GEOLOGY AND GEOCHEMISTRY OF EL TUWAL OPHIOLITIC COMPLEX AND ENVIRON S —_EAST CENTRAL SUDAN

Abstract

ABSTRACT Some 22400 sq. km have been regionally mapped around El Tuwal ophiolitic complex, east-central Sudanhtogether with a detailed geological map of the complex. The area is composed of gneisses, schists, quartzites and marbles, together with the ophiolitic rocks of E1 Tuwal complex, and basic metavolcanics of island-arc character. The layered sequences are intruded by syn-orogenic, I-type granitoids and unfoliated, “younger granite”, A-type complexes. The latter occur as hilly massifs rising above the subdued, flat clayey plains. Numerous dykes and quartz veins cut the older sequences among which is a prominent andesite dyke, some 20 km. long, cutting the syn-orogenic granites. .Late Cretaceous Nubian sediments overlie unconformably the basement sequences along the northeastem margin of the Butana area, and southeast of El Tuwal ophiolitic complex. Tertiary to Quaternary clayey cover usually obscures the subdued layered sequences. The ophiolitic rocks are clearly emplaced onto the other basement rocks by a SE-dipping thrust fault. They are composed of serpentinites and minor pyroxenites, which occupy the base of the complex, followed by gabbros and pillowed basic volcanics. Juxtaposed with structural contacts are the basic island-arc metavolcanics. The syn-orogenic granites cover a wide compositional spectrum exemplified by biotite and hornblende granites, granodiorites and tonalities to gabbros in decreasing abundance. The younger granite complexes are made up of granites, syenites, gabbros and volcanics in decreasing order of abundance. New geochemical data revealed that the ophiolitic rocks, associated arc metavolcanics and the syn-orogenic granitoids have geochemical signatures of subduction zone magmatism found in supra-subduction zone ophiolites and

other arc rocks. The presence of podiform chromites in the studied serpentinites is considered to be typical of supra-subduction ophiolites with Cr# similar to those of subvolcanic ophiolites and boninite-derived chromites particularly those thought to be associated with fore-arc rifling. The “younger granite” complexes represent alkaline, within-plate continental igneous activity. The amphibolite facies gneisses, schists and marbles possibly represent earlier continental shelf sediments of the Saharan Metacraton. The ophiolitic rocks and the metavolcanics possibly represent ensimatic, thrusted material after the collision of the Nubian-Arabian Shield with the older sialic continental Saharan Metacraton during the late Proterozoic, Pan-African tectono-thermal event. The geological setting of the Butana area favours gold mineralization, and the granitoid rocks and the marbles can sustain successful building materials industry.