Abstract

Heavy-mineral assemblages from the rivers and wadis in the River Nile drainage basin reflect their origins in three petrologically and tectonically distinct source terranes that supply sediment to the River Nile and delta. The White Nile carries heavy-mineral assemblages dominated by calcic amphibole, epidote, garnet, staurolite, aluminosilicates, and opaques (ilmenite and titanomagnetite) derived from amphibolite- and granulite-grade metamorphic rocks of the Central African Province. The Blue Nile and Atbara Rivers carry assemblages dominated by calcic amphibole, clinopyroxene, and opaques derived from alkali and tholeiitic basalts of the Ethiopian Highlands. Wadis (dry river beds) draining the Red Sea Hills contribute predominantly clinopyroxene, epidote, calcic amphibole, and opaques eroded from ophiolite complexes, calc-alkaline volcanics, and granitoids as well as minor metamorphic minerals from amphibolite-grade metapelites. Detrital ilmenite grains from the three sources are distinguished on the basis of TiO 2 , Al 2 O 3 , Cr 2 O 3 , MgO, ZnO, and V 2 O 3 but not MnO contents determined by electron microprobe analysis. These data corroborate findings of earlier workers with respect to Al 2 O 3 , MgO, V 2 O 3 and MnO in detrital ilmenite. Submicroscopic inclusions of quartz and sphene recognized on the basis of elevated SiO 2 and CaO contents in ilmenite also distinguish different sources. Statistical and graphical analyses of the compositional data lead to the same conclusions about ilmenite chemistry and source areas. Opaque oxide (ilmenite and titanomagnetite) grains also have textural features unique to their sources. Though most grains are texturally homogeneous, ilmenite from the Central African Province contains exsolution lamellae in one crystallographic direction (0001) and titanomagnetite from the Red Sea Hills showing exsolution textures has octahedral {111} inter-growths. Taken together, compositions and textures of detrital opaque oxide grains from Nile River sediment sources can be used as provenance indicators of Nile delta sediments. This study also provides ties between igneous, metamorphic, and sedimentary petrology that are relevant for provenance studies of ancient river systems.

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