SHRIMP U-PB AGES OF DETRITAL ZIRCONS FROM THE EARLY PROTEROZOIC CONTENDAS-MIRANTE SUPRACRUSTAL BELT, SAO-FRANCISCO CRATON, BAHIA, BRAZIL

SHRIMP U-Pb geochronology has been undertaken on detrital zircons in a orthoconglomerate from the metamorphosed Contendas-Mirante supracrustal belt, Sao Francisco Craton, Bahia, Brazil. The majority of the U-Pb age determinations on the detrital zircons are concordant within uncertainty, and display three principal age groups; 2710-2600 Ma, 2400-2300 Ma and most abundant, 2200-2150 Ma. The conglomerate must have been deposited between ca. 2150 Ma (the youngest detrital zircons) and 2000-1900 Ma, when the Contendas-Mirante supracrustal belt was intruded by peraluminous granites. 3100-3400 Ma gneisses and granitoids crop out as domes within the Contendas-Mirante supracrustal belt, which have been interpreted as reactivated basement to the belt. These gneisses have not contributed a significant amount of detrital material to the conglomerate because the oldest detrital zircons have an age of only ca.2700 Ma. Instead, the source of the detrital material in the conglomerate might be represented by the Jequie Complex to the east, which consists of late Archaean rocks that were migmatized and intruded by granites in the early Proterozoic.