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

摘要

Pb and Sr isotope data have been collected from fluid inclusions trapped in syntectonic quartz veins associated with the Gavarnie thrust in the central Pyrenees, and from lithologies in the footwall and hanging wall of the thrust. These data suggest the occurrence of three distinct phases of fluid movement related to different periods in the structural evolution of the thrust zone. During the first phase, fluid interacting with the Silurian graphitic slates in the immediate hanging wall of the Gavarnie thrust moved along a carbonate mylonite zone beneath the thrust, enriching the deforming carbonates in 87Sr and 207Pb. The second phase of fluid movement occurred during imbricate thrusting and intense veining beneath the Gavarnie thrust. Isotope data from quartz-hosted fluid inclusions show a trend toward high 208Pb/204Pb ratios with increasing 207Pb/204Pb and 206Pb/204Pb and decreasing 87Sr/86Sr. The inclusions contain fluids equilibrated with several source lithologies: deformed Cretaceous and Silurian rocks within the thrust zone, Devonian phyllites located above the Silurian slates in the thrust hanging wall, and Triassic redbeds located beneath Cretaceous limestones in the thrust footwall. The mixture of fluids reflects both the transient chemical interaction of previously isolated rock types in the footwall and hanging wall via fault and fracture networks, and dilatancy pumping of fluid into opening veins. The last phase of fluid movement resulted in the precipitation of late carbonate vein fills and a return to more local fluid movement as stresses relaxed and fracture networks healed. Overpressured fluids trapped within and partially expelled from the thrust zone during phase 1 were still present during phases 2 and 3. This suggests that average permeabilities within the mylonites were low, and that transient high-permeability zones related to fracture networks were of limited extent.