Jos-Bukuru And Shere

Jos-Bukuru is a large ring-complex lying approximately at the centre of the province and covers about 430 km2. In the northeastern corner a small ring structure, the Shere complex, is superimposed on Jos-Bukuru. An elliptical ring-fracture extending around the northern, western and southern margins has determined the form of the complex and extends eastwards to encompass the Jarawa centre (No. 122-00-030). The whole area within the fracture has subsided giving rise, according to MacLeod and Berridge (Buchanan et al., 1971), to large-scale cauldron subsidence. Most of the rocks of Jos-Bukuru are not alkaline only one group, the Naraguta quartz-pyroxene porphyry, being marginally peralkaline. In contrast, many of the rocks building the Shere complex are peralkaline. Jos-Bukuru has been divided by MacLeod and Berridge (Buchanan et al., 1971) into an early volcanic cycle and subsequent early and central granite cycles. The rocks of the early volcanic cycle comprise largely ignimbrites, some vent agglomerates and many felsite dykes. Most of these rocks are rhyolites, the only exception being two somewhat altered basaltic lava flows. The early granite cycle comprises eight distinct granite units and commenced with emplacement of a porphyritic granite in the main ring fractures and along internal fractures; this rock contains hedenbergite and hornblende and, in some facies, fayalite. All the other granites of the early cycle are biotite granites. The central granite cycle commenced with intrusion of a hornblende-fayalite granite similar to the rock opening the early cycle. This was followed by three further biotite granites. Geological and petrographical accounts of all the major rock units of the Jos-Bukuru complex will be found in Buchanan et al. (1971). Modal and chemical data for major elements will be found in Badejoko (1975) and Buchanan et al. (1971) and Imeokparia (1980 and 1989) gives mean and range values for trace and major elements for all the main intrusive units. Some REE data are presented by Bowden et al. (1976), and Aleksiyev (1970) and Ajakaiye (1976) show that the major gravity low of the province is located at Jos-Bukuru. The Shere complex is located in the northeastern sector of Jos-Bukuru where it gives rise to a prominent hill. After initial intrusion of a rock described by Berridge (Buchanan et al., 1971) as quartz-olivine gabbro, in which the olivine is tending to fayalite, the feldspar zoned to albite-oligoclase and generally with rims of perthite, and hornblende and biotite abundant, there was emplacement of hornblende-fayalite granite that forms a substantial intrusion in the middle of the complex and which in one area contains pyroxene and amphibole that sometimes has rims of a blue sodic variety. The next intrusion is of biotite granite but there was then a marked trend to peralkaline compositions with the successive intrusion of three arfvedsonite/riebeckite granites as irregular but somewhat arcuate intrusions around the central hornblende-fayalite granite. The earliest peralkaline granite consists of perthite, abundant riebeckite (arfvedsonite according to Kinnaird, 1981), aegirine, which is generally intergrown with the amphibole, and accessory fluorite, pyrochlore, zircon and in one area astrophyllite. This granite was then followed by riebeckite-biotite and riebeckite granites. The final igneous event was intrusion of a composite dyke over 40 km long and up to 200 m wide which arcs through 90o and cuts across the Shere complex and the northeastern part of Jos-Bukuru as well as the adjacent Jarawa complex (No. 122-00-030). The dyke has gabbroic margins and a granitic core with phenocrysts of quartz, hedenbergite, fayalite and sodic hornblende. Shere is described in some detail by Berridge (Buchanan et al., 1971) and rock analyses will be found in MacLeod et al. (1971), and averaged compositions and ranges for all main rock types are given by Imeokparia and Badejoko (1990). Data for Li and F in 11 Shere granites and for one from Jos-Bukuru are in Bowden (1966a) and for Zr in Bowden (1966b). Pb, Sr and Nd isotope data for three granites will be found in Dickin et al. (1991). Analyses of three riebeckites are given by Borley (1963) and of ferrocolumbites and a zoned columbite by Fadipe (1987).