U-PB EVIDENCE OF LATE ARCHEAN TECTONOTHERMAL ACTIVITY IN THE SOUTHERN SAO-FRANCISCO SHIELD, BRAZIL

The Sao Francisco craton in eastern Brazil is one of the major shield areas in South America. In the Quadrilatero Ferrifero area (southern sector of the craton) the Archean Rio das Velhas greenstone belt is surrounded by granite-gneiss terrane and is overlain by Proterozoic sedimentary successions. The Bonfim Metamorphic Complex is the only area of the granite - gneiss terrane adjacent to the greenstone belt that has been mapped. It comprises two gneissic units, tonalites, and late granitic dykes, which crosscut the regional north-south shear fabric. Samples of gneiss, two tonalites and a granitic dyke were dated by U-Pb. A zircon core from the Alberto Flores gneiss yielded a minimum age of 2920 Ma, whereas the overgrowth is concordant at 2772 +/- 6 Ma. Two tonalites from the vicinity of Serra da Moeda are 2780(-2)+3 Ma old, and a late dyke yielded an age of 2703(+20)+24 Ma. These data, together with previously published U-Pb ages, show that (i) greenstone belt volcanism was coeval with granitoid intrusion and with metamorphism of older units in the granite-gneiss terrane at ca. 2780 Ma and (ii) the main crust-forming event in the Quadrilatero Ferrifero area is about 2780 Ma old. Clear evidence is now available indicating that older magmatism is 2.8-3.2 Ga old. In addition, the last Archean deformation must have occurred in the interval 2780-2703 Ma. Finally, the presence of 2774 +/- 6 Ma titanite in the tonalites indicates that the metamorphism associated with the Trans-Amazonian orogeny (ca. 2.0 Ga) did not reach amphibolite facies in the study area.