Transamazonian orogeny in the Southern S?o Francisco craton region, Minas Gerais, Brazil : evidence for paleoproterozoic collision and collapse in the Quadril?tero Ferr??fero.

The Paleoproterozoic Transamazonian orogeny yielded a series of discrete orogens in Brazil. The present field study in the Quadril?tero Ferr??fero (QF) indicates that the southern S?o Francisco craton region of the Brazilian highlands preserves a portion of one of these orogens. Two sets of Transamazonian structures occur in this region. The first consists of northwest-verging folds and thrusts affecting supracrustal sequences. It is suggested that this set formed in a fold-thrust belt setting shortly after 2.125 Ga, during the closure of a passive-margin basin that had initiated along the margins of a preexisting continental mass at ca 2.5 Ga. The second set consists of structures defining the prominent dome-and-keel architecture of the QF. This set, a consequence of the emplacement of basement domes against supracrustal rocks at 2.095 Ga, may reflect the consequences of orogenic collapse. Narrow, conglomerate-filled intermontane basins may have formed coevally with dome emplacement. Formation of an ocean basin east of the present-day S?o Francisco craton eventually occurred in Late Mesoproterozoic. In effect, the Transamazonian orogen of the QF represents the collision and collapse stages of a Paleoproterozoic Wilson cycle. The contractional phase of the Transamazonian orogeny probably represents accretion of an offshore arc to the eastern and southeastern margin of the present-day S?o Francisco craton region. The arc, and an associated suture, may be traced in the Brasiliano (Pan African) orogen east of the S?o Francisco craton, northwards into the northeastern lobe of the S?o Francisco craton. Clearly, initial assembly of crustal blocks to form a larger continent involving South America occurred during the Paleoproterozoic (2.1 Ga). Post-Transamazonian rifting of this continent created the basins which were later inverted during the Brasiliano assembly of Gondwana

Structural style of basin inversion at mid-crustal levels : two transects in the internal zone of the Brasiliano Ara?ua? belt, Minas Gerais, Brazil.

The Ara?uai belt is the orogenic belt that directly borders the eastern margin of the S?o Francisco Craton in eastern Brazil. Detailed structural investigations in the Governador Valadares region of Minas Gerais indicate that the amphibolite-to granulite-grade internal zones of the Ara?uai belt contain several major, west-vergent, crystalline overthrust sheets. These thrust sheets contain approximately homoclinal east-dipping gneissic banding and are separated from one another by zones of isoclinally and sheath-folded, ductiley sheared, metasedimentary units that behaved as mechanically weak glide horizons during deformation. We interpret this regionally imbricated sequence of basement and cover to be the mid-crustal level manifestation of closure of a mid-Neoproterozoic rift basin that existed to the east of the S?o Francisco Craton. The major thrusts, which are all cratonvergent, are of Brasiliano/Pan-African age (650-450 Ma) becuase they cut the Neoproterozoic Galil?ia batholith. Older fabrics are locally preserved in the basement slices, and these fabrics may be relicts of the Transamazonian orogeny (2.0 Ga). Discrete zones of ductile-brittle extension that were identified in several localities in the study area suggest the occurrence of postorogenic collapse following Brasiliano overthrusting. Alternations of rigid crystalline thrust sheets and highly deformed metasedimentary sequences, such as those of the Governador Valadares region, may be a common structural geometry at a depth of 15?20 km in modern regions of collision and basin closure

How many rifting events preceded the development of the Ara?ua?-West Congo orogen?

