Partial melting of metapelites at ultrahigh-pressure conditions, Greenland Caledonides

Eclogite, orthogneiss and, by association, metapelite from an island at 78°N in North‐East Greenland experienced ultrahigh‐pressure (UHP) metamorphism at approximately 970 °C and 3.6 GPa, at the end of the Caledonian collision, 360–350 Ma. Hydrous metapelites contain abundant leucocratic layers and lenses composed of medium‐grained, anhedral, equigranular quartz, antiperthitic plagioclase and K‐feldspar with minor small garnet and kyanite crystals. Leucosomes are generally parallel to the matrix foliation, are interlayered with residual quartz bands, anastomose around residual garnet and commonly cross‐cut micaceous segregations. Textures suggest that the leucosomes crystallized from a syntectonic melt, but crystallized at the end of local high‐grade deformation. The metapelite outcrop is < 1.5 km from kyanite eclogites with confirmed coesite, but the metapelites lack coesite and palisade textures diagnostic of coesite pseudomorphs. They do contain highly fractured garnet megacrysts with polycrystalline quartz inclusions (some surrounded by radial fractures) and Ti‐rich phengite inclusions that suggest the former presence of coesite. Polyphase inclusions in garnet contain reactants and products of the inferred dehydration melting reaction: Phe + Qtz = Ky + Kfs + Rt + melt. The reactants are thought to have been early inclusions of hydrous phases within garnet that melted and then crystallized new phases. Garnet surrounding these inclusions has patchy zoning with elevated Ca, consistent with experiments that produced similar patchy microstructures in garnet around inclusions with an unequivocal melt origin. The peak UHP metamorphic assemblage in these rocks is inferred to have been phengite, coesite, garnet, kyanite, rutile, fluid ± omphacite ± epidote. Phase diagrams indicate that dehydration melting of phengite in this assemblage would have occurred after decompression from peak pressure, but still above the coesite to quartz transition. Unusual crown‐ and moat‐like textures in garnet around some polycrystalline quartz inclusions are also consistent with the inference that melting took place at UHP conditions.