QUANTIFICAÇÃO DA DEFORMAÇÃO EM MIGMATITOS METATEXITOS ESTROMÁTICOS E ROCHAS ASSOCIADAS DO COMPLEXO ATUBA, PORÇÃO LESTE DO ESTADO DO PARANÁ: Quantification of deformation in stromatic metathexite migmatites and associated rocks of the Atuba Complex, Eastern Paraná State

The Atuba Complex emerges in the eastern portion of the State of Paraná, along an NE-SW elongated swath and comprises the southern part of the Ribeira Belt. This work has as main objective to define the tectonic and deformational regimes of the Atuba Complex. The research was developed through field research, petrography and quantitative structural analysis and complements several tectonic studies already performed. Stromatic metathexite migmatites have medium to high metamorphic grade and are composed of tonalitic residual neosomes interspersed with granitic or tonalitic-granodioritic leukosomes and mafic melanosomes. They can often be classified as belonging to the series of milonites. Amphibolites, schists, granulitic gneisses, schollen diatexites, quartzites and granitoids are also observed. Two foliation planes, both generated by ductile shear, were identified: the first is characterized by Sn-1 foliation associated with thrusting tectonics (Dn-1) and the second by Sn foliation produced by sinistral transpressive tectonics (Dn). The results confirm that the last phase was accommodated by the deformation partition. The quantification of deformation was developed using the Fry, Polar and Rf/φ method, based on analysis of quartz and feldspar crystals on 26 thin section. Samples revealed similar strain ratios for all methods used in the XZ and YZ planes. The ellipsoids are oblate in shape and correspond to the apparent flattening field. Keywords: Structural analysis. Migmatites. Transpression.RESUMO – O Complexo Atuba aflora na porção leste do Estado do Paraná, segundo uma faixa alongada de direção NE-SW e compreende a parte sul do Cinturão Ribeira. Este trabalho tem como objetivo principal elucidar os processos deformacionais do Complexo Atuba. A investigação foi desenvolvida por meio de investigação de campo, petrografia e análise estrutural quantitativa e, complementa vários estudos tectônicos já realizados anteriormente. Os migmatitos metatexitos estromáticos possuem médio a alto grau metamórfico e são compostos por neossomas residuais tonalíticos intercalados com leucossomas graníticos ou granodioríticos-tonalíticos e melanossomas máficos. Muitas vezes podem ser classificados como pertencentes à série dos milonitos. Observam-se também anfibolitos, xistos, gnaisses granulíticos, diatexitos schollen, quartzitos e granitoides. Dois planos de foliação, ambos gerados por cisalhamento dúctil, foram identificados: o primeiro é caracterizado pela foliação Sn-1 associada à tectônica de cavalgamento (Dn-1) e o segundo pela foliação Sn produzido por tectônica transpressiva sinistral (Dn). Os resultados confirmam que a última fase foi acomodada pela partição da deformação. A quantificação da deformação foi desenvolvida por intermédio dos métodos de Fry, Polar e Rf/φ, com base na análise de cristais de quartzo e feldspatos em 26 lâminas delgadas. As amostras revelaram razões de deformação similares para todos os métodos utilizados nos planos XZ e YZ. Os elipsoides possuem forma oblata e correspondem ao campo do achatamento aparente. Palavras-chave: análise estrutural, migmatitos, transpressão

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Trabalhos experimentais recentes têm confirmado as observações feitas por vários autores de que a presença de uma segunda fase mineral, em especial filossilicatos, aumenta consideravelmente a dissolução durante a deformação por dissolução-precipitação por pressão. Muitos autores consideram tal comportamento como exclusivo dos minerais com estrutura em camadas, os quais possuem a habilidade de reter a fase fluida aquosa, mesmo quando submetidos a grandes tensões normais. No entanto, experimentos de dissolução em halita e sílica têm mostrado que a capacidade de retenção da fase fluida parece ser uma característica das interfaces entre diferentes fases (interfases), ao invés de uma exclusividade dos filossilicatos. Um outro fator importante na dissolução do quartzo é a sua orientação cristalográfica. Observações em tectonitos naturais indicam que determinadas orientações reticulares dos grãos de quartzo, sob tensão diferencial, dissolvem mais facilmente que outras orientações. Grande parte dessas observações têm sido feitas em quartzitos, havendo, portanto, uma limitação para a sua aplicação a outros tectonitos de composição diferente, mas deformados em condições semelhantes. Estudos em tectonitos de formações ferríferas bandadas precambrianas constituídos de quartzo, hematita e magnetita, provenientes de diferentes locais do Quadrilátero Ferrífero (MG), mostraram que muitas microestruturas de dissolução comumente encontradas em quartzitos também são comuns nesses tipos de rochas. Tais observações são importantes no sentido de que, apesar de os óxidos de ferro possuírem estruturas atômicas muito diferentes dos filossilicatos, eles praticamente exercem o mesmo efeito nos processos de deformação por dissolução-precipitação por pressão. Além de participarem na dissolução do quartzo de uma maneira determinante, são também dissolvidos e reprecipitados. Além do mais, as transformações de fase magnetita para hematita de baixo grau metamórfico, comumente observadas em alguns dos tectonitos estudados, indicam que tais feições aumentam consideravelmente a ductibilidade dessas rochas, através da liberação da energia de deformação acumulada nos grãos. Para o estudo das transformações da magnetita para hematita, (Capítulo 2) foram selecionadas amostras do setor sul da Serra do Curral. O mesmo conjunto de amostras foi utilizado na análise da influência da presença dos óxidos