Mineral reactions in a calc-mica schist from Gassetts,Vermont, U.S.A.

A petrographic and electron microprobe study of an interbedded calc-mica schist from Gassetts, Vermont reveals the complexities of Fe-Mg crystalline solution and gradients in X _H2O/X _CO2 during regional metamorphism. The common association of microcline+diopside ± zoisite formed from biotite +calcite+quartz may be produced by continuous (Fe-Mg) exchange reactions, despite the implied metastability in the CaO-K₂O-MgO-Al₂O₃-SiO₂-H₂O-CO₂ system. Different assemblages are produced in a reaction zone between carbonate and pelite beds of varying thickness. This illustrates the importance of exhaustion of fluid buffering assemblages due to mineral reactions proceeding on a local scale. Siliceous dolomites at the same metamorphic grade have produced significantly different assemblages to the interlayered carbonatepelite sequence.