Abstract
ABSTRACTIn an effort to have better understanding of the geologic settings and structural disposition of the Nigerian basement rocks, Ilorin area, a notably complex region within the South-western Basement Complex of Nigeria was studied using aeromagnetic data so that existing knowledge on the geology of the area can be appraised and updated. The aeromagnetic data was enhanced using various techniques which include the Total Horizontal Derivative (THD), Standard Euler Deconvolution, Tilt Derivative and Spectral Analysis. Striking features observed over the Residual Magnetic Field map were NE-SW trending anomalies aligned diagonally and running from the SW to the NE, forming a band having a width of between 16,000 and 20,000 m that coincided with the Banded Gneiss region on the geologic map; conspicuous E-W trending magnetic low whose location lies very close to the boundary between Migmatite and Banded Gneiss; and a ring-like magnetic low occurring in the south-eastern part of the residual map with no surface expression on the geologic map. Residual Magnetic Field values ranged from +157 to −202 nT. The THD map showed that the Banded Gneiss was the most deformed unit within the Migmatite–Gneiss–Quartzite complex. Major fractures coincident with Banded Gneiss boundaries appeared to be possible channels from which heat was leached in the regional system during metamorphism such that the Banded Gneiss region attained a relatively lower grade metamorphism than the Migmatite sharing boundary with it. Result from the spectral analysis showed that the magnetic basement in the eastern region was downthrown abruptly relative to that in the western portion. The abruptness was interpreted to mean that such subsidence was fault controlled. The study concluded that the presence of major fractures interfering with metamorphic processes in a regional settings may influence the grades of metamorphism that may result; that the schist region in the study area could be overlying a region of downthrown bedrock; and that the stress pattern that defined the crustal deformation pattern at the near crust in the past had changed compared to the currently dominant pattern.
Highlights
From the series of studies conducted on the “Nigerian Proterozoic crustal development during the PanAfrican Regime” between the 1960s and the early 1990s, three broad lithologies have been identified
Striking features observed over the Residual Magnetic Field map were NE-SW trending anomalies aligned diagonally and running from the SW to the NE, forming a band having a width of between 16,000 and 20,000 m that coincided with the Banded Gneiss region on the geologic map; conspicuous EW trending magnetic low whose location lies very close to the boundary between Migmatite and Banded Gneiss; and a ring-like magnetic low occurring in the south-eastern part of the residual map with no surface expression on the geologic map
During the years that followed this period of extensive regional studies on the development and evolution of the Nigerian basement rocks, except for a few works which were purely geochronology studies (Dada 1998, 1999), lesser scaled studies designed towards more detailed understanding of the peculiar geologic settings across distinguished geologic regions were not conducted to appraise existing discoveries and the implication of this has been that no new, significant insight was gained about the geologic settings and structural disposition of the Nigerian basement rocks
Summary
From the series of studies conducted on the “Nigerian Proterozoic crustal development during the PanAfrican Regime” between the 1960s and the early 1990s, three broad lithologies have been identified These are the polymetamorphic Migmatite–Gneiss complex which is composed largely of migmatites and gneisses of various compositions and amphibolites; low-grade sediments-dominated Schists which form narrow belts to the west of longitude 8° E; and the syntectonic to late tectonic granitic rocks which cut both the Migmatite–Gneiss complex and the Schist Belts (Turner 1983; Ajibade et al 1987). These basic lithologies have been found to be distinguished within the Nigerian Basement Complex (Figure 1). The schist is believed to be intruded by granitic bodies
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