Retrograde Assemblages in the Muscovite-Biotite Gneiss of Oluyole Southwestern Nigeria, an Indication of Shear-Zone Environment

Abstract

Petrographic and whole-rock geochemical study of biotite-muscovite gneiss was determined in order to interpret the metamorphic evolution of the Basement Complex of Southwestern, Nigeria. The gneiss shows a millimetric banding, and in some cases the quartzo-feldspathic bands running up to 10 cm. The gneiss has mineral assemblage biotite + plagioclase + quartz + garnet + K-feldspar + muscovite + chlorite + ilmenite ±titanite. Chlorite occurs along cleavage planes of biotite, and in some cases forms reaction rims around porphyroblasts of garnet. K-feldspar crystals are surrounded by muscovite. Titanite crystals are sub-idioblastic to xenoblastic in form, and have inclusions of ilmenite. Titanite, where present, occurs in close association with biotite and opaque minerals (ilmenite). Also, titanite forms a reaction rim around apatite. Mylonitic texture, fine-grained matrix of mica and quartz ribbons were observed. In addition, there is stretching of the quartz crystals. The SiO2 content is greater than 60 wt %, while CaO ranges from 3.05-6.91 wt %. The M1 foliation comprise of mineral biotite some of which are included in the opaque mineral, M2 represents the metamorphism which gave rise to porphyroblasts of ilmenite, while the M3 gave rise to foliations that forms a wraparound structure on the porphyroblasts of ilmenite. The last metamorphism gave rise to retrograde minerals; chlorite, titanite, and muscovite. The study suggests that this area of the Basement Complex has been subjected to multiple deformations, as well as multiple episodes of metamorphism. The structures observed are similar to those associated with shear zone environment.