Ogudu area is located in Ife-East Local Government Area of Osun State, Southwestern Nigeria, within latitude 7022.25’N and 7025’N, and longitude 4037.5’E and 4040.25’E covering a total area of 25 square kilometers. The terrain falls within the Ife-Ilesha schist belt, which is an integral part of the Nigerian Basement Complex. The Nigerian Basement Complex is part of the late Proterozoic to early Paleozoic mobile belt east of the West African craton. This basement complex comprises Archean and Proterozoic rocks and available geochronological data indicates that this basement is polycyclic and bears the imprint of three thermotectonic events; Liberian (ca. 2700 Ma), Eburnean (ca. 2000 Ma), and Pan-African (ca. 600Ma) orogenic events (Grant, 1970; Oversby,1975; Van Breemen et al., 1977; Fitches et al., 1985; Rahaman, 1988; Dada et al., 1994). The need for a detailed geology of this area forms the main objective of this field mapping.
Lithologies were studied and observed structures were documented in this reported, geometrical analysis of structural fabrics in macro-, meso- and microscale are well documented. Field observations on the various rocks in the study area have shown that the degree of deformation is not equal and are characterized by varying fabrics, grain sizes, structural styles, mineralogy, etc. This independent mapping therefore aims at examining the outcrop scale structures, microstructural and mineralogical features of the various rocks in the study in order to gain understanding of the geology of the basement complex rock.
1.1 AIM AND OBJECTIVES
The aim of this study is to produce the geological map of the study area with the following specific objectives:
1. To map all the lithological units;
2. to examine and report all the structural features and fabrics;
1.2 LOCATION AND ACCESSIBILITY
Ogudu area is located in Ife East Local Government Area of Osun State within the Ife-Ilesha Schist belt in Southwestern Nigeria. Accessibility is poor, most of the routes (mostly footpath) shown on the available topographical map have been covered with vegetation due to abandonment of settlement and/or farmstead.
The topography of the study area is randomly an undulating feature. The eastern part of the study area has a higher elevation. The upper part of the study area has elevation between 1300 and 1995 feets above sea level. Generally the region is characterized by elevations between 1000 feets and 1400 feets above the sea levels.
1.4 CLIMATE AND WEATHER CONDITION
The climate of the area fall under the equatorial climate belt of southwestern Nigeria and the weather condition has a comparatively high temperature throughout the year and high rainfall characteristic of a rain forest belt.
The drainage system is dendritic. The stream channel are generally structurally and stratigraphically controlled with the rivers flowing southwards except in tributaries where they flow in W-E and E-W direction to join the main rivers flowing south.
The study area lies within the Nigeria tropical rain forest belt characterized by dense vegetation with closed canopy, in which most of the treetops touch and creating a shaded forest interior. Cultivation of crops both arable and cash crops also dominate the agricultural activities of the settlement.
1.7 LITERATURE REVIEW
1.7.1 REGIONAL GEOLOGY
The Nigerian Basement Complex is one of the three major lithological components that make up the geology of Nigeria. It forms the southern part of the Trans Saharan mobile belt east of the West African craton and northwest of the Congo craton (Caby, 1989; Ferre et al., 2002; Caby, 2003) and has also explained to be south of the Tuareg shield in Black (1980). This basement complex comprises Archean and Proterozoic rocks which is believed to be the results of three major orogenic cycles of deformation, metamorphism and remobilization, and basement reactivation corresponding to the radiometric ages indicated by Liberian (2700 ± 200 Ma), Eburnean (ca 2000 Ma), Pan-African (ca 600 Ma) which resulted from plate collision between the passive continental margin of the West African craton and the active Pharusian continental margin (Grant, 1970; Oversby, 1975; van Breemen et al., 1977; Fitches et al., 1985; Rahaman, 1988; Dada et al., 1994). Three broad lithological group are usually distinguished within the Nigerian basement complex (Ajibade et al, 1987). These divisions are (i.) polymetamorphic migmatite-gneiss complex, (ii.) low grade sediment dominated schists and (iii.) syntectonic to late tectonic granitic rocks which cut both the migmatite-gneiss complex and the schist belts.
