Migmatite gneiss of the Jaettedal complex, Liverpool Land, East Greenland: protracted high-T metamorphism in the overriding plate of the Caledonian orogen

The Greenland Caledonides (GC) 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 metamorphic patterns, regional structures and the kinematic evolution of the overriding plate of a continental collision. We present new metamorphic petrology and coupled zircon geochronology and geochemistry data from the Jaettedal complex in southern Liverpool Land to document the thermal evolution of the orogenic core of the southern GC. Pelitic migmatite gneisses from the Jaettedal complex document metamorphic conditions of 850-730 degrees C at pressures of 11-9.5kbar. Zircon from these samples yields Archean-Mesoproterozoic detrital cores with positive heavy rare earth element (HREE) slopes, and 440-425Ma rims with flat HREE slopes are interpreted to date the timing of prograde pelite anatexis. Intercalated mafic assemblages record metamorphic conditions of 860-820 degrees C at 12-10kbar. Zircon from mafic gneisses contains cores with ages of c. 458Ma with positive HREE slopes and 413-411Ma rims with flat HREE slopes that are interpreted to record the timing of original mafic dyke intrusion and subsequent partial melting respectively. When placed in the context of correlative rocks from the southern GC, these results suggest the development of a thermally weakened lower to middle crust in the Caledonian overriding plate that spanned >200km perpendicular to orogenic strike during the Silurian. The existing data further suggest Silurian syn-orogenic channel flow and exhumation occurred at the thrust front, while protracted high-T metamorphism continued in the orogenic core. These patterns highlight variations in the thermal and rheologic structure of the Caledonian overriding plate along orogenic strike, and have implications for the development and exhumation of high- and ultrahigh-pressure terranes.