首页|An apatite trace element and Sr-Nd isotope geochemical study of syenites and carbonatite, exemplified by the Epembe alkaline-carbonatite complex, Namibia
An apatite trace element and Sr-Nd isotope geochemical study of syenites and carbonatite, exemplified by the Epembe alkaline-carbonatite complex, Namibia
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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; ε_(Nd(t)) = +2.5 to +3.2), nepheline syenites (~(87)Sr/~(86)Sr_((i)) = 0.7031-0.7037; ε_(Nd(t)) = +1-5 to +4.4) and carbonatite (~(87)Sr/~(86)Sr_((i)) = 0.7031-0.7033; ε_(Nd(t)) = 0 to +3.3) overlap, pointing to a common but heterogeneous source, located in the sub-lithospheric mantle.
Epembe carbonatiteRare-earth elementsU-Pb geochronologySr and Nd isotopes
Mbili Tshiningayamwe、Robert Bolhar、Paul A.M. Nex
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School of Geosciences, University of the Witwatersrand, Johannesburg 2001, South Africa
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