Pressure (P)–temperature (T) pseudosection analyses were carried out on metapelites from Sambagawa belt by using Perple_X 07 so as to determine mineral equilibria and the stability of sodic phases, in the model system MnO–Na2O–K2O–CaO–FeO–MgO–Al2O3–SiO2–H2O–(CO2) under high-pressure (HP) conditions (0.5–2.5 GPa/400–600 °C). A pressure–XNA [=Na/(Na+Al–2K–1.5Ca)] pseudosection at 500 °C is also calculated to examine the effect of Na/Al value of the bulk-rock composition on the stabilities of sodic minerals. The bulk-rock compositions of Sambagawa metapelite are variable in XNA values. The calculation results of stable assemblages in metapelites under the blueschist and eclogite facies conditions indicate that: (i) paragonite and glaucophane are stable throughout the wide XNA range of bulk-rock compositions of host rocks; (ii) stable P–T conditions of sodic pyroxene enlarge with increasing XNA value; and (iii) the stability field of paragonite enlarges with the presence of CO2 in the metamorphic fluid. The suggested wide stability of paragonite in metapelites and the relationships between the stability of sodic pyroxene and the bulk-rock compositions explain the reasons why (i) the occurrence of omphacite in metapelites from several subduction-related terranes is rare; and (ii) paragonite commonly occurs as inclusions in garnet of metapelites from the Besshi region of the Sambagawa belt. Paragonite is an important sodic phase of HP metapelites, and a combination of paragonite and quartz with high residual pressure included in garnet may be a useful indicator to verify the evidence for the eclogite facies metamorphism recorded in metapelites.