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

https://doi.org/10.1016/S0191-8141(01)00135-3Get rights and content

Abstract

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.

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

References (67)

  • Y. Matsuhisa et al.

    Oxygen isotope fractionation in the systems quartz–albite–anorthite–water

    Geochimica Cosmochimica Acta

    (1979)
  • K. O'Hara et al.

    A fluid inclusion study of fluid pressure and salinity variations in the footwall of the Rector Branch thrust, North Carolina, USA

    Journal of Structural Geology

    (1992)
  • M.J. Rickard et al.

    Stress configuration in conjugate quartz-vein arrays

    Journal of Structural Geology

    (1983)
  • Z.D. Sharp

    In situ laser microprobe techniques for stable isotope analysis

    Chemical Geology (Isotope Geoscience Section)

    (1992)
  • B. Stöckhert et al.

    Thermochronometry and microstructures of quartz—a comparison with experimental flow laws and predictions on the temperature of the brittle-plastic transition

    Journal of Structural Geology

    (1999)
  • M. Van Daalen et al.

    Orientation analysis of localized shear deformation in quartz fibres at the brittle-ductile transition

    Tectonophysics

    (1999)
  • Y. Zhang et al.

    Determination of the homogenization temperatures and densities of supercritical fluids in the system NaCl–KCl–CaCl2–H2O using synthetic fluid inclusions

    Chemical Geology

    (1987)
  • C. Audren

    Lithostratigraphie et structure des séries volcano-sedimentaires de Belle-Ile-en-Mer Bretagne méridionale

    Bulletin Societé géologique minéralogique de la Bretagne

    (1984)
  • Audren, C., 1987. Évolution structurale de la Bretagne Méridionale au Palaeozoique. Mémoirs Societé géologique...
  • Audren, C., Plaine, J., 1986. Notice explicative de la feuille Belle-Ile-en-Mer et Iles Houat et Hoedic (Carte...
  • C. Audren et al.

    Analyse géométrique de la propagation des plis dans un milieu naturel stratifié à fort contraste de compétence au cours d'un cisaillement simple

    Comptes Rendues Académie Sciences Paris

    (1995)
  • Audren, C., Triboulet, C., Chauris, L., Lefort, J.-P., Vigneresse, J.L., Audrain, J., Thiéblemont, D., Goyallon, J.,...
  • R.J. Bakker et al.

    Preferential water leakage from fluid inclusion by means of mobile dislocations

    Nature

    (1990)
  • Bons, P.D., 2000. The formation of veins and their microstructures. In: Jessell, M.W. Urai, J.L. (Eds.), Stress, Strain...
  • Brown, P.E., 1989. FLINCOR: a fluid inclusion data reduction and exploration program. 2nd Biennial Pan-American...
  • Brown, P.E., 1998. Fluid inclusion modelling for hydrothermal systems. In: McKibben, M.A. Shanks, W.C. (Eds.),...
  • K. Bucher et al.

    Petrogenesis of Metamorphic Rocks

    (1994)
  • C. Cipriani et al.

    Metamorphic white micas: definition of paragenetic fields

    Swiss Bulletin of Mineralogy and Petrology

    (1971)
  • L.W. Diamond

    Introduction to phase relations of CO2–H2O fluid inclusions

  • G.M. Dipple et al.

    Metasomatism and fluid flow in ductile fault zones

    Contributions to Mineralogy and Petrology

    (1992)
  • J.M. Ferry

    Regional metamorphism of the Waits River Formation, Eastern Vermont: delineation of a new type of giant metamorphic hydrothermal system

    Journal of Petrology

    (1992)
  • D. Fisher et al.

    The character and distribution of mineralized fractures in the Kodiak Formation, Alaska: implications for fluid flow in an underthrust sequence

    Journal of Geophysical Research

    (1990)
  • D. Fisher et al.

    Models of quartz overgrowth and vein formation: deformation and episodic fluid flow in an ancient subduction zone

    Journal of Geophysical Research

    (1992)
  • Cited by (0)

    View full text