Oxygen isotope record of fluid–rock–SiO2 interaction during Variscan progressive deformation and quartz veining in the meta-volcanosediments of Belle-Ile (Southern Brittany)

Belle-Ile in the South Armorican Domain is composed of Palaeozoic volcano-detrital sequences with sericite phyllites and porphyroids. Fine-banded and folded meta-tuffites, microquartzites and graphitic quartzites occur in the basal part at Plage de Bordardoué. Phengite compositions constrain that Variscan metamorphism did not exceed 430 °C/4.5 kbar. Four generations (1–4) of centimeter-thick quartz veins were precipitated during Variscan progressive deformation and recorded changing fluid compositions. Values of 26‰ δ18OSMOW in vein 1 quartz exceed high δ18OSMOW in the host rocks. Decrease of quartz δ18OSMOW from margins to centers can be observed from the syntaxial veins. Younger veins 4 have lower δ18O. Their inclusions indicate lower salinities and traces of CH4 in the fluid when compared with veins 1. Veins 1 were overprinted by shearing and fissuring. Subgrain rotation recrystallization occurred along briquette structures and subgrain boundaries. The initial isotope values have been preserved. Larger domains with small-grained quartz can be identified by lower values of δ18O. Homogeneous isotopic compositions are found in hinges of folded veins 2 with grain boundary migration recrystallization. The small-scale oxygen isotope variations and the changing fluid compositions point to a locally hosted fluid system with a limited contribution of meteoric water during multiphase deformation and vein formation.