Geological Behavior (GBR)

This paper presents the petrography and mineral chemistry of charnockitic gneisses exposed at Ikeram-Ibaram within the Precambrian Basement Complex of southwestern Nigeria. Quartz, plagioclase, perthite, amphibole, biotite and orthopyroxene are essential minerals, while apatite, ilmenite, magnetite and zircon are accessories. Orthopyroxene is ferro-hypersthene (En44Fs56Wo0) with low TiO2, CaO contents, but high in MgO compositions. Orthopyroxene is mantled by hornblende and relicts of biotite grains are found within orthopyroxene as inclusions. Plagioclase is andesine and occur as inclusions in other minerals. Biotite has high concentration of TiO2, but poor in CaO. Ilmenite and magnetite are closely associated with orthopyroxene. Rare earth element (REE) displays enrichment in light REE and depletion in heavy REE with negative Eu anomaly. Biotite as relicts in orthopyroxene and amphibole mantling orthopyroxene are clear evidences of retrograde metamorphic events. The mineral reactions suggest the retrogression of the charnockitic gneisses that are products of rehydration processes. These relationships between pairs of minerals indicate retrogressive form of metamorphism at the transition from granulite facies to amphibolites facies.