Geochemical And Petrographic Studies Of Lokoja Sandstone: Implications On Source Area Weathering, Provenance, And Tectonic Setting

Ten road cut samples collected from five lithological sections of Lokoja sandstone formation in the Middle Niger basin were investigated using integrated granulometric, petrographic and geochemical analyses. Results of grain size analysis; standard deviation and skewness which ranges from 0.08 to -0.05 and 0.59 to -0.07 respectively suggest very well sorted to moderately well sorted sediments. Lokoja sandstone is strongly coarsely skewed and dominantly leptokurtic implying river laid sediments deposited by low energy current. The graphic mean falls between 0.1 and 1.35 suggesting mainly medium to coarse grained sediments. A mineralogical constituent includes quartz, feldspar, mica, rock fragments, clay matrix and cement fraction. The low quantities of quartz and feldspar classify the sandstone as Lithic Arenite. Heavy mineral petrographic results show that the opaque minerals constitute about 72.99% and non-opaque mineral suites of zircon, tourmaline, rutile, staurolite, sillimanite, garnet, apatite and epidote which is indicative of igneous and metamorphic sources, perhaps from the southwest and north central Basement Complex terrains. The calculated mineral maturity index (MMI) and zircon-tourmaline-rutile ratio (ZTR) indices suggest mineralogically immature to sub-mature sediments. The plot of SiO2 versus Al2O3 + K2O + Na2O reveals semi arid to arid conditions for the Lokoja sandstone with varied maturity. The geochemical datasets reveal mature lithic arenites including sub-greywacke and protoquartzites. The chemical index of alteration (CIA) and mineralogical index of alteration (MIA) values (79.37 and 58.74 respectively) implied that their source area underwent ―intense‖ recycling but ―moderate to high‖ degree of chemical weathering. The discriminant function plot shows that the plotted sandstones were predominantly derived from felsic igneous source. Besides, the lower ratios of Ni/Co, Cr/Ni, Cr/Th, Cr/Sc, Th/Sc, La/Co and Th/Co suggest felsic source rock. The K2O/Na2O versus SiO2 binary tectonic diagram shows source materials in the field of oceanic island arc. The Th-Co-Zr/10 and Th-Sc-Zr/10 ternary diagrams also plotted in the field of oceanic island arc.