GEOLOGICAL OBSERVATIONS OF BASEMENT ROCKS, AROUND GANAJA, KOGI STATE, NIGERIA
Imasuen, O.I1*, Olatunji J.A2 and Ibitoye V.T2
1 Department ofGeology, University of Benin, Benin City
2 Department of Earth Sciences, Kogi State University, Anyigba
ABSTRACT
A detailed geologic mapping coupled with petrographic mineral separation and geochemical studies were carried out on various rock types around Ganaja in Kogi state. The area is located between latitudes 7o 42/ and 7o 44/N and longitude 6o 42/ and 6o 45/ E and is about 29 square kilometre (29km2). Mapping was done by transverse mapping method together with systematic sampling of the outcrops on a scale of 1:25,000. Field relations observed on the rock samples collected from this area coupled with available information and literature showed that the rocks belong to the migmatite gneiss complex of the south-western basement complex of Nigeria. The rock types within the area of study include migmatite, augen, gneiss (porphyroblastic granite gneiss) biotite gneiss and pegmatite. There were also minor occurrences of quartz and quartzo – feldspathic veins. Petrographic and chemical analysis 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 chemical analysis data and the petrographic studies of the migmatites mapped in the study area suggest its igneous parentage. However, with their uncomplicated mineralogical composition these migmatites differ from those of the north-eastern part of Lokoja which are of pelitic rocks parentage. It is being suggested that the deformational episode occurred along with the metamorphism of the various rocks of the mapped area and its environs causing varied metamorphic derivatives from the amphibolite facies to a 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 in the rocks during regional metamorphism resulted in the banding of the leucosome (light) and melanosome (dark) minerals. The outcrops and their associated foliation generally trend in NNE-SSW and NNW-SSW directions.KEYWORD: Ganaja migmatites, geochemical , metamorphism leucosome , melanosome