Petrogenetic characterisation and paleo-climatic study of some clay occurrences over pre-Cambrian basement complex in Southwestern Nigeria

Abstract

Bulk clay chemistry from specific areas underlain by Precambrian rocks composed of migmatitic gneiss, schist, charnockite and granite in Ekiti, Southwest Nigeria, was studied for their elemental compositions in order to assess petrogenesis and paleoclimatic conditions. Eleven representative clay samples were collected from various clay occurrences based on the underlying geology. The clay samples were prepared and analysed using X-ray Fluorescence (XRF). The XRF data on eleven representative samples revealed major oxides (wt. %) and trace element (ppm) concentrations. SiO values ranged from 41.29 to 58.03 while Al₂O₃ and Fe₂O₃ values ranged from 15.86 to 36.61 and 0.27 to 13.05 respectively. Standard deviation (S.D) values of SiO₂ (4.2), Al₂ O₃ (6.7) and Fe₂O₃ (4.8) suggested variable mobility. Average value of K₂O (1.46) is observed to be lower than average Upper Continental Crustal UCC) value (3.39) and may be due to intense weathering. Average concentration values of Zr (634.53 ppm), Nb (76.45 ppm) and Ta (29.19 ppm) were higher than their corresponding average values reported for UCC. Average values of Chemical Index of Alteration (CIA) and Plagioclase Index of Alteration (PIA) (CIA, 70 and PIA, 73) showed moderate to high intensity weathering. Ba/Sr vs Y/Sr values discriminated the clay occurrences over schist from migmatitic gneiss and granitoids. Kaolinisation in moderate to high weathering intensities under a dominant oxic condition was inferred from the plots of A-CN-K and Ni/Co vs Mafic Index of Alteration (MIA).