Geologic units in Fluvanna county, Virginia

Metagraywacke, Quartzose Schist, and Melange (Proterozoic Z-Cambrian) at surface, covers 45 % of this area

Metagraywackes are quartzose chlorite or biotite schists containing very fine to coarse granules of blue quartz; primary graded laminations have been transposed by shearing into elongate lozenges that give the rock a distinctive pin-striped appearance in weathered surfaces perpendicular to schistosity. A mylonitic fabric is superimposed in varying degrees, as are late-stage chevron-style folds. In Buckingham, Appomattox, and Campbell Counties, rocks in this unit are progressively more tectonized from east to west across the outcrop belt; in the western portion, the dominant lithology is a polydeformed, mylonitic mica schist with abundant quartz-rich boudins; transposed pinstriped lamination or segregation layering at a high angle to mylonitic schistosity is characteristic. Metagabbroic blocks ranging in size from 5 cm to 3 m across and larger have been identified at widely scattered locations throughout the outcrop belt. Mineralogy: quartz + albite + epidote + chlorite + muscovite + magnetite ± chloritoid ± calcite; biotite- and staurolite-bearing assemblages occur in Appomattox and Campbell Counties. Detrital minerals identifi ed in thin section include plagioclase, perthite, epidote, magnetite, tourmaline, and titanite. Lithic fragments include dacite tuff, gabbro, and monocrystalline quartz with zircon and biotite inclusions (Evans, 1984). The northern portion of the outcrop belt includes melange zone IV of the Mine Run complex of Pavlides (1989; 1990). In Albemarle and Fluvanna counties, CZpm includes the lower chlorite-muscovite unit of Smith and others (1964) and Hardware metagraywacke of Evans (1984). In Appomattox and Buckingham Counties, polydeformed quartzose mica schists in the western portion of the outcrop belt are lithologically indistinguishable from schists mapped as Fork Mountain Formation in structural blocks that occur to the west; these units are considered to be at least in part correlative. In Campbell County, polydeformed metagraywacke and mica schist is intruded by the Cambrian-age Melrose Granite (Cm).

Chopawamsic Formation - Interlayered felsic and mafic metavolcanic rocks (Cambrian) at surface, covers 22 % of this area

Chopawamsic Formation, undivided, (Pavlides, 1981). Includes laterally discontinuous lenses and tongues of metamorphosed felsic, intermediate, and mafic volcanic flows and volcanoclastic rocks, with interlayered quartzite, quartzose graywacke, schist, and phyllite. Volcanic flows are locally highly vesicular; fragmental breccia and tuff are common. Felsic flows are typically light-gray aphanitic rocks with phenocrysts of quartz and feldspar; intermediate flows are dark-green amphibole-bearing rocks with fine-grained quartz-feldspar matrix; greenstone metabasalts contain blue green amphibole, chlorite, albitic plagioclase, and quartz. Geophysical signature: linear strike-elongate pattern of elevated magnetic anomalies. The Chopawamsic is correlated with the James Run Formation in Maryland; the James Run has been dated at 570 to 530 Ma (U-Pb zircon; Tilton and others 1970). The Chopawamsic is unconformably overlain by the Late Ordovician Arvonia and Quantico Formations. Pavlides (1981 and subsequent works) has made the interpretation on the basis of geologic and geochemical data that the Chopawamsic and related plutons represent an ancient is land-arc sequence.

Carysbrook Pluton (Proterozoic) at surface, covers 12 % of this area

Carysbrook pluton. Light-gray, medium- to coarse grained, massive to indistinctly foliated biotite granite. Mineralogy: quartz + potassium feldspar + plagioclase + biotite + chlorite + muscovite + epidote. Geophysical signature: diffuse pattern of elevated radiometric anomalies. Although Smith and others (1964) included the Carysbrook pluton in a granodiorite unit with granitoid rocks in the vicinity of Columbia, Stose and Stose (1948) recognized that the granite at Carysbrook was different in texture and composition from the granodiorite at Columbia. Our mapping affirms that these are separate plutons. The Carysbrook is unconformably overlain by the Arvonia Formation; this relation is well-exposed in an abandoned railroad cut south of Carysbrook (Smith and others, 1964). The pluton intrudes the Chopawamsic Formation.

Mine Run Complex - Melange Zone III (Proterozoic Z-Ordovician) at surface, covers 11 % of this area

Melange zone III (Pavlides, 1989). Phyllite and schist matrix contains abundant euhedral magnetite; many matrix rocks are highly deformed on a mesoscopic and microscopic scale. Mafic exotic blocks (mf) include amphibolite, ultra mafic rocks, serpentinite, and talc; many mafic and ultra mafic blocks are composite. Biotite gneiss blocks (gn) are also present. Metavolcanic olistoliths (vo) are rare. Geophysical signature: Strong positive magnetic anomaly. This unit is intruded by the Ellisville biotite granodiorite (SOe).

