Fundação Carlos Chagas Filho de Amparo a Pesquisa do Estado do Rio de Janeiro / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / A comparação entre o Complexo Juiz de Fora e a Unidade Granulítica Ponte de Zinco (Mangaratiba - RJ) revelou que existem diferenças significativas entre essas unidades. Na Unidade Granulítica Ponte de Zinco, são encontrados ortogranulitos de composição granítica e granodiorítica, que representam o embasamento da Unidade, denominada aqui de Ortogranulitos Ribeirão das Lajes. Dois outros litotipos ortoderivados também foram encontrados: (i) um ortognaisse com granada (MAN-JEF-03a), sendo que a granada ocorre de forma subordinada; (ii) ortognaisse leucocrático (MAN-JEF-04), com características miloníticas. Além dessas, rochas metassedimentares também afloram na Unidade Granulítica Ponte de Zinco. Foi interpretado que um ortogranulito (MAN-JEF-01a) de alto-K, com composição monzogranítica, cristalizou em ca. 2653 37 Ma (U-Pb em zircão por LA-ICPMS), afetado por um evento no Neoproterozóico, que gerou os minerais máficos hidratados, observados na análise petrográfica e como mostram as borda de sobrecrescimento em zircão. Sua idade modelo de Nd de 2,7 Ga e seu εNd positivo de +2,1, apontam para uma gênese mantélica, tendo assimilado rochas crustais, pois são encontrados grãos de zircão herdados de aproximadamente 2996 17 Ma e 3343 3.8 Ma. Os dados de litogeoquímica e sua razão 87Sr/86 Sr(t) (0,70529), são compatíveis com uma geração em um arco continental. O ortognaisses com granada do ponto MAN-JEF-03 possui composição granodiorítica. A idade de cristalização interpretada pela análise geocronológica U-Pb em zircão (LA-ICPMS), foi ca. 2117 15 Ma. Esse litotipo foi metamorfizado no Neoproterozóico, sendo a idade obtida pelo intercepto inferior de 631 40 Ma. Seus dados isotópicos apontam para uma rocha juvenil gerada a partir do manto (TDM ≈ de 2,1 Ga e εNd = +3,4). Sua alta razão 87Sr/86 Sr(t) (0,710 ) juntamente com os grãos de zircão herdados (2,6 Ga) e a presença de enclaves, indicam assimilação de rochas crustais. O ortognaisse leucocrático (MAN-JEF-04) classificado como alto-K, possui composição monzogranítica, idade 2132 9,4 Ma U-Pb em zircão (LA-ICPMS). Um único grão relíquiar de ortopiroxênio encontrado em lâmina, , indica que a rocha já foi submetida a metamorfismo de fácies granulito, porém esse evento não deixou registro nos grãos analisados. O retrometamorfismo pode ter ocorrido em dois momentos, 647 11 Ma e 595 38 Ma, calculados através da concordia age, em sobrecrescimentos homogêneos e, pelo intercepto inferior, respectivamente. Sua baixa razão 87Sr/86 Sr(t) (≈ 0,703) associada com εNd positivo (+2,3) e sua idade modelo de aproximadamente 2,1 Ga, revelam que a rocha foi formada por um material mantélico juvenil. Já as análises geocronológicas em U-Pb em zircão (LA-ICPMS) na região de Juiz de Fora (MG), revelaram a existência de dois litotipos Arqueanos: um ortogranulito granodiorítico (MB-JEF-01b), de baixo-K com idade de 2849 11 Ma e com herança de 2975 10 Ma. Seu εNd positivo (+5,9) aponta para uma gênese a partir do manto depletado, já sua alta razão 87Sr/86Sr(t) (≈0,709) indica contaminação de Rb de fontes externas, talvez causada pela assimilação da crosta, como revelam os zircões herdados e/ou fluidos retrometamorficos. Outro litotipo é uma rocha gabróica do tipo E-MORB, cuja idade foi calculada em 2691 14 Ma, com retrometamorfismo ocorrido no intervalo de 604 67 Ma, obtida pelo intercepto inferior. Seu εNd igual a +3,4 e sua razão 87Sr/86 Sr(t) (≈0,701) mostram extração a partir do manto depletado. Novos dados isotópicos do CJF na região de Três Rios (RJ) e Juiz de Fora (MG), sugerem que os ortogranulitos calcioalcalinos podem representar grupos distintos. Rochas com εNd positivos são consequentemente associadas ao manto depletado, porém rochas com εNd negativos devem ter sido geradas por fusão crustal, que podem ser fusão de crosta inferior, devido a razão 87Sr/86 Sr(t) (0,70514) encontrada no amostra MB-JEF-02a (ortogranulito de alto-K) ou tendo a crosta contribuição nas gênese dessas rochas. Os ortogranulitos básicos possuem εNd positivos com baixas razões 87Sr/86 Sr(t) , o que indica extração a partir do manto depletado, porém sua razões La/YbN e La/NbN maiores que 1, revelam alguma contribuição de uma fonte enriquecida, assim também mostram suas razões Pb/Pb, que são maiores do que as razões calculadas