Shombole is a deeply dissected volcano situated on the floor of the Gregory Rift on the Kenya-Tanzania border (Baker, 1963). It is 10 km in diameter with a relief of about 1000 m. Nephelinitic tuffs and agglomerates were erupted from a central vent, but the inter-layered lavas emanated from radial fissures and parasitic cones (Peterson, 1989a). Flows are short and 1-3 m thick. A central caldera is filled with horizontally bedded tuffs, agglomerates, agglutinates and minor flows. Subordinate phonolite was erupted and the flanks of the volcano are overlain by flood trachytes and trachytic tuffs and by fluvial and lacustrine sediments. Carbonatite occurs as ash tuffs, radial dykes and in zones of intrusive breccia. The nephelinites (Peterson, 1989a) comprise phenocrysts of zoned diopside/hedenbergite/aegirine-augite, nepheline (20-30%), perovskite, titanite, schorlomite, magnetite and a little apatite with zeolite-carbonate globules. These rocks pass into phonolitic nephelinites which contain groundmass alkali feldspar. Analcime and sodalite phenocrysts occur in a few lavas. Analyses of pyroxene, nepheline, perovskite, schorlomite, titanite, magnetite, sodalite and analcime are given by Peterson (1989a) who also gives modal data and chemical analyses with trace elements, including REE, for 31 rocks, and discusses the origin and evolution of the nephelinites (Peterson, 1989b).
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