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)

Abstract

Abstract. The small island of Groix in southern Brittany, France, is well known for\nexceptionally well-preserved outcrops of Variscan blueschists, eclogites, and\ngarnetiferous mica schists that mark a Late Devonian suture between Gondwana\nand Armorica. The kinematics of polyphase deformation in these rocks is\nreconstructed based on 3D microstructural analysis of inclusion trails\nwithin garnet and pseudomorphed lawsonite porphyroblasts using differently\noriented thin sections and X-ray tomography. Three sets of inclusion trails\nstriking NE–SW, NNW–SSE, and WNW–ESE are recognized and interpreted to\nwitness a succession of different crustal shortening directions orthogonal\nto these strikes. The curvature sense of sigmoidal and spiral-shaped\ninclusion trails of the youngest set is shown to be consistent with\nnorthwest and northward subduction of Gondwana under Armorica, provided\nthat these microstructures developed by overgrowth of actively forming\ncrenulations without much porphyroblast rotation. Strongly non-cylindrical\nfolds locally found on the island are reinterpreted as fold-interference\nstructures instead of having formed by progressive shearing and fold-axis\nreorientation. Six samples of a lower-grade footwall unit of the Groix\nophiolitic nappe (Pouldu schists) were also studied. Inclusion trails in\nthese rocks strike E–W, similar to the youngest set recognized on Groix\nisland. They record Carboniferous N–S shortening during continental\ncollision. These new microstructural data from southern Brittany bear a\nstrong resemblance to earlier measured in inclusion-trail orientations in the\nnorthwestern Iberia Massif. A best fit between both regions suggests not more than about\n15 ∘ anticlockwise rotation of Iberia during the Cretaceous opening of\nthe Gulf of Biscay.