Petrogenesis and Ar-40/Ar-39 geochronology of the Brandberg complex, Namibia: Evidence for a major mantle contribution in metaluminous and peralkaline granites

Anorogenic granites of the Brandberg igneous complex in NW Namibia formed d uring early Cretaceous rifting and continental break-up of Gondwana. A meta luminous series [SiO2 = 62-77 wt %. molar (Na + K)/Al = 0.8-0.95] includes an early monzonite body, major biotite-hornblende granite, late biotite gra nite segregations and peripheral dykes of trachydacite. Volumetrically mino r 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 wi th 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 epsilon Nd-(130 Ma) from -0.4 to -5.1 and Sr-87/Sr-86((130 Ma)) from 0 .707 to 0.713. The Nd isotopic composition of the peralkaline granites is w ithin this range [epsilon Nd-(130 ma) from -0.7 to -1.9]. Ar-40-Ar-39 age d eterminations (132-130 Ma) indicate that the metaluminous and peralkaline u nits are indistinguishable in age and that they formed contemporaneously wi th flood basalts and associated felsic volcanism in the Etendeka-Parana pro vince. The metaluminous suite is modelled as a crustally contaminated (10-4 0%) fractionate of a tholeiitic basaltic magma (LTZ.H type), and a common b asaltic parent is inferred for the metaluminous and peralkaline rocks of th e Brandberg complex. Fractional crystallization of plagioclase, clinopyroxe ne and Fe-Ti oxides of a parental monzonitic magma accounts for major and t race element variations within the metaluminous group, but radiogenic isoto pe data require addition of 20 and 40% crustal material. Metaluminous leuco granitic dykes and peralkaline granites formed from highly evolved melts (E u/Eu* < 0.1) and melt inclusion analysis from arfvedsonite pegmatite indica te that enhanced solubilities in an F-rich peralkaline residual melt could account for observed enrichments of high-field strength elements. Compositi onal variations within the peralkaline group reflect at least in part late- magmatic mineral segregation and hydrothermal overgrowth.