No description available.

The origin of tourmaline nodules, bizarre spherical to irregular textures documented worldwide, remains a geologic mystery. Although previously described by numerous researchers, the physical and chemical parameters that govern their formation have yet to be resolved. Commonly containing tourmaline, quartz, and occasionally feldspar, nodules are surrounded by a halo of leucocratic host rock, and are typically eight to ten centimeters in diameter. Tourmaline nodules of the present study are contained within the Paleoproterozoic Scrubber Granite of the southern Gascoyne Complex in Western Australia. </p> <p>This study integrated field observations, nodule petrography, tourmaline crystal chemistry, tourmaline fluid inclusion analyses, whole rock chemistry of nodule cores, leucocratic halo zones and host granite zones, stable and radiogenic isotope signatures of tourmaline separates as well as comparisons with other tourmaline nodule studies to propose the most scientifically sound theory for the formation of tourmaline nodules in the Scrubber Granite. </p> Numerous nodule morphologies, including spherical and C-shaped nodules, along with other features such as tube-like nodules and tourmaline veins occur in massive, porphyritic, foliated and sheared phases of the Scrubber Granite. Microscopically, tourmaline displays prismatic, sub-rounded and massive textures. Microthermometric studies completed on tourmaline fluid inclusions revealed that the nodule-forming fluid contained 14 to 15 weight percent NaCl + CaCl2. Based on stable isotope studies and homogenization temperatures, fluid temperatures were constrained between 450 and 700¢ªC. The ¥ä18O and ¥äD concentrations of the nodule-forming fluid at this temperature range plot above the typical magmatic water field. Epsilon Nd values indicate that the tourmaline nodules of the Scrubber Granite may have been disturbed by a later metamorphic event.</p>Tourmaline nodules of the Scrubber Granite are herein proposed to have formed from the exsolution and rise of buoyant pockets or bubbles of volatile fluid derived from the crystallizing Scrubber Granite magma.

The origin of tourmaline nodules, bizarre spherical to irregular textures documented worldwide, remains a geologic mystery. Although previously described by numerous researchers, the physical and chemical parameters that govern their formation have yet to be resolved. Commonly containing tourmaline, quartz, and occasionally feldspar, nodules are surrounded by a halo of leucocratic host rock, and are typically eight to ten centimeters in diameter. Tourmaline nodules of the present study are contained within the Paleoproterozoic Scrubber Granite of the southern Gascoyne Complex in Western Australia. </p> <p>This study integrated field observations, nodule petrography, tourmaline crystal chemistry, tourmaline fluid inclusion analyses, whole rock chemistry of nodule cores, leucocratic halo zones and host granite zones, stable and radiogenic isotope signatures of tourmaline separates as well as comparisons with other tourmaline nodule studies to propose the most scientifically sound theory for the formation of tourmaline nodules in the Scrubber Granite. </p> Numerous nodule morphologies, including spherical and C-shaped nodules, along with other features such as tube-like nodules and tourmaline veins occur in massive, porphyritic, foliated and sheared phases of the Scrubber Granite. Microscopically, tourmaline displays prismatic, sub-rounded and massive textures. Microthermometric studies completed on tourmaline fluid inclusions revealed that the nodule-forming fluid contained 14 to 15 weight percent NaCl + CaCl2. Based on stable isotope studies and homogenization temperatures, fluid temperatures were constrained between 450 and 700¢ªC. The ¥ä18O and ¥äD concentrations of the nodule-forming fluid at this temperature range plot above the typical magmatic water field. Epsilon Nd values indicate that the tourmaline nodules of the Scrubber Granite may have been disturbed by a later metamorphic event.</p>Tourmaline nodules of the Scrubber Granite are herein proposed to have formed from the exsolution and rise of buoyant pockets or bubbles of volatile fluid derived from the crystallizing Scrubber Granite magma.

Evidence of three glaciations in Paleoproterozoic successions of North America and at least one on three other continents, suggests that these glaciations were of global extent. In common with the Neoproterozoic record, carbonates cap the glacials. However, the relationship between biogeochemical cycling of carbon and ice ages has not been fully appreciated. This research involved the sedimentology and isotope stratigraphy of carbonates and shales in Paleoproterozoic glacially-influenced successions of Wyoming and South Africa. Carbonates of the Vagner Formation cap the middle of three diamictites in the Snowy Pass Supergroup, Medicine Bow Mountains, WY. The Duitschland Formation occurs between two glacial horizons in South Africa. Limestones retain negative d13C values for over 60 m in the Vagner Formation, and for over 100 m in the lower part of the Duitschland Formation. Isotope compositions of TOC from the lower part of the Duitschland Formation reveal pronounced enrichment resulting in significantly lower fractionation between organic and inorganic carbon. This is similar to enrichment noted in Neoproterozoic cap carbonates. Combined with strongly positive carbon isotope compositions in upper Duitschland carbonates, the data from the Vagner Formation underscores strongly positive-to-negative carbon isotope trends bracketing Paleoproterozoic glaciations. These trends mimic those noted in Neoproterozoic glacial successions and possibly indicate a recurrence of global glaciations. The Slaughterhouse and Nash Fork formations significantly postdate the glacial epoch. Both the lower part of the Nash Fork Formation, Medicine Bow Mountains and the Slaughterhouse Formation, Sierra Madre contains carbonates with 13C-enrichment >+6â ° and locally up to +28%, whereas carbonates higher in the Nash Fork Formation have d13C values between 0 and 2.5%. This dramatic change in the composition of the Paleoproterozoic ocean is constrained at ca. 2.1 Ga (Karhu, 1993). Carbonates in the Rawhide Canyon section of the Whalen Group in the Hartville Uplift (the easternmost exposure of the Wyoming Craton) display δ13C values up to +8.2% suggesting correlation with the Slaughterhouse and Nash Fork formations and their deposition on continuous carbonate platform along the margin of the Wyoming Craton. These data support an open-marine, and therefore a global origin for the ca. 2.2-2.1 Ga carbon isotope excursion. / Ph. D.

