SHRIMP U-Pb, 207Pb/206Pb zircon dating, and Nd isotopic signature of the Umburanas greenstone belt, northern Sao Francisco craton, Brazil

The Gavido block in the northern portion of the Sao Francisco craton, northeast Brazil, is one of the oldest Archean fragments of the South American platform. It underwent polycyclic evolution from very old juvenile components dated between 3400 and 3000 Ma. The studied parts of the Umburanas greenstone belt (UGB) formed over 'stable' continental crust. SHRIMP U/Pb isotopic analyses of detrital zircons from conglomeratic quartzites of the UGB's lower unit yield dates between 3335 and 3040 Ma. These dates indicate provenance from crustal materials of different ages and a maximum age of deposition as young as 3040 +/- 24 Ma (single zircon analysis) but certainly after 3147 +/- 16 Ma (three analyses). Pb-207/Pb-206 zircon evaporation analyses from a meta-andesite of the intermediate unit of the UGB yield 2744 +/- 15 Ma, which is interpreted as the time of magmatic crystallization. The Sm/Nd whole-rock systematics of three metakomatiites from the base of the lower unit indicate that they are isotopically disturbed. Additional Sm/Nd analyses of one metabasalt (lower unit) and two meta-andesites (intermediate unit) were compared with those of the country rocks. Their similarity suggests that crustal contamination processes play an important role in the formation of volcanic rocks. As a whole, the isotopic picture is in agreement with an ensialic tectonic setting of the studied parts of the UGB. (C) 2002 Published by Elsevier Science Ltd.