|Name||Lynn Volcanic Complex|
|Geologic age||Lower Devonian, Silurian, or Proterozoic Z|
Igneous > Volcanic > Felsic-volcanic > Rhyolite
Igneous > Volcanic > Felsic-volcanic (Volcaniclastic-volcanic breccia, Pyroclastic-tuff)agglomerate
|Comments||Part of Milford-Dedham Zone (Tertiary and Older Rocks). Secondary unit description per MA023 as reported in USGS Geologic Names lexicon (ref. MA024): Lynn Volcanic Complex is confined to block north of Boston basin between northern border fault and Walden Pond fault. Similar in lithology to Mattapan Volcanic Complex. Both units consist largely of partly porphyritic rhyolite and rhyodacite flows, welded ash-flow tuffs, vitric tuff, lapilli tuff, lithic tuff, flow breccias, breccia pipes, and extrusion domes. Mattapan's rhyolite and rhyodacites are thinner and less varied in composition and texture than Lynn's; volcanic breccias are absent in Lynn. Both units are reported to lie nonconformably on Dedham Granite and unnamed plutonic-volcanic complex of eastern MA. Bell (1976) cites location in Saugus where Lynn overlies both Middlesex Fells Volcanic Complex and diorite intruded into it. Inclusions of Lynn found in Dedham Granite at Pine Hill in Medford (Zarrow, 1978, cited in Naylor, 1981). Lynn has no overlying strata. Both Lynn and Mattapan have been treated as the same age by numerous workers. Billings (1982) accepted Proterozoic Z age of Mattapan (based on U-Th-Pb zircon date of 602+/-3 Ma by Zartman, in Kaye and Zartman, 1980), but questioned reliability of zircon ages from volcanic rocks; he preferred to correlate Lynn rocks with Newbury Volcanic Complex because of Lynn's proximity and lithologic similarity to Newbury. Author feels that proximity argument is not strong because Lynn is geographically and structurally closer to Mattapan rocks. Zarrow (1978, cited in Naylor, 1981) has shown rare-earth element pattern in Lynn rocks to be identical to that in part of Mattapan. Author feels it is reasonable to assume that Lynn and Mattapan are parts of same volcanic complex, and that Lynn in Proterozoic Z in age (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.
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.
Goldsmith, Richard, 1991, Stratigraphy of the Milford-Dedham zone, eastern Massachusetts; an Avalonian terrane, Chapter E, IN Hatch, N.L., Jr., ed., The bedrock geology of Massachusetts: U.S. Geological Survey Professional Paper, 1366-E-J, p. E1-E62.
USGS Geologic Names lexicon found at: http://ngmdb.usgs.gov/Geolex/
|Counties||Essex - Middlesex - Suffolk|