para evolução de Pb na Terra. Essas interpretações ainda podem ser estendidas para um ortoanfibolito da série alcalina, encontrado na região de Três Rios (RJ). / The comparison between the Juiz de Fora Complex JFC and the Ponte de Zinco Granulite Unit, (Mangaratiba RJ) has shown that there are some significant differences between those two units. At the Ponte de Zinco Granulite Unit, one can find ortogranulites with a granitic and granodioritic composition, this is embasement of the area, call here as Ribeirão das Lajes Ortogranulites. Two other ortoderived litotypes have also been found: (i) a orthogneiss with garnet, (MAN-JEF-03a), being that the garnet exists in a subordinated form; (ii) leucocratic orthogneiss, (MAN-JEF-04), with mylonitic characteristics. In addition to the latter, Metasedimentary rocks also spring up in the Unit. It has been interpreted that an organulite (MAN-JEF-01a) of a high-K, with a monzogranitic composition, has crystallized into ca. of 2653 37 Ma (U-Pb into zircon by LA-ICPMS), affected by a Neoproterozoic event generated biotite, hornblende and overgrowth rims on zircon grains. Its model age of Nd of 2,7 Ga and its positive εNd of + 2,1, indicates a genesis from the depleted mantle, having assimilated of the crust, as zircon grains have been found of approximately 2996 17 Ma and 3343 3.8 Ma. The data of litogeochemistry and its ratio 87Sr/86Sr(t) (0,70529), are compatible with a generation in a continental arc. The orthogneiss with garnet (MAN-JEF-03a) has a granodioritic composition. Its crystallizing age, as interpreted by geocronological analysis U-Pb on zircon (LA-ICPMS), was ca. of 2117 15 Ma. This litotype has been metamorphized on the Neoproterozoic, with an age of 631 40 Ma being obtained at the lower intercept. Its isotopic data indicates a young rock generated from the mantle (TDM = de 2,1 Ga e εNd = +3,4), its high ratio 87Sr/86Sr(t) (0,710), together with the inherited zircon grains (2,6 Ga) and the presence of an enclave, indicate the assimilation of crusty rocks. The leucocratic orthogneiss, (MAN-JEF-04) classified as high-K, possesses a monzogranitic composition, age 2132 9,4 Ma U-Pb on zircon (LA-ICPMS). One single grain of orthopyroxene found on lamina, which has been practically consumed by retrometamorphism, which might have happened in two moments, 647 18 Ma and 595 38 Ma, calculated through the age concordia, on homogeneous overgrowth and, by the lower intercept, respectively. Its low ratio 87Sr/86Sr(t) (0,0703) associated to the positive εNd (+2,3) and its model age of approximately 2,1 Ga, reveal that the rock was formed by a young mantle material. Now the geochronological analysis in U-Pb on zircon (LA- ICPMS) in the Juiz de Fora (MG) region, revealed the existence of two litotypes (Arqueanos): one ortogranulite granodioritic (MB-EF-01b), of low-K with an age of 2849 11 Ma and a heritage of 2975 10 Ma. Its e Nd positive (+5,9) indicates a genesis from the depleted mantle, but its high ratio 87Sr/86Sr(t) (0,709) indicates contamination of Rb from external sources, maybe caused by the crust assimilation, as the inherited zircons reveal and/or the retrometamorfic fluids. One other lipotype is a gabbroic of the type E-MORB, its age has been calculated in 2691 14 Ma, with a retrometamorphism that occurred in the interval of 604 67 Ma, obtained by the lower intercept. Its εNd equal to +3,4 and its ratio 87Sr/86Sr(t) (0,701) show the extraction from the depleted mantle. New isotopic data of CJF in the Três Rios (RJ) and Juiz de Fora (MG) regions, suggest that the calcium-alkaline ortogranulites may represent distinctive groups. There are rocks with positive εNd, which are associated to the depleted mantle, however rocks with negative εNd must have been generated by crustal fusion, which can be the fusion of the lower crust, due to the ratio 87Sr/86Sr(t) (0,70514) found in the sample MB-JEF-02a (ortogranulite of high-K) or mixture between the mantle and the crust. The basic ortogranulites possess positive εNd with low ratio 87Sr/86Sr(t), which indicates the extraction from the depleted mantle, nevertheless its ratio La/YbN and La/NbN higher than 1, reveal some contribution from an enriched source, as well as its ratio Pb/Pb, which is bigger than the ones calculated for the evolvement of Pb on Earth. These interpretations can still be extended to an alkaline metabasic, found in the Três Rios (RJ) region.

