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Abstract The Ediacaran-Cambrian transition documents a critical stage in the diversification of animals. The global fossil record documents the appearance of cloudinomorphs and other shelled tubular organisms followed by non-biomineralized small carbonaceous fossils and by the highly diversified small shelly fossils between ~ 550 and 530 Ma. Here, we report diverse microfossils in thin sections and hand samples from the Ediacaran Bocaina Formation, Brazil, separated into five descriptive categories: elongate solid structures (ES); elongate filled structures (EF); two types of equidimensional structures (EQ 1 and 2) and elongate hollow structures with coiled ends (CE). These specimens, interpreted as diversified candidate metazoans, predate the latest Ediacaran biomineralized index macrofossils of the Cloudina-Corumbella-Namacalathus biozone in the overlying Tamengo Formation. Our new carbonate U–Pb ages for the Bocaina Formation, position this novel fossil record at 571 ± 9 Ma (weighted mean age). Thus, our data point to diversification of metazoans, including biomineralized specimens reminiscent of sections of cloudinids, protoconodonts, anabaritids, and hyolithids, in addition to organo-phosphatic surficial coverings of animals, demonstrably earlier than the record of the earliest known skeletonized metazoan fossils.

Zinc isotopes ratios provide qualitative and quantitative information about contributions from different Zn sources in the environment. Here we present the results of a preliminary study to identify the main sources controlling Zn isotope signatures of sediments from Lake Paranoá (Brazil), an urban water reservoir. The Zn isotope ratios were measured by MC-ICP-MS and expressed as δ66/64Zn notation relative to the JMC 3-0749-L solution. The studied sediments showed a narrow isotopic range (+0.00 to +0.13 ‰; n = 10) similar to that reported for intensely weathered soils worldwide (from -0.4 to +0.2‰). The geoaccumulation index (Igeo) also suggests low or negligible influence of Zn anthropogenic source. This research provides a baseline assessment of the Zn isotopes signatures in lake sedimentary environments under the influence of these weathered soils.

The Ediacaran-Cambrian transition is marked by the diversification of metazoans in the marine realm. However, this is not recorded by the Ediacaran-Cambrian Bambuí Group of the São Francisco basin, Brazil. Containing the sedimentary record of a partially confined foreland basin system, the Bambuí strata bear rare metazoan remnants and a major carbon isotope positive excursion decoupled from the global record. This has been explained by changes in the paleogeography of the basin, which became a restricted epicontinental sea in the core of the Gondwana supercontinent, promoting episodes of shallow water anoxia. Here, we report new δ15Nbulk data from the two lowermost second-order transgressive-regressive sequences of the Bambuí Group. The results show a rise of δ15N values from +2 to +5‰ in the transgressive system tract of the basal sequence, which was deposited when the basin was connected to other marginal seas. Such excursion is interpreted as an oxygenation event in the Bambuí sea. Above, in the regressive systems tract, δ15N values vary from +2 to +5‰, pointing to instabilities in the N-cyle that are concomitant with the onset of basin restrictions, higher sedimentary supply/accommodation ratios, and the episodic anoxia. In the transgressive systems tract, the δ15N values stabilise at ∼+3.5‰, pointing to the establishment of an appreciable nitrate pool in shallow waters in spite of the basin full restriction as marked by the onset of a positive carbon isotope excursion. In sum, our data show that the N-cycle and its fluctuations were associated with variations in sedimentary supply/accommodation ratios induced by tectonically-related paleogeographic changes. The instability of the N-cycle and redox conditions plus the scarcity of nitrate along regression episodes might have hindered the development of early benthic metazoans within the Bambuí seawater and probably within other epicontinental seas during the late Ediacaran-Cambrian transition.

Urban air pollution is a matter of concern due to its health hazards and the continuous population growth exposed to it at different urban areas worldwide. Nowadays, more than 55% of the world population live in urban areas. One of the main challenges to guide pollution control policies is related to pollutant source assessment. In this line, U.S. Environmental Protection Agency's Positive Matrix Factorization (EPA-PMF) has been extensively employed worldwide as a reference model for quantification of source contributions. However, EPA-PMF presents issues associated to source identification and discrimination due to the collinearities among the source tracers. Multi-Isotopic Fingerprints (MIF) have demonstrated good resolution for source discrimination, since urban sources are characterized by specific isotopic signatures. Source quantification based on total aerosol mass is the main limitation of MIF. This study reports strategies for PMF and MIF combination to improve source identification/discrimination and its quantification in urban areas. We have three main findings: (1) cross-validation of PMF source identification based on Pb and Zn isotopic fingerprints, (2) source apportionment in the MIF model for total PM mass, and (3) new insights into potential Zn isotopic signatures of biomass burning and secondary aerosol. We support future studies on the improvement of isotopic fingerprints database of sources based on diverse elements or compounds to boost advances of MIF model applications in atmospheric sciences.

