Fluid generation, vein formation and the degree of fluid-rock interaction during decompression of high-pressure terranes: the Schistes Lustres, Alpine Corsica, France

Centimetre- to decimetre-wide quartz +calcite veins in schistes lustres fro m Alpine Corsica were formed during exhumation at 30-40 Ma following bluesc hist facies metamorphism. The delta O-18 and delta C-13 values of the veins overlap those of the host schistes lustres, and the delta O-18 values of t he veins are much higher than those of other rocks on Corsica. These data s uggest that the vein-forming fluids were derived from the schistes lustres. Fluids were probably generated by reactions that broke down carpholite, la wsonite, chlorite and white mica at 300-350 degreesC during decompression b etween c. 1400 and 800 MPa. However, the delta O-18 values of the veins are locally several per mil higher than expected given those of their host roc ks. The magnitude of oxygen isotope disequilibrium between the veins and th e host rock is inversely proportional to the delta O-18 value of the host r ock. Additionally, calcite in some schists is in isotopic equilibrium with calcite in adjacent veins, but not with the silicate fraction of the schist s. Locally, the schists are calcite bearing only within 1-20 cm of the vein s. The vein-forming fluids may have been preferentially derived from calcit e-bearing, high-delta O-18 rocks that are common within the schistes lustre s and that locally contain abundant (>15%) veins. If the fluids were unable to completely isotopically equilibrate with the rocks, due to relatively r apid flow at moderate temperatures or being confined to fractures, they cou ld form veins with higher delta O-18 values than those of the surrounding r ocks. Alteration of the host rocks was probably inhibited by isolation of t he fluid in 'quartz-armoured' veins. Overall, the veins represent a metre- to hectometre-scale fluid-flow system confined to within the schistes lustr es unit, with little input from external sources. This fluid-flow system is one of several that operated in the western Alps during exhumation followi ng high-pressure metamorphism.