|Geologic age||Silurian or Ordovician|
Igneous > Plutonic > Granitic > Granitefoliated, medium-to coarse-grained -muscovite-biotite granite
Igneous > Plutonic > Granitic > Leucocratic-granitic > Pegmatitepegmatite masses common
|Comments||Part of Nashoba Zone (Silurian and Older Rocks). Andover Granite described as light-colored muscovite- and garnet-bearing, mostly gneissic granite. Includes a nongneissic component that is apparently younger than main mass (Hansen, 1956; Castle, 1964). Includes rocks previously assigned to Acton Granite of Hansen (1956) that probably belong to Andover's older phase. Text of report states that gneissic granite is dated at 450 to 445 Ma; younger phase of granite and pegmatite is dated at 415 to 410 Ma. Table 13 reports Rb-Sr whole rock ages of 446 +/-32 Ma (Zartman and Naylor, 1984), 450 +/-23 Ma (Handford and others, 1965), and 460 Ma (Schutts and others, 1976) for the older phase. Rb-Sr whole-rock ages for the younger phase are reported as 408 +/-22 Ma (Zartman and Naylor, 1984), and 415 Ma (Hill and others, 1984). Age in report is Ordovician and Silurian (Wones and Goldsmith, 1991) per MA007.|
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.
|Counties||Essex - Middlesex|