Characteristics of, and a Tectonic Model for, Ultrahigh-Pressure Metamorphism in the Overriding Plate of the Caledonian Orogen

Ultrahigh-pressure (UHP) metamorphism in the Laurentian margin of North-East Greenland formed in the overriding plate of the Caledonian collision with Baltica. The Greenland UHP terrane exhibits a number of characteristics that are incompatible with accepted models for UHP metamorphism in the downgoing slab during continental subduction. UHP metamorphism occurred very late in the collision, and was a long-lived event. Peak temperatures in excess of 950°C are well above the norm for subduction-zone UHP terranes. Exhumation rates, based on new U-Pb SHRIMP results for two boudin neck pegmatites and one hornblende-bearing leucosome, are slower (~3-5 mm/year) than for many other UHP terranes. These atypical features can be explained by intracratonic subduction of Laurentia in the waning phase of the Caledonian collision, analogous to deformation that may be occurring in the Tibetan Plateau today.