A edifica??o do Or?geno Ara?ua?-Congo Ocidental teve in?cio por volta de 630 Ma, como indicam as idades U-Pb mais antigas at? agora obtidas de rochas do seu arco magm?tico. Esta orogenia foi precedida de, pelo menos, seis eventos de rifteamento e/ou magmatismo anorog?nico que afetaram a regi?o ocupada pelo Or?geno Ara?ua?-Congo Ocidental, bem como o dom?nio crat?nico S?o Francisco-Congo a ele adjacente. S?o eles os eventos E1 (Estateriano, 1,77-1,7 Ga), E2 (Calimiano, 1,57-1,5 Ga), E3 (Esteniano, 1,18 - ? Ga), E4 (no limite Esteaniano-Toniano, ca. 1 Ga), E5 (Toniano, 930-850 Ma) e E6 (Criogeniano, 750-670 Ma). O evento E1, chamado de Tafrog?nese Estateriana, teve lugar entre 1,77 e 1,70 Ga e tem como registros mais importantes a deposi??o rifte das unidades sedimentares e vulc?nicas da base do Supergrupo Espinha?o, bem como o alojamento dos plutons anorog?nicos das su?tes Borrachudos e Lagoa Real. O evento E1 ? sucedido por uma nova fase de distens?o durante o per?odo Calimiano, em torno de 1,57 Ga, que ? representada no Espinha?o Setentrional e Chapada Diamantina por rochas sedimentares e vulc?nicas da por??o m?dia do Supergrupo Espinha?o. Manifesta??es do evento E3 (Esteniano) tiveram in?cio em torno de 1,18 Ga com a deposi??o da Forma??o Sopa-Brumadinho (Supergrupo Espinha?o), exposta na Faixa Ara?ua?. Os tr?s primeiros eventos (E1, E2 e E3) parecem n?o ter ocorrido na Faixa Congo Ocidental. Nesta faixa, os granitos Noqui (999 ? 7 Ma) e rochas vulc?nicas associadas representam um magmatismo anorog?nico do limite Esteniano-Toniano, registrando um evento provavelmente local (E4), mas que pode ser correlacion?vel aos diques m?ficos de idade similar que ocorrem no sul da Bahia, no extremo oriental do Cr?ton S?o Francisco. As espessas sucess?es de rochas vulc?nicas bimodais dos grupos Zadiniano e Mayumbiano (930-910 Ma) constituem os principais registros do evento E5 na Faixa Congo-Ocidental. O magmatismo anorog?nico representado pela Su?te Salto da Divisa e enxames de diques m?ficos tonianos (e.g., Pedro Lessa e Espinha?o Setentrional), assim como, muito provavelmente, os dep?sitos pr?-glaciais da bacia Maca?bas s?o manifesta??es de E5, em terrenos do Or?geno Ara?ua? e Cr?ton do S?o Francisco. O evento criogeniano, E6, ? evidenciado pela Prov?ncia Alcalina do Sul da Bahia entre 735 e 675 Ma, e, muito provavelmente, pelas forma??es diamict?ticas do Grupo Maca?bas e unidades correlativas na Faixa Congo Ocidental, e pelo vulcanismo f?lsico La Louila (? 713 Ma) do sudoeste do Gab?o. O evento E6 evoluiu para espalhamento oce?nico no setor centro-sul da Bacia Maca?bas, mas este processo n?o afetou a parte norte desta bacia e, por isto, deixou ?ntegra a liga??o continental (a ponte Bahia-Gab?o) entre a Pen?nsula S?o Francisco e o Continente Congo. De fato, nenhum dos eventos distensionais mencionados logrou levar ? ruptura a placa S?o Francisco-Congo que, aglutinada na transi??o Riaciano-Orosiriano (ca. 2,05 Ga), permaneceu ?ntegra at? o Cret?ceo Inferior quando, ent?o, o rifte Atl?ntico finalmente conseguiu desmembr?-la.The development of the Ara?ua?-West Congo orogen (AWCO) started around 630 Ma, as suggested by U-Pb ages from the oldest rocks yet found in its pre-collisional magmatic arc. This orogeny was preceded by at least six events of rifting and/or anorogenic magmatism, which affected the area occupied by the AWCO and the adjacent S?o Francisco-Congo craton, namely: the Statherian E1 (1.77-1.7 Ga), Calymmian E2 (1.57-1.5 Ga), Early Stenian E3 (1.18 - ? Ga), Stenian-Tonian E4 (ca. 1 Ga), Tonian E5 (930-850 Ma) and Cryogenian E6 (750-670 Ga) events. The E1 event, currently referred to as the Statherian taphrogenesis, took place between ca. 1.77 Ga and 1.7 Ga. It is recorded by the rift-related sedimentary and volcanic rocks of the basal section of the Espinha?o Supergroup, as well as the anorogenic plutons of the Borrachudos and Lagoa Real suites. The E1 event was followed by a Calymmian episode, the E2 (ca. 1.57 Ga), recorded in the northern Espinha?o range and Chapada Diamantina by sediments and volcanics of the middle portion of the Espinha?o Supergroup. The manifestations of the Stenian E3 event started around 1.18 Ga with the syn-rift deposition of the Sopa-Brumadinho Formation (Espinha?o Supergroup), exposed in the Ara?ua? belt. The first three events (E1, E2 and E3) are so far not found in the West Congo belt. In this belt, a magmatism took place at the Stenian-Tonian time boundary, the anorogenic Noqui granites (ca. 999 ? 7 Ma) and related volcanic rocks, recording a probably local extensional event, E4, which can be correlated to mafic dykes of similar age found in southern Bahia (eastern tip of the S?o Francisco craton). The thick bimodal volcanic pile of the Zadinian and Mayumbian groups and related intrusions are the main records of the Tonian E5 event in the West Congo belt. The A-type Salto da Divisa Suite and Tonian mafic dykes (e.g., Pedro Lessa and Northern Espinha?o range), as well as the deposition of the pre-glacial formations of the Maca?bas basin are manifestations of the E5 event in the Ara?ua? orogen and S?o Francisco craton. Records of the Cryogenian E6 event include the Southern Bahia alkaline province (ca. 735-675 Ma) and, probably, the La Louila felsic volcanism in SW Gabon (? 713 Ma), as well as the deposition of the diamictitic formations of the Maca?bas Group and correlatives. The E6 event evolved to oceanic spreading in the central-southern Maca?bas basin, but this process died out toward north keeping unbroken the continental link between the S?o Francisco peninsula and Congo continent (the Bahia-Gabon bridge). In fact, all those extensional events were unsuccessful in splitting the S?o Francisco-Congo plate, which amalgamated in the Rhyacian-Orosirian time boundary (ca. 2.05 Ma) and remained as such until the Lower Cretaceous, as the Atlantic rifting then successfully broke through the old continental mass

