Radiometric age determinations and correlative relations indicate that the Fish Brook Gneiss is Proterozoic Z or Ordovician. Age of informal schist in Hunts Brook in southeastern CT is considered to be same as for Tatnic Hill Formation in CT and MA. Tatnic Hill is almost certainly equivalent to upper part of Nashoba Formation of MA. Nashoba Formation is intruded by Andover Granite, dated as Late Ordovician to Early Silurian by Rb/Sr methods (R.E. Zartman, unpub. data, 1978). This indicates that Nashoba and Tatnic Hill are Ordovician or older. Foliated pegmatite in Tatnic Hill Formation has discordant Pb-Pb zircon age of 520 m.y. (Zartman and others, 1965), indicating a Cambrian or older age, but origin and history of zircons is uncertain. U-Pb concordia plots of a suite of zircons from below Nashoba Formation in eastern MA (Fish Brook Gneiss and Shawsheen Gneiss) give an age of about 742 m.y. (Olszewski, 1978), suggesting that Nashoba and Tatnic Hill could be Proterozoic Z; however, zircons from Middlesex Fells Volcanic Complex were probably included in the above-mentioned suite, according to Olszewski, so that the age is suspect. Age of Nashoba, Tatnic Hill, and schists in Hunts Brook is considered to be Proterozoic Z or Ordovician. [Because Fish Brook Gneiss correlates with lower part of schist in Hunts Brook, its age is changed as well.]

Source: GNU records (USGS DDS-6; Reston GNULEX).

Used as Fish Brook Gneiss of Proterozoic Z or Ordovician age.

Source: GNU records (USGS DDS-6; Reston GNULEX).

Fish Brook Gneiss occurs in Nashoba zone of eastern MA. Described by Castle (1965) as a fine- to medium-grained, pearly white to very light gray, distinctly foliated but generally unlayered biotite-quartz-plagioclase rock. Bell and Alvord (1976) describe an exposure near Billerica, MA, where very thin beds of fine-grained amphibolite (Boxford Member of Nashoba Formation) are interlayered with very thin beds of light-gray Fish Brook Gneiss. Although Castle (1965) considered Fish Brook to be either a premetamorphic intrusive rock or a core gneiss of intrusive or sedimentary ancestry, Bell and Alvord (1976) considered it to be volcanic or volcaniclastic in origin. Zircons in Fish Brook are certainly volcanic in origin and yield a date of 730 +/-26 Ma (Olszewski, 1980). If the rock were a core gneiss, that date would apply only to the Fish Brook and not to surrounding rocks; but, Bell and Alvord (1976) believe Fish Brook to be part of the Marlboro Formation-Nashoba Formation sequence and therefore the date does apply to the sequence. In addition, a 1500 Ma date for Shawsheen Gneiss [reference not given] helps bracket age of Marlboro-Nashoba sequence. An upper limit for the sequence was established from the 430 +/-5 Ma age of intruding Sharpers Pond Diorite and 450 +/-23 Ma age of the intruding Andover Granite (Zartman and Naylor, 1984). 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. [Age is not formally changed in this report.] Report includes geologic maps and correlation charts. [Chapters A-J in U.S. Geological Survey Professional Paper 1366 are intended as explanations and (or) revisions to 1:250,000-scale MA State bedrock geologic map of Zen and others (1983).]

Source: GNU records (USGS DDS-6; Reston GNULEX).

Recent dating by G.R. Dunning has yielded a much younger age than assigned to the Fish Brook of the Nashoba terrane by Olszewski (1980). The new date of 520+14-11 Ma 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 and 430 Ma, as the cross-cutting Sharpners Pond has been dated at 430 Ma. Therefore, no Precambrian rocks have been found in the Nashoba terrane. A 425+/-3 Ma U-Pb age on the monazite from the Fish Brook dates the Silurian metamorphism.

Source: GNU records (USGS DDS-6; Reston GNULEX).

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.

Source: GNU records (USGS DDS-6; Reston GNULEX).