How many rifting events preceded the development of the Ara?ua?-West Congo orogen?

A edifica??o do Or?geno Ara?ua?-Congo Ocidental teve in?cio por volta de 630 Ma, como indicam as idades U-Pb mais antigas at? agora obtidas de rochas do seu arco magm?tico. Esta orogenia foi precedida de, pelo menos, seis eventos de rifteamento e/ou magmatismo anorog?nico que afetaram a regi?o ocupada pelo Or?geno Ara?ua?-Congo Ocidental, bem como o dom?nio crat?nico S?o Francisco-Congo a ele adjacente. S?o eles os eventos E1 (Estateriano, 1,77-1,7 Ga), E2 (Calimiano, 1,57-1,5 Ga), E3 (Esteniano, 1,18 - ? Ga), E4 (no limite Esteaniano-Toniano, ca. 1 Ga), E5 (Toniano, 930-850 Ma) e E6 (Criogeniano, 750-670 Ma). O evento E1, chamado de Tafrog?nese Estateriana, teve lugar entre 1,77 e 1,70 Ga e tem como registros mais importantes a deposi??o rifte das unidades sedimentares e vulc?nicas da base do Supergrupo Espinha?o, bem como o alojamento dos plutons anorog?nicos das su?tes Borrachudos e Lagoa Real. O evento E1 ? sucedido por uma nova fase de distens?o durante o per?odo Calimiano, em torno de 1,57 Ga, que ? representada no Espinha?o Setentrional e Chapada Diamantina por rochas sedimentares e vulc?nicas da por??o m?dia do Supergrupo Espinha?o. Manifesta??es do evento E3 (Esteniano) tiveram in?cio em torno de 1,18 Ga com a deposi??o da Forma??o Sopa-Brumadinho (Supergrupo Espinha?o), exposta na Faixa Ara?ua?. Os tr?s primeiros eventos (E1, E2 e E3) parecem n?o ter ocorrido na Faixa Congo Ocidental. Nesta faixa, os granitos Noqui (999 ? 7 Ma) e rochas vulc?nicas associadas representam um magmatismo anorog?nico do limite Esteniano-Toniano, registrando um evento provavelmente local (E4), mas que pode ser correlacion?vel aos diques m?ficos de idade similar que ocorrem no sul da Bahia, no extremo oriental do Cr?ton S?o Francisco. As espessas sucess?es de rochas vulc?nicas bimodais dos grupos Zadiniano e Mayumbiano (930-910 Ma) constituem os principais registros do evento E5 na Faixa Congo-Ocidental. O magmatismo anorog?nico representado pela Su?te Salto da Divisa e enxames de diques m?ficos tonianos (e.g., Pedro Lessa e Espinha?o Setentrional), assim como, muito provavelmente, os dep?sitos pr?-glaciais da bacia Maca?bas s?o manifesta??es de E5, em terrenos do Or?geno Ara?ua? e Cr?ton do S?o Francisco. O evento criogeniano, E6, ? evidenciado pela Prov?ncia Alcalina do Sul da Bahia entre 735 e 675 Ma, e, muito provavelmente, pelas forma??es diamict?ticas do Grupo Maca?bas e unidades correlativas na Faixa Congo Ocidental, e pelo vulcanismo f?lsico La Louila (? 713 Ma) do sudoeste do Gab?o. O evento E6 evoluiu para espalhamento oce?nico no setor centro-sul da Bacia Maca?bas, mas este processo n?o afetou a parte norte desta bacia e, por isto, deixou ?ntegra a liga??o continental (a ponte Bahia-Gab?o) entre a Pen?nsula S?o Francisco e o Continente Congo. De fato, nenhum dos eventos distensionais mencionados logrou levar ? ruptura a placa S?o Francisco-Congo que, aglutinada na transi??o Riaciano-Orosiriano (ca. 2,05 Ga), permaneceu ?ntegra at? o Cret?ceo Inferior quando, ent?o, o rifte Atl?ntico finalmente conseguiu desmembr?