Abstract The appearance of complex megascopic multicellular eukaryotes in the Ediacaran occurred just when the dynamics of a cooling Earth allowed establishment of a new style of global tectonics that continues to the present as “modern-type plate tectonics”. The advent of this style was first registered in 620 Ma-old coesite-bearing Ultra-High Pressure eclogites within the Transbrasiliano-Kandi mega-shear zone along the site of the West Gondwana Orogeny (WGO). These eclogites comprise the oldest evidence of slab-pull deep subduction capable of inducing continental collisions and producing high-relief Himalayan-type mega-mountains. Life, prior to this time, was essentially microscopic. Yet with increasing Neoproterozoic oxygenation and intensified influx of nutrients to Ediacaran oceans, resulting from the erosion of these mountains, complex macroscopic heterotrophic eukaryotes arose and diversified, taking the biosphere to a new evolutionary threshold. The repeated elevation of Himalayan-type mega-mountains ever since then has continued to play a fundamental role in nutrient supply and biosphere evolution. Other authors have alluded to the influence of Gondwana mountain-building upon Ediacaran evolution, however we claim here to have identified when and where it began.

ABSTRACT: High-resolution chemostratigraphic data of carbonates from the Sete Lagoas Formation (Bambuí Group) show large variations on the C, O, and Sr isotope compositions. Impure limestones at the base show primal δ13C values between -1.0 and 0‰, and δ18O values between -12.0 and -8.0‰. However, some dolostones demonstrate δ13C values varying from +2.8 to -6.8‰, highly radiogenic 87Sr/86Sr ratios (>0.7111), and low Sr concentrations (900 ppm). They are linked to depositional controls on the carbonate platform, such as fluvial and/or submarine water influx, in which carbonates deposited on the proximal sector exhibit significant Sr isotopic variations and those on the distal sector were not subject to such controls, resulting in very homogeneous Sr isotope profiles. However, 87Sr/86Sr ratios of the distal carbonates are less radiogenic than carbonates expected for late Ediacaran (~0.7085). This discrepancy suggests a restricted marine basin without Sr isotopic homogenization with contemporary oceans and, in this case, global correlations based on Sr isotope stratigraphy are not reliable.

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.

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.

Lead analyses in bottom sediments from the hydrographic system of Belem (Para) indicated low contents of this metal for the sediments from the Guama river, with no significant anthropogenic contribution. A concentration of 18.1 ± 1.5 mg kg-1 and 206Pb/207Pb isotopic signature of 1.196 ± 0.002 are assigned for Pb from natural sources. On the other hand, the significant increase of Pb contents in the sediments from the Guajará bay, together with the decrease of 206Pb/207Pb ratios (1.172 < 206Pb/207Pb < 1.188) point to an anthropogenic lead contribution, originated by the industrial and urban activities of the city of Belem.

This study examines the infl uence of pedogenesis on the distribution of rare earth elements in soils derived fromdifferent rock types and formed under tropical humid climate, as well as the possible contribution of airborne Pb to thesesoils. We studied 5 soil profi les developed from different volcanic rocks cropping out in the Fernando de Noronha island.Results show that in the course of weathering, the soils were enriched in REE. The REE patterns of the soils are similarto those of the parent material, except for a slight HREE enrichment. Lead-isotope data indicate the presence of a nonradiogenicanthropogenic component in the upper horizons of the soil profi les.

This study presents the fi rst results of the sample analysis of two boreholes drilled in a quarry located in the Arcos region(MG). Petrographic analysis (macro- and microscopic observations) was performed and made possible to study in detailan approximately 175 meter-section, which shows the basement and 10 sedimentary units. The basement is represented bydark green to gray fi ne- to medium-grained granodiorite. At the base of the sedimentary sequence, Unit 1 is representedby a decimetric layer of polymictic diamictite and is overlain by the impure limestones of Unit 2. Unit 3 is carbonatic andcontains calcilutite and layers of carbonaceous shale. This unit gradually changes to Unit 4, which is composed of marland mudstone. Unit 5 is composed of a thick sequence of massive calcarenites, which begins to show lamellar layers andmicrobial structures in Unit 6. Unit 7 is similar to the previous one, but contains several intraclastic layers. The numberof lamellar layers decreases sharply, giving rise to a thick interval with considerable granulometric variations in Unit 8, which grades to the oolitic calcarenite of Unit 9. At the top of the sequence, Unit 10 is a thick package of stromatolitic dolarenite. Thisstratigraphic arrangement allows the identifi cation of progradational/retrogradational trends. The sedimentological features, especially those present in the basal portion, suggest that at least part of the carbonatic sequence could represent a cap carbonate, supporting a glacial origin for the diamictite. Other data also collected from the basal units suggest that changes occurred in the source area at the beginning of the fi lling of the basin.

