No description available.

The Tillotson Peak Complex (TPC) in northern Vermont records high-pressure (HP) subduction zone metamorphism that occurred during the Ordovician Taconic Orogeny, and subsequent retrograde metamorphism and deformation that occurred during the Silurian Salinic Orogeny. Previous studies have documented a polymetamorphic history, with peak metamorphic pressures possibly up to 2.5 GPa and temperatures of 550°C. Prior to this research, constraints on the timing of metamorphism in the TPC were limited to a single Middle Ordovician 40Ar/39Ar total fusion age for glaucophane. This study integrates 40Ar/39Ar step heating analyses of multiple mineral phases and U-Pb dating of titanite with field and microstructural observations to further constrain the subduction–exhumation history of the TPC. Microstructural and petrologic analyses in thin section on samples of felsic gneiss, pelitic schist, amphibolite, and blueschist suggest deformation during varied P-T conditions. The earliest and highest-pressure metamorphic event documented in the TPC samples is associated with inclusions in garnet and white mica in S1 quartz microlithons. Inclusions of paragonite, titanite, and omphacite in garnets, locally defining S1, suggest that some blueschist may have formed in the retrograde path in association with the S2 foliation. A greenschist-facies metamorphic overprint in most samples is also associated with S2, primarily defined by epidote, white mica, and chlorite. E-W trending F2 intrafolial folds are commonly rootless in outcrop, locally defined by blueschist–eclogite-facies fold noses. Kinematic indicators relative to S2 and L2 stretching lineations give a predominantly top-to-the-E shear sense. S3 crenulation cleavage development is related to folding about E-W trending F3 folds that define the map pattern of the Tillotson Peak Complex. Locally developed S4 crenulations are axial planar to the NW-trending Gilmore Antiform. Additionally, D4 deformation and metamorphism is recorded by microfractures in garnet and epidote, as well as chlorite pseudomorphs after garnet. 40Ar/39Ar step heating of multiple phases and U-Pb dating of titanite yielded ages corresponding with the Taconian to the Salinic orogenies. Ages of ~485–480 Ma are attributed to prograde–peak metamorphism (M1) and S1 development. Ages that span ~471–456 Ma are interpreted to document retrograde M2 metamorphism through greenschist to locally blueschist-facies metamorphic conditions during exhumation and S2 development. Correlation of D3 microstructures in these samples with map-scale folds suggest that E–W trending folds developed in the range of ~455–445 Ma, recorded by minimum apparent ages in the field area, and locally as plateau ages along the margin of the TPC. Younger ages ~435–405 Ma are observed locally in apparent age gradients, and are interpreted to reflect metamorphic overprinting that resulted in the chlorite pseudomorphs after garnet and the growth of actinolite, which may be related to the timing of folding about the Gilmore Antiform. Results presented here suggest the impact of Acadian retrograde metamorphism and deformation on rocks of the TPC may be less significant than previous work suggests.

No description available.

