Abstract A comprehensive tectonic evolution of Proterozoic to Cambrian rocks that integrate the crystalline basement of Rio de Janeiro State, based on the new geological map, is presented. The map combines detailed geological data integrated to a 1:400.000 scale and new geochemical and geochronology data to characterize the distinct terranes amalgamated during three pulses of the Brasiliano orogeny. The Occidental terrane comprises the Tonian to Late Cryogenean passive margin of the São Francisco paleocontinent, including reworked cratonic basement. At the same period, two magmatic arc systems were developed outboard: The Continental Inner arc system represented by the dismembered terranes (Paraíba do Sul, Embú, and Cambuci domain) integrated now in the Central Superterrane: and the Intra-oceanic Outer Arc System, the Oriental Terrane, developed from 860 to 620 Ma. Related active basins were detected in association with the two arc systems that collided at ca. 620 - 595 and 605 - 550 Ma against the São Francisco passive margin. A later collision episode at ca. 535 to 510 Ma brought the Cabo Frio Terrane (Angola tip) and closed the Búzios back-arc basin. Finally, the collapse of the orogenic belt, probably related to slab detachment and asthenosphere upwelling marks transition to Gondwana stabilization.

Abstract New geochemical, U-Pb, Lu-Hf and Sr-Nd data from the Barreiro Suite metabasites in comparison with enclaves within the distal Andrelândia Group and the orthogranulites of the Juiz de Fora Complex are presented. Geochemical data suggest intraplate setting, with high and low-TiO 2 , TDM Nd ages between 1.80 and 1.41 Ga, negative ΕNd t and ( 87 Sr/ 86 Sr)i between 0.714 and 0.747. Results contrast with part of the Juiz de Fora Complex enclaves, with island arc tholeiites- calcalkaline basalts (IAT-CAB) geochemical signatures, TDM Nd ages between 2.58 and 2.16 Ga, positive ΕNd t values and (87Sr/86Sr) i between 0.700 and 0.712. U-Pb data for the Barreiro Suite yielded a crystallization age of 766 ± 13 Ma and a metamorphic overprint of 619 ± 6 Ma. The results indicate three episodes of mafic magmatism in the Occidental terrane of the Ribeira Belt. The two older episodes are related to Rhyacian arc evolution (ca. 2.2 to 2.1 Ga) and to the Statherian (ca. 1.7 Ga) tectonics, and occur only within the Juiz de Fora Complex, while the younger ca. 766 Ma episode constrains the timing of distal passive margin evolution. An important implication is that these late Tonian metabasic rocks could have been a source of detrital zircons for the sedimentation along the distal Andrelândia basin.

Abstract Combined Sm-Nd isotope studies and U-Pb ages of detrital zircons have shown significant differences between the provenance patterns of Neoproterozoic metasedimentary successions in the Southern Brasília belt. The Vazante and Canastra groups are passive margin deposits with source areas in the Archean/Paleoproterozoic São Francisco Craton basement and Mesoproterozoic cover units. In literature, the younging trend of Nd model ages observed in the Vazante Group is interpreted as resulting from the contribution of the Neoproterozoic Goiás Magmatic Arc. Also, detrital zircons younger than 0.90 Ga were not recorded in the Vazante or Canastra groups, which zircon populations are older than 0.93 Ga. This work presents new Sm-Nd and Sr isotope data of metasedimentary rocks from the Vazante and Canastra groups, collected in a 400 km2 area in northwest Minas Gerais, Brazil. The improved database of previously published and new data corroborates with the younging pattern of TDM along the Vazante Group. Isochronic diagrams show that samples from Vazante and Canastra groups scatter along mixing lines between the fields defined by the older cratonic rocks and the more juvenile ones. Here we present the Tonian Intraplate Magmatism within the São Francisco Craton as the juvenile source.

