The geochemistry and evolution of Palaeogene phonolites, central Namibia

Phonolites of Palaeogene age occur at two different localities in central Namibia, occurring as clusters of eroded monogenetic lava domes. Six phonolites at Aris collectively exhibit a narrow compositional range and represent a single magma system of highly evolved phonolite with high concentrations of Na2O, Rb, Th, Zr, Zn, Pb, and REE and very low abundances of MgO, P2O5, Sr and Ba. No mafic rocks are associated with the Aris phonolites and their ultimate petrogenetic origin remains obscure. The more abundant Staalhart phonolites exhibit more variable but scattered compositional variation consistent with fractional crystallization dominated by sanidine (and nepheline) and pyroxene. The behaviour of REE is decoupled from Zr, Nb, and Th in this suite indicating a controlling role for minor phases in their evolution. The least evolved phonolites have initial 87Sr/86Sr ≈ 0.7043, which is identical to a small occurrence of associated plagioclase-bearing nephelinite indicating a possible petrogenetic link. Higher isotopic ratios in more evolved phonolites suggest their evolution was accompanied by some crustal interaction. Comparisons of phonolites world-wide indicate that they all have similar trace element abundance patterns when normalized to associated and possibly parental mafic compositions, regardless of proposed petrogenetic origins (melting, fractional crystallization, AFC, etc.). These patterns of similar pronounced enrichments of Rb, Th, Zr, strong depletions in Ti and P, and YbN > GdN (i.e. depletion of middle REE relative to heavy REE) confirm the suggestion that fractional crystallization is the dominant process by which phonolites evolve from mafic alkaline compositions. Despite similar published values of DEu for sanidine in silicic and phonolitic compositions, highly evolved phonolites develop only modest negative Eu anomalies compared to evolved silicic rocks. This might reflect differences in melt structure, fO2, halogen complexing or fractionating mineral assemblages in phonolite and silicic liquids. Sparse data on Eu partition coefficients in phonolite bulk compositions does not provide insight into the role of these controls of Eu partitioning in phonolite.