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 similar to 1000 degrees C and minimum pressures of similar to 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.

Most field evidence indicates that igneous rocks have formed from upward-moving bodies of either magma, mixtures of magma and crystals, magma and gas bubbles, or even solid rock. Basaltic lava flows cover most of the ocean floor and large areas on continents. Rhyolitic lava flows and volcanic ash also cover large parts of continents. Clearly, magma of a wide range of composition is able to rise to the Earth's surface.

Growing evidence is emerging to support the idea that metamorphism, even in a regional context, may be punctuated – or dominated – by relatively short pulses of heating, fluid flow, and/or mineral growth. Here, we describe data from two Barrovian metamorphic terranes which test this idea. In the Barrovian zones of Scotland, garnet Sm/Nd geochronology from the garnet and sillimanite zones yield the same peak metamorphic ages [1] (~465 Ma). The age is similar to the age of crystallization of large igneous bodies in the area. The contemporaneity of peak ages is explained by an efficient, advective component of heating, perhaps mediated by synchronous fluid flow [2]. The duration of this regionwide pulse of peak metamorphism is constrained by new Srin-apatite diffusion modeling. Apatite grains have detrital cores and metamorphic overgrowths and are included within porphyroblasts (e.g. garnet, staurolite). Modeling of intragrain diffusion of Sr constrains the duration of peak metamorphism to <250 kyr for garnet through staurolite zone samples. Garnet multi-component diffusion modeling from the sillimanite zone corroborates this brief pulse duration. The Wepawaug Schist of Connecticut USA, also yields contemporaneous peak-T garnet Sm/Nd ages from different grades across the terrane (~380 Ma). This age matches a population of texturally young zircons associated with igneous intrusions in these rocks. Garnet cores from the kyanite zone, which have growth textures indicative of extremely rapid growth [3], have been dated by Sm/Nd at 388.6 Ma. This age is matched by another population of zircons also associated with igneous intrusions. This earlier prograde growth event may be related to another pulse of metamorphic growth, brought on by magmatic fluid and heat. Brief pulses of metamorphic heating and mineral reactions, perhaps catalyzed by the introduction of fluids, may be superimposed on regional scale conductive heating at tectonic rates. Such short pulses could help explain the discrepancy between rapid lab-based reaction kinetics and much slower time-integrated field-based reaction kinetics [4].