Estudos de proveniência em zircões detríticos através do método U-Th-Pb (LA-ICP-MS) dos metassedimentos do Complexo Metamórfico Porongos determinaram a existência de duas áreas-fontes principais de onde derivaram idades paleoproterozóicas e mesoproterozóicas para os xistos Arroio Areião, Cerro Cambará e quartzo milonitos associados. Foram obtidas idades entre 967 ± 15 Ma e 2454 ±26 Ma para a seqüência Arroio Areião, entre 894 ± 30 Ma e 2783 ± 18 Ma para a seqüência Cerro Cambará, e idades entre 1621 ± 16 Ma e 3185 ± 117 Ma para os quartzo milonitos associados. A área fonte com idades paleoproterozóicas pode ser correlacionada ás rochas do Complexo Encantadas, que faz parte do embasamento das unidades supracrustrais. As áreas-fontes para os zircões mesoproterozóicos (entre 967 e 1621 Ma) não foram ainda reconhecidas no Cinturão Dom Feliciano. Estes zircões podem ser derivados de associações de rocha pertencentes aos cinturões mesoproterozóicos (orogêneses Elzeverian e Greenville), associados ao sistema geodinâmico que incluí os crátons Amazonas, Kalahari, Congo – São Francisco e Laurencia e cinturões associados. O ambiente geotectônico de formação da paleobacia Porongos é compatível com a margem passiva, onde se estabeleceu um arco magmático. Este trabalho demonstra que do ponto de vista isotópico e estratigráfico, não existem diferenças significativas de idades detríticas entre as seqüências metassedimentares que afloram nos flancos da Antiforme de Santana da Boa Vista. / Provenance studies based upon detritic zircon grain U-Th-Pb methodology (LA-ICP-MS) from metasediments of Porongos Metamorphic Complex have determined the existence of two principal source-areas with paleoproterozoic and mesoproterozoic ages to the schists of Arroio Areião, Cerro Cambará and associated quartz mylonites. Where obtained ages between 967 ± 15 Ma and 2454 ±26 Ma to the Arroio Areião sequence, between 894 ± 30 Ma and 2783 ± 18 Ma to Cerro Cambará sequence, and 1621 ± 16 Ma e 3185 ± 117 Ma to the associated quartz mylonites. The source-area with paleoproterozoic ages can be related to the Encantadas Complex rocks, which is basement of the supracrustal sequences. The source-areas to the mesoproterozoic zircons (between 967 and 1621 Ma) are not recognized in the Dom Feliciano Belt. These zircons can be related to mesoproterozoic petrotectonic associations (Greenville and Elzeverian orogens) associated to the geodynamic system that includes the Amazon, Kalahari, Congo – São Francisco and Laurentia cratons, and associated belts. The tectonic environment of Porongos paleobasin it’s compatible with a passive margin, after where established a magmatic arc environment. This work shows that from isotopic and stratigraphyc point of view there is no significantly differences between the metasedimentary sequences cropping out in both Santana da Boa Vista antiform flanks.

Numerosos terrenos pré-cambrianos de origem enigmática são individualizados nas regiões do vale e serrania do Rio Ribeira e Planalto Alto Turvo (SP, PR), com base em histórias evolutivas contrastantes e diferentes padrões litotectônicos, metamórficos e estruturais. Esses terrenos suspeitos formam três grandes terrenos compostos justapostos no final do Ciclo Brasiliano: Terreno Apiaí, Terreno Curitiba e Terreno Luís Alves. A evolução geotectônica de uma área que abrange estes três terrenos compostos foi investigada por meio da integração de análises petrológicas, microestruturais e estruturais, apoiadas com trabalhos de mapeamento geológico e compilação e integração de dados geocronológicos disponíveis. Os estudos foram concentrados em unidades geológicas representativas de cada um dos três terrenos compostos. Análises petrológicas, estimativas geotermobarométricas e quantificações de trajetórias PT mostram que os terrenos estudados apresentam assinaturas metamórficas contrastantes, refletindo ambientes geotectônicos distintos. O Grupo Votuverava e a Seqüência Serra das Andorinhas (Terreno Apiaí) passaram por metamorfismo barroviano com caminhamento P-T horário sob pressões e temperaturas máximas ao redor de 8 kbar e 550-650ºC. Parte do Complexo Turvo-Cajati (Terreno Curitiba) apresenta gradiente metamórfico acima do barroviano, com pico metamórfico (650-800ºC) sob pressões relativamente altas (9-12 kbar). Parte desta unidade passou por uma trajetória horária com aquecimento isobárico até atingir o pico térmico, enquanto algumas rochas passaram por descompressão praticamente isotérmica. Datações químicas em monazita indicam que o pico metamórfico do Complexo Turvo-Cajati ocorreu entre 590 e 575 Ma, sendo consideravelmente posterior ao clímax metamórfico das unidades do Terreno Apiaí (>= 600-620 Ma). Trajetórias P-T estimadas para o Complexo Atuba (Terreno Curitiba) sugerem um caminhamento retrógrado com resfriamento praticamente isobárico de 750ºC