High-pressure metamorphism in mafic schist from northern Vermont

The only confirmed occurrence of glaucophane and omphacite in the Appalachian Mountains of New England is in mafic schist from Tillotson Peak (44°48', 72°33'), north-central Vermont. Mineral assemblages observed, all with epidote + sphene ± magnetite ± phengitic muscovite ± carbonate (calcite or dolomite) ± apatite ± sulfide, are: (1) actinolite + glaucophane + chlorite + garnet + quartz, (2) + glaucophane + chlorite + albite + garnet, (3) actinolite + glaucophane + omphacite + garnet, (4) glaucophane + chlorite + garnet+ quartz, (5) glaucophane + omphacite + quartz, (6) albite + glaucophane + chlorite + quartz, and (7) albite + paragonite + chlorite + quartz. Electron microprobe investigations show that plagioclase, chlorite, sphene, white mica, carbonate, and magnetite grains are relatively homogeneous. However, amphibole, pyroxene, epidote, and garnet grains are zoned, indicating incomplete equilibration and changing physical conditions during metamorphism. Glaucophane in assemblage (6) has the general formula (rim composition): □ (Na_(1.9^(M4)Ca_(0.1)) (Mg_(1.8Fe_(1.2)^(2+)Al_(1.0)^(VI)Fe_(0.3)^(3+)) (Si_(7.9)Al_(0.1)^(IV)) O_(22) (OH)_(2•) Glaucophane in assemblages (1) to (5) is somewhat poorer in the glaucophane endmember and richer in actinolite. Ca-rich amphibole grains are zoned extensively with barroisite cores and actinolite rims. Omphacite grains in assemblages (3) and (5) have the general formula (rim composition) (Na_(0.5)Ca_(0.5)) (Mg_(0.4)Fe_(0.1)^(2+)Al_(0.4)^(VI)Fe_(0.1)^(3+)) Si_2O_6. spite of the significant zoning, the partitioning of elements among the minerals at their grain margins suggests that all seven assemblages above may coexist at the same physical conditions of metamorphism in mafic rocks with a relatively small range in bulk composition. Correlation of mineral compositions and assemblages with mineral stability data suggests high-pressure facies series metamorphism at about 9 ± 2 kb and 450 ± 100°C with P_(h2O) ≈ P_(Total). It is proposed that the metamorphism occurred during the Ordovician.