Geochemical Characteristics and Tectonic Setting of Amphibolites in Ifewara Area , Ife-Ilesha Schist Belt , Southwestern Nigeria

Trace and Rare Earth Elements (REEs) data are used to constrain the geochemical evolution of the amphibolites from Ifewara in the Ife-Ilesha schist belt of southwestern Nigeria. The amphibolites can be grouped into banded and sheared amphibolites. Major element data show SiO2 (48.34%), Fe2O3 (11.03-17.88%), MgO (5.76-9.90%), CaO (7.76-18.6%) and TiO2 (0.44-1.77%) contents which are similar to amphibolites in other schist belts in Nigeria. The Al2O3 (2.85-15.55%) content is varied, with the higher values suggesting alkali basalt protolith. Trace and rare earth elements composition reveal Sr (160-1077ppm), Rb (0.5-22.9ppm), Ni (4.7-10.2ppm), Co (12.2-50.9 ppm) and Cr (2-7ppm). Chondrite-normalized REE patterns show that the banded amphibolites have HREE depletion and both negative and positive Eu anomalies while the sheared variety showed slight LREE enrichment with no apparent Eu anomaly. The study amphibolites plot in the Mid Oceanic Ridge Basalts (MORB) and within plate basalt fields on the Zr/Y vs Zr discriminatory diagrams. They are further classified as volcanic arc basalt and E-type MORB on the ThHf/3Ta and the Zr-Nb-Y diagrams. The amphibolites precursor is considered a tholeiitic suite that suffered crustal contamination, during emplacement in a rifted crust.


Introduction
The amphibolites of Ifewara in the Ife-Ilesha schist belt (Figure 1) in southwestern Nigeria constitute a major rock unit within the amphibolite complex, with which placer gold and disseminated sulphide mineralization have been associated (Elueze, 1981).The study area is located within latitudes 7º30´N to 7º33´N and longitudes 4º35´E to 4º42´E.The major towns in the study area include Ifewara, Oke Owena, Oke Oloyinbo and Itagunmodi (Figure 2).The area is located within the N-S trending schist belt of the Nigerian Basement Complex.The geology of Ife-Ilesa schist belt (Figure 1) has been studied by Hubbard (1975), Ajayi (1980), Elueze (1981) and Rahaman et al. (1988).They recognized two contrasting lithologies separated by the NNE trending Ifewara faults.The rocks of the study area may be broadly grouped into gneiss-migmatite complex, mafic-ultramafic suite (or amphibolites complex), meta-sedimentary assemblages and intrusive suite of granitic rocks (Odeyemi, 1981 andElueze, 1988).The entire area has been affected by polycyclic episodes of deformation and metamorphism (Odeyemi, 1981).The ratio of metasediments to metavolcanics is much higher in the schists belts of Nigeria than in the typical Archaean greenstone belts (Wright & McCurry, 1970).Mineralization is also not strongly developed in the Ife-Ilesa schist belts unlike most well known greenstone belts in the Canadian, Indian, Australian, and South African Precambrian shields.
The objectives of current investigation are to examine the petrography, determine the geochemical composition, appraise the petrogenetic affinity, and highlight the tectonic significance and crustal evolution of the amphibolites.

Geologica
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Petrog
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Rare Earth Elements (REEs) are amongst the least soluble trace elements and are relatively immobile during low grade metamorphism, weathering and hydrothermal alteration and can represent the original composition of the unaltered parent (Michard, 1989).Chondrite-normalized REE patterns for the amphibolites are given in Figure 4.The banded amphibolites have higher abundances of the light rare element (LREE), medium rare element (MREE) and heavy rare element (HREE).The banded amphibolites show strong HREE depletion with no apparent Eu anomaly.The massive amphibolites showed REE chondrite-normalized patterns with slight LREE enrichment compared to MREE and HREE.It also shows slight HREE depletion relative to the MREE.This relatively flat pattern is characteristic of tholeiitic basalt as shown by Cox et al., (1979).
A negative Europium (Eu) anomaly is caused by removal of feldspar from a felsic melt while positive Eu anomaly can result from increase in hornblende, titanite and orthopyroxene contents.Most of the study amphibolites show no obvious Eu anomaly except samples 6 and 3 which showed strong positive and weak negative Eu anomalies respectively; (Figure 4).The observed disparity in Eu anomalies could result from the increased mafic mineral composition (Table 1) and removal of feldspar from the felsic melt correspondingly.The pattern of the amphibolites on the MORB-normalized spider diagram (Figure 5) shows significant enrichment in Sr compared to Ce indicating volcanic -arc basalts precursor.

Conclusions
The amphibolites of the Ifewara area in the Ife-Ilesha schist belt belong to the Precambrian basement complex of southwestern Nigeria, which consist of Archean to Early Proterozoic migmatite gneissic-quartzite complex that bear the imprint of early to late Precambrian metamorphic episodes.Petrographic and petrochemical data were used to constrain the nature and genesis of the Ifewara amphibolites.Geochemical data showed that the protoliths of the amphibolites were tholeiitic basalts which were emplaced in a rifted crust within an environment of active sedimentation.The Ifewara amphibolite in the Ilesha schist belt has been shown to mark possible site of an oceanic basin, although there is no evidence for a mantle-derived magmatic arc, such that any subduction, if present, must have been short lived and failed to generate calc-alkaline magmas (Bother & Grobler, 1979).The geochemical data agrees with derivation of the magmas of the amphibolites from upper mantle melting.The observed shearing of the amphibolites is not unconnected with the Ifewara fault which provided the pathway for the magma.The fault could have been responsible for magma upwelling from the mantle and subsequent contamination and emplacement of the magma in a Within-plate tectonic setting. esr.ccsenet.

Table 1 .
Modal composition (%) of the study amphibolite rocks from Ifewara area

Table 2 .
Major oxide composition (wt%) of the study amphibolite rocks from Ifewara area

Table 3 .
Comparison of the concentrations (wt %) of major element of the study amphibolites rocks with other localities