Mafic gneiss

Dark-gray to black, fine-grained, magnetite-garnet-hornblende-biotite-diopside-plagioclase gneiss, commonly containing beds of dark-gray vitreous magnetite-garnet quartzite, 2 to 5 cm thick, tremolite-pyroxene gneiss, and biotite-rich, rusty-weathering garnet-quartz schist and gray sulfidic sillimanite quartzite. Occurs as screens within tonalitic gneiss and the Pharaoh Mountain Gneiss and is interpreted as paragneisses older than the tonalitic gneiss (Y2bt). Paragneiss. Rocks of the Adirondacks in Vermont and in the Whitehall, N.Y., area.
State Vermont
Name Mafic gneiss
Geologic age Middle to Early Mesoproterozoic
Lithologic constituents
Major
Metamorphic > Metasedimentary > ParagneissDark-gray to black, fine-grained, magnetite-garnet-hornblende-biotite-diopside-plagioclase gneiss; tremolite-pyroxene gneiss.
Minor
Metamorphic > Schist > Mica-schist > Biotite-schistBiotite-rich, rusty-weathering garnet-quartz schist.
Metamorphic > Metasedimentary > Metaclastic > QuartziteDark-gray vitreous magnetite-garnet quartzite; gray sulfidic sillimanite quartzite.
Comments Original label is Y2bmg, but age is Middle to Early Mesoproterozoic. Changed symbol to Y1Y2bmg.
References
NGMDB product
Counties Rutland