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From granitoid to kyanite-bearing micaschist during fluid-assisted shearing (Ile d’Yeu, France) |
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| Authors: | Caroline Sassier Philippe Boulvais Denis Gapais Ramon Capdevila Hervé Diot |
| Institution: | 1. Géosciences Rennes, UMR 6118 CNRS, Université de Rennes 1, Rennes cedex, 35042, France 3. Laboratoire de Sciences de la Terre, UMR 5570 CNRS, Université de Lyon 1, Villeurbanne cedex, 69622, France 2. P?le Sciences et Technologie, Université de La Rochelle, Av. Michel Crepeau, La Rochelle Cedex 01, 17042, France |
| Abstract: | We describe amphibolite-facies shear zones affecting an orthogneiss from the Armorican Hercynian belt (Ile d’Yeu, western France). The deformation pattern is consistent with top-to-the-South thrusting followed by E–W extension, as documented elsewhere in the region. Shearing was accompanied by channelled fluid flow that transformed the orthogneiss into a peraluminous micaschist. Structural and mineralogical data indicate rather early strain localization. Then, temperature increase associated with crustal thickening favoured more distributed deformations marked by shear zone stretching and the development of a HT regional foliation. Chemical analyses made across five shear zones show mass transfers that mainly implied losses in Ca and Na, and gains in H2O, Mg, and K. Most results indicate constant volume transformation, but some suggest records of either gains or losses of volume (between +20% and −30%). This might reflect variable records of fluid-rock interactions according to the timing of initiation and subsequent evolution of individual shear zones, early thrusting stages being marked by up-temperature flow, and late thrusting stages by down-temperature flow. δ18O analyses suggest that fluids experienced significant isotopic exchange with orthogneisses. |
| Keywords: | Shear zones Mass transfers Fluid-rock interactions High-temperature deformation Granitic rocks |
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