Shawsheen Gneiss

Sillimanite gneiss, sulfidic at base; minor amphibolite.
State Massachusetts
Name Shawsheen Gneiss
Geologic age Ordovician or Proterozoic Z
Lithologic constituents
Major
Metamorphic > GneissSillimanite gneiss, sulfidic at base.
Minor
Metamorphic > Amphibolite
Comments Part of Nashoba Zone (Silurian and Older Rocks). 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. [No formal age change made in this report] (Goldsmith, 1991) per MA017.
References
NGMDB product
Counties Essex - Middlesex