Tectonic Setting of the Dioritic and Ultra-Mafic Intrusives of Obudu Area, South-East Nigeria

This is the first-time report of a ‘back-arc type’ extensional tectonic setting for a granitoid suit of rocks in the Benin-Nigeria Shield. Eastern Nigerian basement complex forms the eastern flank of the Trans-Saharan orogenic belt within the Benin-Nigerian Shield areas. The study area (Obudu massif) which is central among the westerly protruding massifs forming the easternmost margins of the Nigerian metamorphic basement is dominantly underlain by migmatites and gneisses which are intruded by large volumes of granitic, dioritic and ultramafic rocks. Petrographic, whole rock geochemistry and mineral chemistry analyses were focused on the dioritic and especially the ultramafic rocks. The result of this research shows that the meta-quartz diorite is a calc-alkaline (Trachyandesite), while the meta-ultramafite is a picritic, sub-alkaline, N-MORB with arc affinities. Their tectonic affinities and settings are that of an early back-arc spreading center. This discovery of a Precambrian back-arc setting within the Benin-Nigerian Shield will go a long way in the overall description and modelling of the Pan-African plate collisional architecture of the Trans-Saharan Orogen within the Benin-Nigerian Shield.