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. The small island of Groix in southern Brittany, France, is well known forexceptionally well-preserved outcrops of Variscan blueschists, eclogites, andgarnetiferous mica schists that mark a Late Devonian suture between Gondwanaand Armorica. The kinematics of polyphase deformation in these rocks isreconstructed based on 3D microstructural analysis of inclusion trailswithin garnet and pseudomorphed lawsonite porphyroblasts using differentlyoriented thin sections and X-ray tomography. Three sets of inclusion trailsstriking NE–SW, NNW–SSE, and WNW–ESE are recognized and interpreted towitness a succession of different crustal shortening directions orthogonalto these strikes. The curvature sense of sigmoidal and spiral-shapedinclusion trails of the youngest set is shown to be consistent withnorthwest and northward subduction of Gondwana under Armorica, providedthat these microstructures developed by overgrowth of actively formingcrenulations without much porphyroblast rotation. Strongly non-cylindricalfolds locally found on the island are reinterpreted as fold-interferencestructures instead of having formed by progressive shearing and fold-axisreorientation. Six samples of a lower-grade footwall unit of the Groixophiolitic nappe (Pouldu schists) were also studied. Inclusion trails inthese rocks strike E–W, similar to the youngest set recognized on Groixisland. They record Carboniferous N–S shortening during continentalcollision. These new microstructural data from southern Brittany bear astrong resemblance to earlier measured in inclusion-trail orientations in thenorthwestern Iberia Massif. A best fit between both regions suggests not more than about15∘ anticlockwise rotation of Iberia during the Cretaceous opening ofthe Gulf of Biscay.

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Kinematics of subduction in the Ibero-Armorican arc constrained by 3D microstructural analysis of garnet and pseudomorphed lawsonite porphyroblasts from Île de Groix (Variscan belt)