Lithostructural Relationships and Petrogenetic Affinities of the Basement Complex Rocks around Okpella, Southwestern Nigeria

Okpella area is the eastern extension of the Igarra Schist Belt, Southwestern Nigerian Basement Complex. The area comprises granite gneiss, metasedimentary rocks and Pan-African intrusives. Metasedimentary rocks occurring in the area include garnet-biotite schist, marble and calc-silicate gneiss, quartzite and Banded Iron Formation (BIF). The Pan-African intrusives include granite, charnockite, hybrid rocks, pegmatite, aplite and basic dykes. Structural data of rocks in the area suggest the presence of two contrasting structural trends. The older one which appears restricted to the granite gneiss, calc-silicate gneiss and garnet-biotite schist trends ENE-WSW to EW with moderate dips to the south. The latter NW-SE to NS trend is pervasive and occurs in all the metasediments and granite gneiss. These two trends are related to two deformational episodes and probably two orogenic periods. All the metasediments show affinity for sedimentary parentage, the granite gneiss is probably of igneous origin while the intrusives are magmatic. The charnockite-granite hybrid rock probably represents a zone of magma mixing between two contrasting magmas that were emplaced contemporaneously. The occurrence of carbonate bodies in association with deformed pelites as well as the complete absence of metavolcanics in the area suggests deposition in a miogeosynclinal-continental environment along which crustal distention has occurred. A sequence of nine geological events is suggested for the evolution of the area as deduced from field observation.