Relationship between tectonics, metamorphism and metamorphic textures in Oban Massif (Nigeria), southeastern sector of the West African shield

Abstract

Field and petrographic investigations of parts of the Oban Massif, which itself constitutes part of the southeastern margin of the mobile zone separating West Africa Shield, reveals that the metamorphic rocks which consist a thick succession of metapelites, semipelites and metabasic rocks have undergone progressive Dalradian type metamorphism recorded by the index minerals chlorite, biotite and almandine garnet. The distribution of the isogrades indicates a N-S increase in the intensity of metamorphism from greenschist to lower amphibolite facies. The metasediments represent remnants of Proterozoic supracrustal cover rocks infolded within the reactivated Archean migmatite-gneiss complex and were subsequently affected by the Pan African orogeny. Rock microfabrics and textures indicate that the area had been affected by three phases of deformation which are correlatable with episodes of metamorphism. It is obvious from the texturaland metamorphic evidence that the Pan-African event reached a status of an orogeny in southeastern basement complex - Orb