-la.The development of the Ara?ua?-West Congo orogen (AWCO) started around 630 Ma, as suggested by U-Pb ages from the oldest rocks yet found in its pre-collisional magmatic arc. This orogeny was preceded by at least six events of rifting and/or anorogenic magmatism, which affected the area occupied by the AWCO and the adjacent S?o Francisco-Congo craton, namely: the Statherian E1 (1.77-1.7 Ga), Calymmian E2 (1.57-1.5 Ga), Early Stenian E3 (1.18 - ? Ga), Stenian-Tonian E4 (ca. 1 Ga), Tonian E5 (930-850 Ma) and Cryogenian E6 (750-670 Ga) events. The E1 event, currently referred to as the Statherian taphrogenesis, took place between ca. 1.77 Ga and 1.7 Ga. It is recorded by the rift-related sedimentary and volcanic rocks of the basal section of the Espinha?o Supergroup, as well as the anorogenic plutons of the Borrachudos and Lagoa Real suites. The E1 event was followed by a Calymmian episode, the E2 (ca. 1.57 Ga), recorded in the northern Espinha?o range and Chapada Diamantina by sediments and volcanics of the middle portion of the Espinha?o Supergroup. The manifestations of the Stenian E3 event started around 1.18 Ga with the syn-rift deposition of the Sopa-Brumadinho Formation (Espinha?o Supergroup), exposed in the Ara?ua? belt. The first three events (E1, E2 and E3) are so far not found in the West Congo belt. In this belt, a magmatism took place at the Stenian-Tonian time boundary, the anorogenic Noqui granites (ca. 999 ? 7 Ma) and related volcanic rocks, recording a probably local extensional event, E4, which can be correlated to mafic dykes of similar age found in southern Bahia (eastern tip of the S?o Francisco craton). The thick bimodal volcanic pile of the Zadinian and Mayumbian groups and related intrusions are the main records of the Tonian E5 event in the West Congo belt. The A-type Salto da Divisa Suite and Tonian mafic dykes (e.g., Pedro Lessa and Northern Espinha?o range), as well as the deposition of the pre-glacial formations of the Maca?bas basin are manifestations of the E5 event in the Ara?ua? orogen and S?o Francisco craton. Records of the Cryogenian E6 event include the Southern Bahia alkaline province (ca. 735-675 Ma) and, probably, the La Louila felsic volcanism in SW Gabon (? 713 Ma), as well as the deposition of the diamictitic formations of the Maca?bas Group and correlatives. The E6 event evolved to oceanic spreading in the central-southern Maca?bas basin, but this process died out toward north keeping unbroken the continental link between the S?o Francisco peninsula and Congo continent (the Bahia-Gabon bridge). In fact, all those extensional events were unsuccessful in splitting the S?o Francisco-Congo plate, which amalgamated in the Rhyacian-Orosirian time boundary (ca. 2.05 Ma) and remained as such until the Lower Cretaceous, as the Atlantic rifting then successfully broke through the old continental mass

