Andover Granite (MASOagr;0)

State	Massachusetts
Name	Andover Granite
Geologic age	Silurian or Ordovician
Original map label	SOagr
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.
Primary rock type	granite
Secondary rock type	pegmatite
Other rock types
Lithologic constituents	Major Igneous > Plutonic > Granitic > Leucocratic-granitic > Pegmatitepegmatite masses common Igneous > Plutonic > Granitic > Granitefoliated, medium-to coarse-grained -muscovite-biotite granite
Map references	Unpublished Digital Geologic Map of Massachusetts received from Rudi Hon at Boston College in 1998.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. [http://pubs.er.usgs.gov/publication/pp1366EJ]
Geographic coverage	Essex - Middlesex