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As a result of their relative concentration towards the respective Atlantic margins, the silicic eruptives of the Parana (Brazil)-Etendeka large igneous province are disproportionately abundant in the Etendeka of Namibia. The NW Etendeka silicic units, dated at similar to132 Ma, occupy the upper stratigraphic levels of the volcanic sequences, restricted to the coastal zone, and comprise three latites and five quartz latites (QL). The large-volume Fria QL is the only low-Ti type. Its trace element and isotopic signatures indicate massive crustal input. The remaining NW Etendeka silicic units are enigmatic high-Ti types, geochemically different from low-Ti types. They exhibit chemical affinities with the temporally overlapping Khumib high-Ti basalt (see Ewart et al. Part 1) and high crystallization temperatures (greater than or equal to980 to 1120degreesC) inferred from augite and pigeonite phenocrysts, both consistent with their evolution from a mafic source. Geochemically, the high-Ti units define three groups, thought genetically related. We test whether these represent independent liquid lines of descent from a common high-Ti mafic parent. Although the recognition of latites reduces the apparent silica gap, difficulty is encountered in fractional crystallization models by the large volumes of two QL units. Numerical modelling does, however, support large-scale open-system fractional crystallization, assimilation of silicic to basaltic materials, and magma mixing, but cannot entirely exclude partial melting processes within the temporally active extensional environment. The fractional crystallization and mixing signatures add to the complexity of these enigmatic and controversial silicic magmas. The existence, however, of temporally and spatially overlapping high-Ti basalts is, in our view, not coincidental and the high-Ti character of the silicic magmas ultimately reflects a mantle signature.

The bimodal NW Etendeka province is located at the continental end of the Tristan plume trace in coastal Namibia. It comprises a high-Ti (Khumib type) and three low-Ti basalt (Tafelberg, Kuidas and Esmeralda types) suites, with, at stratigraphically higher level, interstratified high-Ti latites (three units) and quartz latites (five units), and one low-Ti quartz latite. Khumib basalts are enriched in high field strength elements and light rare earth elements relative to low-Ti types and exhibit trace element affinities with Tristan da Cunha lavas. The unradiogenic Pb-206/Pb-204 ratios of Khumib basalts are distinctive, most plotting to the left of the 132 Ma Geochron, together with elevated Pb-207/Pb-204 ratios, and Sr-Nd isotopic compositions plotting in the lower Nd-143/Nd-144 part of mantle array (EM1-like). The low-Ti basalts have less coherent trace element patterns and variable, radiogenic initial Sr (similar to0.707-0.717) and Pb isotope compositions, implying crustal contamination. Four samples, however, have less radiogenic Pb and Sr that we suggest approximate their uncontaminated source. All basalt types, but particularly the low-Ti types, contain samples with trace element characteristics (e.g. Nb/Nb-*) suggesting metasediment input, considered source-related. Radiogenic isotope compositions of these samples require long-term isolation of the source in the mantle and depletions (relative to unmodified sediment) in certain elements (e.g. Cs, Pb, U), which are possibly subduction-related. A geodynamic model is proposed in which the emerging Tristan plume entrained subducted material in the Transition Zone region, and further entrained asthenosphere during plume head expansion. Mixing calculations suggest that the main features of the Etendeka basalt types can be explained without sub-continental lithospheric mantle input. Crustal contamination is evident in most low-Ti basalts, but is distinct from the incorporation of a metasedimentary source component at mantle depths.

The Messum complex of NW Namibia, a part of the Parana– INTRODUCTION Etendeka volcanic province, consists of a dominantly felsic central The Etendeka Igneous province in the Damaraland core, surrounded by older gabbros. The igneous rocks of the core region of northwest Namibia (Fig. 1) contains numerous can be divided, in order of decreasing age, into (1) a sub-alkaline anorogenic igneous complexes of Mesozoic age (Martin suite, (2) an outer quartz syenite suite, and (3) an inner silicaet al., 1960; Piranjo, 1994), one of which is the Messum undersaturated suite dominated by nepheline syenite. Compositional complex. The range in chemical composition, both within differences within the quartz syenite suite can be explained by and between the various complexes, is remarkable and fractional crystallization, but Srand O-isotope data indicate that includes both silica-oversaturated and silica-underthese rocks contain a significant crustal component. The younger saturated mafic and felsic rocks, including carbonatite. central nepheline syenites trend towards the 0·1 GPa phonolite Messum was classified as a ‘mixed complex’ by Harris minimum and appear to have evolved by closed system fractional (1995), as it contains substantial amounts of both silicacrystallization of a mantle-derived magma, whereas earlier nepheline undersaturated and silica-oversaturated rock types. syenites appear to be contaminated with quartz syenite. The Dating of several Damaraland complexes yields ages decreasing effects of crustal contamination with time may reflect that lie between 137 and 124 Ma (Siedner & Miller, progressive armouring of the magmatic plumbing system by less 1968; Milner et al., 1995b; Renne et al., 1996). The contaminated material. Basanite dykes that intrude the plutonic alignment of the complexes along two parallel NE–SW rock of the core show trace element and Ndand Sr-isotope trends suggests that they represent a hotspot trace caused characteristics that are similar to those of basanites from Tristan by the motion of the African plate over a fixed mantle da Cunha. This suggests that the mantle-derived component of the plume (e.g. Rhodes, 1971). Because there is no systematic age progression along the lineament, this view has genMessum core rocks came from the Tristan plume. erally been discounted (Marsh, 1973). Recent ideas that large continental flood basalt (CFB) provinces, such as the Parana–Etendeka province, are manifestations of mantle plume activity (White & Mackenzie, 1989) has refocused attention on the relationship between plumes and linear arrays of complexes in CFB provinces. Milner