O campo vulcânico Patagônico é composto pelo vulcanismo datado do Quaternário ao Cretáceo e está amplamente distribuído no ambiente geotectônico de extra back-arc continental. Onze vulcões associados à ocorrência de xenólitos mantélicos estão situados dos 36°13’S aos 44°52’S. Estes vulcões são dominantemente compostos por basanitos e a basaltos alcalinos, que são divididos em dois grupos com base em aspectos petrográficos, geoquímicos e isotópicos. (Grupos I e II). Estes Grupos estão relacionados a fontes mantélicas similares, mas foram submetidos a diferentes processos metassomáticos. Os Grupos I e I foram gerados a partir de baixas taxas de fusão a partir de uma fonte mantélica do tipo OIB na zona de estabilidade da granada, mas o Grupo II tem características de manto enriquecido (EMII) possivelmente herdadas de um agente metassomático relacionado à zona de subducção, enquanto que o Grupo I demonstra assinatura geoquímica de magmas tipo OIB relacionados a fontes mantélicas ricas em flogopita. Os basaltos alcalinos do vulcão Agua Poca (37º01’S - 68º07’W) pertencem ao Grupo II e são traquibasaltos. O vulcão Agua Poca é definido é piroclástico monogenético, é composto por intercalações de camadas de spatter e cinder, hospeda xenólitos mantélicos e está localizado a oeste da Província de La Pampa, no extremo norte da Argentina. As amostras de xenólitos mostram textura protogranular, protogranular a porfiroclástica, porfiroclástica e porfiroclástica a equigranular e são compostos por olivina (fosterita), ortopiroxênio (enstatita), clinopiroxênio (diopsídio) e espinélio (sp). Os xenólitos estudados são peridotitos da fácies espinélio e piroxenitos anidros em basaltos alcalinos do Pleistoceno com #Mg em rocha total de 89 a 91. As assinaturas geoquímicas desses xenólitos mostram correlação negativa entre os principais óxidos quando dispostos contra o #Mg e estão empobrecidos em elementos incompatíveis em relação ao manto primitivo (MP). Os xenólitos do vulcão Agua Poca são caracterizados pelo empobrecimento de ETRP e ETRM normalizados para o MP e pelo fracionamento de ETRL em relação aos ETRP (CeN/YbN = 0,15-0,5), com exceção da amostra HAP10 (1,46). Esse comportamento indica que os xenólitos do terreno Cuyania são o resultado de 1 a 10% de fusão do DMM (Manto Depletado) ou de 8 a 17% do MP (Manto Primitivo). Em geral, os peridotitos mostram anomalias positivas de Ba, U, Ta, Pb, Zr e Ti; e anomalias negativas de Rb, Th, Nb, La e Y, enquanto que os piroxenitos mostram anomalias positivas de Ba, U, Ta e Pb; e anomalias negativas de Th, Nb, La, Zr, Hf, Ti e Y. Curvas de mistura calculadas para o resíduo de fusão do MP/DMM com a composição de fluídos/sedimentos derivados de ambientes de subducção indicam interação do manto com até 3% de fluídos/sedimentos. As razões 87Sr/86Sr (0,702874 - 0,704999, com média de 0,704035) são muito similares àquelas definidas para peridotitos com fonte tipo OIB. Agua Poca tem razões 87Sr/86Sr, que estão abaixo daquelas definidas para peridotitos metassomatizados (usualmente >0,705). As razões de Nb/Ta sugerem a presença de um reservatório eclogítico refratário subductado fusão parcial gerando líquidos alcalinos com razões Nb/Ta supercondríticas. / The Patagonian Volcanic Field composed of late Cretaceous to Quaternary volcanism is widely distributed in a continental extra back-arc geotectonic environment. Eleven monogenetic volcanoes accompanied with ultramafic xenoliths are situated from 36°13′S to 44°52′S. These volcanoes are dominantly composed of basanite to alkaline basalt, which are divided into two groups, based on mineralogy, geochemical and isotope compositions (Groups I and II). These Groups are originated from the similar subcontinental mantle sources, but were undergone to different metasomatism processes. Groups I and II were generated from low melting degrees of an OIB-like garnet peridotite, but the Group II has enriched mantle (EMII) characteristics possibly inherited from on-going subduction related metasomatism, while Group I demonstrates the OIB-like signature, which might result from phlogopite-bearing in the