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Sub-vertical fluid flow during prograde metamorphism in the Nason Terrane, North Cascades crystalline core, Washington


Sub-vertical fluid flow during prograde metamorphism in the Nason Terrane, North Cascades crystalline core, Washington



Western Washington University, Bellingham, WA, United States (USA)



The nature of fluid movement through the lower crust during prograde metamorphism is an important but poorly understood part of the global H2 O and CO2 cycles on this planet. A broad set of samples from some of the structurally deepest exposures of the Nason terrane in the Cascades crystalline core were used to evaluate fluid movement during peak amphibolite-grade metamorphism. XRF bulk-rock chemical analyses, T-X(CO2 ) pseudosections, and thin-section petrography were used to identify equilibrium fluid composition for the solid phases in each specimen; these span the gamut of X(CO2 ) values. The juxtaposition of gneiss and marble-lithologies with differing equilibrium fluid compositions-and the infiltration of fluids during prograde metamorphism resulted in the development of a trend in calculated equilibrium fluid composition across the study area. Linear regression analysis of the fluid chemical data suggests that time-averaged fluid flow throughout peak conditions in the study area occurred largely upward in the study area with a small horizontal component towards the northeast. Vectors fit to the trend of mean fluid composition in each marble body suggest that fluid flowed upward 75 degrees + or -88 degrees above horizontal 64 degrees + or -35 degrees (NE) for the LW Body and 70 degrees + or -42 degrees above horizontal with an azimuth 73 degrees + or -34 degrees (ENE) for the SS Body. This is in agreement with models in the current literature that suggest fluid movement under these conditions is governed by buoyancy forces in the crust, as opposed to meter-to kilometer-scale variations in permeability described by previous studies of aqueous fluid systems in the lower crust. This study embodies a new method that employs petrography, XRF analyses and thermodynamic modeling for the determination of fluid flow directions during metamorphism.

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