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 sed iments in the Palaeoproterozoic Eburnean rocks of Ghana. Gold-bearing and barren Tarkwaian conglomerates in the Tarkwa basin, and a volcanog enic Birimian sediment from the Kumasi basin all show similar age patt erns. Most of the measured detrital zircon ages from the Tarkwaian sed iments are in the range 2194-2132 Ma (20 ages), similar to the range 2 184-2135 Ma (6 ages) found for the Birimian sediment. One Tarkwaian zi rcon grain has a slightly older age of 224 5 +/- 4 Ma. A granitoid sam ple from the Cape Coast pluton gives an age of 2090 +/- 1 Ma from mona zite and zircon. This age appears to represent a regionally significan t time of granitoid plutonism in the sedimentary basins. Many of the z ircon ages for the Tarkwaian and Birimian sediment samples are close t o previously measured ages for synvolcanic granitoid plutons in the Se fwi and Ashanti belts, which flank the Kumasi basin. This suggests tha t both sediments were primarily derived from rocks in the volcanic bel ts. The fact that there were no detrital grain ages found in the inter val 2155-2135 Ma suggests that sedimentation in the Tarkwaian and Biri mian basins was initiated at least 20 m.y. after formation of many of the exposed volcanic and plutonic rocks in the belts, but more extensi ve geochronology is required to determine whether these results are ty pical of the region. Significant epigenetic gold mineralization probab ly began in the short time period between the end of Birimian and the beginning of Tarkwaian sedimentation but continued until after deforma tion of the Tarkwaian sediments. The Tarkwaian placer deposits were li kely to have been derived from erosion of the earliest gold-bearing qu artz veins. The lithologic and geochemical patterns, as well as relati ve age relationships among Eburnean rocks in Ghana all support an accr etionary tectonic model for crustal growth by subduction-collision pro cesses. The overall geologic pattern is strikingly similar to that fou nd in the Archaean Superior Province of Canada and demonstrates that t he Archaean-Proterozoic transition took place at vastly different time s in different parts of the world.