Sobre a evolução tectônica do orógeno Araçuaí-Congo ocidental.

Caracterizada, há 30 anos atrás, como cinturão de dobramentos brasilianos que limitaria o Cráton do São Francisco pelo sudeste e sul, a Faixa Araçuaí é hoje entendida como parte do domínio metamórfico externo do Orógeno Araçuaí-Congo Ocidental. Este componente da grande rede orogênica do Gondwana Ocidental que, na África, compreende a Faixa Oeste-Congolesa e, no Brasil, a Faixa Araçuaí e terrenos adjacentes a leste, possui uma série de atributos singulares. Contornado pelo Cráton do São Francisco-Congo e contíguo, a sul, ao sistema orogênico Ribeira exibe, em mapa, uma forma em ferradura e vergências centrífugas, o que a princípio 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 também o são vários pulsos de volumosa produção de magmas graníticos a partir de fontes tanto mantélicas, quanto crustais. Analisado do ponto de vista tectônico, o Orógeno Araçuaí-Congo Ociental pode ser subdividido em dez compartimentos, os quais desempenharam papéis distintos no curso de sua história. Dada a sua natureza confinada 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 bacia parcialmente assoalhada por crosta oceânica - a Bacia Macaúbas, iniciada por volta de 880 Ma - através um mecanismo que lembra a operação de um quebra-nozes. Ou seja, as peças cratônicas do São Francisco e do Congo, articuladas por meio de riftes interiores, mover-se-iam em sentidos opostos por forças de colisões em suas margens e promoveriam o fechamento da bacia mediterrânea precursora. Ao evento colisional principal, que se desencadeou por volta de 580 Ma, sucederam as fases de escape lateral da porção sul e de colapso 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 não só sobre esta, mas também sobre as cadeias de montanhas de um modo geral.The Araçuaí Belt, portrayed by Almeida (1977) as a brasiliano orogenic domain developed along the southeastern margin of the São Francisco Craton, is now viewed as part of the external zone of the so called Araçuaí-West Congo Orogen. This orogen, which also encompasses the West Congo Belt of Africa and the terrain between the Araçuaí Belt and the Brazilian continental margin, exhibits a whole series of puzzling 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 plutonic rocks including subduction-related granites. Its evolution is thus associated with ocean floor spreading and subduction, 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 its development. Considering the peculiar setting it formed and knowing the kinematic function of the main structures, the tectonic evolution of the Araçuaí-West Congo Orogen can be best explain by a model that involves the closure of a basin partially floored by oceanic crust – the Macaúbas basin, iniciated around 880 Ma - through a mechanism that resembles the operation of a nutcracker. The São Francisco and Congo cratons, like pincers of a nutcracker, and articulated along interior rifts (the Pirapora, Paramirim and Sangha aulacogens), rotated against each other, compressing the Macaúbas basin that lay in-between. The driven forces for the closure are probably triggered by collisions along the margins of the São Francisco-Congo plate during the final assembly of West Gondwana. The main collisional stage around 580 Ma was followed by the lateral escape of the southern portion of the orogen and gravity collapse. We anticipate that the continuation of the study of the Brazilian or Araçuaí portion of the Araçuaí-West Congo Orogen, which correspond to an extraordinary natural lab, will bring solution not only for the enigmas presented by this peculiar orogen, but also for questions related to the anatomy and development of mountain belts in general

Green-schists of the Alto Araçuaí, Minas Gerais : basic volcanism of the neoproterozoic macaúbas rift.

