Granites of the intracontinental termination of a magmatic arc : an example from the Ediacaran Ara?ua? orogen, southeastern Brazil.

The Ara?ua? orogen of southeastern Brazil together with the West Congo belt of central West Africa form the Ara?ua??West Congo orogen 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. The convergence of its margins led to the development of the Rio Doce magmatic arc between 630 Ma and 580 Ma. The Rio Doce magmatic arc terminates in the northern portion of the Ara?ua? orogen. Granitic plutons exposed in the northern extremity of the arc provide a rare opportunity to studymagmatismat arc terminations, and to understand the interplay between calc-alkalinemagma production and crustal recycling. The plutons forming the terminus of the arc consist of granodiorites, tonalites and monzogranites similar to a magnesian, slightly peraluminous, calcic- (68%) to calc-alkaline (24%), with minor alkali-calcic (8%) facies, medium- to high-K magmatic series. Although marked by negative Nb?Ta, Sr and Ti anomalies, typically associatedwith subduction-relatedmagmas, the combined Sr, Nd and Hf isotopic data characterize a crustal signature related to anatexis of metamorphosed igneous and sedimentary rocks, rather than fractional crystallization of mantle-derived magmas. 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 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 dehydrationmelting 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

Contrasting oxygen fugacity of I- and S-type granites from the Ara?ua? orogen, SE Brazil : an approach based on opaque mineral assemblages.

This study presents mineralogical characterization of opaque assemblages from I- and S-type granites from the Ara?ua? orogen, southeastern Brazil that belong respectively, to the pre- and syn-collisional stages of the orogeny. Although these granites are geochemically well-characterized, with a robust geochemical, isotopic and geochronological database, their opaque minerals have not yet been investigated, and they provide important information about the oxygen fugacity and temperature conditions of their magmas. I-type granites (G1 Supersuite) consist of biotite hornblende granites and their opaque assemblage is ilmenite + pyrite + pyrrhotite ? magnetite ? Fe-Ti oxides ? chalcopyrite. S-type rocks (G2 Supersuite) are biotite muscovite sillimanite granites with ilmenite + graphite + pyrrhotite + pyrite as opaques. Our results combined with literature data show that ranges for oxygen fugacity (fO2) are: I-type granitoids containing magnetite and free of pyrite and phyrrhotite likely crystallized under fO2 between 10?15 bars and 10?8.5 bars, whereas magnetite free rocks containing pyrite and pyrrhotite should have crystallized with fO2 higher than 10?18 bars and lower than 10?15 bars. Regarding S-type granites, they must have crystallized under fO2 lower than 10?18 bars

From the plutonic root to the volcanic roof of a continental magmatic arc : a review of the Neoproterozoic Ara?ua? orogen, southeastern Brazil.

The Ara?ua?-West Congo orogen (AWCO) is one of the various components of the Brasiliano/Pan- African orogenic network generated during the amalgamation of West Gondwana. In the reconstructions of Gondwana, the AWCO, encompassing the Ara?ua? orogen of South America and the West Congo belt of Southwestern Africa, appears as a tongue-shaped orogenic zone embraced by the S?o Francisco-Congo craton. Differing from the vast majority of the known orogens owing to its singular confined setting, the AWCO contains a large amount of orogenic igneous rocks emplaced in all stages of its tectonic evolution. We present new and revised information about the oldest Ediacaran granitic assemblage, the G1 Supersuite, which together with the Rio Doce Group defines the Rio Doce magmatic arc, and then we propose a new tectonic setting for the arc. Field relationships and mineralogical compositions of the G1 Supersuite allow us to characterize three lithofacies associations, Opx-bearing rocks, enclaverich Tonalite?Granodiorite and enclave-poor Granite? Tonalite, suggesting different crustal levels are exposed in the central part of the Ara?ua? orogen. The region is interpreted to represent 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, calcalkaline to alkali-calcic and medium- to high-K acidic rocks. The dacitic and rhyolitic rocks of the Rio Doce Group are mainly magnesian, peraluminous, calcic to calc-alkaline, and medium- to high-K acidic rocks. Zircon U?Pb data constrain the crystallization of the granitoids between ca. 625 and 574 Ma, while the age of the metamorphosed volcanic rocks is around ca. 585 Ma. Thus, within errors, these rock associations likely belong to the same magmatic event and might represent the subduction- related, pre-collisional, evolution of the Ara?ua? orogen. In addition, whole-rock Sm?Nd isotopic compositions show variable negative ?Nd(t) values between ?6.7 and ?13.8, and TDM model ages varying from 1.39 to 2.26 Ga, while ?Hf(t) vary between ?5.2 and ?11.7, with TDM ages from 1.5 to 2.0 Ga. Thus, predominantly constructed upon Paleoproterozoic (Rhyacian) basement, the Rio Doce arc shows crustal sources largely prevailing over mantle sources, providing a well-studied example to be compared with similar orogenic settings around the world

