Retrograde Assemblages of Gneisses at Arigidi and Erusu Areas of Southwestern Nigeria

Abstract The study is aimed to unravel the presence of low-grade minerals in the migmatite terrain of Southwestern Nigeria. Mineral and textural relationship of mineral assemblages were determined using a petrographic microscope, while chemical composition was analyzed using XRF and LA-ICP-MS. Petrographic studies show the mineral assemblage, biotite + plagioclase + chlorite + epidote + quartz + muscovite + ilmenite ± calcite. There is intense alteration of both biotite and plagioclase. Textural evidence shows chloritization of biotite and epidotization of both biotite and plagioclase. Epidotization of biotite is basically at the edges of biotite while that of plagioclase in more intense at the core of grains. Primary foliation is defined by mineralogical banding of quartzo-feldspathic and mafic minerals, while secondary foliation is defined by both biotite and chlorite. SiO2 contents are high in both gneisses (> 60 wt %), while there is a high depletion of REE. The results from field observations, mineral assemblages and textural relationship suggests that deformation due to shearing could have enhanced the retrograde chloritization of gneisses. Shearing could have created a path for the influx of fluid (H2O & CO2) thus triggering alteration of both biotite and plagioclase. Hydrothermal fluids containing K+ can be said to have initiated the retrograde process which gave rise to reactions that produces secondary low grade minerals; chlorite, epidote and muscovite.