Mafic dykes as monitors of hp granulite facies metamorphism in the Grenville front tectonic zone (western Quebec)

International audienceMetamorphosed mafic dykes and pods in the Grenville Front Tectonic Zone (GFTZ) are correlated, on the bases of map patterns and geochemical trends, with Paleo- and Mesoproterozoic dykes that cross-cut Archean rocks of the foreland, NW of the Grenville Front. The GFTZ consists of a SE-dipping 30 km thick slab of quartzo-feldspathic granulites and biotite–garnet (±opx) gneisses overriding, along the McLaurin thrust, a 4 km thick zone of muscovite–sillimanite micaschists that abuts on the Grenville Front. The granulite slab in turn dips under amphibolite-grade migmatites of the Réservoir Dozois terrane along the Dorval detachment. According to published U–Pb and new U–Th–Pb ages on monazite, the granulites, the gneisses and the schists of the GFTZ were metamorphosed at ca. 2.7–2.8 Ga and reworked at ca. 1.0 Ga (Grenvillian). Proterozoic mafic dykes and pods cross-cutting the GFTZ are thus monocyclic, i.e. they went through the Grenvillian metamorphic event only. Several of these metamorphosed mafic dykes and pods have preserved evidence of emplacement in a brittle regime such as intersection of the gneissic foliation, chilled margins, magmatic layering, this in spite of a strong metamorphic overprint attested by coronitic fabrics. In particular, metamorphosed mafic dykes of the granulite slab, contain large pyroxenes rimmed by garnet coronas with minute quartz inclusions, suggesting the reaction pl + opx = grt + qtz. These garnet coronas, in turn, are locally replaced by symplectites of opx, hbl, pl and qtz, probably formed through back-reactions grt + cpx + qtz = pl + opx and grt + cpx = hbl + qtz. Thermobarometric calculations using the above assemblages indicate peak equilibration at about 1.2–1.5 GPa at temperatures of about 800 °C followed by a quasi-isothermal decompression down to 0.9 GPa at temperatures around 700 °C. Proterozoic mafic dykes thus record a Grenvillian HP granulite metamorphism resulting from burial of upper crustal levels down to sub-Moho depths with concomitant heating. This event was followed by rapid exhumation of the GFTZ zone probably controlled by normal displacement along the Dorval detachment

Syn-deformational melt percolation through a high-pressure orthogneiss and the exhumation of a subducted continental wedge (Orlica-Śnieżnik Dome, NE Bohemian Massif)

High-pressure granitic orthogneiss of the south-eastern Orlica-Śnieżnik Dome (NE Bohemian Massif) shows relics of a shallow-dipping S1 foliation, reworked by upright F2 folds and a mostly pervasive N-S trending subvertical axial planar S2 foliation. Based on macroscopic observations, a gradual transition perpendicular to the subvertical S2 foliation from banded to schlieren and nebulitic orthogneiss was distinguished. All rock types comprise plagioclase, K-feldspar, quartz, white mica, biotite and garnet. The transition is characterized by increasing presence of interstitial phases along like-like grain boundaries and by progressive replacement of recrystallized K-feldspar grains by fine-grained myrmekite. These textural changes are characteristic for syn-deformational grain-scale melt percolation, which is in line with the observed enrichment of the rocks in incompatible elements such as REEs, Ba, Sr, and K, suggesting open-system behaviour with melt passing through the rocks. The P-T path deduced from the thermodynamic modelling indicates decompression from ~15−16 kbar and ~650-740 ºC to ~6 kbar and ~640 ºC. Melt was already present at the P-T peak conditions as indicated by the albitic composition of plagioclase in films, interstitial grains and in myrmekite. The variably re-equilibrated garnet suggests that melt content may have varied along the decompression path, involving successively both melt gain and loss. The 6-8 km wide zone of vertical foliation and migmatite textural gradients is interpreted as vertical crustal-scale channel where the grain-scale melt percolation was associated with horizontal shortening and vertical flow of partially molten crustal wedge en masse

