Geochemistry and petrogenesis of the Tsirub nephelinite intrusions, southern Namibia

Field studies and geochemical analysis of a nephelinite dyke and two plug-like bodies - Tsirub (south) and Tsirub (north), occurring in southern Namibia are reported. Bulk rock geochemical analyses show that the nephelinites have limited but significant compositional variation (MgO = 8.7-12.1 wt%; Mg# = 0.61-0.71; Zr = 415-525 ppm; Nb = 152-267 ppm; Ni = 105-305 ppm; Zr/Nb = 1.8-3.1; La/Sm_n = 3.8-6.1). In detail, systematic minor variations in incompatible trace element abundances and ratios and Sr and Nd isotope compositions (⁸⁷Sr/⁸⁶Sr = 0.70335-0.70441; ¹⁴³Nd/¹⁴⁴Nd = 0.51278-0.51272) are best accounted for by small differences in degree of partial melting and some source heterogeneity. Quantitative modelling of partial melting processes indicates that compositional variations can be accounted for by low but variable degrees of partial melting (2-6%) of a peridotitic mantle. Negative K and to a lesser extent Ti, anomalies on primitive mantle normalised diagrams require melting to have occurred in the presence of minor amphibole, constraining the depth of melting to less than ~38 kbar, i.e. within the subcontinental lithospheric mantle. Heat required to raise the ambient geotherm (to ~55 mWm^-2) to allow low degrees of melting of a volatile-rich (H₂O-CO₂ undersaturated) peridotitic lithospheric mantle is attributed to the passage of the African plate over the Vema mantle plume at 40-50 Ma. The close spatial proximity of the Tsirub nephelinite to the Dicker Willem carbonatite, Schwarzeberg nephelinite and alkaline intrusive rocks associated with the Klinghardt phonolites is taken to indicate a similar emplacement age of ~30-50 Ma and a related cause of magmatism.