Unidades litológicas, em particular arenitos, muitas vezes, carecem de um posicionamento cronoestratigráfico preciso. Como os arenitos são importantes rochas-reservatório de hidrocarbonetos e aquíferos, a falta de exatidão nestas informações dificulta a exploração destes bens minerais. A datação relativa de rochas sedimentares pode ser obtida por análise do conteúdo fossilífero ou por correlação estratigráfica. Entretanto, em algumas rochas sedimentares, esta análise não é possível ou tem um caráter duvidoso. Este é o caso da Formação Marizal (Bacia do Recôncavo) que apresenta um histórico controverso sobre a real idade deposicional. A Formação Marizal é um arenito flúvio-eólico cuja idade é discutível e, por isso, sua posição na coluna estratigráfica (aproximadamente Albiniano/Aptiniano), ainda é questionável. Em algumas amostras são encontrados overgrowths de K-feldspatos e nos quais é possível aplicar a técnica de datação 40Ar-39Ar visando obter idades que possam ser relacionadas com processos ocorrentes nestes arenitos (em geral, deposição e/ou diagênese). Entre os minerais pesados existentes nas amostras da Formação Marizal, foram encontrados grãos de zircões. A datação U-Pb de zircões detríticos pode fornecer informações sobre a proveniência desta unidade. Assim, zircões da Formação Marizal foram analisados visando complementar as informações sobre esta unidade, permitindo uma melhor interpretação. Os overgrowths de K-feldspatos indicaram valor de 159.89 ± 23.96 Ma e, para o núcleo detrítico, 432.57 ± 11.89 Ma. O valor médio obtido em torno de 160 Ma, considerando-se que todos os cuidados analíticos e de seleção de amostra foram considerados, é mais antigo do que o esperado. Assim, este valor foi interpretado como indicativo de que o overgrowth teria sido desenvolvido numa rocha fonte sedimentar sendo posteriormente transportado. Esta idade pode ser relacionada a fase pré rifte da Bacia do Recôncavo. O valor confirma ideias existentes de remobilização do substrato da bacia durante a fase rifte. Como tem sido discutido, overgrowths de K-feldspato são estáveis e possíveis de serem transportados por pequenas distâncias, o que corrobora a interpretação acima. Já o valor obtido para o núcleo mostra a contribuição do Paleoprotrozóico adjacente à bacia, retrabalhado no Brasiliano. Em relação ao zircão, a idade do núcleo detrítico de 432,53± 6,54 Ma pode ser associada com a cobertura sedimentar do Paleoproterozóico retrabalhada no ciclo Brasiliano, também observada nos valores U-Pb definidos para os zircões. Em relação ao zircão, os dados indicam ausência aparente de fontes arqueanas. Os resultados mostram duas fontes principais para a sedimentação: uma Rhyaciana (Paleoproterozóico onde ± 53 % dos grãos são “Transamazônicos”) e outra Neoproterozóica-Cambriana (30% dos zircões são “Brasilianos”). / Sandstones represent the most important reservoir rocks and aquifers in many sedimentary basins. It is necessary to have a precise chronostratigraphic position in order to provide a better explotation of water or hydrocarbons. Traditionally, the relative dating of sedimentary units is obtained with fossil content or stratigraphic correlation. But in many sedimentary rocks these analyses are not possible and sometimes have a dubious interpretation. This is the case of the Marizal Formation (Recôncavo Basin) where many questions arise when the age of the unit is questioned. The Marizal Formation is a fluvio-eolic sandstone which has been associated with an Albian/Aptian age in the stratigraphic column, although very discussible. Samples of sandstones of Marizal Formation present an important diagenetic overgrowths around K-feldspar detrital cores and they are suitable to 40Ar-39Ar dating concerning the identification of processes in the sandstones (as diagenesis or depositional ages). Among the heavy mineral suite in the Marizal Formation, zircon grains are identified. The U-Pb dating of detrital zircons can provide information about the provenance of the unit allowing better interpretation to the Marizal Formation. The overgrowths of K-feldspar indicated a value of 159.89 ± 23.96 Ma and to the detrital core, 432.57 ± 11.89 Ma. The mean value obtained around 160 Ma, considering that all care and analytical sample selection were considered, is older than expected. So, this was interpreted as indicating that the overgrowth, have been developed in a sedimentary source rock being transported latter to the depositional site. This age may be related to pre-rift stage of the Recôncavo Basin. The value confirms previous ideas of remobilization of the substrate during the rift basin stage. As has been extensively discussed, overgrowths of K-feldspars are stable and can be transported by small distances, which corroborates the above interpretation. The value obtained to the detrital core can be associated with a Paleoproterozoic sedimentary cover reworked in the Brazilian cycle. For zircon U-Pb dating, the data indicate no apparent Archean sources. The results show two main sources for sedimentation: a Rhyacian (Paleoproterozoic where ± 53% of the grains are "Transamazonian") and another Neoproterozoic-Cambrian (30% of zircon are "Brazilian").