Contrasting provenance and timing of metamorphism fromparagneisses of the Ara?ua?-Ribeira orogenic system, Brazil : hints for Western Gondwana assembly.

The Ara?ua? orogen and the Ribeira belt make up a complex Neoproterozoic-Cambrian orogenic system, the Ara?ua?-Ribeira orogenic system(AROS) located fromthe eastern to southeastern Brazil. Along the AROS, the Ediacaran Rio Doce magmatic arc represents a geotectonic connection between the Ara?ua? and the Ribeira orogenic domains. Although the nature and evolution of the Rio Doce plutonic rocks is regionally well established, it lacks detailed studies on the paragneisses found along the western and central regions of this magmatic arc. Besides information on the nature and provenance of their sedimentary protoliths, the paragneisses provide data to unravel the palaeogeographic scenario fromthe precursor to arc-related basins. Six samples of Al-rich gneisses covering a large AROS region were selected for electron microprobe (EMP) mineral analyses in order to obtain geothermobarometric data and monazite ages, as well as for Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS) isotopic analyses on zircon (U-Pb, Lu-Hf) and monazite (U-Pb). The different age spectra fromdetrital zircon grains and contrasting Hf isotopic signatures suggest a complex sedimentary history. Located in thewestern sector of the study region, the samples RC-02 and RC-34, with an 80% age peak of detrital zircon grains from 2158 Ma to 1830 Ma, ?Hf(t) from ?2.2 to ?22.7, and Hf TDM model ages from 3530 Ma to 2440 Ma, suggest sediment sources located in the S?o Francisco craton basement. The samples RC-03, also fromthewestern sector, and RC-46 fromthe southern sector, have a more complex assemblage of detrital zircon grains with an 87% age peak from 987 Ma to 592 Ma, ?Hf(t) from +14.9 to ?2.9, and Hf TDM model ages from 2220Ma to 720 Ma, indicating provenance from mainly juvenile sources of distinct ages. Candidates to be juvenile sources for RC-03 and RC-46 sedimentary protoliths are the Rhyacian Juiz de Fora and Pocrane complexes in the basement of the Rio Doce arc, the Neoproterozoic Rio Negro arc systemof the Ribeira belt, and AROS ophiolite complexes. Samples RC-30 and RC-38 from the eastern sector of the study region, with most detrital zircon ages between 650 Ma and 552 Ma and very negative ?Hf(t) (?25.3 to ?16.5), suggest main sediment sources in the Rio Doce arc. By extending U-Pb analyses on metamorphic zircon and monazite, we have identified a complex timing of metamorphism, represented by metamorphic ages ranging from 621 Ma to 480 Ma, with the main collisional activity between 580Ma and 540 Ma. Geothermobarometric studies on garnet porphyroblasts, syn-kinematic to the D2 regional foliation, show a retrograde metamorphic path typical of continental collision belts, starting with P-T conditions of Tmax = 733 ?C and Pmax = 6.43 kbar. Our data also suggest: i) the studied paragneisses represent distinct Neoproterozoic basin stages, shifting from passive to active margin settings; ii) if the Rio Negro arc system really provided sediments for the basin stage represented by the RC-03 and RC-46 paragneisses, it would have amalgamated with the AROS before 614 Ma; iii) the final amalgamation of Western Gondwana took place around 540 Ma in the focused region; iv) an important re-heating period (520?480 Ma) can be related to the AROS gravitational collapse, afterWestern Gondwana assembly

Magnetometric and gamma spectrometric expression of southwestern S?o Francisco Basin, Serra Selada quadrangle (1:100.000), Minas Gerais state.

Localizada na por??o sudoeste da Bacia do S?o Francisco, a ?rea abrangida pela Folha Serra Selada (1:100.000) cont?m rochas sedimentares deformadas do Grupo Bambu? (Neoproterozoico), localmente recobertas, em pronunciada discord?ncia, pelos sedimentos e dep?sitos vulcanocl?sticos/epicl?sticos cret?ceos dos grupos Areado e Mata da Corda, respectivamente. Os dados aerogeof?sicos mostram dois dom?nios magnetom?tricos principais. Um deles ? caracterizado por anomalias de baixa frequ?ncia (ABF), aparentemente relacionadas a estruturas do embasamento pr?-cambriano local. O outro ? marcado por anomalias de alta frequ?ncia (AAF), representando os dep?sitos vulconocl?sticos/epicl?sticos do Grupo Mata da Corda. O Grupo Bambu?? exibe teores de K e Th intermedi?rios a altos (1 a 3% e 10 a 16 ppm, respectivamente), enquanto os teores de U giram em torno de 2,5 ppm. Esse dados mostram algumas varia??es, geralmente influenciadas pela distribui??o dos diferentes litotipos, fei??es estruturais e ocorr?ncia de emana??es de hidrocarbonetos. Os sedimentos do Grupo Areado, por sua vez, mostram baixos conte?dos de K, Th e U (20 ppm e >3 ppm, respectivamente). Estes valores parecem sofrer grande influ?ncia dos processos de intemperismo. As an?lises realizadas confirmam a grande aplicabilidade dos levantamentos aerogeof?sicos no mapeamento geol?gico e constituem excelentes ferramentas no entendimento do cen?rio tect?nico e dos dep?sitos de hidrocarbonetos da regi?o sudoeste da Bacia do S?o Francisco.Located in the southwestern portion of S?o Francisco Basin, the region covered by the Serra Selada quadrangle (1:100.000) contains deformed rocks of the Neoproterozoic Bambu? Group, unconformably overlain by Cretaceous sedimentary and volcaniclastic rocks of the Areado and Mata da Corda groups, respectively. The aerogeophysical data shows two main magnetometric domains. A low frequency anomalies domain is apparently related to deep structures of Precambrian basement. High frequency anomalies represent the volcaniclastic/epiclastic deposits of Mata da Corda Group. The Bambu? Group exhibits intermediate-to-high K and Th contentes (1 to 3% and 10 to 16 ppm, respectively), while U-levels are around 2.5 ppm. Significant changes in these values are caused by the distribution of rock types, tectonic features and hydrocarbon exudations. Areado Group sediments show low K (20 ppm and >3 ppm, respectively). These values seem to be strikingly influenced by weathering processes. The performed analyses confirm the applicability of aerogeophysical data in geological mapping, and represents an importante tool for the study of both the tectonic scenario and hydrocarbon accumulations in southwestern S?o Francisco Basin

