Heterogenous layered assemblage correlates with the Chopawamsic Formation and Ta River Metamorphic Suite, on strike to the northeast, and in traceable into the Milton belt in North Carolina (Geologic Map of North Carolina, 1985). Quartzofeldspathic biotite gneiss. Heterogeneous layered sequence consists of salt-and-pepper and segregation layered biotite granite gneiss interlayered with biotite schist; dark-gray to black, fine- to coarse-grained, thin- to thickly-laminated hornblende gneiss and schist; lesser quartz-muscovite schist; and, locally, gray to green, medium-grained, calcareous gneiss and calc-silicate granofels (Tobish and Glover, 1969). This unit includes the upper and lower felsic gneiss units and intermediate volcanic rocks in the Hyco Formation as used by Baird (1989, 1991); and biotite gneiss and interlayered gneiss of Kreisa (1980), correlative with the biotite gneiss unit of Marr (1980a; 1980b). Mineralogy: (quartzofeldspathic rocks), (1) quartz + albite + potassium feldspar + muscovite + chlorite + actinolite + epidote + calcite + magnetite + zircon; (2) quartz + oligoclase + muscovite + biotite + garnet + hornblende + magnetite + epidote + rutile + calcite + zircon; (mafic rocks), (1) quartz + albite + chlorite + epidote + actinolite + titanite + magnetite ilmenite. (2) quartz + oligoclase + andesine + hornblende + microcline + biotite + garnet + cordierite + magnetite + rutile + titanite + scapolite; (pelitic rocks), (1) quartz + albite + muscovite + chlorite + epidote + magnetite-ilmenite; (2) quartz + muscovite + biotite + kyanite + oligoclase + potassium feldspar + epidote + magnetite-ilmenite + garnet; (3) quartz + muscovite + sillimanite + magnetite-ilmenite; (calcareous rocks), (1) quartz + calcite + biotite + epidote + chlorite + tremolite + ilmenite; (2) calcite+ quartz + epidote + hornblende + pyroxene + scapolite. Geophysical signature: felsic rocks are delineated by strike-elongate positive radiometric anomalies (Henika and Johnson, 1980); mafic metavolcanic rocks and metasedimentary units are characterized by closed strike-elongate radiometric lows and closed strike-elongate aeromagnetic highs.
Foliated felsite; Heterogenous layered assemblage correlates with the Chopawamsic Formation and Ta River Metamorphic Suite, on strike to the northeast, and in traceable into the Milton belt in North Carolina (Geologic Map of North Carolina, 1985). Foliated felsite. Grayish-orange-pink to white, fine- to medium-grained, foliated to granular metavolcanic rocks range in composition from rhyolite to dacite. Includes muscovite feldspar- quartz schist, gneiss and granofels; massive crystal metatuff; welded ashflow tuff; and, inequigranular metavolcanic breccia. Relict primary volcanic textures are recognizable where metamorphic grade is low (Henika, 1975; 1977). This unit includes felsic gneiss with less common mafic and rare calcareous gneiss mapped by Tobisch (1972), in part the metamorphosed volcanic sequence of Gates (1981), and dominantly felsic-composition units mapped by Nelson (1992). The unit contains numerous granitic dikes, sills, and lit-par-lit injections where it occurs in close proximity to Shelton Formation (Ost). Felsites occur interlayered with amphibolite, amphibole gneiss and schist (Cmv), quartzofeldspathic biotite gneiss (Cbg), sillimanite-quartz-muscovite schist and gneiss (Csg), and ferruginous quartzite (Cfq).
Quartz-muscovite schist and gneiss; Heterogenous layered assemblage correlates with the Chopawamsic Formation and Ta River Metamorphic Suite, on strike to the northeast, and in traceable into the Milton belt in North Carolina (Geologic Map of North Carolina, 1985). Quartz muscovite schist and gneiss. Very-light-gray to light-bluish-gray, fine- to medium-grained, layered kyanite mica schist, kyanite and sillimanite quartzite, and interlayered biotite-garnet schist. Mineralogy: quartz + muscovite + plagioclase ± biotite ± garnet ± sillimanite ± kyanite ± magnetite. Includes the schist and gneiss unit of Tobisch (1972), and muscovite-quartz schist of Baird (1989, 1991).
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.
Includes light-gray, fine- to medium grained, foliated quartz-muscovite schist with relict quartz and plagioclase phenocrysts; lithic and crystal metatuff with relict volcanic fragments; and, minor felsic breccia and tuffaceous greenstone. A penetrative schistosity is defined by aligned mica grains; a lineation is defined by the ellipsoidal relict phenocrysts. These rocks were originally named the Hyco quartz porphyry by Laney (1917); they were renamed the Hyco Formation by Kreisa (1980). The Hyco has been dated at 620±20 Ma (Pb-Pb zircon; Glover and others, 1971).
