Petrology, geochemistry and new U–Pb ages of the Epembe syenites and carbonatite, northwest Namibia

Abstract

The Epembe Alkaline Carbonatite Complex (EACC) in northwestern Namibia, was emplaced along a fault zone into medium-to high-grade Palaeoproterozoic basement rocks of the Epupa Metamorphic Complex (EMC) and extends over a distance of 9 km in a south-easterly direction with a width of 1 km. Alkaline rocks constitute the main lithologies and are cross-cut by a calcite-carbonatite dyke. The alkaline rocks can be classified as syenite and nepheline-bearing syenites, with alkalic, metaluminous and ferroan affinities. An emplacement age of 1220 ± 3 Ma (2 SE, MSWD = 1.3) is determined for the syenite, whereas two nepheline syenite samples give similar magmatic ages of 1209 ± 3 Ma (2 SE, MSWD = 1.1) and 1205 ± 13 Ma (2 SE, MSWD = 2). The nepheline syenite ages correspond with the 1198 ± 5 Ma (2 SE, MSWD = 1.1) age of the carbonatite interpreted as magmatic. The nepheline-normative syenites define broadly linear trends in Harker plots consistent with evolution by fractional crystallization involving pyroxene and apatite. The rare-earth element (REE) pattern of the syenite shows a negative Eu anomaly, consistent with prior plagioclase formation and fractionation. The syenite is characterized by the absence of a negative Nb anomaly on a Primitive Mantle (PRIMA)-normalized diagram with Ce/Pb and Nb/U ratios of 12 and 19, respectively, suggesting that it was not affected by crustal contamination during ascent and emplacement. The absence of a crustal signature in the evolution of the syenite indicates that the syenite is not related to the nepheline syenite by combined assimilation and fractional crystallization and is likely to have been emplaced as a distinct magma batch derived from the mantle. REE patterns of the nepheline syenite and carbonatite are coherent showing LREE enrichment relative to HREE. On a PRIMA-normalized multi-element diagram both the carbonatite and syenite display depletions in Zr, Hf and Pb with positive Sr. Low Mg number and Cr and Ni contents of the carbonatite and nepheline syenite suggest that these rocks crystallized from an evolved parental magma rather than a primary mantle derived-melt. Geochemical similarities between the carbonatite and nepheline syenite also suggest that the origin of the Epembe carbonatite is not a product of immiscible separation between a carbonate and silicate melt. Instead, the carbonate may represent a melt fraction that formed after partial crystallization of the nepheline syenite. Magma assimilation of the country rocks by the carbonatite magma is evident from the occurrence of biotite, feldspars and syenitic xenoliths in the carbonatite.