Phase Equilibria in the Metamorphic Rocks of St. Paul Island and Cape North, Nova Scotia

A suite of schists—one from the garnet zone, 19 from the staurolite zone, 2 from the kyanite isograd, and one from the kyanite zone—were separated into their constituent minerals. Chemical analyses of one chlorite and of 23 sets of coexisting biotites and garnets were carried out by photometric and titrametric procedures. Plots of garnet-biotite tie-lines from divariant assemblages on appropriate phase diagrams result in intersecting tie-lines which cannot be ascribed to experimental error. Theoretical considerations argue that at equilibrium, at the same pressure and temperature or at constant pressure and varying temperature, tie-lines of divariant assemblages should not intersect. Possible explanations require that diffusion equilibrium of Fe and Mg be restricted to volumes smaller than that of a hand specimen or that P as well as T varies considerably. Emission spectrographic determinations of Fe and Mg in biotite indicate that the Fe/Mg ratio varies among biotites little more than a centimeter apart. Such a variation would argue more in favor of a lack of diffusion equilibrium.