Imasuen O. I. and Onyeobi T. U. S.
A detailed geologic mapping of various rock types around Ganaja in Kogi State (Nigeria) was carried out on a scale of 1:25,000. The area lies between latitudes 7042I and 7044IN; and between longitudes 6042I and 6045IE covering about 29km2. Field observations and a study of the rock samples collected indicate that the rocks belong to the migmatite – gneiss complex of the south-western Nigeria Basement Complex. The area is underlain predominantly by migmatite, augen gneiss, biotite gneiss as well as minor occurrences of pegmatite and quartzo – feldspathic veins. Petrographic analyses of the rock samples from this area show the migmatites to generally consist of quartz, feldspar (plagioclase, microcline and orthoclase), biotite and a few accessory minerals like epidote and zircon. The geochemical analysis data and petrographic details suggest an igneous parentage for the migmatites. However, with their uncomplicated mineralogical composition, these migmatites differ from those of the near-by north-eastern part of Lokoja which are of pelitic rock parentage. Consequently, it is suggested that a deformational episode occurred along with the metamorphism of the various rocks of the mapped area and its environs resulting in varied metamorphic derivatives ranging from the amphibolite facies to higher metamorphic facies condition. These migmatites may have been formed from the metamorphism and metasomatism of fractionated igneous bodies during tectonism. The segregation and migration of the melting minerals such as quartz and feldspar during regional metamorphism resulted in the banding of the leucosome and melanosome minerals. The outcrops and their associated foliation generally trend in NNE – SSW directions
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