FundaÃÃo Cearense de Apoio ao Desenvolvimento Cientifico e TecnolÃgico / The Rhyacian Serra das Pipocas Greenstone Belt, is part of Archean/Proterozoic terrene of Cearà Central Domain, in the Setentrional portion of Borborema Province. It is located in Southwestern CearÃ, between the municipalities of Boa Viagem, IndependÃncia, Tauà and Pedra Branca. Geological mapping techniques, thin and polished sections petrography, mineral chemistry, lithogeochemistry, and geochronology have been useful in the distinction of the first Greenstone Belt sequence in CearÃ. The metavolcanic-sedimentary sequence is composed of metasedimentary rocks, marly psammite-pelite, containing alternation of mafic-ultramafic volcanic floods, tholeiitic and komatiitic, respectively, and meta-acidic rocks. The komatiitic meta-ultramafic floods are composed of chlorite-anthophyllite-actinolite-tremolite schist displaying acicular texture, or not, occuring discontinuously near the bottom of the sequence. The tholeiitic metamafic rocks are mainly represented by garnet amphibolites, which continuously extend 30km in length by 500m â 1km wide. Basic and acidic metatuffs, metacherts, and banded iron formation are alternated with amphibolites, which sometimes are deeply hydrotermalized. Metasedimentary rocks are mainly terrigenous, containing biotite, kyanite; however, occasional centimetric alternation of calc-silicatic rocks are observed. These lithotypes are cutted off by mafic-ultramafic intrusions, metagranodiorites and metabasic dykes. The lithological association has been developed in a extensional enviroment, probably a back-arc basin, where the komatiitic rocks Mg-Bearing schists display a transition from Munro-type to Barbeton-type, while the tholeiitic metabasalts (amphibolites) are iron-magnesium-rich. Regarding the structural geology, the area is caracterized by penetrative polyphasic strain, occurred during Brasiliano, with tight, isoclinal and recumbent folds, in addition to thrust faults and shear zones. The thrust faults are best recognized, especially, in the contact between the metavolcanic-sedimentary sequence and granite-gneiss-migmatite of Cruzeta Complex; also, between the mafic-ultramafic subunits. The metamorphic grade of the lithological association varies from high greenschist facies to high amphibolite facies, contrasting with Cruzeta Complex units, which are frequently migmatized. Besides, during the Brasiliano orogenesis, leucocratic granites have been intruded along the border zones of the oldest granites (2181,4Â4.4), near the Queimadas thrust fault, there are deeply hydrotemalized rocks, displaying silicification, potassification, chloritization, and carbonation, sulphidation is also present, and they may contain some mineralisation such as copper-gold Volcanic-hosted Massive Sulfide or auriferous lodes, because this association occurs near the silicified zones and also in shear zones. / O Greenstone Belt Serra das Pipocas, de idade riaciana, està inserido no NÃcleo Arqueano/Paleoproterozoico do DomÃnio Cearà Central, na porÃÃo Setentrional da ProvÃncia Borborema. Encontra-se na porÃÃo sudoeste do Estado do CearÃ, entre os municÃpios de Boa Viagem, IndependÃncia Tauà e Pedra Branca. TÃcnicas de mapeamento geolÃgico, anÃlise petrogrÃfica, atravÃs de estudos de seÃÃes delgadas e polidas, quÃmica mineral, litogeoquÃmica e geocronologia auxiliaram para a distinÃÃo desta primeira sequÃncia do tipo Greenstone Belt no CearÃ. A sequÃncia metavulcanossedimentar à composta por rochas metassedimentares psamo-pelito-margosos, contendo intercalaÃÃes de derrames vulcÃnicos metamÃfico-ultramÃficos de natureza toleÃtica e komatiÃtica, respectivamente, e meta-Ãcidas. Os derrames metaultramÃficos komatiÃticos sÃo constituÃdos por clorita-antofilita-actinolita/tremolita xisto, com texturas aciculares ou nÃo, ocorrendo de maneira descontÃnua prÃximo à base da sequÃncia. As rochas metamÃficas de natureza toleÃticas sÃo representadas principalmente por anfibolitos granadÃferos, que se estendem, de maneira descontÃnua, por uma faixa com comprimento da ordem de 30km e largura entre 500m e 1km. Metatufos bÃsicos a Ãcidos, metacherts e formaÃÃes ferrÃferas bandadas ocorrem intercaladas a estes anfibolitos, que se encontram por vezes, fortemente hidrotermalizados. As rochas metassedimentares sÃo principalmente de natureza terrÃgena, contendo biotita, cianita, mas podem apresentar ocasionais intercalaÃÃes centimÃtricas de calcissilicÃticas. Esses litotipos sÃo recortados por intrusÃes metamÃfica-ultramÃficas, metagranodioritos e dique metabÃsico. Toda a associaÃÃo