The Neoproterozoic Araçuaí-West Congo (A-WC) orogen is one of many Brasiliano/Pan-African orogens that developed during the assembly of West Gondwana. This orogen was split apart in Mesozoic time, due to opening of the South Atlantic-the Araçuaí orogen now underlies eastern Brazil, whereas the West Congo belt fringes central Africa's Atlantic coast. Significantly, at the time it formed, the A-WC orogen was bounded on the west, north, and east by the São Francisco-Congo craton, a crustal block that had the shape of a lopsided, upside-down 'U'. Thus, the orogen was "partially confined" during tectonism, in that it occupied an enclave surrounded on three sides by cratonic crust. Formation of the A-WC orogen resulted in kinematically complex deformation, substantial crustal shortening, and production of a large volume of magma. How such features could develop in this particular setting has long been a mystery. Our field studies in the Araçuaí orogen, together with published data on the West Congo belt, characterize the kinematic picture of the A-WC orogen, and lead to a tectonic model that addresses its evolution. In our model, the A-WC orogen formed in response to closure of the Macaúbas basin. This basin was underlain by oceanic crust in the south, but tapered northward into a continental rift which terminated against the cratonic bridge linking the eastern and western arms of the São Francisco-Congo craton. Closure occurred when the western arm (now the São Francisco craton) rotated counterclockwise towards the eastern arm (now the Congo craton). This closure may have been driven by collision of the Paranapanema, Amazonian, and Kalahari cratons against the external margins of the São Francisco-Congo craton, rather than by slab-pull associated with subduction of the Macaúbas basin's floor. Thus, the process of forming the A-WC orogen resembled the process of crushing of a nut between two arms of a nutcracker. Such "nutcracker tectonics" led to a series of kinematically distinct deformation stages. Initially, internal portions of the orogen flowed northwards. Then, substantial crustal thickening occurred in the orogen's interior, and the deformation front migrated outwards, producing thrust belts that overlapped the internal margins of the craton. With continued closure, space in the enclave became restricted and the orogen's interior underwent lateral escape to the south. Late-stage extensional collapse triggered both production of late-to post-collisional granites and exhumation of high-grade rocks from mid-crustal levels.

The Paramirim aulacogen, hosted in the northern part of the São Francisco craton, corresponds to two superimposed and partially inverted rifts of Paleo and Neoproterozoic ages. The Rio Pardo salient of the Araçuaí belt defines the local limit of the craton and interferes with the aulacogen structures. In order to understand the mechanism and timing of the tectonic interaction between these tectonic features during the inversion processes, a structural analysis was undertaken in the southern Paramirim aulacogen and along the Rio Pardo salient. The results obtained indicate that the Rio Pardo salient formed during an early stage of closure of the Neoproterozoic Macaúbas rift system and consequent initiation of the Araçuaí orogen. The orogenic front propagated further northwards into the craton, causing a first stage of inversion in the southern terminus of the aulacogen trough. Subsequently, the Paramirim aulacogen experienced the main stage of inversion, which led to the development of a NNW-oriented basement involved fold-thrust system. These fabric elements overprint the Rio Pardo salient, and the structures of both the first and second stages of inversion affect the Salitre Formation, the youngest Neoproterozoic unit of the area, clearly indicating a Late Neoproterozoic maximum age for all the inversion stages of the Paramirim aulacogen.

Caracterizada, há 30 anos atrás, como cinturão de dobramentos brasilianos que limitaria o Cráton do SãoFrancisco pelo sudeste e sul, a Faixa Araçuaí é hoje entendida como parte do domínio metamórfico externodo Orógeno Araçuaí-Congo Ocidental. Este componente da grande rede orogênica do Gondwana Ocidentalque, na África, compreende a Faixa Oeste-Congolesa e, no Brasil, a Faixa Araçuaí e terrenos adjacentes aleste, possui uma série de atributos singulares. Contornado pelo Cráton do São Francisco-Congo e contíguo, asul, ao sistema orogênico Ribeira exibe, em mapa, uma forma em ferradura e vergências centrífugas, o que aprincípio sugere uma evolução essencialmente ensiálica. Os estudos realizados no Orógeno Araçuaí mostram,entretanto, que geração e consumo de assoalho oceânico constituem fases da sua evolução, como tambémo são vários pulsos de volumosa produção de magmas graníticos a partir de fontes tanto mantélicas, quantocrustais. Analisado do ponto de vista tectônico, o Orógeno Araçuaí-Congo Ociental pode ser subdividido emdez compartimentos, os quais desempenharam papéis distintos no curso de sua história. Dada a sua naturezaconfinada e as funções cinemáticas desempenhadas pelas peças do seu arcabouço, postulou-se a hipótese,ora em fase de teste, de que o Orogeno Araçuaí-Congo Ocidental tenha evoluído a partir de uma baciaparcialmente assoalhada por crosta oceânica - a Bacia Macaúbas, iniciada por volta de 880 Ma - através ummecanismo que lembra a operação de um quebra-nozes. Ou seja, as peças cratônicas do São Francisco e doCongo, articuladas por meio de riftes interiores, mover-se-iam em sentidos opostos por forças de colisõesem suas margens e promoveriam o fechamento da bacia mediterrânea precursora. Ao evento colisionalprincipal, que se desencadeou por volta de 580 Ma, sucederam as fases de escape lateral da porção sul e decolapso gravitacional. Antevê-se que a continuidade do estudo da porção brasileira desta feição orogênica,que constitui um excepcional laboratório natural, trará respostas para muitas questões ainda em aberto nãosó sobre esta, mas também sobre as cadeias de montanhas de um modo geral.Palavras-chave: Faixa Araçuaí, Orógeno Araçuaí-Congo Ocidental, Evento Brasiliano-Panafricano,Neoproterozóico, Gondwana Ocidental. ABSTRACT: The Araçuaí Belt, portrayed by Almeida (1977) as a brasiliano orogenic domain developed along thesoutheastern margin of the São Francisco Craton, is now viewed as part of the external zone of the socalled Araçuaí-West Congo Orogen. This orogen, which also encompasses the West Congo Belt of Africaand the terrain between the Araçuaí Belt and the Brazilian continental margin, exhibits a whole series ofpuzzling features. Confined to a tongue-shaped enclave between the São Francisco and Congo cratons,the Araçuaí-West Congo Orogen involves, besides Neoproterozoic ophiolites, a large volume of plutonicrocks including subduction-related granites. Its evolution is thus associated with ocean floor spreading andsubduction, processes difficult to reconcile with its confined nature. From a tectonic perspective, the Araçuaí-West Congo Orogen can be subdivided in ten compartments, which played distinct rules in the course of itsdevelopment. Considering the peculiar setting it formed and knowing the kinematic function of the mainstructures, the tectonic evolution of the Araçuaí-West Congo Orogen can be best explain by a model thatinvolves the closure of a basin partially floored by oceanic crust – the Macaúbas basin, iniciated around880 Ma - through a mechanism that resembles the operation of a nutcracker. The São Francisco and Congocratons, like pincers of a nutcracker, and articulated along interior rifts (the Pirapora, Paramirim and Sanghaaulacogens), rotated against each other, compressing the Macaúbas basin that lay in-between. The drivenforces for the closure are probably triggered by collisions along the margins of the São Francisco-Congo plateduring the final assembly of West Gondwana. The main collisional stage around 580 Ma was followed by thelateral escape of the southern portion of the orogen and gravity collapse. We anticipate that the continuationof the study of the Brazilian or Araçuaí portion of the Araçuaí-West Congo Orogen, which correspond to anextraordinary natural lab, will bring solution not only for the enigmas presented by this peculiar orogen, butalso for questions related to the anatomy and development of mountain belts in general.Keywords: Araçuaí belt, Araçuaí-West Congo orogen, Brasiliano-Pan African event, Neoproterozoic,, WestGondwana

