First evidence for ultrahigh-pressure metamorphism in the North- East Greenland Caledonides

Abstract

The first evidence for ultrahigh-pressure ( P ) metamorphism in the Greenland Caledonides is reported from kyanite eclogites and associated host gneisses on an island in Jokelbugt. Polycrystalline quartz inclusions in garnet and omphacite exhibit palisade quartz rims that are a diagnostic feature of quartz pseudomorphs after coesite, thus providing textural evidence for ultrahigh- P conditions. Geothermobarometry on the mineral assemblage garnet + omphacite + kyanite + quartz and/or coesite ± phengite confirms the microstructural interpretation of ultrahigh- P metamorphism. Peak pressure and temperature conditions (∼972 °C at 3.6 GPa) are well within the coesite stability field. The host gneisses are more retrograded than the kyanite eclogites and only record high- P conditions of 2.5 GPa at 826 °C; however, garnet contains polycrystalline quartz inclusions with radial fractures, suggesting that the gneisses also were subject to ultrahigh- P conditions. The presence of high- and ultrahigh- P metamorphism along the Laurentian and Baltica margins, the high temperatures recorded by the ultrahigh- P terranes in Greenland and Norway, and the absence of mantle peridotites in Greenland suggest that crustal thickening may have played an important role in the formation of an extensive orogenic root in the Greenland and Scandinavian Caledonides.