Virgilina greenstone (Laney, 1917). Grayish-green, fine- to coarse-grained basaltic tuff, metavolcanic breccia, and porphyritic to amygdaloidal greenstone, with minor interlayered metasedimentary rocks. Hydrothermally mineralized zones occur within the greenstones; these contain bornite, chalcocite, and chalcopyrite, and have been mined for copper. Mineralogy: plagioclase + hornblende + chlorite + epidote + calcite + apatite + quartz + opaque minerals. Geophysical signature: linear positive magnetic anomaly. Watson (1911) used the term Virgilina Group for a wide belt of rocks in the vicinity of Virgilina, Virginia. Mafic metavolcanic rocks in this sequence were named Virgilina greenstone, and felsic metavolcanic rocks and associated metasedimentary rocks were named Aaron slate by Laney (1917). Kreisa (1980) refined Laney's stratigraphy in defining the Aaron Formation as containing upper and lower members equivalent to Laney's Aaron slate, and a middle member equivalent to the Virgilina greenstone. The stratigraphy was further modified by Harris and Glover (1985), who combined the middle and upper members of Kreisa's Aaron Formation to make the Virgilina Formation. Virgilina greenstone is herein used in the sense of Laney (1917), but is equivalent to the middle member of the Aaron Formation of Kreisa (1980), and the lower portion of the Virgilina Formation of Harris and Glover (1985) (Figure 2).
Amphibolite, hornblende-biotite gneiss, and schist.; Heterogenous layered assemblage correlates with the Chopawamsic Formation and Ta River Metamorphic Suite, on strike to the northeast, and in traceable into the Milton belt in North Carolina (Geologic Map of North Carolina, 1985). Amphibolite, hornblende-biotite gneiss and schist. Black to moderate-olive-brown, medium- to coarse-grained, lineated and foliated; light-greenish-gray quartz-epidote stringers are common. Mineralogy: hornblende + tremolite-actinolite + oligoclase + biotite + epidote + garnet. Includes Blackwater Creek Gneiss and Catawba Creek amphibolite member of Hyco Formation of Baird (1989), hornblende gneiss of LeGrand (1960), gneiss unit of Kreisa (1980), and dominantly mafic-composition units mapped by Nelson (1992). Amphibolite is interlayered with biotite gneiss, as discussed above.
Gray to white, medium-grained, massive to layered granite and granite gneiss range in composition from granite to quartz diorite. Quartz diorite dikes and sills are locally abundant. Mineralogy: quartz + potassium feldspar + plagioclase + biotite + epidote + muscovite ± clinopyroxene + hornblende. This unit intrudes biotite gneiss unit (Cbg).
Pink to gray, coarse-grained, massive, strongly-lineated gneiss ranging in composition from granite to quartz monzonite (Henika, 1977), composed of microcline and perthite augen enclosed by biotite in a quartz-plagioclase matrix. A characteristic linear fabric is produced by rod-like feldspar porphyroblasts and crystalline aggregates. Thin veins of purple fluorite are common. Mineralogy: quartz + potassium feldspar + plagioclase + biotite + muscovite ± pyrite ± fluorite. Geophysical signature: pronounced positive radiometric signature, flat magnetic signature. Originally named Shelton granite (Jonas, 1928), the unit was renamed Shelton Formation (Henika, 1977). Simple linear fabric was cited as evidence for an intrusive origin by Henika (1980). The Shelton has been dated at 424±7 Ma (Rb-Sr whole-rock; Kish, 1983) and 463 Ma (U-Pb; Hund, 1987).
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).
Dominantly leucocratic to mesocratic, medium- to coarse-grained, strongly-lineated granite gneiss; includes porphyritic granite gneiss. Mineralogy: quartz + potassium feldspar + plagioclase + biotite + muscovite ± garnet. Geophysical signature: western portion coincides with strong positive radiometric anomaly. This unit likely includes more than one intrusive body; portions may be derivative of felsic volcanogenic rocks. These rocks intrude and are interlayered with metavolcanic rocks of probable Cambrian age. The unit includes granite gneiss of Nelson (1992), and may be correlative in part with the Shelton Formation (Ost).
Light-greenish-gray to pink-banded, massive, medium- to coarse-grained biotite granite; ranges in composition from quartz monzonite to quartz diorite. Fabric shows a southeastward progression from protomylonite to mylonite to ultramylonite where the unit is cut by the Brookneal shear zone (Gates and others, 1986). Mineralogy: quartz + plagioclase + potassium feldspar + biotite + muscovite + chlorite + epidote + titanite + garnet + mag ne tite-ilmenite + calcite + zircon (Gates, 1981). Geophysical signature: positive radiometric, negative magnetic. The Melrose has been dated at 515 Ma (U-Pb zircon; Sinha and others, 1989).