até um reequilíbrio ao redor de 650-700ºC e 6-7 kbar; a união dos dados petrológicos, microestruturais e geocronológicos sugere que o pico metamórfico teria ocorrido no Paleoproterozóico e o reequilíbrio no Ediacarano. A Suíte Alto Turvo (Terreno Luís Alves) compreende granulitos máficos a intermediários formados no Paleoproterozóico sob temperaturas mínimas ao redor de 850ºC, parcialmente reequilibrados em condições de fácies xisto verde ainda neste período e marginalmente re-metamorfizados em condições de fácies xisto verde a anfibolito no Ediacarano, seguindo uma trajetória horária com pico metamórfico ao redor de 650ºC e 6 kbar. Os padrões estruturais do Terreno Curitiba na área estudada estão associados a uma tectônica de cavalgamentos para oeste e nappismo tardios em relação ao Ciclo Brasiliano e concomitantes com a atuação do sistema transcorrente destral regional na Faixa Ribeira. Os dados multidisciplinares disponíveis indicam que as relações espaciais atualmente observadas para os terrenos estudados não decorrem de relações genéticas e geográficas pretéritas. Os dados convergem para uma evolução geotectônica onde a acresção de diferentes terrenos suspeitos e, em alguns casos, exóticos teria ocorrido em posições diferentes das atualmente observadas, sendo que a justaposição final teria sido controlada por dispersão lateral ao longo das grandes zonas de cisalhamento transcorrentes em períodos tardi- a pós-metamórficos. / Numerous Precambrian terranes of enigmatic origin are recognized in the Ribeira Valley and Alto Turvo Plateau regions (Southeastern Brazil), based on contrasting evolutive histories and distinct lithotectonic, metamorphic and structural patterns. These suspect terranes form three major composite terranes juxtaposed during the late Brasiliano Cycle: Apiaí Terrane, Curitiba Terrane and Luís Alves Terrane. The geotectonic evolution of an area that covers these three composite units was investigated by the integration of petrological, microstructural and structural analysis supported by geological mapping and compilation and integration of available geochronological data. The studies were concentrated on representative geological units from the three composite terranes. Petrological analysis, geothermobarometric estimatives and P-T path quantifications show that the studied terranes exhibit contrasting metamorphic signatures, refleting distinct geotectonic environments. The Votuverava Group and the Serra das Andorinhas Sequence (Apiaí Terrane) underwent barrovian metamorphism with clockwise P-T paths under maximum pressures and temperatures at around of 8 kbar and 550-650ºC. A part of the Turvo-Cajati Complex (Curitiba Terrane) shows a metamorphic field gradient above the barrovian type, with metamorphic peak (650-800ºC) under relatively high pressures (9-12 kbar). A part of this unit underwent a clockwise path with isobaric heating until to reach the thermal peak, while some rocks underwent a near isothermal descompression. Chemical dating of monazite indicate that the metamorphic peak of the Turvo-Cajati Complex occurred between 590 and 575 Ma, a period significantly younger than the metamorphic climax of the Apiaí Terrane units (>= 600-620 Ma). P-T paths from rocks of the Atuba Complex (Curitiba Terrane) suggest a retrograde path following near isobaric cooling from 750ºC to a reequilibration at around of 650-700ºC and 6-7 kbar; the integration of petrological, microstructural and geochronological data suggests that the metamorphic peak would have occurred during the Paleoproterozoic and the metamorphic reequilibration during the Ediacaran. The Alto Turvo Suite (Luís Alves Terrane) comprises mafic and intermediate granulites formed in the Paleoproterozoic under minimum temperatures at about 850ºC, partially reequilibrated under greenschist facies conditions yet in the same period and marginally remetamorphosed from greenschist to amphibolite facies conditions during the Ediacaran, following a clockwise path with metamorphic peak at around of 650ºC and 6 kbar. The structural patterns of the Curitiba Terrane in the studied area are related to a thrust nappe tectonics with westward movement late in relation to the Brasiliano Cycle and concomitant with the activation of the regional dextral transcurrent system in the Ribeira Belt. The available multidisciplinary data indicates that the present spatial relationships between the studied terranes do not imply in previous genetic and geographic relations. The data suggest a geotectonic evolution where the accretion of distinct suspect terranes, and in some cases exotic terranes, would have occurred in different positions in relation to the present array, and the final terrane juxtaposition would have been controlled by lateral dispersion along the major transcurrent shear zones during late- to post-metamorphic periods.

