Phase relations in alkaline and carbonatitic rocks from northern Tanzania

Due to low-degree partial melting of metasomatized mantle sources, the northern Tanzanian segment of the East African Rift hosts volcanoes that produced diverse rock series, ranging from basaltic to trachytic, and from nephelinitic ±melilititic to phonolitic compositions. Some of the latter group also produced carbonatites (Oldoinyo Lengai, Kerimasi, Mosonik, Shombole), while others (Sadiman and Burko) did not.

We present a petrological study of alkaline silicate rocks and associated carbonatites from Kerimasi, Mosonik, and Shombole, and compare them with the nearby Sadiman, Burko, and Oldoinyo Lengai volcanoes. The silicate rocks include olivine-melilitites, various nephelinites and phonolites, and their plutonic counterparts. Whole-rock X_Mg (Mg/(Mg+Fe)) values decrease from ~0.7 in melilitites via 0.6–0.5 in nephelinites to <0.2 in phonolites, indicating progressive magma evolution. Simultaneously, CaO, FeO, TiO₂, and P₂O₅ decrease, while alkalis, Al₂O₃, and SiO₂ increase; in some cases, peralkaline compositions ((Na+K)/Al >1) are reached. Mineral assemblages change from åkermanite +forsterite +perovskite +Fe-Ti oxides in primitive rocks, via diopside +andradite +nepheline in intermediate rocks, to aegirine-augite +nepheline +alkali feldspar +titanite ±sodalite-aenigmatite-eudialyte in evolved rocks. Carbonatites are mostly calciocarbonatites that differ in amount and composition of silicate phases. Apatite, magnetite, mica, and clinopyroxene are present in all volcanoes. Olivine, monticellite, perovskite, periclase occur only at Kerimasi, while Mosonik and Shombole typically contain nepheline, titanite, K-feldspar.

These textural, chemical, and mineralogical variations reflect different storage conditions, cooling histories, as well as heterogeneous mantle sources and partial melting processes, which we aim to constrain by estimating temperature, pressure, redox state, silica activity, and CaO activity.