de ferro e da orientação cristalográfica na dissolução dos grãos de quartzo. Os resultados e discussões referentes a esse estudo são apresentados no Capítulo 3. A presença dos óxidos de ferro na deformação das formações ferríferas vai além dos processos de dissolução. A determinação de orientações cristalográficas de eixos e de quartzo em tectonitos deformados em condições metamórficas de fácies xisto verde, zona de alta temperatura, (setor nordeste da Serra do Curral, Piedade) mostraram que as orientações preferenciais dos eixos c dependem do conteúdo em hematita presente nos diferentes domínios composicionais (bandas composicionais). A apresentação e discussão dos resultados desse estudo é feita no Capítulo 4. As microestruturas presentes nos diferentes domínios composicionais indicam a operação de diferentes mecanismos de deformação e recuperação dos grãos de quartzo. Domínios com conteúdo em hematita entre 10 e 25% em volume possuem microestruturas indicativas de migração de bordas de grãos. Já nos domínios com 50% vol. ou mais em hematita, microestruturas sugerem a atuação de mecanismos de recuperação por saltos de discordâncias (dislocation climb) e de recristalização por rotação de subgrãos. A atuação de diferentes mecanismos de recristalização produziram diferentes padrões de orientações de eixos c de quartzo nos tectonitos estudados. Microscopia eletrônica de varredura (MEV) foi empregada para determinar a distribuição e caracterizar as estruturas dos poros nas bordas dos grãos de quartzo e hematita, e, dessa forma, avaliar a distribuição da fase fluida intergranular (Capítulo 5). Para esse estudo, foram escolhidas amostras de tectonitos de formações ferríferas deformados em condições metamórficas de fácies anfibolito, provenientes da margem leste do QF (flanco leste do Anticlinal de Mariana). Os resultados mostraram que a distribuição da fase fluida aquosa é altamente anisotrópica, concentrando-se especialmente em bordas de grãos normais a direção de extensão máxima. Essa é a direção preferencial do crescimento dos grãos de quartzo. Uma conclusão importante do presente estudo é que a geração de foliação penetrativa e as razões axiais encontradas para o agregado não estão necessariamente relacionadas ao campo de deformação local, podendo, no entanto, refletir o campo de tensão responsável pelas anisotropias na distribuição da fase fluida e no crescimento dos grãos do agregado. No Capítulo 6, é feita uma discussão geral na qual todos os resultados apresentados nos capítulos anteriores são integrados com o intuito de dar uma visão geral dos processos envolvidos no desenvolvimento das microestruturas e na produção das orientações cristalográficas preferenciais. Para tanto, são consideradas as condições de deformação para cada um dos locais de escolha das amostras, avaliando, dessa maneira, a influência de fatores prevalecentes na operação dos mecanismos de deformação. Tais fatores incluem o grau metamórfico (temperatura e pressão), a presença de fase fluida aquosa intergranular e a presença da magnetita e hematita na deformação por dissolução-precipitação por pressão (pressure solution) e por plasticidade intracristalina (dislocation creep). As conclusões gerais do presente estudo estão enumeradas no Capítulo 7.(I) Transformação da magnetita para hematita libera a energia de deformação acumulada no retículo da magnetita, eliminando, assim, grãos resistentes à deformação (hard grains), aumentando a ductibilidade do agregado; microfraturamento e dissolução ao longo das interfaces da magnetita e (0001) da hematita, resultando numa drástica redução do tamanho dos grãos. (II) Dissolução do quartzo é maior nas interfases quartzo/óxido de ferro normais à direção de compressão máxima, mostrando que essa não é uma característica exclusiva das interfases (001)/(0001), filossilicatos/quartzo; a dissolução dos grãos de quartzo é seletiva e depende da orientação cristalográfica dos grãos. Grãos orientados com eixos c paralelos a compressão máxima dissolvem-se mais facilmente. Grãos lenticulares de quartzo com orientação preferencial de forma e cristalográfica (eixo c) na direção da extensão máxima são interpretados como resultantes da precipitação e do crescimento orientado. (III) Padrões de microestruturas e microtramas de eixos c de quartzo em diferentes domínios composicionais indicam que os mecanismos de deformação e recristalização dos grãos de quartzo dependem do conteúdo em hematita. (IV) A distribuição anisotrópica da fase fluida, resultante da anisotropia da energia interfacial da hematita e da ação da tensão diferencial, leva ao crescimento preferencial dos grãos. A fase fluida acumula-se nas bordas normais à compressão mínima, promovendo o crescimento dos grãos nessa direção. Dessa forma, a anisotropia na distribuição da fase fluida, e, como conseqüência, na dissolução e crescimento dos grãos resulta na formação de uma foliação penetrativa de grãos (placas de hematita e grãos tabulares de quartzo).It is well known that phase transformation greatly enhances the ductility and may have important rheological constrains in rocks. While enhanced ductibility due to phase transformation is considered to be more important in the mantle reaction-enhanced ductibility is believed to be mainly a crustal phenomenon. However, the study of tectonitos from a low temperature and high strain deformed Precambrian Iron Formations (Itabira Formation, from Quadrilátero Ferrífero, MG) showed that the phase transformation magnetite to hematite, in a presence of a large amount of intergranular fluid phase, played an important role in the deformation of these crustal rocks. The phase transformation occurs in response to the strain energy stored in the magnetite grains arising from the increase of the dislocation densities along the crystallographic planes . The formation of new