Petrogenesis and 40Ar/39Ar Geochronology of the Brandberg Complex, Namibia: Evidence for a Major Mantle Contribution in Metaluminous and Peralkaline Granites

Abstract

Anorogenic granites of the Brandberg igneous complex in NW Namibia formed during early Cretaceous rifting and continental break-up of Gondwana. A metaluminous series [SiO₂ = 62–77 wt %, molar (Na + K)/Al = 0·8–0·95] includes an early monzonite body, major biotite–hornblende granite, late biotite granite segregations and peripheral dykes of trachydacite. Volumetrically minor peralkaline granites of the Amis complex [SiO₂ = 72–77 wt %, (Na + K)/Al = 1·0–1·5] intrude the main granite and adjacent country rocks. Compared with the metaluminous main granite, these are in part highly enriched in Zr, Nb, Y, U and Th. Initial Nd and Sr isotope ratios of the metaluminous suite are εNd_{(130 Ma)} from −0·4 to −5·1 and ⁸⁷Sr/⁸⁶Sr_{(130 Ma)} from 0·707 to 0·713. The Nd isotopic composition of the peralkaline granites is within this range [εNd_{(130 Ma)} from −0·7 to −1·9]. ⁴⁰Ar–³⁹Ar age determinations (132–130 Ma) indicate that the metaluminous and peralkaline units are indistinguishable in age and that they formed contemporaneously with flood basalts and associated felsic volcanism in the Etendeka–Paraná province. The metaluminous suite is modelled as a crustally contaminated (10–40%) fractionate of a tholeiitic basaltic magma (LTZ.H type), and a common basaltic parent is inferred for the metaluminous and peralkaline rocks of the Brandberg complex. Fractional crystallization of plagioclase, clinopyroxene and Fe–Ti oxides of a parental monzonitic magma accounts for major and trace element variations within the metaluminous group, but radiogenic isotope data require addition of 20 and 40% crustal material. Metaluminous leucogranitic dykes and peralkaline granites formed from highly evolved melts (Eu/Eu^* < 0·1) and melt inclusion analysis from arfvedsonite pegmatite indicate that enhanced solubilities in an F-rich peralkaline residual melt could account for observed enrichments of high-field strength elements. Compositional variations within the peralkaline group reflect at least in part late-magmatic mineral segregation and hydrothermal overgrowth.