Anorogenic granite evolution in Namibia—a fluid contribution

Field observations, petrological interpretations combined with cationic geochemical data and diagrams illustrate the magmatic and postmagmatic events in the Mesozoic anorogenic complexes of northern Namibia. The magmatic lineage of the Damaraland alkaline centres is cationically defined in a modified Q–P diagram (Debon and Le Fort 1988). Major element concentrations for Brandberg and Erongo supported by chemical data from other Damaraland complexes at Cape Cross, Otjihorongo, Messum, Spitzkoppies, and Paresis, have been converted to millications, to delineate a magmatic lineage which follows an alkaline oversaturated trend. Superimposed on this magmatic lineage is textural and geochemical evidence for hydrothermal metasomatic disequilibrium and partial re-equilibration. Volcanic rocks and their granitic equivalents from the Erongo complex show petrological and geochemical criteria indicating substantial potash and boron metasomatism which has given the Erongo rocks a subalkaline potassic geochemical signature. Similar granite compositional trends are also recorded within the Brandberg massif. However, at Brandberg, there is a later-stage hydrothermal overprint generating a texturally variable peralkaline granite, with spectacular examples of linear zones of aegirine and arfvedsonite apparently constrained to microfractures in marginal granite sheets. The zones of mineral ‘layering’ can be divided into three repetitive units and traced horizontally along the southwestern margin of the Brandberg massif for a distance of 3 km and vertically for 200–300m. These marginal rocks are cross-cut by dykes and sheets of brandbergite, a mesocratic fine-grained peralkaline rock. Fenite veins, described elsewhere from marginal zones of strongly alkaline massifs, are found near the inner brecciated contact zone with the main granitic mass of the Brandberg. These fenite veins are interpreted as fluid-channel ways along which mass transfer of peralkaline components occurred during the final stages of cooling and consolidation.