Migmatite formation in a crustal-scale shear zone during continental subduction: an example from a high-pressure granitic orthogneiss from the Orlica-Śnieżnik Dome (NE Bohemian Massif)

Petrological study and pseudosection modelling have been carried out in high-grade orthogneisses of the southern domain of the Orlica- Snieznik Dome (NE Bohemian Massif). The studied samples are from an outcrop dominated by two deformation fabrics, a sub-horizontal S1 foliation defined by bands of recrystallized K-feldspar, quartz and plagioclase folded by centimetre- to several metre-scale close to isoclinal folds associated with development of a new subvertical N-S trending foliation S2. Based on field features and textural observations, a gradual transition from banded mylonitic orthogneiss (Type I) to stromatitic (Type II), schlieren (type III) and nebulitic (type IV) textures typical of migmatities can be distinguished. The banded orthogneiss is composed of almost monomineral recrystallized K-feldspar layers (2 to 10 mm thick) alternating with layers of plagioclase and quartz (1 to 4mm thick), parallel to the S1 limb and the axial planar S2 foliation. The stromatitic migmatite shows 1 to 4 mm thick layers with macroscopically diffuse boundaries between plagioclase, quartz and K-feldspar rich domains. Boundaries between quartz and feldspar layers are poorly defined and interlobed with adjacent minerals. The schlieren migmatite is almost isotropic preserving small K-feldspar-rich domains within a matrix characterized by random distribution of phases, whereas in the nebulitic migmatite the microstructure is completely isotropic characterized by random distribution of phases. The transition from the Type I to IV is characterized by increasing nucleation of interstitial phases along like-like grain boundaries, by a decrease of grain size of all phases and by progressive disintegration of recrystallized K-feldspar grains by embayments of fine-grained myrmekite. The mineral assemblage of all types consists of biotite, white micas, garnet, quartz, K-feldspar and plagioclase, and accessory apatite, ilmenite, zircon and monazite. In the mineral equilibria modelling, the core of garnet (alm0.58, py0.02-0.03, grs0.34, sps0.05) and phengite (Si = 3.38-3.20 p.f.u) is consistent with a P-T peak at 10-13 kbar and 720-750 C in the dominant grt-bt-ph-rt-qtz-pl-kfs mineral assemblage. The garnet rim (alm0.68, py0.02-0.03, grs0.11, sps0.21), white mica rim (Si = 3.10 p.f.u) together with unzoned biotite (XFe = 0.76-0.78) match the modelled isopleths in the middle-P part of the grt-bt-ph-ilm-qtz-pl-kfs field to reach the solidus at 78 kbar and 630650 C. In addition, the absence of prograde garnet zoning in the Type I to III suggests that the garnet was completely re-equilibrated during the retrograde history, whereas in the Type IV the HP garnet chemistry was preserved. This is discussed in frame of melt presence in different migmatite types along their P-T path. Based on mineral equilibria modelling it is argued for fluid/melt-fluxed melting at HP conditions and on exhumation. The migmatite textural types are a result of grain-scale melt migration process and not of a localized melt transport in dykes as known from metasediments

Structure and metabolic potential of the prokaryotic communities from the hydrothermal system of Paleochori Bay, Milos, Greece

IntroductionShallow hydrothermal systems share many characteristics with their deep-sea counterparts, but their accessibility facilitates their study. One of the most studied shallow hydrothermal vent fields lies at Paleochori Bay off the coast of Milos in the Aegean Sea (Greece). It has been studied through extensive mapping and its physical and chemical processes have been characterized over the past decades. However, a thorough description of the microbial communities inhabiting the bay is still missing.MethodsWe present the first in-depth characterization of the prokaryotic communities of Paleochori Bay by sampling eight different seafloor types that are distributed along the entire gradient of hydrothermal influence. We used deep sequencing of the 16S rRNA marker gene and complemented the analysis with qPCR quantification of the 16S rRNA gene and several functional genes to gain insights into the metabolic potential of the communities.ResultsWe found that the microbiome of the bay is strongly influenced by the hydrothermal venting, with a succession of various groups dominating the sediments from the coldest to the warmest zones. Prokaryotic diversity and abundance decrease with increasing temperature, and thermophilic archaea overtake the community.DiscussionRelevant geochemical cycles of the Bay are discussed. This study expands our limited understanding of subsurface microbial communities in acidic shallow-sea hydrothermal systems and the contribution of their microbial activity to biogeochemical cycling

Regional-scale Grenvillian-age UHT metamorphism in the Molleonde-Camana Block (basement of the Peruvian Andes).

