Chemical Geology 124 (1995) 233-252 ISOTOPE GEOSCIENCE Charnockitic and monzonitic Pan-African series from north-central Nigeria: Trace-element and Nd, Sr, Pb isotope constraints on their petrogenesis S.S. Dada aT*, L. Briqueu b, U. Harms ‘, J.R. Lancelot b, G. Matheis ’ ’ Geology Programme, Abubakar Tafawa Balewa University, P.M. B. 0248, Bauchi, Nigeria b Laboratoire de Giochimie Isotopique, U.R.A 13-71. University of Montpellier II, F-34095 Montpellier, France ’ Technical Vniversiry of Berlin, SFB 69, Ackerstrasse 71-76, D-1000 Berlin 65, Germany Received2 September1993;accepted after revision 23 January 1995 Abstract Major- and trace-element studies indicate that the evolution of the talc-alkaline (chamockitic) and sub-alkaline (monzonitic) Pan-African rocks in north-central Nigeria cannot be explained in terms of a unique or classical process of fractional crystallis- ation. 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 quartz monzonites 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 chamockitic 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 tracers (Sr, Nd and Pb) indicates the involvement of pre-existing crust in the formation of these rocks (esrci) = + 30 to + 86, eNdci) = - 15.5 to -5.5, 206Pb/za’Pbi= 16.34-17.71 and 207Pb/2”“Pbi = 15.48-15.58 for Toro Chamockitic Complex; and ?? srci)= + 19 to + 5 1 and eNaci, = - 8.0 to - 4.0 for the quartz-fayalite monzonites of Bauchi) . Long crustal residence ages ( TNI) 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. 1. Introduction and geological setting The basement in northern Nigeria is the most exten- sive and best exposed portion of the Nigerian Basement Complex. The Early Archaean ( > 3.5 Ga) to Late Archaean (2.5-2.7 Ga) Gneiss-Migmatite Complex forms the basement i.n the region (Dada, 1989; Dada et al., 1993; Bruguier et al., 1994). It has been meta- morphosed under medium- to high-amphibolite facies * Corresponding author. [NAI 0009-2541/95/$09.50 0 1995Hsevier Science B.V. All rights reserved SSD10009-2541(95)00010-O conditions. The rocks are largely granitic to granodio- ritic in composition with minor tonalitic types. Amphibolites occur as dispersed blocks or dykes in the migmatitic gneisses. There is a regional vertical NE- SW- and NNl-SSW-trending foliation while locally disturbed, particularly strongly migmatised areas have, in addition, an E-W trend. The general folding pattern consists of open and asymmetric flexures of low plunges to the north and northwest, with fold axes mainly of N-S strike (Wright, 197 1) . Recent geolog- ical mapping in the area adjacent to the Palaeocene