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 ca lc-alkaline (charnockitic) and sub-alkaline (monzonitic) Pan-African r ocks in north-central Nigeria cannot be explained in terms of a unique or classical process of fractional crystallisation. At Toro, the crys tallisation history of the more differentiated members appears more co mplex, with the participation of accessory phases. The quartz-fayalite monzonites show evidence of a cumulative phase during their different iation. The fayalite-bearing quartz monzonites are not common rocks an d 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. The application of multiple radiogenic tracer s (Sr, Nd and Pb) indicates the involvement of pre-existing crust in t he formation of these rocks (epsilon(Sr(i)) = + 30 to + 86, epsilon(Nd (i)) = - 15.5 to - 5.5, Pb-206/Pb-204(i) = 16.34 - 17.71 and Pb-207/Pb -204(i) = 15.48 - 15.58 for Toro Charnockitic Complex; and epsilon(Sr( i)) = + 19 to + 51 and epsilon(Nd(i)) = - 8.0 to - 4.0 for the quartz- fayalite monzonites of Bauchi). Long crustal residence ages (T-Nd) of between 2.0 and 1.5 Ga preclude their complete derivation from the man tle, The bulk of evidence is consistent with the partial melting of a U-poor source typical of the lower crust during the Pan-African orogen y and in particular the involvement of older Precambrian crust.