Petrology of the Alkaline Core of the Messum Igneous Complex, Namibia: Evidence for the Progressively Decreasing Effect of Crustal Contamination

The Messum complex of NW Namibia, a part of the Paraná–Etendeka volcanic province, consists of a dominantly felsic central core, surrounded by older gabbros. The igneous rocks of the core can be divided, in order of decreasing age, into (1) a sub-alkaline suite, (2) an outer quartz syenite suite, and (3) an inner silica-undersaturated suite dominated by nepheline syenite. Compositional differences within the quartz syenite suite can be explained by fractional crystallization, but Sr- and O-isotope data indicate that these rocks contain a significant crustal component. The younger central nepheline syenites trend towards the 0.1 GPa phonolite minimum and appear to have evolved by closed system fractional crystallization of a mantle-derived magma, whereas earlier nepheline syenites appear to be contaminated with quartz syenite. The decreasing effects of crustal contamination with time may reflect progressive armouring of the magmatic plumbing system by less contaminated material. Basanite dykes that intrude the plutonic rock of the core show trace element and Nd- and Sr-isotope characteristics that are similar to those of basanites from Tristan da Cunha. This suggests that the mantle-derived component of the Messum core rocks came from the Tristan plume.