Geochemistry and petrogenetic evolution of metasedimentary rocks in Bunu Area,part of Kabba-Lokoja-Igarra schist belt,SW Nigeria

Field and geochemical studies of the Bunu area, SW Nigeria, were carried out on gneiss-metasedimentary rocks that consist of migmatised gneiss, quartz-mica schist, and foliated and massive quartzites. These gneiss-metasedimentary rocks are interbedded with meta-igneous rock (amphibolite), all of which are intruded by granitoid and cut by basic and felsic dykes. Geochemical data on major, trace, and rare Earth elements of these rocks from the area were used to speculate on the petrogenetic and geodynamic evolution of the rocks in the area. Compositionally, the metasedimentary rocks in the area have a restricted range of major oxides such as SiO₂ and Al₂O₃ with low average values of Fe₂O₃, MnO, CaO, and P₂O₅. Concentrations of average values of HSFE such as Zr, Nb, and Y are moderately high in migmatitic gneiss, quartz-mica schist, and low in both massive and foliated quartzite. ∑LREE average values are 124.25, 132.41, and 18.64 ppm respectively for migmatite gneiss, quartz-mica schist, and low in both massive and foliated quartzite. These rocks are also generally enriched in Ba, Cs, Pb, U, Cr, Rb, and Zr and depleted in Be, Sr, Ti, Mo, Th, and W. This enrichment–depletion in major oxides and trace elements of the rocks in this area is probably a consequence of the movement of metamorphic remobilized fluids within the rocks in the area during the Pan-African or earlier events as noted in the adjacent Egbe-Isanlu Schist belt. A further geochemical characterisation of the rocks in the area using Log (Na₂O/K₂O) vs Log (SiO₂/Al₂O₃ indicates that while the protolith of migmatised gneiss and quartz-mica schist are greywackes to litharenite in compositions that of quartzites are sublitharenite to quartz arenite. On the ternary plot of Al₂O₃-(CaO?+?Na₂O)-K₂O for metasedimentary rocks in the area, most migmatitic gneiss and quartz-mica schist rocks plot close to average shale while quartzites mostly tend to illite compositions and almost toward the Al₂O₃-apex of the diagram attesting to depletion of CaO and NaO with the removal of K₂O. CIA vs PIA plot of the rocks in the area shows that while migmatitic-gneiss and quartz-mica schist are moderately weathered, both massive and foliated quartzites are high to extremely weathered. They are all derived mostly from predominantly felsic igneous to quartzose sedimentary provenance and mostly emplaced in passive continental margins. These nature of the protoliths suggests gradual subsidence of the basin during its genesis, and/or tectonic stable or inactive environment from which the sediments were derived.