The alkaline igneous centres of eastern Uganda comprise a younger Tertiary group represented by dissected volcanoes and an older Cretaceous (?) group composed entirely of equivalent intrusive formations. Both the extrusive and the intrusive suites are characterized by extreme silica undersaturation and richness in alkalis, especially soda; they largely conform to a nephelinite series and an ijolite-carbonatite series, respectively. From field, petrographic, and chemical data it is argued that the immediately parental magma was of nephelinite/melteigite composition. The courses of evolution of the volcanic and plutonic rocks followed odivergent trends, owing to differences in their physico-chemical environments. The volcanic series evolved towards a phonolitic or trachytic residium, whereas the ijolite series maintained silica undersaturation and tended towards final enrichment in lime, alkalis, and volatiles, including carbon dioxide. The final residuum from this course of evolution was a carbonatite or alkali carbonatite. The evidence suggests that the felspar-bearing rocks (trachytes, phonolites, syenites, and nepheline syenites) have had a number of different modes of origin. Apart from evolution in the volcanic series of crystallization differentiation, they appear to have formed by fenitization of Basement rocks and by felspathization of felspathoidal rocks, such as ijolites. Extrusive equivalents may then have arisen by mobilization.
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Basil C. King (1965) studied this question.