STABLE-ISOTOPE VARIATIONS (H, C, O) IN A PROGRADE METAMORPHIC TRIASSIC RED-BED FORMATION, CENTRAL SWISS ALPS

In the Swiss Central Alps, the Upper Triassic red beds range from unme tamorphosed Keuper sediments in the northern foreland to staurolite-gr ade pelites (Quartenschiefer) farther to the south. Individual stratig raphic sequences can be followed intermittently throughout the prograd e metamorphic sequence, allowing for isotopic variations in hydrogen, carbon and oxygen to be traced in a single protolith that has undergon e essentially a single Alpine metamorphic event. Samples were analyzed from each of four metamorphic grades: 1) unmetamorphosed sediments fr om the Lindau borehole in the Molasse Basin (MB), T approximate to 100 degrees C; 2) anchimetamorphosed sediments from the northern Glarus A lps (GA), T approximate to 200-250 degrees C; 3) epimetamorphic-grade (lower greenschist facies) sediments from the Urseren Zone (UZ), T app roximate to 400 degrees C; 4) epi- and mesometamorphic pelites from th e Lukmanier Pass are (LP), T similar or equal to 500-550 degrees C The delta(18)O (parts per thousand vs SMOW) and the delta(13)C (parts per thousand vs PDB) values of the carbonate fraction, the delta(18)O and delta D values of the silicate residue (parts per thousand vs SMOW) a nd the total wt% H2O and wt% carbonate were measured for each sample. Averages and variability for each zone are given as follows: [GRAPHICS ] The silicates are the dominant oxygen reservoir at all but the lowes t grades; hence the delta(18)O(silicate), values are essentially const ant throughout the sequence. The delta(18)O(carbonate), values systema tically decrease with increasing grade, approaching equilibrium with t he silicates. The scattered delta(13)C carbonate values may be due to varying degrees of decarbonation or interaction with organic matter (f rom adjacent black shales?). The delta D values of the hydrous silicat es initially increase, indicating interaction with deuterium-enriched pore waters, and then decrease at higher metamorphic grades due to the removal of water associated with dehydration reactions. The overall i sotopic variations can be interpreted in terms of minor C-O-H fluid lo ss, with no external fluid input.