The potassic series of Karisimbi volcano (Virunga range, Rwanda): Volcanological and petrological aspects

Abstract

The Karisimbi volcano (Rwanda) is one of the principal eruptive centers of the Virunga volcanic range. It is located at the southern tip of a NE-SW volcano-tectonic axis on the western branch of the East African Rift. The different morphological units (caldera “Branca”, pit crater “Muntango”, domes and flow domes) built up on the main edifice make Karisimbi the most complex central volcano of the Virunga range. The products of Karisimbi's activity are essentially fluid lavas of basic or intermediate chemical composition and viscous differentiated lavas. Ash falls and pumice flows are evidence of pyroclastic episodes which occurred during the last stages of volcanic activity around 30,000 years ago. The upper Pleistocene potassic series of Karisimbi is composed of a diverse range of lavas, closely associated in space and time. These lavas show many common features with rocks from rift provinces, such as high K 2O, TiO 2, Rb and Ba contents. The general evolution of the series from potassic basanites to trachytes is qualitatively compatible with the successive separations of the mineral phases observed in the lavas (olivine, clinopyroxene, leucite, ferro-titanic oxides, plagioclase, biotite, alkali feldspar). Coexisting Fe-Ti oxides indicate equilibration temperatures lying between 1100° and 750°C and f O 2 between N-NO and QFM buffers. Felsic magmas are considered to have crystallized at a relatively shallow level in the crust, under near water-saturated conditions, which is consistent with the presence of pyroclastics with trachytic composition and the presence of hydrous mineral phases.