THE GEOLOGIC EVOLUTION OF SOUTH AMERICA DURING THE ARCHAEAN AND EARLY PROTEROZOIC

Abstract

ln the South American continent, Archaean and Lower Proterozoic terranes can be found in all tectonic domains, but especially within the Amazonian and Sao Francisco cratons. They can also be found within the Sao Luis, Luis Alves and Rio de La Plata cratonic fragments, as well as within all mobile belts of the Brasiliano orogenic cycle, as reworked basement. The Arequipa massif is lhe only identified Lower Proterozoic unit in the Andean belt. Within the Amazonian craton, the Central Amazonian province is made up of Archaean rocks, exhibiting conditions since at least the Lower Proterozoic. It is bounded to the north by the Maroni-Itacaiunas mobile belt, in which the major tectonomagmatic episode occurred in the early Proterozoic, the Transamazonico orogenic cycle. It comprises large portions of supracrustal rocks, identified as greenstone belts, associated to gneisses and migmatites, as well as fragments of high-grade polymetamorphic terranes in which Archaean ages were obtained. Within the Sao Francisco craton, three main types of ancient geologic terranes occur: 1) Archaean granite-greenstone terranes, such as the Brumado-Anajo area in Bahia, and the Quadrilatero Ferrifero area in Minas Gerais; 2) Lower Protcrozoic supracrustal belts, such as the Jacobina, Serrinha and Contendas-Mirante sequences in Bahia, and the Minas Group in Minas Gerais; and 3) Medium to high-grade metamorphic terranes, subjected to extensive granitization during the Transamazonic orogeny (Salvador-Juazeiro mobile belt), and including Archaean cratonic fragments, of grunulitic composition, such as the Jequie-Mututpe complex in Bahia. Ancient terranes were also found within the mobile belts of the Brasiliano cycle, where they make up their ensialic basement. Lower Proterozoic type radiometric ages are predominant, especially within the Borborema province and the Ribeira belt. Taking into account the general distribution of the ancient terranes, it seems that a major part of the South American continental crust was already existent as such just alter the Transamazonico cycle. Moreover, since the known Transamazonico belts are ensialic, it is clear that large portions of continental material were formed earlier, in Archaean times.