Abstract
Petrographic and geochemical studies of amphibolites from Wonu-Apomu area in Iseyin-Oyan schist belt and Ife-Ilesa area in Ilesha schist belt, southwestern Nigeria, were carried out with a view to unravel their nature and petrogenesis. The amphibolites in Wonu-Apomu are associated with quartzite, quartz schist, talc-schist, granite and pegmatites, while those of Ife-Ilesha area are associated with anthophyllite schist, talc tremolite and talc chlorite schists. Chemical data of the amphibolites showed high Al2O3 (>14%), Fe2O3 (>12.40%), CaO (>8.10), Na2O (>3.10%) and K2O (>5.0%) contents. Trace element data showed that the amphibolites are enriched in Ba (>137ppm), Sr (>122ppm), Rb (>33ppm), Zr (55pm) and Y (>37ppm), but impoverished in Cr (<62ppm), Ni (<57ppm) and Zn (<83ppm). Petrogenetic indices and discriminant plots of Na2O + K2O vs SiO2, Ga vs Y, Zr/Y vs Zr, Cr vs Ni, Fe2O3 –Na2O + K2O – MgO, indicated tholeiitic basalt affinity of the amphibolites and emplacements of the precursor basaltic rocks within continental crust.
Highlights
A number of investigations have been carried out by various workers on the amphibolites within the schist belts of the Nigerian basement complex
The Proterozoic Ilesa schist belt of southwestern Nigeria has been mapped in various degrees of details by workers, such as, Deswardt (1953), Hubbard (1966, 1975), Elueze (1980), Klemm et al, (1984) and Kehinde-Phillips (1991)
This was largely due to the discovery of gold in the Ilesa area in 1940, as the Ilesa amphibolites Complex was suspected to be the source of the alluvial gold
Summary
A number of investigations have been carried out by various workers on the amphibolites within the schist belts of the Nigerian basement complex. The Proterozoic Ilesa schist belt of southwestern Nigeria has been mapped in various degrees of details by workers, such as, Deswardt (1953), Hubbard (1966, 1975), Elueze (1980), Klemm et al, (1984) and Kehinde-Phillips (1991) This was largely due to the discovery of gold in the Ilesa area in 1940, as the Ilesa amphibolites Complex was suspected to be the source of the alluvial gold. Based on geochemical characteristics, Elueze (1980) and Olade and Elueze (1979) have suggested a tholeiitic basalt precursor and emplacement within the continental crust for the Ilesa amphibolites, Ajayi (1980) proposed a dual parentage for these amphibolites Most of these investigations on the amphibolites occurrences within the Precambrian basement complex of southwestern Nigeria tend to emphasize their field relations, petrological descriptions, mineralogical attributes and chemical compositions. This present investigation largely seeks to generate chemical data to unravel the nature and origin of the amphibolites around Wonu-Apomu in the Iseyin-Oyan schist belt and those of Ilesa in the Ife-Ilesa schist belt, southwestern Nigeria
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