Buji

Approximately circular in shape and extending over 77 km2 the Buji complex is in contact with Rukuba (No. 122-00-033) lying to the west. It comprises two superimposed ring complexes the earlier eastern one being partly obliterated by the western. The eastern centre is composed almost entirely of volcanic rocks and high level intrusions, whereas the western one consists essentially of granites. Activity was initiated by development of vents, of which a dozen were recognised by MacLeod (Buchanan et al., 1971), along a ring-fault which produced a large caldera in which rhyolites, dominantly welded tuffs and pyroclastic flows (Imeokparia, 1986), accumulated and of which only restricted outcrops are now preserved within the caldera. Primary mafic minerals within these rocks have been altered but Imeokparia (1986) reports the presence of tiny replacement laths of aegirine and arfvedsonite-riebeckite. This volcanic sequence was followed by intrusion of rhyolites which are thought to have been emplaced beneath the earlier volcanics and which occupy a major part of the eastern centre. In the south the rhyolites form a swarm of dykes and sheets whereas in the central and northern parts they form ring structures. These rhyolites were then intruded by a large plug of quartz-hedenbergite-fayalite porphyry in which pyroxene phenocrysts are often mantled by needles of blue-green amphibole and arfvedsonite occurs in the groundmass; aegirine is rare. This cycle of activity closed with emplacement of a discontinuous and in places very narrow ring-dyke of quartz-feldspar porphyry. It contains phenocrysts of quartz and alkali feldspar and microphenocrysts of sodic amphibole and pyroxene that poikilitically enclose groundmass feldspar and quartz. There followed intrusion of dykes of gabbro and dolerite. The activity based on the western ring-complex commenced with intrusion of felsite and quartz porphyry cone sheets. There followed emplacement of a narrow ring-dyke of aegirine microgranite which contains phenocrysts of microcline perthite in a groundmass of albite, quartz and aegirine but in the southern section of the dyke blue sodic amphibole and biotite substitute for the aegirine. A pluton of biotite granite up to 8 km in diameter cuts the aegirine microgranite and this was followed on the western margin by intrusion of a riebeckite granite. The latter rock consists of quartz, microcline perthite, albite and intergrowths of riebeckite and aegirine, abundant fluorite and accessory pyrochlore, astrophyllite and monazite. Pegmatites are widely developed in which riebeckite may reach 10 cm in length and assume a wide variety of habits. A sodic amphibole analysis is given by Borley (1963) and analyses of 49 rocks, including some trace elements, are presented and discussed by Imeokparia (1986).