Mineral Resources On-Line Spatial Data
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|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|
Metamorphic > GneissLight-gray, biotite-plagioclase quartz gneiss; distinctive "swirl-form" foliation.
Unpublished Digital Geologic Map of Massachusetts received from Rudi Hon at Boston College in 1998.
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.
|Geographic coverage||Essex - Middlesex|
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