The tectonometamorphic histories of allochthonous nappe units that were deeply buried and subsequently exhumed during the Scandian orogeny in the Western Gneiss Region (WGR) of Norway are poorly constrained and understood. One such unit is the Blåhø Nappe which underlies northern Harøya and Finnøya, two islands in the Nordøyane ultrahigh-pressure (UHP) metamorphic domain. The Blåhø Nappe contains a lower unit of garnet amphibolite gneiss with interlayered garnet-bearing quartzofeldspathic gneiss and marble, and an upper unit of migmatitic aluminous gneiss. Southern Harøya comprises orthogneisses attributed to the Baltican continental crust, and is separated from the Blåhø Nappe by the Finnøya migmatitic shear zone (FMSZ). Field and petrographic observations from these islands suggest that the Blåhø Nappe experienced peak metamorphism at high pressure amphibolite to granulite facies conditions before being overprinted by relatively lower pressure amphibolite facies conditions. In contrast, the adjacent Baltican basement gneiss contains coesite-eclogite pods, which attests to UHP conditions. However, the basement rocks are also overprinted by an amphibolite facies assemblage. To understand the implications of these observations, the metamorphic history of the Blåhø Nappe was investigated, and compared to that of the basement. This thesis presents thermobarometric and geochronologic analyses used to define a metamorphic pressure-temperature-time (PTt) path for the Blåhø Nappe on Harøya and Finnøya. The results suggest prograde metamorphism between ca. 440 Ma and 415 Ma, peak temperature metamorphism at 860?C and 15 kbar at around 410 Ma, equilibration at amphibolite-facies conditions of 680?C and 9 kbar by 395 Ma, and cooling below Ar-closure T in muscovite by ca. 360 Ma. The PTt data from the Blåhø Nappe demonstrate that these rocks did not share the UHP history of the adjacent basement rocks, but that both experienced similar amphibolite facies metamorphism and deformation. These units were likely juxtaposed along the FMSZ during isothermal decompression from their respective maximum burial depths to ~30 km depth.

Les terrains en faciès des schistes bleus dans l'Arc Ibéro-Armoricain sont rares et limités à de petits domaines. Un de ces exemples est l'unité de Ceán qui constitue l'affleurement le plus occidental de l'Allochtone moyen dans le NO du Massif Ibérique et dans la chaîne varisque de l'Europe occidentale. L'unité de Ceán est interprétée comme une séquence volcanosédimentaire qui représente probablement la couverture d'une croûte transitionnelle ou océanique. Les informations sur l'évolution P–T de roches de ce terrain sont donc essentielles pour comprendre les caractéristiques et les mécanismes de la subduction de cette marge. L'unité de Ceán forme la partie supérieure du Complexe de Malpica-Tui (MTC) et comprend des proportions variables de métapélites à glaucophane-chloritoïde et de roches mafiques avec d'abondants pseudomorphes de lawsonite automorphe bien préservés. L'objectif principal de cette recherche est une étude détaillée de l'évolution métamorphique de ces lithologies à l'aide de diagrammes de phases . L'analyse pétrologique montre que l'unité de Ceán a enregistré une évolution métamorphique en trois étapes. (i) Un métamorphisme précoce (M1), lié à la subduction (ii) Un métamorphisme prograde dans le faciès des schistes bleus (iii) Un métamorphisme lié à l'exhumation (M3, post-M3). Les résultats obtenus à partir de la modélisation numérique des effets du H2O et Fe2O3 dans l'évolution des roches du faciès des schistes bleus ont donné des contraintes de premier ordre pour les modèles géodynamiques qui peuvent avoir une application générale. (i) Cette étude propose que le métamorphisme des zones de subduction peut se développer dans des conditions de sous-saturation en H2O, liées à la cristallisation de la lawsonite. La transition entre le faciès schistes bleus à lawsonite et le faciès des amphibolites / schistes verts produit une hydratation significative qui est principalement le résultat de la déstabilisation de la lawsonite. (ii) La proportion du fer ferrique à une forte influence sur les équilibres de phases. Les valeurs analysées du Fe2O3 ne reflètent pas nécessairement l'état d'oxydation pendant les principales étapes de l'évolution métamorphique et sont probablement facilement modifies par l'altération