We report the discovery of granulite facies gneisses that attained ultrahigh temperatures (UHT) above those predicted by typical models of conductive thermal relaxation of over-thickened crust during exhumation. The rocks, which form part of the Acadian (Devonian) metamorphic belt in Connecticut (United States), reached ∼1000 °C and minimum pressures of ∼1 GPa based on Zr-in-rutile thermometry, ternary feldspar compositions, and pseudosection analysis. This is the first regional UHT locality in the United States of which we are aware, and one of relatively few post-Gondwana assembly UHT localities known worldwide. The UHT metamorphism requires heretofore unrecognized contributions to the regional thermal budget. Some possibilities include rapid exhumation from the mantle, underthrusting of extremely radiogenic crust, mechanical strain heating, asthenospheric upwelling, and/or heat input from mantle-derived magmas. The rocks are fairly ordinary looking in outcrop, raising the possibility that other UHT domains remain undiscovered in the orogen.

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