U-Pb Pan-African ages of two charnockite-granite associations from Southwestern Nigeria

U-Pb dating of zircons from two separate charnockite granite associations from S.W. Nigeria, previously considered to be of Kibaran or Archean age, yield PanAfrican emplacement ages of 631±18 Ma and 586±5 Ma respectively when all the data points of each association are regressed together. These U-Pb data on zircons do not permit a confirmation of the field-established sequence of rock emplacement within each charnockite-granite association; thus synchronous ages of 620±20 Ma (charnockite NIG 4), 634±21 Ma (charnockite NIG 7) and 621±10 Ma (granite NIG 12) are obtained for the three analyzed rocks of the Akure-Ikerre-Ado Ekiti association while those of the Idanre association yield younger but synchronous ages of 580±10 Ma (charnockite NIG 2), 593±13 Ma (charnockite NIG 3) and 587±10 Ma (granite NIG 11). The ages obtained for the rocks of the Akure-Ikerre-Ado Ekiti association very well fit the existing data on the tectonic evolution of the basement complex of Nigeria, whereas those from the Idanre area are difficult to reconcile with the current interpretation of tectonic data in this area.     

Characterization and Mineralization Potentials of Granitic Pegmatites of Komu area, Southwestern Nigeria

The geology of the Komu area, south-western Nigeria was investigated in order to determine the nature of occurrence, petrography and chemical composition, and to evaluate the mineral potential of granitic pegmatites. The pegmatites are hosted by para-gneisses, amphibolites and granitic rocks. These host rocks are characterized by low pressure mineral assemblages of upper greenschist to lower amphibolite facies grades. The pegmatites consist of four lithological units: alkali feldspar unit, graphic granite unit, quartz-albite-muscovite-tourmaline unit and saccharroidal albite-muscovite-tourmaline-garnet unit.
A K/Rb ratio of below 100 is indicative of a mineralization potential in most pegmatites, except for the saccharroidal albite-muscovite-tourmaline-garnet unit which has exceptionally high ratios. The discrimination diagrams Rb versus K/Rb, Zn versus K/Rb and Th versus K/Rb show a limited range of fractionation, suggesting that the pegmatites belong to the barren muscovite and rare element classes. The rare element class is moderately enriched in beryl, tourmaline and niobium–tantalum minerals. The pegmatite has affinity for beryl columbite sub-type and lithium–cesium–tantalum (LCT) family. The ores plot mainly in the field of mangano-tantalite of tantalite-columbite quadrilateral diagram. These results indicate that the pegmatites are promising hosts for Nb-Ta ores.