Crustal evolution between 2.0 and 3.5 Ga in the southern Gavião block (Umburanas-Brumado-Aracatu region), São Francisco Craton, Brazil: A 3.5-3.8 Ga proto-crust in the Gavião block?

The main evolution of the Gavião block in the Umburanas-Brumado-Aracatu region, in the state of Bahia, is defined by several sets of tonalitic-trondhjemitic and granodioritic gneisses emplaced during the Paleoarchean. The juvenile Bernada gneisses are emplaced at 3386 ± 9 Ma (SHRIMP zircon age). The Aracatu gneisses, probably derived from the partial melting of ca 3.4 Ga gneisses, are emplaced at 3325 ± 16 Ma. They contain inherited zircon dated at 3366 ± 15 Ma in the range of ages obtained for the juvenile Bernada gneisses. Furthermore, one core in these zircons provides an age of 3487 ± 9 Ma, which is the oldest xenocryst found in the Gavião block. A Neoarchean alkaline granite was emplaced at 2693 ± 5 Ma (Serra de Eixo gneiss) and corresponds to a major crustal reworking stage. All of these rocks were metamorphosed and melted at ca 2.0 Ga, as recorded by monazite ages (EPMA and La-ICPMS) in diatexitic Archean gneisses (Aracatu) and Paleoproterozoic granites (Umburanas). The occurrence of a proto-crust ca 3.5 Ga or older in the Gavião block is discussed based on inherited zircon ages and Sm-Nd isotope signatures of the Archean gneisses.