PETROCHEMISTRY, Pb ISOTOPE SYSTEMATICS, AND GEOTECTONIC SETTING OF THE GRANITE GNEISSES IN ILESHA SCHIST BELT, SOUTHWESTERN NIGERIA

Abstract

The petrology, geochemistry, geotectonic setting and common Pb isotope model ages for the granite gneisses in Ilesha schist belt have been studied and presented in this paper. These gneisses, apart from the normal rock-forming silicates, contain apatite, monazite, ilmenite and zircon in trace amounts. The occurrence of monazite in the assemlage for these rocks indicates crustal input into their original precursor rock. The K/Na ratios, greater than one (1) and AFM plots for the Ilesha schist belt gneisses suggest that they are calc-alkaline in nature, and related to a subduction tectonic setting. Low U/Pb and Th/Pb ratios (0.10 to 0.31 and 0.33 to 1.35, respectively) and normal Pb content (30-47 ppm) show that the common Pb component of these gneisses have not been contaminated with radiogenic Pb. Common Pb model ages obtained for the granite gneisses indicate that they were probably emplaced around 2640 Ma and deformed during the Proterozoic. Plumbotectonic plots for the gneisses suggest that they were formed in an orogen where a crustal plate has been subducted into the mantle to form a mixed hydrous precursor magma for the protocontinent from which the gneisses were derived.



KEY WORDS; plumbotectonic, protocontinent, orogen, tectonic, radiogenic, errochron.


Global Jnl Geological Sciences Vol.2(1) 2004: 1-13