DEVONIAN COLLISION ALONG THE NORTHEASTERN MARGIN OF LAURENTIA, GREENLAND CALEDONIDES

Caledonian collision between Baltica and Laurentia is generally taken to be Silurian or “Scandian” in age based on stratigraphic relationships at the thrust fronts of Scandinavia and Greenland. Our growing data set of U-Pb and Sm-Nd ages for high-pressure metamorphism of Laurentian basement indicates that collision persisted well into the Devonian. Flooding of the Laurentian passive margin by sandy turbidites of the Silurian Lauge Koch Land Formation marks the onset of collision in the Greenland Caledonides. The Lauge Koch Land Formation is also the youngest unit cut by thrusts. No exotic terranes are present, and the structurally highest thrust sheet in North-East Greenland consists of Early Proterozoic Laurentian basement that was metamorphosed to eclogite facies during Caledonian collision. Eight of nine Sm-Nd mineral isochron ages from eclogites between 76.5°-79°N fall in the range of 410-360 Ma. Zircons with metamorphic rims were recovered from six of the same samples; U-Pb SHRIMP ages of the rims corroborate the Sm-Nd data. The youngest ages are found in the eastern part of the eclogite province, which is also the locus of the highest pressure metamorphism, i.e. P=3.6 GPa. Zircons from a kyanite-bearing ultrahigh-pressure eclogite have inclusions of garnet, omphacite, alumino-silicate, and quartz (or coesite), and give a weighted mean 206Pb/238U age of 360 ± 5 Ma. Additional evidence of Devonian metamorphism comes from high temperature - high pressure granulite facies metapsammites and anatectic metapelites of the Payer Land gneiss complex (74°28’-74°47’N, 22°30’-23°40’W). U-Pb SHRIMP analyses of zircon rims from two samples yield weighted mean 206Pb/238U ages of 403 ± 5 Ma and 404 ± 4 Ma. Ages for HP metamorphism spanning 500 km of Laurentian margin are consistently younger than the oft-cited Scandian climax, suggesting that the Baltica-Laurentia collision persisted through the Devonian. Significant continental subduction or crustal thickening continued during the late stages of collision.