State Farm Gneiss

State Farm Gneiss (Brown, 1937). Leucocratic to mesocratic, medium- to coarse-grained, massive to moderately layered, locally migmatitic gneiss ranges in composition from granodiorite to tonalite. Quartzofeldspathic segregations and discordant pegmatites are common; leucocratic biotite-garnet quartz-plagioclase gneiss, metagabbro, and thin pelitic schist interlayers are minor constituents (Poland, 1976; Reilly, 1980; Farrar, 1984). Mineralogy: quartz + plagioclase + microcline + biotite + garnet + hornblende ± clinopyroxene; titanite is a ubiquitous accessory; other accessory minerals include magnetite and zircon. Farrar (1984) reports that the least deformed portions of the State Farm contain relict granulite-facies mineral assemblages. Geophysical signature: characterized by negative magnetic and positive radiometric anomalies. The age of the State Farm is reported as 1031±94 Ma (Rb- Sr whole-rock; Glover and others, 1982). The titanium-rich chemistry of the State Farm, as indicated by the abundance of titanite, is also characteristic of Grenville-age gneisses in the Virginia Blue Ridge.
State Virginia
Name State Farm Gneiss
Geologic age Proterozoic Y
Lithologic constituents
Major
Metamorphic > Gneiss > Hornblende-gneisshornblende-biotite gneiss
Comments Eastern Piedmont
References

Virginia Division of Mineral Resources, 1993, Geologic Map of Virginia: Virginia Division of Mineral Resources, scale 1:500,000.

Virginia Division of Mineral Resources, 2003, Digital Representation of the 1993 Geologic Map of Virginia, Publication 174, CD ROM (ISO-9660) contains image file, expanded explanation in pdf, and ESRI shapefiles, scale 1:500,000.

NGMDB product
Counties Caroline - Goochland - Hanover - Powhatan