Tororo

Tororo rock is a precipitous hill of carbonatite rising over 300 m above the surrounding plain. The carbonatite also extends southwards to form Reservoir Hill. Further to the south again, and separated by an area of fenites, is Limekiln Hill the carbonatites of which invaded ijolites and nepheline syenite. The main mass of carbonatite was apparently emplaced as a series of rings with the dips of the concentric banded structures decreasing outwards (Williams, 1952). A similar structure pertains at Limekiln Hill with the vertical masses separated by bands of xenolithic material derived from the walls. Wall rock xenoliths are feldspathized around the edges, but in ijolitic xenoliths the pyroxene shows alteration to biotite, while pyroxenite-melteigite xenoliths are altered along calcite veins to sodic amphibole similar to that found in fenites. Banding is widespread in the carbonatites and generally defined by layers rich in aegirine-augite, magnetite or biotite. Apatite, fluorite and pyrochlore also occur in the carbonatites. Agglomerate dykes up to 12 m wide cut the carbonatite and include fragments of both basement rocks and ijolite which are carbonated and with nepheline replaced by cancrinite and zeolites. The ijolites, which are restricted to the northern and northwestern lower slopes of Limekiln Hill, are extensively invaded by carbonatite and feldspathized. The contact with syenitic fenite is abrupt and dips outwards at 45°. Fragments of melteigite-pyroxenite occur in the ijolites, which are typically heterogeneous. Pyroxene varies from diopside through to aegirine-augite and the nepheline is commonly replaced by cancrinite. Later veins and patches of nepheline and bladed wollastonite were followed by the development of melanite. Nepheline syenites are characterized by euhedral nepheline and acicular aegirine with K-feldspar forming clear, poikilitic plates and a little albite in some rocks. These rocks are to be distinguished from feldspathized ijolites (King and Sutherland, 1966), but the nepheline syenites have themselves also been feldspathized in the vicinity of carbonatite. The extensive aureole of fenites around the complex is superimposed on Precambrian granites that are shattered along networks of irregular channels and with the quartz rimmed, then replaced completely, by aegirine and sodic amphibole. The ultimate product of the fenitization is a quartz-free syenite of variably perthitic feldspar and sheaves of acicular aegirine. Potassic (K2O = 13.7%) trachyte dykes 30 cm or so wide cut the fenites to the east of the complex. They consist of cloudy K-feldspar in a groundmass of aligned feldspar laths with aggregates of limonite. Sutherland (1980) has described carbonatite with spherical structures of magnetite and calcite and mafic dykes containing rounded fragments, both of which are ascribed to fluidisation processes. Calcite and shortite occurring as inclusions in apatite have been described by Aspden (1981). Analyses of carbonates and apatite are given in Dawson et al. (1996). Data on C and O isotopes in carbonatites will be found in Denaeyer (1970) and Nelson et al. (1988), trace elements, including REE, and Rb-Sr and Sm-Nd isotopic data for a carbonatite in Nelson et al. (1988), Nd and Sr isotopic data in Bell and Blenkinsop (1987), and U, Th and Pb isotopic data in Lancelot and Allegre (1974) and Nelson et al. (1988).