Fitchburg Complex

Dark-gray, strongly foliated biotite granodiorite to tonalite gneiss; resembles Dht; intrudes and contains inclusions of Dl, some mappable; locally cut by sills identical to Dfgrg.
State Massachusetts
Name Fitchburg Complex
Geologic age Lower Devonian or younger
Lithologic constituents
Major
Metamorphic > Gneissstrongly foliated biotite granodiorite to tonalite gneiss
Incidental
Metamorphic > Metasedimentary > Metaclastic > Phylliteintrudes and contains inclusions of Dl (Littleton Formation)
Metamorphic > Schistintrudes and contains inclusions of Dl (Littleton Formation)
Comments Part of Merrimack Belt (Silurian, Devonian and Pennsylvanian Rocks). Secondary unit description per MA007 as reported in USGS Geologic Names lexicon (ref. MA024): Fitchburg Complex mapped as three small stocks intruding Worcester Formation east of Wekepeke fault and west of Clinton. Also intrudes Paxton and Littleton Formations. U-Pb zircon age of 390 +/-15 Ma and Rb-Sr whole-rock age of 402 +/-11 Ma were both reported by Zartman and Naylor (1984) (Wones and Goldsmith, 1991).
References
NGMDB product
Counties Middlesex - Worcester