The polymetamorphic migmatite-gneiss complex is composed largely of migmatites and gneisses of various compositions and amphibolites. Relict of meta-sedimentary rocks represented by medium to-high grade calcareous, pelitic and quartzitic rocks occur within the migmatites and gneisses, and they have been described as “Ancient Metasediments” by Oyawoye, (1972). The migmatite-gneiss complex is considered to be the basement sensustricto, and isotopic ages varying from Liberian to Pan-African have been obtained from the rocks. The Pan-African ages have been interpreted as due to isotopic re-homogenization in preexisting rocks during the Pan-African orogeny. Low-grade sediment dominated schists form narrow belts in the eastern half of the country have been described as “Newer Metasediments” (Oyawoye, 1972), “Younger Metasediments” and “Unmigmatised to Slightly Migmatised Schists” (Rahaman, 1976). These schist belts are believed to be relicts of a supracrustal cover which was infolded into the migmatite gneiss complex. The schist belts are intruded by Pan-African granitoids. Further attempts on the classification of the Nigerian Basement Complex (e.g. Rahaman, 1988) distinguished four major petro-lithological units, namely:
(i) The Migmatite – Gneiss- Quartzite Complex
(ii) The Schist Belt (Metasedimentary and Metavolcanic rocks)
(iii) Order Granites and associated granitic rocks(The Pan-African Granitoids
(iv) Undeformed Acid and Basic Dykes or The minor felsic and mafic intrusives.
184.108.40.206 THE MIGMATITE-GNEISS-QUARTZITE COMPLEX
This group of lithology is generally is considered to be the basement sensustricto (Rahaman, 1988; Dada, 2006 and isotopic ages varying from Liberian to Pan-African have been obtained from the rocks. The Pan-African ages have been interpreted as due to isotopic re-homogenization in preexisting rocks during the Pan-African orogeny (Ajibade et al., 1987). The polymetamorphic migmatite-gneiss complex is composed largely of migmatites and gneisses of various compositions and amphibolites. Relict of meta-sedimentary rocks represented by medium to-high grade calcareous, pelitic and quartzitic rocks occur within the migmatites and gneisses, and they have been described as “Ancient Metasediments” by Oyawoye, (1972).
220.127.116.11 THE SCHIST BELTS
The Schist Belts comprise low grade metasediment-dominated belts trending approximately N—S, which are belt developed in the western province of Nigeria. This Low-grade sediment dominated schists form narrow belts and has been described as “Newer Metasediments” by Oyawoye, (1972), or as “Younger Metasediments” by McCurry, (1976) and as “Unmigmatised to Slightly Migmatised Schists” by Rahaman, (1976). These belts consist of low to medium grade metamorphic rocks of mainly sedimentary and minor igneous origin, which were presumably deposited on the pre-existing migmatite-gneiss-quartzite basement and are, therefore, called supracrustal cover rocks (Adekoya, 1991). The schist belts of Nigeria occupynarrow N-S trending synformal troughs and formstrike ridges which break the monotonous landscapeof the Northern Plains. Each belt is separated from the adjacent belts by migmatites, gneisses or granites.
Theoutcrop pattern has been compared to that ofArchean greenstone belts (Wright and McCurry,1970; Turner, 1983) The schist belts differ fromgreenstone belts only in their bulk compositionwhich is predominantly clastic sediments withminor volcanic rocks. The schist belts are composed mainly of metamorphosed pelitic and semi-pelitic rocks. Howevereach belt differs in the amount and type of associated minor lithologies such as conglomerates greywackes, quartzites and volcanic rocks.The lithological differences between the belts have been used as evidence for suggesting that the schist belts were not deposited in the same basin and for suggesting the presence of two generations of schist belts (Grant, 1978; Turner, 1983; Fitches et al., 1985).
18.104.22.168 THE PAN AFRICAN GRANITES AND ASSOCIATED GRANITIC ROCKS
The Syntectonic to late tectonic granitic rocks cut both the migmatite-gneiss complex and the schist belts. The granitoids include rocks varying in composition from granite to tonalite and Charnockite with smaller bodies of syenite and gabbro. The granitoids have yielded radiometric ages in the range of 750-500 Ma which lie within the Pan-African age spectrum. These Pan-African granitoids are called the Older Granites in Nigeria to distinguish them from the Mesozoic, tin-bearing granite complexes of Central Nigeria which are referred to as the Younger Granites. Charnockites According to Dada (1989), it was at Toro that charnockite was first described within the Nigerian basement where it was then referred to as a “quartzdiorite porphyrite”.
