Katwe -Kikorongo Volcanic Field

This is the most extensive of the volcanic fields of southwestern Uganda, extending from the northern shores of Lake Edward for 30 km to the foothills of Ruwenzori, all of it being within the Western Rift. The field comprises numerous explosion craters, six of them containing permanent lakes, ranging in diameter from 50 m to about 4 km in the case of some composite crater depressions (von Knorring, 1967). Accumulations of tuffs and agglomerates are widespread but only four occurrences of lava are known, these being associated with the Nabugando and Mbuga craters. The fragments of pyroclastic rocks include a wide range of rocks and minerals derived from the underlying Precambrian basement, rocks of deep-seated origin including pyroxenite, peridotite and glimmerite, and bombs of a wide range of lava types. A comprehensive account of the field is not available, the best map being the Mbarara Sheet (Geological Survey of Uganda, 1:250,000, 1961). Holmes and Harwood (1932) and Holmes (1952 and 1956) have described in considerable detail specimens collected, mainly by Combe, from the Katwe, Nabugando, Mbuga and several other craters and localities. Among the rock types described are leucitite tuffs that consist of abundant phenocrysts of leucite and sparse ones of aegirine-augite and biotite with accessories including perovskite and possible altered melilite. Other types of tuff include melilite leucitite, melilite basalt and various varieties rich in carbonate. Among the coarse-grained fragments in the tuffs members of Holmes' ‘B.P.’ (biotite pyroxenite) group are the most abundant, with fewer of pyroxenite, glimmerite and olivine-rich rocks. A full list will be found in Holmes (1956, Table 1). The range of compositions, in terms of clinopyroxene, olivine and hydrous phases, and details of modern terminology i.e. lherzolite, harzburgite, wherlite etc., will be found in Lloyd and Bailey (1975). Lloyd et al. (1999) give numerous analyses of clinopyroxenes and micas from xenoliths from four craters. Data on average modal and chemical compositions are given by Lloyd et al. (1985). These xenoliths, together with those of the Bunyaruguru field (No. 6), in particular, have been interpreted as providing evidence of mantle metasomatism. Among the lava bombs leucite-rich rocks, including katungite and ugandite are widely represented. All rock types are rich in leucite, although in the katungites this may be only occult in the glass. The katungites contain essential melilite, leucite, usually in the glass, and abundant olivine, perovskite and opaque phases; biotite is sometimes present. Some varieties containing augite and biotite Holmes (1956) classified as alnoite, and a sample from Katwe crater in which augite is present to the exclusion of melilite is referred to as olivine ouachitite. The ugandites are rocks comprising augite and leucite, again with olivine, perovskite and opaque phases. Nephelinite has been described from inclusions at one locality only (Holmes and Harwood, 1932). The Nabugando and Mbuga craters lava occurrences are not extensive, but are described in considerable detail by Holmes (1952). He classified the lavas as olivine melaleucitite, leucite ankaratrite and potash ankaratrite, depending on the presence of leucite and/or kalsilite. They contain phenocrysts of pyroxene and opaque minerals, and sometimes olivine and perovskite, in a groundmass of pyroxene, kalsilite and/or leucite plus or minus accessory biotite, perovskite, apatite, analcime, zeolites, calcite and glass. The mineral compositions and textures vary widely, not only between flows, but within them. Analyses of all the major types of lava and included blocks will be found in the Holmes references cited above. Dawson et al. (1985) give petrographic details and rock and mineral analyses of olivine melilitite (katungite) and pyroxene-rich ugandite ejected blocks from Nyamunuka crater. Carbonatitic rocks have been found associated with the Mbuga and Mahega craters (von Knorring, 1967). At the Mbuga crater angular blocks of limestone of a pumiceous character form a roughly circular mound on a ridge of tuff near the crater rim (Holmes, 1956). The highly vesicular rock has the texture of a melilite-rich katungite, but consists almost wholly of calcite; a rock analysis is given by von Knorring (1967). Carbonatitic blocks from the Mahega crater are of two types. The first resemble the Mbuga 'carbonated katungite' and contain accessory apatite, perovskite and magnetite in addition to the essential calcite. The second variety is a coarse-grained, vesicular rock of titanite, aegirine-augite, biotite, apatite and accessory perovskite and magnetite in a matrix of calcite (von Knorring, 1967). The calcite forms rhombs up to 5 cm across enclosing pyroxene poikilitically. Stoppa et al. (2000) describe carbonatite lapilli-bearing tuffs and a dolomite carbonatite bomb from the Murumuli crater, which they take as proof of the presence of carbonatite magma in the Katwe igneous system. Details of strontium isotopes will be found in Bell and Powell (1969), K-Rb values in Bell and Doyle (1971), and trace element data in Higazy (1954), while Holmes (1950) considers the petrogenesis of katungite from this and the nearby volcanic fields. Lloyd (1985) conducted one atmosphere melting experiments on an olivine melilitite lapilli from a crater north of Nabugando.