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Porphyoblastic Biotite-Plagioclase Augen Gneiss

Porphyoblastic biotite-plagioclase augen gneiss
State Virginia
Name Porphyoblastic Biotite-Plagioclase Augen Gneiss
Geologic age Proterozoic Y
Comments Blue Ridge Basement Complex, Middle Proterozoic (Grenville-Age) Plutonic Rocks. From expanded explanation (ref. VA002): Porphyroblastic biotite-plagioclase augen gneiss. Mesocratic, medium- to coarse-grained, biotite-rich quartzofeldspathic gneiss con tains prominent subhedral to euhedral monocrystalline feldspar augen. The ratio plagioclase: potassium feldspar may be as high as 10:1; color index ranges from 30 to 50. Apatite, epidote, muscovite, ilmenite, and titanite are ubiquitous accessories. Plagioclase contains abundant prismatic epidote and white mica; ilmenite is rimmed with masses of anhedral titanite; subhedral hornblende and subhedral to euhedral almandine-grossular garnet occur locally. In the vicinity of adjacent charnockite, anhedral actinolitic amphibole pseudomorphs after pyroxene or rims thoroughly uralitized relict pyroxene. Rock fabric is gradational from granofels to mylonite gneiss. Geophysical signature: negative magnetic signature relative to adjacent charnockite. In northern Virginia, this unit strongly resembles prophyroblastic granite gneiss (Ybp); however, the augen in Ybp are more commonly polycrystalline aggregates rather than single-crystal porphyroblasts. This unit is widespread in the central and southeastern Blue Ridge, encompassing a number of lithologically similar metaplutonic entities: the "biotitic facies"of the Roses Mill and Turkey Mountain ferrodiorites of Herz and Force (1987), the Archer Mountain quartz monzonite of Bartholomew and others (1981), biotite granofels and augen gneiss of Evans (1984, 1991), biotite augen gneiss of Conley (1989), and augen-bearing gneiss of Lukert and Halladay (1980), and Lukert and Nuckols (1976). Historically, most workers have interpreted these rocks as Grenville-age plutons in which the present-day biotite-rich mineral assemblage is a primary igneous assemblage that crystallized from a melt (for example, Bartholomew and others, 1981). Herz and Force (1987) and Evans (1991) presented evidence that these biotite gneisses were derived from charnockite plutons by retrograde hydration reactions. Pettingill and others (1984) reported ages of 1009 +/- 26 Ma (Rb-Sr whole-rock) and 1004 +/- 36 Ma (Sm-Nd whole-rock) for ferrodiorite to quartzmonzonite in the Roseland district. Where this unit has been mapped in the Upperville quadrangle (A.E. Nelson, unpublished data), U-Pb zircon data suggest a crystallization age of 1055 +/- 2 Ma (Aleinikoff and others, 1993).
Primary rock type augen gneiss
Secondary rock type
Other rock types
Lithologic constituents
Major
Metamorphic > Gneiss > Biotite-gneissaugen gneiss
Map references
Digital Representation of the 1993 Geologic Map of Virginia", 2003, CD ROM (ISO-9660) contains image file, expanded explanation in pdf, and ESRI shapefiles, viewing software not included. This is a digital version of "Geologic Map of Virginia" published in 1993. Available from: https://www.dmme.virginia.gov/commerce/
Unit references
Virginia Division of Mineral Resources, 1993, Geologic Map of Virginia: Virginia Division of Mineral Resources, scale 1:500,000
Digital Representation of the 1993 Geologic Map of Virginia - expanded explanation: 2003, Commonwealth of Virginia, Department of Mines, Minerals, and Energy, Divsion of Mineral Resources, Publication 147, 85 pp
Digital Representation of the 1993 Geologic Map of Virginia", 2003, CD ROM (ISO-9660) contains image file, expanded explanation in pdf, and ESRI shapefiles, viewing software not included. This is a digital version of "Geologic Map of Virginia" published in 1993. Available from: https://www.dmme.virginia.gov/commerce/
Counties Albemarle - Amherst - Bedford - Carroll - Culpeper - Floyd - Franklin - Greene - Loudoun - Madison - Nelson - Rappahannock - Roanoke - Lynchburg