79 research outputs found

    50 anos da Geologia Isotópica na América do Sul: Destaques do 9º Simpósio-Sul Americano de Geologia Isotópica

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    On April 2014, over 250 members of the South America Earth Sciences community gathered at the Universidade de São Paulo, Brazil, for the 9th South American Symposium on Isotope Geology – 9th SSAGI. The symposium is a traditional scientific meeting in South America and it has been organized since 1997 in different countries of our continent, including Brazil, Argentina, Chile, Uruguay and Colombia. The motivation to hold the 9th SSAGI in São Paulo was to celebrate the 50th anniversary of the Geochronological Research Center (CPGeo).Centro de Investigaciones Geológica

    50 anos da Geologia Isotópica na América do Sul: Destaques do 9º Simpósio-Sul Americano de Geologia Isotópica

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    On April 2014, over 250 members of the South America Earth Sciences community gathered at the Universidade de São Paulo, Brazil, for the 9th South American Symposium on Isotope Geology – 9th SSAGI. The symposium is a traditional scientific meeting in South America and it has been organized since 1997 in different countries of our continent, including Brazil, Argentina, Chile, Uruguay and Colombia. The motivation to hold the 9th SSAGI in São Paulo was to celebrate the 50th anniversary of the Geochronological Research Center (CPGeo).Centro de Investigaciones Geológica

    A idade e natureza da Fonte do Granito do Moinho, Faixa Ribeira, Sudeste do Estado de São Paulo

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    Este trabalho apresenta novos dados geocronológicos U-Pb SHRIMP em zircões e Sm-Nd em rocha total do Granito do Moinho, que ocorre nas imediações da cidade de Nazaré Paulista, Estado de São Paulo. Este granito apresenta característica sin- a tardi-tectônica em relação à Orogenia Brasiliana e é intrusivo nas seqüências mesoproterozóicas do Grupo Serra do Itaberaba, no segmento central da Faixa de Dobramentos Ribeira. Adicionalmente, foram reavaliados dados isotópicos de Pb obtidos em amostras de rocha total e feldspatos potássicos disponíveis na literatura. A idade U-Pb SHRIMP obtida em monocristais de zircões ígneos do granito foi de 620 ± 11 Ma, sendo interpretada como a época de cristalização desta rocha. As idades modelo Sm-Nd obtidas situaram-se entre 2,0 e 1,9 Ga, mostrando que o magma foi produzido por fusão parcial de rochas pré-existentes, derivadas do manto superior no Paleoproterozóico. As composições isotópicas de Pb das amostras do Granito do Moinho indicam, a exemplo dos isótopos de Nd, uma fonte também crustal e com participação de rochas do Grupo Serra do Itaberaba, de idade mesoproterozóica.U-Pb SHRIMP zircon ages and Sm-Nd isotopic analyses were carried out on the syn- to late tectonic Moinho Granite, intruded within the Mesoproterozoic Serra do Itaberaba Group. Six U-Pb SHRIMP analyses were undertaken on oscillatory zoned zircons. All yielded close to concordant Neoproterozoic ages, with a 206Pb/238U weighted mean age of 620 ± 11Ma, interpreted as the time of the granite crystallization. Sm-Nd isotopic analyses on samples from the Moinho Granite presented Paleoproterozoic depleted-mantle model ages between 2.0 and 1.9 Ga, and negative µNd values, indicating continental sources for the granitic magma. Previous Pb isotopic data suggest that the Mesoproterozoic Serra do Itaberaba metasedimentary sequence contributed to the magma genesis of the studied granitoid

    Analytical procedures for determining Pb and Sr isotopic compositions in water samples by ID-TIMS

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    Few articles deal with lead and strontium isotopic analysis of water samples. The aim of this study was to define the chemical procedures for Pb and Sr isotopic analyses of groundwater samples from an urban sedimentary aquifer. Thirty lead and fourteen strontium isotopic analyses were performed to test different analytical procedures. Pb and Sr isotopic ratios as well as Sr concentration did not vary using different chemical procedures. However, the Pb concentrations were very dependent on the different procedures. Therefore, the choice of the best analytical procedure was based on the Pb results, which indicated a higher reproducibility from samples that had been filtered and acidified before the evaporation, had their residues totally dissolved, and were purified by ion chromatography using the Biorad® column. Our results showed no changes in Pb ratios with the storage time.Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP

    Petrotectonic implications of metabasites of the Eastern Andean Metamorphic Complex at Lago O´Higgins-San Martin, southern Patagonia