Palaeoproterozoic assembly of the S?o Francisco craton, SE Brazil : new insights from U?Pb titanite and monazite dating.

Isotopic U?Pb titanite and monazite data from the southern S?o Francisco craton better constrain the timing of the tectono-thermal event that led to the amalgamation of the craton and the crust that forms the basement of its fringing orogenic belts in the Palaeoproteroic. The data obtained from assemblages exposed in the Quadril?tero Ferr?fero mining district and adjacent Palaeoproterozoic Mineiro belt magmatic arc terrane reveal two different age populations: a first metamorphic event between 2772 and 2613 Ma, followed by recrystallization or/and isotopic resetting in the interval of 2080?1940 Ma. The partial preservation of Neoarchaean ages in the craton interior suggests that the Palaeoproterozoic metamorphism did not exceed the minimum closure temperature for the titanite (between 650 C and 700 C), which is also reinforced by the absence of Palaeoproterozoic metamorphic zircons in the southern S?o Francisco craton. Combining new and existing Palaeoproterozoic data, we infer that the Archaean crust of the southern craton as well as the surrounding magmatic arcs were affected by a long-lived metamorphic event from ca. 2100 Ma to 1940 Ma. These age interval includes an episode of syn-collisional metamorphism between 2100 and 2070 Ma, which represents the amalgamation of the Archaean nuclei of both the S?o Francisco and Congo cratons, along with magmatic arcs and microcontinents. This collision led to closure of the large Palaeoproterozoic Minas basin, followed by orogenic collapse and development of a dome-and-keel architecture in time interval of 2070?2050 Ma. A period of slow cooling ( 1 C/Ma) succeeded these events and lasted until ca. 1940 Ma. Our results correlate with the Palaeoproterozoic metamorphic ages obtained in the various blocks forming the northeastern sector of the craton

Detrital zircons of the Itacolomi Group in its type area, Quadrilátero Ferrífero, Minas Gerais, Brazil: ages, provenance and tectonic significance