subcontinental lithosphere. The alkaline basalts from Agua Poca volcano (37º01’S - 68º07’W) belong to the Group II and are trachybasalts. The Agua Poca volcano is a monogenetic pyroclastic volcano composed by intercalation of spatter and cinder layers, host ultramafic mantle xenoliths and is located in the West of the La Pampa Province, Northernmost of Argentine Patagonia. The xenoliths show protogranular, protogranular to porphyroclastic, porphyroclastic and porphyroclastic to equigranular textures, and are composed of olivine (fosterite), orthopyroxene (enstatite), clinopyroxene (diopside) and spinel (sp). The studied xenoliths are anhydrous spinel-bearing peridotite and pyroxenite xenoliths in Pleistocene alkali basalts with whole rock Mg# from 88 to 91. Geochemical signatures of the mantle xenoliths show negative correlation between main oxides against Mg# and depletion in incompatible elements compared to primitive mantle (PM). Agua Poca mantle xenoliths are characterized by flat Sun & McDonough (1989) primitive mantle (PM) normalized HREE and MREE patterns, and depletion of LREE compared to HREE (CeN/YbN = 0.15-0.5), with exception of the HAP10 (1.46) sample. These characteristics suggest that partial melting event is the main process responsible for the generation of these xenoliths. Model calculations suggest that the xenoliths are the result of 1 to 10% of DMM (Depleted Mantle MORB) or 8 to 17% of PM partial melting. Peridotite samples show positive anomalies of Ba, U, Ta, Pb, Zr and Ti; and negative anomalies of Rb, Th, Nb, La and Y, while the pyroxenite samples show positive anomalies of Ba, U, Ta and Pb; and negative anomalies of Th, Nb, La, Zr, Hf, Ti and Y. Mixing curves calculated to mixtures of melting residue of PM/DMM and fluid or sediment compositions related to subduction tectonic setting end members suggest up to 3% of interaction of the fluid sediment on the depleted mantle residue. 87Sr/86Sr ratios (0.702874 - 0.704999, with average of 0.704035) are similar to those defined to peridotites with OIB source (87Sr/86Sr = 0.70244 to 0.70502), being close to Depleted Mantle (DM; 87Sr/86Sr = 0.7023 to 0.7032) values. Nb/Ta ratios suggest that Agua Poca xenoliths were undergone to partial melting processes that generated alkaline magmas with superchondritic Nb/Ta ratios.

M.Sc. / Please refer to full text to view abstract

Differentiation of the lithospheric mantle occurred principally through partial melting and extraction of melts. Harzburgites are generally considered as melting residues whereas lherzolites are regarded as pristine mantle weakly affected by melting. However, some orogenic peridotites show evidence of igneous refertilization. In this context, this work re-investigates the nature of the Lherz lherzolites (Pyrenees), type-locality of lherzolites, described as a piece of preserved fertile mantle. Structural and geochemical data show that these lherzolites are not pristine but formed through a refertilization reaction between MORB-like melts and refractory lithosphere. Moreover, the Lherz peridotites were partly used to infer the composition of the primitive upper mantle and these results may have important implications for the nature of the late veneer. Additionally, crystal-preferred orientations of minerals (CPO) highlight a strong feedback between melt percolation and finite strain in the percolated rocks. CPO variations are ruled by a subtle balance between instantaneous melt fraction and local strain rate. This work also investigated the effect of melt percolation on Hf, Nd and Sr isotopes. Isotope systematics in Lherz shows that strong isotopic decoupling may arise in a percolation front. The modelling suggests that decoupled isotopic signatures are generated during porous flow and governed by the melt/matrix elements concentrations, chemical diffusivities or efficiency of isotopic homogenization. Melt-rock interactions can generate “intraplate-like” isotopic signatures. This suggests that a part of isotopic signatures of mantle-related rocks could be generated by diffusional processes associated with melt transport.

