U-Pb AGES OF ZIRCONS FROM THE GRÃO-PARÁ GROUP AND SERRA DOS CARAJÁS GRANITE, PARÁ, BRAZIL

Abstract

Zircons collected from weathered rhyolites in the lower metavolcanic sequence of the Grao-Para Group yield a U-Pb upper lntercept age of 2,758 ± Ma. The zircons show no sign of a premagmatic history, and the age is interpreted as the age of volcanism. This Archean agels within the calculated uncertainty of a Rb-Sr whole-rock isochron age for interstratified basalts, reported separately. The U-Pb age is the first indication that the Grao-Para Group, including the Carajas Formation iron deposits, is Archean, much older than previously thought. Zircons were also collected from weathered Serra dos Carajas .Granite adjacent to the quarry near Serra Norte. These yield a U-Pb age of 1,820 ± 49 Ma, which agrees well with previously published age determinations for the granite, and for the Uatuma Supergroup volcanic and shallow plutonic rocks that occur throughout the Amazonian Craton. Zircons from both the Grâo-Para rhyolite and Carajas Granite show evidence for a single stage loss of radiogenic lead, possibly due to dilatancy during Mesozoic uplift of the region. All of the zircons were panned from saprolites, and most exhibit well-defined crystal forms. Tropical weathering is an asset to the collection and field concentration of zircons, and our results confirm that standard U-Pb dating techniques can be successfully applied in deeply-weathered terranes.