SHRIMP U-PB ZIRCON GEOCHRONOLOGY OF ARCHEAN GRANITOIDS FROM THE CONTENDAS-MIRANTE AREA OF THE SAO-FRANCISCO CRATON, BAHIA, BRAZIL

SHRIMP U-Pb zircon geochronology has been undertaken on four samples of granitoids and gneisses from the Contendas-Mirante area of the Sao Francisco Craton of Bahia State, Brazil. A tonalite and a granite from domes of polyphase basement gneisses mantled by the late Archaean-Paleoproterozoic Contendas-Mirante supracrustal belt yielded concordant ages of 3403+/-5 Ma and 3353+/-5 Ma, respectively. These domes are interpreted as basement rocks that were uplifted into their cover during the Paleoproterozoic Transamazonian orogeny. To the west of the Contendas-Mirante supracrustal belt is the Gaviao block, consisting of Archaean greenstones, granites and gneisses. Two samples from the Lagoa do Morro area of the Gaviao block have been dated. A granodiorite yielded a concordant age of 3184+/-6 Ma. The other sample, a metasomatised mafic enclave in granite, contained sparse zircons interpreted to have grown at the time of emplacement of the granite. These zircons are discordant with Pb-207/Pb-206 ages between 2823 and 2446 Ma, apart from one which is concordant at 2845+/-14 Ma, and is taken as giving the best estimate of the age of the surrounding granite. These different ages from only four samples confirm the complexity of the Archaean basement of the region, and cautions against correlation of granitoids and gneisses in the Sao Francisco Craton, even over short distances.