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ÉVOLUTION THERMOMÉCANIQUE DE LA CROÛTEINFÉRIEURE DU SUD DE MADAGASCAR

This work is devoted to the characterization of the thermo-mechanical evolution of the lower crust from the example of Madagascar. The finite strain pattern of Southern Madagascar is derived from the study of satellite images complemented by structural analуsis on the field (foliation, stretching lineation trajectories and kinematics). The strain is partitioned between blocks (dome-and-basin folded domains) surrounded by ductile shear zones. These geometries reflect the superposition of two distincts finite strain pattern D1 and D2. D2 finite strain pattern reworked the sub¬horizontal D1 structures giving rise to a network of anastomozed vertical shear zones (15-25 km wide and> 1000 km long). It is consistent with East-West shortening in a transpressive regime.We determined the metamorphic conditions associated with the two finite strain fields. Estimates have been realized for various lithologies by using several methods (TWEEQ software and usual calibrations). Results show that rather uniform high-temperature conditions can be found everуwhеrе in Southern Madagascar. Ву contrast, there are regional pressure differences controlled by the major shear zones, which in turn control the movements of the crustal blocks. Thus the transpressive D2 strain field is responsible for exhumation of deep-seated rocks.D1 and D2 both occurred under granulite facies conditions. Chronological constrains obtained from monazite (U-Th-Pb) indicate ages at 580-550 Ma for Dl and 520-500 Ma for D2.In order to characterize the rheological behavior of granulites in Madagascar, we carried out a study of microstructures, microfabrics and deformation mechanism of quartz- & feldspar-bearing rock. Our results demonstrate that the continental lithosphere, despite its high composition heterogenеitу, is rheoligiсally homogeneous. Furthermore, the granoblastic textures of granulites are not only annealing textures but thеу are also textures characteristic of high-stained rocks at high-temperature while viscositу cintrasts between phases are very low.Finallу, the pan-African origenic cycle (580-500 Ma) in Madagascar is presented at the scale of the Gondwana super-continent where the D2 deformation stage results from the convergence of cratons. The latter constitute major heterogeneities within the continental lithosphere which control the repartition of the radiochronologic data (diachronous ages), the P-T data (pressure differences), and the strain partitioning (simple shear vs. pure shear, and strain gradient).Ву integrating these direct observations as well as indirect observations derived from seismology and gravimеtry, we infer that the vertical shear zones are deeply rooted in the mantle. This lithospheric anisotropy is associated with fluid transfers allowing heat advection in the orogenic mots responsible for the development and persistent granulite facies metamorphism during tens of million years.L'objectif de cette thèse est de caractériser l'évolution thermomécanique de la croûte continentale inférieure sur l'exemple de Madagascar. Dans ce but le champ de déformation finie du Sud de l'île de Madagascar est caractérisé, à partir de l'étude d'images satellitales complétée par l'analyse structurale de terrain. L'évolution des géométries, dans l'espace, montre une forte partition de la déformation entre des blocs (domaines plissés en dômes-et-bassins) limités par des zones de cisaillement ductiles. Ces géométries résultent de deux champs de déformation finie distincts D1 et D2. Le champ de déformation D2, qui reprend des structures horizontales (D1), est composé d'un réseau de zones de cisaillement anastomosées verticales (15 à 25 km de large par plus de 1000 km de long) où se localise la déformation. I1 est compatible avec un raccourcissement Est-Ouest, en régime transpressif.Nous avons précisé les conditions métamorphiques synchrones des deux champs de déformation finie. Les calculs ont été réalisées sur des lithologies variées en utilisant divers types de méthodes (logiciel TWEEQ et calibrations usuelles). Les résultats mettent en évidence que les conditions de haute température (>750°C) sont généralisées à tout le Sud de l'île. Par contre il existe un gradient régional de pression. Les variations sont contrôlées par les zones de cisaillement majeures qui guident les mouvements différentiels des blocs crustaux. Le champ de déformation D2, le résultat d'une déformation transpressive, permet donc l'exhumation de roches profondes.Les deux champs de déformation finie D1 et D2 sont synchrones du faciès granulite et des estimations chronologiques sur monazite (U-Th-Pb) ont été réalisées afin de préciser leur succession au cours du temps. Les structures D1 sont datées à 580-550 Ma et les structures D2 à 520-500 Ma.Afin de caractériser le comportement rhéologique des granulites de Madagascar, une étude des microstructures, des microfabriques et des mécanismes de déformation des roches quartzofeldspathiques à été réalisée. Nos résultats, en complément des travaux de terrain à plus petite échelle, démontrent que la lithosphère continentale extrêmement hétérogène en composition, montre un comportement rhéologique homogène. De plus les textures granoblastiques des granulites ne sont pas seulement des textures de recuit mais des textures caractéristiques d'une forte déformation de haute température, pendant laquelle les contrastes de viscosités entre les phases sont très faibles.Finalement l'histoire du cycle orogénique panafricain (580-500 Ma) de Madagascar est abordée à l'échelle du super-continent Gondwana où le champ de déformation D2 résulte de la convergence de cratons. Ces derniers constituent des hétérogénéités majeures au sein de la lithosphère continentale, qui influent sur la répartition des données radiochronologiques (diachronisme des âges), des données P-T (variations de pressions) et sur la partition de la déformation ("cisaillement pur/cisaillement simple", gradient de déformation).En intégrant ces données directes naturelles et les données indirectes obtenues notamment par sismique et gravimétrie nous considérons que les zones de cisaillement verticales s'enracinent dans le manteau. Ces anisotropies lithosphériques facilitent les transferts de fluides et de ce fait vont permettre une advection de chaleur dans les racines orogéniques à l'origine du développement et de la persistance du faciès des granulites pendant plusieurs dizaines de millions d'années

