DEGEO - Departamento de Geologia

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The role of tectonic inheritance in the development of a fold-thrust belt and superimposed rift : an example from the São Francisco basin, eastern Brazil.
(2021) Rodrigues, Raiza Toledo; Alkmim, Fernando Flecha de; Reis, Humberto Luis Siqueira; Piatti, Bruno Guimarães
The Sao ̃ Francisco intracratonic basin, eastern Brazil, hosts multiple sedimentary successions younger than 1.8Ga that witnessed major plate reorganizations since its generation. During the Brasiliano/Pan-African Gondwana amalgamation, in the Ediacaran-Cambrian boundary, the western half of the basin was involved in the Brasília foreland fold-thrust belt, which propagated eastwards deforming its Proterozoic-Paleozoic strata. Lately, as the South Atlantic rift evolved giving rise to the Brazilian continental margin, the basin was affected by extensional tectonism leading to the development of the Cretaceous Abaet ́e half-graben. During this episode, elements related to a prominent structure of the Brasília foreland fold-thrust belt, the Joao ̃ Pinheiro fault, were reactivated controlling the graben architecture. Aiming to unravel the evolutionary history of the fault zone since its nucleation until its Cretaceous extensional reactivation, we conducted a structural investigation based on field, well-tied 2D seismic data, and paleostress analysis. Our study revealed that the fault represents an emergent ramp of the basal detachment of the Brasilia foreland fold-thrust belt. Formed under EW-oriented shortening, the structure exhibits a gentle recces-salient geometry in map view, which reflects variations in the detachment depth influenced by pre-orogenic rift structures. Trains of NS-trending chevron folds are abundant and their high symmetry and dominance are expressions of the control exerted by mechanical stratigraphy and low-friction basal detachment. The superposed Abaet ́e half-graben developed under an extensional regime with a NE- trending maximum extension. Under this condition, a stratigraphic interface along the Joao ̃ Pinheiro fault zone was reactivated giving rise to the graben border fault. NS- to NW-oriented preexisting bedding planes were activated as small normal faults. Although newly-formed faults are present, reactivation is the most effective mechanism of extension accommodation. Therefore, this study provides insights on the influence of tectonic inheritance on the architecture, deformation pattern, strain/stress partitioning and evolution of both extensional and compressional intracontinental systems.
Grãos detríticos de zircão do grupo Itacolomi em sua área tipo, Quadrilátero Ferrífero, Minas Gerais : idades, proveniência e significado tectônico.
(2020) Duque, Tiago Rocha Faria; Alkmim, Fernando Flecha de; Lana, Cláudio Eduardo
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.
Geodynamic evolution of the Minas Basin, southern São Francisco craton (Brazil), during the early paleoproterozoic : climate or tectonic?
(2020) Rossignol, Camille Yann; Lana, Cristiano de Carvalho; Alkmim, Fernando Flecha de
Like many other cratons around the world, the São Francisco Craton is characterized by an early Paleoproterozoic (2.5–2.1 Ga) unconformity that is reported worldwide as the result of global glacial events leading to a major sea-level drop. To document the duration and the cause(s) of the stratigraphic hiatus in the São Francisco Craton, this study provides U–Pb dates obtained on detrital zircon grains from the Piracicaba Group, which directly overlies the unconformity. Our results show that this sedimentary unit is younger than 2333 ± 11 Ma and consequently deposited during the late Siderian or the early Rhaycian. This maximum depositional age demonstrates that the early Paleoproterozoic stratigraphic gap lasted for at least 60 Ma. Based on quantitative analyses of the age distribution of the Piracicaba Group, we evidence a major shift in sedimentary provenance between the sedimentary units located below and above the unconformity. This change is interpreted to be directly related to the development of a subduction system along the southern and eastern margins of the São Francisco proto-craton. Our results indicate that regional geodynamic events, and not only global glacial events and their consequences, must be accounted for in Earth system models for the early Paleoproterozoic.
Caracterização faciológica e evolução sedimentar da Formação Moeda (Supergrupo Minas) na porção noroeste do Quadrilátero Ferrífero, Minas Gerais.
(2019) Madeira, Mariana de Resende; Martins, Maximiliano de Souza; Martins, Gustavo Pereira; Alkmim, Fernando Flecha de
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.
