Metamorphic evolution of a metabasite dyke from Ogbagi Akoko area, Southwestern Nigeria

Abstract

Petrography, mineral chemistry and whole rock analyses were conducted on a metabasite dyke hosted by pelitic gneiss (metapelite) in Ogbagi Akoko area with the aim of determining its metamorphic evolution. The dyke is about 40 cm wide and traceable for some meters along strike. It has a discordant contact relationship that is pencil sharp with the host pelitic gneiss and lack any internal fabric. The mineral assemblage includes: garnet +biotite + plagioclase + quartz + orthopyroxene + K-feldspar + ilmenite, with apatite occurring as an accessory mineral. Plagioclase has anorthite content of about 76 (An₇₆) suggesting bytownite (An₇₆ Ab₂₄ Or₀) while garnets are poikiloblastic almandine-pyrope rich with XFe of 0.73-0.75. Orthopyroxene has a composition of Wo₀ En₅₀ Fs₅₀ suggesting hypersthene that is depleted in Ca. The mineral assemblage, presence of bytownite, absence of amphibole and lack of internal fabric indicate that the rock has undergone metamorphism at a high temperature and moderate pressure without deformation at granulite facies metamorphism. The absence of deformation structure in the dyke is an indication that the intrusion occurred after the cessation of the regional metamorphism but prevalent high temperature that recrystallized the rock. The whole rock composition and some elemental ratios of the dyke indicate that the metamorphism had little or no effect on the bulk chemistry of the rock.