We have dated 1060 detrital zircon grains from the Neoproterozoic to lower Palaeozoic Volta basin, Ghana, and from sandstones in the adjoining thrust sheets belonging to the Pan-African Dahomeyide belt. All dated zircons in the lower Voltaian Bombouaka Group are older than ~ 1000 Ma, while samples from the middle Voltaian Oti Group and the upper Voltaian Obosum Group contain numerous zircons of 600-1000 Ma. The samples we have studied from the Dahomeyide thrust sheets (Buem and Togo structural units) have zircon age spectra similar to those from the Bombouaka Group, confirming a correlation of the investigated sandstones with lower Voltaian strata. The Bombouaka Group was deposited between ~ 1000 and 600 Ma, perhaps shortly after 1000 Ma, as suggested by earlier Rb-Sr data on clay minerals. Deposition of the Oti and Obosum Groups took place shortly after 600 Ma, perhaps continuing into the lower Palaeozoic. Most samples contain Palaeoproterozoic zircons with ages of 2000-2200 Ma that probably have been derived from the surrounding crystalline (Birimian) basement. Archaean zircons, present in smaller proportions, may have come from Archaean rocks in the West-African craton. Most zircons of 1000-1900 Ma were probably derived from sources outside the West-African craton, the Amazonian craton being a plausible source region. Zircons with ages around 1200 Ma are believed to have been derived from Grenvillian orogenic belts, perhaps those that fringe the Amazonian craton. If this is so, no major seaways could have been present between these belts and the West-African craton during the early Neoproterozoic. Zircons younger than 1000 Ma were probably derived from an eastern continental block that collided with the West-African craton during the Pan-African orogeny ~ 600 Ma ago.
- Programområde 4: Mineralske råstoffer