Geochronological investigation on the Precambrian granulitic terrain of Bahia, Brazil

Abstract

Rb-Sr and K-Ar isotopic investigations of rocks and minerals were carried out from selected areas in the Precambrian granulite terrain of eastern Bahia, Brazil. The principal results include Rb-Sr isochron ages of 3160±60 Ma, 2730±50 Ma, 2300–2450 Ma, and K-Ar mineral ages in the range of 1800–2000 Ma. The 2730±50 Ma date represents the Jequié event, a major Sr isotopic homogenization episode in the terrain. The 3160±60 Ma date probably indicates the presence of an Archean crust, and the 2300–2450 Ma dates are the youngest Rb-Sr isochron ages obtained in the region, the geological significance of which is still obscure. The Transamazonian Orogeny (1800–2000 Ma) appears to have reset the K-Ar isotopic clocks in the minerals. The isotopic ages of the granulites of Bahia correlate well with similar rocks of the Ntem complex in Southern Cameroon, thereby strengthening the validity of the predrift reconstruction of Africa and South America.