QUATERNARY PERPOTASSIC MAGMATISM IN UGANDA (TORO-ANKOLE VOLCANIC PROVINCE) - AGE ASSESSMENT AND SIGNIFICANCE FOR MAGMATIC EVOLUTION ALONG THE EAST-AFRICAN RIFT

The Toro-Ankole volcanics in the northern sector of the Western Branch of the East African Rift are generally assumed to be of Quaternary ag e. New field observations and comparative K-Ar and Ar-Ar analyses on w hole rock samples and phenocrysts of perpotassic lavas reveal the pres ence of excess Ar and led to the conclusion that apparently no volcani cs 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 occu red in the three areas, with the emission of perpotassic, mildly potas sic or sodic alkaline magmas. The genetic implications of such a synch ronous activity with wide compositional variation are briefly discusse d with respect to basement age and assumed lithosphere characteristics . (C) 1998 Elsevier Science Limited.