It has always been enigmatic which processes control the accretion of the North American terranes towards the Pacific plate and the landward migration of the San Andreas plate boundary. One of the theories suggests that the Pacific plate first cools and captures the uprising mantle in the slab window, and then it causes the accretion of the continental crustal blocks. The alternative theory attributes the accretion to the capture of Farallon plate fragments (microplates) stalled in the ceased Farallon-North America subduction zone. Quantitative judgement between these two end-member concepts requires a 3D thermomechanical numerical modeling. However, the software tool required for such modeling is not available at present in the geodynamic modeling community. The major aim of the presented work is comprised basically of two interconnected tasks. The first task is the development and testing of the research Finite Element code with sufficiently advanced facilities to perform the three-dimensional geological time scale simulations of lithospheric deformation. The second task consists in the application of the developed tool to the Neogene deformations of the crust and the mantle along the San Andreas Fault System in Central and northern California. The geological time scale modeling of lithospheric deformation poses numerous conceptual and implementation challenges for the software tools. Among them is the necessity to handle the brittle-ductile transition within the single computational domain, adequately represent the rock rheology in a broad range of temperatures and stresses, and resolve the extreme deformations of the free surface and internal boundaries. In the framework of this thesis the new Finite Element code (SLIM3D) has been successfully developed and tested. This code includes a coupled thermo-mechanical treatment of deformation processes and allows for an elasto-visco-plastic rheology with diffusion, dislocation and Peierls creep mechanisms and Mohr-Coulomb plasticity. The code incorporates an Arbitrary Lagrangian Eulerian formulation with free surface and Winkler boundary conditions. The modeling technique developed is used to study the aspects influencing the Neogene lithospheric deformation in central and northern California. The model setup is focused on the interaction between three major tectonic elements in the region: the North America plate, the Pacific plate and the Gorda plate, which join together near the Mendocino Triple Junction. Among the modeled effects is the influence of asthenosphere upwelling in the opening slab window on the overlying North American plate. The models also incorporate the captured microplate remnants in the fossil Farallon subduction zone, simplified subducting Gorda slab, and prominent crustal heterogeneity such as the Salinian block. The results show that heating of the mantle roots beneath the older fault zones and the transpression related to fault stepping, altogether, render cooling in the slab window alone incapable to explain eastward migration of the plate boundary. From the viewpoint of the thermomechanical modeling, the results confirm the geological concept, which assumes that a series of microplate capture events has been the primary reason of the inland migration of the San Andreas plate boundary over the recent 20 Ma. The remnants of the Farallon slab, stalled in the fossil subduction zone, create much stronger heterogeneity in the mantle than the cooling of the uprising asthenosphere, providing the more efficient and direct way for transferring the North American terranes to Pacific plate. The models demonstrate that a high effective friction coefficient on major faults fails to predict the distinct zones of strain localization in the brittle crust. The magnitude of friction coefficient inferred from the modeling is about 0.075, which is far less than typical values 0.6 – 0.8 obtained by variety of borehole stress measurements and laboratory data. Therefore, the model results presented in this thesis provide additional independent constrain which supports the “weak-fault” hypothesis in the long-term ongoing debate over the strength of major faults in the SAFS. / Seit jeher rätselhaft sind die Prozesse, die die Akkretion der Nordamerikanischen Terranen in Richtung der Pazifischen Platte sowie die Wanderung der Plattengrenze der San-Andreas-Verwerfung in Richtung Festland bestimmen. Eine Theorie besagt, dass sich