The petrogenetic association of carbonatite and alkaline magmatism. Constraints from the Spitskop Complex, South Africa

The 1341 Ma old Spitskop complex in South Africa is one of a series of intr usions of alkaline affinity, which were emplaced into the central Kaapvaal Craton over the time period 1.4-1.2 Ga. Spitskop contains calcite and dolom ite carbonatite closely associated with pyroxenite; ijolite and nepheline s yenite, and provides an ideal opportunity to study the petrogenetic relatio nships between alkaline silicate and carbonatite magmatism. The pyroxenites are not alkalic and are preserved as xenoliths within a plug-like intrusio n of ijolite. Nepheline syenites are highly peralkaline, though not agpaiit ic, and intrude the ijolites as a series of sheets. These units ale cut by a plug of carbonatite composed of an incomplete marginal zone of calcite an d dolomite-calcite carbonatite, and a larger central zone of ferroan dolomi te carbonatite. Clinopyroxene compositions change systematically from diops ide-rich compositions in the pyroxenites to aegirine-augite-hedenbergite in the ijolites to acmite-dominated compositions in the nepheline syenites. W hole-rock chemical data indicate, however, that the nepheline syenites and ijolites are unlikely to be related through fractional crystallization of a ny reasonable combination of their component minerals (clinopyroxene, nephe line, perthite) from a common parental magma. Low total rare earth element (REE) concentrations and flat to convex-up normalized patterns in rite syen ites contrast strongly with the steep, light REE (LREE)-enriched patterns i n the ijolites The silicate and carbonatite components differ markedly in t heir epsilon(Sr)-epsilon(Nd) compositions, the carbonatites having more dep leted values (epsilon(Sr) -10 to +10; epsilon(Nd) -1 to -8) than the silica tes (epsilon(Sr) 0 to +33; epsilon(Nd) -8 to -13). In addition, the calcite -rich carbonatites have more negative epsilon(Nd) (-6 to -8) than the dolom ite carbonatites (-1 to -4). Contrasting isotopic compositions along with t he geochemical variations within and between rite silicates and carbonatite s argue against them being derived from conjugate immiscible liquids. Inste ad, it is proposed that rite carbonatites evolved from primitive carbonate liquids produced directly by low-degree melting of carbonated mantle perido tite. A preliminary model is presented to explain how mantle carbonatite me lts can ascend through the mantle and into the crust. It proposed that the silicate magmatic rocks associated with the carbonatite are produced by mel ting of enriched mantle lithosphere induced by the influx of deeper-sourced carbonatite melts.