Estudos de proveniência em zircões detríticos através do método U-Th-Pb (LA-ICP-MS) dos metassedimentos do Complexo Metamórfico Porongos determinaram a existência de duas áreas-fontes principais de onde derivaram idades paleoproterozóicas e mesoproterozóicas para os xistos Arroio Areião, Cerro Cambará e quartzo milonitos associados. Foram obtidas idades entre 967 ± 15 Ma e 2454 ±26 Ma para a seqüência Arroio Areião, entre 894 ± 30 Ma e 2783 ± 18 Ma para a seqüência Cerro Cambará, e idades entre 1621 ± 16 Ma e 3185 ± 117 Ma para os quartzo milonitos associados. A área fonte com idades paleoproterozóicas pode ser correlacionada ás rochas do Complexo Encantadas, que faz parte do embasamento das unidades supracrustrais. As áreas-fontes para os zircões mesoproterozóicos (entre 967 e 1621 Ma) não foram ainda reconhecidas no Cinturão Dom Feliciano. Estes zircões podem ser derivados de associações de rocha pertencentes aos cinturões mesoproterozóicos (orogêneses Elzeverian e Greenville), associados ao sistema geodinâmico que incluí os crátons Amazonas, Kalahari, Congo – São Francisco e Laurencia e cinturões associados. O ambiente geotectônico de formação da paleobacia Porongos é compatível com a margem passiva, onde se estabeleceu um arco magmático. Este trabalho demonstra que do ponto de vista isotópico e estratigráfico, não existem diferenças significativas de idades detríticas entre as seqüências metassedimentares que afloram nos flancos da Antiforme de Santana da Boa Vista. / Provenance studies based upon detritic zircon grain U-Th-Pb methodology (LA-ICP-MS) from metasediments of Porongos Metamorphic Complex have determined the existence of two principal source-areas with paleoproterozoic and mesoproterozoic ages to the schists of Arroio Areião, Cerro Cambará and associated quartz mylonites. Where obtained ages between 967 ± 15 Ma and 2454 ±26 Ma to the Arroio Areião sequence, between 894 ± 30 Ma and 2783 ± 18 Ma to Cerro Cambará sequence, and 1621 ± 16 Ma e 3185 ± 117 Ma to the associated quartz mylonites. The source-area with paleoproterozoic ages can be related to the Encantadas Complex rocks, which is basement of the supracrustal sequences. The source-areas to the mesoproterozoic zircons (between 967 and 1621 Ma) are not recognized in the Dom Feliciano Belt. These zircons can be related to mesoproterozoic petrotectonic associations (Greenville and Elzeverian orogens) associated to the geodynamic system that includes the Amazon, Kalahari, Congo – São Francisco and Laurentia cratons, and associated belts. The tectonic environment of Porongos paleobasin it’s compatible with a passive margin, after where established a magmatic arc environment. This work shows that from isotopic and stratigraphyc point of view there is no significantly differences between the metasedimentary sequences cropping out in both Santana da Boa Vista antiform flanks.