Abstract New petrography, geochemistry and Sm-Nd and Sr data from the orthogranulites of the Juiz de Fora complex in southern Rio de Janeiro State and compiled information provide insights on the petrogenetic and tectonic evolution. The complex comprises several geochemical groups including mafic orthogranulites (three tholeiitic and one alkaline) and felsic orthogranulites (three calc-alkaline and one tholeiitic/low K calc-alkaline). New geochemical and isotope data, combined with available U-Pb data suggest a long evolutionary history from the Paleoproterozoic to the Neoproterozoic. The oldest magmatic episode produced juvenile to slightly contaminated arc-type Rhyacian rocks, as well as granitic rocks related to collision or post-collision episodes. Altogether these rocks integrate part of a dismembered Rhyacian orogen within the Ribeira belt. Few TDM Nd model ages yielded 2.75 to 2.58 Ga, suggesting minor Archean contribution for magma genesis. Some of the basic granulites’ bodies yield Meso- to Neoproterozoic TDM Nd model ages, which may refer to an extensional magmatism. Orthogranulites present granulite facies paragenesis, related to the youngest tectonic episode in the Juiz de Fora Complex (Brasiliano Orogeny). The new data are potentially important for Paleoproterozoic reconstruction models, due to the predominantly juvenile character of the Juiz de Fora complex, as similarly worldwide.

RESUMO: O contato do empurrão dos grupos Canastra e Vazante na Faixa Brasília Meridional: evolução estrutural, cristalinidade de mica branca e implicações para a orogenia brasiliana. Duas escamas de cavalgamentos regionais de rochas metassedimentares neoproterozoicas ocorrem na Faixa Brasília Meridional, em Minas Gerais. A inferior compreende o Grupo Vazante, que na área de estudos é formado da base para o topo, pelas formações Serra do Garrote (metasiltitos e filitos carbonosos intercalados), Serra do Poço Verde (metadolomitos estromatolíticos bege a rosados com intercalações de ardósias e margas esverdeadas), Morro do Calcário (metadolomitos estromatolíticos cinza com intercalações de ardósias cinza) e Serra da Lapa (metarritmitos carbonosos com lentes de dolarenitos). A escama superior é composta do Grupo Canastra (formação Paracatu): sericita filitos laminados e filitos carbonosos com intercalações de quartzitos. A deformação resultou em quatro fases de deformação compressivas, contemporâneas ao metamorfismo de baixo grau. D 1 e D2 são de caráter dúctil, enquanto D3 e D4 são de regime rúptil-dúctil. D1 foi responsável pela clivagem ardosiana S1 subparalela ao acamamento sedimentar, com mergulhos baixos a altos para NW. D2 desenvolveu clivagem de crenulação (S2) que mergulha moderadamente para NW e está associada a dobras apertadas a isoclinais. D3 e D4 desenvolveram crenulações espaçadas e dobras abertas com kinks associadas. S3 apresenta eixos para NW, e S4, eixos para NE e SW, ambas com caimento moderados a altos. Medições por cristalinidade de mica branca (índice de Kübler) sugerem que os grupos Canastra e Vazante alcançaram condições de anquizona/epizona, e descontinuidades metamórficas ao longo dos planos de cavalgamento indicam que o pico metamórfico é anterior ou contemporâneo à tectônica de cavalgamento.

ABSTRACT: Two regional thrust-sheets of Neoproterozoic metasedimentary rocks occur in the Southern Brasília Belt, northwest Minas Gerais. The lower one comprises the Vazante Group, that is formed in the studied area, from base to top, by the Serra do Garrote (metapelites interlayered with carbonaceous phyllite), Serra do Poço Verde (beige to pink stromatolitic metadolomite with interlayered greenish slates), Morro do Calcário (gray stromatolitic metadolomite interlayered with gray slates) and Serra da Lapa (phyllite with dolarenitic lenses interlayered with slates) formations. The upper thrust sheet consists of the Canastra Group (Paracatu formation): laminated sericite phyllites and carbonaceous phyllites interlayered with quartzite. The Braziliano orogeny resulted in four phases of contractional deformation, associated with low-grade metamorphism. The first two (D1 and D2) are ductile, while the third and fourth ones (D3 and D4) are brittle-ductile. D1 developed a slaty S1 cleavage subparallel to the primary layering, with shallow to steep dips to NW. D2 developed a crenulation cleavage (S2) that dips moderately to NW and is associated with tight to isoclinal folds. D3 and D4 phases developed crenulations and open folds and kink bands. S3 dips steeply to NW, while S4 has moderate to steep dips to NE and SW. White mica crystallinity (Kübler index) measurements in metapelites indicate that both the Canastra and Vazante groups reached anchizone/epizone conditions, and metamorphic discontinuities along thrusts indicate that the peak of metamorphism is pre or syn-thrusting.