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...

The Tsäkkok Lens of the Scandinavian Caledonides represents the outermost Baltican margin that was subducted in late Cambrian/Early Ordovician time during closure of the Iapetus Ocean. The lens predominantly consists of metasedimentary rocks hosting eclogite bodies that preserve brittle deformation on the μm-to-m scale. Here, we present a multidisciplinary approach that reveals fracturing related to dehydration and eclogitization of blueschists. Evidence for dehydration is provided by relic glaucophane and polyphase inclusions in garnet consisting of clinozoisite + quartz ± kyanite ± paragonite that are interpreted as lawsonite pseudomorphs. X-Ray chemical mapping of garnet shows a network of microchannels that propagate outward from polyphase inclusions. These microchannels are healed by garnet with elevated Mg relative to the surrounding garnet. Electron backscatter diffraction mapping revealed that Mg-rich microchannels are also delimited by low angle (<3°) boundaries. X-ray computed microtomography demonstrates that some garnet is transected by up to 300 μm wide microfractures that are sealed by omphacite ± quartz ± phengite. Locally, mesofractures sealed either by garnet- or omphacite-dominated veins transect through the eclogites. The interstices within the garnet veins are filled with omphacite + quartz + rutile + glaucophane ± phengite. In contrast, omphacite veins are predominantly composed of omphacite with minor apatite + quartz. Omphacite grains are elongated along [001] crystal axis and are preferably oriented orthogonal to the vein walls, indicating crystallization during fracture dilation. Conventional geothermobarometry using omphacite, phengite and garnet adjacent to fractures, provides pressure-temperature conditions of 2.47 ± 0.32 GPa and 620 ± 60°C for eclogites. The same method applied to a mesoscale garnet vein yields 2.42 ± 0.32 GPa at 635 ± 60°C. Zirconium-in-rutile thermometry applied to the same garnet vein provides a temperature of ∼620°C. Altogether, the microchannels, microfractures and mesofractures represent migration pathways for fluids that were produced during glaucophane and lawsonite breakdown. The microfractures are likely precursors of the mesoscale fractures. These dehydration reactions indicate that high pore-fluid pressure was a crucial factor for fracturing. Brittle failure of the eclogites thus represents a mechanism for fluid-escape in high-pressure conditions. These features may be directly associated with seismic events in a cold subduction regime.

Integrated structural, geochemical, and geochronological investigations were conducted on metasedimentary rocks in the eclogite‐bearing Tsäkkok Lens of the Seve Nappe Complex (Scandinavian Caledonides) to resolve its exhumation history. Three deformation events are defined. D1 is likely related to the prograde to peak‐metamorphic stages, represented by a locally preserved S1. D2 resulted in vertical shortening and is defined by a pervasive S2 and cm‐/m‐scale F2 closed folds. D2 terminated with Scandian thrusting, which emplaced the overlying Köli Nappe Complex. D3 records NE‐SW shortening and constitutes m‐/km‐scale F3 open folds that deformed the Tsäkkok Lens and Köli Nappe Complex together. In situ white mica 40Ar/39Ar geochronology was conducted on select metasedimentary samples possessing S1 or S2 to resolve the timing of exhumation. Postdecompression cooling of the Tsäkkok Lens is best recorded by samples containing S1 or S2 that yield homogeneous white mica chemistry and 40Ar/39Ar dates. The timing of cooling is resolved to 477.2 ± 4.1 Ma (S1) and 475.3 ± 3.5 Ma (S2). Vertical shortening of the lens during exhumation may have proceeded until 458.1 ± 9.0 Ma. Later‐stage deformation during Scandian thrusting penetrated the Tsäkkok Lens at 429.9 ± 9.0 Ma, or younger. This resulted in noncoaxial deformation of the metasedimentary rocks, producing heterogeneous white mica chemistry and partially reset the older 40Ar/39Ar cooling record. Temperatures for deformation are resolved to the upper greenschist‐lower amphibolite facies. Altogether, the Tsäkkok Lens records rapid exhumation from eclogite‐facies conditions to midcrustal depths or shallower, followed by emplacement of the overlying Köli Nappe Complex.