litolÃgica desenvolveu-se ambiente extensional, provavelmente do tipo retro arco, cujas rochas komatiÃticas (xistos magnesianos) encontradas apresentam transiÃÃo entre os tipos Munro e Barbeton, enquanto os metabasaltos toleÃticos (anfibolitos) exibem alto teor de ferro e magnÃsio. Estruturalmente, a Ãrea à marcada por deformaÃÃes polifÃsicas penetrativas, ocorridas no Brasiliano, com dobras isoclinais apertadas, recumbentes, alÃm de falhamentos de empurrÃo e cisalhamento dÃctil. Esses falhamentos sÃo reconhecidos, sobretudo, no contato entre esta sequÃncia metavulcanossedimentar e as rochas granito-gnÃissico-migmatÃticas do Complexo Cruzeta e entre as subunidades dominadas pelas rochas ultramÃficas e mÃficas. O grau metamÃrfico das associaÃÃes litolÃgicas à variÃvel, indo da fÃcies xisto verde alto a anfibolito alto, o que contrasta com as unidades do Complexo Cruzeta que se encontra frequentemente migmatizadas. Durante o evento Brasiliano houve tambÃm a intrusÃo dos leucogranitos. Nas bordas dos corpos intrusivos riacianos, prÃximos à zona de empurrÃo Queimadas, hà presenÃas de rochas fortemente hidrotermalizadas, marcadas por silicificaÃÃo, potassificaÃÃo, cloritizaÃÃo e carbonataÃÃo. Estas sÃo acompanhadas de sulfetaÃÃo, sendo hospedeiras de possÃveis mineralizaÃÃes de cobre e ouro do tipo Sulfetos Massivos VulcanogÃnicos ou mesmo lodes aurÃferos, uma vez que esta associaÃÃo ocorre junto Ãs zonas silicificadas ao longo de cisalhamento.

A região de São Félix do Xingu, localizada no centro-sul do estado do Pará e, geologicamente, no contexto da Província Amazônia Central do Cráton Amazônico. Apresenta em seus arredores um registro extremamente preservado das atividades vulcâno-plutônicas ocorridas durante o final do Paleoproterozóico (1870 - 1880 Ma), agrupadas no Grupo Uatumã, que é dividido na região nas formações Sobreiro e Santa Rosa. Estas rochas foram depositadas sobre o embasamento arqueano, representado pelo Terreno Granito-Greenstone do Sul do Pará e pelo Cinturão de Cisalhamento Itacaiúnas, e unidades paleoproterozóicas, tal como o Granito Parauari. Por fim, estas rochas foram invadidas em ~1860 Ma pelos granitos do tipo A da Suíte Intrusiva Velho Guilherme e recobertas pelas rochas sedimentares da Formação Triunfo. A Formação Sobreiro é a unidade basal. Suas rochas tem filiação cálcio-alcalina e são representadas por vulcânicas e piroclásticas predominantemente intermediárias, com componentes ácidos no topo da sequência. Em estudos de campo são reconhecidas ao menos duas sequências de derrames vulcânicos que variam de andesitos basálticos com fenocristais de augita e magnésio-hastingsita nos derrames basais, para andesito e latito com fenocristais de magnésio-hastingsita e de andesina a labradorita e, por fim, quartzo-latito e riolito com fenocristais de plagioclásio sódico e de feldspato potássio, além de quartzo ocasional. Intercalados nestes derames de lava ocorrem corpos de rochas piroclásticas representadas principalmente por tufos máficos de cristais, lapilli-tufo máfico e tufos máficos laminados de cristais. De modo geral essas, são rochas hipocristalinas, maciças, formadas por cristais e fragmentos de cristais líticos e vítreos. Os tufos de cristais máficos laminados apresentam melhor seleção granulométrica, estrutura laminada e arcabouço constituído porcristais e fragmentos de cristais e líticos. A Formação Santa Rosa, como descrita atualmente na literatura, é a unidade superior e representa um vulcanismo intraplaca do tipo A. É composta por rochas vulcânicas, subvulcânicas e piroclásticas com alto teor de \'SiO IND.2\' (> 70% na maioria das amostras). Ao menos três fácies são reconhecidas: a) pórfiros graníticos e riolitos com megacristais de anfibólio, plagioclásio sódico, feldspato potássico e quartzo; riolitos com fenocristais de feldspato potássico e plagioclásio sódico com eventuais megacristais de quartzo; b) álcaliriolitos e pórfiros álcali-riolíticos com fenocristais de feldspato potássico (ortoclásio) e quartzo; c) tufo félsico de cristais hiprocristalino, tufo félsico de cristais levemente soldado com fiamme, tufo soldado laminado, lapilli-tufo acrescionário e tufo vítreo com glass shards. A assembleia de alteração hidrotermal da Formação Sobreiro é composta por epídoto + clorita + clinozoisíta + pirita + quartzo + carbonato + albita + sericita na alteração propilítica; sericita + clorita + quartzo \'+OU-\' pirita \'+OU-\' fluorita \'+OU-\' barita \'+OU-\' alloclasita \'+OU-\' esfalerita na alteração sericítica; e sericita + hematita + quartzo + argilo-minerais \'+OU-\' galena \'+OU-\' ouro na