Calc-Silicate Diffusion Zones between Marble and Pelitic Schist

Four distinct calc-silicate zones are observed between interlayeredmarble and pelite in a roof-pendant (Gile Mountain Fm.) of agranitic intrusive at Lake Willoughby in northern Vermont. Thezones are characterized by garnet, coarse diopside-clinozoisite,fine clinozoisite-diopside and amphibole-plagioclase. The bulkcompositions of the zones, calculated from modal data and microprobeanalyses, deviate significantly from a gradational mixture ofmarble and pelite. The observed zonal sequence is close to thatpredicted by a simple model of cation diffusion metasomatism.Possible locations of the initial marble-pelite boundary aresuggested on the basis of bulk compositional data and petrographicevidence. Calculated mass balance relations for the formationof the zones by diffusion at constant volume are unrealisticfor reasonable locations of the initial boundary. Calculatedmass balance relations at constant Al₂O₃ suggest a volume decreaseof about one third during formation of the diffusion zones whollyfrom the pelite. In addition the data suggest that nearly equalvolumes of initial marble and pelite have disappeared, leadingto a total volume decrease of near 50 percent. The observationssuggest that diffusion of calcium was dominant and that diffusionof aluminium, iron and magnesium was relatively unimportant.Evaluation of relative diffusional mobilities is shown to bepossible but intractable with the quality of available thermochemicaldata and activity-concentration relations of crystalline solutions.