Petrology of the alkaline core of the messum igneous complex, Namibia: Evidence for the progessively decreasing effect of crustal contamination

The Messum complex of NW Namibia, a part of the Parana-Etendeka volcanic pr ovince, consists of a dominantly felsic central core, surrounded by older g abbros. The igneous rocks of the core can be divided, in order of decreasin g age, into (1) a sub-alkaline suite, (2) an outer quartz syenite suite, an d (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 the se rocks contain a significant crustal component. The younger central nephe line syenites trend towards the 0.1 GPa phonolite minimum and appear to hav e evolved by closed system fractional crystallization of a mantle-derived m agma, whereas earlier nepheline syenites appear to be contaminated with qua rtz syenite. The decreasing effects of crustal contamination with time may reflect progressive armouring of the magmatic plumbing system by less conta minated 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.