REASSESSMENT OF PROTEROZOIC GRANITOID AGES IN GHANA ON THE BASIS OF U/PB ZIRCON AND MONAZITE DATING

The West African Craton in Ghana is of Early Proterozoic age and forms alternating volcanic belts and sedimentary basins made up of rocks of the Birimian Supergroup, as well as younger conglomerates and quartzites of the Tarkwaian Group, and extensive granitoid plutons. No consensus exists on absolute and relative ages of plutons within belts and basins. U-Pb geochronology of four different granitoid complexes indicates that, contrary to long-held views, belt granitoid plutons (Ashanti belt, 2172 +/- 2 Ma; Sefwi belt, 2179 +/- 2 Ma) are approximately 60 to 90 Ma older than basin plutons (Kumasi basin, 2116 +/- 2 Ma; Sunyani basin, 2088 +/- 1 Ma). Belt granitoids are approximately coeval with volcanic rocks of the Birimian Supergroup, while basin plutons are late-kinematic and postdate Tarkwaian sedimentation. The first high-precision geochronological data for rocks from Ghana, i.e., the eastern part of the Birimian domain on the West African craton, presented here show that magmatism within this region occurred over a time-span of at least 90 Ma, and provide further evidence that the period between 2.1 and 2.2 Ga was one of important magmatism in West Africa.