U-PB GEOCHRONOLOGY OF ARCHEAN MAGMATISM AND BASEMENT REACTIVATION IN THE CARAJAS AREA, AMAZON SHIELD, BRAZIL

The Archean Carajas Subprovince is a metallogenic district (BIF, bauxite, Cu, Ni, Mn, Au) located in the easternmost Amazonian Craton in Brazil. The main conclusions from a U-Pb study of the Itacaiunas Supergroup and a locality in the Xingu Complex are the following: the Xingu Basement Complex, the oldest unit, underwent migmatization at 2859 +/- 2 Ma and also grew new zircon at 2851 +/- 4 Ma; the Luanga mafic intrusion is 2763 +/- 6 Ma old and coeval with the 2759 +/- 2 Ma felsic volcanics of the Grao Para Group. Amphibolites from the Salobo Group contain 2851 Ma zircons, were metamorphosed (2761 +/- 3 Ma) and intruded by granitic veins (ca. 2758 Ma) at the time of Grao Para magmatism and also grew new zircon at 2732-2742 Ma. The data indicate that the Salobo Group is part of the Xingu Complex and not of the Itacaiunas Supergroup as heretofore believed. The Pojuca Group, possibly coeval with the Salobo Group, was metamorphosed at 2732 +/- 3 Ma. A Late Archean reactivation event of regional extent is marked by titanite growth at 2581 +/- 5 Ma in the Mirim area, intrusion of the Old Salobo Granite at 2573 +/- 2 Ma, new zircon growth in Salobo Group amphibolites at 2555 +4/-3 Ma and monazite growth in a metamorphosed iron formation at 2551 +/- 2 Ma. In addition, K-metasomatism associated with shear zones formed titanite in Salobo Group amphibolites at 2497 +/- 5 Ma and new titanite at 2519 +/- 5 Ma in the Xingu Complex. Events in the 2581-2497 Ma interval are probably associated with the latest activity of a WNW-ESE sinistral shear system which could also be responsible for the establishment of a transtensional basin filled by the volcano-sedimentary sequence of the Grao Para, Bahia and Rio Fresco groups. Finally, undeformed Proterozoic granitoid intrusions were dated at 1883 +/- 2 Ma (Cigano), 1883 + 5/ -2 Ma (Musa), 1880 +/- 2 Ma (Carajas) and 1874 +/- 2 Ma (Pojuca).