hematite grains presumably would reduce the strain energy just because they would at least initially be dislocation-free, so the magnetite containing dislocations and thus having moderate to high strain energy would be replaced by dislocation-free hematite. The new /(0001) interphase boundaries become planes of weakness along which the fluid phase might penetrate, allowing their inner part of the grains to be in contact with the fluid phase. Microfracturing and dissolution occur along the interphase (0001) normal to the maximum compressive stress. Thus phase transformation, microfracturing and dissolution are close-related processes that lead to a grain size reduction and to the production of a smooth and pervasive foliation. Experimental work has confirmed the observations of several authors that a second phase, especially layer silicates, greatly enhance the dissolution of quartz. Most of these observations have been performed on quartzites and this phenomenon is attributed to the ability of the layered silicates to adsorb structured water on the interfaces quartz/mica even under high normal stress gradients. Although the iron oxides have completely different crystal structures, optical and SEM observations in the iron formation rocks indicate that they bring about the same effect. This might be related to the effects of iron oxides on the wetting behavior of water. The presence of oxides enhances wetting thereby enhancing dissolution. Furthermore wetting behavior is usually crystallographically controlled, which is consistent with the observations. Besides the wetting behavior of iron oxides, the dissolution of quartz is strongly dependent on the crystallographic orientation. Extensive quartz c-axes measurements performed for a wide range of quartz-iron oxide rations showed that quartz grains oriented with its c-axes at low angle to the maximum compressive stress are more easily dissolved and tend to disappear as the dissolution proceeds. On the other hand grains with its c-axes at high angle to the maximum compressive stress are more resistant to the dissolution. In chapter 1 the problems and the aims of the present study are defined. In chapter 2 the evidences for the transformation and dissolution of magnetite are reported. Then a microphysical model is presented to explain how the transformation occurs and how it influences the dissolution of magnetite grains. Chapter 3 reports the microstructures and LPOs of quartz grains for several different quartz-magnetite/hematite ratios. The evidences found in these rocks suggest that the dissolution of quartz grains is enhanced by the presence of iron oxide and it is strongly dependent on the orientation of quartz c-axes with respect to the shortening direction (Z). All samples used for the studies reported in chapters 2 and 3 come from low temperature and high strain iron formation rocks collected in a cross-section along the southwestern part of the Serra do Curral Syncline. Chapter 4 presents the results of the study of quartz c-axes patterns taken in several oriented samples of iron formation collected in cross-sections along the northeastern part of the Serra do Curral Syncline (Serra da Piedade). Optical thin sections of the iron formation rocks display a banded structure defined by platelike-hematite-rich domains oriented parallel to the main foliation. The domain boundaries, which define the main foliation (SA), are composed mainly of hematite platelets rich layers (over 50 volume % of hematite). Quartz grains in hematite-rich-domains (50 vol.% of hematie) exhibit a quite equant shape. Internally the domains are made up of hematite platelets (10 vol.% of hematite) and tabular quartz grains. The elongate hematite and the tabular grains define the internal fabric (\'S IND.B\'), oblique to the main foliation (\'S IND.A\'). The specimens were selected according to the volume content of hematite in different admixed layers. Layers containing 10, 25 and 50 volume % of hematite were selected to perform the c-axis measurements. C-Axis patterns for recrystallized grains reveal notable differences in the three selected domains studied. The distinct types of microstructures observed in these domains indicate that the mechanism of recovery (grain boundary migration recrystallization or dislocation climb) is affected by the content of the hematite present in each domain. Consequently, the operation of the different mechanisms of recovery was responsible for the development of different c-axis patterns present in domains with different hematite content. When recovery occurs dominantly by strain-induced grain boundary migration, as it does in quartz domains with hematite content of 10 or 25 vol.% (quartz domains), the resulting c-axis preferred orientations are marked by a maximum concentration around the pole to the internal foliation. Fundamentally, c-axis distributions in these domains occur approximately parallel to the maximum shortening direction (Z). When recovery occurs dominantly by dislocation climb (leading to progressive subgrain rotation recrystallization), as it does in domains with 50% of hematite content (hematite domains), the c-axis preferred orientations are defined by a maximum concentration asymmetrically distributed around the intermediate shortening direction (Y). Chapter 5 reports the study of the microstructures of grain boundaries and the distribution of pores via SEM microscopy in quartz-hematite iron formation rocks. The samples studied were collected along a section of the Mariana Anticline. According to the petrological constrains, the metamorphic grade of the banded iron formation of this part of the Quadrilátero Ferrífero may have reached the amphibolite facies. The higher temperature and lower strain rate metamorphism produced the mineral assemblage quartz + hematite \' + OU -\' kyanite \'+ OU -\' garnet in these banded iron formation. Contrasting with the tabular hematite grains in the tectonitos of the Serra da Piedade, hematite grains in tectonitos of the Mariana Anticline show shapes very similar to the micas in quartzite rocks. They are characteristically foliated and display an excellent basal partition. The main goal of the study of this chapter is to infer the fluid distribution along grain boundaries and together with the grain boundaries microstructures to evaluate the role of fluid distribution on the mechanisms and kinetics of quartz growth in these metamorphic rocks. All observations were performed in oriented samples hence an attempted has been made to establish a correlation between the fluid distribution and the principal compressive directions. Grain boundary microstructures observed in different sections provided important insights on the processes involved in the generation of these microstructures. The different topographies observed along differently oriented boundaries and the heterogeneous distribution of pores along these grain boundaries indicate that fluid geometry and distribution in these rocks are highly anisotropic. The irregular topography of the quartz grain boundaries normal to the maximum elongation direction (X), in addition to the numerous inclusions of tiny hematite grains indicate that a significative grain growth occurred parallel to that direction, driven by reduction of the total grain boundary area. The purpose of chapter 6 is to assemble the information presented in the preceding chapters in order to draw a broad idea about all the processes involved in the deformation of the iron formation rocks and in the generation of the lattice preferred orientations. In addition, a general approach is given to evaluate the influence of the physical and chemical environment on the deformation mechanisms, such as metamorphic grade (T and P, inferred from mineral assemblages), intergranular fluid phase and the presence of a second phase. To assess the geological implications of the results obtained. In conclusion (Chapter 7) the present work has shown that (I) phase transformation greatly enhances the ductility of rocks under crustal conditions, by removing grains which are more resistant to the deformation. (II) Dissolution is easier at the interphase boundaries (magnetite-hematite and quartz-iron oxides). This might be related to the effects of iron oxide on the wetting behavior of water. (III) The dissolution-precipitation of quartz grains is strongly dependent on the crystallographic orientation. This lead to the development of lattice preferred orientations due to anisotropic dissolution/growth rates during solution-transfer creep. For quartz grains, both dissolution and growth parallel to the c-axis are faster than at right angles, resulting in a formation of c-axis maxima parallel to the maximum principal finite elongation direction (X) and c-axis minima parallel to the maximum principal finite shortening direction (Z). (IV) The extensive measurement of quartz c-axes showed that the lattice preferred orientations (LPOs) change significantly as a function of the amount of admixed hematite in the layers. Layers with a low content of hematite exhibit microstructures indicative of grain boundary migration. On the other hand, layers with high content of hematite show microstructures indicative of subgrain formation. Thus quartz grains are recovered and recrystallized by different mechanisms as a function of the amount of hematite that, on its turn, result in different patterns of LPOs. (V) Based on the characterization of pore structures and its distribution, evidences have been found for anisotropic grain growth. The anisotropic grain growth is a result of the anisotropic distribution of the fluid phase due to a combination of the anisotropy of hematite and to the differential stress. Interphase boundaries normal to the maximum shortening direction (Z) are typically pore-free. This might be indicative of the presence of a continuous high-diffusivity fluid film along these boundaries. On the other hand, boundaries normal to the maximum elongation direction (X) display a very irregular topography and profuse distribution of irregular pores, in which well-formed hematite and quartz grains are frequently found. These features indicate that these boundaries are sites of fluid accumulation and precipitation. The highest grain growth rates in the X-direction led to the formation of grains with aspect ratios that exceed the value expected from strain

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Trabalhos experimentais recentes têm confirmado as observações feitas por vários autores de que a presença de uma segunda fase mineral, em especial filossilicatos, aumenta consideravelmente a dissolução durante a deformação por dissolução-precipitação por pressão. Muitos autores consideram tal comportamento como exclusivo dos minerais com estrutura em camadas, os quais possuem a habilidade de reter a fase fluida aquosa, mesmo quando submetidos a grandes tensões normais. No entanto, experimentos de dissolução em halita e sílica têm mostrado que a capacidade de retenção da fase fluida parece ser uma característica das interfaces entre diferentes fases (interfases), ao invés de uma exclusividade dos filossilicatos. Um outro fator importante na dissolução do quartzo é a sua orientação cristalográfica. Observações em tectonitos naturais indicam que determinadas orientações reticulares dos grãos de quartzo, sob tensão diferencial, dissolvem mais facilmente que outras orientações. Grande parte dessas observações têm sido feitas em quartzitos, havendo, portanto, uma limitação para a sua aplicação a outros tectonitos de composição diferente, mas deformados em condições semelhantes. Estudos em tectonitos de formações