Alkaline Rocks and Carbonatites of the World

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Occurrence number: 
Longitude: 30.18, Latitude: -0.47

This isolated volcano stands nearly 200 m above the surrounding valley floor but the lowermost 60-70 m are of Karagwe Ankolean schists. The slopes of the volcano proper are steep and rise to a rim within which is a 230 m-diameter crater. The cone is built principally of blocks of basement rocks, but there are occasional lava bombs and some exposures of tuff along the northern crater rim. Bombs of katungite which have been partly or largely carbonated have been described by von Knorring (1967). Two generally blocky flows issued from the cone (Combe, 1937). The lavas contain phenocrysts of olivine up to 3 mm long and melilite up to 2 mm square in a matrix of leucite, apatite, perovskite, magnetite, rare kalsilite, possible monticellite and zeolite. It is potassic (K2O 4.09%; Na2O 1.32%) and Holmes (1937) gave this rock the name katungite and discussed at length its classification and comparison with similar rocks. The petrogenesis of katungite and associated and similar rocks was also considered (Holmes, 1950). The petrography, mineralogy and chemistry of a specimen of the katungite lava is also described fully by Dawson et al. (1985) and Neuvonen (1956) gives a rock analysis, mode and mineral analyses including one of kalsilite. The tuffs from the crater rim contain lapilli up to 5 mm in diameter consisting of fine-grained katungite, together with crystals of the main katungite phases and fragments and crystals of basement origin (Holmes, 1937). Holmes also found a fragment of biotite pyroxenite and described a number of katungite bombs.

The lack of dissection of the cone and relative freshness of the flows indicate a young age.
COMBE, A.D. 1937. The Katunga volcano, south-west Uganda. Geological Magazine, 74: 195-200.DAWSON, J.B., SMITH, J.V. and JONES, A.P. 1985. A comparative study of bulk rock and mineral chemistry of olivine melilitites and associated rocks from East and South Africa. Neues Jahrbuch fur Mineralogie. Stuttgart. Abhandlungen, 152: 143-75.HOLMES, A. 1937. The petrology of Katungite. Geological Magazine, 74: 200-19.HOLMES, A. 1950. Petrogenesis of katungite and its associates. American Mineralogist, 35: 772-92.VON KNORRING, O. 1967. Carbonatitic rocks from the volcanic field of western Uganda. Annual Report, Research Institute of African Geology, University of Leeds, 11: 30-2.NEUVONEN, K.J. 1956. Minerals of the katungite flow. Bulletin de la Commission Geologique de Finlande, 29(172): 1-7.
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