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

Age of the Ribeir?o da Folha ophiolite, Ara?ua? orogen : the U-Pb zircon (LA-ICPMS) dating of a plagiogranite.

O Or?geno Ara?ua?, de idade neoproteroz?ica, se estende da margem sudeste do Cr?ton do S?o Francisco ao Oceano Atl?ntico, entre os paralelos 15? e 21? S. O est?gio rifte da bacia precursora do Or?geno Ara?ua? ? balizado pela idade U-Pb SHRIMP de ca. 875 Ma dada por granitos anorog?nicos. A evolu??o orog?nica ? subdividida nos est?gios pr?-colisional (ca. 630-585 Ma), sin-colisional (ca. 585-560 Ma), tardi-colisional (ca. 560-530 Ma) e p?s-colisional (ca. 530-490 Ma). Remanescentes de rochas magm?ticas de assoalho oce?nico, localizados no setor central deste or?geno, t?m sido descritos na literatura geol?gica desde 1990. O mais completo destes registros oce?nicos ? o ofiolito de Ribeir?o da Folha, situado nos arredores da vila hom?nima, no munic?pio de Minas Novas, MG. O ofiolito de Ribeir?o da Folha ? uma associa??o litol?gica tectonicamente desmembrada, composta por fatias de rochas meta-ultram?ficas e metam?ficas que foram embutidas por empurr?es em pacotes da Forma??o Ribeir?o da Folha (unidade distal do Grupo Maca?bas). Esta forma??o, na ?rea enfocada, consiste de micaxistos e cianita-grafita xistos (pelitos pel?gicos), com intercala??es de metacherts sulfetados, diopsiditos sulfetados, corpos de sulfetos maci?os, forma??es ferr?feras bandadas dos tipos ?xido, sulfeto e silicato, e orto-anfibolitos finos (metabasaltos), metamorfisados nas zonas da cianita da f?cies anfibolito m?dio. Dados geotermobarom?tricos dos micaxistos peraluminosos revelaram condi??es metam?rficas em torno de 550? C a 5,5 kbar. As assinaturas litoqu?micas das rochas metam?ficas e meta-ultram?ficas revelam afinidade ofiol?tica e origem em assoalho oce?nico. Os dados isot?picos Sm-Nd destas rochas mostram valores positivos de epsilon Nd (+3 a +7), e as idades modelo e isocr?nica sugerem desenvolvimento de litosfera oce?nica durante o Neoproteroz?ico. Todas as tentativas anteriores de recupera??o de zirc?o a partir de volumosas amostras das rochas metam?ficas foram infrut?feras. Contudo, corpos leucocr?ticos semelhantes a plagiogranito foram reconhecidos poucos anos atr?s e se tornaram um dos principais alvos da tese de doutorado da primeira autora. Estes corpos ocorrem sob a forma de veios irregulares com dimens?es milim?tricas a centim?tricas (at? 50 cm), e s?o encaixados por orto-anfibolito bandado de granula??o m?dia a grossa (metadolerito a metagabro). Os corpos leucocr?ticos consistem de metaplagiogranito foliado, composto essencialmente por plagiocl?sio c?lcico com bordas alb?ticas, quartzo, hornblenda e epidoto, com titanita, sulfeto, apatita e zirc?o como os principais minerais acess?rios. Os cristais de zirc?o da amostra de plagiogranito s?o eu?dricos e muito l?mpidos, e mostram morfologia prism?tica elongada (3:1), sugerindo origem magm?tica. An?lises U-Pb por LA-ICPMS (Laser Ablation Inductively Coupled Mass Spectrometry) foram realizadas em dezoito cristais de zirc?o e mostram resultados concordantes, indicando idade de cristaliza??o magm?tica de 660 ? 29 Ma. Esta idade baliza a ?poca de gera??o de crosta oce?nica na bacia precursora do Or?geno Ara?ua?. O espalhamento de algumas das an?lises ao longo da curva conc?rdia sugere perda de Pb devido ao metamorfismo de f?cies 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 magm?tico pr?-colisional, bem como a idade U-Pb (ca. 582 Ma) dos granitos sincolisionais mais antigos

Expression and production of cardiac angiogenic mediators depend on the Trypanosoma cruzi-genetic population in experimental C57BL/6 mice infection.