O Grupo Itacolomi em sua localidade tipo, a serra homônima, situada na porção sudeste do Quadrilátero Ferrífero, Minas Gerais, caracteriza-se por uma sucessão de meta-arenitos, metaconglomerados e filitos. Nessa e demais ocorrências, o grupo é interpretado como uma associação de depósitos fluviais e de leques aluviais, acumulados em bacias intermontanas pós-orogênicas. As suas ocorrências na localidade tipo dão-se na forma de um bloco basal autóctone que jaz em discordância sobre rochas do Supergrupo Minas, e outro superior alóctone, lançado sobre o primeiro por uma falha de empurrão. O bloco autóctone corresponde à porção íntegra e internamente menos deformada do grupo e é constituído de um pacote de aproximadamente 400 m de meta-arenitos e metaconglomerados. No bloco alóctone, exposto no Pico do Itacolomi, meta-arenitos finos a médios com espessura de aproximadamente 145 m encontram-se, em geral, mais intensamente deformados. Os espectros de idades dos grãos de detríticos de zircão obtidos nos blocos autóctone e alóctone são ligeiramente diferentes. Os do primeiro são claramente unimodais, com picos em 2167, 2197 e 2203 Ma. Os do segundo são bimodais, com picos principais em 2156 e 2201 Ma. Eles indicam que as fontes principais do grupo devem ter sido, principalmente, os granitoides paleoproterozoicos do Cinturão Mineiro e do Complexo Mantiqueira, expostos respectivamente a sudoeste e leste da Serra do Itacolomi. A idade máxima de sedimentação do grupo pode ser estimada em 2129±11 Ma. Esses resultados, juntamente com outros disponíveis na literatura, indicam que o Grupo Itacolomi foi depositado em bacia de antepaís do orógeno paleoproterozoico que abarca o Quadrilátero Ferrífero e áreas adjacentes.The Itacolomi Group in its type area, the homonymous ridge located in the southeastern portion of Quadrilátero Ferrífero mineral province in State Minas Gerais, is made up of a succession of metasandstones, metaconglomerates and phyllites, interpreted as fluvial to alluvial fan deposits accumulated in post-orogenic intermontane basins. In the type locality, the group is exposed in the form of a basal autochthonous package that unconformably overlies the 2.6-2.1 Ga Minas Supergroup, and an allochthonous block bounded by thrust faults. The c. 400m-thick autochthonous succession is weakly deformed and composed of metasandstones and metaconglomerates. Exposed in the Itacolomi Peak, the 145 m-thick finegrained metasandstones of the thrust sheet are intensively deformed. The detrital zircon age spectra obtained from these packages are slightly different. The spectra of the basal autochthonous package are clearly unimodal with age peaks at 2167, 2197 and 2203 Ma, whereas the age distributions of the upper thrust sheet are bimodal with peaks at 2156 and 2201 Ma. They indicated that the group was sourced essentially by the Paleoproterozoic arc-related granitoids of the Mineiro belt and Mantiqueira Complex, exposed respectively to the southwest and east of the Itacolomi ridge. The maximum deposition age of the group can be estimated at 2129 ± 11 Ma. These results confirm the post-orogenic nature of the Itacolomi succession, which accumulated in a back-arc/foreland setting in respect to the Paleoproterozoic orogen exposed in the Quadrilátero Ferrífero and adjacent areas

The Jeremoabo transpressional transfer fault, Rec?ncavo-Tucano Rift, NE Brazil.