A Klippe Carrancas, frente do Sistema de Nappes Carrancas, está posicionada na borda sul do Cráton do São Francisco. A klippe é composta por rochas do Grupo Carrancas e estruturada da base para topo por quartzitos puros a micáceos, gradando verticalmente para estaurolita-granada-cloritóide-clorita filito ou cianita-estaurolita-granada xisto cinza porfiroblástico, dependendo das condições do metamorfismo. O metamorfismo gravado nas rochas aumenta para sudeste, da fácies xisto verde superior, na Serra da Estância, até a transição das fácies anfibolito-eclogito na Serra das Bicas, próximo a cidade de Carrancas. Os mapas metamórficos precedentes indicam a posição da isógrada da estaurolita nas proximidades de Itutinga, extremo leste da Serra do Pombeiro, e a ausência de biotita na Serra de Carrancas. A presente investigação reconheceu, a sudoeste de Itumirim, rochas com paragêneses da fácies xisto verde (cianita + clorita + cloritóide + muscovita + quartzo) e na Serra da Estância da fácies anfibolito inferior, zona da estaurolita, que é alcançada com a quebra do cloritóide em presença de cianita (granada + estaurolita + clorita +muscovita + quartzo ± cloritóide ± cianita). Na Serra do Pombeiro as condições da fácies anfibolito médio são alcançadas com o desaparecimento da clorita. Nas Serras de Carrancas e das Bicas ocorre paragênese da fácies anfibolito médio formada por cianita + estaurolita + granada + quartzo + muscovita. A influência da composição química da rocha no desenvolvimento das paragêneses foi avaliada em modelagem usando pseudosseções nos sistemas químicos KFMASH e MnKFMASH. O Mn aumenta consideravelmente o campo de estabilidade da granada. A previsão das associações minerais presentes no restante da região foi examinada a partir da composição química de algumas amostras, havendo boa concordância com os dados de campo. Aliadas às pseudosseções, técnicas de geotermobarometria, via razões catiônicas e geotermobarometria otimizada, permitiram determinar com boa confiabilidade as condições de metamorfismo para rochas dos dois extremos metamórficos descritos na Klippe Carrancas. Os resultados obtidos da comparação dos campos de estabilidade das paragêneses das amostras no sistema MnKFMASH, com os intervalos P-T da geotermobarometria otimizada forneceram condições de pico metamórfico para a Serra da Estância em 10,0 ±1,7 kbar e 577 ±8°C. Para a Serra das Bicas o pico metamórfico foi alcançado em 12,9 ±1,0 kbar e 608,5 ±19,5°C, com paragênese retro-metamórfica estabilizada em 7,0 ±2,2 kbar e 541,5 ±25,5°C. As associações minerais das amostras estudadas (estaurolita-granada-clorita-cloritóide-muscovita-quartzo - Serra da Estância; cianita-estaurolita-granada-quartzo-cianita - Serra das Bicas) são paragêneses típicas das fácies xisto-verde superior e anfibolito, respectivamente. No entanto, os resultados da geotermobarometria otimizada indicam que as rochas foram expostas a regimes mais elevados de pressão, colocando-as na transição com a fácies eclogito. Como as rochas são pelitos verdadeiros, de composição química restrita, as associações minerais de fácies anfibolito persistem em condições de maior pressão. / The Carrancas Klippe, front of the Carrancas Nappe System, is positioned at the southern edge of the São Francisco Craton. The klippe is composed of rocks from Carrancas Group and is structured from base to top by a pure to micaceous quartzite, grading vertically to a staurolite-garnet-chloritoid- chlorite phyllite or a gray porphyroblastic kyanite-staurolite-garnet schist, depending on the conditions of metamorphism. The metamorphism recorded in these rocks increases to the southeast, from upper greenschist facies in the Serra da Estância to the transition to eclogite facies on Serra das Bicas, nearby Carrancas town. Previous metamorphic maps indicate the position of the staurolite-in isograd near Itutinga town, at far east of the Serra do Pombeiro, and the absence of biotite in the Serra de Carrancas. In this research it was recognized that to the southwest of Itumirim town, the rocks bear greenschist facies paragenesis (kyanite + chlorite + chloritoid + muscovite + quartz) and in Serra da Estância a low amphibolite facies, in the staurolite zone, which is achieved with the breakdown of chloritoid in the presence of kyanite (garnet + staurolite + chlorite + muscovite + quartz ± chloritoid ± kyanite). At Serra do Pombeiro conditions of intermediate amphibolite facies are reached with the disappearance of chlorite. On Serra de Carrancas and Bicas occurs intermediate amphibolites facies paragenesis, formed by kyanite + staurolite + garnet + quartz + muscovite. The influence of bulk composition in the development of some parageneses was evaluated using pseudo-section modelling in KFMASH and MnKFMASH systems. Mn increases the stability field of garnet. The prediction of mineral associations present in the rest of the region was done using the bulk composition of some samples, which show good agreement with field data. Allied to pseudo-sections, isopleth-geothermobarometry and optimal geothermobarometry allowed to determine, with good reliability, the metamorphic peak conditions for rocks of the two metamorphic extremes described in the Carrancas Klippe. The results of comparing the stability fields of the parageneses in the MnKFMASH system of the samples, with P-T intervals from optimal geothermobarometry, provide reliable conditions for the metamorphic peak. In Serra da Estância, peak metamorphic conditions were at 10.0 ± 1.7 kbar and 577 ± 8?. For Serra das Bicas, metamorphic peak conditions were reached at 12.9 ± 1.0 kbar and 608.5 ± 19.5°C, with retro-metamorphic paragenesis stabilized at 7.0 ± 2.2 kbar and 541.5 ± 25.5°C. The mineral associations of the studied samples (staurolite-garnet-chlorite-chloritoid-muscovite-quartz - Serra da Estância; kyanite-staurolite-garnet-quartz-kyanite - Serra das Bicas) are typical parageneses of the greenschist facies and upper amphibolites facies, respectively. However, the results of optimal geothermobarometry indicate that the rocks were exposed to higher pressure regimes, reaching the transition to eclogite facies. Since the rocks are true pelites, with restricted bulk composition, the mineral associations of amphibolite facies persist through conditions of greater pressure.

D.Phil. (Geology) / Please refer to full text to view abstract

Quartz and pyrite samples from Pliocene-recent, sub-economic orogenic gold mineralisation in the Southern Alps and Mesozoic economic deposits in the Otago Schist Belt have been analysed for noble gases and halogens. Palaeo-hydrothermal fluids preserved in fluid inclusions were released by crushing and analysed by mass spectrometry. Helium isotope measurements confirm the absence of a mantle-derived fluid component in gold-bearing veins from the Southern Alps and at the large gold deposit at Macraes in Otago. A possible minor mantle helium component is observed in veins within 10km of the Alpine Fault that do not contain gold. Halogen ratios support the absence of mantle-derived fluid and support the presence of a crustal fluid derived from sediments, indicated by high I/Cl ratios. Mixing trends between 40Ar/36Ar and Cl/36Ar indicate mixing between a meteoric-derived fluid or air and crust-derived fluid in all sample types. A correlation between 40Ar/36Ar and 132Xe/36Ar shows that xenon is also sourced from the crustal fluid. Despite a strong crustal-radiogenic Ar signal in some samples, measured neon isotope ratios are atmosphere like. Noble gas elemental ratios show strongly fractionated 20Ne/36Ar away from air-saturated water and air values in deeper formed veins, suggesting air contamination is not dominant. Evidence for the presence of a significant trapped vapour phase is provided by calculated noble gas concentrations in water. Formation of this vapour phase using a two-stage model of de-gassing of meteoric water, and subsequent partial re-dissolution by a Rayleigh fractionation process could account for fractionated 20Ne/36Ar and atmospheric neon isotope ratios. In the Southern Alps, three main types of mineralisation can be identified by noble gas and halogen properties. Deep forming veins contain the most gold and are characterised by 20Ne/36Ar greater than air, 132Xe/36Ar ratios up to 75 times the air value, indicating metamorphic and meteoric fluid components. Some deep ankeritic type veins that display evidence of a CO2-rich component fluid show the highest 20Ne/36Ar ratios. Shallow, late veins have 20Ne/36Ar between air-saturated water and air values, and lower xenon and iodine contents. These veins formed from a boiling rock-exchanged meteoric fluid with a minor metamorphic fluid component and contain less gold. The characteristics of the Nenthorn deposit in Otago are similar to those of the shallow Alpine veins. The economically significant Macraes deposit possibly formed from a meteoric component and a strong metamorphic fluid component derived from the original sediments. This is indicated by the highest levels of excess xenon (relative to air) determined in this study. There is some potential for xenon to be used to fingerprint gold bearing fluids sourced from similar metasedimentary piles in orogenic belts.

