INSTITUTO DE GEOCRONOLOGIA Y GEOLOGIA ISOTOPICA
Unidad Ejecutora - UE
congresos y reuniones científicas
FURTHER CONSTRAINTS ON THE DEPOSITIONAL AGES AND SOURCES OF THE PALEOPROTEROZOIC MINAS SUPERGROUP (QUADRILÁTERO FERRÍFERO): TRACKING THE ARCHEAN MAGMATIC RECORD IN THE SOUTHERN SÃO FRANCISCO CRATON
MARTÍNEZ DOPICO, C.I.; DE CARVALHO LANA, C.; CASSINO, L.; SOUZA MOREIRA, H.
Congreso; The 8th Hutton Symposium of Granites and related rocks; 2015
USP, UFRGS, UFRJ
After the thermal stabilization of the Archean crust in the south São Francisco Craton, the MinasBasin began opening. The infill, the Minas Supergroup, involves a highly deformed 6 km-thickmetasedimentary package that registered the track of a Wilson cycle during early Paleoproterozoictimes . This passive margin sequence encompasses basal alluvial to fluvial quarzites, open marinephyllites (Caraça Group), banded iron-rich and carbonate rocks (Itabira Group), and coastal clasticrocks (Piracicaba Group). At the top, the sequence evolves into a synorogenic foreland basin (SabaráGroup) related to the Rhyacian orogeny . Even though the sequence hosts a world-class iron deposit,the timing of sedimentation and sources have remained elusive. Hf-data combined with U-Pb dating indetrital zircons from the Serra de Ouro Preto are here presented. The base of the Caraça Group, MoedaFm., yields two main detrital zircon peaks at 2720 and 2870 Ma with the youngest cluster occurring at ca.2665 Ma (N=209). These peaks correspond to the Mamona and late Rio das Velhas I magmatic events. The zircons that belong to these events display Hft values between -7 and +3 units (N>200), with asuperchrondritic cluster found for the Rio das Velhas I Event, suggesting a juvenile addition to the crustduring this TTG episode. A clastic lens within the overlying BIF (Itabira Group) exhibited a differentinheritance with a single peak at 2790 Ma and minor low-frequency Meso and Paleoarchean peaks(N=95). The age of its main population coincides with the climax of Rio das Velhas II magmatic event,displaying sub to chondritic Hft values (-4 to -1). A single-highly concordant Paleoproterozoic age wasfound (2453±18 Ma) but it is not fully reliable until more data are produced. On the other hand, an Ferichmetasandstone of Piracicaba Group, which marks the beginning of the closure of the basin, yields amain peak at ca. 2960 Ma followed by a smaller 2870 Ma peak, as well as a wide range of basement agesbetween 2666 and 3462 Ma (N=80). This pattern contrasts with the populations previously obtainedat Serra do Curral  and, with the sources found during the opening of the basin. The deposition ofPiracicaba has to be younger than the 2.42 Ga Pb-Pb age of the carbonates topping the Itabira Group .The lack of Paleoproterozoic detritus in Caraça, Itabira and Piracicaba groups, reinforces the absenceof Syderian magmatic events and the tectonic stability of the area until the inception of the 2.1 GaTransamazonian orogeny. Moreover, this new Hf isotope dataset supports the hypothesis of an evolvedArchean continental crust for the São Francisco Craton.We acknowledge support from FAPEMIG (apoio a participação em eventos). Alkmim, F. & Martins-Neto, M.A. 2012. Marine and Petroleum Geol 33, 127-139 Dorr, J.V.N 1969. Prof Pap U.S. Geol Surv 641 A, 1?110. Lana, C. et al., 2013. Prec Res 231, 157-173. Mendes, M. et al., 2014. Rev Geol USP 14, 55-68. Babinski, M. et al. 1995. Prec Res 72, 235-245.