The phosphatic soils found in the northern part of the Rata island, in the Fernando de Noronha Archipelago, wereformed by reaction of bird guano with weathered mafi c rocks of the Quixaba Formation. Phosphate minerals identifi edinclude crandallite as a major constituent. The unique guano signature preserved in soil is characterized by high levelsof Cu, Pb, Zn, As, U, and Sr. On the other hand, the inheritance of the geochemical signature of the nepheline-basalts isdemonstrated by the anomalous concentrations of Ba, Nb, Ta, Cr, Hf, V and Zr in soils, and by the remarkable similaritybetween REE patterns in rock and soils.

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).

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.

Este trabalho apresenta novos dados geocronológicos U-Pb SHRIMP em zircões e Sm-Nd em rocha total do Granito doMoinho, que ocorre nas imediações da cidade de Nazaré Paulista, Estado de São Paulo. Este granito apresenta característicasin- a tardi-tectônica em relação à Orogenia Brasiliana e é intrusivo nas seqüências mesoproterozóicas do Grupo Serra doItaberaba, no segmento central da Faixa de Dobramentos Ribeira. Adicionalmente, foram reavaliados dados isotópicos de Pbobtidos em amostras de rocha total e feldspatos potássicos disponíveis na literatura. A idade U-Pb SHRIMP obtida emmonocristais 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 derochas pré-existentes, derivadas do manto superior no Paleoproterozóico. As composições isotópicas de Pb das amostras doGranito do Moinho indicam, a exemplo dos isótopos de Nd, uma fonte também crustal e com participação de rochas doGrupo Serra do Itaberaba, de idade mesoproterozóica.

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.

Este estudo focaliza a influência da pedogênese no comportamento dos elementos terras raras em solos derivados de diferentes tipos litológicos que se alteraram sob as mesmas condições de clima tropical úmido. Investiga também a possível contribuição do Pb aerotransportado a esses solos. Foram estudados 5 perfis de solo desenvolvidos sobre diferentes rochas vulcânicas aflorantes na ilha de Fernando de Noronha. Os resultados mostram que, de modo geral, o intemperismo concentra os ETR, mantendo aproximadamente o mesmo padrão de distribuição da rocha parental, com ligeiro enriquecimento preferencial dos ETRP. Os dados isotópicos de Pb indicaram a presença de um componente antrópico não radiogênico nos horizontes superiores dos solos.

Apresenta-se o resultado da investigação de dois furos de sondagem realizados em uma mina na região de Arcos (MG). Foram realizados estudos de petrografia macroscópica e microscópica, que permitiram o detalhamento de uma seção de aproximadamente 175 m, a qual contempla o embasamento cristalino e 10 unidades sedimentares. O embasamento é representado por granodiorito verde-escuro a cinza, de granulação fina a média, inequigranular. Na base da coluna sedimentar, a Unidade 1 representa uma camada decimétrica de diamictito polimítico, sobre o qual se assentam os calcários impuros da Unidade 2. A Unidade 3 é composta por calcilutito e eventuais camadas de folhelho carbonoso, apresentando gradação para a Unidade 4, composta por marga e argilito. A Unidade 5 corresponde a uma espessa sequência de calcarenito claro, maciço e estilolítico, que passa a apresentar estratos lamelares e estruturas microbianas na Unidade 6. A Unidade 7 é semelhante à anterior, porém com diversas camadas intraclásticos. A Unidade 8 representa um espesso intervalo com frequentes variações granulométricas que gradam para o calcarenito oolítico da Unidade 9. Capeando a sequência, a Unidade 10 é um espesso pacote de dolarenito estromatolítico. O arranjo estratigráfico observado permite a identificação de tendências progradacionais/retrogradacionais coerentes com o contexto regional da bacia. As características sedimentológicas presentes, principalmente na porção basal, sugerem que pelo menos parte do pacote carbonático possa representar um cap carbonate, apontando no sentido de uma origem glacial para o diamictito. Ainda nas unidades inferiores, os dados sugerem modificações na área fonte no início do preenchimento da bacia.

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.

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).

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.

