Fernando Flecha de Alkmim

Assembling West Gondwana in the Neoproterozoic: Clues from the São Francisco craton region, Brazil

The Sao Francisco craton, which consists of Archean and Paleoproterozoic basement, now forms eastern Brazil; it once was at the center of West Gondwana. Distinct Neoproterozoic Brasiliano (Pan-African) orogenic belts border the cratons margins. Crosscutting relationships among these belts, along with stratigraphic features of the cover in the cratons interior, provide constraints on the sequence of collisions leading to the assembly of West Gondwana. The earliest collision involved the Sao Francisco–Congo and Rio de la Plata cratons and resulted in the formation of the Southern Brasilia belt. Next came the closure of the Brazilide ocean by collision between the Sao Francisco–Congo and Amazonia cratons. This event produced the Northern Brasilia belt, which now defines the northwest side of the Sao Francisco craton. An intracontinental rift, which merged southward with a narrow sea, once was between the Sao Francisco and Congo cratons; closure of this rift and sea led to the formation of the Aracuai and West Congo orogen, and to the southwestward extrusion of the Ribeira belt. Continued convergence of Amazonia against the Sao Francisco–Congo craton caused the eastward extrusion of the Borborema province. The final stage of West Gondwana assembly closed a basin between the Rio de la Plata and Amazonia cratons and created the Paraguai belt.

Late Neoproterozoic–Cambrian granitic magmatism in the Araçuaí orogen (Brazil), the eastern brazilian pegmatite province and related mineral resources.

Abstract The Araçuaí orogen extends from the eastern edge of the São Francisco craton to the Atlantic margin, in southeastern Brazil. Orogenic igneous rocks, formed from c. 630 to c. 480 Ma, cover one third of this huge area, building up the Eastern Brazilian Pegmatite Province and the most important dimension stone province of Brazil. G1 supersuite (630–585 Ma) mainly consists of tonalite to granodiorite, with mafic to dioritic facies and enclaves, representing a continental calc-alkaline magmatic arc. G2 supersuite mostly includes S-type granites formed during the syn-collisional stage (585–560 Ma), from relatively shallow two-mica granites and related gem-rich pegmatites to deep garnet-biotite granites that are the site of yellow dimension stone deposits. The typical G3 rocks (545–525 Ma) are non-foliated garnet-cordierite leucogranites, making up autochthonous patches and veins. At the post-collisional stage (530–480 Ma), G4 and G5 supersuites were generated. The S-type G4 supersuite mostly consists of garnet-bearing two-mica leucogranites that are the source of many pegmatites mined for tourmalines and many other gems, lithium (spodumene) ore and industrial feldspar. G5 supersuite, consisting of high-K–Fe calc-alkaline to alkaline granitic and/or charnockitic to dioritic/noritic intrusions, is the source of aquamarine-topaz-rich pegmatites but mainly of a large dimension stone production.

Similarities and differences between the Brazilian and African counterparts of the Neoproterozoic Araçuaí-West Congo orogen

Abstract The Araçuaí–West Congo orogen encompasses orogenic domains located to the SE of the São Francisco Craton in Brazil, and to the SW of the Congo Craton in Africa. From the opening of the precursor basin to the last orogenic processes, the evolution of the orogen lasted from the very beginning of the Neoproterozoic up to the Cambrian–Ordovician boundary. After the spreading of the South Atlantic Ocean in Cretaceous time, the Araçuaí–West Congo orogen was split into two quite different but complementary counterparts. The Brazilian side (Araçuaí orogen) inherited two thirds of the whole orogenic edifice, including all the Neoproterozoic ophiolite slivers, the entire magmatic arc and syn-collisional to post-collisional magmatism, and the suture zone. The African counterpart (West Congo Belt), a fold–thrust belt free of Neoproterozoic ophiolite and Pan-African orogenic magmatism, inherited the thick pile of bimodal volcanic rocks of the Early Tonian rift stage, implying that the precursor basin was an asymmetrical rift with the thermal–magmatic axis located in the West Congo Belt. Both counterparts of the Araçuaí–West Congo orogen include Neoproterozoic glaciogenic deposits, allowing tentative lithostratigraphic correlations, but identification of the ice ages remains uncertain because the lack of sufficient well-constrained geochronological data.