Conselho Nacional de Pesquisa e Desenvolvimento Científico e Tecnológico - CNPq / The Propriá, Amparo do São Francisco and Fazenda Alvorada stocks are intrusive in the eastern part of the Macururé Domain and cut the regional orientation. These stocks present similar geological and petrographic features and are essentially constituted by quartz monzonites and porphyritic granites. In these plutons biotite is the predominant mafic mineral and hornblende occurs in some enclaves. Plagioclase crystals exhibit path zoning indicating important instabilities during crystallization. The accessory minerals in these rocks are apatite, zircon, titanite, opaque minerals. In the Propriá and Amparo do São Francisco the stocks the microgranular mafic enclaves are abundant. The crystallization age of the Propriá stock is 615 ± 6 Ma (U-PbSHRIMP). The geochemical data indicate that these stocks correspond to magmatism potassium, orogenic and arc signature and positioned in a post-collision set. / Os stocks Propriá, Amparo do São Francisco e Fazenda Alvorada são intrusivos na parte leste do Domínio Macururé e truncam as orientações regionais. Estes corpos apresentam feições geológicas e petrográficas semelhantes e são essencialmente constituídos por quartzo monzonitos e granitos porfiríticos. Nestes plutons a biotita é o mineral máfico predominante e a hornblenda ocorre em alguns enclaves. Os cristais de plagioclásio exibem forte zoneamento indicando instabilidades importantes durante a cristalização. Os minerais acessórios nestas rochas são apatita, zircão, titanita, minerais opacos. Nos stocks de Propriá e Amparo do São Francisco enclaves máficos microgranulares são abundantes. A idade de cristalização do Stock Propriá é de 615 ± 6 Ma (U-PbSHRIMP). Os dados geoquímicos indicam que estes stocks correspondem a magmatismo potássico, orogênico e com assinatura de arco e posicionados em período pós-colisional. / São Cristóvão, SE

Os depósitos de tipo Cu-Au (Mo) pórfiro de Água Rica e Bajo de la Alumbrera, NW argentino, estão geneticamente associados às atividades do Complexo Vulcânico Farallón Negro, no Mioceno. Estes depósitos de classe mundial estão inseridos no contexto geotectônico dos Andes Centrais, Província Geológica das Serras Pampeanas Ocidentais e são do tipo Cu-Au (Mo) pórfiro, embora o depósito de Água Rica possua uma série de características epitermais e uma fase de enriquecimento supergênico. Estudos isotópicos e geocronologia de alta resolução foram propostos para os dois depósitos, objetivando investigar a fonte da mineralização e a influência crustal em sua gênese, além de idades para os pórfiros de Água Rica. Os isótopos de Pb mostram que as amostras de rocha total de Bajo de la Alumbrera são menos radiogênicas, com valores de razões muito próximos e apresentam-se menos radiogênicas do que as amostras de rocha total de Água Rica. As amostras de rocha total de Água Rica mostram maior dispersão, mas de certo modo alguma relação com o material analisado (pirita, ou rocha total). As amostras de pirita, de ambos os depósitos apresentam valores de razões similares, o que permite agrupá-las. As composições isotópicas de Pb em sulfetos e em rochas hospedeiras para os depósitos de Água Rica e Bajo de la Alumbrera indicaram que, apesar da diferença nas litologias presentes nos dois depósitos, os valores obtidos são próximos e muito homogêneos. As amostras de sulfetos de Bajo de la Alumbrera apresentam razões \'ANTPOT.206 Pb\'/ \'ANTPOT.204 Pb\', \'ANTPOT.207 Pb\'/ \'ANTPOT.204 Pb\' e \'ANTPOT.208 Pb\'/ \'ANTPOT.204 Pb\' muito próximas das razões de rocha total, porém com discreto enriquecimento em Pb radiogênico. As amostras de sulfetos de Água Rica apresentam distribuição pouco mais variável, no entanto, observa-se que as amostras de pirita de diorito e pirita do metarenito encaixante ficam próximas entre si e próximas das razões de pirita de Bajo de la Alumbrera Isto pode ser interpretado como fonte e condições de formação similar para estes sulfetos. Os isótopos de Sr e Nd obtidos e combinados com dados da literatura podem sugerir mais evidências na fonte dos magmas e fluidos para estes depósitos, onde as rochas foram divididas em grupos por afinidades isotópicas. Percebe-se que há rochas de padrão juvenil, incorporadas durante a orogenia Andina, mas também há rochas que indicam a presença de contaminação crustal, não homogênea, com variações nos valores de épsilon Nd(t) positivos e negativos. Idades de U-Pb em zircão SHRIMP-IIe nos pórfiros Trampeadero e Quebrada Seca, do depósito de Água Rica, apresentaram idades de 6,20±0,16 Ma e 5,66±0,22 Ma, respectivamente. Estas idades indicam um intervalo tempo muito curto, de cerca de 0,54 Ma, entre as intrusões que contém as mineralizações de maior teor e sendo as primeiras idades obtidas para as intrusões de Água Rica, por meio de datação por U-Pb SHRIMP em zircão. Estas idades sugerem que as intrusões associadas a mineralizações do depósito Água Rica são as mais jovens das que ocorrem na área do Complexo Farallón Negro. Uma isócrona Re-Os obtida em pirita para o depósito de Água Rica de 6,4±2,8 Ma é condizente com a história evolutiva deste depósito. Esta idade, combinada com a idade U-Pb em zircão condiz com a principal fase magmática ocorrida neste depósito e que é tida como a fase principal da mineralização. Com base nos dados, sugere-se que a gênese do depósito de Água Rica ocorreu concomitantemente aos estágios magmáticos finais de Bajo de la Alumbrera, e que esta marca as idades mais jovens para as rochas do Complexo Vulcânico Farallón Negro. Além disso, a participação crustal foi significativa durante a gênese desses depósitos, com uma componente maior no depósito de Água Rica, devido a maior interação dos fluidos magmáticos e mineralizantes com o embasamento Paleozoico. Com base neste estudo, indicamos que em zonas de subducção com baixo ângulo de mergulho, os depósitos minerais gerados podem apresentar maior interação com