Ammonoosuc Volcanics (VTOa;2)

State	Vermont
Name	Ammonoosuc Volcanics
Geologic age	Ordovician
Original map label	Oa
Comments	Part of Eastern Vermont. U-Pb zircon ages suggest that the Ammonoosuc Volcanics and tuffs of the Partridge Formation were erupted 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	tuff
Secondary rock type	volcanic breccia (agglomerate)
Other rock types	lava flow
Lithologic constituents	Major Igneous > Volcanic > Felsic-volcanic > Rhyolite (Flow)Soda-rhyolite tuff, breccia, and flows. Igneous > Volcanic > Felsic-volcanic > Rhyolite (Volcaniclastic, volcanic breccia)Soda-rhyolite tuff, breccia, and flows. Igneous > Volcanic > Felsic-volcanic > Rhyolite (Pyroclastic, tuff)Soda-rhyolite tuff, breccia, and flows.
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.