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    The Eastern Andean Metamorphic Complex (EAMC) in southwestern Patagonia (4°-52°S) is a 450 km long belt mainly composed by low-grade metasedimentary rocks of Upper Devonian-lower Carboniferous, and Permian-lower Triassic ages. Previous works have suggested a passive margin environment for the deposition of the protolith. The EAMC comprise scarce interleaved tectonic slices of marbles, metabasites, and exceptional serpentinite bodies. At Lago O´Higgins-San Martin (48°30?S-49°00?S) the metasedimentary sucessions are tectonically juxtaposed with lenses of pillowed metabasalts and greenschists having OIB, N-MORB, BABB and IAT geochemical affinities. The Nd-isotopic composition of metabasalts is characterized by εNd(t=350 Ma) of +6 and +7. The metabasalts show no signal of crustal contamination, instead, the mantle source was probably modified by subduction components. New and already published provenance data based on mineralogy, geochemistry and zircon geochronology indicate that the quartz-rich protolith of metasandstones were deposited during late Devonian-early Carboniferous times (youngest single zircon ages around of latest Devonian-earliest Carboniferous times) sourced from igneous and/or sedimentary rocks located in the interior of Gondwana, as the Deseado Massif, for instance. Noticeable, the detrital age patterns of all samples reveal a prominent population of late Neoproterozoic zircons, probably directly derived from igneous and/or metaigneous rocks of the Brasiliano/Pan-African orogen or from reworked material from variably metamorphosed sedimentary units that crops out at the same latitudes in the extra-Andean region of Patagonia. We propose that the protolith of metabasites formed part of the upper part of an oceanic-like lithosphere generated in a marginal basin above a supra-subduction zone, where plume-related oceanic island volcanoes were generated. The closure of the marginal basin, probably in mid-Carboniferous times, or soon after. The oceanic lithosphere was likely underthrusted within an east-to-northeast-dipping subduction zone, where ophiolitic rocks and metasedimentary sequences were tectonically interleaved at the base of an accretionary wedge.Fil: Rojo, Diego. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Estudios Andinos "Don Pablo Groeber". Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Estudios Andinos "Don Pablo Groeber"; Argentina. Universidad Arturo Prat (unap);Fil: Calderón, Mauricio. Universidad Andrés Bello; ChileFil: Ghiglione, Matias. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Estudios Andinos "Don Pablo Groeber". Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Estudios Andinos "Don Pablo Groeber"; ArgentinaFil: Suárez, Rodrigo Javier. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Estudios Andinos "Don Pablo Groeber". Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Estudios Andinos "Don Pablo Groeber"; ArgentinaFil: Quezada, Paul. Universidade Federal do Paraná; BrasilFil: Hervé, Francisco. Universidad Andrés Bello; Chile. Universidad de Santiago de Chile. Dto de Geología; ChileFil: Babinski, Marly. Universidade de Sao Paulo; BrasilFil: Fanning, Mark. Australian National University; AustraliaEuropean Geosciences Union General Assembly 2021AlemaniaEuropean Geosciences Unio

    The Gorutuba Formation: coastal to continental sedimentation on the eastern margin of Bambuí Basin (MG)

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    Propõe-se neste trabalho a formalização da Formação Gorutuba, unidade que engloba os únicos registros remanescentes de ambientes litorâneos e continentais da margem leste da Bacia Bambuí, desenvolvida durante a Orogênese Brasiliana. As rochas da Formação Gorutuba são reconhecíveis em três áreas principais. Nas serras da Jaíba e Colonial, a unidade apresenta um pacote basal de brecha composta por clastos de calcário imersos em matriz arcoseana, que indicam o retrabalhamento do calcário da Formação Jaíba, subjacente. Arcósio ocorre na forma de lentes e camadas intercaladas na brecha, tornando-se predominante em direção ao topo da unidade, onde ocorrem corpos isolados de ortoconglomerado. Na Serra do Gorutuba, a unidade apresenta o mesmo arranjo estratigráfico, porém assenta-se diretamente sobre siltito da Formação Serra da Saudade. No Sinclinal de Buenópolis, por sua vez, arenitos arcoseanos com estratificações cruzadas acanaladas e eventuais lentes de conglomerado e pelito assentam-se sobre siltito da Formação Serra da Saudade, não ocorrendo a brecha basal. A Formação Gorutuba registra um período de soerguimento, exposição e erosão da extensa plataforma marinha representada pelo Grupo Bambuí. A brecha basal, que ocorre nas áreas mais a norte, registra importante discordância erosiva, associada ao retrabalhamento por ondas da plataforma carbonática Jaíba, cujo soerguimento se relaciona à intensa atividade tectônica do período. O aumento da contribuição siliciclástica para o topo está associado à progressiva instalação de um sistema fluvial de alta energia, que marca a fase de colmatação da bacia. A composição dos sedimentos siliciclásticos presentes na matriz da brecha e no restante da unidade sugere a exumação de fontes graníticas no Orógeno Araçuaí.In this study, we formally introduce the Gorutuba Formation, unit that encompasses the remaining coastal and continental records of the eastern margin of the Bambuí Basin, evolved during the Brasiliano/Pan-Africano Orogeny. In Minas Gerais, the Gorutuba Formation occurs in three main localities. In Jaíba and Colonial ridges, the unit shows a basal layer of breccia composed by clasts of limestone in arcosean matrix, recording the erosion and reworking of the underlying Jaíba Formation. Arcose occurs as layers and lenses within the breccia, and becomes the main rock to the top, where shows some lenses of ortoconglomerate. In Gorutuba and Rompe Gibão ridges, the unit shows the same stratigraphic pattern, but lies on the Serra da Saudade siltstones. In Buenópolis Sincline, on the other hand, arcosean sandstone with through cross stratification and conglomerate lenses lie on siltstones from Serra da Saudade Formation, lacking the basal breccia. The Gorutuba Formation records an uplift, exposure and erosion stage of the wide Bambuí marine platform. The basal breccia, which occurs in the northernmost areas, records an important unconformity, where the Jaíba carbonatic shelf was reworked by waves in response to the tectonic driven uplift. The increase of the sandstones to the top is associated to the gradual installation of a high-energy fluvial system, marking the clogging stage of the basin. The composition of the sandstones and the breccia’s matrix suggests the exhumation of granitic sources in the Araçuaí Orogen in this period