fluidos crustais do que aqueles em áreas de com placa de alto ângulo. / The Agua Rica and Bajo de la Alumbrera Cu-Au (Mo) porphyry-type deposits, NW Argentina, are genetically associated with the Farallón Negro Volcanic Complex, Miocene age. These world-class deposits of Bajo de la Alumbrera and Agua Rica, located into the tectonic context of the Central Andes, Geological Province of the Sierras Pampeanas, are Cu-Au (Mo) porphyry-type, although the Agua Rica deposit has a number of epithermal features and a supergene enrichment phase. Isotopic studies and high-resolution geochronology were carried out for the two deposits, aiming to investigate the source of the mineralization and crustal influence in its genesis, and ages for the Agua Rica porphyries. Pb isotopes show that the whole-rock samples from Bajo de la Alumbrera are less radiogenic than those from Agua Rica, with close ratio values. The whole-rock samples from Agua Rica show higher dispersion compared to Bajo de la Alumbrera. The pyrite samples from both deposits present similar ratios, which allows grouping them. The Pb isotopic compositions of sulfides and host rocks for the Agua Rica deposit and Bajo de la Alumbrera indicated that, despite the difference in the lithology present in the two deposits, the values obtained are close and very homogeneous. Sulfide samples from Bajo de la Alumbrera show ratios of \'ANTPOT.206 Pb\'/ \'ANTPOT.204 Pb\', \'ANTPOT.207 Pb\'/ \'ANTPOT.204 Pb\' and \'ANTPOT.208 Pb\'/ \'ANTPOT.204 Pb\' very close to the whole-rock ratios, but with a slight enrichment in radiogenic Pb. Sulfide samples from Agua Rica present variable distribution, however, the pyrite samples from diorite and from metasandstone present ratios close to the pyrite from Bajo de la Alumbrera. This ratios similarity could be interpreted as similar conditions and sources to these sulfides. The Sr and Nd isotopes obtained and combined with literature data may suggest more evidence on the source of magmas and fluids for these deposits. Rocks with juvenile pattern, incorporated during the Andean orogeny, are one group observed, but there are also rocks that indicate the presence of crustal contamination, not homogeneous, with variations in Epsilon Nd(t) positive to negative. U-Pb SHRIMP-IIe zircon ages in the porphyries Trampeadero and Quebrada Seca, from the Agua Rica deposit, presented ages of 6.20 ± 0.16 Ma and 5.66 ± 0.22 Ma, respectively. These ages indicate a short time interval between these intrusions, of about 0.54 Ma, and represent the youngest ages obtained for Agua Rica by U-Pb SHRIMP zircon. A Re-Os isochron obtained in pyrite to the Agua Rica deposit of 6.4 ± 2.8 Ma is consistent with the evolution of this deposit. This age, combined with the U-Pb SHRIMP zircon ages matches the main magmatic phase and shows that the main mineralization phase is coeval to the intrusions. Based on these data, it is suggested that the genesis of the Agua Rica deposit was coeval to the last magmatic stages of Bajo de la Alumbrera, and this marks the youngest ages to the Farallón Negro Volcanic Complex rocks. In addition, the crustal participation was significant in the genesis of these deposits, with a major component in the Agua Rica deposit, because the fluids interaction with the Paleozoic basement. Based on this study, we indicated that in flat-subduction zones, the mineral deposits generated present more significant interaction with the crustal fluids than those normal subduction zones.

A presente dissertação apresenta uma investigação sobre a evolução tectono-metamórfica da Formação São Tomé, uma das unidades metassedimentares pertencente ao Grupo Rio Doce, no Orógeno Araçuaí. Essa unidade, de origem pelítica, ocorre em uma faixa com direção geral aproximadamente N-S, com vergência para NE, contrária ao cráton São Francisco neste setor do orógeno. Possui como litotipo principal quartzo-mica xisto, com proporções variadas de granada, estaurolita, sillimanita e plagioclásio. Duas fases deformacionais estão registradas através de uma foliação continua S1 do tipo clivagem de fratura e uma foliação S2 de clivagem de crenulação. As relações texturais indicam que parte das fases minerais se cristalizaram tanto durante o evento deformacional como em condições ausentes de tensão deviatórica, e registro de condições de pico metamórfico de 6 kbar e 650 °C. Os dados geocronológicos foram obtidos em monazita por meio de datações Th-UPb em microssonda eletrônica. Foram identificadas três populações distintas: (i) A população mais antiga de 641±32 Ma, 614±39 Ma e 607±47 Ma, a qual pode representar grãos detríticos de corpos ígneos, não mais aflorantes, relacionados a um possível estagio pré-colisional ou, tratar-se de monazitas metamórficas formadas em um primeiro evento térmico; (ii) População com idades intermediárias entre 560±32 Ma e 559±29 Ma, compatíveis com as idades do ápice metamórfico regional; (iii) População mais jovem de idades entre 501±28 Ma e 491±34 Ma, condizentes com a formação concomitante à geração dos corpos ígneos tardios. Ainda que não esteja esclarecido se essas idades estão relacionadas a eventos metamórficos distintos ou são reflexo da baixa taxa de resfriamento do orógeno, os três grupos de monazita com idades distintas estão bem estabelecidos. / The present dissertation presents an investigation on the tectono-metamorphic evolution of the São Tomé Formation, one of the metasedimentary units belonging to the Rio Doce Group, in the Araçuaí Orogen. This unit, of pelitic origin, occurs in a band with general direction approximately N-S, with vergence for NE, contrary to the São Francisco craton in this sector of the orogen. It has quartz-mica schist as main lithotype, with varying proportions of grarnet, staurolite, sillimanite and plagioclase. Two deformational phases are recorded through a continuous foliation S1 of the fracture cleavage type and a foliation S2 of crenulation cleavage. The textural relations