The Hartbees River Thrust Belt (HRTS) is a 40-60 km wide, southwest-vergent zone of complex structure, lithostratigraphy and high-grade metamorphism in the northeastern part of the mid-Proterozoic Namaqua Tectonic Province. The HRTS comprises the boundary zone separating the Bushmanland and Gordonia Subprovinces of the Namaqua Province. A knowledge of the movement histories of major ductile shear zones within the HRTS is fundamental to understanding the tectonic development of the belt, and Namaqua tectogenesis as a whole. In spite of this, no detailed microstructural kinematic studies have been attempted and the movement histories and age relationships of these shear zones have not been described in detail. This thesis represents a detailed microstructural kinematic study of a representative suite of orientated samples of mylonitic rocks, collected from five ductile shear zones within the HRTS. These shear zones include the Neusspruit Lineament, the Kakamas shear zone (KSZ), the Hugosput shear system (HSS), the Rozynenbosch-Ganzenmond shear zone (RGSZ) and the Graafwater shear system (GSS). Accepted modern methods of microstructural kinematic analysis were applied to samples of mylonite from these shear zones, in order to determine the precise orientation of the kinematic vectors, and the sense and relative ages of movements on each of the shear zones. Shear sense criteria, including composite SoC planar fabrics and shear band foliations, asymmetrical porphyroclast systems, mica-fish, oblique grain-shape and subgrain fabrics, asymmetrical microfolds, and the displacement of fractured rigid grains, together with a well-developed mylonite elongation lineation, conclusively indicate that SSW-directed thrusting occurred along the HSS, RGSZ, GSS and possibly along the Neusspruit Lineament, while normal, top-to-NE movements occurred on the Neusspruit Lineament, KSZ and HSS. Rare transposition criteria, and textural and paragenetic contrasts between syn-kinematic fabrics, strongly suggest that the phase of normal, top-to-NE movement seen in the northeastern HRTS shear zones is younger than the more widespread top-to-SW thrusting event. On the basis of mesoscopic structural criteria, SSW-directed thrusting is correlated with the D₂ deformation event in the HRTS. The mylonite zones have been refolded by ENE-SSW trending F₃ crossfolds, whose demonstrated coaxial relationship to the mylonite elongation lineation precluded reorientation of primary kinematic vectors. In the southwestern HRTS, primary thrust vectors have been reoriented by right-lateral, strike-slip shearing adjacent to the Pofadder Lineament during D₄. Simple shear dispersion of mylonite lineations related to normal movement, suggests that they too have been modified by D₄ shearing, and this constrains the timing of extensional movements to post-D₂ and pre- or syn-D₄. Syn-kinematic mineral assemblages, rheological criteria and the annealing states of the mylonites, provide insight into the thermotectonic evolution of the shear zones. A model is proposed in which the movement histories of shear zones within the HRTS are explained in terms of a typical orogenic cycle, involving crustal thickening by thrusting during a compressional orogenic phase, followed by collapse of the thickened crust during an extensional taphrogenic phase.

In the Paleoproterozoic Trans-Hudson Orogen, a well exposed section of mid-crust on Hall Peninsula, southeastern Baffin Island, offers an opportunity to improve our understanding of mid-crustal tectonothermal processes in hot, collisional orogens. Additionally, more robust age constraints on the tectonic history of Hall Peninsula are important for plate tectonic reconstructions of the North Atlantic region. Recent mapping shows that the section comprises Archean crystalline basement overlain by Paleoproterozoic supracrustal rocks, which host felsic plutons on the western peninsula. There is a westward increase in peak metamorphic grade, from amphibolite- to granulite-facies, and three regional deformation events are recognized (D1, 2, 3). Equilibrium phase diagram modeling constrained by garnet compositions in pelite indicates peak conditions of ~720–740°C on the eastern peninsula and ~850°C further west, with pressures of ~6.25–7.35 kbar. Modeling and petrographical evidence suggest subsequent cooling, decompression, growth of retrograde biotite and, on the eastern peninsula, retrograde muscovite. In situ U-Pb monazite dating (~450 analyses) and U-Pb zircon depth profiling (~90 analyses) resolve the timing of regional metamorphism and crustal shortening between ca. 1860–1820 Ma, coincident with the accretion of crustal blocks and arc terranes during the amalgamation of the orogenic upper (Churchill) plate. Regionally-occurring ca. 1800–1750 Ma monazite domains and zircon rims are interpreted to result from fluid-assisted dissolution-reprecipitation. They likely record the terminal collision with the lower-plate Superior craton and post-orogenic thermal activity, possibly related to the emplacement of pegmatitic syenogranite dykes. The new data strengthen formerly tentative correlations with southern Baffin Island, West Greenland and northern Labrador. 40Ar/39Ar thermochronology on muscovite, biotite and phlogopite suggests that Hall Peninsula underwent slow cooling at rates of ~1–2.5ºC/Myr after peak metamorphism, remaining hotter than ~400°C until ca. 1670–1660 Ma. Analogous thermochronological ages from elsewhere in the Trans-Hudson Orogen imply orogen-wide slow cooling. Despite significant crustal thickening and elevated paleotemperatures, the Hall Peninsula crustal section does not record evidence of orogenic collapse, implying that it may not be a hallmark of all hot, thickened orogens.