Nouvelles méthodes de cartographie sur le socle protérozoïque du sud de Madagascar (nature et géométrie de la croûte continentale d'un domaine orogénique en convergence, implications économiques)

L'objectif de cette thèse est de contraindre la nature pétrologique et géométrique de la partie profonde d'un domaine orogénique en convergence. Dans ce but, nous avons créé dans un système d'information géographique projeté (Madagascar Laborde 1923), une base de données géologique du tiers Sud de l'île de Madagascar. Les données utilisées sont variées : observations de terrain, images satellites, données topographiques, mesures gravimétriques, radiométriques et aéromagnétiques. L'analyse, le traitement, la confrontation et l'intégration de ces différentes informations (rendues compatibles) nous a permis de réaliser avec précision de nouvelles cartes géologiques de la région, ainsi que des représentations cartographiques nouvelles. Nous mettons notamment en évidence des zones de cisaillements verticales (ZC) majeures et mineures, des fractures, des dykes et des limites lithologiques. Nous précisons la nature des alignements sur l'axe Bongolava Ranotsara : (i) La structure ductile de direction N 140, cartographiée dans plusieurs littératures, n'est qu'une impression régionale (à petite échelle) due à la déflexion N 157 des ZC de direction Nord Sud. La faille de Ranotsara (de direction N 140) n'existe pas en termes de faille régionale, elle est une impression (à petite échelle) de la restitution cartographique causée par le relais de failles de taille <=60 km de long et qui sont orientées N 115, N 140 et N 160. Nous identifions que les ZC sont les lieux de localisation d'anomalie en éléments incompatibles (U-Th). Elles sont également le lieu de concentration d'oxydes de magnétite et titanomagnétite, associé à la disparition de la biotite. Ces observations soulignent la concentration des magmas et de la fusion au sein des ZC. Ainsi, les ZC sont le lieu de concentration de magmas qui permettent l'advection de chaleur. En effet, dans le Sud de Madagascar les conditions de haute température (800C) sont homogénéisées sur de grande surface. Nous établissons une nouvelle carte des ensembles lithologiques qui met en évidence des contrastes de compétence: les ZC se développent préférentiellement dans l'ensemble d'orthogneiss peu alumineux; les ensembles de métasédiments alumineux et les gneiss migmatitiques à amphibole sont plus résistants et forment des boudins à différentes échelles ; l'ensemble de granites et syénites stratoïdes et gneiss migmatitiques à amphibole constituent les blocs les plus rigides. Nous identifions des dykes kilométriques de basaltes, de dolérites et de microgranites qui sont orientés préférentiellement sur les directions N 25 et N 120. Par croisement des données, nous montrons que la position des minéralisations économiques est contrôlée par les anisotropies kilométriques (ZC, charnières de plis, limites lithologiques) qui localisent les circulations fluides et magmatiques. Les minéralisations primaires (métamorphiques ou métasomatiques : corindons, fer et graphite) se localisent au sein des ZC. Les minéralisations secondaires (uranothorianite, or) s'expliquent par des remobilisations au sein des ZC. La position finale des minéralisations s'explique cependant par la géométrie des réseaux hydrographiques et des failles. A l'échelle du Gondwana, la géométrie des ZC majeures néoprotérozoïques contrôle le développement des systèmes de failles et de dykes (170Ma 90Ma) orientés N 160. Dans 4 régions : Satrokala, Fenoarivo, Analavoka, Lavaraty ; existent des anomalies magnétiques et gravimétriques de type bulls eyes qui se localisent à l'intersection des dykes N 160 et des dykes N 30, N 140 et N 170 ; et parfois