Petrology and geochemistry of the Nyamuragira volcano, Zaire

Abstract

Nyamuragira volcano in the Western Rift of the East Africa Rift Valley is characterized by violent volcanic eruptions, the most recent of which occurred in 1967, 1971, 1976–1977, 1980 and 1981–1982. Its extrusives consist of a highly potassic suite of olivine basanite-tephritic phonolites among which phonolitic tephrite is the commonest. The petrographic features of the whole suite such as phenocryst and groundmass mineral assemblages and modal variations are rather simple. Major elements vary smoothly from olivine basanite to phonolitic interstitial glass without any abrupt change with increasing K 2O. In addition, the typical incompatible trace elements, Ba, Ce, F, Nb, Rb and Sr increase continuously with the increase of K 2O. These suggest that chemical variations were controlled mainly by the effective fractionation of olivine and clinopyroxene associated with plagioclase and titanomagnetite. Furthermore, in the K 2O-incompatible trace-element diagrams, trend lines extrapolated to zero, converge on the coordinate axes of the vertical and horizontal lines. This suggests that these incompatible elements were preferentially moved toward the primitive basaltic magma during the partial melting of metasomatized upper-mantle peridotites. Furthermore, these elements are concentrated almost completely in residual liquids during fractional crystallization.