The evolution of the mineral assemblages and P-T conditions during partial melting of upper-amphibolite facies paragneisses in the Orue Unit, Epupa Complex, NW Namibia, is modelled with calculated P-T-X phase diagrams in the Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O system. The close concordance of predictions from the phase diagrams to petrographic observations and thermobarometric results documents that quantitative phase diagrams are suitable to explain the phase relationships in migmatitic upper-amphibolite facies low- and medium-pressure metapelites, which occur in many high-grade metamorphic terranes worldwide. Different mineral assemblages in the migmatitic metapelites of the Orue Unit reflect regional discrepancies in the metamorphic grade: in a Northern Zone, early biotite-sillimanite-quartz assemblages were replaced via melt-producing reactions by cordierite-bearing assemblages. In a Southern Zone, they were replaced via melt-producing reactions by garnet-bearing assemblages while cordierite is restricted to rare metapelitic granofelses, which preserve Grt-Sil-Crd-Bt peak assemblages. Peak-metamorphic conditions of 700-750 °C at 5.5-6.7 kbar in the Southern Zone and of ∼750 °C at 4.5 kbar in the Northern Zone are estimated by integrating thermobarometric calculations with data from calculated mineral composition isopleths. Retrograde back-reactions between restite and crystallizing melt are recorded by the replacement of garnet by biotite-sillimanite and/or biotite-muscovite intergrowths. Keywords: contact metamorphism, geothermobarometry, metapelite, migmatite, phase diagrams.