die Pazifische Platte erst abkühlt und den aufsteigenden Mantel im „Slab Window“ fängt und somit die Akkretion der kontinentalen Krustenblöcke bewirkt. Die andere Theorie geht von einer Akkretion durch das Fangen von Teilen der Farallon-Platte (Mikroplatten) aus, die in der inaktiven nordamerikanischen Farallon-Subduktionszone fest stecken. Die quantitative Beurteilung dieser beiden gegensätzlichen Konzepte erfordert eine thermomechanische numerische 3-D-Modellierung. Das dafür benötigte Software Tool steht jedoch der geodynamischen Modellierung derzeit noch nicht zur Verfügung. Das Hauptziel der vorliegenden Arbeit umfasst im Wesentlichen zwei miteinander verbundene Aufgaben. Die erste besteht in der Entwicklung und Erprobung des Finite-Element-Codes, dessen Eigenschaften den hohen Anforderungen an die Ausführung der dreidimensionalen Simulationen lithosphärischer Deformation auf geologischer Zeitskala gerecht werden müssen. Die zweite Aufgabe ist die Anwendung des entwickelten Tools auf die neogenen Deformationen der Kruste und des Mantels entlang der San-Andreas-Verwerfung in Zentral- und Nordkalifornien. Die Modellierung auf geologischer Zeitskala lithosphärischer Deformation bringt für die Software Tools in Bezug auf Konzept und Durchführung zahlreiche Herausforderungen mit sich. Unter anderem gilt es, den Brittle-Ductile-Übergang in einem einzigen Modell sowie die Gesteinsrheologie in einer breiten Spanne unterschiedlicher Temperaturen und Spannungen adäquat darzustellen und die extremen Deformationen der freien Oberfläche und internen Grenzen aufzulösen. Im Rahmen der vorliegenden Arbeit erfolgte die erfolgreiche Entwicklung und Erprobung des neuen Finite-Element-Codes (SLIM3D). Dieser Code beinhaltet eine gekoppelte thermomechanische Behandlung von Deformationsprozessen und ermöglicht eine elasto-visko-plastische Rheologie mit Diffusion, Dislokation, Peierls Kriechmechanismen und Mohr-Coulomb-Plastizität. Der Code verbindet eine Arbitrary Lagrangian-Eulerian kinematische Formulierung mit freien Oberflächen- und Winkler-Randbedingungen. Das entwickelte Modellierungsverfahren wird für die Untersuchung der Aspekte verwendet, die die neogene lithosphärische Deformation in Zentral- und Nordkalifornien beeinflussen. Die Modellanordnung konzentriert sich auf die Interaktion zwischen drei großen tektonischen Elementen in dieser Region: die Nordamerikanische Platte, die Pazifische Platte sowie die Gorda-Platte, die sich in der Mendocino-Triple-Junction treffen. Unter anderem verdeutlicht die Modellierung den Einfluss des Aufsteigens der Asthenosphäre in das sich öffnende „slab window“ der übergelagerten Nordamerikanischen Platte. Die Modelle beziehen auch die angelagerten Überreste der Mikroplatten in der fossilen Farallon-Subduktionszone, die vereinfachte subduzierende Gorda-Platte sowie markante Heterogenitäten der Kruste, wie beispielsweise den „Salinian Block“, mit ein. Die Ergebnisse zeigen, dass die Erwärmung der Mantellithosphäre unter den älteren Störungszonen sowie die Transpression eine Abkühlung im „Slab Window“ als alleinige Begründung für die Ostwärtsbewegung der Plattengrenze nicht zulassen. Aus Sicht der thermomechanischen Modellierung bestätigen die Ergebnisse das geologische Konzept, welches durch das mehrmalige Fangen von Mikroplatten den Hauptgrund für die Wanderung der Plattengrenze der San-Andreas-Verwerfung in Richtung Festland über die letzten 20 Millionen Jahre sieht. Die Überreste der Farallon-Platte, die in der fossilen Subduktionszone gefangen sind, verursachen im Mantel eine wesentlich stärkere Heterogenität als die Abkühlung der Asthenosphäre und stellen somit den effizienteren und direkteren Weg für die Anlagerung der nordamerikanischen Gebiete an die Pazifische Platte dar. Die Modelle demonstrieren, dass ein hoher effektiver Reibungskoeffizient an großen Störungen nicht in der Lage ist, die eindeutigen Zonen der Dehnungslokalisierung in der spröden Kruste vorherzusagen. Die Größe des Reibungskoeffizienten, die sich aus der Modellierung ableitet, beträgt etwa 0,075 und ist damit wesentlich kleiner als die durch unterschiedliche Bohrlochmessungen und Labordaten ermittelten Spannungswerte zwischen 0,6 und 0,8. Daher liefern die in dieser Arbeit präsentierten Ergebnisse der Modelle in der seit langem geführten Debatte über die Stärke von großen Störungen in der San-Andreas-Verwerfung eine zusätzliche unabhängige Begründung der „Weak-Fault“-Hypothese.

