Precambrian Crustal Development in Western Nigeria: Indications from the Iwo Region

Abstract

The rocks of the Iwo region of southwestern Nigeria comprise a migmatite-gneiss-granite complex and a metasupracrustal sequence. Though dominantly of amphibolite facies grade, pyroxene granulite and charnockite remnants are found in the migmatite-gneiss-granite complex. Old supracrustal remnants, best identified in resistant quartzite horizons, are also represented, and a variety of granitic plutonic rocks form well-marked circumscribed associations. The main metasupracrustal belt is associated with a major dislocation zone and has two principal lithologies — dominant amphibolite and pelite west of the fault zone and quartzite and quartzofeldspathic gneiss to the east. Oligoclase-granite gneiss and pegmatite are associated with the schist. All the rocks of the area have been affected to varying extents by late reworking, metasomatism, and granitic activity. On the basis of the indicated field relations and the limited radiometric age data available, a development sequence for the region is proposed. Archean crustal rocks are considered to have been involved in an Eburnian (1,950 ± 250 m.y.) reactivation of Proterozoic mobile belt type. In the Kibaran period (1,200 ± 200 m.y.), activity of greenstone belt–granite type brought about the formation of the Ife supracrustal–granite gneiss association, with extensive basement reactivation limited to the proximity of the downwarping supracrustal sequence. Late, widespread reworking and granitic plutonism is related to the Pan-African (Older Granite) event (600 ± 150 m.y.). The possible relevance of the proposed scheme to the interpretation of the development of the eastern parts of the West African platform is discussed.