Geologic units in Lunenburg county, Virginia

Interlayered Mafic and Felsic Volcanic Rocks (Proterozoic Z) at surface, covers 29 % of this area

Layered mafic to felsic metavolcanic rocks. Volcanogenic sequence includes felsic pyroclastic and volcaniclastic rocks with intercalated mafic pyroclastic and amygdaloidal flows and phyllitic metasedimentary interbeds. Felsic rocks are crystal, lithic, and vitric tuff and tuff breccia ranging in composition from rhyolite to dacite. Mafic rocks consist of mafic lithic crystal and vitric tuff, with associated amygdaloidal pyroclastic rocks, and greenstone metabasalt.

Biotite Granite (Proterozoic) at surface, covers 12 % of this area

Light-gray, medium-grained, equigranular, broadly-layered, locally migmatitic, foliated. Mineralogy: quartz + plagioclase + microcline + biotite + muscovite ± hornblende + apatite + zircon.

Aaron Slate (Proterozoic Z) at surface, covers 10 % of this area

Very-light-gray, fine-grained, bedded volcaniclastic sediments, conglomerate, lithic feldspathic arenite, micaceous sandstone, siltstone, phyllite, argillite, and vitric tuff, with minor greenstone. Lithic fragments and relict euhedral crystals are common. The lower part of the unit is dominantly grayish-green slate interbedded with light-gray to grayish-green micaceous metasandstone; bedding is conspicuous and graded-bedding is common. The unit grades upward to bedded light-gray to moderate-red phyllite, metasandstone and slate.

Porphyroblastic Biotite Granite (Proterozoic) at surface, covers 9 % of this area

Light-gray, medium to coarse-grained, compositionally layered, well-foliated, commonly lineated gneiss composed of metamorphosed granite, leucogranite, and granodiorite, which locally contains feldspar megacrysts. This unit includes the granite at Lawrenceville; the rocks are variably mylonitic and lineated along the Lake Gordon mylonite zone near Kenbridge (Horton and others, 1993).

Buggs Island Pluton (Mississippian Pennsylvanian) at surface, covers 8 % of this area

Light-gray, medium- to coarse grained, massive to strongly foliated biotite-muscovite granite. The name first appears in the literature as the Buggs Island granite gneiss (Kish and Fullagar, 1978); those workers report an Rb-Sr whole-rock age of 314±16 Ma.

Biotite Gneiss (Proterozoic) at surface, covers 8 % of this area

Light-gray, medium- to coarse grained, compositionally-layered and locally migmatitic rocks, include interlayered biotite gneiss, muscovite-biotite gneiss, muscovite-biotite schist, and sillimanite-mica schist; also includes minor interlayers and lenses of granitic gneiss, biotite-amphibole gneiss, amphibolite, garnet-mica schist, calc-silicate granofels, and rare ultramafic rocks. This unit correlates with Raleigh belt rocks in North Carolina (Parker, 1979; Geologic Map of North Carolina, 1985).

Mylonite, Mylonite Gneiss, and Cataclastic Rocks (Proterozoic - Paleozoic ?) at surface, covers 6 % of this area

Mylonite. Includes protomylonite, mylonite, ultramylonite, and cataclastic rocks. Lithology highly variable, depending on the nature of the parent rock, and on intensive parameters and history of deformation. In most mapped belts of mylonite and cataclastic rock (my), tectonized rocks anastomose around lenses of less-deformed or undeformed rock. In the Blue Ridge, some of these lenses are large enough to show at 1:500,000 scale. In many places mylonitic and cataclastic rocks are gradational into less deformed or undeformed adjacent rocks, and location of contacts between tectonized rocks (my) and adjacent units is approximate or arbitrary. These boundaries are indicated on the map by color-color joins with superimposed shear pattern. Most mapped belts of mylonite represent fault zones with multiple movement histories. In the Blue Ridge, Paleozoic age contractional deformation fabrics are superimposed on Late Precambrian extensional fabrics (Simpson and Kalaghan, 1989; Bailey and Simpson, 1993). Many Piedmont mylonite zones contain dextral-transpressional kinematic indicators that formed during Late Paleozoic collision al tectonics (Bobyarchick and Glover, 1979; Gates and others, 1986). Paleozoic and older faults were reactivated in many places to form extensional faults during the Mesozoic (Bobyarchick and Glover, 1979).

