Geochemistry and U-Pb zircon geochronology of Iwo quartz potassic syenite, southwestern Nigeria: Constraints on petrogenesis, timing of deformation and terrane amalgamation

Abstract

Petrography, whole rock geochemistry, and in-situ U-Pb zircon geochronology on quartz potassic syenite in Iwo area, southwestern Nigeria have been performed. The results indicate that the syenite was syn-tectonically emplaced during the Pan-African orogeny and shows heterogeneous deformation. On the basis of geochemical data, the rock is defined as high potassic calc-alkaline derived from metasomatised mantle with slight crustal contamination. The small negative europium (Eu) anomalies (Eu/Eu∗ = 0.87) displayed by the rock could be an indication of separation of amphibole and pyroxene. In-situ Laser Ablation Inductively Coupled Plasma Mass Spectrometry U-Pb zircon dating yielded a Concordia age of 642 ± 6 Ma which could be taken as that of emplacement and deformation. The relative position of the Iwo quartz potassic syenite in respect to the other undeformed syenite bodies in southwestern Nigeria and the main Pan-African suture zone suggest amalgamation of at least four terranes or micro blocks in the southwestern Nigeria during the Neoproterozoic time. The emplacement and deformation of the Iwo quartz potassic syenite were synchronous with basin closure around Ilesha-Ife and subsequent collision of two terranes/micro blocks. The collision resulted from a west-dipping subduction related tectonics which was different from the main Pan-African subduction zone at the margin of the West African craton. The amalgamation of these terranes contributed to the accretion of the Gondwanaland in the southern part of the Trans Saharan fold belt.