Made available in DSpace on 2014-06-11T19:32:21Z (GMT). No. of bitstreams: 0 Previous issue date: 2009-09-03Bitstream added on 2014-06-13T19:42:57Z : No. of bitstreams: 1 machado_fb_dr_rcla.pdf: 5312808 bytes, checksum: 15fbc455b5a4e01ba9b62b7b87d26ffa (MD5) / Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) / Os basaltos da Formação Serra Geral nos estados de Mato Grasso do Sul e Mato Grosso ocupam uma área aproximada de 180.000 km2 e estão sobrepostos aos arenitos desérticos da Formação Botucatu e sotopostos pelos sedimentos neocretácicos dos Grupos Bauru e Caiuá. Apresentam mineralogia primária constituída de plagioclásio (40% - 55%; An42-67), clinopiroxênios (19% - 40%; augita Wo30-40En34-46Fs35-39), minerais opacos (2% - 10%) e olivina (até 1,5%; Fo31-50). Dados geotermométricos revelam temperaturas de cristalização dos clinopiroxênios em ~1100ºC. A geoquímica dessas rochas mostram tratar-se de basaltos dos tipos Pitanga (ATi; 2,6%<TiO2<4,2%, 396ppm<Sr<438ppm) e Ribeira (BTi;1,7%TiO2<2,4%, 246ppm<Sr<286ppm), onde observam-se diferenças marcantes na distribuição de elementos traços, as quais parecem consistentes com um processo de cristalização fracionada. Processos de contaminação crustal que poderiam interferir na interpretação dos resultados analíticos foram descartados pelos baixos valores de Sri sempre menores de 0,706 e também Rb/Ba < 1,0. O empobrecimento dos basaltos Pitanga e Ribeira em Pbi, (206Pb/204Pbi<17,747; 207Pb/204Pbi<15,522 e 208Pb/204Pbi<38,174), Ndm (0,51235 – 0,51239) e Sri, (0,70549 – 0,70586) são semelhantes com as rochas básicas do sítio 525A da Cadeia Walvis e com o componente mantélico EMI, e ainda, os valores de eNd (-5,21 a - 6,00) sugerem tratar-se de rochas submetidas à fusão parcial pré-cambrianas. Os LILEs possuem padrões de abundância inconsistentes com aqueles de Tristão da Cunha (OIB) e NMORB, e ficam fora da linha de mistura destes. Assim, as razões (La/Yb)n: (4,17-7,70); e (La/Sm)n: (2,15-2,54) e isótopos das amostras representativas dos magmas, em conjunto com aquelas publicadas na literatura, indicam a contribuição do manto litosférico subcontinental na geração da Província Magmática do Paraná... / The Paraná Magmatic Province (PMP) represents one of the greatest volcanic event of our planet, covering more than 70% of the Paraná Basin as flows, sills and dykes. In the Mato Grosso do Sul and Mato Grosso states, the study of basalts has geologic importance because of the absence of published works. In those regions, basaltic rocks occupy 180,000 km2, and are re-covered by the sediments of Bauru Basin, and are associated to sandstones of Botucatu Formation. The mineralogy of basalt is represented by plagioclase (An 42-67; 40% - 55%), pyroxene (augite: Wo30-40En34-46Fs35-39; 19-40%), magnetite (2-10%) and olivine (Fo31-50; <1,5%). Based on rheologic calculations the temperature of crystallization of the pyroxenes is estimated 1100ºC. Also geochemical analyses show that the investigated basalts are of Pitanga (ATi) and Ribeira (BTi) types: ATi: 2,6%<TiO2<4,2%; 396ppm<Sr<438ppm and BTi: 1,7%TiO2<2,4%, 246ppm<Sr<286ppm. LILE abundance for the studied flows evidence differences that are consistent with process of fractional crystallization. Processes of crustal contamination can be discarded considering the values of Sri < 0,70586 e Rb/Ba < 1,0. Moreover, the values of LILE show values that are not consistent with those of OIB - Tristão da Cunha and N-MORB and are outside of the line of mixture of these. In relation to isotopic ratio, the magmas are depleted in Pbi, ,(206Pb/204Pbi<17,747; 207Pb/204Pbi<15,522 e 208Pb/204Pbi<38,174); Ndm (0,51235 – 0,51239) e Sri, (0,70549 – 0,70586) if compared with BTi basalts occurring in southern PMP. These ratios are also similar with that of the basic rocks of the small site 525A of the Walvis Chain and of the mantle component EMI. The values of εNd (-5,21 a -6,00) and of 87Sr/86Sri indicate that the magma types Pitanga and Ribeira were submitted to partial melting in Precambrian times. The ratios (La/Yb)n: (4,17- 7,70); and (La/Sm)... (Complete abstract click electronic access below)