Os xistos verdes do Alto Araçuaí foram divididos em quatro litofácies, com base em texturas primárias macroscópicas preservadas: i) xistos com estruturas em almofada e disjunções poliedrais, ii) xistos acamadados, iii) brechas, e iv) xistos destituídos de estruturas primárias. A interpretação dos ambientes e processos de formação dessas litofácies aponta interação entre vulcanismo basáltico submarino, sedimentação vulcanoclástica e vulcanismo relacionado a fontes de alta produtividade. O arcabouço litoestrutural indica contatos normais entre os xistos verdes e as rochas metassedimentares da Formação Chapada Acauã. Datação U-Pb (SHRIMP) em zircão limita a idade máxima dos protólitos dos xistos verdes em ca. 1,1 Ga. A idade-modelo Sm-Nd (ca. 1,5 Ga) também indica que os xistos verdes são mais novos que o magmatismo do rifte Espinhaço. Os xistos verdes têm assinatura geoquímica de basalto intraplaca e cristais de zircão herdados de rochas félsicas mais velhas. Entretanto, o valor de Nd(900Ma) ligeiramente positivo (+ 0,23) e amostras com assinatura de basalto oceânico sugerem que o magma máfico teria se formado no estágio tardio do rifte Macaúbas e atravessado crosta continental significativamente adelgaçada. Todos os dados embasam a inserção das rochas estudadas no Membro Rio Preto da Formação Chapada Acauã (Grupo Macaúbas).Greenschists (s.s.) of the upper valley of the Araçuaí River were grouped in four lithofacies, based on preserved macroscopic textures: i) schists with pillow structures and columnar joints, ii) banded schists, iii) breccias, and iv) schists with no primary structures. An interpretation of environments and processes involved in the formation of these lithofacies indicates interplay between submarine basaltic volcanism, volcaniclastic sedimentation and fire fountaining. The litho-structural framework indicates normal contact between the greenschists and the metasedimentary rocks of the Chapada Acauã Formation. U-Pb SHRIMP dating constrains the maximum age of the protoliths at ca. 1.1 Ga. The Sm-Nd model age of ca. 1.5 also corroborates that the greenschists are younger than the magmatism of the Espinhaço rift. The greenchists have tholeiitic protoliths, a within-plate basalt signature and inherited zircon crystal from older felsic rocks. However, εNd(900 Ma) slightly positive (+ 0.23) and samples with oceanic signature suggest that the basic magma migrated trough a thinned continental crust, during the late rift stage. All data sets support the correlation between these rocks and the Rio Preto Member of the Chapada Acauã Formation (Macaúbas Group)

Charnockites from southearstern Minas Gerais : the magmatic arc granulitic roots of the Araçuaí Orogen.

Os orógenos Araçuaí e Ribeira constituem sistema orogênico neoproterozóico-cambriano que se es-tende da borda oriental do Cráton do São Francisco até o Oceano Atlântico, a sul do paralelo 15°. A zona de fronteira entre esses orógenos situa-se em torno do paralelo 21°. A região meridional do Orógeno Araçuaí, no sudeste de Minas Gerais, é caracterizada pela exposição de rochas metamórficas de alto grau, ortoderivadas e paraderivadas, refletindo o profundo nível crustal ali exposto. Uma associação de rochas charnockíticas máficas a félsicas, metamorfisadas na fácies granulito, recebeu a designação de Suíte Divino. Esta tem características de série cálcio-alcalina expandida, metaluminosa a ligeiramente peraluminosa, com padrão de elementos traços ca¬racterístico de magmatismo de arco de margem continental. A assinatura isotópica de Nd evidencia significativo envolvimento de fusões derivadas do embasamento paleoproterozóico. Datações U-Pb (LA-ICPMS) de rochas da Suíte Divino (592±7 Ma e 603±4 Ma) indicam idade compatível com o estágio pré-colisional (acrescionário) do orógeno. Desta forma, sugere-se que a Suíte Divino representa a raiz granulítica do arco magmático do Orógeno Araçuaí, cuja eventual continuidade para sul seria importante elo de ligação com o Orógeno Ribeira.The Araçuaí and Ribeira orogens are a Cambrian-Neoproterozoic belt that extends from the eastern edge of the San Francisco craton to the Atlantic Ocean, south of parallel 15 °. The connection between these orogens is roughly situated along the 21 ° S parallel. High-grade metamorphic rocks are exposed in the southern Araçuaí Belt, SE Minas Gerais, reflecting a deep crustal level. The Divino Suite comprises mafic- to felsisc charnockites, metamorphosed in granulite facies, interpreted as an expanded calc-alkaline suite metalu¬minous to slightly peraluminous. Chemical and Nd isotopic signatures suggest the Divino Suite evolved in a con¬tinental magmatic-arc setting, and also that Paleoproterozoic basement was a major source for parental magmas. U-Pb dating of charnockite samples (592±7, and 603±4 Ma) indicates the suite is coeval with the pre-collisional stage of Araçuaí Orogen evolution, and probably represents the magmatic arc granulitic roots. The southward extent of this arc is of great importance in order to unveil the link between Araçuaí and Ribeira orogens

Orógeno Araçuaí : síntese do conhecimento 30 anos após Almeida 1977.