An Early Tonian rifting event affecting the São Francisco-Congo paleocontinent recorded by the Lower Macaúbas Group, Araçuaí Orogen, SE Brazil.
(2019) Castro, Marco Paulo de; Queiroga, Gláucia Nascimento; Martins, Maximiliano de Souza; Alkmim, Fernando Flecha de; Soares, Antônio Carlos Pedrosa; Dussin, Ivo Antonio; Souza, Maria Eugênia Silva de
After it had been assembled in the very early Orosirian, the western São Francisco–Congo Paleocontinent experienced several rifting events since the Statherian (ca. 1750 Ma) to Cryogenian (ca. 700 Ma). Records of anorogenic magmatism and/or associated sedimentation from those events have been found in the Neoproterozoic Araçuaí–West Congo orogenic system (AWCO), located between the São Francisco (eastern Brazil) and Congo (central Africa) cratons. Based on detailed field studies and data from lithochemistry, zircon and titanite U-Pb dating, whole-rock Nd and Hf-in-zircon isotopic analyses, we characterize a previously poorly described Early Tonian, rift-related, volcano-sedimentary succession in order to decipher the evolution of the AWCO precursor basins. That volcano-sedimentary succession, found in the Capelinha Formation type-area, now assigned to the Lower Macaúbas Group, includes quartzites with lenses of ortho-amphibolite (metabasalt) covered by pelitic schists. Zircon grains from ortho-amphibolite samples yielded ages of 957 ± 14 Ma and 576 ± 13 Ma, constraining their magmatic crystallization and regional metamorphism, respectively. These mafic rock show ƐNd(t) from −3.64 to +0.21 and Nd TDM ages from ca. 1.4 to ca. 1.7 Ga. Positive covariation of FeOtot/MgO + FeOtot, TiO2, P2O5, V and Zr, enrichment in light rare earth elements, slightly positive Eu/Eu* anomaly and depletion of high field-strenght elements, suggest tholeiitic protolith related to a continental rifting setting. The metasedimentary rocks show broad spectra of detrital zircon ages from the Early Tonian (ca. 940 Ma) to Paleoarchean, with wide-ranging ƐHf values from predominantly negative (as low as −10.76) to positive (+9.94), evoking well-known sediment sources in the São Francisco–Congo Paleocontinent. The youngest age peak (949 ± 12 Ma) constrains a maximum sedimentation age coeval with the basaltic volcanism represented by the ortho-amphibolite. The Capelinha volcano-sedimentary succession nearly correlates in age and origin with other anorogenic units (e.g., Ilhéus and Pedro Lessa dike swarms, Salto da Divisa granitic suite, Gangila basaltic and Mayumbian felsic volcanisms) found in a large region covered by the AWCO and neighbouring cratonic region. However, the Capelinha magmatism seems to preceed by some 20–30 m.y. the main peak (930–900 Ma) of this Early Tonian anorogenic magmatism, suggesting a long-lived and complex rift system.
Detrital zircons from the Upper Três Marias Formation, São Francisco basin, SE Brazil : record of foreland deposition during the Cambrian?
(2020) Tavares, Túlio Delôgo; Martins, Maximiliano de Souza; Alkmim, Fernando Flecha de; Lana, Cristiano de Carvalho
The Bambuí Group, a succession of mixed carbonate and siliciclastic rocks, accumulated in the intracratonic and poly-historic São Francisco successor basin by the end of the Neoproterozoic. The São Francisco basin, which occupies a substantial portion of the eponymous craton in southeastern Brazil, was converted into a major foreland depocenter in response to orogenic loads generated along the craton margins during the assembly of West Gondwana in the Ediacaran Period. Various studies carried out in recent years have led to a better understanding of many aspects of the evolution of the São Francisco basin during the Neoproterozoic. However, the age of the Bambuí Group, critical for a better understanding of the basin history and establishment of regional and global correlations, remains poorly constrained and matter of a long-standing debate in the literature. In order to contribute to this debate, we performed U-Pb LA-ICPMS age determinations on detrital zircons extracted from the Três Marias Formation, the youngest unit of the group. Our study, based on a stratigraphic survey, was conducted in an occurrence of the formation located near to the eastern border of the basin, i.e., in a proximal position in relation to its potential source, the Brasiliano Araçuaí orogen. The detrital zircon age spectra (involving 432 concordant ages) of the lower alluvial and upper marine units of the Três Marias Formation are characterized by a unimodal distribution with c.83% of the grains dated between 735 and 522 Ma. These results indicated that lithostratigraphic assemblages of the Araçuaí orogen, namely the 630-580 Ma granitoids of Rio Doce magmatic arc and 585-540 Ma syn-collisional granites were the most important sources of the Três Marias sediments. Moreover, post-collisional granite suites younger than 530 Ma also contribute with sediments to the formation. Using four different metrics, we obtained 527 ± 4 Ma as the most accurate and reliable estimation for maximum depositional age of the Upper Três Marias Formation, an age that has major implications for the developmental history of both the São Francisco basin and Araçuaí orogen
Metamorphism and exhumation of basement gneiss domes in the Quadrilátero Ferrífero : two stage dome-and-keel evolution?
