Provenance, tectonic setting and maturity of the ishara sandstone, south western, Nigeria: insight from major element geochemistry

Major elements geochemical study of the Ishara sandstone of the Ise Formation in the Dahomey Basin, southwest Nigeria, was carried out to infer their provenance, maturity and tectonic setting. This study is based on the analysis of outcrop samples located between latitudes 60571 and 60591N; and longitudes 30391 and 30411E. On the basis of whole rock geochemistry (i.e. major elements), the sandstones are considered as continental sands which can be classified as lithic arenites. The ternary plot of Na2O-K2O-(Fe2O3+MgO) depicts ferromagnesian potassic sandstones suggesting acid igneous rocks and gneisses as probable source rock which are common components of the south western Precambrian Basement Complex. The negative correlation of SiO2 with the other major oxides is ascribed to the relative abundance of quartz. The positive correlation of TiO2 and Fe2O3 with Al2O3 implied their association with clay minerals. The relative depletion of Na2O, K2O, TiO2, Fe2O3 and MgO were noticed but SiO2 and CaO were relatively enriched. The bivariate plot of SiO2 versus Al2O3+ K2O+ Na2O depicted that the sandstones were formed under semi-humid/humid conditions and mineralogically immature. The index of compositional variability (ICV) and log (K2O/Na2O) ratios also showed that the studied sandstones are mineralogically immature. Weathering indices such as chemical index of alteration (CIA), chemical index of weathering (CIW) and plagioclase index of alteration (PIA) suggest that the source materials underwent moderate weathering. The weathering trend suggests that the sandstones were once rich in plagioclase before alteration to smectite. The ternary plot of SiO2/Al2O3, MgO and Fe2O3 indicated that the studied samples were deposited in the non marine and deltaic settings. This observed trend is supported by the binary plot of Fe2O3 versus MgO. The high Al2O3/TiO2 ratio and low MgO values indicate these rocks are derived from felsic igneous source. The observed trend is further corroborated by the discriminant function plot which shows studied sandstones in the field of felsic igneous source. The plot of K2O/Na2O versus SiO2 shows most of the studied clastic sediments in the field of active continental margin (ACM) and few samples in the field passive continental margin (PCM) which suggests syn-rift faulting.