REEXAMINING THE GEOTHERMOMETRY OF ACADIAN METAMORPHIC ROCKS IN SOUTHERN MASSACHUSETTS: AN EXTENSION OF THE ULTRAHIGH TEMPERATURE ZONE FROM NORTHEASTERN CONNECTICUT

The recent discovery of ultrahigh temperature metamorphism of Acadian rocks in the Central Maine terrane of northeastern Connecticut requires a reexamination of the geothermometry of the units extending into southern Massachusetts. Previous geothermometry relied significantly on Mg-Fe exchange reactions that often re-equilibrate during retrogression and make it difficult to preserve peak conditions. This study used ternary feldspar reintegration as it has shown to preserve higher-grade conditions. The thermometer was applied using the Theriak/Domino software package (de Capitani & Petrakakis, 2010) and the activity model of Benisek et al. (2010) to calculate the minimum peak temperature of reintegrated ternary feldspars. The chemical compositions of the ternary feldspars are calculated using a weighted average of the chemistry of plagioclase and k-feldspar in antiperthite crystals within metapelites. The antiperthite from rocks in a ~9 km transect across the Southbridge and Wales quadrangles show metamorphic temperatures increasing from 750-1000 °C moving west to east. Pressure estimates range from 1.0-1.8 GPa. Additional geothermometry was completed to test retrogression of Mg-Fe exchange reactions using WinTWQ v. 2.34 (Berman, 1991). Two-pyroxene thermometry in presumably metavolcanics units record an average of 649 °C at 1.0 GPa and 642 °C at 1.8 GPa. Garnet-biotite thermometry in metapelites show average temperatures of 721±22 (2σ) to 764±14 °C at 1.0 GPa, and from 747±22 to 792±15 °C at 1.8 GPa. In addition, crystallographically oriented, multiphase acicular inclusions in garnet were found in one sample in the Bigelow Brook Formation in Massachusetts. There are only two reported localities of oriented multiphase inclusions like these, both of which are extreme metamorphic environments. They were previously found in the same units farther south in Connecticut (Axler and Ague, 2015) and in the ultrahigh pressure rocks of the Erzgebirge, Germany (Perchuk, 2008). This study extends a previously found ultrahigh temperature metamorphic zone found in northeastern Connecticut into southern Massachusetts and expands the only regional ultrahigh temperature metamorphic zone in the United States.