Transformation of calcite-bearing ijolites to biotite calcite carbonatites – A case study from Dicker Willem, southern Namibia

Abstract

The Eocene Dicker Willem carbonatite complex (southern Namibia) primarily comprises fine-grained calcite carbonatites hosting xenoliths and irregular inclusions of calcite ijolites, calcite syenites, nepheline silicocarbonatites, and coarse-grained calcite carbonatites. A 7 by 6 m xenolith within the complex exhibits zonal lithological transitions, evolving from calcite-bearing ijolite via calcite syenite and silicocarbonatite to coarse-grained calcite carbonatite. This succession reflects initial carbohydrothermal alteration (fenitisation), followed by interaction with carbonatitic melt, as evidenced by textures, mineralogy and element transport. In the central ijolitic zone, primary nepheline is replaced by orthoclase, while the outer margins show intense resorption of original silicate minerals, replaced by coarse calcite, biotite ± aegirine-rich clinopyroxene and magnetite, ultimately forming coarse calcite carbonatite. Mass transfer reveals enrichment in Ca, Sr, and REE, alongside depletion in Na, Mg, Al, Si, and K. Sample sets from other parts of Dicker Willem, transitioning from ijolite to nepheline silicocarbonatite, exhibit increasing replacement of nepheline by cancrinite, driven by magmatic interaction as well. These findings indicate that nepheline silicocarbonatite is a transitional lithology, not an orthomagmatic rock, formed through in-situ dissolution-reprecipitation of silicates and replacement by calcite, with significant silica introduction into the carbonatitic magma. Scaling this process up to the entire Dicker Willem Complex suggests that even silicate-bearing coarse-grained calcite carbonatites could result from carbonatite magma-induced transformation, consistent with published isotopic and mineral data. • Ijolite xenoliths in carbonatitic fluids/magma undergo systematic change. • Two-stage process: fluid-driven fenitisation, then carbonatite mineral formation. • Nepheline replacement by orthoclase or cancrinite reveals mode of transformation. • Elemental and isotopic changes occur, especially Nb and REE enrichment. • Origin of ijolite is reinterpreted as an interaction product, not a cumulate.