Geochemical Characterisation of Anatexite within High Grade Migmatite Complex Terrain from Ogbomoso, Southwest of the Nigerian Precambrian Basement Complex

Migmatite rocks are complex rocks largely due to the degree of partial melting and nature of parent rocks of this rock type. Partial melting or anatexis yield metatexite and diatexite components with variable mineralogical and chemical compositions. The nature of fractionation of elements during anatexis is not clearly understood. The migmatite quartzite gneiss complex of southwestern Nigeria exposed at Ogbomoso was studied in order to constrain the geochemical character of its metatexite and diatexite components.  Field mapping revealed migmatite with stromatic, surreitic and dictyonitic structures represented the metatexite while parts of the migmatite with schollen structure and granitic component represent the diatexite. Several sections of representative samples were cut and examined for mineralogical compositions. Thin section analysis revealed occurrence of hornblende and garnet with plagioclase, microcline, biotite and muscovite implying greenschist to amphibolite facies for the migmatite quartzite gneiss complex. Twelve representative samples were selected and analysed for their elemental concentrations using the Inductively Coupled Plasma Mass Spectrometry (ICP-MS). Major oxide geochemical data revealed a silica enriched migmatite with schollen structure having moderate alkali concentration while the granitic component and porphyroblastic gneiss with lesser silica concentrations showed relative Na2O and K2O enrichments respectively. Classification plots demonstrated that the migmatite of the study are ortho gneisses and have a Tonalite Trondjhemite Granodiorite (TTG) parentage. Trace element ratios revealed that the diatexite had lower than average upper continental crustal values for Nb/Ta ratio while the metatexite revealed average values slightly above 12. Normalised REE plot discriminated the migmatite with schollen structure as parts having HREEs closest to chondrite values