Amphibolite is an important lithologic unit of the Nigerian basement complex that are commonly intercalated within metasedimentary sequences. Disseminated grains of sulphide minerals and base metal deposits are reported to occur in amphibolites and some other lithologies in other schist belts of Nigeria apart from the Iseyin-Oyan schist belt. Detailed geochemical study of amphibolitic rocks in this schist belt is scarce in literature, whereas none exist for the area under study, thus, this study is aimed at delineating the amphibolites within the Iseyin-Oyan schist belt around Itasa area with detail appraisal of their petrography and geochemical characteristics. A geologic field mapping was undertaken, and fresh representative amphibolite samples were obtained. Petrographic and geochemical studies of the samples were carried out with a view to understand their nature and petrogenetic characteristics. Geochemical data were elucidated using diverse geochemical discrimination diagrams. The amphibolites occur as lenses of small to large rocky boulders trending NNE-SSW and are essentially basal to quartz mica schist. Modal compositions revealed that they are dominantly composed of hornblende and plagioclase. XRD and mineral chemistry revealed that the plagioclase ranged from andesine to anorthite while hornblende is mainly magnesiohornblende. Geochemical data and discrimination diagrams revealed that the amphibolites are formed from basaltic to basaltic-andesite protolith that are of tholeiitic nature and had suffered crustal contamination. Tectonic discrimination diagrams indicated that the rocks were emplaced in the Mid Ocean Ridge but had been affected by collision due to Pan African Orogenic tectonic event.
Background/Objectives: The Iseyin-Oyan schist belt is made up metasedimentary rocks, gneisses, granites and pegmatite intrusions. The study was aimed at identifying the schist within this belt and assess their metamorphism, geochemical characteristics and tectonic origin. Methods: Detailed geologic field mapping was undertaken where rocks were located, studied in-situ and identified. Samples of the schist were prepared for petrographic studies. Mineralogical contents were determined using X-Ray Diffraction technique. Polished sections were studied for mineral chemistry using Scanning Electron Microscope-Energy Dispersive Spectroscopy. Rock samples were analysed using X-Ray Fluorescence Spectroscopy and Inductively Coupled Plasma Emission Spectrometry. Geochemical data were elucidated using diverse geochemical discrimination diagrams. Findings: The schists are quartz mica schists and occur in close association with amphibolites, intrusive granitoids and pegmatites. The Mineral assemblage indicates upper (at the western part) to lower (at the central part) amphibolite facies grade metamorphism in the area. Pyrope-almandine garnets occur in quartz mica schist at the western parts reinforcing higher pressure-temperature metamorphic conditions. The concentration (in %) of SiO2 ranged from 56.4-71.6; Al2O3, 13.7-21.1; Fe2O3, 2-8; MgO, 0.7-2.4; and K2O, 2.1-5.5 supporting the evidence for differential degrees of metamorphism. Large iron lithophile and high field strength elements are similar to the average upper continental crust. Pronounced negative Europium anomaly pointed to the major roles played by feldspars during the geological processes. Plagioclase ranged from albite-oligoclase and oligoclase- andesine. The precursors of the quartz mica schist are possibly arkosic and greywacke sands deposited within the active continental margins. Evidence of uplift and overturning suggested for the differential metamorphism may be due to these events usually associated with active continental margins. Applications: This study has identified the once named undifferentiated schist in the study area to be quartz mica schist with details in their grades of metamorphism elucidated. Keywords: Quartz mica schist; geochemistry; mineral chemistry; Iseyin-Oyan schist belt; precambrian basement complex
The Rare Earth Elements (REE) composition of granitoids in and around Ila-Orangun area Southwestern Nigeria was assessed in order to ascertain their potential for possible exploitation. Detailed lithological mapping of the area was undertaken followed by whole rock geochemical analysis of representative samples of the granitoids using Inductively Coupled Plasma Mass Spectrometry (ICP-MS) technique. Petrographic study of the samples was carried out as well as the interpretation of the geochemical data using diverse geochemical discrimination plots. The rock units mapped were biotite granite gneiss, granite gneiss and hornblende biotite granite. Biotite hornblende gneiss, quartzite, talc-chlorite-tremolite-schist, mica schist and pegmatites were the surrounding country rocks. The REE concentrations (in ppm) revealed higher concentrations of the light REEs compared to the heavy REEs. The fractionation ratio, (La/Yb)N ranged from 4.35-15.04 (granite gneiss) and 13.78-18.48 (hornblende biotite granite) indicating enrichment in LREEs over the HREEs. The spider plot for the REEs also showed that the granitoids are LREE-enriched and HREE-depleted suggesting fractional crystallisation and a distinct negative Eu anomaly indicating plagioclase fractionation. Enrichment plot also revealed that the REEs in the granitoids are significantly enriched. Comparison with other areas showed that the granitoids of the study area especially the hornblende biotite granite has higher concentrations of REEs and may be a possible pointer of REE mineralisation.
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