It was assumed to present a certain affinity with the basic members of the charnockitic series of the Ivory Coast. Wright (1970) described it as an Undeformed Acid and Basic Dykes annular complex of hypersthene diorite at the centre of three circular, concentric granites. He considered the hypersthene diorite as older than the granites from contact relations. Cooray (1975) in his review of charnockitic rocks of Nigeria came to the same conclusion, using for argument the presence of granitic veins In the diorite,of dioritic xenoliths in granites and microcline porphyroblasts in the diorite. A field study of the same hybrid rocks led Rahaman (1981) to consider both the granites and the charnockites as either contemporaneous or the latter emplaced shortly after the former. The basement in Toro area consists of gneisses and migmatites into which the Toro Charnockitic Complex intruded (Dada et al., 1989); Older Granites and charnockites which constitute the complex proper; and undeformed basic (doleritic) dykes considered to be later than the Pan-African Granites.
22.214.171.124 THE MINOR FELSIC AND MAFIC INTRUSIVES
The Minor Felsic and Mafic intrusives are also refers to as the undeformed acid and basic dykes are late to post-tectonic Pan African that cross-cut the Migmatite-Gneiss Complex, the Schist Belts and the Older Granites in the Nigerian basement. This group of lithological units include:
a. Felsic dykes that are associated with Pan African granitoids on the terrain such as the muscovite, tourmaline and beryl bearing pegmatites, microgranites, aplites and syenite dykes. The age of the felsite dykes has been put at between 580 and 535 Ma from Rb-Sr studies on whole rocks (Dada, 2006)
b. Basic dykes that are generally regarded as the youngest units in the Nigerian basement such as dolerite and the less common basaltic, felsite and lamprophyricdykes. The basic dykes have a much lower suggested age of ca. 500 Ma (Grant, 1970).
1.7.2 TECTONIC SETTING AND EVOLUTION OF THE BASEMENT ROCKS
It is believed by some authors (e.g. Ajibade et al., 1987) that the evolution of the Nigerian basement can best be understood in the Western province of Nigeria. By the time of their report, detailed work has been done in two widely separated areas of this province in the northwest and southwest, and that correlated from these two areas is often uncertain but large of the knowledge of pre-African event comes from the southwestern terrain but field relationship of the low-grade schist belt comes from the northwest. Pioneering and recent works on the evolution of the Nigerian basement rocks have been reviewed and are documented in this work. Since pioneering works, it has been argued that this Nigerian basement complex comprises Archean and Proterozoic rocks which is believed to be the results of three major orogenic cycles of deformation, metamorphism and remobilization, and basement reactivation corresponding to the radiometric ages indicated by Liberian (2700 ± 200 Ma), Eburnean (2000 ± 200 Ma),Pan-African (600 ± 1500 Ma) which resulted from plate collision between the passive continental margin of the West African craton and the active Pharusian continental margin. It has been argued that the existence of the Kibaran (1100 ± 200Ma) event in Nigeria claimed by some workers (e.g.Grant et al., 1972; Ogezi, 1977).on the basis of some Rb/Sr dates on metamorphic rocks is not generally accepted i.e. the Kibaran (1100 Ma) event is not considered to have been a major tectonic event in Nigeria (Ajibade et al., 1987).
Geochronological and field evidence have confirmed the polycyclic nature of the Nigerian basement rocks, with the Pan-African event the last to have affected the basement. Evidence from pre-Pan-African events has been extremely difficult because of poor exposure and the overprinting effect of the Pan-African event (Ajibade et al., 1987), because the Pan-African metamorphism is in the upper greenschist and lower amphibolite facies, close to the grade attained by the pre-existing rocks which is predominantly in the middle to upper amphibolite facies. The migmatite-gneiss complex is considered to contain rocks of Archean age which have been deformed and modified several times prior to the Pan-African orogeny. Evidence from the Ibadan area indicates that the Archean rocks included metasedimentary and metavolcanic rocks which were deformed before the emplacement of the Eburnean granite gneiss. The granite gneiss is also considered to have been derived from pre-existing rocks by partial fusion.
