Late-stage extensional exhumation of high-pressure granulites in the Greenland Caledonides

New ages from high-pressure (P) granulites in Payer Land, East Greenland, suggest that exhumation of the deepest crustal levels occurred during extensional collapse late in the collisional history of the Caledonian orogen. The ages set limits on the time of movement on the Payer Land detachment fault, which places a little-deformed low-grade metasedimentary sequence on the granulite gneisses. U-Pb SHRIMP (sensitive high-resolution ion microprobe) analyses of metamorphic rims on zircon from high-P (1.4–1.5 GPa) metapsammite yield a weighted mean ²⁰⁶Pb/²³⁸U age of 403 ± 5 Ma. Zircon analyses from a high-P kyanite-bearing melt pod derived from metapelites yield a weighted mean ²⁰⁶Pb/²³⁸U age of 404 ± 4 Ma. The high-P metamorphism is distinctly younger than the extension associated with generation of 430–424 Ma leucogranites and migmatites at mid-crustal levels. Exhumation of deep-level rocks in Payer Land some time after 405 Ma is roughly coeval with exhumation of eclogite terranes in the Scandinavian Caledonides. Orogenic collapse in the Caledonides appears to have taken place in two stages: synorogenic extension dated by leucogranites, followed 24–30 m.y. later by late orogenic exhumation of the deepest levels. The history of this ancient, well-exposed collisional orogen is an analogue for evolution of the Himalayan system and perhaps offers a predictive model for future exhumation of high-P rocks currently forming at the base of the overthickened Himalayan crust.