Claudia Sayão Valladares

UPb 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, Paraiba 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 Paraiba 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 Paraiba 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 Sao Francisco craton and indicate that the foreland was more extensive than the presently defined ‘craton’.

Ages of Detrital Zircon from Siliciclastic Successions South of the São Francisco Craton, Brazil: Implications for the Evolution of Proterozoic Basins

Abstract In this work we report 207Pb/206Pb LA-ICPMS ages of 152 detrital zircons from lower greenschist facies quartzites from Proterozoic basin successions of the southern border of the Sao Francisco Craton, southern Minas Gerais State, Brazil. These are the intracratonic Sao Joao del Rei basin, the intraplate continental margin Andrelândia basin, and the Serra do Ouro Grosso sequence, developed on a crystalline basement older than 1.8 Ga, and deformed and metamorphosed during the Brasiliano Orogeny, ca. 0.59–0.50 Ga. The data constrain both the ages of the sources and the interval of sedimentation. The detrital zircons of the Serra do Ouro Grosso sequence were derived predominantly from the erosion of a Neoarchean crust, 2.5–2.8 Ga old, with only one grain showing a Paleoproterozoic age (2, 245±83 Ma) older than the Transamazonian event. Zircons extracted from a shelf quartzite of the lowermost sequence of the Sao Joao del Rei basin indicate derivation from the 1.8–2.2 Ga Transamazonian crust, with subordinate contribution from the 2.5–2.9 Ga Archean crust. The 1, 809±41 Ma age is interpreted as the maximum limit for sedimentation in this basin. The results confirm the regional correlation with the Espinhaco Rift successions. The zircons extracted from an autochthonous quartzite of the Andrelândia sequence yielded ages in the 1.0–2.2 Ga range, with a modal class at 1.2–1.3 Ga. Only two of the forty analyzed zircons yield Archean ages. The youngest zircon yields 1, 086±85 Ma. The zircons from the allochthonous quartzite yield ages between 1.0–2.7 Ga, with a modal class at 2.1–2.2 Ga. Only five of 45 analyzed grains yield Archean ages. The youngest zircon has an age of 1, 047±77 Ma. The results indicate that the detrital sediments deposited during the second marine flooding event of the Andrelândia sedimentation were mainly derived from the erosion of Mesoproterozoic and Paleoproterozic rocks. The 1, 047±77 Ma age is interpreted as the maximum depositional age for the described association.