Detailed oxygen-isotope profiles were measured across two very similar amphibolite-grade, paragneiss – mafic gneiss contacts located 2 km apart within the Kapuskasing structural zone. The first profile is asymmetrical about die contact. δ18O values for whole rocks and minerals are homogeneous in the paragneiss (whole-rock δ18O = +10.4 to +11.4‰), smoothly decrease by about 2‰ over 2.5 m moving into the mafic gneiss, and are homogeneous in the remainder of the mafic gneiss (whole-rock δ18O = +7.0 to +7.4‰). The second profile is flat, with homogeneous whole-rock and mineral δ18O values in bom lithologies, and mafic gneiss values (whole-rock δ18O = +8.7 to +9.8‰) that are higher by about 3‰ relative to typical mafic gneiss values. Mineral–mineral isotopic fractioations are fairly constant in all samples and are typical of high-grade metamorphic rocks. Despite their differences, both profiles can be explained by the advection of a mixed H2O–CO2 fluid in equilibrium with the paragneiss across the contact into the mafic gneiss, coupled with diffusion of oxygen in the fluid. Modeling constrains the net fluid flux required to form me first profile to be 1–3 m3/m2, and for me second profile to be ≥ 10 m3/m2. Thus the net fluid flux during metamorphism varied probably by at least a factor of five over 2 km. These fluxes could have been generated locally within the Kapuskasing structural zone during metamorphism either by metamorphic devolatilization reactions or by the release of volatiles from crystallizing magmas.

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