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Quartz–calcite sandstones experienced the reaction calcite + quartz=wollastonite + CO2 during prograde contact metamorphism at P=1500 bars and T=560°C. Rocks were in equilibrium during reaction with a CO2–H2O fluid with XCO2=0·14. The transition from calcite-bearing, wollastonite-free to wollastonite-bearing, calcite-free rocks across the wollastonite isograd is only several millimeters wide. The wollastonite-forming reaction was driven by infiltration of quartz–calcite sandstone by chemically reactive H2O-rich fluids, and the distribution of wollastonite directly images the flow paths of reactive fluids during metamorphism. The mapped distribution of wollastonite and modeling of an O-isotope profile across a lithologic contact indicate that the principal direction of flow was layer-parallel, directed upward, with any cross-layer component of flow <0·1% of the layer-parallel component. Fluid flow was channeled at a scale of 1–100 m by pre-metamorphic dikes, thrust and strike-slip faults, fold hinges, bedding, and stratigraphic contacts. Limits on the amount of fluid, based on minimum and maximum estimates for the displacement of the wollastonite reaction front from the fluid source, are (0·7–1·9) × 105 cm3 fluid/cm2 rock. The sharpness of the wollastonite isograd, the consistency of mineral thermobarometry, the uniform measured 18O–16O fractionations between quartz and calcite, and model calculations all argue for a close approach to local mineral–fluid equilibrium during the wollastonite-forming reaction.