    Nova unidade litoestratigráfica registra glaciação ediacarana em Mato Grosso: formação Serra Azul

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    A new succession of diamictites and siltstones was found above post-Marinoan carbonates from the Araras Group in the northern Paraguay Belt (Brazil), in discontinuous outcrops. This new stratigraphic unit, named Serra Azul Formation in this work, has a variable thickness reaching up to 300 meters. The diamictites were deposited in the first 70 m and present glacial evidences, while the siltstones represent the upper part of the succession and show some sandstone layers towards the top of the succession. This glacial record is the youngest Neoproterozoic glacial event that has been found on South America and probably is related to Gaskiers Glaciation (582 Ma).Uma nova sucessão de diamictitos e siltitos foi encontrada acima dos carbonatos pós-Marinoanos do Grupo Araras, na porção norte da Faixa Paraguai, em afloramentos descontínuos. Esta nova unidade estratigráfica é denominada Formação Serra Azul neste trabalho, possui espessura muito variável, de 60 a 300 m. Na seção tipo, os diamictitos foram depositados nos primeiros 70 m e possuem evidências glaciais, como clastos polimíticos facetados e estriados, enquanto que os siltitos ocorrem ao longo do restante da sucessão, sendo intercalados por camadas de arenito no topo. Este registro glacial é o primeiro encontrado na América do Sul que pode estar relacionado à Glaciação Gaskierana (582 Ma)

    Proveniência e análise sedimentar da porção basal do Grupo Bambuí em Arcos (MG)

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    A origem e evolução da Bacia Bambuí é tema de grandes debates, em muito intensificados pela recente corrida exploratória de empresas públicas e privadas em busca de hidrocarbonetos gasosos. Na porção sudoeste da bacia, são raras as oportunidades de se observar o contato basal das rochas sedimentares com seu embasamento, cujas características são cruciais para o entendimento dos processos de instalação da bacia e do início de seu preenchimento. Por meio de testemunhos de sondagem, teve-se acesso a toda a porção basal do Grupo Bambuí na região de Arcos (MG), incluindo seu embasamento. A seção amostrada exibe como embasamento um granodiorito arqueano (c. 2,8 - 2,9 Ga), fortemente fraturado à época da sedimentação. A unidade basal do Grupo Bambuí é um tilito de alojamento maciço, polimítico, de espessura decimétrica. Grãos de zircão separados da matriz do tilito foram datados e exibem um pico principal de idades em c. 2,8 Ga, indicando proveniência do próprio embasamento. Sobre o tilito repousam calcários impuros, que passam gradativamente para um pacote pelítico, em tendência de preenchimento retrogradacional. Fragmentos terrígenos no calcário impuro sugerem que o embasamento tenha continuado a atuar como fonte durante a sedimentação dos metros iniciais da seção. As rochas pelíticas que ocorrem para o topo têm assinatura litoquímica compatível com fontes ácidas, e possuem idades-modelo Sm-Nd de 1,7 Ga. O acervo de dados sugere que rochas da Faixa Brasília tenham atuado como fonte para as rochas pelíticas, corroborando a interpretação de um caráter foreland para a bacia.The origin and evolution of Bambuí Basin has been a matter of debate, in much intensified by the recent exploratory efforts carried out by public and private companies looking for natural gas. In the southeastern portion of the basin there are rare opportunities to access the contact between the sedimentary rocks and its basement, whose characteristics are crucial to understanding the processes of basin installation and the early sedimentation. The analysis of drill cores allowed us to describe the lowermost Bambuí Group in Arcos (MG) region, including its basement. The sampled section displays as basement an archean granodiorite (ca. 2.8 - 2.9 Ga), fractured at the time of sedimentation. The Bambuí Group basal unit is dm-thick, massive lodgment tillite. Grains of zircon separated from the tillite matrix were dated and show a main age peak at ca. 2.8 Ga, indicating provenance from the own basement. On the tillite rests an impure limestone that passes gradually to a muddy unit, in a retrogradational filling trend. Terrigenous fragments in the impure limestone suggests that the basement has continued to be a source of sediments in the bottom of the section. The pelitic rocks that occurs to the top has a litochemical signature compatible with acidic source rocks, and shows Sm-Nd model ages of 1.7 Ga. These data suggests that rocks from the Brasília Belt have acted as a source for the pelitic rocks, supporting the interpretation of a foreland set for the basin

