Karisimbi

Karisimbi is the highest of the Virunga volcanoes (4507 m) and its products cover about 600 km2 within Rwanda, but the northwestern part of the volcano lies within the Democratic Republic of Congo. The volcano comprises a summit cone of pyroclastic and viscous lava flows, the 1 km diameter and 150 m deep pit crater of Muntango lying immediately south of the summit cone, the circular, 2 km diameter, 40 m deep Branca caldera, and a gently sloping lava plain of some 400 km2. Photographs and aerial views of all these features are given by Mulder (1985). The edifice is built principally of lava flows, of both pahoehoe and block type, that are generally 2-3 km in length but a series of flows more than 10 km long erupted from satellite cones east of the caldera while some flows on the lava plain extend to over 20 km. Mulder (1985) lists 106 adventive cones which, together with other morphological features, are described and illustrated in some detail. The volcanic rocks comprise a continuous potassic series to which half a dozen terminologies have been applied, as tabulated by Mulder (1985) and Marcelot et al. (1985), but in both papers a system based on differentiation index is adopted which generates the terms K-ankaratrite (‘Primitive suite K-basanites’ in Mulder et al., 1986), K-basanite, K-hawaiite, K-mugearite, K-benmoreite and K-trachyte. Olivine and clinopyroxene are present throughout the series, generally as both phenocrysts and in the groundmass, the former varying from Fo89 in the ankaratrites to Fo44 in the trachytes, the latter from diopside to augite and salite. Plagioclase is present in the groundmass of all but the trachytes and develops as labradorite phenocrysts in hawaiites and mugearites. Sanidine is a groundmass phase from mugearite through to trachyte but in the last also forms phenocrysts; it varies from Or62 Ab34 An4 to Or44 Ab47 An9. Biotite forms phenocrysts in the benmoreites and trachytes and also occurs in the groundmass of the latter. Cr-spinel is found in the ankaratrites and Ti-magnetite in the intermediate rocks. Apatite, analcime and hornblende occur in some rocks (Mulder et al., 1986). Analyses of 68 rocks, including some trace element data, are given in Mulder (1985) who also presents 87Sr/86Sr and 143Nd/144Nd data for eleven rocks. Further rock analyses, including much trace element data, are given by Marcelot et al. (1985), Mulder et al. (1986) and Rogers et al. (1992) and considerable Sr, Nd and Pb isotopic data are given in the last two papers. Petrogeneses including crustal contamination and evolution from an enriched mantle source are considered in these geochemical papers.