avec les ZC. Ces points correspondent à des creux topographiques circulaires (10 m - 2,5 km de diamètre) isolés ou associés en champ de point. Ces zones sont potentiellement des intrusions kimberlitiques. La présence de ces pipes à l'intersection d'anisotropies kilométriques est favorable à l'ascension rapide des magmas silicatés, soit la présence de kimberlites diamantifères.The objective of this thesis is to investigate the petrologic nature and geometry of the outcropping Madagascar granulitic crust. We have created a geological database of the southern third of Madagascar (in the Madagascar Laborde 1923 coordinate system). The data used are varied: field observations, satellite images, topographic, gravity, radiometric and aeromagnetic data. The analysis, processing, comparison and integration of these different datasets (after being made compatible) have enabled us to create precise and reliable new geological maps of the region, as well as new map representations. We will highlight particular the areas of major and minor vertical shear zones (SZ), fractures, dykes and lithological boundaries. Thus, we clarify the nature of the structural-lineaments, the so-called Bongolava Ranotsara shear zone: the ductile structure oriented 140 N is on a regional scale due to the North SZ deflections oriented N 157, and the Ranotsara fractures (orientation 140 N) does not exist in terms of regional fault, but is the results of faults relays with a size <= 60 km long which are oriented 115 N, 140 N and 160 N. SZ are the sites of localisation of anomalies rich in incompatible elements (U-Th). They are also the location of concentration of oxides of magnetite and titanomagnetite, together with the disappearance of biotite. These observations emphasize the concentration of magmas and the merger within the SZ. Thus, the SZ are focus the concentration of the magmas that provide heat by advection. In southern Madagascar, the conditions of high temperature (800 C) were homogenous over a wide area. We establish a new lithological map, highlighting competency contrasts: SZ grow preferentially in the aluminous orthogneiss; aluminous metasediments and ampibolo-migmatites are more resistant and form boudins at different scales; all of the granites, syenites and stratoïdes amphibolo-migmatites are the most rigid blocks. We identify kilometers of dykes: basalt, dolerites and microgranite, they're preferentially oriented at 25 N and 120 N. By cross-correlating our data, we show that the position of economic mineralization is controlled by the kilometer scale anisotropies (SZ, hinges folds, lithological boundaries) that localize interchange and magmatic fluids. The primary mineralization types (metamorphic or metasomatic: corundum, iron and graphite) are located within the SZ. The secondary mineralization types (uranothorianite, gold) are explained by remobilizations in the SZ. The final mineralization is controlled by the geometry of river systems and faults. On the scale of Gondwana, the geometry of Neoproterozoic major SZ controls the geometries of faults and dykes (170 Ma-90 Ma) which are oriented in 160 N. In four regions: Satrokala, Fenoarivo, Analavoka, Lavaraty; we identified magnetic and gravity "bulls eye" type anomalies which are located at the intersection of 160 N oriented dykes with other ones oriented 30N, 140 N and 170 N; and sometimes with SZ. These points correspond to circular topographic depressions (10 m - 2.5 km in diameter) isolated or in clusters. These areas are potential kimberlite intrusions. Their presence at the intersection of kilometric anisotropies supports the rapid ascent of silicate magmas, resulting in the possible presence of diamondiferous kimberlites.GRENOBLE1-BU Sciences (384212103) / SudocSudocFranceF

