La chaîne hercynienne française résulte de la collision continentale entre le Gondwana au Sud et la Laurussia au Nord. Le Massif Central constitue un fragment méridional de la chaîne dans lequel l'agencement structural indique un développement polycyclique. La lithologie est composée de micaschistes de l'Unité Para-autochtone, de Gneiss para et orthodérivés de l'Unité Inférieure, des Gneiss de l'Unité Supérieure et l'Unité de Thiviers-Payzac composée de métagrauwacke, de rhyolites et de quartzite. La chaîne comporte des migmatites et des granitoïdes qui ont été datés par la méthode de datation ponctuelle de monazite à la sonde électronique pour situer l'anatexie dans son contexte dynamique. Le caractère ponctuel des datations (point de ~2 µm de diamètre) permet de mettre en évidence plusieurs événements enregistrés sur un grain unique et éviter les problèmes de mélange rencontrés avec d'autres méthodes de datation. Trois événements thermiques sont identifiés : un évènement entre 385 et 375 Ma, un autre autour de 330 et 325 Ma et un dernier vers 310 et 290 Ma. Ils sont contrôlés par l'épaississement et le des-épaississement crustal et des injections asthénosphériques.

The Mesoproterozoic rocks of the Bysteek and Koenap Formations of the Arribees Group are exposed within a NW-SE striking antiformal structure comprised of mafic granulites and metapelitic diatexites, and a number of marble and calc-silicate rock layers. The mafic granulites of the Bysteek Formation show a typological variety of anatectic features, including nebulitic, stromatitic mesosomes, melanosomes, quartz syenitic leucocratic vein networks and syenitic pools. Melanosomes consist of hedenbergitic to diopside-rich clinopyroxene (XMg: 0.40), anorthitic plagioclase (An90), with some quartz, minor apatite and titanite. Anatexis was caused by biotite dehydration melting and formed a melt of probably granitic composition. The leucosome composition ranges from either alkali-feldspar-granitic to plagioclase rich or granitic. This variation is interpreted as a result of variable extraction of melt from the source to granitic pools. The diatexites of the Koenap Formation are most likely of metapelitic or meta-greywacke origin. They are texturally variable but always contain high modal contents of alkali feldspar and quartz which generally form magmatic textures. Almandine-rich garnet (XMg: 0.18-0.25), cordierite (XMg: 0.71) form secondary biotite, sillimanite and magnetite during retrograde breakdown. Thermodynamic modelling of mafic granulite compositions suggests peak P-T conditions of ~865 °C and 8.6 kbar. Occasionally, garnet rich in ferric iron (XAdr: 0.55) forms by plagioclase-clinopyroxene breakdown under oxidising conditions at ~6 kilobar and ~ 800 °C. At the same stage amphibole forms in some melanosomes. P-T estimations for the diatexites based on thermodynamic modelling suggest the equilibration of the assemblage garnet, cordierite, alkali feldspar and melt at ~860 °C and 5.5 kbar. Conditions comparable to the peak pressure in the mafic granulites could not be established. However, since the diatexites and the mafic granulites are closely related in the field and no evidence of juxtaposition after the thermal peak exists, the P-T record of the diatexites might be incomplete

Submitted by Igor Matos (igoryure.rm@gmail.com) on 2017-01-31T13:33:54Z No. of bitstreams: 1 DISS_2015_Newton Diego Couto do Nascimento.pdf: 10404936 bytes, checksum: fb842bb71d9f222883dbd33e72e27db8 (MD5) / Approved for entry into archive by Jordan (jordanbiblio@gmail.com) on 2017-01-31T14:04:02Z (GMT) No. of bitstreams: 1 DISS_2015_Newton Diego Couto do Nascimento.pdf: 10404936 bytes, checksum: fb842bb71d9f222883dbd33e72e27db8 (MD5) / Made available in DSpace on 2017-01-31T14:04:02Z (GMT). No. of bitstreams: 1 DISS_2015_Newton Diego Couto do Nascimento.pdf: 10404936 bytes, checksum: fb842bb71d9f222883dbd33e72e27db8 (MD5) Previous issue date: 2015-02-27 / CAPES / O Migmatito Furna Azul foi primeiramente descrito como um ortognaisse bandado de composição tonalítica a granodiorítica pertencente a Suíte Intrusiva Serra do Baú e correlato ao Complexo Gnaisse Chiquitania na Bolívia. Os aspectos petrograficos e estruturais permitem classificar o migmatito como metatexitos transicionais, sendo possível distinguir fácies ricas em melanossoma de ricas em leucossoma. Enclaves anfibolíticos e injeções dioríticas ocorrem com frequência. A paragênese essencial dos metatexitos é constituída por quartzo, plagioclásio, feldspato alcalino, biotita e granada, que se orientam formando níveis hololeucocráticos e mesocráticos de melanossoma e leucossoma intercalados. Os enclaves são constituídos por hornblenda marron, plagioclásio, quartzo e clinopiroxênio, distribuídos em bandas concordantes com a estruturação dos metatexitos. Essa assembleia mineral, apesar de não diagnóstica, indica um pico metamórfico de fácies anfibolito alto. As injeções de dioritos são formadas por plagioclásio, feldspato alcalino e quartzo (< que 13%), tendo comumente biotita, granada, epidoto e monazita como acessórios. Em termos de evolução da deformação nota-se que os metatexitos e os enclaves foram intensamente deformados, enquanto que as injeções são discretamente foliadas, provavelmente sin a tardi cinemáticas. O padrão geoquímico sugere uma evolução crustal em dois estágios, primeiro houve a formação do protólito em 1,43 Ga, precoce a Orogenia San Ignácio, a partir do retrabalhamento de uma crosta orosiriana (1,9 Ga). Em um intervalo de 100 mil anos, ocorreu o metamorfismo de médio grau responsável pela fusão crustal e formação dos metatexitos durante o estágio colisional da Orogenia San Ignácio evidenciado também pela geração de estruturas típicas de migmatitos associados a cristalização das injeções dioríticas em torno de 1,34 Ga a partir da fusão de uma crosta continental mais jovem extraída do manto em 1,47 Ga. / The Migmatite Furna Azul was first described as a banded orthogneiss of tonalitic to granodioritic belonging to Intrusive Suite Serra do Bau correlate the Gneiss Complex Chiquitanía described in Bolivia. Petrographic and structural features to classify the migmatite as transitional metatexites, being possible to distinguish rich facies melanosome from another leucosome. Amphibolites enclaves and dioritics injections occur frequently. The essential paragenesis of metatexites consists of quartz, plagioclase, alkali feldspar, biotite and garnet, which are oriented forming hololeucocratics levels and mesocratic of melanosome and leucosome interspersed. The enclaves are made up of brown hornblende, plagioclase, quartz and clinopyroxene distributed in concordants bands with the structure of metatexites. This mineral assemblage, although not diagnostic, indicates a high peak metamorphic in the amphibolite facies. The diorites injections are composed of plagioclase, alkali feldspar and quartz (<13%) and commonly biotite, garnet, epidote and monazite as accessories. In terms of evolution of deformation note that the metatexites and enclaves were intensely deformed, while injections are slightly foliated, probably the sin and tardi cinematic. The geochemical pattern suggests a crustal evolution in two stages, first there was the formation of the protolith in 1.43 Ga, early to Orogeny San Ignacio, from the reworking of a orosirian crust (1.9 Ga). In an interval of 100,000 years, was the average degree of metamorphism responsible for crustal melting and forming of metatexites during the collisional stage of orogeny San Ignacio, also evidenced by the generation of migmatites of typical structures associated with crystallization of dioritics injections around 1.34 Ga, from the melting of the continental crust younger extracted from the mantle at 1.47 Ga.

