U-PB AGE CONSTRAINTS ON DEPOSITION AND PROVENANCE OF BIRIMIAN AND GOLD-BEARING TARKWAIAN SEDIMENTS IN GHANA, WEST-AFRICA

U-Pb ages have been measured on single detrital zircon grains from sediments in the Palaeoproterozoic Eburnean rocks of Ghana. Gold-bearing and barren Tarkwaian conglomerates in the Tarkwa basin, and a volcanogenic Birimian sediment from the Kumasi basin all show similar age patterns. Most of the measured detrital zircon ages from the Tarkwaian sediments are in the range 2194-2132 Ma (20 ages), similar to the range 2184-2135 Ma (6 ages) found for the Birimian sediment. One Tarkwaian zircon grain has a slightly older age of 224 5 +/- 4 Ma. A granitoid sample from the Cape Coast pluton gives an age of 2090 +/- 1 Ma from monazite and zircon. This age appears to represent a regionally significant time of granitoid plutonism in the sedimentary basins. Many of the zircon ages for the Tarkwaian and Birimian sediment samples are close to previously measured ages for synvolcanic granitoid plutons in the Sefwi and Ashanti belts, which flank the Kumasi basin. This suggests that both sediments were primarily derived from rocks in the volcanic belts. The fact that there were no detrital grain ages found in the interval 2155-2135 Ma suggests that sedimentation in the Tarkwaian and Birimian basins was initiated at least 20 m.y. after formation of many of the exposed volcanic and plutonic rocks in the belts, but more extensive geochronology is required to determine whether these results are typical of the region. Significant epigenetic gold mineralization probably began in the short time period between the end of Birimian and the beginning of Tarkwaian sedimentation but continued until after deformation of the Tarkwaian sediments. The Tarkwaian placer deposits were likely to have been derived from erosion of the earliest gold-bearing quartz veins. The lithologic and geochemical patterns, as well as relative age relationships among Eburnean rocks in Ghana all support an accretionary tectonic model for crustal growth by subduction-collision processes. The overall geologic pattern is strikingly similar to that found in the Archaean Superior Province of Canada and demonstrates that the Archaean-Proterozoic transition took place at vastly different times in different parts of the world.