indicate that part of the mineral phases crystallized both during the deformational event and in conditions absent of deviatore voltage, and record of metamorphic peak conditions of 6 kbar and 650 ° C. The geochronological data were obtained in monazite by means of Th-U-Pb dating in electron microprobe. Three distinct populations were identified: (i) The oldest population of 641 ± 32 Ma, 614 ± 39 Ma and 607 ± 47 Ma, which may represent detrital grains of igneous bodies, no longer outcrops, related to a possible pre- collisional or, being metamorphic monazites formed in a first thermal event; (ii) Populations with intermediate ages between 560 ± 32 Ma and 559 ± 29 Ma, compatible with regional metamorphic apex ages; (iii) Younger population between 501 ± 28 Ma and 491 ± 34 Ma, consistent with the formation concomitant with the generation of late igneous bodies. Although it is unclear whether these ages are related to distinct metamorphic events or are reflective of the low orogenic cooling rate, the three monazite groups of different ages are well established.

Located in the south of the Moroccan Sahara, the Adrar Souttouf Massif is the northern continuation of the Mauritanides at the western margin of the West African Craton. The massif itself exhibits a complex polyphase geologic history and contains four geologically different, SSW-NNE trending main units named from west to east: Oued Togba, Sebkha Gezmayet, Dayet Lawda, Sebkha Matallah. They are thrusted over each other in thin-skinned nappes with local windows of the discordantly overlain Archaean Reguibat basement. The eastern margin of the massif is bordered by the Tiris and Tasiast-Tijirit areas of the Reguibat Shield as well as its (par-) autochthonous Palaeozoic cover sequence, termed Dhloat Ensour unit. More than 5.500 U-Th-Pb age determinations and over 1.000 Hf isotopic measurements on single zircon grains from igneous, metamorphic, and sedimentary rocks of all the massifs units and its vicinity have yet been obtained. Most of the zircons were studied with respect to their morphological features. This method improves the accuracy of provenance studies by detecting varying zircon morphologies in space and time. These data are accompanied by U-Th-Pb age determinations on apatite as well as rutile. Together, they allow proposing a model of the geologic evolution of this poorly mapped area for the last 635 Ma. A combination of the obtained data with extensive zircon age databases of the surrounding cratons and terranes facilitates continental-scaled palaeogeographic reconstructions. Regarding the geologic evolution of the Adrar Souttouf Massif, the assembly of the first units began prior to 635 Ma. Although containing all the major zircon age and Hf-isotope populations of the West African Craton as well as some Mesoproterozoic grains, the Sebkha Gezmayet unit lies to the west of the Dayet Lawda unit of oceanic island arc composition. Hence, the Sebkha Gezmayet unit must have been rifted away from the craton prior to the formation of the oceanic unit within the West African Neoproterozoic Ocean at about 635 Ma. Recently published Hf and zircon age data of this unit suggest that the island arc was derived from a juvenile mantle source. Subsequently, the accretion of precursors of the Oued Togba and Sebkha Gezmayet units as well as a partial obduction of the oceanic Dayet Lawda unit and the Neoproterozoic sediments of a foreland basin (Sebkha Matallah unit) onto the Reguibat Shield took place. Peak metamorphism in the obducted oceanic rocks was reached at about 605 Ma. Magmatism in the western units between 610 and 570 Ma suggests on-going tectonic activity. The Early and Middle Cambrian is characterised by the erosion of the Ediacaran orogen and deposition of thick sedimentary sequences at the Sebkha Matallah unit, which acted as foreland basin. These sediments show a mostly West African zircon record with only some Mesoproterozoic grains provided by the westernmost parts of the massif. Initial rifting of the Oued Togba and Sebkha Gezmayet units from the remaining areas presumably occurred during the Late Cambrian. Coeval granitoid intrusions occurred on both sides of the rift. The two rifted units were likely involved to the polyphased Appalachian orogenies, which is emphasised by Devonian magmatism. Thus, and with respect to the isotopic data, the Oued Togba unit is interpreted to be of Avalonia affinity, while the Sebkha Gezmayet unit can likely be linked to Meguma. The units which remained at the West African Craton underwent intense sediment recycling during the entire Ordovician to Devonian times. Final accretion of all units and formation of the current massif was achieved during the Variscan-Alleghanian orogeny. This was accompanied by magmatism in the Sebkha Gezmayet unit and intense metamorphism of the Reguibat basement, whose zircons often show lower discordia intercepts of Carboniferous or Permian age. The post-Variscan period is characterised by erosion of the orogen and subjacent alternating cycles of sedimentation and deflation. The Adrar Souttouf Massifs importance for palaeogeographic reconstructions is given by the striking differences in the zircon age and Hf-isotope record of its westernmost Oued Togba unit and the remaining area. The results obtained from the Oued Togba unit resemble the published data of the Avalonia type terranes including prominent Mesoproterozoic, Ediacaran-Early Cambrian, as well as Early Devonian age populations. Many Mesoproterozoic zircons, which are exotic for the West African Craton prior to 635 Ma, form a ca. 1.20 to 1.25 Ga age peak that is an excellent tracer for detrital provenance studies and source craton identification of the sedimentary rocks. This is also valid for some sedimentary samples that do not show ages younger than 700 Ma, but large quantities of Mesoproterozoic zircon. These rocks can be correlated to similar sediments in Mauritania and W-Avalonia and are thought to be of pre-pan-African", i.e. pre-Ediacaran or even pre-Cryogenian age. They may give direct insights to the source area in Early to Mid Neoproterozoic times. Accordingly, comparison with published data of Amazonia and Baltica, allows setting up new hypotheses for the pre-Ediacaran history of the Avalonian type terranes. Lacking of magmatism in Amazonia between ca. 1200 and ca. 1300 Ma favours Baltica as source craton for the Avalonian terranes and requires a new point of view for the Neoproterozoic palaeogeography.