ABSTRACT: We present new Sm-Nd, U-Pb and Hf isotope geochronological data for the siliciclastic rocks in the southern sector of the São Francisco Basin. An abrupt change in the Sm-Nd data is observed from the Carrancas Formation’s oligomitic conglomerates (TDM ages between 2.7 and 3.3 Ga; εNd(550 Ma) values between -25.09 and -17.98), its finer facies, and the finer facies of the the Moema Laminite (TDM ages from 1.4 to 2.1 Ga; εNd(550 Ma) values between -9.46 and -5.59). No further significant changes in the Sm-Nd data occur farther upwards in the Bambuí Group (Sm-Nd TDM ages within the 1.3-2.0 Ga interval; εNd(550 Ma) values from -9.53 to -4.09), showing a lack of reorganization in the source areas throughout the deposition of the basin. This is yet another argument to dismiss an unconformity within the Bambuí Group. The presence of the index fossil Cloudina sp. in the lower Sete Lagoas Formation makes the glaciation probably Late Ediacaran in age. U-Pb ages for detrital zircons of the Bambuí Group range from the Archean to the early Ediacaran, but the current data is insufficient to distinguish between the contribution from sources in the Brasília and Araçuaí belts.

ABSTRACT: The Bambuí Group, the most extensive carbonate-siliciclastic cover on the São Francisco craton, has been a matter of debate because of its potential correlations to global glacial events. Unfortunately, most available chemostratigraphic data came from samples collected on surface rock exposures, ever susceptible to the aggressive chemical weathering that characterizes the southeastern Brazil. On the other hand, we present here high-resolution chemostratigraphic studies based on C, O and Sr isotopic data from 53 samples collected along a weathering-free, continuous, 175 m thick sedimentary succession. This succession was recovered by borehole drilling in the southwestern São Francisco craton, where occur the Carrancas and Sete Lagoas formations, the lowermost units of the Bambuí Group. The drill cores reveal extremely irregular contacts between the basal diamictite and its basement, an Archaean foliated granodiorite. Geochronological and sedimentological data strongly suggest that the diamictite represents a lodgement till. This glaciogenic deposit is covered by a limestone succession which starts with impure carbonates showing aragonite pseudomorph fans and thin bands of black shale. The limestone pile grades to a marl-mudstone interval, which turns to a carbonate with biological components, succeeded by stromatolitic dolomite at the top. C and O isotopic signatures (referred to V-PDB) allow to the subdivision of the lower carbonate-pelite section into three intervals. The first isotopic interval corresponds to a cap carbonate, and displays negative values of δ13C (c . -4‰), and a large oscillation of the δ18O (-6 to -15‰). The Interval II shows a striking homogeneity in δ13C and δ18O, around 1‰ and -7‰, respectively. At the top, Interval III shows a large positive excursion of the δ13C (up to 8‰) and δ18O (-8 to -3‰) values. Unaltered 86Sr/87Sr ratios range from 0.7075 to 0.7077, mainly at the top of the section. The geochemistry of the carbonates is controlled by their terrigenous content (mostly quartz and clay minerals) which is concentrated in the lower units. Samples free of terrigenous contamination show Y/Ho ratios ranging from 25 to 50, suggesting a freshwater input during carbonate deposition. It is concluded that the diamictite has a glaciogenic origin and is covered by a cap carbonate. This pair has been identified along the basin and is related to one of the main Neoproterozoic glaciations. Discrepancy between the 86Sr/87Sr values and the global variation curves can be related to freshwater input during the carbonate deposition. Based on the regional tectonic context, the Bambuí Basin may have been a restricted marine basin, totally or partially surrounded by mountain ranges within Gondwana, in the Neoproterozoic/Paleozoic boundary. In its early stages, the sedimentation was influenced by a global glacial event, whose melting phase was responsible by freshwater input in the basin. The gradual rise of the temperature was followed by an increase of the biological activity. Finally, a sudden increase in the biological activity could have been driven by paleogeographic changes caused by the active tectonic.

ABSTRACT: The first results of Pb isotope compositions of the high-Ti Mesozoic dykes of the Southern Espinhaço are presented. The results do not show large variations and are significantly more radiogenic than the Pb isotope compositions of the high-Ti tholeiites from the Paraná Continental Flood Basalts. The data combined with published geochemical and Sr-Nd isotope results rule out crustal contamination processes in the genesis of the dykes, requiring magma generation in metasomatized subcontinental lithospheric mantle with the involvement of HIMU-type and carbonatite components. The magmas may have been also derived from a mantle source containing ~4 - 5% of pyroxenite and ~1% of carbonatite melts, agreeing with published Os isotope compositions of high-Ti rocks from the Paraná Continental Flood Basalts. These metasomatizing agents could be responsible for mantle source refertilization, as was also proposed in the literature to explain the characteristics of xenoliths of the Goiás Alkaline Province, which also occurs in the border of the São Francisco Craton. Additionally, to evaluate the risks of Pb contamination during sample preparation for analysis, several experimental tests were accomplished, which indicate the need of sawed surface removal and a careful washing of small-sized rock fragments before powdering, especially for rocks with [Pb] < 7 µg/g.