DEGEO - Departamento de Geologia
URI permanente desta comunidadehttp://www.hml.repositorio.ufop.br/handle/123456789/8
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Item Late Neoproterozoic–Cambrian granitic magmatism in the Araçuaí orogen (Brazil), the eastern brazilian pegmatite province and related mineral resources.(2011) Soares, Antônio Carlos Pedrosa; Campos, Cristina Maria Pinheiro de; Noce, Carlos Maurício; Silva, Luiz Carlos da; Novo, Tiago Amâncio; Roncato Júnior, Jorge Geraldo; Medeiros, Silvia Regina de; Castañeda, Cristiane; Queiroga, Gláucia Nascimento; Dantas, Elton Luiz; Dussin, Ivo Antonio; Alkmim, Fernando Flecha deThe Araçuí 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.Item Orógeno Araçuaí : síntese do conhecimento 30 anos após Almeida 1977.(2007) Soares, Antônio Carlos Pedrosa; Noce, Carlos Maurício; Alkmim, Fernando Flecha de; Silva, Luiz Carlos da; Babinski, Marly; Cordani, Umberto Giuseppe; Castañeda, CristianeThe Araçuaí Fold Belt was defined as the southeastern limit of the São Francisco Craton in the classical paper published by Fernando Flávio Marques de Almeida in 1977. This keystone of the Brazilian geologic literature catalyzed important discoveries, such as of Neoproterozoic ophiolites and a calc-alkaline magmatic arc, related to the Araçuaí Belt and paleotectonic correlations with its counterpart located in Africa (the West Congo Belt), that provided solid basis to define the Araçuaí-West-Congo Orogen by the end of the 1990th decade. After the opening of the Atlantic Ocean in Cretaceous times, two thirds of the Araçuaí-West-Congo Orogen remained in the Brazil side, including records of the continental rift and passive margin phases of the precursor basin, all ophiolite slivers and the whole orogenic magmatism formed from the pre-collisional to post-collisional stages. Thus, the name Araçuaí Orogen has been applied to the Neoproterozoic-Cambrian orogenic region that extends from the southeastern edge of the São Francisco Craton to the Atlantic coastline and is roughly limited between the 15º and 21º S parallels. After 30 years of systematic geological mapping together with geochemical and geochronological studies published by many authors, all evolutionary stages of the Araçuaí Orogen can be reasonably interpreted. Despite the regional metamorfism and deformation, the following descriptions generally refer to protoliths. All mentioned ages were obtained by U-Pb method on zircon. The Macaúbas Group records rift, passive margin and oceanic environments of the precursor basin of the Araçuaí Orogen. From the base to the top and from proximal to distal units, this group comprises the pre-glacial Duas Barras and Rio Peixe Bravo formations, and the glaciogenic Serra do Catuni, Nova Aurora and Lower Chapada Acauã formations, related to continental rift and transitional stages, and the diamictitefree Upper Chapada Acauã and Ribeirão da Folha formations, representing passive margin and oceanic environments. Dates of detrital zircon grains from Duas Barras sandstones and Serra do Catuni diamictites suggest a maximum sedimentation age around 900 Ma for the lower Macaúbas Group, in agreement with ages yielded by the Pedro Lessa mafic dikes (906 ± 2 Ma) and anorogenic granites of Salto da Divisa (875 ± 9 Ma). The thick diamictite-bearing marine successions with sand-rich turbidites, diamictitic iron formation, mafic volcanic rocks and pelites (Nova Aurora and Lower Chapada Acauã formations) were deposited from the rift to transitional stages. The Upper Chapada Acauã Formation consists of a sand-pelite shelf succession, deposited after ca. 864 Ma ago in the proximal passive margin. The Ribeirão da Folha Formation mainly consists of sand-pelite turbidites, pelagic pelites, sulfide-bearing cherts and banded iron formations, representing distal passive margin to oceanic sedimentation. Gabbro and dolerite with plagiogranite veins dated at ca. 660 Ma, and ultramafic rocks form tectonic slices of oceanic lithosphere thrust onto packages of the Ribeirão da Folha Formation. The pre-collisional, calc-alkaline, continental magmatic arc (G1 Suite, 630-585 Ma) consists of tonalites and granodiorites, with minor diorite and gabbro. A volcano-sedimentary succession of this magmatic arc includes pyroclastic and volcaniclastic rocks of dacitic composition dated at ca. 585 Ma, ascribed to the Palmital do Sul and Tumiritinga formations (Rio Doce Group), deposited from intra-arc to fore-arc settings. Detrital zircon geochronology suggests that the São Tomé wackes (Rio Doce Group) represent intra-arc to back-arc sedimentation after ca. 594 Ma ago. The Salinas Formation, a conglomerate-wacke-pelite association located to northwest of the magmatic arc, represents synorogenic sedimentation younger than ca. 588 Ma. A huge zone of syn-collisional S-type granites (G2 Suite, 582-560 Ma) occurs to the east and north of the pre-collisional magmatic arc, northward of