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.
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Alkaline rocks are concentrated in two areas in Nigeria; the Jos Plateau of central and northern Nigeria and the Benue Valley in the southeast and along the border with Cameroon. The Jos Plateau suite consists of a remarkable series of essentially granitic intrusions, which are known as the Younger Granites and also as the Nigerian Anorogenic ring complexes, with less extensive extrusive rocks. Many of the granite complexes form spectacular ring structures, some formed of numerous intrusions. Non-alkaline rocks are more voluminous than alkaline ones amongst the Younger Granites but most occurrences include a greater or lesser proportion of peralkaline granite and/or syenite, with extrusive peralkaline rocks also present in some centres. A preliminary review of the province was given by Jacobson et al. (1958) with much fuller accounts of most of the individual complexes by MacLeod et al. (1971) and Buchanan et al. (1971). Further detailed accounts of nine of the complexes are in Bennett et al. (1984) and Turner (1976) also reviews the province. The province as a whole is clearly depicted on a geological map at a scale of 1:500,000 compiled by Kinnaird (1981), while a map of the northern part of the province only, at a scale of 1:250,000, has been compiled by Turner (1979). Reviews of the geology and mineralization are given by Bowden et al. (1984), Bowden and Kinnaird (1984), and Pastor and Turaki (1985) while Hossain and Turaki (1983) have compiled a bibliography of the province and the associated tin mineralization.
The volcanic rocks of the Benue Valley consist of a number of lava plateau together with several concentrations of plugs and other minor intrusions. Little detailed work has been done on these rocks and although a proportion of the plugs are known to be alkaline information on the lavas is sparse. Wright (1976) reviewed the data available at that time.
Cutting Archaean basement gneisses and migmatites the Kigom complex of peralkaline granites is approximately circular and covers 356 km2 The earliest intrusion is an arfvedsonite granite that forms a roof pendant enveloped by later riebeckite-aegirine granite.
Kagoro is a substantial granite pluton lying southwest of Kigom. Three types of granite are described by Buchanan et al.
The Ropp complex has a somewhat irregular, elongated triangular shape and has extensive, but outlying, arcuate and polygonal ring-dykes. The complete complex extends over 180 km2 but two thirds of this is underlain by basement rocks within the ring-dykes.
Rising some 60-70 m above surrounding plains of basalt, Umat Hill consists of a plug of phonolitic rocks with basalts on the lower western flank.
The Sara-Fier complex is about 50 km from north to south and consists of five major ring structures and a syenite stock. The four most northerly centres overlap so their relative ages can be determined with certainty and there is a general younging southwards (Turner, 1963 and 1976).
This is an approximately circular, 3 km diameter intrusion lying east of the Pankshin syenite. It is a fayalite-hedenbergite-calcic-sodic amphibole syenite and quartz syenite with a marginal granite (Kinnaird, 1981). Little other information has been gleaned.
According to Kinnaird (1981) this complex, which is situated at the extreme southeastern margin of the Jos Plateau, consists of syenite, granite porphyry and arfvedsonite granite. No other information has been traced.
The third largest complex of the Nigerian Younger Granite province, Sha-Kaleri extends over 640 km2. More than 20 units have been distinguished of which two comprise pre-caldera and intra-caldera agglomerates, tuffs and ignimbrites (MacLeod in Buchanan et al., 1971).
Wei Wei consists of two precipitous peaks rising to over 300 m which have not been mapped in detail. The northern one consists principally of ignimbrites and the southern is probably of biotite granite with an outer zone of riebeckite-annite granite.