Fluid evolution during burial and Variscan deformation in the Lower Devonian rocks of the High-Ardenne slate belt (Belgium): sources and causes of high-salinity and C-O-H-N fluids

摘要

Fluid inclusions in quartz veins of the High-Ardenne slate belt have preserved remnants of pro-grade and retrograde metamorphic fluids. These fluids were examined by petrography, microthermometry and Raman analysis to define the chemical and spatial evolution of the fluids that circulated through the metamorphic area of the High-Ardenne slate belt. The earliest fluid type was a mixed aqueous/gaseous fluid (H_2O-NaCl-CQ_2-(CH_4-N_2)) occurring in growth zones and as isolated fluid inclusions in both the epi-zonal and anchizonal part of the metamorphic area. In the central part of the metamorphic area (epizone), in addition to this mixed aqueous/gaseous fluid, primary and isolated fluid inclusions are also filled with a purely gaseous fluid (CO_2-N_2-CH_4). During the Variscan orogeny, the chemical composition of gaseous fluids circulating through the Lower Devonian rocks in the epizonal part of the slate belt, evolved from an earlier CO_2-CH_4-N_2 composition to a later composition enriched in N_2. Finally, a late, Variscan aqueous fluid system with a H_2O-NaCl composition migrated through the Lower Devonian rocks. This latest type of fluid can be observed in and outside the epizonal metamorphic part of the High-Ardenne slate belt. The chemical composition of the fluids throughout the metamorphic area, shows a direct correlation with the metamorphic grade of the host rock. In general, the proportion of non-polar species (i.e. CO_2, CH_4, N_2) with respect to water and the proportion of non-polar species other than CO_2 increase with increasing metamorphic grade within the slate belt. In addition to this spatial evolution of the fluids, the temporal evolution of the gaseous fluids is indicative for a gradual maturation due to metamorphism in the central part of the basin. In addition to the maturity of the metamorphic fluids, the salinity of the aqueous fluids also shows a link with the metamorphic grade of the host-rock. For the earliest and latest fluid inclusions in the anchizonal part of the High-Ardenne slate belt the salinity varies respectively between 0 and 3.5 eq.wt percent NaCl and between 0 and 2.7 eq.wt percent NaCl, while in the epizonal part the salinity varies between 0.6 and 17 eq.wt percent NaCl and between 3 and 10.6 eq.wt percent for the earliest and latest aqueous fluid inclusions, respectively. Although high salinity fluids are often attributed to the original sedimentary setting, the increasing salinity of the fluids that circulated through the Lower Devonian rocks in the High-Ardenne slate belt can be directly attributed to regional metamorphism. More specifically the salinity of the primary fluid inclusions is related to hydrolysis reactions of Cl-bearing minerals during prograde metamorphism, while the salinity of the secondary fluid inclusions is rather related to hydration reactions during retrograde metamorphism. The temporal and spatial distribution of the fluids in the High-Ardenne slate belt are indicative for a closed fluid flow system present in the Lower Devonian rocks during burial and Variscan deformation, where, fluids were in thermal and chemical equilibrium with the host rock. Such a closed fluid flow system is confirmed by stable isotope study of the veins and their adjacent host rock for which uniform 8180 values of both the veins and their host rock demonstrate a rock-buffered fluid flow system.