| Guilford Quartz Monzonite | |
| Type: | igneous |
| Age: | Silurian |
| Period: | Paleozoic |
| Prilithology: | monzonite |
| Namedby: | Cloos and Broedel, 1940 |
| Region: | Piedmont of Maryland |
| Extent: | central Maryland |
The Guilford Quartz Monzonite is a Silurian or Ordovician quartz monzonite pluton in Howard County, Maryland. It is described as a biotite-muscovite-quartz monzonite which occurs as discontinuous lenticular bodies[1] which intrude mainly through the Wissahickon Formation (gneiss).
The extent of this intrusion was originally mapped in 1940[2] as the "Guilford granite". It was given its current name in 1964 by C. A. Hopson.[3] Hopson grouped the Guilford Quartz Monzonite with the Ellicott City Granodiorite and the Woodstock Quartz Monzonite as "Late-kinematic intrusive masses."
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.[4] He provides this detailed description of the granite:
Hopson[3] 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 |
The 1898 account of Edward B. Mathews of the Maryland Geological Survey[4] of the quarry at Guilford (now within the town of Columbia) is as follows:
A. A. Drake argued that the Guilford is of Ordovician age[5] because it is probably comagmatic with the Woodstock Quartz Monzonite dated at 444 Ma. An earlier source gives the date of 420 +/-50 Ma.[6] More recently, radiometric dating (U-Pb-TIMS) of zircon crystals extracted from the Guilford Quartz Monzonite yielded an age of 362 +/- 3 Ma (Devonian).[7]