    Petrology and geochemical characterization of the mantle \ud sources in the northwest region of the Paraná continental \ud flood basalt province

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    The Paraná continental flood basalt (PCFB) province, one of the largest continental provinces preserved on Earth, has been the focus of many studies and is still a matter of debate. The PCFB covers more than 70% of the Paraná Basin as flows, sills, and dykes. The mineralogy of basalt is represented by plagioclase (An42-67; 40-55%), pyroxene (augite: W030-40En34-46Fs35-39;19-40%). magnetite (2 -10%), and olivine (F031-5O; <1.5%). Based on rheological calculations the lemperature of crystallization of the pyroxenes is estimated at 11OO°C. From a geochemical point of view, lhe origin \ud of the tholeiitic basalts with low-Ti02 (S 2%; Ribeira) in the northern PCFB province, characterized by the occurrence of basalts with higher concentrations of Ti02 (TiO2>2%) and incompatible trace elements, are here investigated through the deterrnination of Sr, Nd and Pb isotope ratios. The isotope data, along with trace element ratios are used to assess the possible role of the interaction with the continental crust and/or the Sub-Continental Lithospheric Mantle (SCLM). The Ribeira basalts investigated in this study have initial (134 Ma) 87Sr/86Sr, ratios of 0.705341 - 0.705931, 143Nd/144Nd, of \ud 0.512308 - 0.512097, 206Pb/204Pb, of 17.628 -17.810, 207Pb/204Pb, of 15.506 -15.541, and 208Pb/204Pb, of 37.859 - 38.154. These isotopic compositions do not display any correlation with Nb/Th, Nb/La or P2O5/K2O ratios, which also reflect thal these rocks were not significantly aftected by low-pressure crustal contamination. The geochemical composition of the northern PCFB may be explained through lhe involvement of fluids and/or small volume melts related to metasomatic processe

    Geochemical evidence for the formation of soils by interaction between guano and volcanic rocks, Rata Island, Fernando de Noronha (Pernambuco)

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    The phosphatic soils found in the northern part of the Rata island, in the Fernando de Noronha Archipelago, were formed by reaction of bird guano with weathered mafic rocks of the Quixaba Formation. Phosphate minerals identified include crandallite as a major constituent. The unique guano signature preserved in soil is characterized by high levels of Cu, Pb, Zn, As, U, and Sr. On the other hand, the inheritance of the geochemical signature of the nepheline-basalts is demonstrated by the anomalous concentrations of Ba, Nb, Ta, Cr, Hf, V and Zr in soils, and by the remarkable similarity between REE patterns in rock and soils.Os solos fosfáticos encontrados na porção norte da ilha Rata, no Arquipélago de Fernando de Noronha, foram formados pela reação do guano de aves marinhas com as rochas máficas intemperizadas da Formação Quixaba. Os minerais fosfáticos determinados no solo incluem a crandalita como o constituinte mais importante. A assinatura do guano preservada no solo é caracterizada por altos teores de Cu, Pb, Zn, As, U, e Sr. Por outro lado, a herança da assinatura geoquímica dos nefelina basaltos é demonstrada pelas concentrações anômalas de Ba, Nb, Ta, Cr, Hf, V e Zr nos solos e pela notável similaridade entre os padrões de ETR na rocha e nos solos
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