A combined study of O, H, and C isotope variations in the Ruby-East Humboldt Range metamorphic core complex has identified infiltration of aqueous fluids originating from both the surface of the earth, and from deep crustal or mantle levels. At deep structural levels, metamorphic fluids flowed upward and were rich in water, had mantle-like isotopic compositions, and were probably derived from crystallizing magmas. Conversely, at higher structural levels, a major meteoric-hydrothermal system developed in response to rapid uplift, causing meteoric fluids to penetrate downward into plastically deforming metamorphic rocks at depths of 10 to 15 km below the surface. Infiltration fronts associated with both processes have been recognized, allowing us to determine fluid fluxes and flow geometries at mid-crustal levels.

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