Siderian

Abstract: The Sao Francisco shield is one of the major cratonic units forming the backbone of the South American continent. The Quadrilatero Ferrifero area, in the south of the shield, is underlain by three major units: granitic-gneissic terrains with a minimum age of 2860 Ma; the Rio das Velhas greenstone belt, emplaced at 2772–2780 Ma; and the Minas Supergroup-Itacolomi Group Paleoproterozoic sedimentary sequences. The 207 Pb 206 Pb and UPb ages of more than 400 detrital zircons obtained by Laser ablation-ICPMS and isotope dilution lead to the following conclusions: (1) the Rio das Velhas greenstone belt contains exclusively cratogenic sediments and developed on, or proximal to, old, evolved continental crust with a minimum age of ca. 2.85 Ga; (2) the lower detrital sequence of the Minas Supergroup was supplied with sediments derived from both the granite-greenstone terrain and the surrounding older crust; (3) the upper unit of the Minas Supergroup (Sabara Formation) contains 2125 Ma zircon and the overlying Itacolomi Group contain zircons with minimum ages of ca. 2.06 Ga, identical to the ages of regional metamorphism. Together with other considerations, these ages lead to the novel proposal that the tectogenic sedimentation represented by these units occurred in a foreland basin of the Transamazonian orogen. Basin evolution was closely associated with the tectonic development of the orogen and uplift of basement complexes between 2125 Ma and 2030 Ma. The scarcity of 2.6-2.4 Ga zircons in the Proterozoic units of the Quadrilatero Ferrifero confirms previous results for the Sao Francisco shield and is symptomatic of the absence of magmatic-metamorphic activity in that period of time.