UPb geochronology of the central Ribeira belt (Brazil) and implications for the evolution of the Brazilian Orogeny

Abstract

The Ribeira belt in southeastern Brazil was generated during the Brazilian Orogeny, the South American equivalent of the Pan-African Orogeny (∼ 700-450 Ma). The central sector of the belt is characterised by a series of northwest-vergent crustal-scale thrust stacks emplaced under amphibolite-grade conditions. Major shear zones separate four lithotectonic domains: Andrelândia to the northwest, Juiz de Fora, Paraíba and Coastal domains. Analyses of zircon, monazite and titanite from a variety of rock types from the four domains lead to the following conclusions. All domains record an important tectono-thermal event at 590-565 Ma, represented by partial melting, granitoid intrusion and remobilisation of older basement gneisses, and corresponding to thrusting and development of dextral shear zones. Also recorded are older metamorphic ages (611-604 Ma) of unknown significance. Only the Paraíba and Coastal domains, in the south, record a younger metamorphic event (M2) at 535-520 Ma most likely associated with another episode of thrusting and shear zone development. The absence of M2 metamorphism in the Andrelândia and Juiz de Fora domains can be explained by post-535-520 Ma emplacement of the Paraíba domain. Late- to post-tectonic activity is represented by 503-492 Ma metamorphism. Precursors of the basement gneisses are 2185-2134 Ma old rocks generated during the Transamazonian Orogeny some of which contain a significant Archean component with minimum ages of 2981-2846 Ma. The ages presented above define specific metamorphic events for the Brazilian Orogeny for the first time in southeastern Brazil. In addition, together with previously published data they indicate that the basement rocks found in the allochthonous domains of the central Ribeira belt represent an extension of the units present in the southern São Francisco craton and indicate that the foreland was more extensive than the presently defined ‘craton’.