alteração argílica. Indícios de alunita sugerem que a Formação Sobreiro pode hospedar em suas rochas sistemas epitermais do tipo low-e high-sulfidation. Já na Formação Santa Rosa, a assembleia mineral de alteração hidrotermal é composta por feldspato potássico + biotita + quartzo + sericita na alteração potássica; e sericita + quartzo + pirita + clorita \'+OU-\' fluorita \'+OU-\' carbonato na alteração sericítica. Tais assembleias podem hospedar nas rochas da Formação Santa Rosa mineralizações do tipo Intrusion Related Gold Systems. Estudos litoquímicos revelam a naturezacálcio-alcalina de alto potássio da Formação Sobreiro, com enriquecimento em elementos litófilos como K, Ba, Sr, Rb, e baixa concentração de elementos de alto potencial iônico como Nb e Ta. Esta unidade mostra rochas enriquecidas em elementos terras raras leves em relação a terras raras pesados, indicada pela razão \'(La/Yb) IND.N\' ~ 12, sem anomalias de Eu nas rochas menos evoluídas e com anomalias levemente negativas nas rochas mais evoluídas. Tais características são típicas de andesitos orogênicos e estudos comparativos revelam que as rochas da Formação Sobreiro são bastante semelhantes às de alguns arcos magmáticos mais jovens, como o Arco Eólio na região da Sicília. Isto corrobora a hipótese de que a Formação Sobreiro é relacionada a um vulcanismo associado a um evento de subducção. A Formação Santa Rosa mostra resultados mais heterogêneos. Algumas das amostras analisadas apresentam afinidade cálcio-alcalina metaluminosa, com enriquecimento em Ba, Rb e Sr semelhantes às rochas evoluídas da Formação Sobreiro. Nestas amostras são observadas anomalias negativas de nióbio e tântalo em diagramas normalizados de elementos traços, além de suaves anomalias negativas de Eu em diagramas de elementos terras raras, com enriquecimento em terras raras leves em relação aos pesados similar à Formação Sobreiro. Estas características, junto às razões de Ba/Ta > 450 e Rb/Nb > 7, mostram mais semelhanças com as rochas cálcio-alcalinas da Formação Sobreiro do que com as rochas do tipo A da Formação Santa Rosa. O outro grupo de amostras da Formação Santa Rosa apresenta um comportamento completamente diferente, sendo caracterizado por rochas alcalinas, peraluminosas, com enriquecimento em elementos de alto potencial iônico (principalmente Nb e Ta) e fortes anomalias negativas para elementos litófilos (principalmente Ba e Sr, além de CaO, P e Ti) emdiagramas normalizados de elementos traços. Em relação aos elementos terras raras, este grupo apresenta enriquecimento muito mais discreto em elementos terras raras leves em relação aos pesados, evidenciado pela razão \'(La/Yb) IND.N\' ~ 4, com forte anomalia negativa de Eu. As razões Rb/Nb < 7 indicam que estas amostras atendem a maioria dos critérios classificatórios para rochas subalcalinas do tipo A e, portanto devem ser classificadas como pertencentes à Formação Santa Rosa. Por fim, os resultados sugerem que embora agrupados somente na Formação Santa Rosa nos trabalhos anteriores, ao menos uma parte dos riolitos mostra características que apresentam associação ao vulcanismo cálcio-alcalino da Formação Sobreiro / The São Felix do Xingu region, located in the center-south region of the state of Pará - Brazil, under the Central Amazonian province of theAmazonian Craton context, presents in its surroundings extremely well preserved volcano-plutonic activities occurred during the Paleoproterozoic (1870 - 1880 Ma), where units are grouped into the Uatumã Group which is therefore divided into formations Sobreiro e Santa Rosa. These rocks are intrusive in units of the Archean basement represented by the South Pará Granite-Greenstone Terrain and by the Itacaiúnas Shear-belt; and rocks of Paleoproterozoic such as the Parauari Granite. Thus, these rocks are intruded in ~1860 Ma by A-type granites of the Velho Guilherme Intrusive Suite and covered by sedimentary rocks of the Triunfo Formation. The Sobreiro Formation is the basal unity. It is calc-alkaline, composed of volcanic and pyroclastic rocks of mainly intermediate composition, with acid components on the top of the sequence. In field study, at least two sequences of volcanic flows which vary from andesi-basalts with phenocrysts of augite and magnesium-hastginsite in the basal flows, to andesites and latites with phenocrysts of magnesium-hastginsite and plagioclase (andesine to labradorite) and finally, quartz-latites and rhyolites with phenocrysts of sodic plagioclase and potash feldspar, besides occasional quartz, are recognized. Interspersed with these lava flow rocks, pyroclastic