A Faixa Araçuaí foi formada no Neoproterozóico a partir da colisão E-W entre os continentes Sul-Americano e Africano. Sua porção leste compreende uma extensa área migmatítica (~300 km de comprimento por 50-100 km de largura) onde afloram anatexitos e leucogranitos (unidade Carlos Chagas), kinzigitos e granulitos migmatizados, que provavelmente são o registro de uma ampla fusão parcial da crosta intermediária a inferior. Observações de campo associadas com evidências micro-estruturais indicam que a deformação ocorreu quando as rochas estavam incompletamente solidificadas. Estimativas de temperaturas sincinemáticas realizadas a partir do geotermômetro TitaniQ (titânio-em-quartzo) indicam que a temperatura mínima para a cristalização de cristais de quartzo é ~750°C. Tais temperaturas combinadas com composição química de leucossomas dos anatexitos sugerem que a viscosidade das rochas crustais foi reduzida para pelo menos 108 Pa s. Baixo valor de viscosidade associado às evidências de campo e de micro-estruturas são consistentes com a geração de no mínimo 30% de volume de magma durante a orogênese. Grandes quantidades de magma promovem um drástico enfraquecimento da resistência mecânica das rochas à deformação, e atestam que a crosta anatética do extremo leste da Faixa Araçuaí representa um análogo de litosferas quentes (hot orogen), tal como a Himalaiana. Investigação mineralógica detalhada permitiu caracterizar um comportamento dominantemente paramagnético para os anatexitos e ferromagnético para os granulitos. Medidas de orientação preferencial cristalográfica (OPC) a partir da técnica de EBSD (electron backscatter diffraction) revelam que a foliação magnética surge, sobretudo, a partir da orientação preferencial dos eixos [001] da biotita orientados perpendicularmente ao plano de fluxo. Contudo, dada a fraca anisotropia linear desse mineral, apenas uma secundária contribuição de sua subtrama foi observada para a origem da lineação magnética (k1). A correspondência entre os eixos [001] de feldspatos e k1 ocorre devido a OPC de pequenas inclusões de ilmenita que imitam a OPC de seus minerais hospedeiros. Correlação entre k1 da Anisotropia de Remanência Anistéretica (ARA) e k1 da Anisotropia de Suscetibilidade Magnética (ASM) demonstra que, na escala do espécime, a lineação magnética tem uma contribuição da anisotropia dos minerais ferromagnéticos. Assim sendo, a lineação magnética nos anatexitos é o resultado da combinação da trama cristalográfica de feldspatos e de biotita com o alinhamento preferencial de grãos ferromagnéticos. Medidas de ASM realizadas para recuperar a trama mineral e investigar o fluxo nos migmatitos revela um padrão de deformação complexo, no qual, em função das direções de lineação, especialmente, é possível caracterizar três setores estruturais. A porção norte (região estrutural 1) com foliações dominantemente sub-horizontais e lineação fortemente orientada na direção NW-SE representa uma região de escape tectônico que ocorre através de um fluxo horizontal de canal (channel flow). Fluxos de canais possivelmente resultam da atuação de forças gravitacionais (gravity-driven flow). O setor sul (regiões estruturais 2 e 3) com variadas direções de foliação (NE-SW, E-W e NW-SE) e lineações com caimentos para Norte e Oeste, provavelmente refletem um regime de fluxo influenciado, sobretudo, pela tectônica de convergência E-W (collision-driven flow). Ambos os setores sugerem que na escala regional o fluxo crustal registrado pelos migmatitos resulta de um regime de deformação que envolve forças gravitacionais, devido a carga topográfica da crosta superior sobreposta à crosta intermediária parcialmente fundida, com viscosidade baixa, e forças tectônicas, associadas à colisão entre os continentes Sul-Americano e Africano. / The Araçuaí belt was formed by the collision between South American and African protocontinents during the Neoproterozoic. Its eastern part consists of an extensive migmatitic area (~300 km long x 50-100 km wide) where crop out anatexites and leucogranites (Carlos Chagas unit), migmatitic kinzigites and granulites that probably are the record of a widespread partial melting of the middle to lower crust. Field observations associated with microstructural evidences indicate that the deformation occurred when the rocks were incompletely solidified. Synkinematic temperature estimates realized using the TitaniQ (titaniun-in-quartz) geotermomether suggest that the minimum temperature for the quartz crystallization is ~750°C. Such temperatures combined with bulk rock composition of leucosome in the anatexites suggest that the viscosity of crustal rocks was dropped to at least 