The evolution of the Nigerian basement during the late Proterozoic is believed to be related to the activities taking place at the plate boundary (Caby et al., 1989). This can be best discussed in the regional context within the Trans-Saharan framework (Figure 8) of the Pan-African mobile belt of West Africa. This approximately 3000km long orogenic belt is said to occurs to the north and east of the ca 2 Ga West African Craton within the Anti-Atlas and bordering the Tuareg and Nigerian shields. It consists of pre-Neoproterozoic basement which was strongly reworked during the Pan-African event and of Neoproterozoic oceanic assemblages. The presence of ophiolites, accretionary prisms, island-arc magmatic suites and high-pressure metamorphic assemblages makes this one of the best documented Pan-African belts, revealing ocean opening, followed by a subduction and collision-related evolution between ca 900 and ca 520Ma. According to Caby (1989), the southern part of the Trans-Saharan Belt is exposed in Benin, Togo and Ghana where it is known as the Dahomeyan Belt. The western part of this belt consists of a passive margin sedimentary sequence in the Volta basin which was overthrust, from the east, along a well delineated
1.7.3 THE GEOLOGY OF IFE- ILESHA SCHIST BELT
The geology of Ilesha schist belt (Fig. 5 and 6) falls within this western sector and has been studied by Hubbard (1975), Elueze (1977), Ajayi (1981) and Rahaman and Ajayi (1988). The last authors recognized two contrasting lithologies separated by the Ifewarafault system. The eastern lithology is composed of metasediments dominated by quartz muscovite schist, quartzschist, quartzites, quartzo-feldspathic gneisses and biotite garnet schist. The western group consists of volcanosedimentary sequence of pelitic schists, quartzites and metabasites or mylonites. The entire area has been affected by polycyclic episodes of deformation and metamorphism (Odeyemi, 1981). In general, the major petrological units recognized in the area by Elueze (1988) and Odeyemi (1993) includemigmatites, gneisses, amphibolites and metaclastics. The amphibolites occur as lenticular bodies of differentvarieties within Ife-Ilesa area while the metaclastics comprise strongly sheared varieties of mylonites and biotiteschists. The quartzitic sequence occurs as massive quartzites, schistose quartzites and quartz schist (Anifowoseand Borode, 2007).
They also reported the existence of a series of faults that offset the fold trend as well as thedisplay of strong foliation and shearing by the schists and schistose quartzites (an indication of displacements along sub-parallel planes). Structures are well developed in the rocks of the area. Boesse and Ocan (1988) recognised two phases of deformation which produced folds of variable styles and large vertical fault zones in most rocks of the area. The area can be divided into two contrasting structural domains (Onyedim and Ocan, 2001). The NNE-SSW trending shear system (the Ifewara fault zone) in which the main structure is a NNE-SSW trending mylonitic foliation. This corresponds to the cataclastic rocks in the study area. Lithological units that include ridges of quartzite andquartzo-feldsparthic gneisses are parallel to this shear zone. The other dominant structures are NNW-SSE trending. It is interesting to note that an attempt to locate the study sites on the Geological Map of Okemesi Fold Belt (Odeyemi, 1993) indicated that the study sites plot within the fault zone and close to one of the fault traces of the map. Olarewaju and Ajayi, (1993) studies of the main ferromagnesian minerals in the amphibolites of the Ife-Ilesha schist belts shows that the overall chemical features of the ferromagnesian minerals suggest variable metamorphic load pressure conditions during regional metamorphism.
The amphiboles in the massive melanocratic amphibolites were syntectonic while that in the foliated leucocratic amphibolites were post-tectonics. In Okunlola and Okoroafor (2009), whole rock analytical results of major, trace and rare earth elements show that the rock units are comparable to those of post Archean pelitic-supracrustal rocks. The variation plots involving (Na2O3, Al2O3, and K2O) and (TiO2 and SiO2) reveal arkosic sedimentary progenitors for the rocks. Their analysis of La/Th and Th/U ratio suggest that the rocks especially biotite schist is associated with post Archean recycled Upper Crustal sources while Chondrite normalized rare earth signatures of samples further indicate low grade post Archean terrigenous sedimentation of rocks derived from possible mixture of granite- tonalities. Their study additional elucidates the possibility of the rocks evolving in a rifted environment of rapid subsidence, followed by closure which led to contemporaneous deformation of the sediments.
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