Abstract
The ages of three precambrian orogenic cycles have been defined hy means of U/Ph, Rb/Sr and K/Ar determinations carried out on igneous and metamorphic rocks of southeast Brazil. This work confirms the preliminary results obtained by the same authors on the granitoid rocks of Serra dos Orgaos (which belong to the Brazilian orogenic cycle, about 620 m.y.), and on the granulitic gneisses of the Paraiba do Sul Formation (Trans-Amazonian orogenic cycle, about 2070 m.y.). For the Mantiqueira gneisses, considered, by field relationships, to be the oldest rocks of the region, new results establish an age of about 2800 m.y. (λ Rb 87 = 1,47 X 10 - 11 years -1 ). The Juiz de Fora Formation has the same petrologic characteristics as the Paraiba do Sul Formation, and it is supposed to be discordant over the Mantiqueira gneisses. However, the geochronological results seem to demonstrate that it is, at least in part, as old as the latter formation. In addition, this work demonstrates a practically complete redistribution of Sr isotopes among the minerals of all rocks of the region, including the oldest ones, during the late precambrian Brazilian orogeny.
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