POST-GOE IRON FORMATIONS OF THE SãO FRANCISCO CRATON: FILLING THE GAP IN IRON FORMATION RECORDS

The Archean, proto-cratonic core of the São Francisco craton has grown during the Trans-Amazonian orogeny (2.2-2.0 Ga) via accretion of magmatic arcs (Teixeira et al., 2014) along its southern boundary and addition of terranes with continental crust to the north (Barbosa and Sabaté, 2004). The Orosirian Fe-enriched sequences extending from the NW border (Colomis Gr.) to the SE into the Espinhaço Belt (<1.99 Ga S. da Serpentina Gr.; Rolim et al., 2016) record opening of an intracratonic basin with episodic development of Fe-rich waters prior to the initiation of the Espinhaço rift. In the same basin, contemporaneous schists of the 2049 ± 16 Ma Barão de Guaicuí Fm. (Machado et al., 1989) are associated with greenstones and iron formations of the Pedro Pereira Fm. that comprise the Costa Sena Gr. Deposition of iron formations continued in the Espinhaço rift where the Canjica Iron Formation of the <1.7 Ga Serra de São José Gr. (Rolim et al., 2016) is contemporaneous with felsic magmatism (Conceição do Mato Dentro Rhyolite and Borrachudos Granitic Suite) and deposition of extensive sandstones of the <1666 ± 32 Ma Itapanhoacanga and 1.683 ± 11 Ma São João da Chapada fms. In the upper Sao Joao da Chapada Fm., banded hematitic phyllite records input of Fe-rich fluids. Young age of these iron formations, absence of granular facies (GIFs), and yet close association with psammitic rocks indicate an unusual post-GOE setting developed in a large, previously unrecognized basin, following the Transamazonian orogeny. In this basin, iron formations precipitated in association with mafic volcanics that were a potential source of Fe. The ~1.7 Ga mantle plume could have been the driving force for the opening of the Espinhaço rift, the Paramirim aulacogen, and a proposed third rift basin (Jequitinhonha rift) truncating to the south the eastern Bahia Belt. Together these three rifts define a system with the Espinhaço and Jequitinhonha rifts evolved to a passive margin and later partially obliterated within the Araçuaí orogenic belt during the Neoproterozoic to Early Paleozoic Brasiliano Orogeny. The Fe input diminished in the Espinhaço time with iron formations deposited as discontinuous lenses and Fe-enriched sediments, while GIFs are entirely absent in the geologic record of both intracratonic basins formed after the Trans-Amazonian orogeny.