|Geologic age||Ordovician or Proterozoic Z|
Metamorphic > Schistsillimanite schist and gneiss, partly sulfidic.
Metamorphic > Gneisssillimanite schist and gneiss, partly sulfidic; biotite gneiss, calc-silicate gneiss
Metamorphic > Metasedimentary > Calc-silicate-rock
Metamorphic > Amphibolite
Metamorphic > Metasedimentary > Metacarbonate > Marble
|Comments||Part of Nashoba Zone (Silurian and Older Rocks). Secondary unit description per MA017. Age debated. MA017 = Although age on MA State bedrock map is shown as Proterozoic Z or Ordovician (due to uncertainty regarding actual rocks sampled by Olszewski and a strong belief that rocks of Nashoba zone correlated with Ordovician rocks to the west), author now feels that rocks of Nashoba zone (except for Tadmuck Brook Schist) are all Proterozoic, but that they are unlike the Proterozoic rocks of neighboring Milford-Dedham zone (Goldsmith, 1991). MA018 = Fish Brook Gneiss of the Nashoba terrane has been dated by G.R. Dunning at 520+14-11 Ma. This not only establishes a Cambrian age for this unit, but constrains the age of the overlying Marlboro and Nashoba Formations to the interval between 520 Ma and 430 Ma, the age of the cross-cutting Sharpners Pond Diorite. No Precambrian rocks, therefore, are known from the Nashoba terrane (Hepburn and others, 1993). MA019 = Fish Brook Gneiss has yielded a Cambrian to Ordovician U/Pb abraded zircon crystallization age of 499+/-6/-3 Ma. This date constrains the age of the metavolcanic and metasedimentary rocks of the Nashoba terrane to the interval between Late Cambrian and Early Silurian (Hepburn and others, 1994).|
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.
Goldsmith, Richard, 1991, Stratigraphy of the Nashoba zone, eastern Massachusetts; An enigmatic terrane, IN Hatch, N.L., Jr., ed., The bedrock geology of Massachusetts: U.S. Geological Survey Professional Paper, 1366-F, 22 p.
Hepburn, J.C., Hon, Rudolph, Dunning, G.R., Bailey, R.H. and Galli, Kenneth, 1993, The Avalon and Nashoba terranes (eastern margin of the Appalachian Orogen in southeastern New England), IN Cheney, J.T., and Hepburn, J.C., eds., Field trip guidebook for the northeastern United States; 1993 Boston GSA; Volume 2: University of Massachusetts, Geology Department Contribution, no. 67, Joint annual meeting of Geological Society of America, Annual Meeting, and New England Intercollegiate Geological Conference, 85th Annual Meeting, Boston, MA, October 25-28, 1993, p. X1-X31.
|Counties||Essex - Middlesex - Worcester|