Carbonatite, aillikite and olivine melilitite from Zandkopsdrift, Namaqualand, South Africa: Constraints on the origin of an unusual lamprophyre-dominated carbonatite complex and the nature of its mantle source

Abstract

We present a chemical and stable and radiogenic isotope data set for carbonatite, aillikite (ultramafic lamprophyre) and olivine melilitite from the Zandkopsdrift carbonatite complex in Namaqualand, South Africa. This carbonatite complex is associated with the ≈500-km-long, NE-SW oriented, age-progressive (40–80 Ma) Namaqualand-Bushmanland-Warmbad ultramafic alkaline magmatic lineation that consists mainly of olivine melilitite and kimberlite. Zandkopsdrift has a radiometric age of 55 ± 4 Ma and is unusual among southern African carbonatites because the dominant rock type exposed is aillikite (a variety of carbonated ultramafic lamprophyre), with carbonatite occurring mainly as a network of subvertical dykes in the southwest of the complex and olivine melilitite occurring as meter-sized blocks of float derived from the subsurface. Calcite and ferroan dolomite carbonatites are present but Mg-rich dolomitic carbonatite varieties are lacking. All rock types appear to have experienced at least moderate differentiation and all show elevated δ18OSMOW values for carbonate minerals, whereas phlogopite and clinopyroxene from these rock types fall near mantle values (with ranges of +13.3 to +21.8‰ and + 4.9 to +5.4‰ for carbonates and silicates, respectively), indicative of interaction with hydrous fluids at low-temperature. Sr-Nd-Pb isotope compositions indicate moderate to strong HIMU signatures (e.g., 206Pb/204Pbi = 19.4 to 21.0) with limited SrNd isotope variation and subvertical NdHf isotope arrays extending well below the ocean island basalt field and terrestrial array. Uncertainty regarding the differentiation history of the three rock types complicates estimation of primary magma compositions but strong chemical similarities between the carbonatites and aillikites, plus their close physical intermingling, strongly suggests that they are related by fractional crystallization to a common parental magma, whereas their relationship to the melilitites is less straightforward. The aillikites and melilitites have compositions of the REE and other elements similar to near-primary versions of these rock types from other alkaline igneous provinces globally, which have been explained as the products of low degrees of partial melting at 4–7 GPa (for aillikites) and slightly higher degrees of melting at 3–3.5 GPa (for melilitites). Lower pressures of generation and distinct trace element characteristics of the Zandkopsdrift melilitites could be explained by these originating as melts of a garnet wehrlite halo around the magmatic conduit formed by metasomatic reaction between carbonated melts and lithospheric peridotite. Most Zandkopsdrift carbonatites have high Lu/Hf ratios which, combined with their relatively low 176Hf/177Hf values, suggests that their mantle sources could not have formed more than 30–50 Myr prior to emplacement of the complex. This implies that a mantle plume had a role in their formation, either as a direct source of melts or as a source of heat with which to melt pre-existing metasomatic heterogeneities.