Field Geology and Structures of Migmatitic Gneisses Around Ibillo-Okene Area, Southwest Nigeria

Abstract

Ibillo-Okene migmatites are a typical example of the Basement Complex rocks of southwest Nigerian, which is part of the larger West African shield. They lie directly north and constitute the northern closure of the Igarra Schist Belt. They are interbanded with relics of metasedimentary units called older metasediments as well as overlain by younger metasediments. These units are affected by co-genetic or similar episodes of deformation that has both petrogenetic and economic significance as observed in Igarra Belt south of this area. Though this area is interesting in terms of geology and tectonic setting, detailed geological and structural interpretation of features in the area is lacking due to limited accessibility. Land Sat 8 and shuttle radar topographic mission (SRTM), data constitute useful tools for preliminary reconnaissance survey as well as structural interpretation. Based image interpretation, a minimum of three deformation events were identified: The first event (D1) which is essentially a ductile deformation produced foliations and isoclinal folds which are axial planar such that both the eastern and western limbs of the folds are parallel with regional metamorphic foliation trend S1 (NNW-SSE) and dominant lineament trends (ENE-WSW). The second event (D2) is believed to have produced a progressive reactivation of the D1 event with accentuation of the E-W compression leading to folding of the axes of F1 folds. The third event (D3) has two deformational stages: a ductile deformation at the onset at a higher temperature and pressure and a brittle deformation at the later stages with lower temperature and pressure. Geologic fieldwork which was further conducted to validate structures and units identified from image processing revealed that the migmatites are essentially metatexites which to the northwest is dialational, dialational net-structured in the northeast and straumatic in the southeast. The southwest of the area constitute the basinal closure of the Igarra area overlain by relics of the metasedimentary units. This research underscore the relevance of remotely sensed data in geological investigation whose results have important implications in terms of regional tectonics and geological mapping as well as in land-use planning and other areas such as hydrogeology or geotechnics.

Keywords: Migmatite, Ibillo, Okene, metasediments, Remote sensing.

DOI: 10.7176/JEES/9-2-08