Phase relations in metabasic rocks are documented from the results of phase modelling, experiments and ACF analysis. A P–T pseudosection for a mid-ocean-ridge basalt (MORB) composition was calculated in the P–T range of 1–32 kbar and 400–1100°C using THERMOCALC 3.45. Phase relations in this pseudosection are mostly consistent with the results from experiments but complicated. ACF analyses suggest that the complicated phase relations can be simplified into 10 invariant assemblages involving orthopyroxene, clinopyroxene, hornblende, glaucophane, actinolite, garnet, chlorite, epidote, lawsonite, kyanite and plagioclase, assuming quartz, NaAlO₂ and H₂O/melt being are excess. Phase relation analyses in the P–T projections that are constructed according to Schreinemakers’ rules and qualitative P–T pseudosections that are obtained for a MORB composition indicate that 22 subfacies assemblages are recognized for metabasic rocks. A four-fold classification of metamorphism is proposed on the basis of the phase relations for metabasic rocks, including low-, medium-, high- and very-high-P/T types, with apparent geothermal gradients >110°C/kbar, between 110 and 55°C/kbar, between 55 and 28°C/kbar, and <28°C/kbar, respectively. This four-fold classification of metamorphism is better able to match various tectonic settings.