|Geologic age||Middle Devonian|
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).|
Zen, E-An (ed.), Goldsmith, R. (comp.), Ratcliffe, N.M. (comp.), Robinson, P. (comp.), Stanley, R.S. (comp.), Hatch, N.L., Jr., Shride, A.F., Weed, E.G.A., and Wones, D.R., 1983, Bedrock Geologic Map of Massachusetts: U.S. Geological Survey, Reston, VA, scale 1: 250,000.
Wones, D.R. and Goldsmith, Richard, 1991, Intrusive rocks of eastern Massachusetts, Chapter I, IN Hatch, N.L., Jr., ed., The bedrock geology of Massachusetts: U.S. Geological Survey Professional Paper, 1366-E-J, p. I1-I61.
USGS Geologic Names lexicon found at: http://ngmdb.usgs.gov/Geolex/