Fish Brook Gneiss (MAOZf;0)

State	Massachusetts
Name	Fish Brook Gneiss
Geologic age	Ordovician or Proterozoic Z
Original map label	OZf
Comments	Part of Nashoba Zone (Silurian and Older Rocks) 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) per MA018. 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) per MA019.
Primary rock type	granitic gneiss
Secondary rock type
Other rock types
Lithologic constituents	Major Metamorphic > GneissLight-gray, biotite-plagioclase quartz gneiss; distinctive "swirl-form" foliation.
Map references	Unpublished Digital Geologic Map of Massachusetts received from Rudi Hon at Boston College in 1998.
Unit 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., Wones, D.R., 1983, Bedrock Geologic Map of Massachusetts: U.S. Geological Survey, Reston, VA, scale 1: 250,000. 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. Hepburn, J.C., Dunning, G.R. and Hon, Rudolph, 1994, Geochronology and regional tectonic implications of Silurian deformation in the Nashoba terrane, southeast New England [abs.]: Atlantic Geology, Journal of the Atlantic Geoscience Society, v. 30, no. 2, p. 167-168.
Geographic coverage	Essex - Middlesex