EARLY ARCHEAN COMPONENT (GREATER-THAN-3.5-GA) WITHIN A 3.05-GA ORTHOGENEISS FROM NORTHERN NIGERIA - U-PB ZIRCON EVIDENCE

Conventional U-Pb analyses on selected single zircon grains and on zircon fractions from a granodioritic orthogneiss at Kaduna (northern Nigeria) show that the Nigerian basement contains Archaean components that have not been recognized until now. A distinct U-rich zircon group records an age of 3040 +/- 60 Ma, which is interpreted as the age of emplacement and crystallization of the granodioritic magma. The occurrence of a magmatic event at ca. 3000 Ma is confirmed by the addition of core-free abraded grains (3050 +/- 23 Ma). These ages are identical to the less precise value (2980 +/- 90 Ma) given by zircon fractions. These results also demonstrate that zircons underwent Pb loss during the Pan-African orogeny at 612 +/- 23 Ma, which is well supported by an identical age on a single monazite grain (618 +/- 8 Ma). Moreover, U-Pb analyses of abraded U-poor core-bearing crystals clearly identify the presence of source material of early Archaean age (> 3500 Ma), the oldest age for crust initiating processes ever found in this area. In contrast to earlier Rb-Sr geochronology no evidence for an Eburnean (2000 +/- 200 Ma) or Yibaran (1100 +/- 200 Ma) orogeny is present in the U-Pb systems of these zircons.