Abstract

Bode Saadu area, southwestern Nigeria is underlain by metasedimentary and metaigneous rocks including orthogneisses which have been intruded by largely undeformed granitic rocks of probable Pan-African (ca. 600Ma) age. The metamorphic rocks have been subjected to several phases of both ductile and brittle deformation. Three suites of orthogneisses of granodioritic composition have been identified in the area, locally they contain xenoliths of metasediments. Geochemical analyses show that they are calc-alkaline, magnesian, metaluminous and possess the characteristics of granitoids emplaced in magmatic arcs.LA-ICP-MS in-situ dating of zircons in these orthogneisses has yielded upper Concordia intercept Palaeoproterozoic ages of 2236±29Ma, 2228±32Ma and 2179±28Ma. These indicate the times of crystallization of these rocks and attest to a long period of magmatism associated with the Eburnean orogeny in this part of Nigeria. The gneisses bear the imprints of several deformation events in the form of foliations, folds and shear zones. The geochronological data enable constraints to be placed on the timing of the development of some of these structures and also indicate that some of the metasedimentary units are at least of Palaeoproterozoic age.Similar Palaeoproterozoic orthogneisses have been documented in some other parts of southwestern Nigeria, in Central Hoggar area of the Tuareg Shield and in the Borborema Province of Brazil and support their correlation as part of West Gondwana.

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