Named for Columbia, Fluvanna Co., central VA. Consists of biotite quartz monzonite. Intrusive into Glenarm series. Age is Precambrian.

Source: GNU records (USGS DDS-6; Reston GNULEX).

Columbia used on State map as informal Columbia pluton. Described as light-gray, medium- to coarse-grained, and foliated. Includes biotite-muscovite granite, granodiorite, tonalite, and granitic pegmatite. Contains xenoliths of biotite gneiss, amphibolite, and felsic metavolcanic rocks. Predominant minerals are plagioclase, quartz, and microcline. Common accessories include biotite, muscovite, epidote, zircon, apatite, garnet, magnetite and pyrite. Original name Columbia granite was objected to by later workers because of the relatively small percentage of true granite. The Columbia includes, in part, the Hatcher complex of Brown (1969). Multiple intrusive phases are indicated by its heterogeneous nature. Tonalite in the eastern part of the pluton has yielded ages of 590+/-80 Ma (Rb-Sr whole-rock, Fullagar, 1971). Rb-Sr whole-rock age of 454+/-9 Ma reported by Mose and Nagel (1982). This last date, supposedly from the western portion of the Columbia, may well have come from the unit herein mapped as the Carysbrook pluton, however map symbol indicates an Ordovician age for the Columbia.

Source: GNU records (USGS DDS-6; Reston GNULEX).