Mineralogy, petrology, and geochemistry of charnockites and associated rocks in the Ikare area, Ondo State, Southwestern Nigeria

In this study, the petrography, the mineralogy, the mineral chemistry, and the geochemical characteristics are documented and interpreted for a series of Pan African igneous and orthogneissic charnockites, granites, granitic gneisses, metapelites, and grey gneisses from the basement complex making up the Ikare area, northern Ondo State, southwestern Nigeria. The whole rock chemistry of the magmatic rocks and their deformed and metamorphosed equivalents show calc-alkaline to high-K, calc-alkaline affinity. The REE patterns of these rocks are typical for the rocks from the continental crust, suggesting a deep crustal melt source. Low Sr/Y-ratios point to a post-Archean magmatic arc environment. Pressure-temperature estimation indicates peak temperatures of ca. 850 °C for the igneous charnockites and 800 °C for the orthogneissic charnockites assuming a regional pressure of 500 MPa; 725–750 °C / 430–490 MPa for the metapelites; 725 °C / 430–490 MPa for the granitic gneisses; and 750 °C / 460 MPa for the grey gneisses. The relationship between the igneous and associated patchy charnockites, and the orthogneissic charnockites is described and further elucidated upon with respect to their relationship to the granites, granitic gneisses, metapelites, and grey gneisses in the study area. The proposed model for the crustal evolution of the Ikare area starts with a pre-existing grey gneissic and metapelitic basement, followed by intrusion of a charnockite-granite complex. While the magmatic granite was characterized by a high H 2 O activity, the charnockitic magma was characterized by a low H 2 O activity due to a predominance of CO 2 in the fluid associated with the melt. This was subsequently followed by the formation of charnockite patches in the grey gneiss in the vicinity of the igneous charnockite via the solid-state conversion of biotite and amphibole to orthopyroxene +/- clinopyroxene due to the limited influx of CO 2 -rich, low H 2 O activity fluids from the crystallizing igneous charnockite. Lastly, deformation of the charnockite-granite complex to a gneiss occurred with only low-strain islands of granite and areas of the igneous charnockite remaining nearly undeformed, thereby preserving their magmatic origins.