Evaluation of Major and Trace Elements Geochemistry of BasementComplex Rocks in South -Eastern Part of Ado-Ekiti, SouthwesternNigeria

Abstract

Ten fresh rock samples (3 migmatite gneisses, 3 banded gneisses and 4 granites gneisses) were geochemically analyzed for major oxides and trace elements at Activation Laboratories Limited Ancaster, Ontario, Canada employing Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-AES) technique with a view to unravel their petrogenesis. The geochemical composition of the country rocks revealed that they are felsic rocks with average SiO2 content of 67.23%. Other oxides with average chemical compositions include Al2O3 (14.934), FeO (4.223), MnO (0.0646), MgO (0.986), CaO (4.668), Na2O (3.382), K2O (3.179), TiO2 (0.4401) and P2O5 (0.397). Compositional features revealed a descending trend in the oxides of the basement rocks as shown; SiO2 > Al2O3 > CaO > Fe2O3 > Na2O >K2O > MgO >MnO> TiO2 > P2O5. The compositional trend of the rocks suggests geologic setting of continental crust origin possibly formed during orogenic activity. Discriminant plots revealed strong negative correlation of all the oxides with SiO2 except Na2O that has positive correlation indicating pyroxene – hornblende and plagioclase fractionations respectively. Evaluation of the alumina variation of the rocks indicates met aluminous rocks produced from the partial melting of meta-igneous source at low pressure. The rocks are not corundum normative as aluminum saturated index (ASI)<1, typical of type I magmatic rocks. There are great variations among the trace elements compare to the average granite and crust values. The concentrations of Zr and Ba are exceptionally high indicating felsic source of derivation. Also, the low average ratio of Rb/Sr (0.284) for the rocks in the study area signifies possible depletion of Rb and/or enrichment of Sr in the gneisses. The rocks display a great range of rare earth element (REE) concentrations with Ce and Nd enrichment. The migmatite gneiss is more enrich in the REE compare to the banded gneiss and granite gneiss. The chondrite normalized diagram displayed a high degree of fractionation with steep pattern which was confirmed by the high values of normalized ratio of La to Yb, Ce to Yb and La to Sm. The negative Eu anomalies in the gneissic rocks show that high amount of plagioclase was removed from the magma during fractional crystallization while
similarities in the rare earth elements (REE) pattern suggest they have similar origin. The rocks in the area are felsic and met aluminous with igneous origin.