The Jeremoabo fault, located in the Early Cretaceous Rec?ncavo?Tucano Rift, northeastern Brazil, represents an example of transfer fault. A field-based structural analysis along 20 km of exposures revealed an E?W-trending and steeply dipping reverse-sinistral fault that resulted from transpressional reactivation of a pre-existent weak zone. The fault is associated with a 5-km-wide footwall deformation zone, in which sandstone layers are folded and locally overturned becoming parallel to the fault. The deformation zone contains a pervasive system of NE- and NNW-trending conjugate Riedel shear fractures of outcrop to kilometer scale. A paleostress analysis based on these Riedel fractures indicated ?1 and ?3 trending, respectively, 016? and 108?, and ?2 subvertical, consistent with transpression and local switch between ?1 and ?2 relative to the regional paleostress field. The Jeremoabo fault links smaller rift border faults and has sense of slip consistent with the mapped offset of the basin border. Its transfer function is attested by linkage with a major dextral transfer fault and a transfer zone during the rifting period. By acting together these structures accommodated the change in asymmetry between the two major half-grabens of the rift

Caracterização faciológica e evolução sedimentar da Formação Moeda (Supergrupo Minas) na porção noroeste do Quadrilátero Ferrífero, Minas Gerais

The Moeda Formation, along the northwestern region of the Quadrilátero Ferrífero, registers the first stages of the Minas basin, during the Neoarchean/Paleoproterozoic, south of the São Francisco Craton. This paper examines this unit from six stratigraphic sections where nine sedimentary facies were characterized: four conglomerate facies (Gms, Gm, Gt and Gp), three sandstones (St, Sp and Sh) and two predominant mudstones (Fl and Fsc). The stratigraphic sections were correlated allowing the grouping of the facies in five genetically related associations. The associations of facies AF1 and AF2 presents an alluvial fan system which evolved into interlaced fluvial plains. AF3 is related to a shallow marine associated lake system in the distal portions. Finally, the association of facies AF4 and AF5 represent interlaced fluvial plains enclosed by a marine transgression in the final evolution stage of the basin. Based on the geological-structural detail mapping of these facies associations and in a restored section, it is interpreted that AF1, AF2, AF3, and the basal portion of AF4 were deposited during the initial stages of the continental rift, and the other associations materialize the transition between the stages rift and passive margin.A Formação Moeda, ao longo da região noroeste do Quadrilátero Ferrífero, registra os primeiros estágios da Bacia Minas, desenvolvida no limite Neoarqueano/Paleoproterozoico no sul do Cráton do São Francisco (CSF). Este trabalho analisa essa unidade a partir de seis perfis estratigráficos de detalhe nos quais foram identificadas nove fácies sedimentares: quatro conglomeráticas (Gms, Gm, Gt e Gp), três essencialmente areníticas (St, Sp e Sh) e duas predominantemente pelíticas (Fl e Fsc). As seções estratigráficas foram correlacionadas, possibilitando o agrupamento das fácies em cinco associações geneticamente relacionadas. As associações de fácies AF1 e AF2 representam sistemas de leques aluviais que evoluíram para planícies fluviais entrelaçadas. AF3 está relacionada a um sistema lacustre associado a marinho raso nas porções distais. Por fim, as associações de fácies AF4 e AF5 representam planícies fluviais entrelaçadas encerradas por uma transgressão marinha no estágio final de evolução da bacia. Com o auxílio do mapeamento geológico-estrutural de detalhe dessas associações e da confecção de uma seção restaurada foi possível interpretar que as AF1, AF2, AF3 e a porção basal da AF4 foram depositadas durante os estágios iniciais do rifteamento continental, e as demais associações materializam a transição rifte-margem passiva

O lineamento Piúma, sul do estado do Espírito Santo: expressão estrutural e significado tectônico

