Fluid expulsion and dilatancy pumping during thrusting in the Pyrenees: Pband Sr isotope evidence

Pb and Sr isotope data have been collected from fluid inclusions trapped in syntectonic quartz veins associated with the Gavarnie thrust in the centra l Pyrenees, and from lithologies in the footwall and hanging wall of the th rust. These data suggest the occurrence of three distinct phases of fluid m ovement related to different periods in the structural evolution of the thr ust zone. During the first phase, fluid interacting with the Silurian graph itic slates in the immediate hanging wall of the Gavarnie thrust moved alon g a carbonate mylonite zone beneath the thrust, enriching the deforming car bonates in Sr-87 and Pb-207. The second phase of fluid movement occurred du ring imbricate thrusting and intense veining beneath the Gavarnie thrust. I sotope data from quartz-hosted fluid inclusions show a trend toward high Pb -208/Pb-204 ratios With increasing (PP)-P-207/Pb-204 and Pb-206/Pb-204 and decreasing Sr-87/Sr-86. The inclusions contain fluids equilibrated with sev eral source lithologies: deformed Cretaceous and Silurian rocks within the thrust zone, Devonian phyllites located above the Silurian slates in the th rust hanging wall, and Triassic redbeds located beneath Cretaceous limeston es in the thrust footwall. The mixture of fluids reflects both the transien t chemical interaction of previously isolated rock types in the footwall an d hanging wall via fault and fracture networks, and dilatancy pumping of fl uid into opening veins. The Last phase of fluid movement resulted in the pr ecipitation of late carbonate vein fills and a return to more local fluid m ovement as stresses relaxed and fracture networks healed. Overpressured flu ids trapped within and partially expelled from the thrust zone during phase 1 were still present during phases 2 and 3. This suggests that average per meabilities within the mylonites mere low and that transient high-permeabil ity zones related to fracture networks were of limited extent.