Partridge Formation (VTOp;2)

State	Vermont
Name	Partridge Formation
Geologic age	Ordovician
Original map label	Op
Comments	Part of Eastern Vermont U-Pb zircon ages suggest that the Ammonoosuc Volcanics and tuffs of the Partridge Formation were erupted between 453 to 449 Ma, meaning they are of late Middle Ordovician and earliest Late Ordovician age. The stratigraphic relationships of these two units is very complex with current thought being that the Partridge lies between the lower and upper members of the Ammonoosuc Volcanics per VT010.
Primary rock type	slate
Secondary rock type	phyllite
Other rock types	tuff
Lithologic constituents	Major Metamorphic > Metasedimentary > Metaclastic > PhylliteDull gray to sooty black carbonaceous slate and phyllite; mostly fissile, but locally massive and tough. Metamorphic > Metasedimentary > Metaclastic > SlateDull gray to sooty black carbonaceous slate and phyllite; mostly fissile, but locally massive and tough. Incidental Igneous > Volcanic > Felsic-volcanic > Rhyolite (Pyroclastic, tuff)Contains a few beds of both schistose and massive soda-rhyolite tuff (not shown on map).
Map references	Nicholson, S.W., Dicken, C.L., Horton, J.D., Foose, M.P., Mueller, J.A.L., and Hon, Rudi, 2006, Preliminary integrated geologic map databases for the United States: Connecticut, Maine, Massachusetts, New Hampshire, New Jersey, Rhode Island, and Vermont: U.S. Geological Survey Open-File Report 2006-1272. [http://pubs.er.usgs.gov/publication/ofr20061272]
Unit references	Doll, C.G., Cady, W.M., Thompson, J.B., Jr., and Billings, M.P., 1961, Centennial Geologic Map of Vermont: Vermont Geological Survey, Miscellaneous Map MISCMAP-01, scale 1:250,000. Tucker, R.D. and Robinson, Peter, 1990, Age and setting of the Bronson Hill magmatic arc; a re-evaluation based on U-Pb zircon ages in southern New England: Geological Society of America Bulletin, v. 102, no. 10, p. 1404-1419.