EM - Escola de Minas
URI permanente desta comunidadehttp://www.hml.repositorio.ufop.br/handle/123456789/6
Notícias
A Escola de Minas de Ouro Preto foi fundada pelo cientista Claude Henri Gorceix e inaugurada em 12 de outubro de 1876.
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Item A interação tectônica embsamento/cobertura em aulacógenos invertidos : um exemplo da Chapada Diamantina Ocidental.(2007) Cruz, Simone Cerqueira Pereira; Dias, Vilson Marques; Alkmim, Fernando Flecha deUm cinturão de dobramentos e cavalgamentos com embasamento envolvido está exposto ao longo da borda ocidental da Chapada Diamantina, um platô recoberto por unidades proterozóicas no interior da porção setentrional do Cráton do São Francisco. Esse cinturão, com trend NNW e vergência ENE, envolve uma parte substancial da porção leste do Aulacógeno do Paramirim, que corresponde a um rifte intracontinental que experimentou uma longa história de desenvolvimento iniciada em 1,75 Ga com a sedimentação do Supergrupo Espinhaço. Após a deposição da segunda maior unidade de preenchimento, o Supergrupo São Francisco, de idade neoproterozóica (Tonian-Cryogeninan), o rifte do Paramirim experimentou uma vigorosa inversão positiva que gerou um sistema de falhas e dobras com trend NNW. Com o intuito de contribuir com o entendimento da tectônica de inversão envolvendo o embasamento, foi realizada a análise estrutural baseada em dados de campo ao longo da borda oeste da Chapada Diamantina. Os resultados indicam que o processo de inversão ocorreu em quatro fases coaxiais de deformação progressiva. Sob um campo de encurtamento geral orientado segundo WSW-ENE, a primeira fase de deformação (Dpdesc) nucleou um sistema em que a cobertura se descolada do embasamento nucleando estruturas vergentes para ESE, incluindo descolamentos intra-estratais, leques imbricados, duplexes confinados a estratos e uma variedade de estruturas de pequena escala. Reativando estruturas pré-existentes, a fase deformacional subseqüente (Dp1/Dp2) foi responsável pela geração de falhas reversas envolvendo o embasamento, zonas de cisalhamento reversas a oblíquas e grandes dobras com trend NNW que domina o cenário estrutural da borda oriental da Chapada Diamantina. Durante a fase Dp3 um trem de dobras orientadas segundo NNW, vergente para WSW e associada com uma proeminente clivagem de crenulação desenvolveu-se sobre as estruturas previamente deformadas em zonas de alta deformação da borda oriental da Chapada Diamantina.Item Structural style of basin inversion at mid-crustal levels : two transects in the internal zone of the Brasiliano Araçuaí belt, Minas Gerais, Brazil.(1996) Cunningham, Dickson; Marshak, Stephen; Alkmim, Fernando Flecha deThe Araçuai belt is the orogenic belt that directly borders the eastern margin of the São 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 Araçuai 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 São Francisco Craton. The major thrusts, which are all cratonvergent, are of Brasiliano/Pan-African age (650-450 Ma) becuase they cut the Neoproterozoic Galiléia 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.Item Tectonic implications of precambrian Sm-Nd dates from the southern São Francisco craton and adjcent Araçuaí and Ribeira belts, Brazil.(2000) Brueckner, Hannes K.; Cunningham, Dickson; Alkmim, Fernando Flecha de; Marshak, StephenThe Archean and Paleoproterozoic São Francisco craton of eastern Brazil is surrounded on all sides by Brasiliano (=Pan African) orogens. The N–NE trending orogen that separates the eastern edge of the southern São Francisco craton from the Atlantic coast can be divided into the largely greenschist and amphibolite facies Araçuaı́ belt on the west and the largely granulite facies Ribeira belt on the east. A pronounced linear gravity and magnetic anomaly, the Abre Campo discontinuity, defines the boundary between these two belts. We obtained Sm–Nd mineral ages and whole-rock Sm–Nd model ages for garnet-bearing metamorphic rocks along an E–W transect across the southern São Francisco craton, the southern Araçuaı́ belt, and the Ribeira belt at about latitude 20°S. A recrystallization age of 2.1 Ga from metasediments recrystallized during the development of the classic dome-and-keel province of the southern São Francisco craton (the ‘Quadrilátero Ferrı́fero’) indicates that dome emplacement occurred during the waning stages (extensional collapse) of the Transamazonian collisional orogeny. Seven mineral ages from the southern Araçuaı́ and Ribeira belts date the thermal peak of metamorphism at between 538 and 589 Ma, confirming that these belts were pervasively remobilized during the Brasiliano event. Samples from the Araçuaı́ belt yield either Archean (>2.6 Ga) or Transamazonian (2.1–2.3 Ga) TDM model ages, indicating that the protoliths were either fragments of the São Francisco craton crust or were sediments derived from that craton, which presumably had been stretched to form a thinned continental margin during Meso- and Neo-Proterozoic rifting events. Notably, a mixed meta-pelite and metabasite sequence in the southern Araçuaı́ belt, the Dom Silvério Group, has Transamazonian ancestry and thus may represent oceanic sediments deposited on or east of this stretched margin and then thrust back onto the continent to mark a collisional suture between the São Francisco block and an Archean (?) crustal sliver to the east. The rocks of the Ribeira belt (i.e. the region east of the Abre Campo discontinuity) have younger model ages (TDM=1.6–2.0 Ga), indicating that this belt was not originally part of the São Francisco craton — it may represent an accreted Transamazonian terrane. Therefore, the Abre Campo discontinuity marks an important crustal boundary, possibly a suture. Brasiliano shear zones appear to steepen and root into the Abre Campo discontinuity, suggesting that it formed in Brasiliano time.Item Proterozoic first-order sedimentary sequences of the São Francisco craton, eastern Brazil.(2012) Alkmim, Fernando Flecha de; Martins Neto, Marcelo AugustoThe São Francisco craton in eastern Brazil hosts sedimentary sequences deposited between the Paleoarchean (∼3300 Ma) and Late Neoproterozoic (∼580 Ma). Proterozoic successions occurring in this region comprise five 1st-order sedimentary sequences, which besides episodes of global significance record major basin-forming events. The ca. 8000 m-thick Minas-Itacolomi 1st-order sequence, exposed in the Brazilian mining district of the Quadrilátero Ferrífero and containing as marker bed the Lake Superior-type Cauê Banded Iron Formation, tracks the operation of a Wilson cycle in the Paleoproterozoic Era. The quartz-arenite dominated Espinhaço I and II sequences record at least two major rift-sag basin-forming events, which affected the host continent of the São Francisco craton at around 1.75 Ga and 1.57 Ga. The Macaúbas sequence and its correlatives in the extracratonic domains witness the individualization of a São Francisco-Congo plate in synchronicity with the break-up of Rodinia in the Cryogenian period. The São Francisco-Congo plate together with various fragments derived from Rodinia reassembled to form Gondwana in the Ediacaran period. In the course of the Gondwana amalgamation, orogenic belts developed along the margins of the craton; its interior, converted into foreland basins, received the shallow water carbonates and pelites of the Bambuí 1st-order sequence and its correlatives.Item Stabilization of the southern portion of the São Francisco craton, SE Brazil, through a long-lived period of potassic magmatism.(2012) Romano, Rafael Cotta; Lana, Cristiano de Carvalho; Alkmim, Fernando Flecha de; Stevens, Gary; Armstrong, RichardAlthough the southernportion of the SãoFranciscocraton in southeast Brazil encompasses one of the largest segments of Meso- to Neoarchaean crust in the South American plate, there is little understanding of the processes leading to cratonization of this region. Our combined field and U-Pb age data show that the crust experienced massive intrusions of potassic (K-rich) granitoids between 2750 and 2600 Ma, and that this magmatic event marked the beginning of a fundamental change in the tectonics of the terrain, with the ensuing billion years being dominated by the deposition of a series of thick clastic and chemical sedimentary successions. Thus, deep crustal partial melting and subsequent transport and intrusion of these granites in the upper crust is a likely mechanism by which this segment of the SãoFrancisco crust attained buoyancy to survive recycling. Regional mapping indicates that the potassic granitoids are distributed over an area of ∼25000 km2, and are related to one of the most prolific periods of potassicmagmatism in the craton. LA-ICP-MS and SHRIMP U-Pb data from 16 samples of potassic granitoids together with published TIMS ages suggest alonglivedperiod of magmatism from 2750 to 2700 Ma, and a small pulse of granite crystallisation at ca. 2612 Ma. The U-Pb SHRIMP and LA-ICP-MS data also indicate that the Archaean granitoids do not record evidence of U-Pb zircon resetting. Our interpretation is that partial melting and transport of granitoid melts (rich in heat producing elements) to the upper crust gave rise to a thermally stable lower crust and that this lower crustal segment (in the craton area) became sufficiently refractory and resistant to further partial melting.