Methods
Detailed geological mapping was done through traversing and positioning with the aid of a GPS. This was carried out in order to understand the geology of the area and determine the different rock types present. Petrographic analyses of selected rock samples were done. The samples were selected based on physical similarities and hand specimen mineralogical composition.
Seven samples (one pegmatite, two granites, and four gneisses) were collected for whole rock analyses. On petrological grounds, the best-preserved samples were selected for analyses and thin section preparation. Samples were crushed to powder form for whole rock analyses using a 9900 OASIS Integrated X-ray Analyzer at the Lafarge Cement WAPCO Nigeria laboratory. The result obtained from the XRF analysis was normalized using the CIPW Norm.
The values from the XRF results were recalculated to total of 100%. The total may not exactly be 100% because of rounding error. The normative minerals were estimated based on the recalculated percentages of the oxides. The Weight norm% is the normative components of the minerals calculated weight percent. The Volume% norm was calculated in terms of the volume percentage in the normative rock, rather than their weight percentage. The Fe3+/ (total Fe) in rock is the molar ratio of ferric iron to total iron in the rock composition used in the norm (specifically the rock in the corrected analysis). This ratio is a measure of the oxidation state of the rock, expressed out of 100%. The Mg/ (Mg + total Fe) in rock, is the molar ratio of Mg to the total of Mg and total of iron in the rock (specifically in the corrected analysis). It is a measure of the differentiation of an igneous rock. This ratio is independent of the degree of oxidation of the iron in the rock. The Mg/(Mg + Fe2+) in rock is the molar ratio of Mg to the total of Mg and total of Fe2+ in the rock (specifically in the corrected analysis); it is also a measure of the differentiation of an igneous rock. Ca/ (Ca + Na) in rocks and Ca/ (Ca + Na) in plagioclase (albite and anorthite) are also measures of differentiation.
Differentiation index used in the normalization is the Thornton Tuttle differentiation which is the ratio of normative (quartz + albite + orthoclase + nephiline + Leucite + calcite + sodium + carbonate + sodium sulphate) to the weight total of the norm. The calculated density is the solid rock density calculated from the volume norm except for the exclusion of water in the norm (admittedly a substantial problem) while the calculated liquid density g/cc was determined assuming a dry magma and the crudely estimated liquid temp (Mc Birney, 1993). The calculated viscosity, dry, is the log base 10 of the liquid viscosity in Pascal seconds assuming a dry magma and a crudely estimated liquidus temp (Mc Birne, 1993) while the calculated viscosity wet, log10 of the liquid viscosity in Pascal seconds. Percentage water in the magma was estimated is from “Estimated water content” of Mc Birney, 1993.