The Araçuaí Fold Belt was defined as the southeastern limit of the São Francisco Craton in the classical paper published by Fernando Flávio Marques de Almeida in 1977. This keystone of the Brazilian geologic literature catalyzed important discoveries, such as of Neoproterozoic ophiolites and a calc-alkaline magmatic arc, related to the Araçuaí Belt and paleotectonic correlations with its counterpart located in Africa (the West Congo Belt), that provided solid basis to define the Araçuaí-West-Congo Orogen by the end of the 1990th decade. After the opening of the Atlantic Ocean in Cretaceous times, two thirds of the Araçuaí-West-Congo Orogen remained in the Brazil side, including records of the continental rift and passive margin phases of the precursor basin, all ophiolite slivers and the whole orogenic magmatism formed from the pre-collisional to post-collisional stages. Thus, the name Araçuaí Orogen has been applied to the Neoproterozoic-Cambrian orogenic region that extends from the southeastern edge of the São Francisco Craton to the Atlantic coastline and is roughly limited between the 15º and 21º S parallels. After 30 years of systematic geological mapping together with geochemical and geochronological studies published by many authors, all evolutionary stages of the Araçuaí Orogen can be reasonably interpreted. Despite the regional metamorfism and deformation, the following descriptions generally refer to protoliths. All mentioned ages were obtained by U-Pb method on zircon. The Macaúbas Group records rift, passive margin and oceanic environments of the precursor basin of the Araçuaí Orogen. From the base to the top and from proximal to distal units, this group comprises the pre-glacial Duas Barras and Rio Peixe Bravo formations, and the glaciogenic Serra do Catuni, Nova Aurora and Lower Chapada Acauã formations, related to continental rift and transitional stages, and the diamictitefree Upper Chapada Acauã and Ribeirão da Folha formations, representing passive margin and oceanic environments. Dates of detrital zircon grains from Duas Barras sandstones and Serra do Catuni diamictites suggest a maximum sedimentation age around 900 Ma for the lower Macaúbas Group, in agreement with ages yielded by the Pedro Lessa mafic dikes (906 ± 2 Ma) and anorogenic granites of Salto da Divisa (875 ± 9 Ma). The thick diamictite-bearing marine successions with sand-rich turbidites, diamictitic iron formation, mafic volcanic rocks and pelites (Nova Aurora and Lower Chapada Acauã formations) were deposited from the rift to transitional stages. The Upper Chapada Acauã Formation consists of a sand-pelite shelf succession, deposited after ca. 864 Ma ago in the proximal passive margin. The Ribeirão da Folha Formation mainly consists of sand-pelite turbidites, pelagic pelites, sulfide-bearing cherts and banded iron formations, representing distal passive margin to oceanic sedimentation. Gabbro and dolerite with plagiogranite veins dated at ca. 660 Ma, and ultramafic rocks form tectonic slices of oceanic lithosphere thrust onto packages of the Ribeirão da Folha Formation. The pre-collisional, calc-alkaline, continental magmatic arc (G1 Suite, 630-585 Ma) consists of tonalites and granodiorites, with minor diorite and gabbro. A volcano-sedimentary succession of this magmatic arc includes pyroclastic and volcaniclastic rocks of dacitic composition dated at ca. 585 Ma, ascribed to the Palmital do Sul and Tumiritinga formations (Rio Doce Group), deposited from intra-arc to fore-arc settings. Detrital zircon geochronology suggests that the São Tomé wackes (Rio Doce Group) represent intra-arc to back-arc sedimentation after ca. 594 Ma ago. The Salinas Formation, a conglomerate-wacke-pelite association located to northwest of the magmatic arc, represents synorogenic sedimentation younger than ca. 588 Ma. A huge zone of syn-collisional S-type granites (G2 Suite, 582-560 Ma) occurs to the east and north of the pre-collisional magmatic arc, northward of latitude 20º S. Partial melting of G2 granites originated peraluminous leucogranites (G3 Suite) from the late- to post-collisional stages. A set of late structures, and the post-collisional intrusions of the S-type G4 Suite (535-500 Ma) and I-type G5 Suite (520-490 Ma) are related to the gravitational collapse of the orogen. The location of the magmatic arc, roughly parallel to the zone with ophiolite slivers, from the 17º30’ S latitude southwards suggests that oceanic crust only developed along the southern segment of the precursor basin of the Araçuaí- West-Congo Orogen. This basin was carved, like a large gulf partially floored by oceanic crust, into the São Francisco-Congo Paleocontinent, but paleogeographic reconstructions show that the Bahia-Gabon cratonic bridge (located to the north of the Araçuaí Orogen) subsisted since at least 1 Ga until the Atlantic opening. This uncommon geotectonic scenario inspired the concept of confined orogen, quoted as a new type of collisional orogen in the international literature, and the appealing nutcracker tectonic model to explain the Araçuaí-West-Congo Orogen evolution

Idade máxima de sedimentação e proveniência do Complexo Jequitinhonha na área-tipo (Orógeno Araçuaí) : primeiros dados U-Pb (LA-ICP-MS) de grãos detríticos de zircão.