The Piúma lineament, located in the Southern Espírito Santo State, Brazil, is one of the most prominent linear morphostructural features of the northern Mantiqueira Province. The lineament stands out in satellite images and topographic maps, extending for about 70 km in a N50°W direction, between the towns of Iconha and Conceição do Castelo (Espírito Santo). In order to investigate the nature and role played by the Piúma lineament in the tectonic evolution of the Mantiqueira Province and adjacent Campos Basin, we carried out a detailed structural analysis based on fieldwork, as well as geophysical data processing and interpretation. The results we obtained indicate that the Piúma lineament is a brittle shear zone characterized by N50W-trending and SW-dipping shear fractures, faults and joints. These structures record two distinct and incompatible motions, thereby recording the operation of two tectonic regimes: NNE-SSW extension and E-W transtension. Comparing our results with the literature on the Southeastern Brazilian Continental Margin, we conclude that the Piúma lineament more likely developed after the Ediacaran Brasiliano orogenic event in response to an NNE-SSW extension, experiencing later a reactivation as normal dextral to dextral transtensional zone, probably in connection with the South Atlantic rifting in the Lower Cretaceous.The Piúma lineament, located in the Southern Espírito Santo State, Brazil, is one of the most prominent linear morphostructural features of the northern Mantiqueira Province. The lineament stands out in satellite images and topographic maps, extending for about 70 km in a N50°W direction, between the towns of Iconha and Conceição do Castelo (Espírito Santo). In order to investigate the nature and role played by the Piúma lineament in the tectonic evolution of the Mantiqueira Province and adjacent Campos Basin, we carried out a detailed structural analysis based on fieldwork, as well as geophysical data processing and interpretation. The results we obtained indicate that the Piúma lineament is a brittle shear zone characterized by N50W-trending and SW-dipping shear fractures, faults and joints. These structures record two distinct and incompatible motions, thereby recording the operation of two tectonic regimes: NNE-SSW extension and E-W transtension. Comparing our results with the literature on the Southeastern Brazilian Continental Margin, we conclude that the Piúma lineament more likely developed after the Ediacaran Brasiliano orogenic event in response to an NNE-SSW extension, experiencing later a reactivation as normal dextral to dextral transtensional zone, probably in connection with the South Atlantic rifting in the Lower Cretaceous.The Piuma lineament, located in the Southern Espirito Santo State, Brazil, is one of the most prominent linear morphostructural features of the northern Mantiqueira Province. The lineament stands out in satellite images and topographic maps, extending for464531546sem informaçãosem informaçãoO lineamento Piúma, situado na porção sul do estado do Espírito Santo, está entre as feições morfo-estruturais mais proeminentes da província Mantiqueira Setentrional. Constitui uma feição linear regional, bem marcada em imagens de sensoriamento remoto e mapas topográficos, com aproximadamente 70 km de comprimento e direção N50°W, entre as cidades de Iconha e Conceição do Castelo (Espírito Santo). Para investigar sua natureza e o papel que desempenha na evolução tectônica da Província Mantiqueira e da bacia de Campos, adjacente, foi feita uma analise estrutural detalhada, levada a efeito através de trabalhos de campo, processamento e interpretação de dados geofísicos. Os resultados obtidos indicam que o lineamento Piúma se traduz numa zona de cisalhamento rúptil, caracterizada pela presença de falhas e juntas com direção preferencial N50W e mergulho para SW. Essas estruturas registram a ação de dois movimentos distintos e incompatíveis, que marcam a ação de dois regimes tectônicos: distensão NNE-SSW e transtração E-W. Comparando-se os resultados com a literatura disponível para a margem continental sudeste brasileira, conclui-se que o lineamento Piúma, provavelmente, se desenvolveu em algum momento após o evento orogenético ediacarano Brasiliano, em resposta a uma distensão NNE--SSW, posteriormente sendo reativado como uma zona normal dextral a transtracional dextral, possivelmente relacionada à abertura do rifte do Atlântico Sul, no Cretáceo InferiorO lineamento Piúma, situado na porção sul do estado do Espírito Santo, está entre as feições morfo-estruturais mais proeminentes da província Mantiqueira Setentrional. Constitui uma feição linear regional, bem marcada em imagens de sensoriamento remoto

U?Pb ages and Hf-isotope data of detrital zircons from the late Neoarchean-Paleoproterozoic Minas Basin, SE Brazil.

