The geology of Chishanya, Buhera District, Southern Rhodesia

The area described consists of Pre-Cambrian granite, overlain unconformably in the south-east by sediments of the Umkondo System. An alkaline complex, consisting of syenites, nepheline-syenite, ijolites and carbonatites, occurs in the granite, and both granite and Umkondo sediments are cut by dykes of alkaline rocks, nepheline-basalts, nepheline-basanites and phonolites. Evidence is presented indicating that the alkaline dykes are magmatic in origin and that the carbonatite is an intrusive rock which is not derived from limestone of the Umkondo System. The syenite is regarded as fenitised granite and it appears possible that the ijolite may also be a fenitised rock. The origin of the nepheline-syenite is discussed. Some comparisons are made with the alkaline ring-complexes at Shawa and Dorowa (Southern Rhodesia) and the opinion is expressed that the carbonate cores of these are also magmatic carbonatites.