Abstract

Carbonatites from a complex in southwestern Namibia retain features such as flow-aligned phenocrysts of calcite or dolomite enclosed in a granular or spinifex-textured calcite groundmass, comb layering, and gravity layering. All textures are compatible with Ca-rich carbonatite magmas carrying liquidus calcite or dolomite quenching on emplacement into a subvolcanic environment. Oxygen isotope fractionation between silicate and oxide minerals in sovite cumulates indicate high temperatures (600-900 °C), compatible with those determined experimentally for crystallization of liquidus calcite or dolomite. We propose that a Ca-rich carbonatite magma (proto-alvikite) separated immiscibly from a carbonate-rich ijolite and evolved by fractional crystallization of magnetite, acmitic pyroxene, and calcite to a dolomite-phyric alvikite.

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