The hot back-arc zone of the Araçuai Orogen, Eastern Brazil: From sedimentation to granite generation

This article presents new lithochemical and geochronological data obtained from gneisses and granites occurring in the region located to the east of the Rio Doce calc-alkaline arc (630 - 580 Ma), which corresponds to the back-arc basin of the Aracuai orogen. The Nova Venecia Complex, represents the most fertile source of peraluminous granitic melts in the studied back-arc zone. It mostly consists of migmatitic Al-rich paragneisses, ranging from biotite-rich gneisses to biotite-free cordierite-rich granulites, whose main protoliths were graywacky sediments. An EW-oriented section across the northern back-arc region reveals a zone rich in cordierite granulites of the Nova Venecia Complex at the base, followed by migmatites that gradually pass to the Ataleia foliated granites rich in metasedimentary enclaves, which in turn lay beneath the Carlos Chagas batholith. To the south of the Carlos Chagas batholith, orthopyroxene-bearing rocks often occur in both the Nova Venecia Complex and the Ataleia Suite, suggesting a deeper crustal level. Our U-Pb data suggest that melting processes started on the Nova Venecia Complex during the late development of the Rio Doce arc, around 590 Ma, forming autochthonous peraluminous melts related to the Ataleia Suite. Progressive anatexis and melt accumulation attained the climax around 575 Ma, leading to the development of the syn-collisional Carlos Chagas batholith. Around 545 - 530 Ma, a late to post-collisional anatectic episode formed garnet-cordierite leucogranites, mostly from the re-melting of the Ataleia and Carlos Chagas granites. A remarkable post-collisional plutonism caused widesperead re-heating of the back-arc domain from ca. 520 Ma to 480 Ma. This long lasting history (ca. 110 Ma) of granite generation in the back-arc zone requires distinct heat sources, such as asthenosphere ascent under the back-arc region in the pre-collisional stage, thrust stacking of the hot arc onto the back-arc, radiogenic heat release from the collisional thickened crust and, finally, asthenosphere uprising during the gravitational collapse of the Aracuai orogen.

The southern Araçuaí belt and the Dom Silvério Group: geologic architecture and tectonic significance

A Faixa Aracuai corresponde a porcao ocidental externa do orogeno neoproterozoico Aracuai-Congo Ocidental. Margeia a borda leste do Craton do Sao Francisco e e separada do nucleo cristalino do orogeno pela descontinuidade geofisica de Abre Campo. A porcao meridional da Faixa Aracuai envolve quatro unidades litologicas principais: os ortognaisses arqueanos e paleoproterozoicos do Complexo Mantiqueira, os charnoquitos Pedra Dourada, os granitoides paleoproterozoicos da Suite Borrachudos e as rochas metavulcanossedimentares do Grupo Dom Silverio. O Grupo Dom Silverio ocorre em uma faixa NNE-SSW e engloba um pacote de rochas metapeliticas com intercalacoes de quartzitos, anfibolitos, meta-ultramaficas, formacoes ferriferas, gonditos e marmores. Todas as unidades da porcao meridional da Faixa Aracuai foram envolvidas em quatro fases deformacionais sin-metamorficas no curso do Evento Brasiliano. A primeira fase, sincronica a um metamorfismo regional de facies anfibolito, associou-se a um transporte tectonico geral para norte ao longo da zona de cisalhamento sinistral de Dom Silverio e no segmento de baixo ângulo a ela conectado. A segunda e terceira fases representam estagios progressivos de um encurtamento com movimentacao geral para oeste, com desenvolvimento de empurroes localizados e intenso dobramento em todas as escalas. A quarta fase e extensional e reflete o colapso do orogeno.

The Tectonic interaction between the Paramirim Aulacogen and the Araçuaí Belt, São Francisco craton region, Eastern Brazil

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 youngestNeoproterozoic unit of the area, clearly indicating a Late Neoproterozoic maximum age for all the inversion stages of the Paramirim aulacogen.

Structural style of basin inversion at mid-crustal levels: two transects in the internal zone of the Brasilizno Araçuai Belt, Minas Gerais, Brazil

The Araguai belt is the orogenic belt that directly borders the eastern margin of the Silo Francisco Craton m eastern Brazil. Detailed structural investigations in the Governador Valadares region of Minas Gerais indicate that the amphibolite-to granulitegrade internal zones of the Ara9uai 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 Sao Francisco Craton. The major thrusts, which are all cratonvergent, are of Brasiliano/Pan-African age (650--450 Ma) because they cut the Neoproterozoic Galil6ia 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.

