Browsing by Author "Peres, Guilherme Gravina"

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    Kinematic evolution of the Araçuáı-West Congo orogen in Brazil and Africa : nutcracker tectonics during the Neoproterozoic assembly of Gondwana.
    (2006) Alkmim, Fernando Flecha de; Marshak, Stephen; Soares, Antônio Carlos Pedrosa; Peres, Guilherme Gravina; Cruz, Simone Cerqueira Pereira; Whittington, Alan
    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.
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    Metamorphic imprints on units of the southern Araçuaí belt, SE Brazil : the history of superimposed Transamazonian and Brasiliano orogenesis.
    (2018) Cutts, Kathryn Ann; Lana, Cristiano de Carvalho; Alkmim, Fernando Flecha de; Peres, Guilherme Gravina
    The extensive Neoproterozoic Brasiliano belts that surround the São Francisco craton in southern Brazil are a consequence of tectonic processes that resulted in the formation of the supercontinent Gondwana in the Ediacaran to Cambrian. One of these Brasiliano-age orogens, the Araçuaí belt, overprints an earlier orogen developed during the so-called Transamazonian event at the transition between the Rhyacian and Orosirian periods. Archean and Paleoproterozoic basement units in the southern Araçuaí belt have been shown to have experienced both the Transamazonian and Brasiliano orogenies and have the potential to have preserved the P-T conditions of both events. This study investigates a series of granulite- and amphibolite-facies samples from the southern Araçuaí belt. Granulite-facies samples from the Acaiaca Complex and Pedra Dourada Granulite give monazite U-Pb ages in the range: 2015–1960 Ma. Both the granulite samples preserve two metamorphic textures. Coarse-grained garnet + biotite + orthopyroxene + K-feldspar + plagioclase + quartz + ilmenite + inferred melt and later, fine-grained garnet + biotite + hornblende + quartz that grows at the expense of orthopyroxene. Peak conditions for the earlier event were c. 750–800 °C and 6 kbar, whereas those of the later event are poorly constrained. A garnet-bearing amphibolite-facies sample from the Mantiqueira Complex contains monazite included within garnet, which gives U-Pb ages of c. 1940 Ma, whereas matrix monazite grains give ages of c. 590–540 Ma. Garnet core compositional zonation indicates P-T conditions of 6 kbar and 700 °C for the c. 1940 Ma metamorphism, while the matrix assemblage records a P-T path culminating in peak conditions of 8–9 kbar at 650 °C. A second amphibolite-facies sample was obtained from the Dom Silvério Group, whose depositional age post-dates the Transamazonian event. This sample gives two monazite U-Pb age populations, 597 ± 3 Ma and 547 ± 5 Ma. Garnet core zonation indicates a prograde P-T evolution with peak conditions of c. 8–9 kbar and 650 °C, although it is uncertain at this time what age population this event corresponds to. The results of this study provide better constraints on the chronology and metamorphic regimes of both the Transamazonian and Brasiliano events in the southern Araçuaí belt.
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    Pb/Pb single-zircon dating of paleoproterozoic calc-alkaline/alkaline magmatism in the southeastern São Franciso Craton region, Brazil.
    (2000) Evangelista, Hanna Jordt; Peres, Guilherme Gravina; Macambira, Moacir Jose Buenano
    Two chemically and petrographically distinct intrusive bodies (a calc-alkaline trondhjemite and an alkaline quartz-syenite cut by granitic aplites) occur in the vicinity of Piranga (Minas Gerais State), near the southeastern border of the São Francisco craton, Brazil. They intrude the Rio das Velhas greenstone belt and the Mantiqueira complex, both of Archean age. Both intrusive bodies are variably deformed and mylonitized. Age determinations using the Pb/Pb single-zircon evaporation method yielded Paleoproterozoic ages for the emplacement/crystallization of the trondhjemite (2058 ± lOMa), quartz-syenite (2036 ± 4Ma), as well as the younger aplites (2012 ± 8Ma). These ages allow us to interpret the calc-alkaline as well as the alkaline magmatism as manifestations of the Transamazonian event in the southeastern São Francisco craton region. Furthermore, the calc-alkaline trondhjemite is interpreted to have intruded during compressional deformation, while the alkaline quartz-syenite and its aplitic differentiates are post-orogenic, possibly extension-related. In terms of the Rb vs. (Y +Nb) contents, the trondhjemite plots in the field of volcanic arc granites, whereas the syenite and the aplites plot in the field of post-collision granites. The mylonitic overprinting of the syenite and the trondhjemite is also Paleoproterozoic, as deduced from the 2012 ± 8Ma age of the undeformed younger aplitic dykes which cut the syenite. It is likely that this alkaline magmatism correlates with a Paleoproterozoic post-collisional magmatic event well documented in the Transamazonian mobile belts of the northern São Francisco craton.
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    The southern Araçuaí belt and the Dom Silvério Group : geologic architecture and tectonic significance.
    (2004) Peres, Guilherme Gravina; Alkmim, Fernando Flecha de; Evangelista, Hanna Jordt
    A Faixa Araçuaí corresponde à porção ocidental externa do orógeno neoproterozóico Araçuaí-Congo Ocidental. Margeia a borda leste do Cráton do São Francisco e é separada do núcleo cristalino do orógeno pela descontinuidade geofísica de Abre Campo. A porção meridional da Faixa Araçuaí envolve quatro unidades litológicas principais: os ortognaisses arqueanos e paleoproterozóicos do Complexo Mantiqueira, os charnoquitos Pedra Dourada, os granitóides paleoproterozóicos da Suíte Borrachudos e as rochas metavulcanossedimentares do Grupo Dom Silvério. O Grupo Dom Silvério ocorre em uma faixa NNE-SSW e engloba um pacote de rochas metapelíticas com intercalações de quartzitos, anfibolitos, metaultramáficas, formações ferríferas, gonditos e mármores. Todas as unidades da porção meridional da Faixa Araçuaí foram envolvidas em quatro fases deformacionais sinmetamórficas no curso do Evento Brasiliano. A primeira fase, sincrônica a um metamorfismo regional de fácies anfibolito, associou-se a um transporte tectônico geral para norte ao longo da zona de cisalhamento sinistral de Dom Silvério e no segmento de baixo ângulo a ela conectado. A segunda e terceira fases representam estágios progressivos de um encurtamento com movimentação geral para oeste, com desenvolvimento de empurrões localizados e intenso dobramento em todas as escalas. A quarta fase é extensional e reflete o colapso do orógeno.
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    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.