Petrography, Geochemistry and Petrogenesis of grey Gneisses of part of Oke-Ode Area, South-Western Nigeria
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Date
2022
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Abstract
Oke-Ode area, northeast of Ilorin, is part of the Precambrian Basement Complex of
southwestern Nigeria. There are no published data on the gneisses, which are well exposed,
unlike the adjacent terrains. The area was mapped in order to determine the petrographic,
geochemical and petrogenetic attributes of the grey gneisses. The acquired sets of field,
mineralogical, and geochemical data indicated that the medium-to coarse-grained gneisses
are of igneous origin, and have witnessed multiple tectono-thermal readjustments. Occurrence
of xenoliths in the rocks points to magmatic origin. The range of anorthite molecular contents
of the plagioclase (An26-34) suggests an andesine to oligoclase composition, indicating
derivation from granodioritic to tonalitic progenitors. Petrographic studies showed that the
rocks witnessed complex interplay between metamorphism, deformation, and migmatization,
which culminated in grain-boundary migration under a relatively stable, low-strain, hightemperature
conditions involving mechanical rotation, during grain-scale dynamic
recrystallization. Geochemically, plots of Ni vs Zr/TiO₂ and K₂O/Al₂O₃ vs Na₂O₃/Al₂O₃ also
constrained the the gneisses to the igneous field while the TiO2 - K2O - P₂O₅ ternary plot
further indicated a continental tectonic setting prior to the widespread Pan African
magmatism. The rocks are essentially peraluminous calc-alkaline rocks, which are moderately
saturated with respect to silica. They are products of fractional crystallization of a basaltic
magma in a continental setting but later reworked during the Pan African time. The grey
gneises have similar geo-chemical characteristics with some gneisses from other parts of the
Basement Complex of Nigeria, but at distinct variance with others.
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Research Paper
Keywords
Grey gneisses, geochemistry, xenolith, tonalitic gneiss, peraluminous, precambrian