Release faults, associated structures, and their control on petroleum trends in the Recôncavo rift, northeast Brazil

Release faults are rift cross faults, which develop to accommodate the variable displacements of the hanging-wall block along the strike of normal faults. Release faults are nearly perpendicular or obliquely oriented to the strike of the normal fault they are related to. They have maximum throws adjacent to the parent normal fault and die out in the hanging wall away from it. They form to release the bending stresses in the hanging wall and do not reflect the orientation of the regional stress field in a basin. Commonly, they show normal-oblique displacements and are preferentially located along the strike ramps. Release faults may also act at the scale of an entire basin, reaching displacements of thousands of meters. Joints, shale, and salt diapirs may develop in association with release faults. Because all these structures represent domains of stress release, they may work as conduits for oil migration and oil traps in extensional basins. This is the case of the Reconcavo basin in northeastern Brazil, a Cretaceous failed rift, connected to the eastern Brazilian continental margin basins. In the Reconcavo basin, two large-scale release faults, with displacements in the order of 3 km, developed in the hanging wall of the rift border faults and control the location of the main oil fields.

The Araçuaí Belt

The Aracuai belts extends along the curved southeastern margin of the Sao Francisco craton between the Brazilian coast and Lat 21°S, where it merges with the Ribeira belt. It represents the external, basement-involved fold-thrust belt of the Aracuai-West Congo confined orogen (AWCO), which formed due to the closure of the terminal branch of the Adamastor ocean during the amalgamation of West Gondwana in the Ediacaran and beginning of the Cambrian. Bounded to the east and southeast by the high grade and granitic core of the AWCO, the Aracuai belt involves a basement assemblage older than 1.8 Ga, the 1.7–0.9 Ga rift to rift-sag successions of the Espinhaco Supergroup, the Tonian-Edicaran rift-passive margin Macaubas Group, as well as the syn-orogenic Salinas Formation and crustal derived granitic intrusions. The Macaubas Group, the type unit of the belt, contains a glaciomarine sequence made up of thick diamictites, sandstones and Rapitan-type banded iron formations. The units exposed along the belt were metamorphosed under greenschist to amphibolite facies conditions and affected by thrusts, reverse faults and cratonward verging folds, developed between 575 and 530 Ma. The Aracuai orogenic front propagates into the craton interior and interacts with preexistent rift structures. This chapter describes the stratigraphic framework and overall structure of the Aracuai belt, emphasizing the paleogeographic and tectonic significance of its sedimentary and volcanic assemblages.

Idade, proveniência e ambiente tectônico do Complexo Jequitinhonha de alto grau, Orógeno Araçuaí

The Jequitinhonha Complex of the northeastern Aracuai orogen is an extensive sedimentary unit metamorphosed in the amphibolite-granulite facies transition around 580-545 Ma. The unit consists of Al-rich (kinzigitic) paragneisses with decametric intercalations of graphite gneisses and quartzites, and centimetric to metric lenses of calcsilicate rocks. A new detrital zircon U-Pb age spectrum is reported for a sample of quartzite, and whole-rock geochemical (major and trace elements, 9 samples) and Sm-Nd isotope data (10 samples) for Jequitinhonha Complex paragneiss. Together with published data these show that: (1) the geochemistry of paragneiss samples of the Jequitinhonha Complex are similar to those of passive margin sedimentary protoliths; (2) detrital zircon data yield U-Pb age populations between ca. 0.9 and 2.5 Ga; and (3) Sm-Nd TDM model ages range from 1.6 to 1.8 Ga and eNd(575 Ma) around -7.5. The data reveal a mixture of Cryogenian to Mesoproterozoic rift-related igneous rocks with the Palaeoproterozoic-Archaean basement rocks of the Sao Francisco-Congo palaeocontinent as the main source areas, and also support the correlation between the Jequitinhonha Complex and the passive margin units of the upper Macaubas Group, constituting the precursor basin of the orogen. Our results, with the absence of ophiolites in the Jequitinhonha Complex, reinforce the interpretation that the Sao Francisco-Congo palaeocontinent was not divided to the north of the focused region, suggesting an ensialic termination of a gulf during the Neoproterozoic.2Département des Sciences de la Terre et de l’Atmosphère, GEOTOP, Université du Québec à Montréal, Montréal, Québec, Canada. E-mail: stevenson.ross@uqam.ca 3Geological Survey of Brazil, CPRM-SUREG-BH, Belo Horizonte (MG), Brazil. E-mail: luiz.silva@cprm.gov.br 4Universidade Federal de Ouro Preto, Departamento de Geologia, Morro do Cruzeiro, Ouro Preto (MG), Brazil. E-mail: alkmim@degeo.ufop.br 5IG-Laboratório de Geocronologia, Universidade de Brasília, Asa Norte, Brasília (DF), Brazil. E-mail: marcio@unb.br, marcio.pimentel@ufrgs.br