Mammalian cardiac cells are important targets to the protozoan Trypanosoma cruzi. The inflammatory reaction in the host aims at eliminating this parasite, can lead to cell destruction, fibrosis and hypoxia. Local hypoxia iswelldefined stimulus to the production of angiogenesis mediators. Assuming that different genetic T. cruzi populations induce distinct inflammation and disease patterns, the current study aims to investigate whether the production of inflammatory and angiogenic mediators is a parasite strain-dependent condition. C57BL/6 mice were infectedwith the Y and Colombian strains of T. cruzi and euthanized at the 12th and 32nd days, respectively. The blood and heart tissue were processed in immune assays and/or qPCR (TNF, IL-17, IL-10, CCL2, CCL3, CCL5, CCR2, CCR5 and angiogenic factors VEGF, Ang-1, Ang-2) and in histological assays. The T. cruzi increased the inflammatory and angiogenicmediators in the infectedmicewhen theywere compared to non-infected animals.However, the Colombian strain has led to higher (i) leukocyte infiltration, (ii) cardiac TNF and CCL5 production/expression, (iii) cardiac tissue parasitism, and to higher (iv) ratio between heart/body weights. On the other hand, the Colombian strain has caused lower production and expression VEGF, Ang-1 and Ang-2, when it was compared to the Y strain of the parasite. The present study highlights that the T. cruzi-genetic population defines the pattern of angiogenic/inflammatory mediators in the heart tissue, and that itmay contribute to themagnitude of the cardiac pathogenesis. Besides, such assumption opens windows to the understanding of the angiogenic mediator's role in association with the experimental T. cruzi infection

Dep?sitos de alexandrita de Malacacheta, Minas Gerais.

Since 1975, alexandrite [Be(Al2-xCrx)2O4], the chromium-bearing crysoberyl variety, has been exploited from alluvial and paleo-alluvial deposits in the Malacacheta region. The alluvial deposits consist of resedimented gravel along the present drainage streams. The paleo-alluvial deposits are richer in alexandrite, and show well-developed soil horizons covering the alexandrite-bearing gravel layer. Alexandrite grains show angular shapes with very sharp edges, suggesting transport for short distances. The country rocks are quartz-mica schist and peraluminous mica schist (Salinas Formation), covered by alternating mica schist and quartzite layers (Capelinha Formation). Both formations are of Neoproterozoic age. They host tectonic slabs of metaultramafic rocks, and are cut by intrusive granites of Cambrian age. No alexandrite-bearing rock has been found in the area, probably due to the intense tropical weathering. However, some of the mapped rocks are sources for Be (granites), Cr (metaultramafics) and Al (peraluminous schists), the essential elements for alexandrite crystallization. We suggest a metasomatic system of Cambrian age for alexandrite genesis in the area, involving the interaction of granite-related Be-rich fluids with metaultramafic rocks and peraluminous schists

O embasamento arqueano e paleoproteroz?ico do or?geno Ara?ua?.

