LASER RAMAN MICROPROBE IDENTIFICATION OF COESITE IN KYANITE ECLOGITES AND THEIR HOST GNEISSES, NORTH-EAST GREENLAND CALEDONIDES

Coesite inclusions have been unambiguously identified by laser Raman spectroscopy in zircon from kyanite-eclogites and their quartzofeldspathic host gneisses that are exposed on a small island in Jøkelbugt (78°00’N, 18°04’W) off the northeast coast of Greenland. The kyanite eclogites occur as decimeter scale layers within banded, mafic eclogite boudins. Individual blocks are on the order of 50 x 100 meters and are surrounded by clinopyroxene-bearing quartzofeldspathic gneisses with pegmatitic melts in the boudin necks. Ultrahigh-pressure (UHP) metamorphism was previously recognized on the basis of palisade, polycrystalline quartz inclusions in garnet and omphacite with radial fractures, as well as by geothermobarometry, which calculates pressure-temperature conditions well within the coesite stability field (approx. 970°C and 3.6 GPa). The UHP mineral assemblage in the kyanite eclogites is garnet + omphacite + kyanite + phengite + coesite + rutile. The host gneisses consist of a disequilibrium assemblage of quartz + plagioclase + garnet + clinopyroxene + amphibole + biotite + epidote/clinozoisite + titanite ± kyanite with eclogite facies minerals preserved as inclusions in garnet. Three kyanite eclogite samples and one host gneiss yield clear, sub-spherical, 50-200 micron diameter, metamorphic zircons. Five out of 960 zircons analyzed at the Laser Raman Microprobe Lab, Washington University, St. Louis, MO contain coesite phases that are 15 to 50 micrometers in diameter. The zircons also contain inclusions of other eclogite facies phases such as garnet, omphacite, kyanite and rutile. U-Pb dating of zircon domains that contain coesite is in progress.

Discovery of coesite confirms the existence of UHP metamorphism in the easternmost part of the Greenland eclogite province. Coesite in the host gneisses demonstrates unequivocally that a coherent slab of the Laurentian continental margin experienced UHP conditions during the Caledonian collision with Baltica. UHP metamorphism on both the Laurentian margin (NE Greenland) and Baltica margin (Western gneiss region, Norway) requires that closure of the Iapetus Ocean produced a more symmetric collisional orogen caused by crustal thickening on both sides rather than by simple asymmetric subduction of Baltica beneath Laurentia, as is the current view.