Arvonia Formation - Slate and porphyroblastic schist (Ordovician) at surface, covers 4 % of this area

Dominantly dark gray to grayish-black, lustrous, very-fine-grained, graphitic slate (northeastern sector); and, medium-grained, porphyroblastic garnetiferous biotite schist (southwestern sector). Discontinuous beds of quartzose muscovite schist, coarse grained to pebbly micaceous quartzite, and conglomeratic schist occur along the margins of the outcrop belt, stratigraphically at the base of the section. Interbeds of dacite metatuff occur in the western portion of the slate outcrop belt. Graded laminated metasiltstone and metasandstone are interbedded with slate in the central and eastern portions of the outcrop belt at the latitude of the James River (Evans and Marr, 1988); these rocks pass into porphyroblastic schists at higher metamorphic grades to the southwest. A distinctive garnet-amphibole-quartz interbed occurs within porphyroblastic schist south of the James River (volcanogenic marker?; Brown, 1969); north of the river, this passes down metamoprphic grade into what is described as an oolitic chlorite schist (Smith and others, 1964). Mineralogy: (slate) chlorite + mus co vite + plagioclase + quartz + magnetite ± biotite ± calcite ± graphite ± pyrite; (porphyroblastic schist) biotite + muscovite + garnet + quartz + plagioclase + magnetite ± kyanite ± calcite; tourmaline and zircon are common accessories. Geophysical signature: The Arvonia is marked by positive magnetic and radiometric anomalies. Originally referred to as the slate in the Arvonia belt by Rogers (1884), the unit was named Arvonia slate by Stose and Stose (1948), and raised to formation status by Brown (1969). An Upper Ordovician age for the Arvonia has been established from fossils collected by Watson and Powell (1911), Stose and Stose (1948), Tillman (1970), and Kolata and Pavlides (1986). The Arvonia has long been considered unconformable on top of adjacent units. Micaceous quartzite, pebbly mus covite schist, and conglomeratic schist are common at the base of the section where that boundary is not faulted (Stose and Stose, 1948; Smith and others, 1964; Brown, 1969; Evans and Marr, 1988). The base of the Arvonia is exposed in an old railroad cut near Carysbrook, Fluvanna County (Smith and others, 1964); there, a micaceous quartzite containing quartz pebbles rests on granite of the Carysbrook pluton. The Stoses (1948) considered the Arvonia a sequence deposited on a post-Taconic-orogeny regional unconformity, and folded and metamorphosed during subsequent orogenies; that interpretation is consistent with geologic constraints as we know them today. The Arvonia is correlated with the Quantico Formation. History of the Arvonia district slate industry is discussed by Brown (1969) and Evans and Marr (1988).

Chopawamsic Formation - Mafic to intermediate metavolcanic rocks (Cambrian) at surface, covers 3 % of this area

Dark greenish-gray, fine- to medium-grained, foliated, lineated, amphibole-bearing gneiss and schist. Also includes tuffaceous volcaniclastic rocks. Mineralogy: amphibole + acicular actinolite + epidote + chlorite + titanite + plagioclase + magnetite.

Interlayered Mafic and Felsic Metavolcanic Rocks (Cambrian) at surface, covers 0.9 % of this area

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.

Quantico Formation - Slate and porphyroblastic schist (Ordovician) at surface, covers 0.8 % of this area

Gray to black, graphitic, pyritic phyllite and slate (northern Piedmont); metamorphic grade increases to the southwest to produce porphyroblastic staurolite-, kyanite-, and garnet-biotite muscovite schists. Locally the unit contains felsic metatuff, metagraywacke, and micaceous quartzite interbeds; thickness has been estimated at as much as 3000 feet (Pavlides, 1980). Mineralogy: quartz + muscovite + biotite ± garnet ± staurolite ± kyanite + opaque minerals; chlorite is a common secondary mineral. Geophysical signature: strike-elongated positive linear magnetic and radiometric anomalies. The unit was originally named Quantico Slate by Darton (1894), and modified to Quantico Formation by Pavlides (1980). An Ordovician age for the Quantico is indicated by fossils collected by Watson and Powell (1911) and more recently by Pavlides and others (1980). The Quantico unconformably overlies older units in the northeastern Pied mont, and is correlated with the Arvonia Formation to the southwest.

