Postdepositional dolomite occurs in the shallow subsurface of several Caribbean Islands in Tertiary and Pleistocene carbonates, e.g., on Barbados, Grand Cayman, Jamaica, and St. Croix. Recent investigations have interpreted these dolomites as relatively early-diagenetic formations from "chemically modified seawater", mainly on the basis of petrography, stratigraphic association, and stable isotope values. Interpretations of the hydrologic regimes during dolomitization vary and usually include freshwater-seawater mixing, thermal or density-driven convection of seawater, or a combination thereof. This paper offers two alternative interpretations based on the relatively low Sr isotope ratios of the dolomites (0.7085 to 0.7092) and their proximity to active plate margins. Dolomitization may have been driven by plate tectonics, i.e., either by thermal convection of pore fluids at active spreading centers or by tectonic expulsion of fluids from accretionary prisms. The dolomitizing solution was seawater that was enriched in 86Sr by magmatic-hydrothermal solutions or by interaction with siliciclastics in an accretionary prism, respectively.