Alkaline Rocks and Carbonatites of the World

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Occurrence number: 
Longitude: 34.2, Latitude: 2.83

The Toror Hills comprise a central carbonatite complex around which are a number of trachyte and phonolite intrusions. The central complex consists of a partial ring of carbonatite, which cuts and fenitizes the Precambrian country rocks, and is itself invaded by a mass of phonolitic breccia. Fragments of phonolite are found in the breccia, but it is itself also cut by similar rock types. The youngest intrusions are numerous phonolite and tinguaite plugs, dykes and sheets one of which forms Toror Peak. Many of the outlying phonolite masses were considered by Du Bois (1959) to be plugs but King and Sutherland (1966) note that many have low angle lower contacts with the basement and are apparently remnants of sheets. Several small intrusions of nephelinite are also present. A small, approximately circular intrusion of carbonatite 400 m in diameter occurs at Theno Hill some 8 km west of Toror; it has a fenite envelope. King and Sutherland (1966) distinguish four main varieties of carbonatite: sovite, dolomitic sovite, ferruginous carbonatite and feldspathic carbonatite. Apatite is plentiful in the sovite, in which pyrochlore is sporadic and a pale green pyroxene rare. With the rock-forming carbonates of the dolomitic sovite are a fibrous green amphibole, mica, magnetite and occasional pyrochlore. The most widespread carbonatite is the ferruginous type in which magnetite may be abundant, and veining by hematite and limonite is common. Clouded orthoclase is abundant in some varieties. Apatite concentrations are widespread in the outer zones of the carbonatite, and this rock may contain locally concentrations of pyrochlore. Fenitization is not generally intense enough at Toror to generate quartz-free rocks; it is normally of a potassic type, K-feldspar developing usually along narrow channels. There is some development of aegirine, sodic amphibole and biotite. There is an intense brecciation of the country rocks in places, which may extend up to 200 m from the carbonatite contact. The trachytes consist of fluidally textured K-feldspar with a little limonite, and with sparse phenocrysts of orthoclase and sanidine. Fluorite is abundant in one trachyte sheet together with largely altered aegirine; there is also a distinctive group of aegirine trachytes which are free of fluorite. Sutherland (1965) describes the trachytes in some detail, shows that many are highly potassic and that they encompass orthoclasites, as well as potassic feldspathic fenites and fenite breccias. The phonolites contain phenocrysts of nepheline, feldspar and pyroxene, the last of which are commonly zoned from diopsidic cores to aegirine-augite and occasionally aegirine. Nepheline may be more abundant than feldspar. The fluidal groundmass is of feldspar, nepheline and aegirine with accessories including titanite, zircon, magnetite, natrolite, cancrinite, calcite, rare phlogopite, sodalite or analcime and pectolite. Biotite phonolites are rare; they contain a colourless garnet and aenigmatite. The nephelinites comprise up to 20% nepheline, microphenocrysts of unzoned aegirine-augite, natrolite, cancrinite, iron oxides and aegirine. One intrusion of nephelinite contains numerous xenoliths of ijolite with melilite, wollastonite and melanite. Analyses of phonolite and trachyte are given by King and Sutherland (1966) and of various types of trachyte by Sutherland (1965). Bell and Powell (1970) give Sr isotope data and Rb and Sr abundances for five specimens of phonolite, nephelinite, trachyte and carbonatite.

K-Ar on an analcime phonolite gave 15.5(0.5 Ma (N.J. Snelling, quoted by King et al., 1972).
BELL, K. and POWELL, J.L. 1970. Strontium isotopic studies of alkalic rocks: the alkalic complexes of eastern Uganda. Bulletin of the Geological Society of America, 81: 3481-90.Du BOIS, C.G.B. 1959. The Toror Hills alkaline complex, central Karamoja, Uganda. 20th International Geological Congress, Mexico; Session Asociation de Servicios Geologicos Africanos, 303-8.HYTONEN, K. 1959. On the petrology and mineralogy of some alkaline volcanic rocks of Toror Hills, Mt. Moroto, and Morulinga in Karamoja, northeastern Uganda. Bulletin de la Commission Geologique de Finlande, 184: 75-132.KING, B.C. and SUTHERLAND, D.S. 1966. The carbonatite complexes of eastern Uganda. In O.F. Tuttle and J. Gittins (eds) Carbonatites. 73-126. John Wiley, New York.KING, B.C., LE BAS, M.J. and SUTHERLAND, D.S. 1972. The history of the alkaline volcanoes and intrusive complexes of eastern Uganda and western Kenya. Journal of the Geological Society, London, 128: 173-205.SUTHERLAND, D.S. 1965. Potash-trachytes and ultra-potassic rocks associated with the carbonatite complex of the Toror Hills, Uganda. Mineralogical Magazine, 35: 363-78.
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