O Complexo Jequitinhonha, situado no nordeste de Minas Gerais, é uma das unidades metassedimentares mais extensas do Orógeno Araçuaí. Na área-tipo, situada na região de Jequitinhonha -Almenara, este complexo consiste de paragnaisse peraluminoso (kinzigítico) migmatizado, com intercalações de quartzito, grafita gnaisse e rocha calcissilicática. Os dados isotópicos U-Pb (LA-ICP-MS) de 80 grãos detríticos de zircão de uma amostra de quartzito, coletada em corte da BR-367 cerca de 12 km a SW de Almenara, permitem identificar seis principais intervalos de idades, cujas médias das modas sugerem as seguintes fontes de sedimentos: o embasamento São Francisco-Congo (2541 ± 8 Ma e 2044 ± 6 Ma), o sistema Espinhaço-Chapada Diamantina (1819 ± 6 Ma, 1487 ± 5 Ma e 1219 ± 3 Ma) e o sistema de rifteamento Noqui-Zadiniano-Mayumbiano-Salto da Divisa (956 ± 4 Ma). A idade máxima de sedimentação em 898 ± 8 Ma é dada pelo zircão mais novo. Os espectros de idades desta amostra do Complexo Jequitinhonha e de rochas do Grupo Macaúbas são muito similares, indicando correlação entre estas unidades. Contudo, no Complexo Jequitinhonha inexiste evidência de glaciação. Assim, o Complexo Jequitinhonha na área-tipo é interpretado como depósito de margem passiva da bacia precursora do Orógeno Araçuaí, mais novo que a glaciação Macaúbas e, portanto, equivalente às formações Chapada Acauã Superior e Ribeirão da Folha.The Jequitinhonha Complex is one of the most extensive metasedimentary units of the Araçuaí Orogen. In the type-area, located in the Jequitinhonha-Almenara region, this complex includes migmatized peraluminous (kinzigitic) paragneiss with intercalations of quartzite, graphite gneiss and calcsilicate rock. U-Pb (LA-ICP-MS) data from 80 detrital zircon grains extracted from a quartzite sample, collected in a BR-367 road cut around 12 km SW from Almenara, yielded six main age intervals, which mode mean values suggest the following sediment sources: the São Francisco-Congo basement (2541 ± 8 Ma and 2044 ± 6 Ma), the Espinhaço-Chapada Diamantina system (1819 ± 6 Ma, 1487 ± 5 Ma and 1219 ± 3 Ma), and the Noqui-Zadinian-Mayumbian-Salto da Divisa rift system (956 ± 4 Ma). The maximum sedimentation age of 898 ± 8 Ma is constrained by the youngest zircon grain. Spectra of detrital zircon ages for this sample of the Jequitinhonha Complex and rocks of the Macaúbas Group are very similar and suggest a correlation between these units. However, this complex shows no evidence of glaciation. Therefore, the Jequitinhonha Complex in its type-area is interpreted as a passive margin deposit of the precursor basin of the Araçuaí Orogen, younger than the Macaúbas glaciation and, thus, equivalent of the Upper Chapada Acauã and Ribeirão da Folha formations

U-Pb geochronology of the Lagoa Real uranium district, Brazil : implications for the age of the uranium mineralization.

The Lagoa Real uranium district in Bahia, northeastern Brazil, is the most important uranium province in the country and presently produces this metal in an open-pit mine operated by Indústrias Nucleares do Brasil. Uranium-rich zones are associated with plagioclase (dominantly albite ± oligoclase) -rich rocks, albitites and metasomatized granitic-gneisses, distributed along NNW/SSE striking shear zones. We have used the ID-TIMS U-Pb method to date zircon and titanite grains from the São Timóteo granitoid, and albite-rich rocks from the Lagoa Real district in order to assess the age of granite emplacement, deformation/metamorphism and uranium mineralization. The isotopic data support the following sequence of events (i) 1746 ± 5 Ma - emplacement of the São Timóteo granitoid (U-Pb zircon age) in an extensional setting, coeval with the beginning of the sedimentation of the Espinhaço Supergroup; (ii) 956 ± 59 Ma hydrothermal alteration of the São Timóteo granitoid and emplacement of the uranium mineralization (U-Pb titanite age on an albite-rich sample); (iii) 480 Ma metamorphism, remobilization and Pb loss (U-Pb titanite age for the gneiss sample), during the nucleation of shear zones related to the collision between the São Francisco-Congo and Amazonia paleoplates. The 956 ± 59-Ma mineralization age is apparently associated with the evolution of the Macaúbas-Santo Onofre rift. This age bracket may bear an important exploration implication, and should be included in the diverse age scenario of uranium deposits worldwide