Four main classes of tectonic entities may be considered for the Ribeira Belt and southwest African counterparts: (1) cratonic fragments older than 1.8 Ga and their passive margin successions, (2) reworked basement terranes with Mesoproterozoic and/or Neoproterozoic deformed cover, (3) magmatic arc associations, (4) terranes with Palaeoproterozoic basement and deformed Neoproterozoic back-arc successions. Based on comparative investigation, a tectonic model of polyphase amalgamation is proposed with c. 790 and 630 -610 Ma major episodes of intra-oceanic and cordilleran arc magmatism along both sides of the Adamastor Ocean. Subsequent diachronous collision of the arc terranes and small plates followed at c. 630, 600, 580 and 530 Ma. The tectonic complexity reflects an accretionary evolution from Cryogenian to Cambrian times. The São Francisco-Congo and Angola palaeo-continents did probably not behave as one consolidated block, but rather may have accommodated considerable convergence during the Brasiliano/Pan-African episodes. The final docking of Cabo Frio and Kalahari in the Cambrian was coeval with the arrival of Amazonia on the opposite side, resulting in lateral reactivation and displacement between the previously amalgamated pieces. The transition between the Cambrian and the Ordovician is marked by the extensional collapse of the metamorphic core zones of the orogens.

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The Brasília Belt comprises terranes and thrust-sheets that were tectonically transported towards the western passive margin of the São Francisco–Congo palaeocontinent during an orogenic episode resulting from collision of the Paranapanema and Goiás blocks and the Goiás magmatic arc against São Francisco–Congo at 0.64–0.61 Ga. The tectonic zones of the belt are, from east to west: a foreland zone with Archaean–Palaeoproterozoic granite–greenstone basement covered by Neoproterozoic anchimetamorphic sedimentary rocks (Bambuí Group); a low metamorphic grade thrust-fold belt of proximal shelf successions, mostly siliciclastic, containing rare basement slivers; metamorphic nappes in upper greenschist to granulite facies of distal shelf and slope metasediments and subordinate tholeiitic metabasalts; the Goiás massif, possibly a microcontinent; and the Goiás magmatic arc. The accretion of these terranes against the western margin of the São Francisco–Congo palaeocontinent took place during an early phase of Gondwana supercontinent amalgamation, when terranes accreted around São Francisco–Congo to create a proto-West Gondwana landmass, around which subsequent collisional and accretionary events followed, such as those in the Borborema–Trans-Saharan province (c. 0.62–0.60 Ga); in the Ribeira–Araçuaí belt (c. 0.58 Ga); along the Araguaia and Paraguay belts (collision of Amazonia, c. 0.54–0.52 Ga); and the accretion of Cabo Frio terrane in the Ribeira Belt (c. 0.53–0.50 Ga).

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As unidades litológicas da região noroeste do Estado do Rio de Janeiro e sul do Espírito Santo estão situadasno segmento setentrional da Faixa Ribeira. O conhecimento da estruturação tectônica deste segmento dafaixa possibilita sua correlação com o segmento sul da Faixa Araçuaí. A compartimentação tectônica da FaixaRibeira, estabelecida no seu setor central, compreende quatro terrenos tectono-estratigráficos: Ocidental,Oriental, Paraíba do Sul/Embú e Cabo Frio. Os dois primeiros terrenos são separados por uma zona decisalhamento complexamente redobrada (Limite Tectônico Central- LTC) com mergulhos subverticais amoderados para NW na porção centro-sul fluminense, e mergulhos para SE na porção noroeste fluminensee sul capixaba. O limite basal dos terrenos Cabo Frio e Paraíba do Sul/Embú é representado por uma zona decisalhamento de baixo ângulo, com mergulhos para SE e NW. Os três primeiros terrenos foram amalgamadosentre ca. 600 e 570 Ma, enquanto que Terreno Cabo Frio foi acrescionado ao