superficielle, même dans les échantillons frais en apparence. L'utilisation des pseudosections P/T–X(H2O/Fe2O3) avec une analyse pétrographique détaillée (incluant une bonne connaissance de la composition chimique des minéraux et de leurs relations texturales) est alors nécessaire pour estimer le degré de saturation en fluide et l'état réel oxydation afin d'évaluer correctement les conditions P–T pendant le métamorphisme de subduction. L'âge du pic du métamorphisme dans le faciès des schistes bleus a été contraint à environ 363±2 Ma par la méthode 40Ar/39Ar sur muscovite phengitique des schistes pélitiques. Les datations sur les muscovites des mylonites quartzo-feldspathiques du détachement de Bembibre-Ceán, à la base de l'unité de Ceán a donné un âge d'environ 337±3 Ma. Cet âge est interprété comme le début de la tectonique en extension qui mène au collapse gravitationnel de l'orogène. Les différences entre l'événement HP/BT et le début de la tectonique post-nappes suggèrent une vitesse d'exhumation de 2–2,5 mm/an pour le complexe de Malpica-Tui. Ces âges supportent l'équivalence de l'unité de Ceán avec l'unité supérieure de l'Ile de Groix dans le Massif Armoricain et suggèrent que les deux terrains partagent le même événement en faciès des schistes bleus vers 360–370 Ma qui peut représenter la subduction tardi-dévonienne carbonifère précoce de la marge nord du Gondwana sous le Laurussia, au début de la tectonique varisque. / Blueschist-facies (BSF) terranes in the Ibero-Armorican Arc are restricted to scarce and relatively small areas. One of these examples is the Ceán Unit (CU), the westernmost exposure of the Middle Allochthon in the NW Iberian Massif, and in the European Variscan belt. The CU is a volcano-sedimentary sequence interpreted as a part of the cover of a transitional (continental to oceanic) crust of the north Gondwana margin during its subduction below Laurussia. Thus, constraints on the pressure–temperature (P–T) evolution of rocks from this terrain are essential to understand the subduction of this margin. The CU forms the upper tectonic sheet of the Malpica-Tui Complex (MTC) and comprises variable proportions of glaucophane-chloritoid-bearing metapelites and mafic rocks with abundant well-preserved pseudomorphs after euhedral lawsonite. The main objective of this research consists in a detailed study of the metamorphic evolution of these lithologies using pseudosection approach. Petrological analysis shows that the CU recorded a three-stage metamorphic evolution comprising (i) early subduction-related MP/LT metamorphism (ca. 350–380 ºC, 12–14 kbar), which is only preserved in the basal part of the sequence. (ii) Subduction-related blueschist/LT-eclogite-facies prograde metamorphism characterized by a H2O-undersaturated prograde P–T path peaking at 19–22 kbar, corresponding to a maximum burial of ca. 65–70 km. (iii) Exhumation-related metamorphism occurred in two stages (1) a slower nearly isothermal decompression and a phase of fast cooling once the rocks have reached an upper crustal level. Furthermore, thermodynamic modelling of the effects of H2O and Fe2O3 in the metamorphic evolution of BSF rocks shows that (i) subduction zone metamorphism may occur in H2O-undersaturated conditions induced by the crystallization of a significant modal amount of lawsonite, and (ii) the analysed values of Fe2O3 may not reflect the oxidation state during the main metamorphic evolution and are probably easily modified by superficial alteration even in apparently fresh samples. Then, the use of P–T–X(H2O/Fe2O3) pseudosections together with a thorough petrographic investigation, and an extensive knowledge on the mineral chemistry and the textural relationships is necessary to estimate the extent of fluid-saturation during subduction zone metamorphism and the real oxidation state of the rocks to correctly evaluate the P–T conditions. The age of the peak BSF metamorphism has been constrained at ca. 363±2 Ma by 40Ar/39Ar step-heating of phengitic muscovite from the pelitic schists, supporting the equivalence of the CU and its counterpart in the Armorican Massif, the Upper Unit of Ile de Groix. This suggest that both terranes share a BSF event constrained at ca. 360–370 Ma, that may represent the Late Devonian-Early Carboniferous subduction of the northern margin of Gondwana beneath Laurussia, at the onset of the Variscan collision.

