The Pan-African reactivation of Eburnean and Archaean provinces in Nigeria: structural and isotopic data

Abstract

Eastern Nigeria experienced high-grade metamorphism during the Pan-African collision event marked by a pervasive migmatization. Structural evidence demonstrates that anatexis was contemporaneous with regional deformation and emplacement of calc-alkaline granite-diorite plutons. New Pb-evaporation data on single zircons from the migmatites confirm that partial melting and plutonism occurred synchronously around 580 ± 10 Ma. The Pan-African reactivation is almost ubiquitous and therefore the protolith age is still debated. However, Sm-Nd analysis of migmatitic gneisses from Eastern Nigeria yielded Eburnean model ages of the protolith between 1.7 and 2.0 Ga, in agreement with previous model ages of 1.3 and 2.0 Ga from granitoids and gneisses of the same area. These results suggest that the protoliths in Eastern Nigeria are distinct from those from the Western Archaean province. Furthermore, recent work has shown that, further north, in the Tuareg shield, the Pan-African belt consists of amalgamated terranes. Thus, a terrane model could explain the contrast between Eastern Nigeria, characterized by Eburnean protoliths and calc-alkaline granitic rocks and Western Nigeria, characterized by Archaean protoliths and units having tonalite-trondhjemite-granodiorite and greenstone affinities. The contrasted terranes also correspond to a gold province in the West and to a tin province in the East of Nigeria.