Abstract
Abstract. The small island of Groix in southern Brittany, France, is well known for exceptionally well-preserved outcrops of Variscan blueschists, eclogites, and garnetiferous mica schists that mark a Late Devonian suture between Gondwana and Armorica. The kinematics of polyphase deformation in these rocks is reconstructed based on 3D microstructural analysis of inclusion trails within garnet and pseudomorphed lawsonite porphyroblasts using differently oriented thin sections and X-ray tomography. Three sets of inclusion trails striking NE–SW, NNW–SSE, and WNW–ESE are recognized and interpreted to witness a succession of different crustal shortening directions orthogonal to these strikes. The curvature sense of sigmoidal and spiral-shaped inclusion trails of the youngest set is shown to be consistent with northwest and northward subduction of Gondwana under Armorica, provided that these microstructures developed by overgrowth of actively forming crenulations without much porphyroblast rotation. Strongly non-cylindrical folds locally found on the island are reinterpreted as fold-interference structures instead of having formed by progressive shearing and fold-axis reorientation. Six samples of a lower-grade footwall unit of the Groix ophiolitic nappe (Pouldu schists) were also studied. Inclusion trails in these rocks strike E–W, similar to the youngest set recognized on Groix island. They record Carboniferous N–S shortening during continental collision. These new microstructural data from southern Brittany bear a strong resemblance to earlier measured in inclusion-trail orientations in the northwestern Iberia Massif. A best fit between both regions suggests not more than about 15∘ anticlockwise rotation of Iberia during the Cretaceous opening of the Gulf of Biscay.
Highlights
Structural analysis in metamorphic terrains is traditionally based on a combination of geological mapping, study of structures in outcrops, and microstructures in thin sections that are commonly cut parallel to the stretching lineation aimed at determining a shear sense
A major breakthrough in this state of affairs was claimed by Bell (1985), Bell et al (1986), and Bell and Johnson (1989) after recognizing the polyphase origin of inclusion-trail patterns that were previously assumed to have formed by progressive shearing and porphyroblast rotation (e.g. Zwart, 1962; Spry, 1963; Rosenfeld, 1970)
In this paper we report a similar microstructural study in the Armorican Massif focusing on high-pressure metabasites of Île de Groix, and paying attention to a lowergrade footwall unit known as the Pouldu schists
Summary
Structural analysis in metamorphic terrains is traditionally based on a combination of geological mapping, study of structures in outcrops, and microstructures in thin sections that are commonly cut parallel to the stretching lineation aimed at determining a shear sense. 15 km2) is a national reserve famous for exceptionally well-preserved coastal outcrops of Variscan blueschists, eclogites, and interlayered garnetiferous mica schists (Audren et al, 1993) These are generally accepted to represent the remains of the floor of a narrow ocean that, in the late Silurian and lower Devonian, separated Gondwana in the south from the Armorica microplate to the north. The significance of glaucophane orientation in the basic rocks of Groix needs to be studied: the orientation is extremely variable and may not be related to a shear direction Along these lines of suggested further research, we have performed detailed 3D microstructural analyses of 10 garnetiferous blueschist samples of the Île de Groix ophiolitic nappe, 4 samples of albite-porphyroblast, bearing greenschists (Pouldu schists) of a footwall unit cropping out along the mainland coast, plus 2 kyanite-staurolite schists collected further inland from the North Armorican Zone (Fig. 2). A possible correlation is proposed between inclusion-trail trends in NW Iberia and in the Armorican Massif, which if correct, implies not more than 15◦ anticlockwise rotation of Iberia during opening of the Gulf of Biscay
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