Hornblende-plagioclase Gabbro (Proterozoic) at surface, covers 4 % of this area

Dark-grayish-green, coarse- to medium-grained, massive to foliated metagabbro. Mineralogy: amphibole + plagioclase + clinopyroxene + quartz + biotite + muscovite + epidote ± magnetite. Geophysical signature: small circular positive magnetic anomalies. Plutons of these gabbros intrude interlayered mafic and felsic metavolcanic rocks.

Metatonalite (Proterozoic) at surface, covers 3 % of this area

White to light-gray, medium- to coarse-grained, faintly foliated, locally porphyritic; ranges from granodiorite to quartz diorite. Mineralogy: quartz + plagioclase + biotite + microcline. This unit includes the Vance pluton of Horton and others (1993), dated at 571±17 Ma (U-Pb zircon; LeHuray, 1989).

Migmatitic Paragneiss (Proterozoic) at surface, covers 3 % of this area

Leucocratic to mesocratic, medium- to coarse-grained layered gneiss contains interlayered biotite-rich and quartzofeldspathic zones, locally migmatitic; includes lesser amounts of biotite schist, muscovite schist, and thin lenticular amphibolite bodies. Mineralogy: biotite + muscovite + plagioclase + potassium feldspar + garnet ± hornblende.

Biotite Granodiorite (Proterozoic) at surface, covers 2 % of this area

Light-gray, medium-grained, foliated. Mineralogy: quartz + potassium feldspar + plagioclase + biotite + muscovite; accessory minerals include epidote, apatite, and opaque minerals.

Quartzofeldspathic Gneiss (Proterozoic Y-Pennsylvanian) at surface, covers 1 % of this area

Quartzofeldspathic gneiss (Bobyarchick and others, 1981). Light-gray, fine- to coarse-grained, foliated, layered muscovitie-bearing quartzofeldspathic gneiss; contains intercalated quartz-muscovite schist. Mineralogy: quartz + plagioclase + microcline + garnet + muscovite + biotite.

Porphyritic Granite (Proterozoic) at surface, covers 0.9 % of this area

Light-gray to pink, medium to coarse-grained, coarse granular texture common; locally pegmatitic. Mineralogy: quartz + porphyroblastic microcline + biotite + muscovite ± epidote ± titanite ± opaque minerals.

Biotite-Muscovite Granite (Proterozoic) at surface, covers 0.9 % of this area

Light-gray, fine- to coarse grained, muscovite-biotite granite, biotite-muscovite granite, and leucogranite with accessory garnet. The granite is undated but interpreted as part of the Pennsylvanian-Permian suite of granites, and considered as part of the Wise pluton, which can be traced into North Carolina (McSween and others, 1991).

North View Granite (Proterozoic) at surface, covers 0.8 % of this area

North View granite (Bentley, 1992). Light-gray, fine to coarse-grained, lineated to foliated to massive alkali granite composed of blocky alkali feldspar mesoperthite, quartz, magnetite, and minor aegirine, riebeckite, biotite, allanite, titanite, fluorite, zircon, and garnet. Minor microcline and albite form separate phases with recrystallization. Garnet, which may be metamorphic in origin, is grossular-almandine-spessartine with up to three percent yttrium; riebeckite is in the plane of foliation and is metamorphic in origin. The northern and western borders of the pluton are fine-grained and granophyric, with phenocrysts of mesoperthite, albite, and quartz; accessory and secondary minerals include magnetite, biotite, garnet, titanite, chlorite, epidote, muscovite, and calcite.

Biotite Gneiss and Schist (Proterozoic) at surface, covers 0.8 % of this area

Dark-gray, medium grained, foliated and broadly-layered. Mineralogy: biotite + plagioclase + potassium feldspar + quartz + muscovite; accessory minerals include titanite, epidote, and opaque minerals.

Felsic Volcanic Rocks (Proterozoic Z) at surface, covers 0.3 % of this area

Very-light-gray, fine- to medium-grained crystal, lithic, and lithic-crystal andesitic metatuff with minor light-gray to white, fine-grained metasedimentary interbeds.

Mafic Volcanic Rocks (Proterozoic Z) at surface, covers < 0.1 % of this area

Greenish-gray to dusky green, fine- to medium-grained, massive to amygdaloidal metabasalt with dark-gray to white, medium-grained mafic lithic and crystal tuff, and minor purple phyllite and metasedimentary rocks. Geophysical signature: linear positive magnetic and negative radiometric anomalies.