Located in the south of the Moroccan Sahara, the Adrar Souttouf Massif is the northern continuation of the Mauritanides at the western margin of the West African Craton. The massif itself exhibits a complex polyphase geologic history and contains four geologically different, SSW-NNE trending main units named from west to east: Oued Togba, Sebkha Gezmayet, Dayet Lawda, Sebkha Matallah. They are thrusted over each other in thin-skinned nappes with local windows of the discordantly overlain Archaean Reguibat basement. The eastern margin of the massif is bordered by the Tiris and Tasiast-Tijirit areas of the Reguibat Shield as well as its (par-) autochthonous Palaeozoic cover sequence, termed Dhloat Ensour unit. More than 5.500 U-Th-Pb age determinations and over 1.000 Hf isotopic measurements on single zircon grains from igneous, metamorphic, and sedimentary rocks of all the massifs units and its vicinity have yet been obtained. Most of the zircons were studied with respect to their morphological features. This method improves the accuracy of provenance studies by detecting varying zircon morphologies in space and time. These data are accompanied by U-Th-Pb age determinations on apatite as well as rutile. Together, they allow proposing a model of the geologic evolution of this poorly mapped area for the last 635 Ma. A combination of the obtained data with extensive zircon age databases of the surrounding cratons and terranes facilitates continental-scaled palaeogeographic reconstructions. Regarding the geologic evolution of the Adrar Souttouf Massif, the assembly of the first units began prior to 635 Ma. Although containing all the major zircon age and Hf-isotope populations of the West African Craton as well as some Mesoproterozoic grains, the Sebkha Gezmayet unit lies to the west of the Dayet Lawda unit of oceanic island arc composition. Hence, the Sebkha Gezmayet unit must have been rifted away from the craton prior to the formation of the oceanic unit within the West African Neoproterozoic Ocean at about 635 Ma. Recently published Hf and zircon age data of this unit suggest that the island arc was derived from a juvenile mantle source. Subsequently, the accretion of precursors of the Oued Togba and Sebkha Gezmayet units as well as a partial obduction of the oceanic Dayet Lawda unit and the Neoproterozoic sediments of a foreland basin (Sebkha Matallah unit) onto the Reguibat Shield took place. Peak metamorphism in the obducted oceanic rocks was reached at about 605 Ma. Magmatism in the western units between 610 and 570 Ma suggests on-going tectonic activity. The Early and Middle Cambrian is characterised by the erosion of the Ediacaran orogen and deposition of thick sedimentary sequences at the Sebkha Matallah unit, which acted as foreland basin. These sediments show a mostly West African zircon record with only some Mesoproterozoic grains provided by the westernmost parts of the massif. Initial rifting of the Oued Togba and Sebkha Gezmayet units from the remaining areas presumably occurred during the Late Cambrian. Coeval granitoid intrusions occurred on both sides of the rift. The two rifted units were likely involved to the polyphased Appalachian orogenies, which is emphasised by Devonian magmatism. Thus, and with respect to the isotopic data, the Oued Togba unit is interpreted to be of Avalonia affinity, while the Sebkha Gezmayet unit can likely be linked to Meguma. The units which remained at the West African Craton underwent intense sediment recycling during the entire Ordovician to Devonian times. Final accretion of all units and formation of the current massif was achieved during the Variscan-Alleghanian orogeny. This was accompanied by magmatism in the Sebkha Gezmayet unit and intense metamorphism of the Reguibat basement, whose zircons often show lower discordia intercepts of Carboniferous or Permian age. The post-Variscan period is characterised by erosion of the orogen and subjacent alternating cycles of sedimentation and deflation. The Adrar Souttouf Massifs importance for palaeogeographic reconstructions is given by the striking differences in the zircon age and Hf-isotope record of its westernmost Oued Togba unit and the remaining area. The results obtained from the Oued Togba unit resemble the published data of the Avalonia type terranes including prominent Mesoproterozoic, Ediacaran-Early Cambrian, as well as Early Devonian age populations. Many Mesoproterozoic zircons, which are exotic for the West African Craton prior to 635 Ma, form a ca. 1.20 to 1.25 Ga age peak that is an excellent tracer for detrital provenance studies and source craton identification of the sedimentary rocks. This is also valid for some sedimentary samples that do not show ages younger than 700 Ma, but large quantities of Mesoproterozoic zircon. These rocks can be correlated to similar sediments in Mauritania and W-Avalonia and are thought to be of pre-pan-African", i.e. pre-Ediacaran or even pre-Cryogenian age. They may give direct insights to the source area in Early to Mid Neoproterozoic times. Accordingly, comparison with published data of Amazonia and Baltica, allows setting up new hypotheses for the pre-Ediacaran history of the Avalonian type terranes. Lacking of magmatism in Amazonia between ca. 1200 and ca. 1300 Ma favours Baltica as source craton for the Avalonian terranes and requires a new point of view for the Neoproterozoic palaeogeography.

