SILICA-UNDERSATURATED GARNET + SPINEL + CORUNDUM MINERAL ASSEMBLAGES RECORD UHT METAMORPHISM, CENTRAL MAINE TERRANE, CONNECTICUT, USA

We report new evidence for ultrahigh-temperature (UHT) metamorphism in the Acadian-Neoacadian orogen, northeastern United States. Fault-bounded slices of the Brimfield Schist and Upper member of the Bigelow Brook Formation in the Central Maine Terrane, NE Connecticut, USA are known to preserve evidence for UHT metamorphism including exsolved, oriented rutile ± ilmenite needles in garnet, as well as antiperthite and mesoperthite ternary feldspars (Ague and Eckert, 2012; Ague et al., 2013; Axler and Ague, 2015). The silica-undersaturated rocks we study are proximal to the previously-described UHT rocks and also record evidence of an extreme thermal history. The undersaturated rocks contain two feldspars + biotite + garnet + spinel + corundum + ilmenite, together with multi-cm leucosomes of K-feldspar ± plagioclase ± biotite. The coarsest-grained mineral assemblages, including garnet crystals and masses of spinel as much as ~6 cm across, are found in a ductile shear zone. Temperature estimates from ternary feldspar reintegration thermometry fall between 975 and 1050°C. Pseudosection modeling yields a pressure estimate of ~15 kbar for this temperature range – a significantly higher pressure than previously recognized from the Central Maine Terrane. Textural evidence including sillimanite inclusions limited to garnet cores, late matrix sillimanite, and the replacement of large spinel grains by corundum supports a counter-clockwise P-T path. The assemblage garnet + spinel + corundum is rare in nature, and has been reported from a few UHT localities as well as the corgaspinites of Mazzone and Haggerty (1989), which may represent a similar rock from which melt has been extracted fully. Relatively refractory silica-undersaturated assemblages such as the one we present apparently preserve UHT conditions readily. Consequently, they may provide more complete records of P-T-time history in the form of inclusions in high-grade minerals, melt textures, and inferred peak assemblages than more siliceous rocks prone to higher degrees of partial melting at UHT conditions (e.g. Kelsey et al., 2005).