Earth SciencesThe geochemistry, tectonic setting and origin of the massive melanocratic amphibolite in the Ilesha schist belt, Southwestern Nigeria.

Abstract

A massive melanocratic amphibolite, (MMA) occurs in Ilesha schist belt within a series of muscovite schists and amphibolite gneiss. Though metamorphosed, MMA shows no obvious textural deformation. Actinolite, tremolite, hornblende and boitite constitute the major minerals in MMA. Minor minerals in MMA include calcite, plagioclase and pyroxene while monazite, zircon and apatite form the accessory minerals. Chemical studies revealed that MMA contains low K2O and Na2O. Its Mg1, Cr and Ni contents are considerably lower than those of similar basalts derived from purely primitive mantle. Enrichment of LREE, negative Eu/Eu* anomaly and occurrence of monazite in its mineralogy are all indications that the precursor magma of MMA contains a sedimentary input. The plots of immobile trace (Zr, Ti, Nb, Y) and rare earth elements during the greenschist-amphibolite metamorphic grade for MMA show that it was derived from a low-K-Tholeiitic magma in a volcanic geotectonic setting (back arc basin).

Global Journal of Pure and Applied Sciences Volume , No 1 January (2001) pp. 85-90