The Guilford Quartz Monzonite is a Devonian quartz monzonite pluton in Howard County, Maryland. It is described as a biotite-muscovite-quartz monzonite which occurs as discontinuous lenticular bodiesUSGS Mineral Resources On-Line Spatial Data which intrude mainly through the Wissahickon Formation (gneiss).
The extent of this intrusion was originally mapped in 1940Cloos, Ernst, and Broedel, C.H., 1940, Geologic map of Howard County and adjacent parts of Montgomery and Baltimore Counties (Maryland): Maryland Geological Survey County Geologic Map, 1 sheet, scale 1:62,500 as the "Guilford granite". It was given its current name in 1964 by C. A. Hopson.Hopson, C. A., 1964, The crystalline rocks of Howard and Montgomery Counties: Maryland Geological Survey County Report, 337 p., (Reprinted from Cloos, Ernst, and others, "Geology of Howard and Montgomery Counties," p. 27-215) Hopson grouped the Guilford Quartz Monzonite with the Ellicott City Granodiorite and the Woodstock Quartz Monzonite as "Late-kinematic intrusive masses."
Description The Guilford Quartz Monzonite was described in 1898 as "perhaps the most attractive stone in the state" by Edward B. Mathews of the Maryland Geological Survey.Maryland Geological Survey Volume 2, 1898, The Johns Hopkins Press, Baltimore. He provides this detailed description of the granite:
Hopson reported the chemical composition (by %) of the Guilford Quartz Diorite from 1.5 miles west-southwest of Guilford along the Middle Patuxent River, as follows: Chemical % Chemical % SiO2 73.08 CaO 1.71 TiO2 0.27 Na2O 3.70 Al2O3 13.86 K2O 4.09 Fe2O3 0.65 H2O+ 0.46 FeO 1.40 H2O− 0.10 MnO 0.05 CO2 0.05 MgO 0.48 P2O5 0.06
Early quarrying The 1898 account of Edward B. Mathews of the Maryland Geological Survey of the quarry at Guilford (now within the town of Columbia) is as follows:
Age A. A. Drake argued that the Guilford is of Ordovician ageDrake, A.A., Jr., 1998, Geologic map of the Kensington quadrangle, Montgomery County, Maryland: U.S. Geological Survey Geologic Quadrangle Map, GQ-1774, scale 1:24,000 because it is probably comagmatic with the Woodstock Quartz Monzonite dated at 444 Ma. An earlier source gives the date of 420 +/-50 Ma.Wetherill, G.W., Tilton, G.R., Davis, G.L., Hart, S.R., and Hopson, C.A., 1966, Age measurements in the Maryland Piedmont: Journal of Geophysical Research, v. 71, p. 2139-2155. More recently, radiometric dating (U-Pb SHRIMP) of zircon crystals extracted from the Guilford Quartz Monzonite yielded an age of 362 +/- 3 Ma (Devonian).John N. Aleinikoff, J. Wright Horton, Avery Ala Drake, C. Mark Fanning, 2002, SHRIMP and Conventional U-Pb ages of Ordovician granites and tonalites in the central Appalachian Piedmont: Implications for Paleozoic tectonic events: American Journal of Science, 302 (1) 50-75; DOI: 10.2475/ajs.302.1.50
References
External links Guilford Granite Guilford's Quarries