rocks represented by mafic crystal tuffs, mafic crystal lapilli-tuffs and laminated mafic crystal tuffs occur. The Santa Rosa Formation, as described in literature nowadays, is the superior unit and represents an A-type intraplate volcanism. It is composed by volcanic, subvolcanic and pyroclastic rocks with high \'SiO IND.2\' content (>70% in most of the samples). At least three facies are identified, the first consisting of granitic porphyry and rhyolite with megacrysts of amphibole, sodic plagioclase, potash feldspar and quartz; the second of rhyolites with phenocrysts of potash feldspar and sodic plagioclase with occasional quartz; and the last of alkali-rhyolites and alkali-rhyolitic porphyries with phenocrysts of potash feldspar (orthoclase) and quartz. Hipocrystalline felsic crystal tuff, lightly welded crystal tuff with fiamme, laminated welded tuff, accretionary lapilli-tuff and vitreous tuff with glass shards represent the pyroclastic rocks associated with Santa Rosa Formation. The hydrothermal alteration mineral assemblage of the Sobreiro Formation is composed of epidote + chlorite + clinozoisite + pyrite + quartz + carbonate + albite + sericite in the propylitic alteration; sericite + chlorite +quartz + carbonate ± pyrite ± fluorite ± barite ± alloclasite ± sphalerite in the sericitic alteration; and sericite + hematite + quartz + clay minerals ± galena ± gold in the argillic alteration. In addition to evidences of alunite, the vii Sobreiro Formation may host low and high-sulfidation epithermal systems in its rocks. The Santa Rosa Formation, on the other hand, presents ahydrothermal alteration mineral assemblage composed of potash feldspar + biotite + quartz + sericite in the potassic alteration; and sericite + quartz + pyrite + sericite ± fluorite ± carbonate in the sericitic alteration. Such assemblages may host Intrusion Related Gold Systemsmineralization type into the rocks of the Santa Rosa Formation. Lithochemistry studies reveal the high potassium calc-alkaline nature for the Sobreiro Formation, with enrichment in lithophile elements such as K, Ba, Sr, Rb and low concentration of high field strength elements such as Nb and Ta. This unit shows rocks enriched in light rare earth elements in relation to heavy rare earth elements indicated by the \'(La/Yb) IND. N\' ~ 12 ratio, with absence of Eu anomalies in the less evolved and lightly negative anomalies in the most evolved rocks. Such characteristics are typical of orogenic andesites and comparative studies reveal that the Sobreiro Formation chemical characteristics are rather similar to some younger magmatic arcs, like the Aeolian Arc in the Sicily\'s region. This data corroborates with the hypothesis that the Sobreiro Formation is related to a calc-alkaline volcanism related to a subduction regime. The Santa Rosa Formation shows more heterogeneous results. Some of the analyzed samples present calc-alkaline affinity, metaluminous, with enrichment in Ba, Rb and Sr similar to the evolved rocks of the Sobreiro Formation. Such samples also present negative Nb and Ta anomalies in normalized trace elements diagrams, besides presenting light negative anomalies in normalized rare earth elements diagrams with enrichment in light rare earth elements in relation to heavy rare earth elements similar to the Sobreiro Formation. These characteristics, allied to Ba/Ta > 450 and Rb/Nb > 7 ratios, show much more similarities to the calc-alkaline rocks of the Sobreiro Formation than to the A-type rocks of the Santa Rosa Formation. The other group of samples of this unity shows a completely different behavior, being characterized by alkaline rocks, peraluminous, withenrichment in high field strength elements (mainly Nb and Ta) and strong negative anomalies for lithophile elements (mainly Ba and Sr, besides CaO, P and Ti) in normalized trace elements diagrams. In relation to rare earth elements, the Santa Rosa Formation presents much more discrete enrichment in light rare earth elements in relation to heavy rare earth elements, highlighted by the \'(La/Yb) IND.N ~ 4 ratio, with strong negative anomalies of europium. The Rb/Nb < 7 ratio indicate that these samples attend to the most of the classificatory criteria for sub-alkaline type-A rocks and, therefore must be classified as belonging to the Santa Rosa Formation. viii All things considered, the results suggest that besides grouped exclusively in the Santa Rosa Formation in the previous works, at least somepart of the rhyolites shows characteristics which presents association to the calc-alkaline volcanism of the Sobreiro Formation.