Columbia Pluton (Ordovician) at surface, covers 0.7 % of this area

Light-gray, medium- to coarse grained, foliated. Includes biotite-muscovite granite, granodiorite, tonalite, and granitic pegmatite; contains xenoliths of biotite gneiss, amphibolite, and felsic metavolcanic rocks. Mineralogy: plagioclase + quartz + microcline; common accessories include biotite, muscovite, epidote, zircon, apatite, garnet, magnetite, and pyrite (Bourland and Glover, 1979). Geophysical signature: diffuse magnetic lows and radiometric highs. The pluton was originally named Columbia Granite by Jonas (1928); this name was objected to by later workers because of the relatively small percentage of true granite present. The pluton includes the southeastern portion of the granodiorite unit of Smith and others (1964). Granitic rocks in the Carysbrook area of Fluvanna County are here considered part of a separate Carysbrook pluton, following the usage of Stose and Stose (1948). The Columbia includes, in part, the Hatcher complex of Brown (1969). Bourland and Glover (1979) refer to the pluton as the Columbia metagranite. Given the heterogeneous nature of the pluton, multiple intrusive phases are likely present. Tonalite in the eastern part of the pluton has yielded ages of 590+/-80 Ma, (Rb-Sr whole-rock; Fullagar, 1971). Mose and Nagel (1982) report a Rb-Sr whole-rock age for the Columbia of 454±9 Ma. Because samples for this age are described as coming from the western portion of the Columbia, it is possible that the rocks dated were taken from what is herein mapped as the Carysbrook pluton (grc).

Ellisville Biotite Granodiorite (Ordovician-Silurian) at surface, covers 0.7 % of this area

Ellisville biotite granodiorite (Pavlides, 1990). Mesocratic, coarse- to medium-grained, equigranular to porphyritic, massive to strongly foliated granodiorite. Mineralogy: quartz + plagioclase + potassium feldspar + biotite; accessories include epidote, allanite, titanite, and apatite. Porphyritic rocks contain potassium feldspar megacrysts up to 1.5 cm across; myrmekite commonly occurs adjacent to potassium feldspar. Brownish-green, strongly pleochroic biotite is associated with, and in places poikilitically encloses epidote, allanite, titanite, and apatite. Subhedral epidote locally encloses euhedral titanite. Pleochroic green amphibole and muscovite are minor constituents locally. The Ellisville has been dated at 440±8 Ma (Rb-Sr whole rock; Pavlides and others, 1982).

Ta River Metamorphic Suite (Cambrian) at surface, covers 0.3 % of this area

Ta River Metamorphic Suite, (undivided). Layered sequence consists dominantly of greenish-gray to black, medium- to coarse-grained, poorly to well-lineated, massive to well-layered amphibolite and amphibole-bearing gneiss and schist; includes interlayered ferruginous quartzite, and minor biotite gneiss, felsic volcanic rocks, gabbro and granite. Amphibolitic rocks commonly contain quartz-epidote lenses and veins. Porportion of biotite gneiss and schist in creases from northeast to southwest along strike, as does grade of regional metamorphism. Mineralogy: (horn blende, tremolite-actinolite, and cummingtonite) + quartz + calcic oligoclase ± epidote ± biotite ± garnet. Geophysical signature: linear positive and negative magnetic and radiometric anomalies. Pavlides (1981) correlated the Ta River with the Chopawamsic and James Run Formations, and considered the Ta to be a more oceanward facies of a Chopawamsic island arc sequence, on the basis of geologic and geochemical factors. The Quantico Formation generally overlies the boundary between the Chopawamsic and the Ta, obscuring the contact relationships.

Arvonia Formation - Bremo quartzite. Quartzite (Ordovician) at surface, covers 0.2 % of this area

Bremo quartzite (Stose and Stose, 1948). Light-gray, fine- to medium-grained, thick-bedded and locally crossbedded quartzite; includes quartz-muscovite schist, and quartz-pebble conglomerate. Mineralogy: quartz + muscovite + chlorite ± plagioclase ± potassium feldspar ± calcite ± magnetite-hematite ± zircon. The Bremo reportedly contains crinoid stems and brachiopods (Smith and others, 1964; Brown, 1969). Stose and Stose (1948) and Smith and others (1964) considered the quartzite stratigraphically above the slates and schists; Brown (1969) cited structural evidence that the Bremo does not occupy a position at the top of the Arvonia, but occurs locally in the middle or lower part. Smith and others (1964) and Brown (1969) considered the Bremo a member of the Arvonia Formation.

Quantico Formation - Micaceous quartzite (Ordovician) at surface, covers 0.2 % of this area

Light-gray, fine- to medium grained quartzite and quartzose muscovite schist. Mineralogy: quartz + muscovite + plagioclase ± microcline. This lithology occurs as thin discontinuous lenses at the base of the Quantico; thin diopsidic calcsilicate layers are also found locally in the lower part of the Quantico (Pavlides, 1980).

Interlayered Mafic and Felsic Metavolcanic Rocks - Amphibolite, hornblende-biotite gneiss, and schist. (Cambrian) at surface, covers < 0.1 % of this area

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.

Chopawamsic Formation - Foliated felsite (Cambrian) at surface, covers < 0.1 % of this area

Light-gray to white, medium-grained, foliated felsite ranges in composition from rhyolite to dacite. Mineralogy: quartz + perthitic microcline + muscovite + biotite; beta-form quartz phenocrysts are characteristic.