Garnet composition and zoning in the determination of temperature and pressure of metamorphism, central Massachusetts

The distribution of Fe, Mg, Ca, and Mn within individual zoned garnets of medium- to high-grade metamorphosed pelitic rocks from west-central Massachusetts has been studied in detail with an automated electron microprobe, and is consistent with variations predictdd for the Fe-Mg-Mn continuous reactions taking place at each metamorphic grade. This regular behavior allows use of compositions of garnet and coexisting phases for geothermometry and geobarometry tf P-T-X relations of appropriate continuous reactions can be calibrated. Preliminary calibrations of garnet-biotite and garnet-cordierite Fe-Mg exchange reactions and several Fe-Mg-Mn continuous reactions have been used to evaluate P-I conditions of metamorphism in central Massachusetts. Results indicate pressure in the range of 5 to 7 kbar and a west to east temperature gradient from about 580-605C for kyanite-staurolite grade to 650-700.C for sillimanite-orthoclase-cordierite grade. Pressure and temperature estimates are consistent with regional tectonic reconstructions indicating synmetamorphic uplift in the Bronson Hill anticlinorium and downbuckling in the Merrimack synclinorium.