Exhumation of high-pressure and ultrahigh-pressure eclogites in large orogens and associated petrological change during the process remain enigmatic problems. This dissertation examines eclogites from high-pressure (HP) and ultrahigh-pressure (UHP) terranes in the North-East Greenland Eclogite Province, aiming to decipher their metamorphic pressure-temperature (P-T) paths, evaluate spatial variation of P-T paths, and understand petrological changes during the exhumation. Kyanite-bearing UHP eclogites from North-East Greenland contain a peak mineral assemblage of phengite, garnet, omphacite, kyanite, coesite, rutile and probably epidote-group minerals. Thermodynamic modeling with an XRF-derived bulk composition yielded a peak P-T condition of 3.4 GPa and 920 °C. Petrographic textures, such as graphic intergrowth of amphibole and plagioclase, cusps of plagioclase into garnet and quartz, and neoblasts of garnet indicate that the eclogites were partially melted through dehydration melting of phengite and epidote-group minerals. Since thermodynamic modeling could not yield a satisfactory solidus curve, experimental phase relations were considered in interpreting the melting process, and show a near isothermal decompression path across the epidote mineral melting curve. Additional thermodynamic modeling of a symplectite after omphacite, consisting of amphibole, plagioclase and clinopyroxene, yields a P-T condition of ~ 1.2 GPa and 800 °C. Thermodynamic modeling of a melt pocket yields a further P-T constraint of 1.4 GPa and 740 °C. The HP zoisite eclogites from the Storstrømmen shear zone in the Sanddal area preserve partial melting textures both in garnet and in the matrix. The textures include multiphase solid inclusions of albite and K-feldspar in garnet, graphic intergrowth of amphibole and plagioclase, cuspate textures, and leucosome. Thermodynamic modeling combined with mineral composition and modes yielded an exhumation P-T path from subsolidus conditions at ~1.95 GPa and ~670 °C, to ~1.85 GPa and 715 °C at suprasolidus, to ~1.45 GPa and 640 °C. Paragonite, phengite, and amphibole were the major dehydration melted phases along the exhumation path. The HP kyanite eclogite from the Danmarkshavn area contains disequilibrium textures developed during retrograde stages. Petrographic observation documents two groups of textures: a strongly zoned plagioclase (anorthite to andesine) enclosing a poorly developed symplectite of sapphirine + spinel + plagioclase after kyanite, and a less zoned plagioclase (labradorite to andesine) enclosing a fully developed symplectite after kyanite. Thermodynamic modeling of the bulk rock returns a peak P-T condition of 1.9 GPa and 840 °C. Thermodynamic modeling of a symplectite domain yields poor P-T constraints of 0.8 – 1.3 GPa and 700 – 900 °C. Modeling also indicates the plagioclase development would be richer in Ca during decompression while progressive replacement of kyanite induced the plagioclase rim to be less Ca-rich. This study reveals that HP and UHP eclogites may experience partial melting on their exhumation path. Dehydration melting of hydrous minerals (e.g. phengite and zoisite) is the most plausible way in partially melt the eclogites, because of limited amounts of free fluid. The partial melting does not trigger exhumation of the eclogites, but may facilitate the exhumation process. The near-isothermal exhumation path for the UHP terranes suggests that it was initially exhumed through vertical extrusion. Lateral extrusion by the Storstrømmen and Germania Land shear zones is suggested to have further exhumed the HP and UHP rocks, which is analogous to the lateral escape tectonics in the Tibetan Plateau

