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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Resultados da Pesquisa
Item The Ediacaran Rio Doce magmatic arc revisited, Araçuaí-Ribeira orogenic system, SE Brazil.(2016) Tedeschi, Mahyra; Novo, Tiago Amâncio; Soares, Antônio Pedrosa; Dussin, Ivo Antonio; Tassinari, Colombo; Silva, Luiz Carlos da; Gonçalves, Leonardo Eustáquio da Silva; Alkmim, Fernando Flecha de; Lana, Cristiano de Carvalho; Dantas, Elton Luiz; Medeiros, Silvia Regina de; Campos, Cristina Maria Pinheiro de; Corrales, Felipe; Heilbron, Monica da Costa Pereira LavalleDescribed half a century ago, the Galil eia tonalite represents a milestone in the discovery of plate margin magmatic arcs in the Araçuaí-Ribeira orogenic system (southeastern Brazil). In the 1990's, analytical studies on the Galil eia tonalite finally revealed the existence of a Late Neoproterozoic calc-alkaline magmatic arc in the Araçuaí orogen. Meanwhile, the name Rio Doce magmatic arc was applied to calc-alkaline plutons found in the Araçuaí-Ribeira boundary. After those pioneer studies, the calc-alkaline plutons showing a pre-collisional volcanic arc signature and age between 630 Ma and 585 Ma have been grouped in the G1 supersuite, corresponding to the Rio Doce arc infrastructure. Here, we revisit the Rio Doce arc with our solid field knowledge of the region and a robust analytical database (277 lithochemical analyses, and 47 UePb, 53 SmeNd, 25 87Sr/86Sr and 7 LueHf datasets). The G1 supersuite consists of regionally deformed, tonalitic to granodioritic batholiths and stocks, generally rich in melanocratic to mesocratic enclaves and minor gabbroic to dioritic plutons. Gabbroic to dioritic enclaves show evidence of magma mixing processes. The lithochemical and isotopic signatures clearly reveal a volcanic arc formed on a continental margin setting. Melts from a Rhyacian basement form the bulk of the magma produced, whilst gabbroic plutons and enclaves record involvement of mantle magmas in the arc development. Tonalitic stocks (UePb age: 618e575 Ma, εNd(t): 5.7 to 7.8, Nd TDM ages: 1.28e1.68 Ga, 87Sr/86Sr(t): 0.7059e0.7118, and εHf(t): 5.2 to 11.7) form the northernmost segment of the Rio Doce arc, which dies out in the ensialic sector of the Araçuaí orogen. At arc eastern and central zones, several batholiths (e.g., Alto Capim, Baixo Guandu, Galil eia, Muniz Freire, S~ao Vítor) record a long-lasting magmatic history (632e580 Ma; εNd(t): 5.6 to 13.3; Nd TDM age: 1.35e1.80 Ga; 87Sr/86Sr(t): 0.7091 e0.7123). At arc western border, the magmatic evolution started with gabbro-dioritic and tonalitic plutons (e.g., Chaves pluton, UePb age: 599 ± 15 Ma, εNd(t): 4.8 to 6.8, Nd TDM ages: 1.48e1.68 Ga, 87Sr/86Sr(t): 0.7062e0.7068, and εHf(t): 4.3 to 9.7; and Brasil^andia pluton, UePb age: 581 ± 11 Ma, εNd(t): 8.2 to 10.2, Nd TDM ages: 1.63e1.68 Ga, 87Sr/86Sr(t): 0.7088e0.7112, εHf(t): 12.3 to 14.9),followed by late granodioritic intrusions (e.g., Guarataia pluton, UePb age: 576 ± 9 Ma, εNd(t): 12.52 to 13.11, Nd TDM age: 1.74e2.06 Ga, 87Sr/86Sr(t): 0.7104e0.7110, εHf(t): 12.9 to 21.6). The Muria e batholith (UePb age: 620e592 Ma, εNd(t): 8.2 to 13.6, Nd TDM age: 1.41e1.88 Ga) and the Conceiç~ao da Boa Vista (586 ± 7 Ma) and Serra do Valentim (605 ± 8 Ma) stocks represent a segment of the Rio Doce arc correlated to the Serra da Bolívia and Marceleza complexes, making the link between the Araçuaí and Ribeira orogenic domains. We suggest three phases of arc development: i) eastward migration of arc front (632e605 Ma), ii) widespread magma production in the whole arc (605e585 Ma), and iii) late plutonism in the western arc region (585e575 Ma). Usual processes of volcanic arc development, like subduction of oceanic lithosphere under a continental margin, followed by asthenosphere ascent related to slab retreating and break-off may explain the Rio Doce arc evolution.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.