Granulite facies metamorphism and charnockite plutonism: examples from the Neoproterozoic Belt of northern Nigeria

Abstract

Incipient granulite facies conditions and charnockitization affected metasedimentary formations of northern Nigeria during the early stage of the Late Neoproterozoic (Pan-African) orogeny. This domain is characterized by recumbent foliations consistent with crustal thickening. Far from the Pan-African plutons, initial Barrovian-type metamorphism (medium temperature) evidenced by kyanite and staurolite inclusions is documented only from garnet cores. This early stage was followed by a temperature increase up to ≧700°C, with decompression coeval with the generation of garnet-cordierite melts and emplacement of garnet granitoid sheets. Hectometric-wide layers of non-retrograded granulites survived conformably within anatexites and S-type garnet-bearing anatectic granites from the Toro area. Temperatures of ≧800°C required for the crystallization of granulites facies rocks at c. 0.7 GPa pressure and coeval dry anatexis, were reached regionally as a result of heat supply from abundant anhydrous monzo-dioritic to noritic magmas injected as metre-wide veins and as stocks and plutons (charnockites, monzodiorites). The available geochronological results suggest a long-lived thermal perturbation (638–585 Ma) in this former continental domain of northern Nigeria rooted by Archaean crust that was later the site of the Jurassic anorogenic magmatism.