Permian high-temperature metamorphism in the Western Alps (NW Italy)

During the late Palaeozoic, lithospheric thinning in part of the Alpine realm caused high-temperature low-to-medium pressure metamorphism and partial melting in the lower crust. Permian metamorphism and magmatism has extensively been recorded and dated in the Central, Eastern, and Southern Alps. However, Permian metamorphic ages in the Western Alps so far are constrained by very few and sparsely distributed data. The present study fills this gap. We present U/Pb ages of metamorphic zircon from several Adria-derived continental units now situated in the Western Alps, defining a range between 286 and 266 Ma. Trace element thermometry yields temperatures of 580-890°C from Ti-in-zircon and 630-850°C from Zr-in-rutile for Permian metamorphic rims. These temperature estimates, together with preserved mineral assemblages (garnet-prismatic sillimanite-biotite-plagioclase-quartz-K-feldspar-rutile), define pervasive upper-amphibolite to granulite facies conditions for Permian metamorphism. U/Pb ages from this study are similar to Permian ages reported for the Ivrea Zone in the Southern Alps and Austroalpine units in the Central and Eastern Alps. Regional comparison across the former Adriatic and European margin reveals a complex pattern of ages reported from late Palaeozoic magmatic and metamorphic rocks (and relics thereof): two late Variscan age groups (~330 and ~300 Ma) are followed seamlessly by a broad range of Permian ages (300-250 Ma). The former are associated with late-orogenic collapse; in samples from this study these are weakly represented. Clearly, dominant is the Permian group, which is related to crustal thinning, hinting to a possible initiation of continental rifting along a passive margin

Meteoric fluid‐rock interaction in Variscan shear zones

AbstractVariscan shear zones in the Armorican Massif represent sites of strong fluid‐rock interaction. The hydrogen isotope composition of muscovite (δDMs) from syntectonic leucogranite allows to determine the source of fluids that infiltrated the footwall of three detachment zones and the South Armorican Shear Zone. Using temperatures of hydrogen isotope exchange estimated from microstructural data, we calculate the hydrogen isotope ratios of water (δDwater) present within the shear zones during high‐temperature deformation. A ~40‰ difference in δDwater values from deep to shallow crustal level reveals a mixing relationship between deep crustal fluids with higher δD values that range from −34 to −33‰, and meteoric fluids with δD values as low as −74‰ in the upper part of detachment footwalls.</jats:p

Evolution microstructurale de métabasaltes à l’intérieur d’une auréole de métamorphisme de contact : une approche quantitative bi-dimensionnelle

Cette étude a pour but de décrire et de quantifier les transformations microstructurales des roches qui, à l’intérieur d’une auréole de métamorphisme de contact, recristallisent statiquement sous l’effet thermique d’une intrusion. La diorite de Saint Brieuc (533 ± 12 Ma ; Bretagne, France) developpe une étroite auréole de contact dans les amphibolites de Lanvollon. Cet encaissant est essentiellement constitué de métabasaltes qui ont été auparavant soumis à une déformation et un métamorphisme régionaux aux environs de 590-570 Ma. Afin de quantifier l’évolution texturale des cornéennes, une analyse d’image a été réalisée à partir de lames minces. Les caractéristiques morphologiques des hornblendes et des minéraux opaques des amphibolites mesurées sont : les facteurs d’anisotropie de forme (étirement et élongation), l’orientation préférentielle et la distribution de taille (surface) des grains. La quantification de ces différents paramètres montre que la recristallisation statique, qui augmente vers le contact avec l’intrusion, est (i) responsable de la diminution des paramètres d’anisotropie de forme et d’élongation des minéraux, (ii) cause une augmentation de la taille de grain, et (iii) est responsable de la disparition de l’orientation préférentielle des minéraux. Ensembles, la description qualitative et les mesures quantitatives montrent que les transformations à l’état solide dues au métamorphisme de contact tendent à faire disparaître toute trace d’anisotropie préexistante en faisant évoluer la texture des cornéennes vers des textures isotropes et équigranulaires. Cette étude met également en avant les difficultés et problèmes liés à la segmentation des objets avant traitement, en particulier pour les roches métamorphiques multiphasées où les phases minérales avec fractures et clivages "décorés" sont fréquents

Trace element and isotopic fingerprints in HP-LT metamorphic rocks as a result of fluid-rock interactions (Ile de Groix, France)

International audienceWhole rock trace element and isotopic compositions of different HP-LT metamorphic rocks of the Ile de Groix were analysed to characterise geochemical fingerprints during subduction and exhumation in a late Palaeozoic HP metamorphic terrain. Massive metabasites of hydrothermally altered enriched mid-ocean ridge basalt (E-MORB) origin are in association with banded metabasic rocks of volcano-sedimentary origin and metapelites. Fluid-rock interactions that likely occurred during seafloor hydrothermal alteration and early subduction metasomatism increased δ18O values, as well as K2O, Na2O, MgO, and LILE contents and decreased CaO contents of metabasites. Most metabasites have retained their early-subduction and pre-HP trace element and isotopic composition, even for rocks metamorphosed to lower eclogite-facies P-T conditions. Micaschists also preserved apparent pelitic protolith trace element values and oxygen isotopic compositions. During retrograde metamorphism related to the exhumation, metabasites were rehydrated by fluids in equilibrium with the host rock compositions, which were likely derived from the basic rocks. This style of fluid-rock interaction formed a greenschist facies mineral assemblage. Metabasites that underwent pervasive alteration by seafloor hydrothermal and metasomatism processes prior to peak metamorphism, show greater effects of retrogression and albitisation, probably because they were richer in H2O and Na2O. The variety of metamorphic assemblages on the Ile de Groix is thus directly related to the pre-HP rock composition. The extent of retrogression in the western part of the Ile de Groix primarily reflects stronger metasomatic intensities than in the eastern part