High-P/T metamorphism of the Franciscan Complex represents evolving subduction-zone processes at depth. Varied peak P-T conditions (100--350 ¿C and 3--10 kbar) promote differences in porosity, permeability, and rheology, potentially impacting fluid mobility and entrainment of seawater. Local-scale isotopic buffering of CaCO3 veins by exchange with adjacent host-rocks obscures assessment of fluid sources (most vein Δ13CVPDB=-11.0 to -3.0?, Δ18OVSMOW=+12.0 to +18.5?). Some veins with elevated Δ18O (relative to rock-buffered values) may reflect up-dip flow of H2O released at greater depths. Differences in vein Δ13C in adjacent coherent greywacke and shaley m¿lange zones of lower-grade rocks may be due to preferential infiltration of m¿lange zones by deeply derived CH4-bearing fluids or to vein formation over a range of T. Calculated fluid-Δ18O for lower-T veins spans the range of fluids venting in active accretionary prisms and producing forearc serpentinite seamounts. Calcite cement in the Coastal Belt is absent in higher-grade rocks, suggesting that cement is lost to decarbonation reactions like those invoked to elevate fluid alkalinity in Marianas seamounts. ¿ 2001 American Geophysical Union