Orientador: Antonio José Ranalli Nardy / Banca: Leila Soares Marques / Banca: Marcos Aurélio Farias de Oliveira / Banca: Evandro Fernandes de Lima / Banca: Antonio Carlos Artur / Acompanha 2 mapas anexos / Resumo: Os basaltos da Formação Serra Geral nos estados de Mato Grasso do Sul e Mato Grosso ocupam uma área aproximada de 180.000 km2 e estão sobrepostos aos arenitos desérticos da Formação Botucatu e sotopostos pelos sedimentos neocretácicos dos Grupos Bauru e Caiuá. Apresentam mineralogia primária constituída de plagioclásio (40% - 55%; An42-67), clinopiroxênios (19% - 40%; augita Wo30-40En34-46Fs35-39), minerais opacos (2% - 10%) e olivina (até 1,5%; Fo31-50). Dados geotermométricos revelam temperaturas de cristalização dos clinopiroxênios em ~1100ºC. A geoquímica dessas rochas mostram tratar-se de basaltos dos tipos Pitanga (ATi; 2,6%<TiO2<4,2%, 396ppm<Sr<438ppm) e Ribeira (BTi;1,7%TiO2<2,4%, 246ppm<Sr<286ppm), onde observam-se diferenças marcantes na distribuição de elementos traços, as quais parecem consistentes com um processo de cristalização fracionada. Processos de contaminação crustal que poderiam interferir na interpretação dos resultados analíticos foram descartados pelos baixos valores de Sri sempre menores de 0,706 e também Rb/Ba < 1,0. O empobrecimento dos basaltos Pitanga e Ribeira em Pbi, (206Pb/204Pbi<17,747; 207Pb/204Pbi<15,522 e 208Pb/204Pbi<38,174), Ndm (0,51235 - 0,51239) e Sri, (0,70549 - 0,70586) são semelhantes com as rochas básicas do sítio 525A da Cadeia Walvis e com o componente mantélico EMI, e ainda, os valores de eNd (-5,21 a - 6,00) sugerem tratar-se de rochas submetidas à fusão parcial pré-cambrianas. Os LILEs possuem padrões de abundância inconsistentes com aqueles de Tristão da Cunha (OIB) e NMORB, e ficam fora da linha de mistura destes. Assim, as razões (La/Yb)n: (4,17-7,70); e (La/Sm)n: (2,15-2,54) e isótopos das amostras representativas dos magmas, em conjunto com aquelas publicadas na literatura, indicam a contribuição do manto litosférico subcontinental na geração da Província Magmática do Paraná... (Resumo completo, clicar acesso eletrônico abaixo) / Abstract: The Paraná Magmatic Province (PMP) represents one of the greatest volcanic event of our planet, covering more than 70% of the Paraná Basin as flows, sills and dykes. In the Mato Grosso do Sul and Mato Grosso states, the study of basalts has geologic importance because of the absence of published works. In those regions, basaltic rocks occupy 180,000 km2, and are re-covered by the sediments of Bauru Basin, and are associated to sandstones of Botucatu Formation. The mineralogy of basalt is represented by plagioclase (An 42-67; 40% - 55%), pyroxene (augite: Wo30-40En34-46Fs35-39; 19-40%), magnetite (2-10%) and olivine (Fo31-50; <1,5%). Based on rheologic calculations the temperature of crystallization of the pyroxenes is estimated 1100ºC. Also geochemical analyses show that the investigated basalts are of Pitanga (ATi) and Ribeira (BTi) types: ATi: 2,6%<TiO2<4,2%; 396ppm<Sr<438ppm and BTi: 1,7%TiO2<2,4%, 246ppm<Sr<286ppm. LILE abundance for the studied flows evidence differences that are consistent with process of fractional crystallization. Processes of crustal contamination can be discarded considering the values of Sri < 0,70586 e Rb/Ba < 1,0. Moreover, the values of LILE show values that are not consistent with those of OIB - Tristão da Cunha and N-MORB and are outside of the line of mixture of these. In relation to isotopic ratio, the magmas are depleted in Pbi, ,(206Pb/204Pbi<17,747; 207Pb/204Pbi<15,522 e 208Pb/204Pbi<38,174); Ndm (0,51235 - 0,51239) e Sri, (0,70549 - 0,70586) if compared with BTi basalts occurring in southern PMP. These ratios are also similar with that of the basic rocks of the small site 525A of the Walvis Chain and of the mantle component EMI. The values of εNd (-5,21 a -6,00) and of 87Sr/86Sri indicate that the magma types Pitanga and Ribeira were submitted to partial melting in Precambrian times. The ratios (La/Yb)n: (4,17- 7,70); and (La/Sm)... (Complete abstract click electronic access below) / Doutor

