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Geochronology of the southern part of the São Francisco Craton

1982; Sociedade Brasileira de Geologia; Volume: 12; Linguagem: Inglês

10.5327/rbg.v12i1-2-3.269

2317-4889

Wilson Teixeira,

Geological and Geochemical Analysis

The Sao Francisco Craton, a large tectonically stable area in Late Proterozoic times, is still poorly known, in respect to its geologic evolution. This work deals with the geochronological interpretation of all available data for the southern part of the cratonic area, about 150 radiometric results, many of which were added to recently, by the work of the Centro de Pesquisa Geocronologica (CPGeo) do Instituto de Geociencias da Universidade de Sao Paulo (IG-USP). Several nuclei, with Late Archean ages, were brought to light by Rb-Sr whole rock measurements. The values range between 3,000 and 2,750 Ma indicating the existence of an ancient terrain formed mainly of granitoid rocks (gneisses, migmatites, granites, amphibolites), in part associated to supracrustals interpreted as greenstone belts (Nova Lima, Piui, etc). Some other Rb/Sr isochron diagrams, and many of the K-Ar apparent ages indicate that the Transamazonico orogeny was the last important tectonothermal event, which overprinted the Archean terrain. A relation was also noted between the radiometric patterns and its geographic situation. The older rocks are situated mainly in the western part of the studied area, where the last regional cooling is related to the Minas diastrophism, while only in the southeastern corner, the Quadrilatero Ferrifero area, a Late Proterozoic thermal episode is charactcrized by the K-Ar radiometric data.