ferríferas bandadas precambrianas constituídos de quartzo, hematita e magnetita, provenientes de diferentes locais do Quadrilátero Ferrífero (MG), mostraram que muitas microestruturas de dissolução comumente encontradas em quartzitos também são comuns nesses tipos de rochas. Tais observações são importantes no sentido de que, apesar de os óxidos de ferro possuírem estruturas atômicas muito diferentes dos filossilicatos, eles praticamente exercem o mesmo efeito nos processos de deformação por dissolução-precipitação por pressão. Além de participarem na dissolução do quartzo de uma maneira determinante, são também dissolvidos e reprecipitados. Além do mais, as transformações de fase magnetita para hematita de baixo grau metamórfico, comumente observadas em alguns dos tectonitos estudados, indicam que tais feições aumentam consideravelmente a ductibilidade dessas rochas, através da liberação da energia de deformação acumulada nos grãos. Para o estudo das transformações da magnetita para hematita, (Capítulo 2) foram selecionadas amostras do setor sul da Serra do Curral. O mesmo conjunto de amostras foi utilizado na análise da influência da presença dos óxidos de ferro e da orientação cristalográfica na dissolução dos grãos de quartzo. Os resultados e discussões referentes a esse estudo são apresentados no Capítulo 3. A presença dos óxidos de ferro na deformação das formações ferríferas vai além dos processos de dissolução. A determinação de orientações cristalográficas de eixos e de quartzo em tectonitos deformados em condições metamórficas de fácies xisto verde, zona de alta temperatura, (setor nordeste da Serra do Curral, Piedade) mostraram que as orientações preferenciais dos eixos c dependem do conteúdo em hematita presente nos diferentes domínios composicionais (bandas composicionais). A apresentação e discussão dos resultados desse estudo é feita no Capítulo 4. As microestruturas presentes nos diferentes domínios composicionais indicam a operação de diferentes mecanismos de deformação e recuperação dos grãos de quartzo. Domínios com conteúdo em hematita entre 10 e 25% em volume possuem microestruturas indicativas de migração de bordas de grãos. Já nos domínios com 50% vol. ou mais em hematita, microestruturas sugerem a atuação de mecanismos de recuperação por saltos de discordâncias (dislocation climb) e de recristalização por rotação de subgrãos. A atuação de diferentes mecanismos de recristalização produziram diferentes padrões de orientações de eixos c de quartzo nos tectonitos estudados. Microscopia eletrônica de varredura (MEV) foi empregada para determinar a distribuição e caracterizar as estruturas dos poros nas bordas dos grãos de quartzo e hematita, e, dessa forma, avaliar a distribuição da fase fluida intergranular (Capítulo 5). Para esse estudo, foram escolhidas amostras de tectonitos de formações ferríferas deformados em condições metamórficas de fácies anfibolito, provenientes da margem leste do QF (flanco leste do Anticlinal de Mariana). Os resultados mostraram que a distribuição da fase fluida aquosa é altamente anisotrópica, concentrando-se especialmente em bordas de grãos normais a direção de extensão máxima. Essa é a direção preferencial do crescimento dos grãos de quartzo. Uma conclusão importante do presente estudo é que a geração de foliação penetrativa e as razões axiais encontradas para o agregado não estão necessariamente relacionadas ao campo de deformação local, podendo, no entanto, refletir o campo de tensão responsável pelas anisotropias na distribuição da fase fluida e no crescimento dos grãos do agregado. No Capítulo 6, é feita uma discussão geral na qual todos os resultados apresentados nos capítulos anteriores são integrados com o intuito de dar uma visão geral dos processos envolvidos no desenvolvimento das microestruturas e na produção das orientações cristalográficas preferenciais. Para tanto, são consideradas as condições de deformação para cada um dos locais de escolha das amostras, avaliando, dessa maneira, a influência de fatores prevalecentes na operação dos mecanismos de deformação. Tais fatores incluem o grau metamórfico (temperatura e pressão), a presença de fase fluida aquosa intergranular e a presença da magnetita e hematita na deformação por dissolução-precipitação por pressão (pressure solution) e por plasticidade intracristalina (dislocation creep). As conclusões gerais do presente estudo estão enumeradas no Capítulo 7.(I) Transformação da magnetita para hematita libera a energia de deformação acumulada no retículo da magnetita, eliminando, assim, grãos resistentes à deformação (hard grains), aumentando a ductibilidade do agregado; microfraturamento e dissolução ao longo das interfaces da magnetita e (0001) da hematita, resultando numa drástica redução do tamanho dos grãos. (II) Dissolução do quartzo é maior nas interfases quartzo/óxido de ferro normais à direção de compressão máxima, mostrando que essa não é uma característica exclusiva das interfases (001)/(0001), filossilicatos/quartzo; a dissolução dos grãos de quartzo é seletiva e depende da orientação cristalográfica dos grãos. Grãos orientados com eixos c paralelos a compressão máxima dissolvem-se mais facilmente. Grãos lenticulares de quartzo com orientação preferencial de forma e cristalográfica (eixo c) na direção da extensão máxima são interpretados como resultantes da precipitação e do crescimento orientado. (III) Padrões de microestruturas e microtramas de eixos c de quartzo em diferentes domínios composicionais indicam que os mecanismos de deformação e recristalização dos grãos de quartzo dependem do conteúdo em hematita. (IV) A distribuição anisotrópica da fase fluida, resultante da anisotropia da energia interfacial da hematita e da ação da tensão diferencial, leva ao crescimento preferencial dos grãos. A fase fluida acumula-se nas bordas normais