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Pour mieux comprendre le processus géodynamique qui a permis la destruction du craton Nord Chinois (NCC), le rôle des fluides mantelliques a été examiné. Pour cela, les distributions des teneurs en eau et des compositions isotopiques du Lithium dans le manteau lithosphérique NCC ont été déterminées à partir des xénolites de péridotite entrainés par les basaltes mésozoïques et cénozoïques. Une variation temporelle des teneurs en eau est observée. Le manteau lithosphérique cénozoïque est appauvri en eau, sans doute suite à l'amincissement crustal et au réchauffement du manteau résiduel par un flux ascendant asthénosphérique. Le manteau lithosphérique mésozoïque montre des teneurs en eau intermédiaire entre les teneurs élevées du Crétacé et les teneurs basses cénozoïques, indiquant une déshydratation du manteau commençant dès le début de sa destruction. Cette déshydratation, facilitée par la destruction du manteau lithosphérique profond, permet de renforcer la rigidité de la lithosphère et lui permet de résister à la convection mantellique. Les distributions élémentaire et isotopique du Li montrent une grande hétérogénéité, aux échelles intra et inter-cristallines. Par simulation numérique, nous démontrons que deux enrichissements successifs ont affecté le manteau, un enrichissement limité (<5ppm) avec une signature pauvre en 7Li ([delta]7Li ~ -20 [pour mille]), suivi d'un enrichissement important (> 100 ppm) avec une signature riche ([delta]7Li ~ +20 [pour mille]), précédent de peu l'exhumation des xénolites. La formation des liquides métasomatiques responsables de ces enrichissements nécessite une distribution hétérogène dans le manteau NCC d'éléments recyclés lors de la subduction à l'est du NCC / In order to investigate the geodynamic cause for destruction of the North China Craton (NCC), the role of mantle fluids is examined. The aim of the PHD work is to clarify H2O contents and lithium isotopic compositions of the NCC lithospheric mantle by studying peridotite xenoliths hosted by Mesozoic-Cenozoic basalts across eastern NCC. A temporal variation of H2O content has been revealed, and it has deep implications for processes of craton destruction. The Cenozoic lithospheric mantle was featured by low H2O content, interpreted to be the relict mantle that survived the lithospheric thinning and has been dewatered by reheating from upwelling asthenospheric flow. The late-Mesozoic lithospheric mantle showed relatively high H2O content, a hydrous status intermediate between the Cretaceous hydration and the Cenozoic dryness, indicating the dehydration of the NCC mantle with time during NCC destruction. The dehydration, facilitated by thinning of weak mantle pieces at bottom, is one way by which the lithosphere strengthens itself to survive in the convecting mantle. Extreme Li and isotopic disequilibria were observed intra- and inter-mineral in the peridotites. With numerical simulations, we demonstrate two superimposed Li enrichment events occurring at the mantle: a limited Li enrichment (< 5 ppm) and large delta7Li depletion (-20~-10[per 1000]) of the mantle domain, followed by a recent and transient infiltration of high Li and delta7Li (up to +20 [per 1000]) melts/fluids. The anomalous Li isotopic compositions of mantle metasomatic agents call upon the same of their mantle sources, and we assume recycled components, both Li isotopically heavy and light, in the mantle beneath the eastern NCC

