The Geology of the Londiani Area of the Kenya Rift Valley

An area of about 900 square miles (2300 k:m2 ) at the junction of the Kenya and lCa.virondo Rift Valleys was studied and a map of it on a scale of 1 : 50,000 is presented. The fonmations present are alkaline lavas and tuffs ranging in age from 12 m.y. b.p. to recent and can be divided into a basanite to phonolite series older than 7 m.y. and a basalt to trachyte series younger than 7 m.y. The fonna.tions are grouped into four assemblages, each .consisting of rocks derived fr.om sources in about the same area. A series of trachytio ash flows about 4m.y. old, the Eldama Ravine Tuff; and two trachyte volcanoes, Londiani of 3 m.y. b.p. and Kilornbe of 2 m.y. b.p., together with their associated syenite bombs are described in detail. The structure of the area is dominated by the Equator and Mau Monoclines which form the western margin of the Kenya Rift Valley. Faults are relatively unimportant but show three distinct trends which can be related to structures in the basement. Chemical analysis was carried out on about 200 rocks, particularly concentrating on the Eldama Ravine Tuff and the Londiani and Kilambe Trachytes. This, with the petrography, showed that the. rocks within the ba.sa·ni te to phonolite series and the basalt to trachyte series are related in general but not in detail by fractional crystallisation. It is also shown that in the trachytic rocks Na, Fe, Y and the Lanthanides are very mobile