à compressão mínima, promovendo o crescimento dos grãos nessa direção. Dessa forma, a anisotropia na distribuição da fase fluida, e, como conseqüência, na dissolução e crescimento dos grãos resulta na formação de uma foliação penetrativa de grãos (placas de hematita e grãos tabulares de quartzo).It is well known that phase transformation greatly enhances the ductility and may have important rheological constrains in rocks. While enhanced ductibility due to phase transformation is considered to be more important in the mantle reaction-enhanced ductibility is believed to be mainly a crustal phenomenon. However, the study of tectonitos from a low temperature and high strain deformed Precambrian Iron Formations (Itabira Formation, from Quadrilátero Ferrífero, MG) showed that the phase transformation magnetite to hematite, in a presence of a large amount of intergranular fluid phase, played an important role in the deformation of these crustal rocks. The phase transformation occurs in response to the strain energy stored in the magnetite grains arising from the increase of the dislocation densities along the crystallographic planes . The formation of new hematite grains presumably would reduce the strain energy just because they would at least initially be dislocation-free, so the magnetite containing dislocations and thus having moderate to high strain energy would be replaced by dislocation-free hematite. The new /(0001) interphase boundaries become planes of weakness along which the fluid phase might penetrate, allowing their inner part of the grains to be in contact with the fluid phase. Microfracturing and dissolution occur along the interphase (0001) normal to the maximum compressive stress. Thus phase transformation, microfracturing and dissolution are close-related processes that lead to a grain size reduction and to the production of a smooth and pervasive foliation. Experimental work has confirmed the observations of several authors that a second phase, especially layer silicates, greatly enhance the dissolution of quartz. Most of these observations have been performed on quartzites and this phenomenon is attributed to the ability of the layered silicates to adsorb structured water on the interfaces quartz/mica even under high normal stress gradients. Although the iron oxides have completely different crystal structures, optical and SEM observations in the iron formation rocks indicate that they bring about the same effect. This might be related to the effects of iron oxides on the wetting behavior of water. The presence of oxides enhances wetting thereby enhancing dissolution. Furthermore wetting behavior is usually crystallographically controlled, which is consistent with the observations. Besides the wetting behavior of iron oxides, the dissolution of quartz is strongly dependent on the crystallographic orientation. Extensive quartz c-axes measurements performed for a wide range of quartz-iron oxide rations showed that quartz grains oriented with its c-axes at low angle to the maximum compressive stress are more easily dissolved and tend to disappear as the dissolution proceeds. On the other hand grains with its c-axes at high angle to the maximum compressive stress are more resistant to the dissolution. In chapter 1 the problems and the aims of the present study are defined. In chapter 2 the evidences for the transformation and dissolution of magnetite are reported. Then a microphysical model is presented to explain how the transformation occurs and how it influences the dissolution of magnetite grains. Chapter 3 reports the microstructures and LPOs of quartz grains for several different quartz-magnetite/hematite ratios. The evidences found in these rocks suggest that the dissolution of quartz grains is enhanced by the presence of iron oxide and it is strongly dependent on the orientation of quartz c-axes with respect to the shortening direction (Z). All samples used for the studies reported in chapters 2 and 3 come from low temperature and high strain iron formation rocks collected in a cross-section along the southwestern part of the Serra do Curral Syncline. Chapter 4 presents the results of the study of quartz c-axes patterns taken in several oriented samples of iron formation collected in cross-sections along the northeastern part of the Serra do Curral Syncline (Serra da Piedade). Optical thin sections of the iron formation rocks display a banded structure defined by platelike-hematite-rich domains oriented parallel to the main foliation. The domain boundaries, which define the main foliation (SA), are composed mainly of hematite platelets rich layers (over 50 volume % of hematite). Quartz grains in hematite-rich-domains (50 vol.% of hematie) exhibit a quite equant shape. Internally the domains are made up of hematite platelets (10 vol.% of hematite) and tabular quartz grains. The elongate hematite and the tabular grains define the internal fabric (\'S IND.B\'), oblique to the main foliation (\'S IND.A\'). The specimens were selected according to the volume content of hematite in different admixed layers. Layers containing 10, 25 and 50 volume % of hematite were selected to perform the c-axis measurements. C-Axis patterns for recrystallized grains reveal notable differences in the three selected domains studied. The distinct types of microstructures observed in these domains indicate that the mechanism of recovery (grain boundary migration recrystallization or dislocation climb) is affected by the content of the hematite present in each domain. Consequently, the operation of the different mechanisms of recovery was responsible for the development of different c-axis patterns present in domains with different hematite content. When recovery occurs dominantly by strain-induced grain boundary migration, as it does in quartz domains with hematite content of 10 or 25 vol.