Geochemical evolution of the Nyiragongo volcano (Virunga, Western African Rift, Zaire)

Abstract

Strongly undersaturated foiditic lavas (nepheline—melilitites, nephelinites, leucitites) from the active Nyiragongo volcano have been analysed for trace elements (Sc, V, Cr, Co, Ba, REE, Hf, Ta, Th and U) by instrumental neutron activation analysis. The overall low Sc and Cr contents and low Mg/Fe ratios suggest that the Nyiragongo parent magma evolved from less undersaturated mantle magmas (olivine—nephelinitic, basanitic) by high-pressure pyroxene fractionation. Pyroxene fractionation seems to be the main cause of the pronounced petrological differences between the adjacent Nyiragongo and Nyamuragira volcanoes. The distribution of incompatible trace elements within the melilite—nephelinitic magma column was strongly affected by migrating CO2-rich fluids. This is most clearly reflected in the unusually large fractionation of U from Th (Th/U = 4.5 down to 0.15), and the high and variable REE contents of the carbonated nepheline‒melilitites (bergalites). Major element differentiation within the magma column was dominated by melilite fractionation in the early stages. Later on, subtraction of pyroxene ± magnetite) and flotation of leucite (± nepheline) gave rise to the formation of highly foiditic porphyritic leucite‒nephelinites and nepheline‒leucitites.

Keywords

• volcanic rocks
• geochemistry
• trace elements
• alkaline rocks
• nephelinites
• African Rift