Este trabalho apresenta um estudo paleomagnético realizado em rochas ácidas da Suíte Colíder e rochas máficas associadas, situadas no Norte do Mato Grosso, parte sudoeste do Cráton Amazônico. Foram coletados 5 cilindros e 42 blocos orientados de rochas ácidas e intermediárias (12 sítios) da Suíte Colider e 75 cilindros e 3 blocos orientados de rochas máficas (11 sítios) pertencentes à Intrusiva Guadalupe e à Suíte Intrusiva Flor da Serra localizados nas proximidades de Alta Floresta, Colíder, Terra Nova e Matupá. A Suíte Colíder foi datada pelo método UPb em zircões e apresenta idades entre 1,80 e 1,78 Ga. Para a Intrusiva Guadalupe ainda não existem determinações radiométricas que estabeleçam a sua idade. A investigação da mineralogia magnética feita através de curvas termomagnéticas, de indução magnética e de histerese, além do tratamento térmico e por campos magnéticos alternados, indica a hematita como sendo o principal portador magnético da maioria das rochas ácidas. Por outro lado, as rochas máficas apresentam grãos de titanomagnetita pobre em Ti, a maioria com estrutura de pseudo-domínio simples (PSD). Espécimes destas rochas após as desmagnetizações térmicas e por campos magnéticos alternados apresentaram duas direções de magnetização característica. Uma delas (componente B) é representada por direções norte (sul) com inclinações negativas (positivas) (Dm=187.4°, Im=50.9°, N=16 ?95=11.3°, K=11.7) que foram isoladas para a maioria das rochas da Suíte Colíder e máficas associadas. A outra (componente A) é representada por direções norte com inclinações positivas ((Dm=356.5°, Im=47.6°, N=3, ?95=15.8°, K=61.9) e foi encontrada para o gabro Guadalupe e outros dois diques máficos. Dois pólos paleomagnéticos foram determinados para estas componentes, os quais estão localizados em 289.4°E, -65.4°N (?95=12.9°) (pólo SC, Suíte Colíder) e 298.8°E, 50.4°N (?95=15.1°) (pólo IG, Intrusiva Guadalupe). O pólo SC (fator de confiabilidade Q=5) de idade 1789 ± 7 Ma é considerado um pólo de referência para o Cráton Amazônico. Reconstruções paleogeográficas obtidas através de pólos de referência do Paleoproterozóico sugerem que a Laurentia, a Báltica, o Norte da China e o Cráton Amazônico formavam uma grande massa continental (Supercontinente Columbia) entre 1830-1770 Ma. Esta hipótese é reforçada por dados geológicos que evidenciam a existência de cinturões paleoproterozóicos bem alinhados nestas quatro massas continentais, sendo estes formados por cinturões magmáticos relacionados a subducções com a mesma polaridade. / This work presents a paleomagnetic study perfomed on felsic volcanic rocks of the Colider Suite (5 oriented cores and 42 oriented hand samples, 12 sites), Flor da Serra and Guadalupe mafic rocks (75 oriented cores and 3 oriented hand samples, 11 sites). These units are situated in the northern Mato Grosso State (southwestern Amazonian Craton), close to the Alta Floresta, Colider, Terra Nova, and Matupá cities. The Colider Suite rocks have been dated by the U-Pb (zircon) method, and ages vary between 1.80 and 1.78 Ga. Radiometric determinations are not yet available for the Guadalupe Intrusive. Two very stable magnetization components were isolated after AF and thermal demagnetization: northern (southern) directions with moderate to steep downward (upward) inclinations (component B - Dm=187.4°, Im=50.9°, N=16, ?95=11.3°, K=11.7) were isolated for most Colider Suite rocks and associated mafic rocks. Northern upward direction with moderate inclination (component A - Dm=356.5°, Im=47.6°, N=3, ?95=15.8°, K=61.9) was found for the Guadalupe gabbro and other two mafic dikes. Rock magnetism experiments show that the magnetization is carried by hematite, which is probably primary in origin, in the felsic rocks. In the mafic rocks the main magnetic carrier is Ti-poor titanomagnetite in the PSD magnetic structure. Two paleomagnetic poles were determined for these components, which are located at 289.4°E, -65.4°N (?95=12.9°) (SC pole, Colider Suite) and 298.8°E, 50.4°N (?95=15.1°) (IG pole, Guadalupe Intrusive). An age of 1.780 ± 7 Ma is attributted to pole SC (quality factor Q=5), which is considered a key pole for the Amazon Craton. Paleogeographic reconstructions using Paleoproterozoic key poles suggest that Laurentia, Baltica, North China and the Amazon Craton were located in laterally contiguous positions forming a huge continental mass (Columbia Supercontinent) at 1830-1770 Ma ago. This hypothesis is reinforced by the geological evidence that paleoproterozoic mobile belts of these four continental masses fit well in the reconstruction, being formed by contemporaneous subduction-related magmatic arc belts with the same tectonic polarity.

