EARLY PROTEROZOIC CRUSTAL EVOLUTION IN THE BIRIMIAN OF GHANA - CONSTRAINTS FROM GEOCHRONOLOGY AND ISOTOPE GEOCHEMISTRY

Representative suites of Ghanaian granitoids, metavolcanics and metasediments were selected from Birimian rock units for Rb-Sr, Pb/Pb and Sm-Nd analyses. Isotopic data on the igneous rocks provide evidence for a major Early Proterozoic crust-forming episode which took place over a maximum time interval from approximately 2.3 to approximately 2.0 Ga by differentiation from a depleted mantle source. There is little evidence for the involvement of significantly older (e.g. Archaean) crust in the genesis of the igneous rock units. The only exception so far found is the Winneba granitoid, which appears to contain a magmatic contribution from presumably underlying Archaean basement. Sm-Nd model ages indicate that Birimian sediments were derived from adjacent penecontemporaneous volcanic belts. The Birimian of Ghana forms part of a major Proterozoic (Eburnean) episode of juvenile crustal accretion which has been recognised in the surrounding areas of Mauretania, Senegal, Ivory Coast, Burkina Faso, Mali and Niger, where it has been dated at 2.1-2.2 Ga (Abouchami et al.). The Birimian terranes bridge a major gap in mantle activity and associated crustal evolution for a period considered as quiescent on other continents.