Microdiamonds associated with phlogopite, quartz, paragonite, phengite, apatite, and rutile are found as regular constituents of minute polyphase inclusions in garnet of ultrahigh-pressure (P) metamorphic gneiss lenses within migmatites of the gneiss-eclogite unit, Erzgebirge, Germany. These aggregates are interpreted to represent original inclusions of a supercritical dense COH fluid rich in K, Na, and SiO2. From this fluid, diamond was precipitated as a daughter crystal due to cooling at ultrahigh-P conditions. Brittle failure of the garnet host due to overpressure during release of confining pressure is demonstrated by healed radial cracks. During further cooling, the silicate phase assemblage of the inclusions crystallized at reduced internal pressures outside the stability field of diamond, as indicated by the presence of quartz, paragonite, and plagioclase. It is proposed that the mica-dominated mineral assemblage of the inclusions formed by reaction between the fluid and the garnet host, the previously formed diamond daughter crystals being preserved metastably. These diamond-bearing inclusions provide an unequivocal record of dense supercritical COH fluids rich in alkalies and silica within subducted continental crust during ultrahigh-P metamorphism.