Dedham Granite

Dedham Granite - Light grayish-pink to greenish-gray, equigranular to slightly porphyritic, variably altered, granite south and west of Boston. Includes dioritic rock near Scituate and Cohasset and Barefoot Hills Quartz Monzonite of Lyons (1969) and Lyons and Wolfe (1971). Intrudes Zdi, Zgb, Zb, Zv. Extensive calc-alkaline plutons separated by Boston basin have long been mapped as Dedham. Those to the north of Boston and studied in this report, are referred to as Dedham North. Crystallization ages for the Dedham North suite (based on titanites and zircons) have been determined at 607+/-4 Ma, while ages for the Lynn are slightly younger at 596+/-3 Ma. Both are clearly part of the Late Proterozoic magmatic event. Dates on two samples from Sheffield Heights indicate that the diorite and granite are part of the Dedham North suite. The Dedham south and west of Boston has been dated at 630+/15 Ma (Zartman and Naylor, 1984). Dedham North Granite has a compositionally highly variable suite ranging from leucogranites to granodiorites, tonalites, and quartz diorite. The granites originated by partial melting of a sedimentary protolith, while the intermediate members show a mixing of granitic magma and mafic magma (Hepburn and others, 1993).
State Massachusetts
Name Dedham Granite
Geologic age Proterozoic Z
Comments Part of Milford-Dedham Zone (Tertiary and Older Rocks) Secondary unit description per MA018. Proterozoic Z age of Dedham is based on the following isotopic dates: . 1. 630 +/-15 Ma by U/Pb methods on zircon (Zartman and Naylor, 1984) from Dedham Granite. 2. 595 +/-17 Ma by Rb/Sr whole rock methods (Kovach and others, 1977) from Dedham Granite north of Boston. 3. 640 +/-14 Ma by K/Ar methods on hornblende (Zartman and Naylor, 1984) per MA007.
Primary rock type granite
Secondary rock type diorite
Other rock types quartz monzonite
Lithologic constituents
Igneous > Plutonic > Granitic > GraniteLight grayish-pink to greenish-gray, equigranular to slightly porphyritic, variably altered, granite south and west of Boston.
Igneous > Plutonic > DioriticIncludes dioritic rock near Scituate and Cohasset.
Igneous > Plutonic > Syenitic > Quartz-monzoniteBarefoot Hills Quartz Monzonite of Lyons (1969) and Lyons and Wolfe (1971).
Map references
Unpublished Digital Geologic Map of Massachusetts received from Rudi Hon at Boston College in 1998.
Unit references
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., 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.
Hepburn, J.C., Hon, Rudolph, Dunning, G.R., Bailey, R.H. and Galli, Kenneth, 1993, The Avalon and Nashoba terranes (eastern margin of the Appalachian Orogen in southeastern New England), IN Cheney, J.T., and Hepburn, J.C., eds., Field trip guidebook for the northeastern United States; 1993 Boston GSA; Volume 2: University of Massachusetts, Geology Department Contribution, no. 67, Joint annual meeting of Geological Society of America, Annual Meeting, and New England Intercollegiate Geological Conference, 85th Annual Meeting, Boston, MA, October 25-28, 1993, p. X1-X31.
Counties Bristol - Middlesex - Norfolk - Plymouth - Suffolk