Pb, Sr and Nd isotope constraints on the Archaean evolution of gneissic-granitoid complexes in the southern São Francisco Craton, Brazil

Abstract

Three Archaean gneissic complexes (Bonfim, Belo Horizonte and Campo Belo) in the southern part of the São Francisco craton were studied by means of UPb (zircon and titanite, monazite) and SmNd, RbSr and PbPb (whole rock) methods. In the Bonfim metamorphic complex the period 2780-2700 Ma is marked by in situ anatexis of gneisses, intrusion of tonalites, mafic dikes and late granites, and deposition of the Rio das Velhas greenstone belt. SmNd T DM model ages on its gneisses, amphibolites 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 2860, 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 uncertainties) and 87Sr/ 86Sr initial ratios of 0.700-0.710. The highest initial ratio (0.710) obtained for the migmatites suggests that ensialic episodes participated in the tectonomagmatic evolution. Existence of older crust is suggested by UPb zircon ages of ∼ 3030, 2920 and 2880 Ma from inherited grains in metavolcanics and orthogneisses. Rb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 lithologies have yielded ages of ∼ 3380-3000, 2900 and 2650 Ma by UPb zircon and RbSr and PbPb whole rock methods. T DM ages on its granites, gneisses and granulites have yielded ages from 3070 to 2780 Ma indicating 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 to 3380 Ma old. Late granites were injected between 2700-2600 Ma, preceding final assembly and stabilization of the Archaean crust.