Deformation Traits in the Charnockitic Rocks of Akure Area, Southwestern Nigeria

摘要

The charnockitic rocks of Akure constitute one of the petrologic units of the precambrian basement complex of Southwest Nigeria. The field studies indicate fresh outcrops that show little signs of weathering although the surfaces are rough. The rock is in contact with migmatite and granite gneiss of the migmatite gneiss complex. On the outcrops are the occurrences of pegmatite, aplite and quartz veins as minor intrusions. Petrographic studies revealed hornblende, plagioclase, hypersthene, biotite, quartz, muscovite and opaque minerals as major minerals while zircon is accessory. Bent lamellae in plagioclase, distorted twinning in plagioclase and micro-cracks in virtually all the minerals in the thin sections indicate some element of deformation. Joint sets trend N-S, E-W and NNE-SSW with the N-S set predominating. The trends of the joint sets tally with those of the minor intrusions. The directions of maximum compressional and tensional stresses inferred from the rose diagram are in consonance with the general directions imprinted by the Pan African orogenic events which affected the basement complex rocks. Since, virtually all minerals in the thin sections exhibit traits of deformation indicating that the deformation is penetrative, it is inferred that the cause of the deformation might be tectonic.