Late Archaean UPb age for the reactivated basement of Northeastern Nigeria

A fundamental question in the evolution of the Pan-African/Brazilian domain in West Africa and Latin America concerns the degree and extent to which its continental crust was affected by the Late Proterozoic event. There are only a few ages on rocks that constitute the older nucleii within the remobilised zone. We present UPb data on zircons from the reactivated migmatite-gneissic basement in North-eastern Nigeria that constitute the first evidence of Late Archaen basement. The data can be explained by an important imprint of the Pan-African thermotectonic event resulting in considerable loss of the accumulated radiogenic lead in zircons from a 2.7 Ga. basement to give minimum upper (2.5 Ga.) and lower (550-500 Ma.) intercept ages for the gneissic basement around Toro and a better defined Pan-African lower intercept age of 599 Ma. for the anatectic Jada granite.

The young lower intercept ages are similar to available KAr and RbSr metamorphic/cooling ages and support the long interval of at least 200 Ma (>650 to 450 Ma) for the Pan-African orogeny. This lends credence to the translational hypothesis of the deformational activity from the west (near the suture) towards the east with time. These results confirm the heterogenous nature of the Pan-African event and call for a combination of methods to more precisely determine the primary age of crystallisation of the old basement in multiply metamorphosed/deformed terrains. The Eburnean (2.2-2.0 Ga.) event is not identified by the present work nor by UPb data on single zircon grains and calls for further study to properly define its spatial significance in the region.