Identifying serpentinite reworked during high-T metamorphism
Of course, any ultramafic rock will ultimately melt at about 2000 deg C, but long before that there will be some interesting phase transitions, possibly involving serpentine > talc > olivine + orthopyroxene. 
Various subtle changes occur in the rock during its original hydration from peridotite to serpentinite. There is high- and low-temperature serpentinite. Isotopic studies have shown that most exposed serpentinite exposed in outcrop has hydrated at ambient groundwater temperature. Consider what happens. The Ca from the clinopyroxene in the original peridotite ends up as Ca- or Ca-Mg- carbonates, of which much is lost or precipitated elsewhere. The Fe in the original orthopyroxene, clinopyroxene and olivine is either lost from the system or re-precipitates as a mesh-texture of goethite. Cr from the silicate phases is excluded from the serpentine crystal lattice and from the surface of chromite crystals leaving a zoned Magnetite-Chromite structure in previously homogeneous chromite crystals. Since these are inert and refractory, they should survive any subsequent dehydration process. Any Ni will probably survive within the serpentine lattice. Summing up, when the serpentine is metamorphosed and turns back into peridotite, look for zoned chromite, reduced Ca (as clinopyroxene), and either reduced Fe, or Fe as a relict ghost sieve structure. Depending upon local circumstances there may, or may not, be a slight loss of silica during the hydration-dehydration cycle.