O embasamento do Or?geno Ara?ua? compreende essencialmente os complexos Guanh?es, Gouveia, Porteirinha, Mantiqueira e Juiz de Fora. Os quatros primeiros possuem car?ter aut?ctone a para-aut?ctone e representam o embasamento crat?nico retrabalhado no dom?nio orog?nico. O Complexo Juiz de Fora est? tectonicamente justaposto ao Complexo Mantiqueira por meio de extensa zona de cisalhamento neoproteroz?ica, a Falha de Abre Campo. Os complexos Guanh?es, Gouveia e Porteirinha assemelham-se ao arcabou?o arqueano do Quadril?tero Ferr?fero, incluindo gnaisses e migmatitos TTG (tonalito-trondhjemito-granodiorito, plutons gran?ticos e seq??ncias do tipo greenstone belt. As idades conhecidas de eventos e unidades geol?gicas, entretanto, n?o s?o perfeitamente correlatas ?quelas do Quadril?tero Ferr?fero. No Complexo Guanh?es os gnaisses/migmatitos TTG foram datados entre 2867 e 2711 Ma, e um corpo gran?tico em 2710 Ma. No Complexo Gouveia ocorre uma sequ?ncia greenstone belt de 2971 Ma, e uma intrus?o gran?tica foi datada em 2839 Ma. Data??es U-Pb n?o est?o dispon?veis para o Complexo Porteirinha. O Complexo Mantiqueira ? composto predominantemente por ortognaisses bandados, cuja cristaliza??o magm?tica ocorreu no intervalo 2180-2041 Ma. Os dados isot?picos de Sr e Nd sugerem uma origem por processos de fus?o de crosta continental antiga. J? os ortognaisses granul?ticos do Complexo Juiz de Fora possuem assinatura isot?pica dominantemente juvenil e cristalizaram entre 2134 e 2084 Ma. A associa??o de rochas do Complexo Mantiqueira corresponde a arco(s) magm?tico(s) desenvolvido(s) sobre a margem do paleocontinente arqueano, enquanto o Complexo Juiz de Fora parece representar um arco intra-oce?nico. Estes complexos constituem segmentos de um or?geno riaciano, desenvolvido entre 2,2 e 2,05 Ga, e retrabalhado pela Orogenia Brasiliana.Basement units of the Ara?ua? Orogen are the Guanh?es, Gouveia, Porteirinha, Mantiqueira, and Juiz de Fora complexes. The first four units are autochthonous to parautochthonous, and are considered to be the reworked margin of the S?o Francisco Craton, now located in the Ara?ua? Orogen. The Juiz de Fora complex is juxtaposed against the Mantiqueira complex by a dextral-reverse, high-angle shear zone of Brasiliano age (the Abre Campo Fault). The Archean basement of the Quadril?tero Ferr?fero region can be traced into the Guanh?es, Gouveia, and Porteirinha complexes, which display TTG gneisses and migmatites, granite plutons and greenstone belt sequences. However, ages of geologic units and events are not exactly the same as in the Quadril?tero Ferr?fero. TTG gneisses within the Guanh?es Complex yielded magmatic ages between 2867 and 2711 Ma, and a granite pluton was dated at 2710 Ma. A greenstone belt sequence and a granite pluton of the Gouveia Complex were dated at 2971 Ma and 2839 Ma, respectively. U-Pb ages are not available for the Porteirinha Complex. The Mantiqueira complex consists predominantly of banded biotite-amphibole orthogneiss, with magmatic ages at 2180-2041 Ma. Sr and Nd signatures suggest that the gneiss protholits were essentially generated by partial melting of crustal material. On the other hand, granulite facies orthogneisses of the Juiz de Fora Complex make up calc-alkaline suites of juvenile affinity, and crystallized at 2134-2084 Ma. The Mantiqueira Complex is a magmatic arc built up along the Archean margin of the S?o Francisco Paleocontinent, whilst the Juiz de Fora Complex probably evolved within an oceanic magmatic arc setting, or on a very stretched continental crust. They are envisaged as segments of a Rhyacian orogen that evolved at 2.2-2.05 Ga, latter reworked by he Brasiliano Orogeny

Geocronologia U-Pb (SHRIMP) e Sm-Nd de xistos verdes bas?lticos do Or?geno Ara?ua? : implica??es para a idade do Grupo Maca?bas.

