Inferring intermediate‐scale fluid flow in a heterogeneous metasedimentary multilayer sequence during progressive deformation: evidence from the Monts d’Arrée slate belt (Brittany, France)

Abstract

Quartz veins in the early Variscan Monts d'Arré e slate belt (Central Armorican Terrane, Western France), have been used to determine fluid-flow characteristics. A combination of a detailed structural analysis, fluid inclusion microthermometry and stable isotope analyses provides insights in the scale of fluid flow and the water-rock interactions. This research suggests that fluids were expelled during progressive deformation and underwent an evolution in fluid chemistry because of changing redox conditions. Seven quartz-vein generations were identified in the metasedimentary multilayer sequence of the Upper Silurian to Lower Devonian Plougastel Formation, and placed within the time frame of the deformation history. Fluid inclusion data of primary inclusions in syn-to post-tectonic vein generations indicate a gradual increase in methane content of the aqueous-gaseous H 2 O-CO 2 -NaCl-CH 4 -N 2 fluid during similar P-T conditions (350-400°C and 2-3.5 kbar). The heterogeneous centimetre-to metre-scale multilayer sequence of quartzites and phyllites has a range of oxygen-isotope values (8.0-14.1& Vienna Standard Mean Ocean Water), which is comparable with the range in the crosscutting quartz veins (10.5-14.7& V-SMOW). Significant differences between oxygen-isotope values of veins and adjacent host rock (D = )2.8& to +4.9& V-SMOW) suggest an absence of host-rock buffering on a centimetre scale, but based on the similar range of isotope values in the Plougastel Formation, an intraformational buffering and an intermediate-scale fluid-flow system could be inferred. The abundance of veins, their well-distributed and isolated occurrence, and their direct relationship with the progressive deformation suggests that the intermediate-scale fluid-flow system primarily occurred in a dynamically generated network of temporarily open fractures.

Evolution of Cu–Co mineralizing fluids at Nkana Mine, Central African Copperbelt, Zambia

“…The latter two values lie in the field of metamorphic waters (Sheppard, 1986) and such a variation in oxygen isotopic composition is common in metamorphic settings (Gray et al, 1991;Marquer and Burkhard, 1992;Dewaele et al, 2004). Variations are due to different source rocks of the fluids, migration pathways, varying water-rock ratios and the scale of buffering by the rocks (Knoop et al, 2002;Kenis et al, 2005;Berwouts et al, 2008).…”

Diagenesis to very low-grade metamorphism in lower Palaeozoic sediments: A case study from deep borehole Tobolka 1, the Barrandian Basin, Czech Republic

“…Comparable veins were also employed to constrain p-T conditions during Variscan orogenic deformations in other parts of the European Variscan orogen (e.g. Berwouts et al, 2008;Marsala et al, 2013;Slobodník et al, 2006).…”

Fluid composition changes in crystalline basement rocks from ductile to brittle regimes

“…A broad variety of fluid flow types occurs in the upper to middle crust, including flow along brittle faults and ductile shear zones, convection and pervasive fluid flow, which are driven by deformation and temperature gradients (Oliver, 1996(Oliver, , 2001). These fluid flow types cover a wide range of scales, from centimetric to regional scale (Oliver, 1996;Berwouts et al, 2008).…”

“…The study of veins, the best evidence of fluid flow, from a structural, petrological and geochemical point of view, provides insight about the relationship among deformation, metamorphism and fluid flow (Dewaele et al, 2004;Miller and Cartwright, 2006;Berwouts et al, 2008;Van Noten et al, 2008;Pitcairn et al, 2010;Depoorter et al, 2014). Quartz veins are present in most of exhumed metamorphic rocks and are generally believed to represent active fluid paths during or slightly after deformation and metamorphism, thus registering the P-T path and fluid chemistry during and after metamorphism (Nesbitt and Muelenbachs, 1989;Wood and Walther, 1986;Yardley, 1986;Ferry and Dipple, 1991;Birtel and Stöckhert, 2008;Berwouts et al, 2008).…”