% (quartz domains), the resulting c-axis preferred orientations are marked by a maximum concentration around the pole to the internal foliation. Fundamentally, c-axis distributions in these domains occur approximately parallel to the maximum shortening direction (Z). When recovery occurs dominantly by dislocation climb (leading to progressive subgrain rotation recrystallization), as it does in domains with 50% of hematite content (hematite domains), the c-axis preferred orientations are defined by a maximum concentration asymmetrically distributed around the intermediate shortening direction (Y). Chapter 5 reports the study of the microstructures of grain boundaries and the distribution of pores via SEM microscopy in quartz-hematite iron formation rocks. The samples studied were collected along a section of the Mariana Anticline. According to the petrological constrains, the metamorphic grade of the banded iron formation of this part of the Quadrilátero Ferrífero may have reached the amphibolite facies. The higher temperature and lower strain rate metamorphism produced the mineral assemblage quartz + hematite \' + OU -\' kyanite \'+ OU -\' garnet in these banded iron formation. Contrasting with the tabular hematite grains in the tectonitos of the Serra da Piedade, hematite grains in tectonitos of the Mariana Anticline show shapes very similar to the micas in quartzite rocks. They are characteristically foliated and display an excellent basal partition. The main goal of the study of this chapter is to infer the fluid distribution along grain boundaries and together with the grain boundaries microstructures to evaluate the role of fluid distribution on the mechanisms and kinetics of quartz growth in these metamorphic rocks. All observations were performed in oriented samples hence an attempted has been made to establish a correlation between the fluid distribution and the principal compressive directions. Grain boundary microstructures observed in different sections provided important insights on the processes involved in the generation of these microstructures. The different topographies observed along differently oriented boundaries and the heterogeneous distribution of pores along these grain boundaries indicate that fluid geometry and distribution in these rocks are highly anisotropic. The irregular topography of the quartz grain boundaries normal to the maximum elongation direction (X), in addition to the numerous inclusions of tiny hematite grains indicate that a significative grain growth occurred parallel to that direction, driven by reduction of the total grain boundary area. The purpose of chapter 6 is to assemble the information presented in the preceding chapters in order to draw a broad idea about all the processes involved in the deformation of the iron formation rocks and in the generation of the lattice preferred orientations. In addition, a general approach is given to evaluate the influence of the physical and chemical environment on the deformation mechanisms, such as metamorphic grade (T and P, inferred from mineral assemblages), intergranular fluid phase and the presence of a second phase. To assess the geological implications of the results obtained. In conclusion (Chapter 7) the present work has shown that (I) phase transformation greatly enhances the ductility of rocks under crustal conditions, by removing grains which are more resistant to the deformation. (II) Dissolution is easier at the interphase boundaries (magnetite-hematite and quartz-iron oxides). This might be related to the effects of iron oxide on the wetting behavior of water. (III) The dissolution-precipitation of quartz grains is strongly dependent on the crystallographic orientation. This lead to the development of lattice preferred orientations due to anisotropic dissolution/growth rates during solution-transfer creep. For quartz grains, both dissolution and growth parallel to the c-axis are faster than at right angles, resulting in a formation of c-axis maxima parallel to the maximum principal finite elongation direction (X) and c-axis minima parallel to the maximum principal finite shortening direction (Z). (IV) The extensive measurement of quartz c-axes showed that the lattice preferred orientations (LPOs) change significantly as a function of the amount of admixed hematite in the layers. Layers with a low content of hematite exhibit microstructures indicative of grain boundary migration. On the other hand, layers with high content of hematite show microstructures indicative of subgrain formation. Thus quartz grains are recovered and recrystallized by different mechanisms as a function of the amount of hematite that, on its turn, result in different patterns of LPOs. (V) Based on the characterization of pore structures and its distribution, evidences have been found for anisotropic grain growth. The anisotropic grain growth is a result of the anisotropic distribution of the fluid phase due to a combination of the anisotropy of hematite and to the differential stress. Interphase boundaries normal to the maximum shortening direction (Z) are typically pore-free. This might be indicative of the presence of a continuous high-diffusivity fluid film along these boundaries. On the other hand, boundaries normal to the maximum elongation direction (X) display a very irregular topography and profuse distribution of irregular pores, in which well-formed hematite and quartz grains are frequently found. These features indicate that these boundaries are sites of fluid accumulation and precipitation. The highest grain growth rates in the X-direction led to the formation of grains with aspect ratios that exceed the value expected from strain

Fluid-assisted grain boundary sliding in bedding-parallel quartz veins deformed under greenschist metamophic grade.

