Compositional and petrogenetic features of schistose rocks of Ibadan area, southwestern Nigeria

Amphibole and quartz schist which occur in association with migmatite gneiss, granitic gneiss and Pan African Older Granite bodies around Ibadan area, southwestern Nigeria, were studied with a view to elucidate their compositional characteristics and their evolution. Mineralogical determinations from optical studies show a high proportion of granular quartz and accessory muscovite in the quartz schist. The amphibole schist on the other hand comprises mainly dark colored bands of hornblende with subordinate tremolite, chlorite and minor amounts of plagioclase and quartz. Chemical analysis of the samples obtained by Inductively Coupled Plasma Mass Spectrometry (ICPMS) instrumentation method, involving major and trace elements reveal the silicieous nature of the quartz schist and and the amphibole samples of both rock units are also marked by relatively elevated contents of Ba, Zr, Rb, La and Ce and Zr. In addition, variation plots using [Na.sub.2]O, [Al.sub.2]O, [K.sub.2]O suggests arenaceous sedimentary ancestry for the quartz schist and an igneous ancestry most probably mafic extrusive volcanics for the amphibole schist. Provenance indicators, such as Ba, in the quartz schists suggest derivation of this sedimentary protolith from the weathering of largely granitic rocks. Similarity of the amphibole schist progenitors with subalkaline basaltic andesite is also implied by the [Na.sub.2]O + [K.sub.2]O versus Si.[O.sub.2] bivariate plot, while the [Na.sub.2]O + [K.sub.2]O-[Fe.sub.2][O.sub.3][sub.(t)-]MgO ternary plot reveal their calc-alkaline affinity. Tectonically, the quartz schists evolved within the passive margin environment, whereas the MgO-[Fe.sub.2][O.sub.3]- [Al.sub.2][O.sub.3] ternary plots reveal an Ocean island basalt tectonic evolution for the amphibole schist. Key words: Amphibole, Quartz, Schist, Schistose rocks, Ibadan-Nigeria, Petrogenetic Anfiboles y esquistos de cuarzo que ocurren en asociacion con gneiss migmatiticas, gneiss de granito y cuerpos antiguos de granito Pan Africano alrededor del area de Ibadan, a sudoeste de Nigeria, fueron estudiados con el fin de aclarar las caracteristicas composicionales y su evolucion. Determinaciones mineralogicas a partir de estudios opticos muestran una alta proporcion de cuarzo granular y moscovita accesorios en el esquistos de cuarzo. Por otro lado, los esquistos anfiboliticos incluyen bandas de color oscuro principalmente de hornblenda con tremolita, clorito y de pequenas cantidades de plagioclasa y