EVIDENCE FOR ULTRAHIGH-TEMPERATURE (UHT) METAMORPHISM IN THE ADIRONDACK HIGHLANDS

Isochemical phase equilibria modeling reveals ultrahigh-temperature (UHT) metamorphism of felsic granulites at Ledge Mountain in the Adirondack Highlands. We ran hundreds of models for 38 samples of sillimanite-rich felsic migmatites from Ledge Mountain. We achieved consistent P-T results; and mineral chemistries, assemblages, and modal percentages in models correspond well to estimates from hand specimens and petrography. “Pseudosection” models suggest that the peak mineral assemblage is similar to the distinctive and relatively rare assemblage: Ky + hypersolvus Fsp (now mesoperthite) + Grt + Rt formed at ~7-17 kb / 950-1050^°C which is higher than previously reported for granulites in the region based on biotite-garnet and GASP barometry (~750-850°C at 6-9 kbar). The leucocratic melt phase comprises 20-30 vol.% of the rock at peak temperature conditions. Widespread textural evidence for the presence of anatectic melt includes abundant lobate Qz inclusions in peritectic Grt, Bt–Qz symplectites, and pseudomorphs after melt films. Evidence for retrograde reactions and the presence of a late fluid include sericitized feldspars, retrograde Bt and Ms, corroded Sil, and Grt rims replaced by Bt-Pl-Al₂SiO₅ intergrowths. Isopleth models for Alm, Grs, Sps, and Pl chemistries demonstrate phase equilibria models record peak P-T conditions for metamorphism. Retrograde Bt is not in equilibrium with these models, and the prograde history of these rocks may have been completely obliterated by UHT metamorphism. Sillimanite-rich felsic migmatites exposed at Ledge Mountain represent the only location in the Adirondack Highlands where Ky has been found. Where Sil and Ky are in contact, Ky is texturally late and overgrows Sil in undeformed pegmatites. The inferred peak mineral assemblage is found in the Bohemian massif is widely considered to reflect P–T conditions of at least ∼14 kbar / 920°C; this is consistent with our models and suggests there may be other petrotectonic similarities between the Grenville and Variscan orogens. The definitive Antarctic UHT granulite-facies terranes follow isothermal decompression paths that fit our exhumation path for Ledge Mountain suggesting they have some petrotectonic processes in common.