Abstract

The group of nephelinite volcanoes and associated carbonatite centres extending 600 km through eastern Uganda and western Kenya on the flanks of the main rift structure range from early Tertiary to Pleistocene in age, most of the volcanicity having occurred in the Miocene. An older (Mesozoic) land surface still represented extensively in central and western Uganda and preserved as eroded relics rising eastwards towards the crest of the Kenya Rift arch had in all probability already been removed entirely from eastern Uganda where the degraded landscape was updomed and penetrated by the early centres. These complexes comprise carbonatite ring-intrusions up to 6 km diameter, associated with ijolite, nepheline syenite and fenites, but volcanic superstructures were probably small if they existed at all. The Miocene volcanoes, predominantly nephelinitic, were built up after further erosion had exposed the early intrusive complexes; the landscape preserved under the volcanics shows considerable relief and modification by local doming and tectonic depression and has locally been infilled with sediments and basal tuffs yielding Miocene faunas.

In western Kenya the volcanoes are associated with the transverse Kavirondo rift. The Kisingiri volcano, to the west, has been eroded to reveal an older complex of intrusive ijolite, uncompahgrite, carbonatite and fenite, pierced by the later pyroclastic vent of Rangwa. This nephelinitic volcano is truncated by faults associated with the Kavirondo rift structure, and the trough is partly infilled by Pleistocene sediments. Eastwards, on the fringe of the volcano, the Nyanzian basement is penetrated by the carbonatite cone-sheet complexes of the Ruri Hills, the ijolite-urtite mass of Usaki which is surrounded by feldspathic fenites probably related to an older carbonatite episode, and a number of minor carbonatite centres and dykes of varying ages. Phonolites and feldspathic nephelinites occur abundantly as plugs and dykes. Isotopic ages vary, but are of the order of 10 to 5 m.y. The carbonatite cone-sheet complex of Homa Mountain farther east is associated with earlier ijolite and fenites; after a period of erosion, igneous activity was resumed with the eruption of vents of phonolitic nephelinite, melilitite and carbonatite. Apart from a few remnants of bedded pyroclastics at Ruri, and fragmental rocks of uncertain origin at Homa, evidence for extrusive activity in this region is confined to the small phonolitic volcano of Nyamaji, to the north-east of Ruri.

At the eastern end of the Kavirondo trough and close to the main rift, occur the volcanoes of Tinderet and Londiani; minor occurrences of carbonatite are associated with the earliest activity of Tinderet. The volcanics of Tinderet bury the early fault scarp of the Kavirondo rift, but are displaced by later movements; the flood phonolites of Uasin Gishu and Kericho are intercalated in this succession. The volcanoes and intrusives of the whole region have been affected by later (end-Tertiary) erosion accompanied by lateritisation of the slopes and valley bevels. The varying pattern of Pleistocene sedimentation and erosion in the Kavirondo trough reflects the effects of the latest tectonic events.

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