Petrology and spatial correlation of Black Belly tuff cone basalts, Tanzania

Abstract

Abstract Most of the tuff cones in the North Tanzanian region have not been studied in depth. Therefore, the range of known rock types is limited, and the relationships between these cones are unknown. Our study focused on the Black Belly tuff cone located close to Oldoinyo Lengai. Petrographically, the studied Black Belly samples are alkaline basalts with microcrystalline groundmass and phenocrysts. The groundmass consists of plagioclase, clinopyroxene, Ti-magnetite, apatite, and the phenocrysts are predominantly clinopyroxene, altered (to calcite and serpentinite) olivine, and carbonate minerals. While Oldoinyo Lengai and Black Belly rocks are not similar or directly related, the mineralogy and mineral chemistry of the studied Black Belly samples are similar to those representing the Crater Highlands’ volcanoes. Based on their compositions, the Black Belly clinopyroxenes define one end-member, and Oldoinyo Lengai clinopyroxenes represent the other end-member, while the Crater Highlands’ samples are of a transitional composition. The compositions of the other minerals (olivine, feldspar, and magnetite) confirm this theory. According to mineral chemical data, the Black Belly samples crystallised in several steps under high pressures (5.6 to 6.6 kbar) and temperatures (750 to 900°C) from an alkali basaltic melt. Considering the new chemical and thermobarometry mineral data presented here and the structural evolution of the region, Black Belly cannot be petrologically part of Oldoinyo Lengai. Instead, it defines an end-member of a series, with Oldoinyo Lengai representing the other extreme and the Crater Highlands volcanoes in a transitional position.