108 Pa s. Low viscosity values associated with field and microstructural evidences are consistent with the generation of at least 30% volume of melt during the orogeny. The presence of large volumes of melt promotes a drastic weakening of the mechanical strength of rocks and suggests that the anatectic crust of the eastern Araçuaí belt represents an analogue of present day hot orogen such the Himalayas. Detailed mineralogy investigation permitted to characterize the paramagnetic behaviour of the anatexites and the ferromagnetic behaviour of the granulites. Crystallographic preferred orientation (CPO) measurements using the EBSD (Electron Backscatter Diffraction) technique reveal that the magnetic foliation results from the preferred orientation of the biotite [001] oriented normal to the flow plane. However, given the feeble linear anisotropy of this mineral, only a subsidiary contribution of its subfabric to the origin of the magnetic lineation (k1) was observed. Correspondence between [001] of feldspars and k1 is due to the CPO of small inclusions of ilmenite that mimic the CPO of their host minerals. Correlation between k1 of the Anisotropy of Anhysteretic Remanent Magnetization (AARM) and k1 of the Anisotropy of Magnetic Susceptibility (AMS) demonstrate that, at the specimen scale, the magnetic lineation has a contribution of the anisotropy of the ferromagnetic minerals. AMS measurements realized to recover the mineral fabric and investigate the migmatitic flow field revealed a complex strain pattern in which, considering the lineation trends, especially, it is possible to characterize three structural sectors. The north region (structural sector 1) with foliations dominantly sub-horizontal and lineation trending NW-SE is interpreted as a region of tectonic escape that may represent a horizontal channel flow. This oblique tectonic escape probably results from gravity forces (gravity-driven flow). The Southern region (structural sectors 2 and 3) with variable trending foliations (NE-SW, E-W and NW-SE) and lineation plunging to North and West, probably reflect a flow regime dominantly influenced by the E-W convergence of the African and South-American continents (collision-driven flow). Altogether, the characteristics of the various domains suggest that the deformation of the partially molten middle crust of the Araçuaí belt was the result of the combination of gravity forces due to the topographic load and tectonic forces due to the convergence between the African and South-American continents.

The accretionary Mesoproterozoic Grenville Orogeny (ca. 1300 – 980 Ma) involving the Central Metasedimentary Belt is a key building block of the eastern Laurentian margin. A petrographic, mineralogical, geochemical and geochronological study of the migmatite complex in Otter Lake (QC) within the Marble Domain is used to resolve regional metamorphic and magmatic events primarily recorded in the leucosome accessory minerals (i.e. monazite). The relationship between the different stages of monazite and garnet growth and dissolution during the tectonic evolution of the orogenic history for the interpreted metasomatic (injected) and anatectic (in situ) monazite-bearing neosomes from this study supports published thermochronological work in the area and challenges the claim that the Central Metasedimentary Belt was not heated above 500 °C during the Ottawan phase. Instead, the region shows Grenville magmatic and anatectic events were overprinted by high-temperature, fluid-rich Ottawan-phase metamorphism recorded within both injected (monazite-bearing) and in situ (monazite- and garnet-bearing) neosomes.

<p> Glamorgan township in southeastern Ontario, is underlain by Precambrian rocks of the Grenville province. Prominent amongst these are migmatite, paragneiss, and granite gneiss, VJhich collectively form a series of rocks (the Glamorgan gneiss aeries). </p> <p> Field work revealed that this series is completely gradational from a geological aspect, and that the geological gradation is complemented by a geochemical gradation. <p> An explanation of these gradational relationships constitutes the main contribution of this study. The conclusions reached are that partial melting of paragneiss produced migmatite and a trondhjemitic melt, which later produced granitic (in the strict sense) derivatives. </p> <p> In developing the main conclusions, a number of subsidiary problems are discussed, chief of which are the possible metavolcanic or metasedimentary o'rigin of the paragneiss and the possible origin of so-called diorite as a differentiate of an alkaline gabbro. Metamorphism was concluded to be of Miyashiro's