UMA CONTRIBUIÇÃO PARA A SUBDIVISÃO DO PRÉ-CAMBRIANO NA AMÉRICA DO SUL

On the shield areas of the South America continent (about 5,000,000 km 2 large) there are about 17,000 geochronological determinations available. More than 75% of this amount has been carried out in the Centro de Pesquisas Geocronologicas da Universidade de Sao Paulo, CPGeo-USP, in the last 25 years. The majority of these data were accomplished through Rb-Sr (ca. 60%) and K-Ar (ca. 40%) methods The authors tried to work this amount of data on a coherent way according to the progress observed in the geologic and geotectonic knowledge during the last two decades. One of the fundamental goals was to outline an up-to-date subdivision for the Precambrian of this continent, having in mind a contribution to the International Commission on Stratigraphy (ICS), Subcommission of Precambrian Stratigraphy (SPS) of the IUGS. A threefold subdivision for the Proterozoic is adopted and discussed. Such a subdivision differs, in terms of geochronometric boundaries between eras, from that proposed by SPS and published by ICS during the last International Geological Congress in Washington. The geological reasons for these differences are extensively discussed in this article.