Because of its world-class iron ore deposits and promising Au and U mineralizations, the late Neoarchean to Paleoproterozoic Minas Basin (Minas Supergroup, SE of Brazil) is one of the best-studied basins in South America. However, the lack of datable interlayered volcanic rocks prevented discourse over ages of the strata, the sources and the nature of its ore deposits. In this paper, we present detrital zircon U? Pb age patterns coupled with Lu?Hf data for 18 samples, representing different stages of the Minas Basin evolution ( 2000 analyzed zircons). Age spectra for the main basal unit (Moeda Formation) show a classic rift-related detrital zircon pattern, characterized by multiple autochthonous sources, which in turn are much older than the age of deposition. Maximum age for the rifting event is constrained at ca. 2600 Ma. Detritus accumulated at the base of the Minas Supergroup were derived from Archean source rocks and their sedimentation was marked by differential uplift of the Archean crust, shortly after the 2730?2600 Ma high-K calc-alkaline magmatism (Mamona Event). The age of the BIF deposits is younger than 2600 Ma, most likely coinciding with the great oxygenation event between 2400 and 2200 Ma and the precipitation of banded iron deposits worldwide. Detrital zircons from the topmost units of the Minas strata suggest that tectonic inversion and closure of the basin took place at ca. 2120 Ma with the deposition of the synorogenic Sabar? Group. Rhyacian zircon supply showing juvenile Hf signatures gives evidence of a late Rhyacian amalgamation between the Mineiro Belt and the craton. The eHf signatures support the hypothesis that the Archean crystalline crust of the craton was mostly built by crust?mantle mixing processes, with a successive decrease of eHf values in zircons crystallized after 3250 Ma and minor mantle-like additions after Paleoarchean times. Regionally, our dataset supports previous interpretations of a long-lived evolution of the southern S?o Francisco Craton comprising a succession of convergent island arcs, small microplate collisions, and developmen

Metamorphism and exhumation of basement gneiss domes in the Quadril?tero Ferr?fero : two stage dome-and-keel evolution?

The presence of dome-and-keel provinces in Archean cratons has been connected with the initiation of plate tectonics on Earth as these features are most commonly observed in Archean rocks. The Quadril?tero Ferr?fero in Brazil has been identified as a Paleoproterozoic dome-and-keel province for more than three decades. The prevailing model suggests that it formed during the Rhyacian Transamazonian orogeny, making it unique among dome-and-keel provinces. However, a lack of appropriate lithologies, datable minerals and the metamorphic overprint of later orogenesis has resulted in a cryptic metamorphic record for the formation of this dome-and-keel province. A clinopyroxene-bearing migmatite from the core of the Ba??o dome has peak PeT conditions of 5e7 kbar and 700e750 C and a published age of ca. 2730 Ma based on UePb ages of zircon from leucosomes, suggesting that this age represents the migmatisation event. A fine-grained epidote-albite-titanite assemblage overprints the coarse-grained clinopyroxene and amphibole, giving PeT conditions of 8e9 kbar and 550 C with an associated titanite age of ca. 2050 Ma. A garnet-bearing amphibolite sample also from the core of the dome has peak PeT conditions of 7e8 kbar and 650e700 C, and texturally late titanite from this sample produces an age of ca. 2060 Ma. Three additional samples were collected from the edges of the dome. A garnet-gedrite bearing felsic schist produces peak PeT conditions of 8e9 kbar and 650e700 C on a clockwise PeT evolution. This sample has a UePb zircon age of ca. 2775 Ma, which could date metamorphism or be the age of its volcaniclastic protolith. Texturally unconstrained titanite from the sample gives an age of ca. 2040 Ma. A garnet-bearing amphibolite that occurs as a boudin within the felsic schist gives both zircon and titanite ages of ca. 2050 Ma and has peak PeT conditions of 5e6 kbar and 650e700 C on a near isobaric PeT path. An amphibolite dike, observed to cross-cut the felsic schist produces a zircon UePb age of ca. 2760 Ma. Altogether this data suggests that the samples were metamorphosed in the Archean (ca. 2775e2730 Ma) and again during the Transamazonian event. The most plausible explanation for this data is that dome-and-keel formation occurred in the Archean with migmatisation and high-temperature metamorphism occurring at this time. The Paleoproterozoic event is interpreted as a reactivation of the dome-and-keel formation structures, with Paleoproterozoic keels crosscutting Archean keels and producing metamorphic aureoles. The high radiogenic heat production and the presence of dense sedimentary successions in Archean terranes make dome-and-keel provinces a uniquely Archean feature, but they are susceptible to reworking, resulting in an enigmatic record of formation