Generation of Volatiles During Very Low-Grade to
Low-Grade Metamorphism: Examples from the
Ardennes Mts. (Belgium)

Ulrich F. Hein IGDL - Universität Göttingen, Goldschmidtstr. 3, D-37077 Göttingen, Germany

Frank Dorendorf IGDL - Universität Göttingen, Goldschmidtstr. 3, D-37077 Göttingen, Germany

Tobias Schöttler IGDL - Universität Göttingen, Goldschmidtstr. 3, D-37077 Göttingen, Germany

Uwe Buczko IGDL - Universität Göttingen, Goldschmidtstr. 3, D-37077 Göttingen, Germany

The Ardennes Mts. are part of the Rhenohercynian belt of the Mid-European Variscides. They are made up of a series of Cambro-Ordovician and Lower Devonian silici-clastites, Mid Devonian and Lower Carboniferous carbonaceous rocks with intercalated pelites and sandstones and Upper Carboniferous molasse sediments. Major deformation of Variscan age lead to folding and thrusting of the series (Bless et al., 1989; von Winterfeld and Walter, 1993, Hollmann and Walter, 1995).

Along the northern margin of the belt (Faille du Midi
= Variscan front) the rocks are non-metamorphic. Towards the south metamorphism slightly increases to very low-grade. Low-grade metamorphic rocks (greenshist facies) of Ordovician to Lower Devonian age are restricted to a narrow zone which is developed from the southern margin of the Rocroi Massif in the southwest along the Serpont Massif (Stainer, 1930) to the southern margin of the Stavelot-Venn Massif in the northeast (Schreyer, 1975; Kramm, 1982) on a length of 120 km. Peak metamorphism of these rocks predates folding. Intrusive rocks are restricted to local occurences of tonalites.

Fluid inclusions have been studied in mineral veins hosted by various rocks of varying metamorphic grade The veins are mostly composed of quartz and carbonates and may contain minor biotite, albite, chlorite and hematite. They may be pre-, syn- and late-kinematic thus permitting the reconstruction of the P-T-X-t development of fluids in the individual rock units

Syn- to late-kinematic veinlets in non- to very low-grade metamorphic coal-bearing sequences along the Faille du Midi are characterised by CH4-rich fluids of well defined dry-gas composition (XCH4 0.7, XCO2 £ 0.3, no HHC detected) and cogenetic low-salinity aqueous fluids. The gas composition is constant over a striking length of 150 km.

Synkinematic mineralization in very low-grade pelites and shales contains gas inclusions of pure CH4 besides low-salinity aqueous fluids somolar to standard profiles described by Mullis (1987). Secondary graphite may occur in tectonically overprinted CH4 inclusions.

CO2-rich fluids with XCH4 £ 0.2 are common in the pre- and syn-kinematic veins of the "metamorphic zone" but may also occur in late-kinematic veins in nappe units adjacent to the north. No decarbonatization of the host rocks has been observed. Accordingly the fluids are interpreted as being pumped into their position from the metamorphic pile at depth similar to synmetamorphic carbonate veins of the Rhenish Massif (Hein, 1993).

N2-rich fluids are restricted to pre- to syn-kinematic structures of the "metamorphic zone" and some nappe units adjacent to the north. Generally N2 is introduced into the series during the metamorphic peak or later. The distribution of H2O-N2-CO2-CH4 inclusions does not correlate with vein paragenesis of the "metamorphic zone" and may serve as an indicator of the spatial and temporal extent of "low-grade fluids". The retrograde N2-rich fluids trapped in the late-kinematic quartz-biotite veins at Bastogne (Darimont, 1986) remain unique.


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