High-temperature/low-pressure (high T/P) metamorphism in the overlying plate of the Alpine orogenesis of the Cyclades (Greece) is restricted to eroded nappes within the Upper Tectonic Unit. A combination of cation exchange thermobarometers, phase diagram pseudosection calculations and an oxygen isotope geothermometer allows us to derive the P–T conditions of the sillimanite/amphibolite-grade metamorphic event on the island of Donoussa and their relation to accretionary orogenic development. The key focus of the study is the critical role of bulk-rock MnO mol% in promoting garnet formation in metapelites, thereby allowing sillimanite + garnet + biotite and sillimanite + biotite assemblages to develop at the same P–T. Thermobarometry of zoned garnets in metapelites reveals a decompression path from peak metamorphic values at ∼650°C, 4 kbar, to ∼520°C, 2.5 kbar. Correspondingly, both the measured and calculated Mn isochores in garnet record the cooling and decompression from peak metamorphic P–T. Amphibolite thermometry (690°C ± 40°C) and a P–T pseudosection substantiate the peak temperature-decompression path and predict retrograde sphene formation at ∼550°C. Refractory accessory mineral oxygen isotope thermometry of prograde quartz + andalusite ± sillimanite lenses gives high temperatures (625–665°C). The P–T path proposed in this study shows that outcrop-scale MnO variation produces metamorphic assemblage variations in high T/P metapelites that would normally be assigned to the garnet to sillimanite zone transition. Parallels with the Buchan metamorphism possibly suggest tectonic switching during the transition from early active subduction to slab roll-back.