Age and origin of the annular charnockitic complex at Toro, Northern Nigeria: UPb and RbSr evidence

Abstract

The Toro Charnockitic Complex (T.C.C.) is composed essentially of hypersthene diorite and biotite hornblende granites intrusive in a basement composed mainly of migmatites and orthogneisses. The latter have been deformed and metamorphosed during the Pan-African orogeny. UPb data from T.C.C. zircons indicate that the hypersthene diorite and the associated porphyritic granite were emplaced at 585 ± 7 Ma. and 607 ± 11 Ma. respectively. These ages disagree with previous assumptions of Kibaran of Archaean time of emplacement for this type of charnockitic complex in Nigeria. Our results place the Toro Complex precisely in the series of Pan-African granitoids. UPb geochronology and Sr isotope results neither imply different sources for the granites and the charnockites, nor suggest an entirely mantle source at 600 Ma.; the initial isotope ratios ( 87Sr/ 86Sr)i range from 0.70619 to 0.71015 for the two rock types. It is in effect a case of significant crustal contamination. We envisage the melting of lower crustal material during the Pan-African. Charnockitic magma, produced by fusion of tonalitic restite under dry granulite facies conditions, was contaminated by granitic magma under more hydrous amphibolite facies conditions at higher crustal levels; a phenomenon corroborated by the occurence of hybrid rocks at the contact between the two rock types. This model is reinforced by UPb data which implies a contemporaneous emplacement for both granites and charnockites of the Toro Complex.