INSIGHTS INTO THE DECOMPRESSION PATH OF UHP ROCKS OF THE NE GREENLAND CALEDONIDES PROVIDED BY PARTIALLY MELTED METAPELITES

Evidence of ultrahigh pressure metamorphism has been found on one small island in the NE Greenland Caledonides. Kyanite eclogites and felsic host gneisses contain coesite inclusions in zircon and distinctive coesite pseudomorphs. Rare metapelitic rocks show extensive evidence of partial melting and preserve textures that are suggestive, but not diagnostic, of coesite having been present. We infer that the metapelites, like adjacent mafic eclogites and host gneisses, experienced peak metamorphic conditions of ~970°C at ~3.6 GPa in the coesite stability field. Evidence of partial melting of the metapelites is preserved in leucosome layers and lenses with granitic composition and igneous textures that are generally parallel to the matrix foliation, but anastomose throughout the matrix and around residual garnet megacrysts and cross-cut layering. Antiperthitic alkali feldspar is common in the leucosome, and small garnet and kyanite crystals are euhedral and appear to have crystallized from a melt.

As a first approximation we assume that the Greenland Caledonides, like other UHP terranes, experienced a steep decompression path. Partially melted metapelites preserve evidence of conditions and reactions at various stages along the decompression path. Partial melting of the metapelites would have occurred when the decompression path crossed the Phengite+Coe/Qtz dehydration melting reaction. Available experiments suggest that this reaction was crossed at pressure above the coesite-quartz transition; however, the exact position of the melt reaction depends on mineral and melt compositions. Textures in the matrix leucosome and an inferred partial melt inclusion in garnet are compatible with melting either before or after the transformation of coesite to quartz. An unusual “crown-like texture” around some feldspar-rimmed quartz inclusions in garnet suggests; however, that melt was already present to be forcefully injected along garnet-inclusion grain boundaries and into cracks in garnet when coesite transformed to quartz. Melt crystallized in the plagioclase stability field, at T above the alkali feldspar solvus, and P above the kyanite-sillimanite transition, and cooled at mid-crustal depths. We are currently examining garnet zoning profiles to put further constraints on the decompression path.