Quaternary perpotassic magmatism in Uganda (Toro-Ankole Volcanic Province): age assessment and significance for magmatic evolution along the East African Rift

Abstract

The Toro-Ankole volcanics in the northern sector of the Western Branch of the East African Rift are generally assumed to be of Quaternary age. New field observations and comparative KAr and ArAr analyses on whole rock samples and phenocrysts of perpotassic lavas reveal the presence of excess Ar and led to the conclusion that apparently no volcanics have been erupted prior to 50 ka. The present geochronological data, compared with previous ones for the Virunga and South Kivu volcanic fields, suggest that a Late Quaternary pulse of volcanic activity occured in the three areas, with the emission of perpotassic, mildly potassic or sodic alkaline magmas. The genetic implications of such a synchronous activity with wide compositional variation are briefly discussed with respect to basement age and assumed lithosphere characteristics.