An apatite trace element and Sr-Nd isotope geochemical study of syenites and carbonatite, exemplified by the Epembe alkaline‑carbonatite complex, Namibia

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. Nepheline syenite with minor syenite constitute the main lithologies, cross-cut by a calcite‑carbonatite dyke. Apatite grains from one syenite, six nepheline syenite and five carbonatite samples were studied using cathodoluminescence (CL) imaging, trace element and Sr-Nd isotope compositions as well as U-Pb geochronology. Syenite-hosted apatite is homogenous in CL and contains the highest concentration of REE (9189–44,100 ppm) with light rare-earth element (LREE) enrichment (La N /Yb N = 4–91) relative to heavy (H) REE consistent with a magmatic origin. Negative Eu anomalies (Eu/Eu* = 0.4–0.9) in syenite apatite are attributed to the formation of apatite in an evolved mantle-derived melt associated with plagioclase fractionation. Nepheline syenite and carbonatite-hosted apatite is also commonly homogeneous in CL, while core-rim zoning and patchy textures are observed occasionally. Both texturally homogeneous and core-rim zoned apatite are enriched in LREE (La N /Yb N = 24–9) relative to HREE, consistent with a magmatic origin. Core-rim zoned apatite is characterized by rim-ward increase in REE concentrations, which can be attributed to mineral fractionation. Patchy apatite is depleted in Na, Y and REE, particularly the LREE (La N /Yb N = 4–19) relative to other nepheline syenite apatite, reflecting interaction with fluids (metasomatism). The strontium isotope composition of metasomatic apatite and magmatic apatite is indistinct suggesting a magmatic origin of the alteration fluids. No Eu anomalies (Eu/Eu* = 1) in chondrite-normalized REE patterns are observed in any apatite hosted by nepheline syenite and carbonatite. An LA-ICPMS U-Pb age of 1216 ± 11 Ma (MSWD = 4.3, 2 SE) for apatite constrains emplacement of the syenite, while magmatic nepheline syenite apatite ages are 1193 ± 14 Ma, 1197 ± 17 Ma and 1194 ± 16 Ma (MSWDs <4.0, 2 SE). The Sr and Nd isotopic composition of apatite in syenite ( 87 Sr/ 86 Sr (i) = 0.7035–0.7048; Ɛ Nd(t) = +2.5 to +3.2), nepheline syenites ( 87 Sr/ 86 Sr (i) = 0.7031–0.7037; Ɛ Nd(t) = +1.5 to +4.4) and carbonatite ( 87 Sr/ 86 Sr (i) = 0.7031–0.7033; Ɛ Nd(t) = 0 to +3.3) overlap, pointing to a common but heterogeneous source, located in the sub-lithospheric mantle. • Core-rim zoned apatite is consistent with mineral fractionation. • Patchy apatite in nepheline syenite signify metasomatism. • Epembe syenite was emplaced at 1216 ± 16 Ma, whereas nepheline syenite ca. 1194 Ma. • Epembe syenite, nepheline syenite and carbonatite have a sub-lithospheric mantle origin.