Contribui??o ? geoqu?mica, geocronologia, estrutura e evolu??o dos segmentos central e setentrional do arco magm?tico Rio Doce, Or?geno Ara?ua?, MG.

Programa de P?s-Gradua??o em Evolu??o Crustal e Recursos Naturais. Departamento de Geologia. Escola de Minas, Universidade Federal de Ouro Preto.O Or?geno Ara?ua?, localizado no sudeste do Brasil, juntamente com o cintur?o do Congo Ocidental, situado na parte central oeste da ?frica, formam o Or?geno Ara?ua?-Oeste Congo, o qual foi gerado durante o fechamento do segmento terminal do Oceano Neoproteroz?ico Adamastor. Confinado entre os cr?tons do S?o Francisco e Congo, esta por??o do Adamastor era somente parcialmente oceanizada. Embora este sistema orog?nico seja diferente da vasta maioria dos or?genos conhecidos, ele cont?m uma grande quantidade de rochas ?gneas orog?nicas formadas em todos os est?gios de sua evolu??o, desde o Ediacarano at? o Ordoviciano. No presente estudo ser?o fornecidas novas informa??es sobre os segmentos central e setentrional da assembl?ia gran?tica Ediacarana mais antiga, a Supersuite G1, a qual em conjunto com o Grupo Rio Doce materializa o Arco Magm?tico Rio Doce. No segmento central, situado entre as cidades de Governador Valadares e Ipanema, MG, rela??es de campo e composi??es mineral?gicas permitiram a individualiza??o de tr?s associa??es de litofacies, denominadas de zona com rochas portadoras de Opx, Tonalito-Granodiorito rico em enclaves e Granito a Tonalito pobre em enclaves, as quais foram interpretadas como exposi??es de diferentes n?veis crustais na parte central do Or?geno Ara?ua?. Portanto, essa regi?o parece representar uma se??o crustal inclinada, sendo que as ra?zes mais profundas do arco est?o agora expostas ao longo de sua borda oeste. Quimicamente, essas associa??es plut?nicas consistem majoritariamente de rochas magnesianas, metaluminosas a levemente peraluminosas (m?dia ASI = 1,02), com assinatura c?lcio-alcalina a alcali-c?lcica de m?dio a alto-K. Nessa mesma regi?o, rochas dac?ticas e riol?ticas do Grupo Rio Doce s?o essencialmente magnesianas, peraluminosas, c?lcicas a c?lcio-alcalinas e de m?dio a alto-K. Por outro lado, os plutons que formam a termina??o norte do arco, entre os munic?pios de Te?filo Otoni e Rio do Prado, MG, consistem de granodioritos, tonalitos e monzogranitos, os quais quimicamente s?o similares ? uma s?rie magnesiana, levemente peraluminosa (m?dia ASI = 1,07), c?lcica (68%) a c?lcio-alcalina (24%), e em menor propor??o alcali-c?lcica (8%), de m?dio a alto-K. As rochas gran?ticas G1 de ambas as ?reas s?o marcadas por anomalias negativas de Nb-Ta, Sr e Ti, que seriam t?picas de magmas relacionados ? subduc??o. Entretanto, a combina??o de dados isot?picos de Sr, Nd e Hf (?HF(t) = -5,2 e -11,7 para os granitos G1 do norte), caracterizam o predom?nio de uma assinatura crustal relacionada ? anatexia de rochas meta-?gneas e meta-sedimentares, sobre cristaliza??o fracionada de magmas de deriva??o mant?lica. Novos dados U-Pb obtidos em zirc?o do segmento central do arco restringem a cristaliza??o das rochas gran?ticas entre ca. 625 e 579 Ma, enquanto as idades das rochas meta-vulc?nicas variam entre ca. 585 e 578 Ma. Esse evento, o qual antecede, com alguma superposi??o, a coloca??o dos granitos do tipo-S sin-colisionais do or?geno, deve assim representar o evento pr?-colisional, relacionado ? subduc??o, que teve lugar na forma??o do Or?geno Ara?ua?. Adicionalmente, composi??es isot?picas Sm-Nd de rocha total mostram valores vari?veis de ?Nd(t) entre -13,02 e -6,52, e idades modelo TDM variando entre 1,51 e 1,87 Ga. Portanto, predominantemente constru?do sobre um arco magm?tico Riaciano entre 630 e 580 Ma, a por??o central do Arco Rio Doce mostra que fundidos crustais largamente prevalecem sobre magmas mant?licos, fornecendo dessa forma um bom estudo de caso, que em estudos futuros, poder? ser comparado a contextos orog?nicos similares em outras partes do mundo. Para o segmento norte, novas idades U-Pb em zirc?o individualizaram dois grupos de granit?ides. O mais antigo deles cristalizou entre 630 e 590 Ma e experimentou migmatiza??o em torno de 585 Ma, ao passo que o grupo mais jovem cristalizou entre 570 e 590 Ma, e n?o mostra qualquer evid?ncia de migmatiza??o. V?rias das amostras estudadas mostram boa correla??o, em termos qu?micos, com campos de composi??es experimentais de fus?o de anfibolitos, com algumas amostras plotando no campo de fus?o de grauvacas. As rochas G1 do segmento norte apresentram diferen?as se comparadas aos t?picos granitos do tipo-I ou granitos do tipo-S, sendo particularmente similares aos granitos transicionais I/S descritos no arco Ordoviciano Famatiniano (NW Argentina). Sugere-se, portanto, um modelo h?brido, o qual envolveria fus?o de rochas meta-?gneas (anfibolitos) e meta-sedimentares (meta-grauvacas) para a produ??o de magma na termina??o norte do Arco Rio Doce. Entretanto, a real contribui??o entre os membros finais nesse processo ? um trabalho desafiador, ainda a ser realizado.The Ara?ua? orogen of southeastern Brazil together with the West Congo belt of central West Africa form the Ara?ua?