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Metapelitic gneisses exposed along the Moose River in the west-central Adirondack Highlands contain uncommon Mg- and B-rich mineral assemblages. Phases identified in these rocks include cordierite, garnet, sillimanite, spinel, orthopyroxene, tourmaline, and locally prismatine, the B-rich end-member of the kornerupine group. Four distinct mineral assemblages are identified, two of which are interpreted to form by partial melting. Prismatine is interpreted to have formed by the melting of tourmaline + biotite + cordierite + sillimanite. Thermobarometry calculations from net transfer and exchange equilibria record temperatures and pressures of 850 ± 20 °C and 6.6 ± 0.6 kilobars for orthopyroxene + garnet assemblages and 675 ± 50 °C and 5.0 ± 0.6 kilobars for cordierite + garnet + sillimanite + quartz assemblages. Mineral textures and compositions are consistent with formation along a nearly isobaric cooling path similar to that proposed by Spear and Markussen (1997).

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