The Guilford Quartz Monzonite is a

Silurian The Silurian ( ) is a geologic period and system spanning 24.6 million years from the end of the Ordovician Period, at million years ago ( Mya), to the beginning of the Devonian Period, Mya. The Silurian is the shortest period of the Paleozo ...

Ordovician The Ordovician ( ) is a geologic period and System (geology), system, the second of six periods of the Paleozoic Era (geology), Era. The Ordovician spans 41.6 million years from the end of the Cambrian Period million years ago (Mya) to the start ...

quartz monzonite Quartz monzonite is an intrusive, felsic, igneous rock that has an approximately equal proportion of orthoclase and plagioclase feldspars. It is typically a light colored phaneritic (coarse-grained) to porphyritic granitic rock. The plagioclase ...

pluton In geology, an igneous intrusion (or intrusive body or simply intrusion) is a body of intrusive igneous rock that forms by crystallization of magma slowly cooling below the surface of the Earth. Intrusions have a wide variety of forms and com ...

in Howard County, Maryland. It is described as a

biotite Biotite is a common group of phyllosilicate minerals within the mica group, with the approximate chemical formula . It is primarily a solid-solution series between the iron-endmember annite, and the magnesium-endmember phlogopite; more alumino ...

muscovite Muscovite (also known as common mica, isinglass, or potash mica) is a hydrated phyllosilicate mineral of aluminium and potassium with formula K Al2(Al Si3 O10)( F,O H)2, or ( KF)2( Al2O3)3( SiO2)6( H2O). It has a highly perfect basal cleavage ...

quartz Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical form ...

monzonite Monzonite is an igneous intrusive rock, formed by slow cooling of underground magma that has a moderate silica content and is enriched in alkali metal oxides. Monzonite is composed mostly of plagioclase and alkali feldspar. Syenodiorite is an o ...

which occurs as discontinuous lenticular bodies which intrude mainly through the

Wissahickon Formation The Wissahickon Formation is a mapped bedrock unit in Pennsylvania, New Jersey, and Delaware. It is named for the Wissahickon gorge in Fairmount Park, Philadelphia. In Maryland formations, the term "Wissahickon" is no longer used. Rocks in this cl ...

(

gneiss Gneiss ( ) is a common and widely distributed type of metamorphic rock. It is formed by high-temperature and high-pressure metamorphic processes acting on formations composed of igneous or sedimentary rocks. Gneiss forms at higher temperatures an ...

). 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 The Ellicott City Granodiorite is a Silurian or Ordovician granitic pluton in Howard and Baltimore Counties, Maryland. It is described as a biotite granodiorite along the margin of the intrusion which grades into a quartz monzonite in its cor ...

and the

Woodstock Quartz Monzonite The Woodstock Quartz Monzonite is a Silurian or Ordovician quartz monzonite pluton in Baltimore County, Maryland. It is described as a massive biotite-quartz monzonite which intrudes through the Baltimore Gneiss at a single locality surrounding ...

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:

Early quarrying

The 1898 account of Edward B. Mathews of the Maryland Geological Survey of the

quarry A quarry is a type of open-pit mine in which dimension stone, rock, construction aggregate, riprap, sand, gravel, or slate is excavated from the ground. The operation of quarries is regulated in some jurisdictions to reduce their envi ...

Guilford Guildford is a town in Surrey, England. It gives its name to the Borough of Guildford, the Diocese of Guildford and the Parliamentary constituency of Guildford. Guildford, Guilford, or Gildford may also refer to: Places Australia * Guildfor ...

(now within the town of Columbia) is as follows:

Age

A. A. Drake argued that the Guilford is of

age because it is probably comagmatic with the

dated at 444 Ma. An earlier source gives the date of 420 +/-50 Ma. More recently, radiometric dating ( U-Pb- TIMS) of zircon crystals extracted from the Guilford Quartz Monzonite yielded an age of 362 +/- 3 Ma (

Devonian The Devonian ( ) is a geologic period and system of the Paleozoic era, spanning 60.3 million years from the end of the Silurian, million years ago (Mya), to the beginning of the Carboniferous, Mya. It is named after Devon, England, whe ...

). 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

{{Reflist Howard County, Maryland Columbia, Maryland Granitic rocks Igneous petrology of Maryland Silurian magmatism Ordovician magmatism