Fort Portal And Kasekere (Rusekere)

The volcanic rocks of the Fort Portal area extend over 142 km2 but have a volume of only 0.25 km3 (Nixon and Hornung, 1973). Kasekere is a contemporaneous field lying some 20 km to the northeast of the main Fort Portal field but extending over only 2.5 km2. Forty nine volcanic craters have been distinguished along several linear zones within the Fort Portal field. Most of the craters are associated with tuff cones which have steep outer slopes; the cones reach 150 m in height. The majority of the craters have diameters within the range 50-160 m but the largest (Kyeganywa) is 650 m in diameter, the smallest only 10 m. Many of the smaller craters are 'blow holes' with little ejectamenta. Nine of the craters contain lakes. The principal volcanic rocks are tuffs which are divided by Nixon and Hornung (1973) into friable lapilli tuffs and welded flaggy tuffs. The former are the more abundant and build most of the volcanic cones. They consist of lava droplets up to 15 mm in diameter cored commonly by calcite or phlogopite and, more rarely, by clinopyroxene, magnetite or basement rock fragments; the rims contain calcite and melilite. Precambrian basement fragments occur in the tuffs and lower crustal granulite xenoliths are described by Thomas and Nixon (1987). Garnet and spinel lherzolite xenoliths are reported from the Kalyango cone (Kapustin and Polyakov, 1985). The flaggy tuffs occur mostly away from the volcanic centres as beds up to several metres thick. Approximately 15% of the tuff is basement fragments, 25% carbonate lapilli, 10% xenocrysts of quartz and plagioclase, and the remainder calcite and hydrous calcium silicates (Barker and Nixon, 1989). These tuffs are surge deposits and in one area of the field the surface is marked by thousands of blisters, from 10 cm to 6 m in diameter and up to 1.5 m high; these mark the sites of springs produced when the surges covered swampy ground. Less than 1% of the Fort Portal igneous rocks are carbonatite lava flows and dykes. The largest occurrence is a flow which breached the northwestern part of the Kalyango crater, has a length of over 500 m, and varies between 1 and 5 m in thickness. Carbonatite also forms a 140 metre-diameter ring-dyke at Makome, a radial dyke at Kyenganywa, and a plug at Lake Balituma (Nixon and Hornung, 1973). The rock of the Kalyango flow contains phenocrysts and xenocrysts of phlogopite, which enclose sparse clinopyroxene and olivine, the phenocrysts having rims of monticellite and reinhardbraunsite, together with octahedra of magnetite, rimmed by spinel, and rare quartz xenocrysts rimmed by rosenhahnite. The groundmass in the freshest rocks consists of calcite (55%), hydroxyapatite (10%), spurrite (30%), spinel, periclase (3%), perovskite (3%) and sparse barite and pyrrhotite. Irregular vesicles up to a centimetre long contain clear calcite, succeeded inwards by radiating tufts of jennite and 'hairs' of thaumasite. Photomicrographs illustrating this rock are given by Barker and Nixon (1989) and whole rock analyses will be found in the same paper and Nixon and Hornung (1973). 87Sr/86Sr ratios of 0.7039 and 0.7041 on two samples of carbonatite lava from Kalyango were determined by Bell and Powell (1969), and Bell and Blenkinsop report 87Sr/86Sr and 143Nd/144Nd ratios. Deines and Gold (1973) and Vinogradov et al. (1980) give C and O isotope data for the Kalyango flow. Romanchev and Sokolov (1980) report homogenization temperatures from fluid inclusions in calcite of 630-650°C and in apatite from Kalyango lava of 690-710°C.Kasekere comprises several scoria cones, one with a crater lake, but the widespread carbonate-rich tuffs of Fort Portal do not occur here. However, the present author has seen two dykes cutting the Kasekere tuffs which contain acicular calcite crystals up to several centimetres long orientated perpendicular to the dyke margins.