|Geologic age||Middle Devonian|
Igneous > Plutonic > Granitic > Alkali-feldspar-granite > Alkali-graniteAlkalic granite 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): The Peabody Granite occurs in two plutons that total about 50 square kilometers. The large mass at Peabody (Peabody pluton) is the main body. The smaller mass near Reading (Reading pluton) is lithologically similar to the Peabody pluton. Both masses intrude the rocks of the Late Proterozoic mafic volcanic-plutonic complex. Although the major minerals and bulk composition of the Quincy, Cape Ann, and Peabody Granites are similar, their petrography, fabric, and ages of intrusion indicate distinct magmatic events. The occurrence of aplite and pegmatite in the Peabody distinguishes it from the Cape Ann Complex. Devonian age is based on Rb-Sr whole-rock age of 359 +/-24 Ma (Zartman and Marvin, 1971) and U-Pb zircon age of 395 +/-20 Ma (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/
|Counties||Essex - Middlesex|