Petrology of Newly Exposed Basement Rocks of Ouka Area Southwestern Nigeria

In less than 50 years, Uokha area has been stripped bare of its original Cretaceous sedimentary cover and the underlying Precambrian basement is exposed, making the study area the southeastern most fringe of the southwestern Nigerian Basement, where the Precambrian Basement rocks disappear below the unconformable Cretaceous sediments. This work is the first report of the petrology of the exposed Precambrian Basement Complex rocks. The exposed rocks are dominated by schist with subordinate gneiss, quartzite, dioritic, granodioritic, granitic and pegmatitic rocks. Detailed field work and sample collection was followed by thin sectioning and petrographic analyses of the rocks. The peak metamorphic grade of middle amphibolite facies (600-670°C) attained by the rocks was constrained by the textural display, and index mineralogical assemblage of Staurolite-Garnet-Biotite association in the quartzite sample. Retrograde metamorphism is inferred by the rampart occurrence of bulbous myrmeckites and sericitization of feldspathic minerals, and by cordieratization of quartz and biotite. The rate of sediment erosion is estimated to be about 1 meter in 5-10 years. This work will add value to the data pool of the P-T conditions of the Basement Complex rocks of the Benin-Nigerian Shield needed for the proper understanding of the metamorphic intensity of the Pan-African orogeny, and set the pace for investigation of the rates of sediment removal and exhumation of buried rocks.