Iron formation rocks of Quadrilátero Ferrífero, Brazil, were deformed at greenschist facies. Quartz grains in bedding parallel veins were sheared and deformed by a combination of mechanisms assisted by aqueous fluids. Veins in the outcrop appear to be stretched parallel to the compositional layering. The overall vein shapes resemble those of boundinage and pinch and swell. In thin sections, veins show microstructures similar to those observed in hand samples, where domains of large quartz crystals are pulled apart for several millimeters. The voids between quartz fragments are filled with domains of polycrystalline quartz. The microstructural and orientation data show that the strain imposed on the vein as a rigid and competent layer was not accommodated in the quartz polycrystals exclusively by crystal plastic deformation or dynamic recrystallization. The new grains are strain-free, with straight boundaries and with weak to random crystallographic fabrics. We interpret these features to have resulted from a combination of processes, which included grain boundary sliding accomplished by solution transfer. We propose that the coeval operation of both mechanisms allows the aggregate to deform at higher strain rates without necking of the vein layer in a type of flow similar to those described in superplastic regimes

Sheared-bedding parallel quartz vein as an indicator of deformation processes.

Monomineralic veins are well known as good recording of filling and precipitation processes. However, they are also able to register the action of deformation in rocks. We used monomineralic, quartz veins from Quadriláterro Ferrífero, Brazil, to represent how the deformation actuated during the transformation of different structures. They were analyzed using a combination of a u-stage and EBSD. Two main types of quartz aggregates are distinguished: single crystals with subgrain development and strain-free grains of quartz. We interpret the microstructures and textures as the result of a combination of concurrent crystal plasticity, microfracturing, solution transfer and recrystallization. The single quartz grains were deformed by dislocation glide. However, as the c-axis orientation of the grain was not favorable for further glide on basal planes, the deformation was accommodated by microfracturing. New grains were formed along the deformed zones with crystallographic orientations suitable for gliding on basal planes. As the deformation proceeded, the new grains continue to develop until the vein was completely recrystallized into an aggregate of granular grains

Crystallographic texture of the magnetite-hematite transformation : evidence for topotactic relationships in natural samples from Quadrilátero Ferrífero, Brazil.

The transformation of magnetite to hematite is described and analyzed in three natural samples of banded iron formation, from Quadrilátero Ferrífero, Brazil. In each sample, a particular microstructure related to the transformation process is described. In the first, magnetite crystals are large and euhedral, and they display the beginning of the transformation into hematite. In the second sample, a relict crystal of magnetite was found and the fabric of the transformed hematite was evaluated. In the last sample, the foliation was the main observed structure and the correlations of magnetite and hematite lattices were measured. All the microstructures were analyzed in a scanning electron microscope equipped with a detector for electron backscatter diffraction allowing the complete analysis of crystallographic orientations of hematite and magnetite on a local scale. The results show that the orientations of the basal planes of hematite coincide with the orientations of the octahedral planes of magnetite, indicating that the hematite crystals are a direct product from the magnetite transformation

Nucleation and growth of new grains in recrystallized quartz vein : an example from banded iron formation in Iron Quadrangle, Brazil.

Intracrystalline microcracks developed in quartz single crystals deformed in greenschist metamorphic conditions. A detailed study of samples collected in tabular to lens shape quartz vein was carried out to investigate how the microcracks initiated and how the microstructures evolved with the progressive deformation. A combination of light and EBSD (electron backscatter diffraction) techniques was used to analyze the microstructures and determine the crystallographic orientation of quartz grains. The crystallographic orientations of microcracks indicate that they might have initiated parallel to the direction of one of the rhombohedral planes of the host crystals. It is suggested that new grains nucleated by rotation of broken fragments from the host grains. c-axes the of host are distributed in a small-circle close to the foliation plane while the c-axes of the new grains in microcracks are more scattered when compared with the host orientations. New grains grew with their c-axes approximately perpendicular to the shortening direction