Oxygen isotope geochemistry of the Mesozoic volcanics of the Etendeka Formation, Namibia

Abstract

The Etendeka Formation volcanics consist of a bimodal association of basalts and quartz latites. Forty three new whole rock oxygen isotope analyses are reported for all the major magma types. All the rocks except a minor suite of dolerites have higher δ¹⁸O values than normal mantle. The basic rocks (average of 29=8.8‰) have significantly different δ¹⁸O to the acid rocks (average of 10=14.4‰) These data are apparently consistent with previously published petrogenetic models, which propose that the basalts were affected by crustal contamination and that the quartz latites are crustally derived. However, mineral oxygen data show that there is significant oxygen isotopic disequilibrium between phenocryst and whole rock, the latter being significantly higher in most cases. One of the basic magma types (the Tafelberg basalts) shows mutual positive correlations between δ¹⁸O, SiO₂ and ɛ_Sr. If these correlations are due to crustal contamination, then as much as 45% contamination is required by material having a δ¹⁸O value of 15‰ which is the maximum observed value in the Damaran basement rocks. In the absence of pyroxene phenocryst δ¹⁸O data for the high ɛ_Sr Tafelberg basalts (they are aphyric), it is not possible to confirm that contamination has taken place. An alternative explanation is that the correlation between ɛ_Sr and SiO₂ resulted from assimilation coupled with fractional crystallization (AFC) (before emplacement). Post-eruption alteration resulted in a correlation between SiO₂ δ¹⁸O because the material with the most Si-O bonds was able to concentrate ¹⁸O more effectively. The limited mineral data for the quartz latites suggests that there is some source heterogeneity. A pyroxene δ¹⁸O value of 10% for a southern Etendeka quartz latite is consistent with a crustal source.