PROTRACTED HIGH-PRESSURE-GRANULITE FACIES METAMORPHISM PRIOR TO HIGH-PRESSURE EXHUMATION IN AN OROGENIC HANGINGWALL, LIVERPOOL LAND, EAST GREENLAND

The East Greenland Caledonides formed in the overriding Laurentian plate during the closure of the Iapetus Ocean and the subduction of Baltica, and offer a unique opportunity to study (U)HP exhumation processes in orogenic hangingwalls. Recent work in Liverpool Land, located in the hinterland of the Greenland Caledonides, differentiates the granulite-facies Jaettedal complex from the eclogite-facies Tvaerdal complex, and suggests that at least some of the East Greenland HP terranes may represent fragments of subducted Baltican crust that were incorporated into the Laurentian hangingwall during the exhumation process. Here, we present new thermobarometry, and LA-ICP zircon geochronology and trace element data from the Jaettedal complex that place constraints on the conditions of the Laurentian hangingwall prior to continental collision and the exhumation of the Liverpool Land HP terranes.

The Jaettedal complex consists of a series of intercalated pelites and mafic bodies that display a variety of high-temperature assemblages and migmatitic textures. Pelitic assemblages record pressures of 10-13 kbar at temperatures >750°C, and contain zircons with Archean-Mesoproterozoic detrital cores with positive HREE slopes and 440-425 Ma zircon rims with flat HREE slopes. Mafic assemblages record pressures of 12-15 kbar at 850-930°C, and contain zircon cores with ages of 460-455 Ma with positive HREE slopes and 418-410 Ma zircon rims with flat HREE slopes. While Archean detrital zircon populations in pelites and Ordovician magmatic ages in mafic protoliths link the Jaettedal complex to the Laurentian craton and Caledonian thrust sheets farther west, zircons with flat HREE profiles in indicate growth of zircon in the presence of garnet, and are interpreted to record the timing of prograde metamorphism in the Jaettedal complex en route to high-pressure granulite facies conditions by 410 Ma. This protracted record of high-temperature metamorphism, possibly related to magmatic underplating associated with the closure of the Iapetus, implies a rheologically weak Caledonian hangingwall prior to the collision with Baltica, and has important implications for the style of deformation within orogenic upper plates including the development of orogenic plateaux and the location of UHP terrane exhumation.