SHRIMP zircon U-Pb constraints on the age of the Carajas formation, Grao Para Group, Amazon Craton

The approximate to 400 m-thick Carajas Formation, of northern Brazil, is on e of the most extensive Precambrian banded iron-formations (BIFs) of the Go ndwanaland continents. The formation lies within the Grao Para Group, a gen tly folded and weakly metamorphosed volcanosedimentary sequence which uncon formably overlies older gneissose and granitoid rocks of the Xingu Complex. Zircons extracted from five samples selected to provide age constraints fo r the Carajas Formation have yielded good U-Pb data following ion-microprob e analysis. Our results, combined with those available from earlier U-Pb zi rcon work, constrain deposition of the Carajas Formation between approximat e to 2750 and approximate to 2740 Ma. A 2681 +/- 5 Ma age from an overlying sandstone with abundant zircons, apparently derived from syndepositional v olcanism, suggests that deposition in the Grao Para "Basin" may have lasted less than 100 Ma. Our data hint at late metamorphic and/or metasomatic eve nts, as young as 569 +/- 11 Ma, whose significance remains unresolved. Our results are in accord with previously published U-Pb zircon data constraini ng the maximum age of the Carajas Formation, but also provide close constra int on its minimum age that is not reliant on regional stratigraphic correl ation. (C) 1998 Elsevier Science Ltd. All rights reserved.