The connection between perovskite, magnetite, titanite and schorlomitic garnet in nephelinitic rocks from Burko volcano, Tanzania

Burko is a nephelinitic volcano in the Gregory Rift of the East African Rift System (EARS), situated in northern Tanzania. The rocks are olivine-free and phonolitic nephelinites, deposited as tuffs, agglomerates and lavas that frequently contain plutonic inclusions. Based on geochemical data, a carbonatite-metasomatized mantle source has been assumed for these rocks (Mana et al., 2015).

We present a detailed mineralogical and petrological study of the variably evolved Burko rocks (whole-rock Mg numbers <50), which contain either nepheline + Fe-Ti oxide + perovskite + diopsidic pyroxene or nepheline + Fe-Ti garnet + titanite + hedenbergitic pyroxene ± alkali feldspar assemblages. Such an evolution of relatively oxidized and strongly SiO₂-undersaturated alkaline magmas towards peralkaline compositions may be a common process and is probably controlled by T-a_SiO2-f_O2 changes. Based on a comparison with spatially associated nephelinitic volcanos like Sadiman and Ol Doinyo Lengai, we discuss similarities and differences in their genesis.

Mana, S., Furman, T., Turrin, B. D., Feigenson, M. D., and Swisher, C. C., III, 2015, Magmatic activity across the East African North Tanzanian Divergence Zone: Journal of the Geological Society, v. 172, no. 3, p. 368-389.