Wenham Monzonite

Monzonite containing ferro-hornblende.
State Massachusetts
Name Wenham Monzonite
Geologic age Middle Devonian
Lithologic constituents
Major
Igneous > Plutonic > Syenitic > MonzoniteMonzonite containing ferro-hornblende.
Comments Part of Milford-Dedham Zone (Tertiary and Older Rocks). Secondary unit description per MA007 as reported in USGS Geologic Names lexicon (ref. MA024): Wenham Monzonite intrudes unnamed Proterozoic Z mafic volcanic-plutonic complex. Occurs over a 10 sq km area north of Peabody. Described as gray, weathering to cream, and medium grained. Distinguished from rocks of Cape Ann Complex by presence of plagioclase and biotite, and interstitial nature of quartz. Devonian age is based on Rb-Sr whole-rock date of 395 +/-20 Ma [reference cited is Zartman (1977), but Goldsmith and others (1982) state that this dates supersedes that of Zartman (1977)] (Wones and Goldsmith, 1991).
References
NGMDB product
Counties Essex