SEAWATER-SEDIMENT-BASALT INTERACTIONS - STABLE-ISOTOPE (H, O) AND ELEMENTAL FLUXES WITHIN THE ORDOVICIAN VOLCANO-SEDIMENTARY SEQUENCE OF ERQUY (BRITTANY, FRANCE)

The Ordovician volcano-sedimentary succession of Erquy (northern Britt any) is made of immature sediments thermally metamorphosed at the cont act of intruding basic sills. Pillow lavas constitute the upper part o f the sequence. The trace element geochemistry of sills and pillow lav as suggests that they were derived from a tholeiitic source located be neath a passive margin. This volcanic sequence was metamorphosed under low to moderate greenschist facies conditions. In this study the dire ction and amplitude of chemical and isotopic fluxes in the basalt-sedi ment-water system were established and the oxygen and hydrogen isotope compositions of the aqueous fluid that reacted with the volcanic rock s were characterized. Cationic thermometry on chlorites and isotopic t hermometry on plagioclase-chlorite pairs indicate closure metamorphic temperatures in the range 200-250 degrees C for the basaltic sills. St able isotope compositions of iron-rich chlorites (delta(18)O = 5.5 par ts per thousand; delta D from -60 to -50 parts per thousand) and plagi oclases (delta(18)O from +9 to +10 parts per thousand) reveal that the source of the fluid was certainly seawater. The delta(18)O variations within the sills are strongly correlated with the rate of progress of the main metamorphic reaction:clinopyroxene + plagioclase + ilmenite --> chlorite + albite + epidote + quartz + sphene that produced major element mobility at the scale of the volcanosedimentary sequence. Calc ulation of elemental fluxes by mass balance combined with oxygen isoto pic compositions of basalts shows that the highest water-rock ratios ( greater than or equal to 1) were at sill-sediment boundaries and withi n pillow lavas at the top of the pile. The volcano sedimentary sequenc e of Erquy was a net sink for Na and a source for Ca. No Mg uptake cou ld be detected whereas the hydrothermal alteration of the sediments re leased Fe, Si, and K trapped by the volcanic rocks. The delta(18)O val ue of the fluid reacting with sills appears to have shifted no more th an +4 parts per thousand after percolation at low temperature through immature sediments (delta(18)O = +12 parts per thousand). The Erquy vo lcano-sedimentary sequence represents a marine hydrothermal system dom inated by low-temperature exchange which allowed a general O-18-enrich ment of the volcanic rocks and a O-18-depletion of sediments.