No vale do Rio Preto, setor ocidental do Or?geno Ara?ua? (ca. 60 km a NE de Diamantina), ocorrem xistos verdes de filia??o bas?ltica, cuja idade e posi??o estratigr?fica foram motivo de controv?rsia, pois os autores dividiram-se naqueles que os atribu?ram ao Grupo Maca?bas (Neoproteroz?ico) e naqueles que os correlacionaram ao Supergrupo Espinha?o inferior (ca. 1,7 Ga). Entretanto, estudos detalhados demonstram que os xistos verdes representam derrames bas?lticos submarinos, sedimenta??o vulcanocl?stica e vulcanismo relacionado a fontes de alta produtividade, relacionados ? deposi??o da Forma??o Chapada Acau? do Grupo Maca?bas (Gradim et al., 2005). Os dados geoqu?micos indicam que os prot?litos dos xistos verdes evolu?ram em ambiente continental intraplaca. An?lises isot?picas U-Pb (SHRIMP) foram realizadas em doze cristais de zirc?o extra?dos de uma amostra de xisto verde, cujo p? de rocha-total foi utilizado para an?lise Sm-Nd. A idade-modelo Sm-Nd (ca. 1,52 Ga) sugere que os prot?litos dos xistos verdes s?o mais novos que o magmatismo do rifte Espinha?o. A maioria dos cristais de zirc?o analisados mostra-se como gr?os detr?ticos. As idades mais antigas indicam gr?os herdados do embasamento arqueano-paleoproteroz?ico e de rochas magm?ticas do rifte Espinha?o. Os cristais mais jovens limitam a idade m?xima dos prot?litos dos xistos verdes em ca. 1,16 Ga.The age and stratigraphic position of the basaltic greenschists of the Rio Preto valley, located in the western part of the Ara?ua? Belt (ca. 60 km north of Diamantina, Minas Gerais) were a matter of controversy in the geologic literature, because several authors correlated them to the Neoproterozoic Maca?bas Group but others to the lower Espinha?o Supergroup (ca. 1.7 Ga). However, detailed studies demonstrate that these greenschists represent an interplay of submarine basaltic volcanism, volcanoclastic sedimentation and fire fountaining, and that they belong to the Chapada Acau? Formation of the Maca?bas Group (Gradim et al., 2005). Geochemical studies indicate that the greenschist protoliths evolved in a continental intraplate environment. Zircon crystals were separated from a greenschist sample and analyzed by the U-Pb SHRIMP method. A Sm-Nd whole-rock isotopic analysis was obtained from the same sample. The Sm- Nd model age of ca. 1.52 Ga suggests that the greenschist protoliths are younger than the magmatism of the Espinha?o rift. Most analyzed zircon crystals show features of detrital grains. The older ages indicate zircon grains inherited from the Archean- Paleoproterozoic basement and from magmatic rocks of the Espinha?o rift. The younger U-Pb values constrain the maximum age of the greenschist protoliths at ca. 1,16 Ga

Metamorphic record of collision and collapse in the Ediacaran?Cambrian Ara?ua? orogen, SE?Brazil : insights from P?T pseudosections and monazite dating.

he Ara?ua? orogen is the Brazilian counterpart of the Ara?ua??West Congo orogenic system (AWCO), a component of the Ediacaran?Cambrian orogenic network formed during the amalgamation of West Gondwana. The northwestern portion of the Ara?ua? orogen is dominated by a succession of metasedimentary rocks made up of Meso? to Neoproterozoic rift, passive margin and syn?orogenic sequences, locally intruded by post?collisional granites. These sequences are involved in three distinct tectonic units, which from west to east are: the southern Espinha?o fold?thrust system (SE?thrust system), the normal?sense Chapada Acau? shear zone (CASZ) and the Salinas synclinorium. Three deformation phases were documented in the region. The first two phases (D1 and D2) are characterized by contractional structures and represent the collisional development stage of the orogen. The third phase (D3) is extensional and currently viewed as a manifestation of orogenic collapse of the system. The distribution of the metamorphic mineral assemblages in the region characterizes two metamorphic domains. The M?Domain I on the west, encompassing the SE?thrust system and the CASZ, is marked by a syn?collisional (syn?D1) Barrovian?type metamorphism with P?T conditions increasing eastwards and reaching ~8.5 kbar at ~650?C between 575 and 565 Ma. The M?Domain II comprises the Salinas synclinorium in the hangingwall of the CASZ, and besides the greenschist facies syn?collisional metamorphism, records mainly a Buchan?type metamorphic event, which took place under 3?5.5 kbar and up to 640?C at c. 530 Ma. The northwestern Ara?ua? orogen exhibits, thus, a paired metamorphic pattern, in which the Barrovian and Buchan?type metamorphic domains are juxtaposed by a normal?sense shear zone. Lithospheric thinning during the extensional collapse of the orogen promoted ascent of the geotherms and melt generation. A large volume of granites was emplaced in the high grade and anatectic core of the orogen during this stage, and heat advected from these intrusions caused the development of Buchan facies series over a relatively large area. Renewed granite plutonism, hydrothermal activities followed by progressive cooling affected the system between 530 and 490 Ma