ULTRAHIGH-PRESSURE METAMORPHISM IN THE GREENLAND CALEDONIDES–MICROSTRUCTURAL AND THERMOBAROMETRIC EVIDENCE

The first evidence for ultrahigh-pressure metamorphism (UHPM) in the Greenland Caledonides has been found in kyanite eclogites and associated host gneisses on a remote island in Jøkelbugt (78°00’ N, 18°04’ W). The UHP locality lies at the eastern edge of the North-East Greenland eclogite province, which constitutes a coherent 400 km long slab of MT/HP, Laurentian continental crust. The kyanite eclogites form a 2 km long string of boudins; individual blocks are 50x100 m and lie within clinopyroxene-bearing, quartzofeldspathic gneisses. Polycrystalline quartz inclusions in garnet, omphacite and kyanite are surrounded by radial fractures. Some inclusions contain a palisade quartz outer rim that encircles a region of very fine-grained subgrains to coarser, polygonized recrystallized grains of quartz. Palisade quartz is diagnostic of quartz pseudomorphs after coesite, thus providing textural evidence for UHP conditions. Geothermobarometry on the mineral assemblage garnet + omphacite + kyanite + quartz/coesite ± phengite confirms the microstructural interpretation of UHPM. P-T estimates are based on a combination of the garnet-clinopyroxene Fe-Mg exchange thermometer and internally consistent calibrations derived from the net-transfer reactions: (1) grossular + pyrope + 2 coesite/quartz=3 diopside + 2 kyanite, and (2) 3 celadonite + 2 grossular + pyrope=6 diopside + 3 muscovite. We used the most sodic omphacite, garnet with maximum (XCa2)(XMg), and white mica with the highest Si content to calculate peak P conditions for three eclogite samples. The samples define maximum P conditions of 3.6-4.2 GPa at temperatures ranging from 928 -972 °C. These PT estimates fall well within the coesite stability field. Temperatures are probably overestimated because of the assumption that all Fe in omphacite is ferrous; however, any Fe3+ in omphacite would only increase the pressure estimates because of the negative slope of reaction (1). The host gneisses are more retrogressed than the kyanite eclogites and only record HP conditions of 2.5 GPa at 826°C; but garnet contains polycrystalline quartz inclusions with radial fractures that are evidence for UHPM. If our research develops as in other UHP terranes (e.g. Western Gneiss Region, Dora Maira, Erzbebirge), then we have begun to define a UHP province in Greenland.