Geometria, petrografia e deforma??o de corpos ultram?ficos metamorfisados da regi?o de Santa Rita de Ouro Preto, MG, Brasil.

A regi?o de Santa Rita de Ouro Preto ? historicamente conhecida pelo artesanato em pedra-sab?o, designa??o popular do esteatito, cujo conte?do de talco ? superior a 75% e ? produto do metassomatismo de rochas ultram?ficas peridot?ticas. Assim, o estabelecimento da geometria externa dos corpos ultram?ficos metamorfisados, de onde se extrai o esteatito, est? diretamente relacionado ao melhor aproveitamento econ?mico dos mesmos. Um modelo inicialmente proposto de zoneamento litol?gico, de geometria externa circular, cuja diferencia??o petrogr?fica do n?cleo em dire??o ? borda, reflete em um n?cleo quimicamente menos modificado. No presente trabalho, tal zoneamento litol?gico em linhas gerais se mantem, com suaves modifica??es. Entretanto, quanto a sua morfologia, prop?e-se, uma geometria assim?trica para os corpos ultram?ficos metamorfisados encontrados na regi?o de Santa Rita de Ouro Preto e arredores, uma vez que a geometria arrendondada desconsidera a a??o de um campo de tens?es. Por meio de mapeamento geol?gico, em escala 1:10.000, caracterizaram-se corpos com geometria na forma de pods, cuja associa??o com diques de anfibolito reflete as dire??es aproximadas, NE-SW, de um campo de tens?es, uma vez que os pods apresentam alinhamento NW-SE. Esses corpos foram deformados e sua geometria, em mapa observada na forma de ?diques?, reflete, de fato, a dire??o principal de alongamento dos pods. Neste contexto, esse manuscrito visa fornecer subs?dios para o melhor entendimento dos processos deformacionais ocorridos, da geometria e do posicionamento tect?nico destas rochas

The Ediacaran Rio Doce magmatic arc revisited, Ara?ua?-Ribeira orogenic system, SE Brazil.

Described half a century ago, the Galil eia tonalite represents a milestone in the discovery of plate margin magmatic arcs in the Ara?ua?-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 Ara?ua? orogen. Meanwhile, the name Rio Doce magmatic arc was applied to calc-alkaline plutons found in the Ara?ua?-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, ?Nd(t): 5.7 to 7.8, Nd TDM ages: 1.28e1.68 Ga, 87Sr/86Sr(t): 0.7059e0.7118, and ?Hf(t): 5.2 to 11.7) form the northernmost segment of the Rio Doce arc, which dies out in the ensialic sector of the Ara?ua? orogen. At arc eastern and central zones, several batholiths (e.g., Alto Capim, Baixo Guandu, Galil eia, Muniz Freire, S~ao V?tor) record a long-lasting magmatic history (632e580 Ma; ?Nd(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, ?Nd(t): 4.8 to 6.8, Nd TDM ages: 1.48e1.68 Ga, 87Sr/86Sr(t): 0.7062e0.7068, and ?Hf(t): 4.3 to 9.7; and Brasil^andia pluton, UePb age: 581 ? 11 Ma, ?Nd(t): 8.2 to 10.2, Nd TDM ages: 1.63e1.68 Ga, 87Sr/86Sr(t): 0.7088e0.7112, ?Hf(t): 12.3 to 14.9),followed by late granodioritic intrusions (e.g., Guarataia pluton, UePb age: 576 ? 9 Ma, ?Nd(t): 12.52 to 13.11, Nd TDM age: 1.74e2.06 Ga, 87Sr/86Sr(t): 0.7104e0.7110, ?Hf(t): 12.9 to 21.6). The Muria e batholith (UePb age: 620e592 Ma, ?Nd(t): 8.2 to 13.6, Nd TDM age: 1.41e1.88 Ga) and the Concei?~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 Bol?via and Marceleza complexes, making the link between the Ara?ua? 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