Ammonoosuc Volcanics

Amphibolite, felsic gneiss, garnet-amphibole quartzite, and marble too thin to show separately at map scale. Gedrite, anthophyllite, cummingtonite locally abundant in amphibolite layers.
State Massachusetts
Name Ammonoosuc Volcanics
Geologic age Middle Ordovician
Lithologic constituents
Major
Metamorphic > Gneissfelsic gneiss
Metamorphic > Metasedimentary > Metaclastic > Quartzitegarnet-amphibole quartzite
Metamorphic > AmphiboliteGedrite, anthophyllite, cummingtonite locally abundant in amphibolite layers.
Minor
Metamorphic > Metasedimentary > Metacarbonate > Marble
Comments Part of Bronson Hill Zone (Ordovician and Older Rocks). 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 (Tucker and Robinson, 1990) per MA015.
References

Zen, E-An (ed.), Goldsmith, R. (comp.), Ratcliffe, N.M. (comp.), Robinson, P. (comp.), Stanley, R.S. (comp.), Hatch, N.L., Jr., Shride, A.F., Weed, E.G.A., and Wones, D.R., 1983, Bedrock Geologic Map of Massachusetts: U.S. Geological Survey, Reston, VA, scale 1: 250,000.

https://pubs.er.usgs.gov/publication/70046713

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.

NGMDB product
Counties Franklin - Hampden - Hampshire - Worcester