Submitted by Regiane Andrade Fumes null (regianefumes@gmail.com) on 2017-01-23T17:15:22Z No. of bitstreams: 1 dissertacao_Regiane.pdf: 29489950 bytes, checksum: 8c1114d1d274099ae6173d689a7d7124 (MD5) / Approved for entry into archive by LUIZA DE MENEZES ROMANETTO (luizamenezes@reitoria.unesp.br) on 2017-01-26T12:23:04Z (GMT) No. of bitstreams: 1 fumes_ra_me_rcla.pdf: 29489950 bytes, checksum: 8c1114d1d274099ae6173d689a7d7124 (MD5) / Made available in DSpace on 2017-01-26T12:23:05Z (GMT). No. of bitstreams: 1 fumes_ra_me_rcla.pdf: 29489950 bytes, checksum: 8c1114d1d274099ae6173d689a7d7124 (MD5) Previous issue date: 2017-01-18 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) / Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) / A Nappe Luminárias corresponde a uma estrutura alongada de orientação NNE-SSW com cerca de 40 km de extensão, situada na porção sul do Orógeno Brasília (idade neoproterozoica), bordejando o Cráton do São Francisco. Tal estrutura é composta majoritariamente por metapelitos e quartzitos do Grupo Carrancas. O presente trabalho foca na caracterização metamórfica de metapelitos e quartzitos do Grupo Carrancas na Nappe Luminárias. Para tal, utiliza-se modelagem metamórfica através de pseudosseções (THERMOCALC), química mineral e os geotermômetros Zr em rutilo e Ti em quartzo. Com base na mineralogia e nas relações texturais e estruturais observadas em lâmina, foram identificadas paragêneses distintas nas porções norte, centro-norte e sul da Nappe Luminárias. Na porção norte, a paragênese é Cld+Chl+Ky+Rt+Qtz+Ms. Na porção centro-norte, ocorre a paragênese St+Grt+Rt+Qtz+Ms, com biotita, clorita e ilmenita retrometamórfica. A assembleia de pico metamórfico registrada nas rochas da porção sul é Grt+Ky+St+Rt+Qtz+Ms com biotita, clorita e ilmenita retrometamórfica. Os resultados indicam a presença de um gradiente metamórfico com condições variando de fácies xisto-verde na porção norte (560˚C e 10kbar) e centro-norte (610˚C e 12,5kbar) a fácies anfibolito / eclogito na porção sul (630˚C e 15kbar). As rochas metapelíticas da Nappe de Luminárias evoluíram através de trajetórias P-T-t horárias, que indicam aquecimento seguido de uma forte descompressão. Análises de elementos traço em grãos de rutilo derivados de quartzito indicam que os mesmos podem ser utilizados para cálculo de temperatura utilizando-se o geotermômetro Zr no rutilo. Todavia, os dados indicam que a homogeneização, ou reequilíbrio, da concentração de Zr em rutilos detríticos em quartzitos ocorre em temperaturas mais elevadas que nos metapelitos, em torno de 580˚C. Não foi observada sillimanita nas rochas estudadas. Estes dados colocam em dúvida a extensão da Zona de Interferência entre as Faixas Brasília e Ribeira até a região de Luminárias. Além disso, os dados de modelagem do presente trabalho mostram que seria necessária descompressão isotérmica de aproximadamente 8 kbar para a cristalização de sillimanita, o que é incompatível com a superposição da Faixa Ribeira sobre a Faixa Brasília, que levaria a um soterramento ainda maior das unidades. / The Luminárias Nappe is a 40 km long, NNE-SSW elongated structure, located in the southern portion of the Neoproterozoic Brasília Orogen, which borders the São Francisco Craton (Minas Gerais, Brazil). It is composed of high aluminium metapelites and quartzites from the Carrancas Group. The present work focuses on the metamorphic characterization of the metapelites and the quartzites of the Luminárias Nappe by means of pseudosection modelling, mineral chemistry and the Zr-in-rutile thermometer. In the northern portion, the paragenesis is Cld + Chl + Ky + Rt + Qtz + Ms. In the center-north portion, the paragenesis is St + Grt + Rt + Qtz + Ms, with retro-metamorphic biotite, chlorite and ilmenite. The metamorphic peak assembly recorded in rocks from the southern portion is Grt + Ky + St + Rt + Qtz + Ms with retro-metamorphic biotite, chlorite and