PETROCHEMICAL EVIDENCE OF ANATECTIC SOURCE FOR QUARTZO-FELDSPARTIC GNEISSES AND GRANITE IN IKARE-AKOKO, SOUTHWESTERN NIGERIA

Ikare-Akoko in southwestern Nigeria is underlain by grey gneiss, granite-gneiss, pelitic gneiss and granite. The gneissic units form a major component grouped together as Migmatite-gneiss-quartzite complex. Field investigation reveals the gneisses are low-lying structurally heterogenous units bearing pervasive tectonic signatures attributable to polymetamorphic and multiple orogenic activities. The granite occurs as massive intrusive bodies within the gneissic country rock. Mineralogical investigation (modal composition) reveals the gneisses contain dominantly of quartz, K-feldspar and plagioclase with subordinate biotite, and hornblende interlocked into granoblastic textures. The granite is porphyritic and contains K-feldspar phenocrysts, plagioclase, microcline and quartz. Major and trace elements composition from ICP-MS techniques in Bureau Veritas, Vancouver Canada, reveals average SiO2 contents of the grey gneiss (72.15%), granite gneiss (72.97%), pelitic gneiss (70.35%) and granite (73.10%) classifies them as leucocratic or quartzo-feldspartic. The overall average magnesia (MgO; 0.39%) and iron (Fe2O3; 3.82%) contents for the gneisses, granite (MgO, 0.62%; Fe2O3, 4.02%) and FeOt/(FeOt+MgO) versus SiO2 variation plot classifies all the rock units as ferroan. Total alkali (Na2O+K2O) versus SiO2 (TAS) classifies the rocks as granite. AFM [(Na2O+K2O)-Fe2O3-MgO] ternary diagram reveal the rocks are calc-alkaline. A consistently high ACNK (>1.1) and SiO2 versus ACNK binary diagram indicate the rocks are metaluminous. Similar geochemical trends in the basement rocks and the binary plot of TiO2 versus SiO2 suggest the crystalline rocks from Ikare-Akoko area originated from igneous precursor. The lithologies were possibly derived from anatectic melts of same ancient igneous protolith indicating the gneisses and granite are genetically related and tectonically connected.