Abstract
Petrographic approach is used to separate the Pan-African basement rock assemblages and younger within-plate igneous rocks into three contrasting rock groups: namely, metamorphic, igneous and volcaniclastic rocks. The metamorphic rocks, the mineralogy of which reflect pelitic to semi-pelitic sedimentary protolith, are dominated by phyllites and schists and subcrop mostly in the Concession 32 area. This rock association represents deep marine sedimentary facies that had been subsequently regionally metamorphosed to greenschist facies. Banded gneisses are encountered mostly in the Concession 71 area. Cordierite and aluminium silicate minerals are lacking. This, together with the presence of well- developed biotite flakes within the gneisses, suggests that these rocks were
metamorphosed under low to medium grade regional metamorphism. The igneous products are petrographically diverse ranging from mafic to felsic in character. Fourteen rock types are distinguished: granites, granodiorites, quartz monzonites, diorites,
gabbros, syenites, basalts, trachyte, andesites, dacites, rhyolites as well as volcaniclastics. Some of the basaltic rocks are somewhat evolved and could be described as basaltic andesites, basaltic trachyandesites and trachyandesites. Granites, granodiorites and quartz monzonites, occupy the greatest part of the subcroped surface area while syenites, trachytes, dacites and gabbros are the least represented rocks. The volcaniclastic rocks have been described as tuff-breccia, lapilli tuffs and pyroclatics. Most of these rocks are unlikely to be in-situ pyroclastites but more likely they represent the many reworked volcaniclastics derived from scattered highly explosive eruptive craters or maars.