M.Sc. (Geology) / Please refer to full text to view abstract

Les intrusions magmatiques au sein d’une lithosphère soumise aux contraintes tectoniques sont souvent considérées comme des instabilités thermomécaniques à même de stimuler transitoirement et localement la localisation de la déformation. Dans le but de tester ce modèle et de déterminer les possibles mécanismes gouvernant l’initiation d’une déformation localisée au contact et au sein de corps magmatiques en cours de consolidation, ce travail de thèse se propose de coupler une étude de terrain à une approche expérimentale. Les Cyclades (Mer Égée, Grèce) constituent un domaine de croûte continentale en extension dans lequel se sont mises en place au cœur de cinq dômes métamorphiques extensifs (MCCs) des intrusions magmatiques, elles-mêmes coiffées par des systèmes de détachements. Les diverses campagnes de terrain, combinées aux données de la bibliographie, ont permis de proposer un modèle d’interaction régional dans lequel ces corps magmatiques impactent l’évolution tardive de MCCs cycladiques. Au vu des âges de mise en place des intrusions (15-9 Ma), soit plusieurs millions d’années après le début de l’extension et les premiers stades d’exhumation de roches métamorphiques, le magmatisme dans les Cyclades ne peut être considéré comme un candidat réel pour la genèse de MCCs. Néanmoins, les continuums de la déformation enregistrés en bordure des intrusions (depuis l’état magmatique jusqu’aux conditions ductile/cassant) et les relations géométriques avec les détachements laissent supposer un rôle majeur des complexes magmatiques dans les processus de redistribution et de localisation de la déformation, notamment sur le développement séquentiel de détachements. Les vecteurs de localisation de la déformation au sein de magmas partiellement cristallisés ont été en parallèle recherchés par la voie expérimentale. L’étude du comportement structural de magmas, chimiquement et texturalement proches de systèmes naturels, a permis d’une part de confirmer que la déformation se localise préférentiellement le long d’interfaces à rhéologie contrastée tels aux abords de filons syn-plutoniques. Il est d’autre part montré que la présence initiale d’inhomogénéités texturales (e.g. concentration de cristaux en amas) au sein de magmas moyennement cristallisés peut de manière drastique influencer le degré de localisation de la déformation au cours des stades de refroidissement ultérieurs. / Magma intrusions within the lithosphere are often considered as thermomechanical instabilities capable to locally and transiently stimulate strain localization. With the aim of testing this model and determining possible mechanisms that govern the initiation of localized deformation at the contact and within magmatic bodies, this thesis combine a fieldwork with an experimental approach. The Cyclades (Aegean Sea, Greece) form a highly extended continental domain in which five metamorphic core complexes (MCCs) were intruded by magmatic complexes, themselves capped by detachment systems. All collected structural and kinematic data, combined with previous investigations, converge toward a regional scheme in which magmatic bodies dynamically impacted the late evolution of the Cycladic MCCs. Granitoids were emplaced in relatively short time period (15-9 Ma) while metamorphic domes were largely exhumed after more than 10 Myrs of extension. None of those intrusions thereby proves to be a real candidate for the genesis of MCCs. However, continuums of deformation recorded within granitoids (magmatic to ductile/brittle states) as well as geometrical relationships with detachments suggest a pivotal role of magmatic complexes in redistribution and localization processes of the deformation, with in particular the sequential development of detachments. Precursors of strain localization within partially cristallized magmas have been concurrently deciphered by an experimental study. The investigation of the structural behavior of magmas, chemically and texturally similar to natural systems, corroborates that strain localization is efficiently activated along interfaces with contrasting rheology such as in the vicinity of synplutonic dikes. It has been also shown that the initial presence of textural inhomogeneities (e.g. like clusters) in medium-crystallized magmas can drastically influence the degree of strain localization during subsequent cooling stages.