Este trabalho apresenta um estudo paleomagnético realizado em rochas ácidas da Suíte Colíder e rochas máficas associadas, situadas no Norte do Mato Grosso, parte sudoeste do Cráton Amazônico. Foram coletados 5 cilindros e 42 blocos orientados de rochas ácidas e intermediárias (12 sítios) da Suíte Colider e 75 cilindros e 3 blocos orientados de rochas máficas (11 sítios) pertencentes à Intrusiva Guadalupe e à Suíte Intrusiva Flor da Serra localizados nas proximidades de Alta Floresta, Colíder, Terra Nova e Matupá. A Suíte Colíder foi datada pelo método UPb em zircões e apresenta idades entre 1,80 e 1,78 Ga. Para a Intrusiva Guadalupe ainda não existem determinações radiométricas que estabeleçam a sua idade. A investigação da mineralogia magnética feita através de curvas termomagnéticas, de indução magnética e de histerese, além do tratamento térmico e por campos magnéticos alternados, indica a hematita como sendo o principal portador magnético da maioria das rochas ácidas. Por outro lado, as rochas máficas apresentam grãos de titanomagnetita pobre em Ti, a maioria com estrutura de pseudo-domínio simples (PSD). Espécimes destas rochas após as desmagnetizações térmicas e por campos magnéticos alternados apresentaram duas direções de magnetização característica. Uma delas (componente B) é representada por direções norte (sul) com inclinações negativas (positivas) (Dm=187.4°, Im=50.9°, N=16 ?95=11.3°, K=11.7) que foram isoladas para a maioria das rochas da Suíte Colíder e máficas associadas. A outra (componente A) é representada por direções norte com inclinações positivas ((Dm=356.5°, Im=47.6°, N=3, ?95=15.8°, K=61.9) e foi encontrada para o gabro Guadalupe e outros dois diques máficos. Dois pólos paleomagnéticos foram determinados para estas componentes, os quais estão localizados em 289.4°E, -65.4°N (?95=12.9°) (pólo SC, Suíte Colíder) e 298.8°E, 50.4°N (?95=15.1°) (pólo IG, Intrusiva Guadalupe). O pólo SC (fator de confiabilidade Q=5) de idade 1789 ± 7 Ma é considerado um pólo de referência para o Cráton Amazônico. Reconstruções paleogeográficas obtidas através de pólos de referência do Paleoproterozóico sugerem que a Laurentia, a Báltica, o Norte da China e o Cráton Amazônico formavam uma grande massa continental (Supercontinente Columbia) entre 1830-1770 Ma. Esta hipótese é reforçada por dados geológicos que evidenciam a existência de cinturões paleoproterozóicos bem alinhados nestas quatro massas continentais, sendo estes formados por cinturões magmáticos relacionados a subducções com a mesma polaridade. / This work presents a paleomagnetic study perfomed on felsic volcanic rocks of the Colider Suite (5 oriented cores and 42 oriented hand samples, 12 sites), Flor da Serra and Guadalupe mafic rocks (75 oriented cores and 3 oriented hand samples, 11 sites). These units are situated in the northern Mato Grosso State (southwestern Amazonian Craton), close to the Alta Floresta, Colider, Terra Nova, and Matupá cities. The Colider Suite rocks have been dated by the U-Pb (zircon) method, and ages vary between 1.80 and 1.78 Ga. Radiometric determinations are not yet available for the Guadalupe Intrusive. Two very stable magnetization components were isolated after AF and thermal demagnetization: northern (southern) directions with moderate to steep downward (upward) inclinations (component B - Dm=187.4°, Im=50.9°, N=16, ?95=11.3°, K=11.7) were isolated for most Colider Suite rocks and associated mafic rocks. Northern upward direction with moderate inclination (component A - Dm=356.5°, Im=47.6°, N=3, ?95=15.8°, K=61.9) was found for the Guadalupe gabbro and other two mafic dikes. Rock magnetism experiments show that the magnetization is carried by hematite, which is probably primary in origin, in the felsic rocks. In the mafic rocks the main magnetic carrier is Ti-poor titanomagnetite in the PSD magnetic structure. Two paleomagnetic poles were determined for these components, which are located at 289.4°E, -65.4°N (?95=12.9°) (SC pole, Colider Suite) and 298.8°E, 50.4°N (?95=15.1°) (IG pole, Guadalupe Intrusive). An age of 1.780 ± 7 Ma is attributted to pole SC (quality factor Q=5), which is considered a key pole for the Amazon Craton. Paleogeographic reconstructions using Paleoproterozoic key poles suggest that Laurentia, Baltica, North China and the Amazon Craton were located in laterally contiguous positions forming a huge continental mass (Columbia Supercontinent) at 1830-1770 Ma ago. This hypothesis is reinforced by the geological evidence that paleoproterozoic mobile belts of these four continental masses fit well in the reconstruction, being formed by contemporaneous subduction-related magmatic arc belts with the same tectonic polarity.

