Paper Number: 2944
Pressure–temperature conditions and inherent metamorphic signatures in the Nigerian basement complex: A review
Amuda, A.K1, Danbatta, U.A1, Najime, T1, Ibrahim, A.A1, Salau, S.L2, Elatikpo, S.M1 and Girei, M.B3
1Department of Geology, Ahmadu Bello University, Zaria, Kaduna State, Nigeria; akabdulgafar@abu.edu.ng
2Nigeria Mining Cadastre Office, Ministry of Mines and Steel Development, Abuja, Nigeria.
3 Nigerian Institute of Mining and Geosciences, Jos, Nigeria.
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A general review of the transformation of pre-existing rocks through a continuum of changes in mineralogy and fabric produced by various types of metamorphism as documented in the Nigerian basement complex has been attempted. The basement is mainly characterized by a regional penetrative metamorphic fabric and development of new minerals to such an extent that original stratigraphic features and relations are obscured. However, the contrasts in protoliths compared with adjoining terrains (older, younger granites and volcanics) are demonstrable. The major metamorphic lithologies include migmatite gneisses, gneisses, metasediments and metavolcanics. Regional metamorphism predominates, but contact and dynamic/cataclastic metamorphism is also common, with a few cases of hydrothermal metamorphism. The metamorphic lithologies within the basement also comprise variable metamorphic mineral assemblages (greenschist through amphibolite to granulite facies) with characteristic index minerals (chlorite, biotite, garnet, kyanite, sillimanite, staurolite and others). The near absence of staurolite in the southeastern area and its presence in the southwestern and northern parts is indicative of high and low oxygen fugacities in the areas respectively. The distribution of isograds from geothermometric and geobarometric measurements indicates a general zonation from low to medium grade, Buchan type metamorphism (≤ 550⁰C and ≤ 1.5–3 kbar) to limited higher grade, Barrovian type metamorphism (550–700⁰C and 3–8 kbar or more) in southwestern and northwestern Nigeria. While the northeastern and southeastern parts mainly had medium to high grade, Barrovian type metamorphism with anatexis in places. Structurally, the basement records polyphase deformation. The earlier phases were characterized by predominant NNW–SSE trends of foliation and were pre-Pan-African. Conversely, the other phase occurred during the Pan-African Orogeny with imprints that dominantly trend NNE–SSW.