ilmenite. Results indicate the presence of a metamorphic gradient with conditions increasing from green-schist facies in the northern portion (560˚C and 10kbar) and center-north (610˚C and 12,5kbar) to amphibolite / eclogite facies in the southern portion (630˚C and 15kbar). Metapelitic rocks of the Luminárias Nappe followed a clockwise P-T-t path, characterised by an initial heating stage that is followed by strong decompression. Analyses of trace elements in rutile grains derived from quartzite indicate that they can be used for temperature calculation using the geothermometer Zr in rutile. However, the data shows that the homogenisation, or reequilibration, of the Zr content detrital rutile in quartzites occurs at higher temperatures than in the metapelites, at about 580˚C. No sillimanite has not been described in the studied rocks. Therefore, it suggests that the Interference Zone between the Brasília and Ribeira belts does not extent to the Luminárias Nappe region. In addition, our modelling shows that it would be require an isothermal decompression of approximately 8 kbar for the crystallization of sillimanite, which is incompatible with the overlapping of the Ribeira Belt over the Brasilia Belt, which would lead to an even greater burial of the units. / FAPESP: 2015/05230-0 / FAPESP: 2015/07750-0

The St. Cyr klippe hosts well preserved to variably retrogressed eclogites found as sub-meter to hundreds of meter scale lenses within quartzofeldspathic schists in the Yukon-Tanana terrane, Canadian Cordillera. The St. Cyr area consists of structurally imbricated, polydeformed, and polymetamorphosed units of continental arc and oceanic crust. The eclogite-bearing quartzofeldspathic schists form a 30 by 6 kilometer thick, northwest-striking, coherent package. The schists consist of metasediments and felsic intrusives that are intercalated on the tens of meter scale. The presence of phengite and Permian age zircon crystallized under eclogite facies metamorphic conditions indicates that the eclogite was metamorphosed in situ with its quartzofeldspathic host. I investigated the metamorphic evolution of the eclogite-facies rocks in the St. Cyr klippe using isochemical phase equilibrium thermodynamic (pseudosection) modeling. I constructed P-T pseudosections in the system Na2O-K2O-CaO-FeO-O2-MnO-MgO-Al2O3-SiO2-TiO2-H2O for the bulk-rock composition of an eclogite and a host metatonalite. In combination with petrology and mineral compositions, St. Cyr eclogites followed a five-stage clockwise P-T path. Peak pressure conditions for the eclogites and metatonalites reached up to 3.2 GPa, well within the coesite stability field, indicating the eclogites reached ultrahigh-pressure conditions. Decompression during exhumation occurred with a corresponding temperature increase. SHRIMP-RG zircon dating shows that the protolith of the eclogites formed within the Yukon-Tanana terrane during early, continental arc activity, between 364 and 380 Ma, while the metatonalite protolith formed at approximately 334 Ma, during the Little Salmon Cycle of the Klinkit phase of Yukon-Tanana arc activity. Both the eclogites and the metatonalites were then subducted to mantle depths and metamorphosed to ultrahigh-pressure conditions during the late Permian, between 266 and 271 Ma. The results of our study suggest portions of the Yukon-Tanana terrane were subducted to high-pressure and ultrahigh-pressure conditions. This is the first report of ultrahigh-pressure metamorphism in the accreted terranes of the North American Cordillera. Petrological, geochemical, geochronological, and structural relationships link the eclogites at St. Cyr to other eclogite localities in Yukon, indicating the high-pressure assemblages form a larger lithotectonic unit within the Yukon-Tanana terrane.

No description available.