Abstract:The Paleoproterozoic tectono-metamorphic evolution of the pre-Athabasca basement (ϳ1.7 Ga) within the WollastonMudjatik Transition Zone (WMTZ) (Saskatchewan, Canada) has been characterized using both exposed basement and drill cores from the Wolly-McClean exploration drilling project. The finite ductile strain pattern of the WMTZ results from the superposition of two tectono-metamorphic events M1-D1 and M2-D2. M1-D1 is associated with the development of a gently dipping foliation striking N90°-N100°and a southward decrease in peak pressures from up to 10 kbar (1 kbar = 100 MPa) in the Cochrane River area down to 6 kbar in the Wolly-McClean exploration drilling project. The M2-D2 event is responsible for the main northeasterly trend of the WMTZ that developed in a sinistral transpressional tectonic regime during the final oblique collision of the Trans-Hudson Orogeny. Thermobarometric estimations on M2-D2 assemblages show that the studied area was reequilibrated at about 4-5 kbar and 750-825°C. The basement has thus been affected by a differential isothermal decompression event between D1 and D2 that allowed the juxtaposition of the deepest northeastern domains and the Wolly-McClean exploration drilling project, at the same structural level. These results suggest that the basement exposed to the northeast of the Athabasca Basin is not an analog of the basement located beneath the eastern Athabasca Basin where uranium-enriched granitic pegmatites and granites are known. We also suggest that uranium-enriched melts produced during the early M1-D1 stage of partial melting in the deep crust were transferred to the midcrust, owing to D2 shear zones, where they have differentiated to produce uranium-bearing pegmatites.Résumé : L'évolution tectono-métamorphique Paléoprotérozoique du socle pré-Athabasca (ϳ1.7 Ga) de la Zone de Transition Mudjatik-Wollaston (ZTMW) (Saskatchewan) a été définie à partir du socle affleurant et des forages du projet Wolly-McClean. Le champ de déformation finie ductile résulte de la superposition de deux évènements tectono-métamorphique M1-D1 et M2-D2. M1-D1 est associé au développement d'une foliation peu inclinée orientée N90°-N100°et à un pic de pression décroissant vers le sud avec plus de 10 kbar (1 kbar = 100 MPa) à la rivière Cochrane, jusqu'à 6 kbar dans la zone de forage du projet Wolly-McClean. L'évènement M2-D2 est responsable de la structuration majeure Nord-Est de la ZTMW qui s'est développée dans un régime transpressif senestre au cours de la phase finale de la collision oblique Trans-Hudsonienne. Les estimations thermobaromé-triques sur les assemblages M2-D2 mettent en évidence un rééquilibrage à 4-5 kbar et 750-825°C. Le socle a donc été affecté entre D1 et D2 par un épisode de décompression isotherme, ramenant au même niveau structural, la partie nord-est la plus profonde de la zone d'étude et la zone de forage Wolly-McClean. Ces résultats suggèrent que le socle affleurant au Nord-Est du bassin de l'Athabasca n'est pas l'analogue du socle situé sous le bassin là où les...

Chemical mass transfer was quantified in a metacarbonate xenolith enclosed within the granodiorite of the Quérigut massif (Pyrenees, France). Mass balance calculations suggest a strong decrease of CaO, SrO and CO 2 contents (up to-90%), correlated with a decrease of modal calcite content as the contact is approached. Most other chemical elements behave immobile during metasomatism. They are therefore passively enriched. Only a small increase of SiO 2 , Al 2 O 3 and Fe 2 O 3 contents occurs in the immediate vicinity of the contact. Hence, in this study, skarn formation is characterized by the lack of large chemical element influx from the granitoid protolith. A large decrease of the initial carbonate volume (up to-86%) resulted from a combination of decarbonation reactions and loss of CaO and CO 2. The resulting volume change has potentially important consequences for the interpretation of stable isotope profiles: the isotope alteration could have occured over greater distances than those observed today.

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Highlights  Accurate and continuous P-T-D-t path for the Wollaston-Mudjatik Transition Zone.  The first tectono-metamorphic event (M1-D1) took place between ca.1840 and 1813 Ma  The second tectono-metamorphic event (M2-D2) took place between ca.1813 and 1770 Ma  Multistage genetic model for Uranium-enriched pegmatite based on this P-T-D-t evolution

Melting experiments of calcite were performed on the join CaCO 3 -H 2 O at a pressure of 1000 bars. The system evolves to the ternary CaO-H 2 O-CO 2 system during melting experiments. Our experiments show that partial melting of calcite begins at a low temperature, below 650°C. Such a low partial melting temperature for carbonates revives the debate about the presence of carbonate melts in the upper crust. More specifically, the conditions for carbonate partial melting are present in carbonate host rocks undergoing contact metamorphism at high temperatures in the presence of water-rich fluid. The presence of carbonate melts influences physical parameters such as viscosity and permeability in contact aureoles, and, furthermore, decarbonation reactions release massive amounts of CO 2 .

The kaolinite-to-chlorite conversion is one of the chloritization processes that occurs in low temperature diagenetic and hydrothermal systems. The mechanism of this mineralogical transformation is still under discussion, since direct transformation, conversion via berthierine as intermediate phase or direct formation of berthierine/chlorite mix, either by dissolution-crystallization or by solid state transformation (or a combination of both), are all hypotheses put forward. In this context, each description of a kaolinite-to-chlorite conversion occurrence becomes an opportunity to shed new light and to renew this debate. Studying Carboniferous shale–crosscut by large quartz-kaolinite veins–from the mining basin of the North of France, we report therefore an uncommon kaolinite-Fe-rich chlorite assemblage. This assemblage appears as a chlorite fringe 20 µm wide along the interfaces between the shale and the quartz-kaolinite veins. All petrographical, mineralogical and chemical data suggest that the Fe-chlorite results from the interaction between the shale, providing the Fe,Mg supply, and the Si,Al-rich veins, leading to the chloritization of the kaolinite at a small scale via at least one dissolution-recrystallisation step. High-resolution observations highlight that neoformed Fe-rich chlorite contains some 7Å isochemical layers, as relict of berthierine. Therefore, we advance that the conversion takes place either through the precipitation of berthierine following by a second step involving solid state berthierine-chlorite conversion, or through the direct precipitation of a chlorite-rich/berthierine-poor mix driven by the Fe/(Fe + Mg) ratio, at low temperature and in reducing conditions. The comparison of our data with the recent literature allows to prefer the second hypothesis.

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Determining the Mn valence variation at the nanometer scale will be an important advance in the study of heterogeneous natural silicates. Here, the potential of the scanning transmission X-ray microscopy at the Mn L2,3-edges (640 -655 eV) as a probe for manganese redox state is evaluated. For this purpose, several natural Mn-silicates (rhodonite, ardennite, piemontite, Mn 4+ -silicate, jacobsite), covering several Mn valence, were analysed to identify the spectral parameters most sensitive to the Mn valence, regardless of the coordination environment, the crystal field strength, the nature and the length of the metal-ligand bonds, and the intraatomic Coulomb and spin-orbit interactions. Two suitable spectral empirical calibrations are thus proposed, linking the Mn valence to two peak intensity ratios: one ratio of intensities from two energy points of the L2 peak (at 651.7 and 655.2 eV), and one ratio of intensities from one energy point of the L2 peak (at 655.2 eV) and one of the L3 peak (at 641.6 eV).Thank to them, the first quantitative Mn valence maps are constructed, with a high spatial resolution (< 40 nm pixel size), opening the way to exhaustive crystallochemical studies of silicates containing Mn with different valences.