CHEMICAL, MINERALOGICAL AND TEXTURAL DIFF ERENCES BETWEEN CORE AND MARGINS OF THE GRANDE-COMMUNE DIABASE DYKE (MASSIF-DE-ROCROI, ARDENNES) - EVIDENCES FOR AN INFILTRATION EPISODE BY A H2O-CO2 FLUID DURING A VARISCAN SYNCLEAVAGE METAMORPHISM IN THE GREENSCHIST FACIES

After its consolidation, the Grande Commune dyke has suffered a Varisc an polyphased metamorphism in greenschist facies conditions. The last major phase is a syncleavage metamorphism that is registered only at t he dyke margin. A progressive and continuous chemical, mineralogical a nd textural differentiation between the core and the margin evidences mass transfer by fluids during this metamorphic phase. Mass balance ca lculations indicate C, O, H, Mn gains and Na,Sr losses at the margin. The C and O gains account for the main part of the total rock mass var iation (almost-equal-to 9 %), the decrease in the rock density (3.05 t o 2.81) and the volume increase (almost-equal-to 20 %) in the margins. They indicate the infiltration of the margin by a CO2-H2O rich fluid that promotes metamorphic reactions. Toward the margin, calcite, chlor ite, albite, quartz and rutile replace progressively actinolite, oligo glase, ilmenite and sphene that are only preserved in the undeformed c ore of the dyke. The oxidation of a low amount of CH4 in the fluid (<0 .1 % of the initial rock mass) could account for the reduction of iron thal results from the replacement of epidote by chlorite. The amount of CO2 and H2O consumed (almost-equal-to 7 % and almost-equal-to 1 %) by the calcite and the chlorite growth implies a volumetric fluid/rock ratio higher than 0.5. The fluid cannot originate from the enclosing rocks of the dyke and requires a deeper source which could be metamorp hic or magmatic.