THE RELATIONSHIP BETWEEN GEOLOGY AND TRACE ELEMENTS STUDIES OF GRANITE AT GBALEOSU SOUTH WESTERN NIGERIAN

CHAPTER ONE

INTRODUCTION

1. Statement of the problem

Granite is a common type of felsic intrusive igneous rock that is granular and phaneritic in texture. Granites can be predominantly white, pink, or gray in color, depending on their mineralogy. The word “granite” comes from the Latin granum, a grain, in reference to the coarse-grained structure of such a holocrystalline rock. Strictly speaking, granite is an igneous rock with at least 20% quartz and up to 60% alkali feldspar by volume. The term “granitic” means granite-like and is applied to granite and a group of intrusive igneous rocks with similar textures and slight variations in composition and origin. These rocks mainly consist of feldspar, quartz, mica, and amphibole minerals, which form an interlocking, somewhat equigranular matrix of feldspar and quartz with scattered darker biotite mica and amphibole (often hornblende) peppering the lighter color minerals. Occasionally some individual crystals (phenocrysts) are larger than the groundmass, in which case the texture is known as porphyritic. A granitic rock with a porphyritic texture is known as granite porphyry. Granite differs from granodiorite in that at least 35% of the feldspar in granite is alkali feldspar as opposed to plagioclase; it is the potassium feldspar that gives many types of granite a distinctive pink color. The extrusive igneous rock is equivalent to granite rhyolite. Granite is classified according to the QAPF for coarse grained plutonic and named according to the percentage of quartz (Harvey Blatt & Robert J. Tracy 1997).Granite is nearly always massive (lacking any internal structures), hard and tough, and therefore it has gained widespread use throughout human history, and more recently as a construction stone.

The study area lies within the Western region of Nigeria. Nigeria can broadly be subdivided into three major geological components and these are: the Basement, Granites and Sedimentary Basins (Obaje, 2009). The basement (Precambrian in age) is further subdivided into five regions on the basis of the occurrence of sedimentary basins. These are the Western Nigerian Basement, North Central Nigerian Basement, Adamawa Highland, Eastern Nigerian Basement and the Oban Massif. As discussed by Obiora et al (2009) and references cited within, the basement consists of “migmatitic gneisses, including banded varieties; the schist belts constituted by mica-schists, tremolite-schists, graphite-schists, with occasional marbles and dolomites, calc-silicate rocks, meta-conglomerates and banded iron formation (BIF) and Precambrian granites including porphyritic/porphyroblastic muscovite granites, biotite granites, hornblende-biotite granites, non-porphyritic/non-porphyroblastic granites, aplites, granodiorites, diorites, quartz diorites, syenites, quartz-enstatite granites and enstatite granites (charnockites).”

Two generations of granites can be identified and these are prominent within the basement rocks. The older granites as termed by Falconer (1911) range widely in composition and age from 750 Ma to 450 Ma (Obaje, 2009). The younger granites (Jurassic in age) are prominently distributed in the North Central Nigerian Basement and occur as ring complexes which form part of a wider province of alkaline anorogenicmagmatism (Obaje., 2009).  Broadly speaking, the sedimentary basins can be stratigraphically divided into formations, the older Cretaceous sedimentary basins of which the Benue Trough, Bida-, and Sokoto Basins are the most prominent. The youngest sequence is the Tertiary Sedimentary Basins of which the Chad Basin is the most prominent as well as the sedimentary sequences of the Niger Delta. The geology of the area of interest in this present study falls within the Western Nigerian Basement of which the basement rocks cover almost 100% of the total land surface in this state (Oyedokun and Igonor, 2013). The study presents the results of the characterisation of granite samples obtained from a deposit site in Gbaleosu, South Western Nigeria and focused on the geology, major elements and composition of granite.

1. Aim and objectives of study

The main aim of this study is to undertake the trace elements study of granite from Gbaleosu. Other objectives of this study include:

• To produce an accurate geological map of the area.
• To identify the different rock lithologies present in the study area.
• To understand and establish the geologic history of the area.
• To analyze structural data acquired and their relevance to the current geology of the study area.
• To generally analyze the economic value of the rock lithology found in the area
• To examine and report all structural features