low pressure intermediate type. </p> <p> By the use of experimentally determined reactions and stability fields a metamorphic grid was devised, which led to the following upper limits of metamorphic conditions: 550 to 650°C and 3 to 6.5 kilobars total pre5sure. These estimates in turn lead to the following limiting geothermal gradients: 25 to 55°C per kilometre. </p> <p> The Glamorgan occurrence was found to share three characteristics with many other Precambrian terrains : 1. migmatisation and emplacement of granite accompanied high grade metamorphism; 2. an early sodium-rich granite was followed by a more potassic one; and 3. the more sodic granite is associated with a small amount of basic igneous rock. These three generalisations were used to formulate a possible model for deep crustal petrogenesis. </p> / Thesis / Doctor of Philosophy (PhD)

Pour ce secteur central du socle du Massif central français, autour de la ville de Brioude, une carte géologique en planche hors-texte (pl. 3) à 1/440.000 couvre environ 150 km x 80 km. Deux autres cartes (pl. 4-5) recensent sur ce fond géologique (1) les affleurements d'amphibolites, pyroxénites et péridotites serpentinisées (2) les gîtes de minéralisation en stibine, en fluorine et en uranium. Les terrains cristallophylliens étudiés sont compris entre des massifs, pour l'essentiel postérieurs, de granites hercyniens (Velay à l'Est, Livradois-Forez au Nord, Margeride au S et SW) et sont recouverts par les coulées de basalte néogènes du Cantal à l'W. Les formations cartographiées, cristallines et hautement métamorphiques, sans possibilité d'y établir une stratigraphie par absence de litage sédimentaire et du moindre fossile, sont l'objet d'une description pétrographique et structurale appuyée par une trentaine d'analyses chimiques nouvelles. Est notamment défini le "groupe leptyno-amphibolique brivadois" (GLAB), une succession pluri-kilométrique litée en grand, d'ectinites à faciès felsiques (leptynites, à chimie pour certaines de volcanites acides) et à faciès mafiques subordonnés (péridotites, serpentinites, amphibolites). Peut-être à rapporter au Précambrien, le GLAB sert de repère structural dans un fond de gneiss et de micaschistes à biotite ± muscovite, sillimanite, parfois disthène. La cartographie permet de dessiner le "synclinorium de Massiac" (SM, pl. 1), une structure de 40 km x 20 km axée NW-SE. Ses bords correspondent à une couronne de migmatites (anatexites de la migmatisation-I), d'abord schisteuses puis grenues en allant vers l'extérieur du SM, où existent des granites d'anatexie subordonnés. Ce synclinorium d'ectinites-I et migmatites-I serait zonéographiquement dans les gneiss inférieurs Z1 (cf. nomenclature ci-après). Le SM est ceinturé (au SW, SE et NE) par une structure arquée, le "synclinorium Desges-Senouire-Doulon" (pl. 2), qui aurait comporté initialement les zones Z1+Z2+Y1 d'ectinites-I et subi la migmatisation-I, mais aurait été largement affecté par un second métamorphisme (ectinites-II dans la zone Y1) et une migmatisation-II ultérieurs considérés comme hercyniens. On serait ainsi en présence de deux zonéographies superposées. La thèse alternative pour cette région d'une structuration en nappes hercyniennes est réfutée, notamment les travaux de A. Demay qui proposait de voir un synclinal de nappe hercynien dans le synclinorium de Massiac. Il est rappelé (p. 12) que la méthode zonéographique due à J. Jung et M. Roques divisait le terrain cristallophyllien en : --Ectinites : formations où la recristallisation s'est faite dans un climat topochimique. Les ectinites de la séquence pélitique repère comprennent les zones d'isométamorphisme suivantes: X zone non métamorphique; Y2 zone des micaschistes supérieurs = schistes à chlorite et séricite; Y1 zone des micaschistes inférieurs = micaschistes à biotite et muscovite; Z2 zone des gneiss supérieurs = gneiss à biotite et muscovite; Z1 zone des gneiss inférieurs = gneiss à biotite et sillimanite; U zone des gneiss ultra-inférieurs = gneiss à orthose et cordiérite. --Migmatites: formations ou la recristallisation s'est faite dans un climat métasomatique. Les migmatites sont des gneiss riches en feldspath (surtout alcalins) ; la biotite, la muscovite, la cordiérite, la sillimanite, le grenat y sont les minéraux les plus courants. Leur texture litée est : soit régulière, réalisant le faciès embréchite ; soit irrégulière (nébulitique, artéritique, agmatique...) et le faciès anatexite apparaît. Dans les formations migmatiques, les deux faciès peuvent apparaître simultanément, les embréchites occupant alors volontiers la partie supérieure de la formation, ou indépendamment l'un de l'autre.