The Kenya Rift volcanics: A note on volumes and chemical composition

Abstract

Cainozoic alkaline volcanics associated with the Kenya Rift Valley have an estimated total volume of about 144,000 km3. Basalts were erupted repeatedly and account for more than half this volume. The most strongly alkaline basalts occur in Miocene flows which flooded a proto-rift depression in northern Kenya. Post-Miocene basalts of the rift floor are less undersaturated (Niggli qz. value = −28) than those east of the rift (qz = −37). Some basalts at the rift margins show affinities with lavas of the rift floor; others more closely resemble rocks of multi-centre fields east of the rift. Nephelinites are frequently accompanied by highly undersaturated phonolites (phonolitic nephelinites) at central volcanoes in western Kenya (Miocene) and southern Kenya (Pliocene and Quaternary). Voluminous late Miocene phonolites (25,000 km3) tend to be more undersaturated to the west than to the east. Excluding phonolitic nephelinites, post-Miocene phonolites of the rift floor are on average less undersaturated than phonolitic lavas outside the rift valley; they are also less undersaturated than the Miocene phonolites. Alkali trachytes, rhyolites and ignimbrites (10,000 km3) which flooded the rift floor in southern Kenya and locally overflowed on to the plateaus were erupted during an important phase of domal uplift in Plio-Pleistocene times. Pliocene alkali rhyolites and ignimbrites in the northern part of the Kenya Rift evidently have a similar volume.