(2019) Cutts, Kathryn Ann; Lana, Cristiano de Carvalho; Alkmim, Fernando Flecha de; Farina, Federico; Moreira, Hugo Souza; Coelho, Viviane Viana
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 Quadrilátero Ferrífero 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 Bação 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.
Twenty million years of post-orogenic fluid production and hydrothermal mineralization across the external Araçuaí orogen and adjacent São Francisco craton, SE Brazil.
(2019) Gonçalves, Guilherme de Oliveira; Lana, Cristiano de Carvalho; Buick, Ian S.; Alkmim, Fernando Flecha de; Cipriano, Ricardo Augusto Scholz; Queiroga, Gláucia Nascimento
The Araçuaí orogen (AO) is a hot orogenic system that is known for its widespread and long-lived magmatism (630–480 Ma).Within the last c. 20 million years of its evolution, the AO saw the widespread development of hydrothermal systems in its lower-grade external fold-and-thrust belt and its Foreland Domain (eastern margin of the adjoining São Francisco craton). U\\Pb crystallization ages for hydrothermal monazite, rutile, and xenotime indicate that fluid circulation along the entire AO occurred between 515 to ca. 495 Ma, largely overlapping with U\\Pb ages of hydrothermal systems and mineralized zones in the adjacent São Francisco craton. Titanium-inquartz thermometry suggests that most hydrothermal systems closed at temperatures of ca. 360 °C. The Sm\\Nd isotope composition of the vein monazite samples ranges from εNd500Ma −16.8 to −17.8, and one less-evolved sample with εNd500Ma of −5.9. The Sm\\Nd isotope compositions overlap with the bulk rock Sm\\Nd data for the Espinhaço Supergroup, suggesting that the hydrothermal veins in this study are mainly related to metamorphic fluids derived from the prograde dehydration of low-grade metasedimentary rocks. The U\\Pb ages, Sm\\Nd isotope composition, and vein mineralogy are all consistent with a 20 My period of channelized fluid flow, characterized by an oxidizing, low pH, REE-, P-, and Ti-bearing aqueous metamorphic fluid. The expression of supercritical fluid flow in the core of the AO is given by the emplacement of large volumes of bare/mineralized pegmatite bodies and regional resetting of monazite ages in host rocks. On a regional context, this widespread fluid flow is associated with a high-T, low-P metamorphism that marks the beginning of the post-collisional decompression of the orogeny, which is associated with lithospheric mantle delamination. The reorganization of mass and heat, due to extensional collapse, resulted in multi-sourced fluid production and mineralization from deep to shallow crustal levels that affected an area of over 400.000 km2 along the eastern border of the São Francisco Craton.
Zircon U-Pb and Lu-Hf record from high-grade complexes within the Mantiqueira Complex : first evidence of juvenile crustal input at 2.4–2.2 Ga and implications for the Palaeoproterozoic evolution of the São Francisco Craton.