final da colagem orogênica, em ca.530-510 Ma. Estes terrenos representariam paleoplacas convergentes durante a formação do supercontinenteGondwana na transição Neoproterozóico/Cambriano. O Terreno Ocidental corresponderia à paleoplacainferior (Placa Sanfranciscana), e o Terreno Oriental à placa superior, na qual se instalou o arco magmáticoresponsável pela colisão Arco/Continente. Para leste, por trás do Terreno Oriental, o fechamento do espaçoback-arc resultou na colisão com a paleoplaca do Terreno Cabo Frio. O Terreno Ocidental é representadopelo Domínio Tectônico Juiz de Fora, que integra rochas paleoproterozóicas do Complexo Juiz de Fora euma seqüência metassedimentar neoproterozóica conhecida como Megasseqüência Andrelândia. O TerrenoParaíba do Sul aflora como uma klippe sinformal complexamente dobrada sobre o Terreno Ocidental. ÉGEONOMOS 15(1): 67 - 79, 200768constituído por ortognaisses paleoproterozóicos do Complexo Quirino e por um conjunto metassedimentarrico em intercalações de mármores dolomíticos e de idade ainda incerta, denominado de Complexo Paraíba doSul. O Terreno Oriental, que contem as rochas geradas em ambientes de arco magmático e metassedimentosneoproterozóicos, foi subdividido na região noroeste fluminense em três domínios estruturais distintos: a)o Domínio Cambuci, em posição basal, compreende uma seqüência metavulcano-sedimentar com lentesde mármore e ortognaisses calcioalcalinos com ambiência tectônica de arco magmático; b) o DomínioCosteiro é constituído por metassedimentos pelíticos em fácies granulito a anfibolito alto, com intercalaçõesde quartzitos impuros intrudidos por ortognaisses e metagabros do Arco Magmático Rio Negro (ca. 790 a620 Ma); c) a Klippe de Italva aflora sobre o Domínio Costeiro e compreende um conjunto metavulcanosedimentarcom mármores calcíticos, anfibolitos (ca. 840 Ma) e paragnaisses com provável contribuiçãovulcânica. O Terreno Cabo Frio não aflora na região noroeste fluminense, sendo limitado por uma falharúptil de direção NWW-SEE na região de Macaé. A comparação entre este segmento da Faixa Ribeira e osegmento meridional da Faixa Araçuaí, ainda em andamento, sugere a continuidade lateral do Domínio Juiz deFora para o denominado Domínio Externo e o prolongamento dos Domínios Cambuci e Costeiro do TerrenoOriental para o Domínio Interno da Faixa Araçuaí. Neste sentido, os metassedimentos do Grupo Rio Doce eos ortognaisses equivalentes ao Tonalito Galiléia poderiam ser correlacionados às unidades litoestratigráficasdo Domínio Cambuci, enquanto os metassedimentos de alto grau atribuídos ao Complexo Paraíba do Sul eortognaisses da porção leste do Estado do Espírito Santo poderiam ser correlatos às unidades do DomínioCosteiro, incluindo o arco Rio Negro. Restam ainda dois domínios com aloctonia completa, ou seja, comuma superfície de descolamento em sua base e sem ligação com sua raiz, que seriam representadas pelasklippen Paraíba do Sul e Italva, que possuem posicionamento paleogeográfico ainda incerto. ABSTRACT: The northern sector of the Ribeira Belt is located in parts of the states of Rio de Janeiro and Espírito Santo,and its tectonic organization helps to understand the correlations between the Ribeira and Araçuaí belts.Four tectono-stratigraphic terrains have been already described in the central sector of the Ribeira Belt:Occidental, Oriental, Paraíba do Sul/Embú and Cabo Frio. The Central Tectonic Boundary (CTB), a foldedshear zone, separates the Occidental and Oriental terrains. It changes from a low to high angle NW dippingto a low angle SE dipping surface, from the Central to Northern Ribeira Belt. Subhorizontal to verticallyfolded shear zones that dips to SE or NW are interpreted as base thrusts of the Cabo Frio and Paraíba doSul/Embú terrains. The amalgamation of the first three terrains occurred between 600 and 570 Ma, whilethe Cabo Frio terrain collided during 530 to 510 Ma, at the end of the orogenic collage. These terrains areinterpreted as colliding paleoplates