Subduction zones are an important recycling center at which material from the exterior of the Earth is transported to Earth's interior. The processes that occur along subduction zones have important implications for elemental cycles, geodynamics, and material mass transport. The cold, dense subducting lithosphere experiences prograde metamorphism as it transitions from blueschist to eclogite facies resulting in the breakdown of volatile-bearing minerals and producing anhydrous minerals and a free fluid phase. Previous works attempting to understand the evolution of subducted lithologies have provided a firm foundation in which to apply field work, computational thermodynamic modeling, and geochronological techniques in order to better constraint the Pressure-Temperature-time (P-T-t) paths and dehydration of subducted lithologies. This dissertation; 1.) Explores novel approaches to modeling and predicting fluid/rock interactions during deep (>60km) subduction, and 2.) Questions what the calculated P-T-t path from eclogite lithologies reveals about early exhumation of subducted terrains. The second chapter focuses on how externally-derived hydrous fluids can decarbonate subducted basalt, liberate carbon and transfer it to the overlying mantle wedge, where it can be incorporated into melt that forms volcanic arcs. Here, the thermodynamic response to the infiltration of external fluids assuming open system, pervasive fluid flow, is quantified. It was determined that while hotter subduction zones have more favorable P-T conditions in which to facilitate decarbonation than colder subduction, the extent of decarbonation is largely dependent on the availability of fluid from the dehydration of underlying serpentine. The third chapter constrains the P-T-t paths of subducted lithologies from Syros, Greece using a combination of thermodynamic modeling, 147Sm/144Nd garnet geochronology, and quartz-in-garnet geobarometry. This provides insight into early exhumation of subducted lithologies, and allows for the exploration of assumptions made in thermodynamic modeling and in quartz-in-garnet geobarometry. Results suggest that garnet grew over a 4.31my period from 45.71±0.98Ma to 41.4±1.7Ma, during initial exhumation from maximum subducted depths. Calculated exhumation rates are a relatively rapid, 0.4-1.7 cm/yr. Because field relationships on Syros suggest the width of the subduction channel along the slab/mantle interface is not adequate to facilitate buoyancy-driven ascension of metabasic blocks, initiation of southward retreat of the Hellenic Subduction Zone and subsequent slab rollback is proposed to have played an important role in the exhumation of subducted lithologies. The final chapter investigates the compositional controls on the P-T conditions at which dehydration due to the breakdown of hydrous minerals occur during subduction (blueschist/eclogite boundary), and the implications they have on the rheology, seismicity, and densification of the down going slab. Total Alkali Silica (TAS) diagrams reveal that eclogites are more alkali rich, implying that initial alteration of the seafloor controls the mineral evolution of subducted basalt in many cases. / Ph. D. / Subduction zones are an important ‘recycling center’ at which material from the exterior of the Earth is transported to Earth’s interior. The processes that occur along subduction zones have important implications for elemental cycles, geodynamics, and material mass transport. The cold, dense rock that gets heated and deeply buried (high pressure metamorphism) releases fluid as water-bearing minerals breakdown. Previous works attempting to understand the history of subducted rocks have provided a firm foundation in which to apply field work, modeling, and dating techniques in order to better constraint the depths and temperatures a rock experienced, and over what time interval metamorphism and subsequent fluid loss occurred. This dissertation; 1.) Explores novel approaches in modeling and predicting fluid/rock interactions during deep (>60km) subduction, and 2.) Asks what does the calculated Pressure-Temperature-time (P-T-t) path from eclogite lithologies reveal about early exhumation of subducted terrains. The second chapter focuses how water-rich fluids can infiltrate a subducted rock and liberate carbon, which migrates upwards and can be incorporated into melt that forms volcanic arcs. It was determined that while the potential to release more carbon is higher in hotter subduction zones, the extent of decarbonation is largely dependent on the availability of fluid from the dehydration of the water bearing mineral., serpentine. The third chapter constrains the P-T-t paths of subducted lithologies from Syros, Greece using a combination of techniques, which not only provides insight into the return of subducted rocks back to the surface, but allows for the exploration of assumptions made in various analytical methodologies. Results suggest that garnet grew over a span from 45.71±0.98Ma to 41.4±1.7Ma, as the rock moved at a rate of 0.4–1.7 cm/yr to travel approximately 18km back to the surface, which suggests that a large-scale tectonic mechanism is needed to facilitate the rock’s return journey. The final chapter investigates the compositional controls on the P-T conditions in which the breakdown of hydrous minerals, and subsequent fluid loss, occur during subduction (blueschist/eclogite boundary), and the implication they have on the physical properties and behavior of the subducting rock. Chemical data reveal that subducted rocks that dehydrate earlier are more enrich in K and Na, implying that initial alteration of the seafloor controls the mineral evolution of subducted basalt.

Metamorphic hydrous, CO2-bearing fluids play a critical role in the global carbon cycle. However, how big this influence is on the global carbon cycle and therefore on global climatic processes, is unknown. The actual amount of CO2 which is released into the atmosphere due to metamorphic processes is still debated. For this purpose, fluid-driven reactions in metamorphic rocks must be studied by tracking fluid-rock interactions along pathways of ancient fluids. In the study presented in this thesis, we study fluid-rock interaction in the southeastern part of the Greek island Syros in the Cycladic Archipelago (Aegean). On Syros fluid-rock interaction is recorded by the preservation of blueschist facies assemblages at greenschist facies conditions along a normal shear zone. Blueschist preservation is caused by a combination of metasomatic addition of SiO2 and Na2O and elevated XCO2 which is maintained by high fluxes of a CO2-bearing, hydrous fluid along the shear zone. This research aims to provide a better understanding of the role of mountain building in the carbon cycle. Flux estimates for climate-forcing fluid components (e.g. carbon) require that their concentration in the fluid, fluid volumes and velocities are known. This will be the focus of future work. Further, whole rock chemistry and the availability of specific minerals will be studied to achieve knowledge about which kind of parameters influence and enhance the propagation of fluids through rocks.

No description available.

No description available.

