Displacement of isotopic compositions at boundary layers across strata of contrasting composition is commonly used to investigate hydrothermal fluid flow during orogeny. This study investigates whether hydrothermal fluid flow was focused along the Islay Anticline, Islay, SW Highlands of Scotland, as shown in the axial zone of the neighbouring Ardrishaig Anticline. Four localities from the limb to the axial plane of the Islay Anticline were investigated for isotopic homogenization of metacarbonate units to silicate values. At Mull of Oa on the limb of the anticline, metacarbonate samples show limited isotopic resetting and the fluid flux is estimated to be <1 m3 m−2. Within the axial zone of the Islay Anticline, metacarbonate units from Port a' Chotain and Bagh an Da Dhoruis show complete isotopic homogenization to silicate values indicating higher fluid fluxes. Fluid flow was enhanced along localized parasitic folds such as at Port an t-Sruthain, where metacarbonates have been isotopically reset, and there are abundant quartz–carbonate veins that precipitated during D1–D2 deformation. Metamorphic fluid flow was higher in the axial zone of the Islay Anticline and in localized antiformal structures. Fluid fluxes are estimated to be considerably lower than at the neighbouring Ardrishaig Anticline.

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