involved in the Neoproterozoic/Cambrian Gondwana formation: a) theOccidental terrain was the lower plate (São Francisco Paleoplate); b) the Oriental terrain was the upper platewith the magmatic arc that acted as a collisional backstop; c) the closure of the back-arc basin to the eastresulted in the final collision of the Cabo Frio paleoplate/terrain. In the studied areas, the Occidental Terrainis represented only by the Juiz de Fora tectonic domain which comprises palaeoproterozoic granulites ofthe Juiz de Fora Complex and a neoproterozoic metasedimentary sequence (Andrelândia Megasequence).The Paraíba do Sul/Embú Klippe is structured as a noncylindrical sinform over the Occidental Terrain. Itcontains palaeoproterozoic orthogneisses of the Quirino Complex and a carbonatic-pelitic metasedimentarysequence (Paraíba do Sul Group) with uncertain depositional age. Arc-related meta-plutonic, volcanic andsedimentary rocks constitute the Oriental terrain. At the NW region of the Rio de Janeiro State, it can bedivided into three tectonic domains: a) the Cambuci Domain, with calc-alkaline metaplutonic rocks and acarbonatic to pelitic meta- vulcanosedimentary sequence; b) the Costeiro Domain, comprising the Rio NegroMagmatic Arc (790 a 620 Ma); c) the Italva Klippe which overlays the Costeiro Domain and consists of acarbonatic and metamafic volcano sedimentary sequence dated at 840 Ma, with plutonic fragments of theRio Negro Arc. The Cabo Frio terrain is limited, to the west, by a NNW-SSE trending brittle shear zonenear Macaé town and probably do not extends into the Araçuaí Belt. The correlation between the northernsection of the Ribeira Belt and the southern section of the Araçuaí Belt, still at work, suggests a link betweenthe Juiz de Fora Domain and the External Domain of the Araçuaí Belt, and also between the Cambuci andCosteiro domains and the Internal Domain of the Araçuaí Belt. The Cambuci Unit and the calc-alkalinemetaplutonic rocks of the Cambuci Domain can be correlated, respectively, to the Rio Doce Group and theGaliléia Tonalite in the Araçuaí Belt. The high-grade metasediments and orthogneisses of the Costeiro Domain(including the Rio Negro Complex) can be easily followed northward up to the kinzigites and high-grademetassediments at eastern Espírito Santo State, formerly included in the Paraíba do Sul Complex. Availablegeological data does not yield reliable information about the paleogeographic context of the allochtonousParaíba do Sul/Embu and the Italva klippe.

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This chapter presents a synthesis of the pre-break-up plate tectonics of western Gondwana and the pre- and syn-rift magmatism in the SW South Atlantic margin (Brazil, Uruguay and Argentina) and the conjugate African continental margin (Angola, Namibia and South Africa). An updated database of the geographical locations of the primary available radiometric ages is also included in this work. A systematic analysis of the K–Ar and Ar/Ar ages from outcrops and boreholes shows a marked Mid to Late Jurassic peak in the southernmost segment of the South Atlantic, related to the emplacement of the Karoo volcanics in South Africa and in Argentina (including the Falkland Islands), and an important Early Cretaceous peak with age distributions that are related to the Gondwana break-up and formation of rift basins along the incipient continental margins. In both the southern Brazilian and Argentinian margins, as well as in the conjugate Namibian and South African margins, several igneous centres and basaltic lava flows are suggestive of the influence of mantle plumes in the Early Cretaceous, which were heralded by mafic dyke swarms in Argentina, Brazil, South Africa and Namibia.Supplementary material:A complete table with Radiometric ages of Jurassic–Early Cretaceous magmatism in the southern portion of West Gondwana is available at: www.geolsoc.org.uk/SUP18589

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