Nature and Origin of the Amphibolites in the Precambrian Basement Complex of Iseyin and Ilesha Schist Belts, Southwestern Nigeria

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 Al 2 O 3 (>14%), Fe 2 O 3 (>12.40%), CaO (>8.10), Na 2 O (>3.10%) and K 2 O (>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 Na 2 O + K 2 O vs SiO 2 , Ga vs Y, Zr/Y vs Zr, Cr vs Ni, Fe 2 O 3 -Na 2 O + K 2 O -MgO, indicated tholeiitic basalt affinity of the amphibolites and emplacements of the precursor basaltic rocks within continental crust.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. Various efforts to unravel the petrogenetic affinity have been dogged with controversy. Therefore, 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.

Geochemistry and mineral chemistry of quartz mica schists within Iseyin-Oyan Schist Belt, Southwestern Nigeria

“…(5,6) Previous studies revealed undifferentiated schists as the most widespread lithology; quartzites occur near the margins of the schist belt while massive amphibolites represent mafic igneous rocks. (5) More recent geological mapping (7)(8)(9)(10)(11) within the schist belt reported the occurrence of lithologic units similar to those reported in earlier studies. Based on mineral assemblages recorded in pelitic schist mapped within the Belt, higher grade metamorphism was suggested in the belt than in most northern schist belts (12) , whose structural style and magnetic properties are well elucidated.…”