latitude 20º S. Partial melting of G2 granites originated peraluminous leucogranites (G3 Suite) from the late- to post-collisional stages. A set of late structures, and the post-collisional intrusions of the S-type G4 Suite (535-500 Ma) and I-type G5 Suite (520-490 Ma) are related to the gravitational collapse of the orogen. The location of the magmatic arc, roughly parallel to the zone with ophiolite slivers, from the 17º30’ S latitude southwards suggests that oceanic crust only developed along the southern segment of the precursor basin of the Araçuaí- West-Congo Orogen. This basin was carved, like a large gulf partially floored by oceanic crust, into the São Francisco-Congo Paleocontinent, but paleogeographic reconstructions show that the Bahia-Gabon cratonic bridge (located to the north of the Araçuaí Orogen) subsisted since at least 1 Ga until the Atlantic opening. This uncommon geotectonic scenario inspired the concept of confined orogen, quoted as a new type of collisional orogen in the international literature, and the appealing nutcracker tectonic model to explain the Araçuaí-West-Congo Orogen evolution.Item Age of the Ribeirão da Folha ophiolite, Araçuaí orogen : the U-Pb zircon (LA-ICPMS) dating of a plagiogranite.(2007) Queiroga, Gláucia Nascimento; Soares, Antônio Carlos Pedrosa; Noce, Carlos Maurício; Alkmim, Fernando Flecha de; Pimentel, Márcio Martins; Dantas, Elton Luiz; Martins, Maximiliano de Souza; Castañeda, Cristiane; Suita, Marcos Tadeu de Freitas; Prichard, HazelO Orógeno Araçuaí, de idade neoproterozóica, se estende da margem sudeste do Cráton do São Francisco ao Oceano Atlântico, entre os paralelos 15° e 21° S. O estágio rifte da bacia precursora do Orógeno Araçuaí é balizado pela idade U-Pb SHRIMP de ca. 875 Ma dada por granitos anorogênicos. A evolução orogênica é subdividida nos estágios pré-colisional (ca. 630-585 Ma), sin-colisional (ca. 585-560 Ma), tardi-colisional (ca. 560-530 Ma) e pós-colisional (ca. 530-490 Ma). Remanescentes de rochas magmáticas de assoalho oceânico, localizados no setor central deste orógeno, têm sido descritos na literatura geológica desde 1990. O mais completo destes registros oceânicos é o ofiolito de Ribeirão da Folha, situado nos arredores da vila homônima, no município de Minas Novas, MG. O ofiolito de Ribeirão da Folha é uma associação litológica tectonicamente desmembrada, composta por fatias de rochas meta-ultramáficas e metamáficas que foram embutidas por empurrões em pacotes da Formação Ribeirão da Folha (unidade distal do Grupo Macaúbas). Esta formação, na área enfocada, consiste de micaxistos e cianita-grafita xistos (pelitos pelágicos), com intercalações de metacherts sulfetados, diopsiditos sulfetados, corpos de sulfetos maciços, formações ferríferas bandadas dos tipos óxido, sulfeto e silicato, e orto-anfibolitos finos (metabasaltos), metamorfisados nas zonas da cianita da fácies anfibolito médio. Dados geotermobarométricos dos micaxistos peraluminosos revelaram condições metamórficas em torno de 550º C a 5,5 kbar. As assinaturas litoquímicas das rochas metamáficas e meta-ultramáficas revelam afinidade ofiolítica e origem em assoalho oceânico. Os dados isotópicos Sm-Nd destas rochas mostram valores positivos de epsilon Nd (+3 a +7), e as idades modelo e isocrônica sugerem desenvolvimento de litosfera oceânica durante o Neoproterozóico. Todas as tentativas anteriores de recuperação de zircão a partir de volumosas amostras das rochas metamáficas foram infrutíferas. Contudo, corpos leucocráticos semelhantes a plagiogranito foram reconhecidos poucos anos atrás e se tornaram um dos principais alvos da tese de doutorado da primeira autora. Estes corpos ocorrem sob a forma de veios irregulares com dimensões milimétricas a centimétricas (até 50 cm), e são encaixados por orto-anfibolito bandado de granulação média a grossa (metadolerito a metagabro). Os corpos leucocráticos consistem de metaplagiogranito foliado, composto essencialmente por plagioclásio cálcico com bordas albíticas, quartzo, hornblenda e epidoto, com titanita, sulfeto, apatita e zircão como os principais minerais acessórios. Os cristais de zircão da amostra de plagiogranito são euédricos e muito límpidos, e mostram morfologia prismática elongada (3:1), sugerindo origem magmática. Análises U-Pb por LA-ICPMS (Laser Ablation Inductively Coupled Mass Spectrometry) foram realizadas em dezoito cristais de zircão e mostram resultados concordantes, indicando idade de cristalização magmática de 660 ± 29 Ma. Esta idade baliza a época de geração de crosta oceânica na bacia precursora do Orógeno Araçuaí. O espalhamento de algumas das análises ao longo da curva concórdia sugere perda de Pb devido ao metamorfismo de fácies anfibolito em ca. 580 Ma. A idade de ca. 660 Ma plagiogranito precede a maior idade U-Pb (ca. 630 Ma) de tonalitos deformados do arco magmático pré-colisional, bem como a idade U-Pb (ca. 582 Ma) dos granitos sincolisionais mais antigos.