late-stage ferroan dolomitic carbonatites. and carbonate rocks immiscible liquid pairs. This paper presents field, mineralogical and geochemical evidence from the Dicker Willem Complex, southwest Namibia, relevant to the ijolite–sovite as