Charnockitic and monzonitic Pan-African series from north-central Nigeria: Trace-element and Nd, Sr, Pb isotope constraints on their petrogenesis

Major- and trace-element studies indicate that the evolution of the calc-alkaline (charnockitic) and sub-alkaline (monzonitic) Pan-African rocks in north-central Nigeria cannot be explained in terms of a unique or classical process of fractional crystallisation. At Toro, the crystallisation history of the more differentiated members appears more complex, with the participation of accessory phases. The quartz-fayalite monzonites show evidence of a cumulative phase during their differentiation. The fayalite-bearing quartzmonzonites are not common rocks and their occurrence in the Nigerian basement remains the only reported exposure in the Pan-African belt of West Africa. Like the charnockitic suite at Toro, they were emplaced during the Pan-African and at deep structural levels unlike the adjacent high-level Jurassic peralkaline granites of Jos Plateau. plication of multiple radiogenic tracers (Sr, Nd and Pb) indicates the involvement of pre-existing crust in the formation of these rocks (ϵSr(i) = + 30 to + 86, ϵNd(i) = − 15.5 to − 5.5, 206Pb204Pbi = 16.34–17.71 and 207Pb204Pbi = 15.48–15.58 for Toro Charnockitic Complex; and ϵSr(i) = + 19 to + 51 and ϵNd(i) = − 8.0 to − 4.0 for the quartz-fayalite monzonites of Bauchi). Long crustal residence ages (TNd) of between 2.0 and 1.5 Ga preclude their complete derivation from the mantle. The bulk of evidence is consistent with the partial melting of a U-poor source typical of the lower crust during the Pan-African orogeny and in particular the involvement of older Precambrian crust.