The Franciscan Complex forms the structurally complicated, locally chaotic basement of the Northern and Central California and southwestern Oregon Coast Ranges. It is an accretionary wedge formed during the Late- Jurassic-Tertiary subduction along the west coast of North America. In northern California, the Franciscan is subdivided into three belts, the Western, Central, and Coastal belts, which show a zonation in age, metamorphic “grade”, and structural style. Franciscan mélanges are present in the Central belt, as well as the Diablo Range --a tectonic window within the structurally overlying Great Valley Group--, and the Nacimiento Block, where the study area is located. One of the best exposures of Franciscan mélange, where contact relationships between blocks and matrix can be observed, crops out along 6 km of seacliffs near San Simeon. Boudinaged blocks of graywacke, greenstone, chert, and much rarer blueschist and graphite-schist are ix dispersed in the shale matrix. Block sizes range from 10 cm to 15 m. The discovery of interlayered blueschist and graphite-schists, and the presence of lawsonite in some graphite-schists demonstrates that these two lithologies were metamorphosed together. Graphite-schist blocks in the Franciscan have not been reported prior to this study. Two main mineral assemblages were recognized among the studied 34 mafic blueschist blocks: 1) Lws + Na-amp + Pmp + Phe + Ttn + Chl recrystallized at ~5 kbar and 200-250°C, and 2) Lws + Na-amp + Pmp + Phe + Ttn + Ep + Chl, which recrystallized under slightly higher temperatures but similar pressures (300-350°C, at 5 kbar). A pre-blueschist facies metamorphic event under greenschist facies conditions is recorded by calcic cores overprinted by Na-amp rims in about half of the blocks. Sodic amphibole rims with a higher Fe3+ content probably developed due to the breakdown of epidote during a decrease in T. These mafic blueschists followed a counterclockwise P-T path. Remnants of “actinolitic rinds”, which are reaction zones formed when the blocks were in contact with serpentine, were found associated with nine of the studied mafic blueschist blocks. Such rinds were thought to be unique to better-studied Franciscan high-T blocks. Graphite-schist blocks (30 studied) contain Qtz + Phen + Ab + Gr, and are of two types. One type has relict sedimentary textures with a weak foliation defined by graphite and pressure solution seams. The other type has a compositional layering with layers containing well recrystallized quartz. Nine of these blocks also contain lawsonite within the more graphitic layers. The geochemistry of the blueschist blocks indicates that they were derived from the MORB-like oceanic crust, and seamounts underplated during the initiation of subduction. The mainly mafic protolith contained a small volume of interlayered sediment, as indicated by the presence of associated metasedimentary graphite-schists. Blueschist facies conditions were attained at the bottom of the overriding plate during the initial states of Franciscan subduction (150-155 Ma). Mafic material continued to be underplated, and low-T dynamic blueschist-facies metamorphism continued to form until at least ~137 Ma. A model is proposed in which exhumation was facilitated by normal faulting near the surface driven by gravitational collapse driven by decrease in subduction plate dip around 80 Ma that is known to have caused the Laramide orogeny. During this time, blueschist and graphite-schist blocks were plucked from the bottom of the hanging wall, incorporated into the shaleand water-rich shear zone at the plate interface, and exhumed during the upward flow of mélange driven by the movement of the downgoing plate. Blocks were extended and boudinaged, and metasomatically altered in the cataclastic zones that developed along necks and margins of the blocks during upwelling and the dewatering that led to final compaction near the surface. / text

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.

Θεματικός πυρήνας της συγκεκριμένης εργασίας είναι η αναγνώριση, ιεράρχηση και ερμηνεία των παραμορφωτικών χαρακτηριστικών ενός τμήματος του καλύμματος της Όχης, που καταλαμβάνει τα υψηλότερα τεκτονοστρωματογραφικά επίπεδα της ενότητας κυανοσχιστολίθων Εύβοιας (EBU). Μολονότι θεματικό πυρήνα της εργασίας αποτελεί η τεκτονική ανάλυση τμήματος του καλύμματος της Όχης, έμφαση δίνεται και στους μηχανισμούς που έχουν προταθεί για τον εκταφιασμό λιθοτεκτονικών ενοτήτων που έχουν υποστεί συνθήκες υψηλών και υπερ - υψηλών πιέσεων. Καταληκτικά, εκτίθενται επίσης τεκτονοστρωματογραφικά δεδομένα, τεκτονικά δεδομένα και πετρολογικά δεδομένα απο την ευρύτερο τμήμα της Νοτίου Ευβοίας δίνοντας έμφαση στην μεταμορφική πληροφορία που εσωκλείουν τα πετρώματα που υπέστησαν τις συνθήκες ενός καναλιού καταβύθισης. / The core subject of the present study is the recognition, relative dating and structural interpretation of the deformational characterestics of a part of the Ochi nappe. The Ochi nappe possess the highest tectonostratigraphic levels between the stacked nappes which constitute the Evia Bluschist Unit (EBU). Emphasis in this study is given to the mechanisms which have been proposed that contribute to the exhumation of (U)HP metamorphic terranes. In general, this study expose the to date published information about the stratigraphy, structural geology and metamorphic petrology of Southern Evia blueschist belt.