The ring complexes of the Nigeria–Niger Younger Granite province occur in three separate districts: Air, the Zinder–é area of S Niger Republic and the Jos Plateau area of Nigeria. Together these form a north-south zone 1300 km long, characterized petrographically by alkaline granites and associated volcanic rocks, and structurally by ring dykes, volcanic cauldrons and high level granite plutons. The three districts are separated by areas of younger sedimentary rocks. Between Air and the S Niger district are Cretaceous rocks, filling the saddle between the sedimentary basins of Iullemmeden and Chad. Between the S Niger and Nigerian districts is an embayment of the Chad basin containing deposits of Pliocene to Pleistocene age (Fig. 1).
The Nigerian district is the best known (Jacobson et al. 1958; Buchanan et al., 1971). Many of the complexes contain rhyolites within areas of cauldron subsidence. Intrusions into and beneath these rhyolites are mainly granitic but include minor syenites and gabbros. The granites contain such mafic minerals as fayalite, sodic amphibole and pyroxene and biotite. By far the most abundant type is biotite granite, often with tin and niobium mineralization. A Jurassic age of about 160 m.y. has been established for the Nigerian Younger Granites (Jacobson et al., 1963).
In the S Niger district (Karpoff 1948; Black 1963) most of the Younger Granite rocks are concentrated in the Mounio massif, which extends for over 100 km south from Gouré Rhyolites occur extensively, and the main intrusive rocks are peralkaline granites and syenites. Traces of cassiterite have