SHRIMP zircon U–Pb constraints on the age of the Carajás formation, Grão ParáGroup, Amazon Craton

The ≈400 m-thick Carajás Formation, of northern Brazil, is one of the most extensive Precambrian banded iron-formations (BIFs) of the Gondwanaland continents. The formation lies within the Grão ParáGroup, a gently folded and weakly metamorphosed volcanosedimentary sequence which unconformably overlies older gneissose and granitoid rocks of the Xingu Complex. Zircons extracted from five samples selected to provide age constraints for the Carajás Formation have yielded good U–Pb data following ion-microprobe analysis. Our results, combined with those available from earlier U–Pb zircon work, constrain deposition of the Carajás Formation between ≈2750 and ≈2740 Ma. A 2681±5 Ma age from an overlying sandstone with abundant zircons, apparently derived from syndepositional volcanism, suggests that deposition in the Grão Pará“Basin” may have lasted less than 100 Ma. Our data hint at late metamorphic and/or metasomatic events, as young as 569±11 Ma, whose significance remains unresolved. Our results are in accord with previously published U–Pb zircon data constraining the maximum age of the Carajás Formation, but also provide close constraint on its minimum age that is not reliant on regional stratigraphic correlation.