Cráton do São Francisco, geocronologia, evolução tectónica
Abstract
The geochronologic data base for the São Francisco Craton (SFC) includes more than 400 K-Ar age determinations. The K-Ar ages on crystalline basement rocks characterize regional cooling patterns within the SFC, that permit definition of its tectonothermal history during the Proterozoic. This history is also substantiated by the crustal evolution postulated for the SFC and by the U-Pb, Rb-Sr, Pb-Pb, Sm-Nd) data including that of its adjacent Neoproterozoic fold belts. Regional cooling of large domains of the SFC occurred mostly during the Transamazonian cycle as a result of progressive uplift accompanying the tectonic stability of the Itabuna, Correntina-Guanambi and Mineiro belts, around 2,1 - 1,8 Ga ago. The 1,72 - 1,65 Ga age pattern identified in sectors of the Itabuna (Salvador region) and Mineiro belts (west of Belo Horizonte) suggests a relationship with the geodynamics associated with the extensional collapse of the Espinhaço rift originated at ca. L75 Ga ago. The 0,80/0,65 Ga and 0,65/0,50 Ga age intervals are characteristic for the basement rocks located at the borders of the SFC and in the Paramirim province. These patterns may have a tectonic link with the collision processes against the SFC, of the marginal Neoproterozoic belts. In special, the K-Ar resetting in the Paramirim province accompanied deformation and metamorphic overprint of the crystalline basement, probably in association with the collision of the Rio Preto and Araçuaí belts. As a whole, the K-Ar age patterns coupled with the geologic and structural scenario and the crustal evolution of the SFC reveal it consists a foreland unit for the surrounding tectonics that took place in eastern Brazil, during the Neoproterozoic. Nevertheless, the basement rocks located in the southeast and east borders of the SFC and in the Paramirim province have been reactivated due to the Brasiliano cycle evolution marginal to the craton.
