|Geologic age||Ordovician, Cambrian, or Proterozoic Z|
Metamorphic > Amphibolite
Metamorphic > Gneisslayered to massive biotite-plagioclase gneiss, microcline augen gneiss
|Comments||Part of Bronson Hill Zone (Ordovician and Older Rocks). Swanzey, Pauchaug, Monson, and Fourmile Gneisses represent metamorphosed intrusive igneous rocks and yield ages from 454 to 442+3/-2 Ma. Therefore, ages range from early Late through latest Ordovician. Authors suggest that they represent the plutonic root of a calc-alkaline magmatic arc at least 250 km long that was produced partly or entirely on a continental margin in the Late Ordovician. The overlying Ammonoosuc Volcanics and Partridge Formation, rather that resting unconformably on these plagioclase-rich plutonic gneiss, are now known to have had a magmatic history overlapping with them and are approximately the same age (Tucker and Robinson, 1990) per MA015.|
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.
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.
|Counties||Franklin - Hampden - Hampshire - Worcester|