Sandstone, very fine- to coarse-grained, reddish-brown to gray, micaceous, minor conglomerate beds. Siltstone, reddish-brown to gray, micaceous. Shale, reddish-brown, greenish-gray, gray, yellowish-brown, laminated, fossiliferous. Upward-fining sequences, discontinuous vertically and horizontally.
Rounded to subangular pebbles, cobbles, and boulders of mixed lithologies including quartz, phyllite, quartzite, gneiss, schist, greenstone, and marble in a matrix of medium- to very-coarse-grained, reddish-brown to gray, locally arkosic, sandstone.
White to gray, fine- to medium grained, massive to layered quartz diorite gneiss, contains minor biotite and epidote; lenses of gray to black, medium-grained, layered hornblende-plagioclase gneiss and quartz-epidote clinopyroxene- hornblende-plagioclase gneiss occur locally (Tobisch, 1972).
Diorite, gray, fine- to medium-grained, well-foliated, locally trondhjemitic; composition locally grades into granodiorite; quartz diorite, white to gray, fine- to medium-grained, massive to layered, foliated. Mineralogy: plagioclase + quartz + biotite + amphibole + epidote.
Porphyroblastic mica schist, characterized by 1- to 2-mm garnet porphyroblasts in an anastomosing, greenish-black biotite-rich, schistose matrix. Most exposures show complex microstructures suggestive of polyphase deformation and superimposed shearing. In Appomattox and Campbell counties, and locally elsewhere,this unit includes quartzose muscovite schist along the western and eastern margins of the outcrop belt (presumed base of the stratigraphic section); locally the unit contains thin interbeds of calcareous mica schist and marble. Mineralogy: biotite + garnet + muscovite + quartz + plagioclase + magnetite ± kyanite ± calcite. Geophysical signature: characterized by elongate positive magnetic and radiometric anomalies. This unit was mapped in strike-belts southwest of, and not physicaly connected to the type section at Arvonia.
Buffalo granite (Laney, 1917). Light-gray, coarse-grained, porphyritic granite to granodiorite. Mineralogy: quartz + potassium feldspar + plagioclase + biotite; accessory minerals include apatite and zircon. Hydrothermal alteration to sericite, epidote, kaolin, and chlorite is common. Geophysical signature: positive radiometric anomaly associated with elevated potassium and thorium values. This unit is included in the Clarksville batholith of Horton and others (1993).
Metaconglomerate (Kreisa, 1980). Metamorphosed sandy conglomerate, conglomeratic sandstone, and granule conglomerate containing pebbles and cobbles of polycrystalline quartz, with lesser diorite, diabase, chert, rhyolite, granite, schist, and quartz arenite. Cobbles are generally less than 2 cm but may range up to 25 cm in diameter (Kreisa, 1980). Unit occurs as beds of 0.3 to 6.1 meters thickness at base of and locally within the lower portion of Aaron section. Aaron slate (Zas) and metaconglomerate (Zac) herein includes the upper and lower members of the Aaron Formation of Kreisa (1980), and the Aaron Formation and upper portion of the Virgilina Formation of Harris and Glover (1985).
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.
Abbeyville gabbro (Laney, 1917). Dark-greenish gray, coarse-grained metamorphosed gabbro; includes some soapstone. Mineralogy: uralitic hornblende + plagioclase + (zoisite, clinozoisite, epidote) + magnetite ± quartz; kaolin, sericite, chlorite, and calcite are also present. Geophysical signature: positive magnetic anomaly.
White to light-gray, medium- to coarse-grained, well-foliated, locally crinkle-folded, quartzose kyanite schist and kyanite-bearing quartzite. Mineralogy: kyanite + quartz + muscovite ± graphite ± pyrite ± garnet; kyanite constitutes as much as 30 percent of the rock. Kyanite-bearing quartzites and schists have been correlated with quartzose muscovite schists that occur locally at the base of the Arvonia Formation (Conley and Marr, 1980). Those workers report primary sedimentary structures in kyanite quartzite including wedge-shaped quartzite layers and quartzite-metapelite couplets; fining-upward sequences; channel fillings; and, large-and small-scale cross-beds.
Shale, light-greenish gray, light- to dark-gray, carbonaceous, and reddish-brown in cyclic sequences, laminated, silty to sandy, fossiliferous. Siltstone, typically reddish-brown to gray, sandy, micaceous, with minor fine-grained sandstone beds.
Fine- to coarse-grained, reddish-brown to gray, primary bedding features such as cross-beds, channel lags, and ripple marks , minor conglomerate, siltstone, and shale beds.
Light-gray, medium-grained, foliated granite to granodiorite. Mineralogy: quartz + potassium feldspar + plagioclase + biotite; accessory minerals include muscovite, apatite, and zircon. Hydrothermal alteration to sericite, epidote, kaolin, and chlorite is common. Geophysical signature: positive radiometric anomaly. This unit is equivalent in part to the Red Oak granite as defined by Laney (1917) and Horton and others (1993).