Oxygen isotope record of fluid–rock–SiO2 interaction during Variscan progressive deformation and quartz veining in the meta-volcanosediments of Belle-Ile (Southern Brittany)
Introduction
Quartz veins are geological features giving evidence of past fluid flow in rocks. The vein geometry and structure provide information about vein formation and deformation, and when combined with fluid inclusion and oxygen isotope studies, a changing of fluid–rock interaction parameters during geological time and space can be revealed (Rumble, 1977, Rumble, 1994, Cox and Etheridge, 1989, Fisher and Byrne, 1990). Stable isotope and fluid inclusion studies on veins and their host rocks allow the evaluation of the regional scales of the fluid systems, of their character, open or closed, and of the main fluid transfer and driving mechanisms (Kerrich, 1986, Ferry, 1992, O'Hara and Haak, 1992, Dipple and Ferry, 1992a, Oliver et al., 1993, Slater et al., 1994, Henderson and McCaig, 1996). Dependent on the scale, a discrimination among closed systems, fractured and unfractured, and open systems, pervasive or channellized, with wallrock interaction or not, has been proposed by Oliver (1996).
Many studies on veins consider a regional scale of kilometer or tens of kilometers. Some attention has been dedicated to isotope variations in the centimeter- or millimeter-scale within single veins and the interpretation of such observations (Rye and Bradbury, 1988, Kirschner et al., 1993, Slater et al., 1994). Furthermore, the small-scale changes and second order effects upon the stable isotope systems during deformational processes in such veins are poorly known (Kirschner et al., 1995). In the Variscan low-grade meta-volcanosedimentary sequence of Belle-Ile-en-mer in southern Brittany (France), several generations of quartz veins crystallized during successive phases or steps of bulk rock deformation. The structural and microstructural evolution of the veins together with the related variations of oxygen isotope values and the fluid-inclusion compositions in quartz are described. This allowed the evaluation of the dimension, the changing compositions, and the provenance of the fluid(s) during a progressive deformation.
Section snippets
Regional geology
The island Belle-Ile-en-mer in the South Armorican Domain of the Armorican Massif (Fig. 1a–c) is composed of a 3000-m-thick volcanic and volcano-detrital sequence of presumably Lower to Middle Palaeozoic (Ordovician?) age (Audren and Plaine, 1986). Monotonous sericite phyllites (meta-tuffites) and porphyroids (former ignimbrites) dominate the lithological sequence; graphite quartzites, graphite phyllite, phyllitic tuffites, former keratophyric tuffs and chert are found at its base (Fig. 1d).
Conditions of metamorphism
Temperatures and pressures of the syndeformational Variscan metamorphism have not yet been quantified. A low pressure–low temperature evolution has been suggested from petrographical observations (Audren, 1984, Audren, 1987). Samples from porphyroids (KerdoO, Coter, Penvraz, PTal5, Talut), phyllitic tuffites (Kerdo1, KerdoT), a phyllite (Kerzo) and a quartzite (Bor) represent bulk rock compositional variations within the silicic volcano-sedimentary sequence (Fig. 1c). The major element
Structural evolution of quartz veins
Spectacularly folded centimeter- to millimeter-scale interlayering of coarse- and fine-grained meta-tuffites, quartzites, graphitic quartzites and microquartzites occurs at the plage de Bordardoué (Fig. 1c). The structural position of these layered rocks is below a thick porphyroid series, which presumably belongs to the base of the volcano-detrital sequence (Fig. 1d). In general, the foliation of the banded rocks strikes NW–SE, dips to the SW (Fig. 3a and e) and is cut by several normal faults
Petrography and oxygen isotope data of quartz veins
Quartz grains and quartz microstructures in the vein fillings were studied in polished thin sections cut perpendicular to the vein margins (Fig. 5). Veins enclosing fragments of the host rocks are not abundant and were not considered for further study. The quartz grain size in the microquartzite host rocks is <0.01 mm; the quartz grain sizes in the veins range from 0.1 to >1 mm. Internal structures, geometry of grains and morphology of grain boundaries in quartz under polarized light were
Fluid inclusion study
As quartz microstructures and oxygen isotope values in veins 1 and 4 show marked differences, the fluid inclusion study concentrates on these vein generations. The question arises, whether fluid compositions and fluid entrapment conditions may have changed during the successive vein formation. Gravimetric estimates (quartz veins have 2.60–2.58 g/cm3, trigonal pure quartz has 2.65 g/cm3) led to a maximum 3–7% proportion of fluid inclusions (1.0 g/cm3) within the veins. Microthermometric
Discussion of quartz–H2O–CO2 interactions
Vein quartz is precipitated from silicious solutions (e.g. Fournier, 1985). Fisher and Brantley (1992) discussed models of quartz overgrowth and vein formation in the light of petrographic observations. They favored diffusive flux from adjacent matrix rocks as a silica transport mechanism after each dilatational event to explain the textural features of crack-seal veins. Periodically repeated incremental dilatation and sealing, induced and triggered by seismic events (Sibson et al., 1975) can
Conclusions
At Bordardoué, the quartz vein generations 1–4 successively crystallized in fine-banded quartz-rich host rocks at conditions between 300 and 400 °C during a multistage Variscan deformation. Inclusion lines parallel to the vein margins are not always obvious, but elongated blocky textures provide the argument for the crack-seal mechanism of vein opening and quartz precipitation. At least veins 3 and 4 developed as conjugated arrays of stress-related primary tension gashes in sites of brittle
Acknowledgements
Oxygen isotope analyses performed by BS at the Institut de Minéralogie de l'Université de Lausanne, were made possible and accompanied by Z. Sharp, D. Kirschner and J. Hunziker. H.-P. Meyer and H. Remy assisted during the microprobe analyses at Mineralogisches Institut Heidelberg and Laboratoire de Pétrologie Minéralogique, Paris. A. Roostai, Institut für Geologie und Mineralogie, Erlangen, Germany, helped with the XRF analyses. The cathodoluminescence microscopy of the samples was possible by
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