Coys Hill Porphyritic Granite Gneiss

Hornblende gneiss inclusions in granitic gneiss of the Coys Hill.
State Massachusetts
Name Coys Hill Porphyritic Granite Gneiss
Geologic age Lower Devonian
Lithologic constituents
Major
Metamorphic > Gneiss > Hornblende-gneissHornblende gneiss inclusions in granitic gneiss of the Coys Hill.
Comments Part of Merrimack Belt (Silurian, Devonian and Pennsylvanian Rocks). Secondary unit description per MA003 as reported in USGS Geologic Names lexicon (ref. MA024): Coys Hill Granite named by Emerson (1898) of Early Devonian age is here renamed Coys Hill Porphyritic Granite Gneiss to more accurately reflect its character in MA. Characterized by variously deformed tabular microcline and locally sodic plagioclase phenocrysts 3 to 10 cm long in a finer matrix of quartz, plagioclase, and biotite with or without muscovite, garnet, and sillimanite (Field, 1975). May be continuous with Cardigan pluton of Kinsman Quartz Monzonite of Billings (1956) (P.J. Thompson, 1983, unpub. data). (Authors note that Kinsman should also be called a granite under current terminology.) Lenses of hornblende-pyroxene gneiss within the Coys Hill unit are mapped separately (Robinson and Luttrell, 1985).
References
NGMDB product
Counties Worcester