The Fish Creek-Vallecito Basin exposes an archive of sediment related to early rifting of the Gulf of California beginning at 8.0 Ma followed by Colorado River delta progradation from 5.3-3.0 Ma. Mio-Pliocene deposits from the Fish Creek-Vallecito Basin of southern California and a sample from the modern Colorado River delta were analyzed through detrital zircon U-Pb (n=1996) and (U-Th)/He (n=280) double-dating in order to better constrain sediment provenance, hinterland exhumation, and Colorado River evolution. Coupling this dataset with outcrop study of the first Colorado River-sourced turbidites into the basin at 5.3 Ma, allows for evolution of the Colorado River system to be viewed from a source-to-sink perspective. Detrital zircon U-Pb and (U-Th)/He (ZHe) ages obtained in this study suggest earliest derivation of sediment was from the Peninsular Ranges followed by more distant sediment sourcing from the Colorado River. Initial Colorado River-sourced deposits show Yavapai-Mazatzal U-Pb ages with Laramide ZHe ages suggesting that the river was sourcing from Laramide basement cored uplifts at the onset of deposition into the Gulf of California, supporting a top-down model of river evolution. An increased percentage of Grenville U-Pb age grains as well as a wider range of ZHe ages associated with western US basement-derived zircon from a modern Colorado River delta sample indicate erosion into older stratigraphic units through time which is consistent with deep erosion on the Colorado Plateau since ~6 Ma. Vertically measured sedimentology logs through the Wind Caves Member, the first Colorado River-sourced unit deposited, were used to determine slope and basin floor architecture as the Colorado River and delta dispersed subaqueous sediment gravity flows into the marine Gulf. Measured sections arrayed along depositional strike show a 4.5 km wide pod of sand-rich turbidites that were delivered through a broad Fish Creek exit point from the paleo-Colorado shelf. The vertical sedimentation trend is one showing thick bedded, amalgamated channelized and sheet-like sandstones initially, shifting to thinnerbedded sheets and more isolated channels higher in the increasingly muddy section. The facies variability up section is interpreted as a change from a submarine basin floor fan to a lower slope environment as the Colorado River prograded its delta into the Gulf. / text

The Eagle Ford Shale and equivalent Boquillas Formation (Late Cretaceous) contain abundant volcanic ash beds of varying thickness. These ash beds represent a unique facies that displays a range of sedimentary structures, bed continuity, and diagenetic alteration. They are prominent not only in West Texas outcrops, but also in the subsurface of South Texas where hydrocarbon production is actively occurring. The ash beds have the potential to be used for stratigraphic correlation for understanding early diagenesis and — most importantly — for obtaining high-resolution geochronology, which can then be used for defining depositional rates and chronostratigraphy. Study of the ash beds was conducted at outcrops along U.S. 90, west of Comstock, Texas, the subsurface in Atascosa and Karnes County, and at a construction site in South Austin. Bed thicknesses range from 0.1–33 cm and were collected throughout the entirety of the Eagle Ford succession. Mineral separation yielded abundant non-detrital zircons for U-Pb dating. Dating was conducted using LA-ICP-MS at The University of Texas at Austin, to attain a base level understanding of the age range for the Eagle Ford. High-resolution ages for the base and top of the Eagle Ford were obtained, in addition to radioisotopically defining the Cenomanian-Turonian boundary within the section. U-Pb ages for the Eagle Ford Shale range from Early Cenomanian to Late-Coniacian near Comstock, Mid-Cenomanian to the Turonian-Coniacian boundary in the subsurface, and Early Cenomanian to Late Turonian in Austin area. These findings contrast with many of the regional biostratigraphic studies across the Eagle Ford and indicate a more prolonged period of Eagle Ford deposition than previously observed. / text

No description available.