a b s t r a c tThis paper aims at establishing a tectonic and temporal framework to characterize the metallogenic processes that contributed to the origin of the mineral provinces in the northern São Francisco Craton. Many Archean mineralizations (eg. massive sulfide zinc, lead, zinc and copper, besides magnesiteetalc, ironetitaniumevanadium, iron, chromite and manganese) were generated before the assembly of the Craton. Deposits of chromite, nickel, gold and emerald were produced during the Paleoproterozoic orogenic cycle, when the Craton was amalgamated into the Atlantica paleocontinent. An extension event is recorded in the Neoproterozoic, during the breakup of Rodinia, associated with deposits of phosphorite and uranium. Kimberlite diamond and gold mineralization were generated during the Brasiliano orogenic cycle, coeval with the amalgamation of West Gondwana. A long-lasting and rather uniform crustal stress is recorded in the area during the Cambrian period. Resetting of the isotopic and magnetic systems that affected the Neoproterozoic sediments of the Irecê Basin at about 520 Ma was attributed to the regional-scale fluid migration and mineralization in the aftermath of the Brasiliano orogenic cycle.Ó 2010 Elsevier Ltd. All rights reserved. r e s u m oEste artigo visa estabelecer um quadro tectônico e temporal para caracterizar os processos metalogenéticos que contribuíram para a origem das províncias minerais no setor norte do Cráton São Francisco. Muitas mineralizações arqueanas (por ex. sulfeto maciço de zinco, chumbo, zinco e cobre, além de magnesitaetalco, ferroetitânioevanádio, ferro, cromo e manganês) foram geradas antes da formação do Cráton. Depósitos de cromo, níquel, ouro e esmeralda foram produzidos durante a orogênese paleoproterozóica, quando o cráton foi amalgamado dentro do paleocontinente Atlântica. Um evento extensional é registrado no Neoproterozóico, durante a fragmentação do paleocontinente Rodinia, associado a depósitos de fosforita e urânio. Kimberlitos diamantíferos e mineralizações de ouro foram geradas durante o ciclo orogenético Brasiliano, contemporâneo com a aglutinação do paleocontinente Gondwana Ocidental. Um regime de tensão crustal bastante uniforme e de longa duração foi registrado na região durante o período Cambriano. A reconfiguração dos sistemas isotópico e magnético que afetou os sedimentos neoproterozóicos da bacia de Irecê há cerca de 520 Ma foi atribuída ao evento tectônico de escala regional, à migração de fluidos e mineralização, como consequência do ciclo orogenético Brasiliano.

Resumo O Complexo Lagoa Real, constituído por meta-granitóides com idade de colocação em torno de 1,7 Ga, ocorre no vale do Paramirim e engloba as rochas denominadas genericamente de Granito São Timóteo e um conjunto de granitóides milonitizados em graus variados gerados no curso de uma deformação compressional sin-metamórfica. A análise estrutural permitiu identificar duas famílias de estruturas: uma compressiva, representada por zonas de cisalhamento, bandamento composicional, foliação milonítica e dobras; e outra distensiva, marcada pela nucleação de zonas de cisalhamento normais. As estruturas deformacionais identificadas assemelham-se com as que são encontradas no sinclinal de Ituaçu, onde afloram as formações Salitre e Bebedouro do Grupo Una. Como não foi encontrada uma trama mais antiga que a primária, sugere-se que a deformação do Complexo Lagoa Real ocorreu durante o Neoproterozóico.