Pages: (115-134 )
Abstract
An inorganic geochemical study of a clay and shale sediments sequence around Araromi and Irogun-akere (the Southern part of Yewa River), Eastern dahomey basin, south western Nigeria, was carried out to determine the sediment’s depositional conditions, provenance and tectonics. Fifteen shale and clay sediment samples underwent mineralogical and geochemical analysis involving major, trace and rare earth element analysis at acme analytic laboratory, Ontario, Canada using an ICP mass spectrometer (Perkin-Elmer, Elan 6000). Mineralogical studies using X-ray diffraction analysis revealed prominent kaolinite, montmorillonite and illite peaks; accessory minerals included quartz and microcline. Major element abundance showed that the shale samples had SiO2 (44.77%), Al2O3 (15.01%) and Fe2O3 (5.75%), constituting more than 65% of bulk chemical composition. Thirty-four elements trace metals, heavy rare earth elements and light rare earth elements analysis were quantified from the geochemical analysis. The shale sediments exhibit higher Ni, Ba, CO, Zr, Rb and Th concentrations than clay sediments, whereas clay samples are Sr-, Zn,, Pb- and U-enriched .Light rare earth element (La, Ce, Nd,Sm,Pr) were enrich in shale and clay sediment samples while, heavy rare earth elements (Tb, Yb, Lu, Gd ,Dy ,Ho, TM and Eu) are depleted. High TiO2 and Rb/K2O values also indicated that the shale and clay samples were matured. Geochemical parameters such as U, U/Th, Ni/Co and Cu/Zn ratios indicated that these shales were deposited in oxic conditions; the Al2O3/TiO2 ratio suggested that intermediate igneous rocks were probable source rocks for the shale, while mafic rocks were suggested as being source rocks for the claystone. However, the La/Sc, Th/Sc, Th/Co ratios and shale and clay sediment plots revealed that they came within the range given for felsic rocks as provenance, thereby suggesting mixed provenance for the sediments. A passive-margin tectonic setting was adduced for the sedimentary sequences.
Keywords: Trace metals,Light Rare Earth Element, Heavy Rare Earth Element, Inductively coupled mass spectrometer, X-ray diffraction analysis, provenance, tectonic setting.,