Alkaline ring complexes in Sudan

Abstract

An anorogenic petrographic province containing nearly 100 alkaline ring complexes extends across Sudan. These anorogenic complexes are distinct from late-orogenic granite-gabbro, calcalkaline plutonic centres particularly in the eastern part of Sudan, many of which also take the form of ring complexes. The biggest, most closely spaces, and most numerous ring complexes occur in a discontinuous belt of alkali granites and syenites, rarer foid syenites, and associated extrusive trachytes, rhyolites and ignimbrites exposed intermittently across 100 km from the Bayuda Desert and Nile River Valley near Khartoum to northern Kordofan Province and the Nuba Mountains region of central Sudan. These complexes range in age from Ordovician to Jurassic, but show no progressive change in age or distribution pattern other than a tendency to align locally in NW-trending bands, which might reflect zones of weakness in the underlying basement rocks. Mesozoic syenites and alkali granites forming plugs and ring complexes lie in a belt parallel to the coast in the Red Sea Hills of eastern Sudan. In the north-west of the country, alkali granites and foid syenites of Tertiary age crop out in the basement inlier of J. Uweinat, and Mesozoic granites, syenites and foid syenites form igneous plutons in Equatoria Province in the extreme south.