Petrology and amphibole-plagioclase chemistry of amphibolites in the Ilesa area, Southwestern Nigeria

Abstract

Amphibolites of Itagunmodi and Igun, within the Ilesa Area were studied in order to determine their petrochemical affinities, tectonic setting and conditions of formation; ultimately for an improved understanding of the evolution of the Ilesa Schist Belt. The amphibolites are massive in texture and occur as small to large lensoid boulders associated with quartz-biotite schist, quartzite and talc-chlorite schist. They comprise magnesio-hornblende, actinolite, labrodorite, albite and accessory titanite. Average elemental composition and ratios in the amphibolites were: Ba (872 ppm), Sr (609 ppm), Ni (89 ppm), Rb (72 ppm), Y (27 ppm), Zr (168 ppm), Nb (83 ppm), Cr (200 ppm), La (34 ppm), Cr/Ni (2.30), Y/Nb (0.63), Rb/Sr (0.10), Nb/La (2.55), and Zr/Nb (4.62). The major and trace element characteristics revealed that they are ortho-amphibolites with tholeiitic and calc-alkaline affinities, plotting in Within Plate Basalt field of the Zr vs Zr/Y diagram. The amphibolites exhibit enhanced mid-ocean ridge basalt (E-MORB) elemental characteristics with LREE > HREE enrichment and slight negative Eu anomaly. The calc-alkalic affinity of some of the amphibolites is a result of crustal contamination, and not an arc-related signature as earlier proposed. Elemental abundances and ratios revealed that the amphibolites were formed in an extensional tectonic setting where mantle plume activity and lithospheric contamination occurred. The mineralogical assemblage reflects amphibolite facies metamorphism, at temperature of formation of 450–687 °C and pressure of 0.5–4.3 kbar constrained by plagioclase-hornblende thermobarometry.