Terrestrial shallow water hydrothermal outflow characterized from out of space

International audienceWe investigate the potential of satellite imagery to map and monitor the activity of shallow-water hydrothermal systems, which are often found around volcanic islands. For this study, we used publicly available data and proprietary WorldView-2 satellites images, with spectral bands that can penetrate up to water depths of 30 m. Shallow water hydrothermal sites are visible on satellite imagery, primarily with publicly available data, demonstrating the potential of satellite imagery to study and monitor shallow water hydrothermal activity. We focus our work on volcanic islands, showing intense near-shore, shallow-water hydrothermal activity, and distinct styles of hydrothermal venting. Satellite imagery constrains regional outflow geometry and the temporal variability or stability of these systems. Milos Island shows hydrothermal outflow associated with reflective mineral precipitates and/or bacterial mats, which are stable over time (2010–2014). These outflows locally define polygonal patterns likely associated with hydrothermal convection in porous media. In Kueishantao Island individual hydrothermal plumes charged with particles are visible at the sea surface, and display great variability in intensity and distribution of plume sources (2002–2019). Worldwide we have identified ~15 shallow water hydrothermal sites with satellite imagery, that are similar to either the Milos system (e.g., Vulcano and Panarea, Italy), or the Kueishantao system (numerous sites in Pacific volcanic islands). This study demonstrates that satellite imagery can be used to map and monitor different types of shallow-water hydrothermal systems, at regional scale, and monitor their evolution. Satellite data provide not only regional and temporal information on these systems, unavailable to date, but also the regional context for follow-up in situ field data and observations (e.g., instrumental monitoring, sampling, observations and mapping with divers or AUVs) to understand both the nature and dynamics of these systems, and ultimately the associated fluxes

Granulite microfabrics and deformation mechanisms in southern Madagascar

International audienceOptical microstructures and crystallographic preferred orientations were studied in naturally deformed granulite- to high-amphibolite facies quartzo-feldspathic rocks in southern Madagascar. The microstructures of coarse-grained granulite suggest that feldspar and quartz accommodated deformation by both dislocation and diffusion creep in the absence of melt. The extreme ductility of feldspar in dynamically recrystallized granulite is explained by activity of dislocation creep, in conjunction with stress-controlled intracrystalline diffusion. In the studied rocks, the considerable volume of quartz is not interconnected even at high strain. The lack of its interconnectivity in coarse-grained granulites and in amphibolite facies granoblastic platy quartz rocks is explained by an extreme stability of the load-bearing framework structure at high-temperatures. In dynamically recrystallized granulite, the feldspar viscosity decreases so that quartz becomes enveloped by a weak feldspar matrix which inhibits its coalescence and interconnectivity. We predict an important decrease in strength of quartzo-feldspathic granulites due to activity of diffusional creep and convergence of viscosity of recrystallized feldspar and quartz