Amo And Rukuba

Amo is perhaps the best example of a ring-complex in the Nigerian Younger granite province, being built of a succession of granite ring-dykes and plutons. The concentric distribution is marred, however, by a later intrusion cutting the complex and extending to the northwest, while on the southern side the rings are also lost against the northern part of the Rukuba complex, although poor exposure has prevented determination of the relative ages of the two. Activity was initiated by cone-sheet injection these being particularly abundant around the northeastern margin. There then followed emplacement of a narrow ring-dyke of porphyritic granite, which is only preserved along the northern margin, containing crowded phenocrysts of orthoclase and rare pyroxene in a matrix of alkali feldspar, quartz, a green amphibole, altered fayalite and a little biotite. The following ring-dyke encompassing the eastern and northern sides of the complex is 19 km in length and of riebeckite-biotite granite, which is mineralogically and texturally variable. Near the outer margin arfvedsonite is partly tranformed to riebeckite (Imeokparia, 1983), while inwards biotite increases and becomes more abundant than amphibole near the inner contact, as does astrophyllite. The amphibole is sometimes mantled by aegirine; fluorite is the most plentiful accessory. The next granite to be emplaced contains hornblende and biotite and this was followed, according to the mapping of MacLeod (Buchanan et al., 1971), by six different biotite granites in one of which moderate amounts of columbite and cassiterite are present. A small riebeckite granite then followed which includes a little aegirine and cryolite, amblygonite and pyrochlore as accessories. Activity closed with injection of a swarm of felsite dykes cutting two of the biotite granites. The Rukuba complex to the south of Amo consists of three biotite granites and a hornblende-biotite granite, the latter being confined to two large roof pendants. Analyses of six samples of the Amo outer riebeckite-biotite ring-dyke including REE, Cs, Hf, Ta and other trace elements are given by Bowden et al. (1976). A geochemical study of the Amo complex has been made by Imeokparia (1983) who only published, however, average values of major and trace elements for each of the major units. U and Th data for a biotite granite are given by Bowden et al. (1981), for Li by Bowden (1966a) and for Zr by Bowden (1966b). Pb, Sr and Nd isotope data for a biotite granite from Amo are in Dickin et al. (1991). Sodic amphibole analyses will be found in Borley (1963) and biotite in Borley (1976a).