(2020) Cutts, Kathryn Ann; Lana, Cristiano de Carvalho; Moreira, Hugo Souza; Alkmim, Fernando Flecha de; Peres, Guilherme Gravina
The Mantiqueira Complex together with the Mineiro Belt and Juiz de Fora Complex, characterize a long-lived system of oceanic and continental magmatic arcs. The proposed accretion of these terranes in the Rhyacian resulted in the closure of the Minas Basin, a fundamental turning point for the geology of this area. Understanding this event is critical for determining the evolution of the South American continent. Despite its importance, there is a paucity of zircon U-Pb and Lu-Hf data available for the Mantiqueira Complex and no data from two complexes within the later. U-Pb zircon data were obtained from interpreted metasediments of those two complexes, the granulite facies Acaiaca Complex and Pedra Dourada Granulite. Four targeted samples yield cores with ages between 3.5 and 2.2 Ga and a large proportion of metamorphic grains and rims at ca. 2.1–2.0 Ga. The similarity between the detrital zircon signatures of the Acaiaca Complex and Pedra Dourada Granulite, and the Archaean inheritance of the Mantiqueira Complex, indicates that these three units are related. Potentially the Acaiaca Complex and Pedra Dourada Granulite either sourced the Mantiqueira Complex or the Mantiqueira intrusive suites reworked the protoliths of the Acaiaca Complex and Pedra Dourada Granulite. Hf-in-zircon data indicate two discrete sources for both the Acaiaca Complex and Pedra Dourada Granulite. An Archaean source (model ages of 3.2–2.6 Ga), which is consistent with the suggested origin of the Mantiqueira Complex as recycled Archaean crust sourced from the São Francisco Craton and reworked during the ca. 2.1 Ga Transamazonian orogeny. A second more juvenile source (model ages of 2.4–2.2 Ga) recorded exclusively within the more mafic samples is identical in age and Hf to that found in the Juiz de Fora Complex. This could indicate a close link between the Mantiqueira and Juiz de Fora Complexes in the period 2.2 to 2.0 Ga. Alternatively, these juvenile zircons may represent an as yet unidentified juvenile portion of the Mantiqueira arc. The results of this study are consistent with the prevailing theories for formation of the Mantiqueira Complex as a continental arc with cumulative proportion curves indicating an active-margin type depositional setting.
Metamorphic record of collision and collapse in the Ediacaran‐Cambrian Araçuaí orogen, SE‐Brazil : insights from P–T pseudosections and monazite dating.
(2017) Peixoto, Eliza Inez Nunes; Alkmim, Fernando Flecha de; Soares, Antônio Carlos Pedrosa; Lana, Cristiano de Carvalho; Chaves, Alexandre de Oliveira
he Araçuaí orogen is the Brazilian counterpart of the Araçuaí‐West Congo orogenic system (AWCO), a component of the Ediacaran‐Cambrian orogenic network formed during the amalgamation of West Gondwana. The northwestern portion of the Araçuaí orogen is dominated by a succession of metasedimentary rocks made up of Meso‐ to Neoproterozoic rift, passive margin and syn‐orogenic sequences, locally intruded by post‐collisional granites. These sequences are involved in three distinct tectonic units, which from west to east are: the southern Espinhaço fold‐thrust system (SE‐thrust system), the normal‐sense Chapada Acauã shear zone (CASZ) and the Salinas synclinorium. Three deformation phases were documented in the region. The first two phases (D1 and D2) are characterized by contractional structures and represent the collisional development stage of the orogen. The third phase (D3) is extensional and currently viewed as a manifestation of orogenic collapse of the system. The distribution of the metamorphic mineral assemblages in the region characterizes two metamorphic domains. The M‐Domain I on the west, encompassing the SE‐thrust system and the CASZ, is marked by a syn‐collisional (syn‐D1) Barrovian‐type metamorphism with P–T conditions increasing eastwards and reaching ~8.5 kbar at ~650°C between 575 and 565 Ma. The M‐Domain II comprises the Salinas synclinorium in the hangingwall of the CASZ, and besides the greenschist facies syn‐collisional metamorphism, records mainly a Buchan‐type metamorphic event, which took place under 3–5.5 kbar and up to 640°C at c. 530 Ma. The northwestern Araçuaí orogen exhibits, thus, a paired metamorphic pattern, in which the Barrovian and Buchan‐type metamorphic domains are juxtaposed by a normal‐sense shear zone. Lithospheric thinning during the extensional collapse of the orogen promoted ascent of the geotherms and melt generation. A large volume of granites was emplaced in the high grade and anatectic core of the orogen during this stage, and heat advected from these intrusions caused the development of Buchan facies series over a relatively large area. Renewed granite plutonism, hydrothermal activities followed by progressive cooling affected the system between 530 and 490 Ma.

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