Petrology of alkali gabbro from the Messum Crater, Namibia (Paranà-Etendeka Province) – Melting of metasomatized carbonated lithospheric mantle sources

Abstract

Foid gabbros (alkali gabbros) are late-stage intrusive rocks into the alkaline core of the Cretaceous Messum complex, an anorogenic ring complex belonging to the Etendeka Large Igneous Province in NW Namibia. The mineralogy of the alkali gabbros is characterized by primary olivine, clinopyroxene (Ti-augite), nepheline and plagioclase and late-stage or secondary brown Ti-amphibole and biotite plus magnetite. The bulk rock compositions resemble low-SiO₂ alkaline melts with low TiO₂ and K₂O/Na₂O < 1. Some of them have Cr (up to 490 ppm) and Ni (up to 265 ppm) contents that approach values characteristic for primary mantle melts. The alkali gabbros are characterized by variable enrichment in incompatible trace elements (Ba: 750–1020 ppm, Zr: 131–173 ppm; Hf: 3.0–3.4 ppm, Nb: 66–90 ppm, Ta: 3.7–5.2 ppm, La: 32–43 ppm, Th: 4.0–5.5 ppm). Zr/Hf and Nb/Ta ratios are high and negatively correlated. The alkali gabbros display a small spread in initial ⁸⁷Sr/⁸⁶Sr (0.7044–0.7046), εNd (+1.2 to +1.7) and Pb isotope ratios (²⁰⁶Pb/²⁰⁴Pb, 17.86–18.00, ²⁰⁷Pb/²⁰⁴Pb: 15.54–15.56, ²⁰⁸Pb/²⁰⁴Pb: 37.65–37.81). Variations of major and trace element data and Sr, Nd, and Pb isotopic compositions indicate that the more differentiated alkali gabbro samples evolved by fractional crystallization from the most mafic parent involving mainly olivine, clinopyroxene, and minor plagioclase and FeTi oxides, whereas crustal contamination was negligible. In primitive mantle-normalized multi-element diagrams, alkali gabbros show depletions in Rb, Th, U, Pb, Hf, Zr and Ti and enrichments in Sr. Notably, high molar Ca/Al, high Ti/Eu, high Zr/Hf and Nb/Ta, low TiO₂, low Zr/Nb at high Nb concentrations and low Zr/Sm in samples with high Lu/Hf and low Hf/Sm point to a carbonatitic component in the source of the alkali gabbros. REE modelling suggests that the parental melts form within the garnet stability field and may be modified either by partial re-equilibration or mixing with melts generated in the spinel stability field. Application of the thermobarometer proposed by Lee et al. (2009) (Lee et al., 2009. Constraints on the depths and temperatures of basaltic magma generation on Earth, and other terrestrial planets using new thermobarometers for mafic magmas. Earth Planet. Sci. Lett. 279, 20–33.) implies generation of the most primitive alkali gabbros at ∼4.5 GPa and ∼ 1480 °C, the latter value is in agreement with previous temperature estimates using Al-in-olivine thermometry and melt inclusion studies. The Sr-Nd-Pb isotope compositions of the alkali gabbros do not overlap with those of mafic lavas attributed to the Tristan or Gough hot spot or the newly defined Doros component. Trace element and isotope data indicate derivation of the Messum alkali gabbros from a heterogeneous mantle source enriched in incompatible trace elements (Rb, Ba, NbTa, LREE, ZrHf). The alkali gabbros were formed by melts of carbonatite-metasomatized peridotite and melts from the subcontinental lithospheric mantle. These melts were mobilized by edge-driven convection or lithospheric drip at the time of continental break-up.