The system Na₂O-CaO-MgO Al₂O₃-SiO₂-H₂O is proposed to model phase equilibria and mineral parageneses for low-temperature metamorphism of basaltic rocks. Univariant reactions marking the transitions between various sub-greenschist facies are identified and some have been experimentally determined. The introduction of Fe₂O₃ into the model system at fixed FeO/MgO ratio creates continuous reactions for facies boundaries and discontinuous reactions for invariant points of the model system. Both qualitative and quantitative effects on P-T displacement and phase compositions are discussed. The X_Fe³⁺ isopleths for epidote were plotted to exemplify the transition from the zeolite through prehnite-pumpellyite to prehnite-actinolite facies. T-X_Fe³⁺ relations were established for continuous and discontinuous reactions relating such facies transitions. Because of the common occurrence of two or three Ca-Al hydrosilicates in low-grade metabasites, an isobaric Al-Ca-Fe³⁺ projection from chlorite may be used to illustrate mineral assemblages and compositions of the coexisting Ca-Al silicates in the presence of quartz, albite, and chlorite. Reported occurrences in several classic burial metamorphic terrains and ocean-floor metabasites in ophiolites are described. Only the composition of a mineral from a buffered assemblage can constrain the intensive properties for metamorphism; previously reported compositional trends for pumpellyite and epidote with increasing metamorphic grade are oversimplified.