Nós usamos tomografia sísmica de tempo de percurso para estudar o manto superior sob as regiões Sudeste e Centro-Oeste do Brasil. Este método baseia-se na inversão de resíduos relativos de tempo para as ondas P e S (VanDecar, 1991), que foram obtidos para mais de 80 estações em uma área de 20 x 20 graus. Mais de 11000 e 8000 resíduos de tempo foram obtidos para as ondas P (P direta e PKPdf) e S (S direta, ScS, SKS e SKKS), respectivamente, utilizando correlação cruzada de forma de ondas para até 12 estações operando simultaneamente. Para avaliar a robustez dos resultados com respeito aos dados, nós utilizamos o método estatístico de re-amostragem Jackknife, o qual inerentemente leva em conta a altamente variável cobertura dos raios e os erros das medidas, e pode fornecer limites de confiança para as anomalias. Inversões regionais foram realizadas para estudar a influência da parametrização nas anomalias sísmicas. Nossos resultados mostraram boa correlação das anomalias sísmicas com as principais estruturas tectônicas e revelaram novas anomalias que ainda não haviam sido observadas nos trabalhos anteriores. Anomalias de alta velocidade na porção oeste do Cráton do São Francisco apóiam a hipótese de que este cráton foi parte de uma placa Neoproterozóica maior. Anomalias de baixa velocidade sob a Província Tocantins (principalmente nas faixas móveis entre os Crátons Amazônico e do São Francisco) foram interpretadas como causadas por afinamento litosférico, consistente com a boa correlação entre a sismicidade intraplaca e as anomalias de baixa velocidade nesta região (Assumpção et al., 2004b). A melhora na resolução da anomalia de alta velocidade sob a Bacia do Paraná (~200 km) permitiu uma discussão sobre a geometria do núcleo cratônico desta Bacia. A subducção da Placa de Nazca foi observada como uma anomalia de alta velocidade sob a Bacia do Paraná (profundidades entre 700 e 1200 km). Nestas profundidades, uma grande anomalia de baixa velocidade aparece próxima da Placa de Nazca. Testes sintéticos mostraram que esta anomalia é um artefato da inversão gerado pela presença da Placa de Nazca. / We used travel time seismic tomography to study the upper mantle beneath SE and Central Brazil. This method is based on the inversion of P- and S-wave relative travel time residuals (VanDecar, 1991) obtained from more than 80 stations in an area of 20 x 20 degrees. More than 11000 P and PKP residuals, and more than 8000 S, ScS, SKS, and SKKS residuals were obtained from waveform cross-correlations for up to 12 simultaneous stations. To evaluate the robustness of the tomographic results with respect to the data, we use the Jackknife re-sampling method, which inherently take into account the highly variable ray coverage and measurement errors, and can provide confidence limits for the anomalies. Regional inversions were carried out to study influence of the parameterization on the seismic anomalies. Our results show correlations of seismic anomalies with the main tectonic structures and reveal new anomalies not yet observed in previous works. High velocity anomalies in the western portion of the São Francisco Craton support the hypothesis that this craton was part of a major Neoproterozoic plate. Low velocity anomalies beneath the Tocantins Province (mainly fold belts between the Amazon and São Francisco cratons) are interpreted as due to lithospheric thinning, consistent with a good correlation between intraplate seismicity and low velocity anomalies in this region (Assumpção et al., 2004b). The resolution improvement of the high velocity anomalies beneath the Parana Basin (at ~200 km) allows a discussion about the geometry of the cratonic nucleous of this Basin. The slab of the Nazca Plate is observed as a high velocity anomaly beneath the Parana basin (at 700-1200 km depth). At these depths, large low velocity anomaly appears next to the slab. Synthetic tests show that these anomalies are an artifact of the inversion generated by the presence of the slab.