PB, SR AND ND ISOTOPE CONSTRAINTS ON THE ARCHEAN EVOLUTION OF GNEISSIC-GRANITOID COMPLEXES IN THE SOUTHERN SAO-FRANCISCO CRATON, BRAZIL

Three Archaean gneissic complexes (Bonfim, Belo Horizonte and Campo Be lo) in the southern part of the Sao Francisco craton were studied by m eans of U-Pb (zircon and titanite, monazite) and Sm-Nd, Rb-Sr and Pb-P b (whole rock) methods. In the Bonfim metamorphic complex the period 2 780-2700 Ma is marked by in situ anatexis of gneisses, intrusion of to nalites, mafic dikes and late granites, and deposition of the Rio das Velhas greensrone belt. Sm-Nd T-DM model ages on its gneisses, amphibo lites and granites range from 2800 to 3000 Ma, indicating involvement of older sialic crust in the 2780-2700 Ma event. In the Belo Horizonte metamorphic complex U-Pb zircon geochronology has yielded ages of 286 0, 2776 and 2712 Ma and Rb-Sr whole rock isochrons on its gneisses and migmatites yielded ages in the range 2800-2750 Ma (with large uncerta inties) and Sr-87/Sr-86 initial ratios of 0.700-0.710. The highest ini tial ratio (0.710) obtained for the migmatites suggests that ensialic episodes participated in the tectonomagmatic evolution. Existence of o lder crust is suggested by U-Pb zircon ages of similar to 3030, 2920 a nd 2880 Ma from inherited grains in metavolcanics and orthogneisses. R b-Sr ages of 2250-2130 Ma on granitoids and U-Pb (titanite, monazite) ages from 2320 to 2030 Ma on granitoids, felsic veins and amphibolite enclaves indicate reworking of this complex in the Palaeoproterozoic. In the Campo Belo metamorphic complex granite and greenstone lithologi es have yielded ages of similar to 3380-3000, 2900 and 2650 Ma by U-Pb zircon and Rb-Sr and Pb-Pb whole rock methods. T-DM ages on its grani tes, gneisses and granulites have yielded ages from 3070 to 2780 Ma in dicating involvement of older sialic material during the 2650 Ma event . These complexes indicate major events during the period 2860-2700 Ma , including reworking of established sialic crust with components up t o 3380 Ma old. Late granites were injected between 2700-2600 Ma, prece ding final assembly and stabilization of the Archaean crust.