O significado da idade do magmatismo, reconhecimento das séries magmáticas e ambientes tectônicos que controlaram a origem das rochas metavulcânicas do Greenstone Belt da Vila Nova (GBVN), é uma importante ferramenta para entender a evolução do Escudo das Guianas durante o Paleoproterozóico. Este trabalho apresenta novos dados geoquímicos de rochas metavulcânicas e de U/Pb em zircão (LA-MC-ICPMS), que foram suplementados por petrografia, dados estratigráficos e estruturais obtidos através de mapeamento geológico e descrição de furos de sondagem. Na região de Vila Nova, as unidades do GBVN repousam sobre o embasamento Arqueano composto por ortognaisses, metagranitos e anfibolitos do Complexo Tumucumaque. A base do GBVN é composta por metabasaltos e metabasaltos andesíticos, sotoposto por um domínio superior metassedimentar com rochas químico exalativas e metavulcânicas subordinadas. As rochas metavulcânicas incluem anfibolitos e anfibólio xistos, cujos corpos estão alongados segundo a xistosidade regional de direção NW-SE. A geocronologia pelo método U-Pb em zircão mostra uma idade de 2.154 ± 6 Ma para um meta-andesito da porção inferior do GBVN. O evento de metamorfismo orogênico esteve associado a três eventos deformacionais. Os eventos D1 e D2 formaram a xistosidade (S1), preservada como dobras intrafoliais (F2) e a clivagem de crenulação (S2), originadas a partir de movimentos de cavalgamento. O pico metamórfico (M1) está marcado pela assembleia plagioclásio+hornblenda+granada e, plagioclásio+hornblenda+diopsídio, indicando temperaturas entre 450 e 650 °C e pressão entre 4 e 6 kbares. Os metabasitos compreendem Fe-toleítos e Mg-toleítos com afinidade komatitica, composições geoquímicas enriquecidas em LILE e ETR e empobrecidas em HFSE (com anomalias negativas de Nb, Ti e P) e padrões de ETR semelhantes ao MORB. As características observadas indicam um magmatismo toleítico relacionado à bacias de retro-arco e arco de ilhas de 2.15 Ga no Escudo das Guianas. / The significance of the age of magmatism, magma series and tectonic settings that controls the origin of metavolcanic rocks of Vila Nova Greenstone Belt (VNGB) is an important issue in order to understand the evolution of Guiana Shield in Paleoproterozoic times. This work presents new U-Pb LA-ICP-MS geochronological and geochemical analyzes carried out on zircon grains and metavolcanic rocks of the Vila Nova greenstone belt (VNGB) which were supplemented by petrography, and stratigraphic and structural data acquired through description of boreholes and field work. In the Vila Nova region, the VNGB units rest on the Archaean basement composed of orthogneisses of the Tumucumaque Complex. The lower portion of the VNGB is composed of metabasalts and andesitic metabasalts, supported by an upper metasedimentary domain with exhalative chemical rocks and subordinate metavolcanic rocks. The metavolcanic rocks include amphibolites and amphibole schists, whose bodies are elongated according to the regional NW-SE schistosity. U-Pb zircon geochronology data showed an age of 2154 ±6 Ma for a meta-andesite of the lower portion of VNGB. Orogenic metamorphism event followed by three deformation events were recognized. The D1 and D2 events formed the schistosity (S1), preserved as intrafolial folds (F2) and the crenulation cleavage (S2), derived from thrust movements. The assemblages plagioclase + hornblende + garnet and plagioclase + hornblende + diopside define the metamorphic peak (M1-M2) with temperatures from 450-650 °C and lithostatic pressure between 4 and 6 kbars. The metabasites comprise Fe-tholeiites and Mg-tholeiites with komatiitic affinity, geochemical compositions enriched in LILE and REE and depleted in HFSE (with negative Nb, Ti and P anomalies) and MORB-like REE patterns. The observed features indicate an expressive magmatism related to back-arc basins and island arcs at 2.15 Ga in Guiana Shield.

Two experimental seismic surveys were collected in the 1991 LITHOPROBE Trans-Hudson Orogen (THO) data acquisition program. The purpose of the coincident dynamite and vibroseis reflection surveys was to compare crustal images obtained using high-fold low-energy and low-fold high-energy sources. On single-fold field records, signal amplitudes from explosive sources are consistently 50 dB higher than on the corresponding vibroseis records. The vibroseis final stack exhibits better defined upper-crustal reflectivity due primarily to the higher fold. However, at lower-crustal and Moho levels, the dynamite data provides images which are equal or superior to those obtained from the vibroseis data. The dynamite source not only allowed deeper signal penetration but also succeeded in mapping of a number of subcrustal reflections not identified in previous vibroseis data. These new seismic images indicate that the crustal root is not simple depression on the upper-mantle as was inferred initially but a broad (3 s) zone of reflectivity that dips west and extends more than 10 km below the younger regional Moho. Moreover, the dynamite data also indicates that diffraction patterns, detected at lower crustal and Moho depths, have large apertures which permitted proper migration of these lower crustal events. Four vibroseis expanding spread profiles (ESP) were also acquired during the data acquisition program to obtain more detailed and accurate velocity structure. These profiles, with a maximum offset of 18 km, were centered on areas where prominent crustal reflectivity was detected by the regional vibroseis survey. The small source stepout distance (100 m) generated high-fold ($>$30) data. Extensive modeling was carried out to estimate the offset range within which each traveltime approximation and velocity analysis technique may be implemented. The results reveal that velocity estimation becomes more robust and accurate when crustal seismic surveys utilize longer offsets than commonly used. These larger source-receiver separations, however, must be generally limited to offset/depth ratios not exceeding 1.5 when conventional velocity analysis techniques, based on the hyperbolic moveout assumptions, are implemented. Besides the semblance method two velocity estimators adapted to crustal studies, namely the covariance and the $\tau$ - p techniques, were tried. The former yielded the highest resolution followed by the semblance and the $\tau$ - p methods. Resolution of the semblance estimator for a maximum offset of 36 km is equal to that of the covariance method with a corresponding offset of 18 km for mid-crustal reflectors. The advantages provided by the long-offset data acquisition include increased S/N ratio and a greater number of traces with sufficiently large moveouts whichimproved velocity resolution, especially below mid-crustal depths. To achieve similar advantages in a regional crustal reflection survey would require the adoption of longer spread lengths than those presently implemented in standard data acquisition procedures.