-West Congo orogen (AWCO) generated during closure of a terminal segment of the Neoproterozoic Adamastor Ocean. Corresponding to an embayment in the S?o Francisco-Congo craton, this portion of the Adamastor was only partially floored by oceanic crust. Although AWCO differs from the vast majority of the known orogens owing to its singular confined setting, it contains a large amount of orogenic igneous rocks emplaced in all stages of its tectonic evolution, from the Early Ediacaran to the Early Ordovician. In this study, we provide new information about the central and northernmost segments of the oldest Ediacaran granitic assemblage, the G1 Supersuite, which together with the Rio Doce Group materializes the Rio Doce magmatic arc. In the central segment, between the towns of Governador Valadares and Ipanema, MG, field relationships and mineralogical compositions of the G1 Supersuite allowed us to characterize three lithofacies associations, which are a zone with Opx-bearing rocks, enclave-rich Tonalite-Granodiorite and enclave-poor Granite to Tonalite, suggesting different crustal levels exposed in the central part of the Ara?ua? orogen. This region seems thus to be a tilted crustal section, with deep arc roots now exposed along its western border. Chemically, these plutonic associations consist mostly of magnesian, metaluminous to slightly peraluminous (average ASI = 1.02), calc-alkaline to alkali-calcic and medium- to high-K acidic rocks. In the same region, dacitic and rhyolitic rocks of the Rio Doce Group are mainly magnesian, peraluminous, calcic to calc-alkaline and medium- to high-K acidic rocks. On the other hand, the plutons forming the northern terminus of the arc, between the towns of Te?filo Otoni and Rio do Prado, MG, consist of granodiorites, tonalites and monzogranites similar to a magnesian, slightly peraluminous (average ASI = 1.07), calcic- (68%) to calc-alkaline (24%), with minor alkali-calcic (8%) facies, medium- to high-K magmatic series. The G1 granitic rocks from both areas are marked by negative Nb-Ta, Sr and Ti anomalies, typically associated with subduction-related magmas, however the combined Sr, Nd and Hf (?Hf(t) = -5.2 and -11.7 for the northern G1 granites) isotopic data characterize a crustal signature related to anatexis of metamorphosed igneous and sedimentary rocks, rather than fractional crystallization of mantle-derived magmas. New zircon U-Pb data from the central segment constrain the crystallization of the granitic plutonic rocks between ca. 625 and 579 Ma, while the ages for the metavolcanic rocks range between ca. 585 and 578 Ma. Thus, within errors, the studied plutonic and volcanic rocks might belong to the same magmatic event. This event, predating (with some overlap) the ages of emplacement of syn-collisional, S-type granites, might represent the subduction-related, pre-collisional, event that took place in the Ara?ua? orogen. In addition, whole-rock Sm-Nd isotopic compositions show variable negative ?Nd(t) values between -13.07 and -6.52, and TDM model ages varying from 1.51 Ga to 1.87 Ga. Thus, predominantly constructed upon a Rhyacian magmatic arc from 630 Ma to 580 Ma, the central Rio Doce arc shows crustal melts largely prevailing over mantle magmas, providing a well-studied example to be compared with similar orogenic settings around the world. For the northern portion, zircon U-Pb ages characterizes two groups of granitoids. The older group, crystallized between 630 and 590 Ma, experienced a migmatization event at ca. 585 Ma. The younger granitoids, emplaced between 570 and 590 Ma, do not show any evidence for migmatization. Most of the investigated samples show good correlation with the experimental compositional field of amphibolite dehydration-melting, with some samples plotting into the field of greywacke dehydration-melting. The studied northern G1 rocks are not typical I-type or S-type granites, being particularly similar to transitional I/S-type granitoids described in the Ordovician Famatinian arc (NW Argentina). We suggest a hybrid model involving dehydration-melting of meta-igneous (amphibolites) and metasedimentary (greywackes) rocks for magma production in the northern termination of the Rio Doce arc. The real contribution of each end-member is, however, a challenging work still to be done