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CHAPTER ONE: INTRODUCTION
Schist is a medium-grade metamorphic rock with medium to large, flat, sheet-like grains in a preferred orientation (Allaby, 2013). It is defined by having more than 50% platy and elongated minerals, often finely interleaved with quartz and feldspar (Jackson, et al., 2005). According to Bishop et al. (1999), these lamellar (flat, planar) minerals include micas, chlorite, talc, hornblende, graphite, and others. Quartz often occurs in drawn-out grains to such an extent that a particular form called quartz schist is produced. Schist forms at a higher temperature and has larger grains than phyllite. Foliation (metamorphic arrangement in layers) with medium to large grained flakes in a preferred sheetlike orientation is called schistosity.
The names of various schists are derived from their mineral constituents. For example, schists rich in mica are called mica schists and include biotite or muscovite (Allaby, 2013). Most schists are mica schists, but graphite and chlorite schists are also
common. Schists are also named for their prominent or perhaps unusual mineral constituents, as in the case of garnet schist, tourmaline schist, and glaucophane schist.
The individual mineral grains in schist, drawn out into flaky scales by heat and pressure, can be seen with the naked eye. Schist is characteristically foliated, meaning that the individual mineral grains split off easily into flakes or slabs. Most schists are derived from clays and muds that have passed through a series of metamorphic processes involving the production of shales, slates and phyllites as intermediate steps. Certain schists are derived from finegrained igneous rocks such as basalts and tuffs. Schists are frequently used as dimension stone, which is a stone that has been selected and fabricated to specific shapes or sizes.
During metamorphism, rocks which were originally sedimentary, igneous or metamorphic are converted into schists and gneisses. If the composition of the rocks was originally similar, they may be very difficult to distinguish from one another if the metamorphism has been great. A quartz-porphyry, for example, and a fine grained feldspathic sandstone, may both be converted into a grey or pink mica-schist. Usually, however, it is possible to distinguish between sedimentary and igneous schists and gneisses. If for example, the whole district occupied by these rocks has traces of bedding, clastic structure, or unconformability, then it may be a sign that the original rock was sedimentary. In other cases intrusive junctions, chilled edges, contact alteration or porphyritic structure may prove that in its original condition a metamorphic gneiss was an igneous rock. The last appeal is often to the chemistry, for there are certain rock types which occur only as sediments while others are found only among igneous masses, and, however, advanced the metamorphism may be, it rarely modifies the chemical composition of the mass very greatly. Such rocks as limestones, dolomites, quartzites and aluminous shales have very definite chemical characteristics which distinguish them even when completely recrystallized.
The schists are classified principally according to the minerals they consist of and on their chemical composition. For example, many metamorphic limestones, marbles, and calc-schists, with crystalline dolomites, contain silicate minerals such as mica, tremolite, diopside, scapolite, quartz and feldspar (Wikipedia, 2016). They are derived from calcareous sediments of different degrees of purity. Another group is rich in quartz (quartzites, quartz schists and quartzose gneisses), with variable amounts of white and black mica, garnet, feldspar, zoisite and hornblende. These were once sandstones and arenaceous rocks. According to Encyclopedia Britanica (2014), the graphitic schists may readily be believed to represent sediments once containing coal or plant remains; there are also schistose ironstones (hematite-schists), but metamorphic beds of salt or gypsum are exceedingly uncommon.
The majority of mica-schists, however, are altered claystones and shales and pass into the normal sedimentary rocks through various types of phyllite and mica-slates. They are among the most common metamorphic rocks; some of them are graphitic and others calcareous. The diversity in appearance and composition is very great, but they form a well-defined group not difficult to recognize, from the abundance of black and white micas and their thin, foliated, schistose character. A subgroup is the andalusite-, staurolite-, kyanite- and sillimanite-schists which usually make their appearance in the vicinity of gneissose granites, and have presumably been affected by contact metamorphism (Albert and Slate, 1920).
The branch of petrology that deals with the classification and description of rocks by microscopic examination is petrography. Petrography focuses on the detailed description of rocks. The mineral content, textural relationships and the characteristics of the rock are described in detail. The most important tool in the petrographic analysis is the petrographic microscope.
The aim of this study is to undertake a petrography study of schist in Aiyegunle Akoko-Edo Local Government Area of Edo State. Specifically, the study will:
Aiyegunle is in Akoko-Edo Local Government Area of Edo State, that lies within Latitude 7014I 3.2 IIN to Longitude 605I 52II E; Latitude 7017IN to Longitude 6010IE.
Fig 1: Accessibility Map of Aiyegunle
The climate condition of Aiyegunle falls within the warm humid tropical climate region where the wet and dry seasons are noticed prominently in the area with a temperature range of 36.70 especially within the hottest period of February and April. Vegetation has been altered thousands of years ago and agricultural crops such as cassava, cocoa, yam are grown there.
The distinct relief regions in the state include the swamps/creeks, the Esan Plateau, Orie Valley and the dissected uplands of Akoko-Edo Local Government Area which ranges from 183 to 305m. Outstanding features of the upland include granite peaks, which rise above 610 meters and sandstone in the South. During the rainy season, the aquifer is recharged and water flows through but in the dry season, the aquifer is not recharged resulting in the drying up of the streams.
The soil type in Akoko-Edo environs consists of shallow/stony reddish clay at the feet of inselberges in the higher sections, lateritic clay and fine grained to sandy soil in the upper slope, lateritic and ferruginous soils on the crystalline acid rocks of the Basement Complex.
A thorough geological mapping was done in the study area aided by field maps and Global Position System (GPS), Samples were collected and examined in–situ. The samples collected were taken for laboratory analysis. The results were interpreted in this study and various deductions like mineralogical composition was made.
According to Ako and Onoduku (2012), no documented information is presently available for the Pan–African geology of Aiyegunle and the information utilized in this research project is mainly from intense field work and microscopic study.
The ancient crystalline basement had been reactivated by the Pan–African Orogeny during which series of granite suite were emplaced and into which supracrustal sediments were unfolded (Grant, 1969). The schist belts are believed to be relicts of an ancient supracrustal lower which has folded into the migmatite-gneiss complex (Russ, 1957).
Gneiss from North central Nigeria has been suggested to be oldest basement rocks in Nigeria (Oyawoye, 1984; Kroener, et al., 1978).
Important work done on the geology of North Central Nigeria include those of Ejueme (1982) Onyegocha (1984) Ayi, (1988). The Nigerian basement lies within the reactivated part of the Pan–African belt (Turner, 1980, Ajibade and Fitches, 1988) structural trends in the Basement Complex have been greatly influenced by orogenic cycles which includes the Liberian (Oversby, 1972) and Pan–African (Ogezi, 1977) which produced structural trends, granitization and migmatite gneiss, quartzite, the schist belt, the older granite series and the undeformed acid and basic dykes (McCurry, 1971). The majority of radiometric ages from the Nigerian crystalline rock lies in the range 600_+ 150 ma (Grant, 1969) dating the final imprint of the Pan African.