Geochronologic Age Constraints on Tectonostratigraphic 

Units of the Central Virginia Piedmont, USA

Professional Paper 1861

U.S. Department of the Interior

U.S. Geological Survey
Cover. Scanning electron microscopy–back-scatter electron detector image of a zircon aggregate from biotite-muscovite migmatitic 

paragneiss at the northern terminus of the Elk Hill Complex, central Virginia Piedmont. Image captured by Ryan J. McAleer, U.S. Geological 

Survey, using a Hitachi SU5000 field emission scanning electron microscope.
Geochronologic Age Constraints on 

Tectonostratigraphic Units of the Central 

Virginia Piedmont, USA

By Mark W. Carter, Ryan J. McAleer, Christopher S. Holm-Denoma, 

David B. Spears, Sean P. Regan, William C. Burton, and Nick H. Evans

Professional Paper 1861

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

DAVID BERNHARDT, Secretary

U.S. Geological Survey

James F. Reilly II, Director

U.S. Geological Survey, Reston, Virginia: 2020

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Suggested citation:

Carter, M.W., McAleer, R.J., Holm-Denoma, C.S., Spears, D.B., Regan, S.P., Burton, W.C., and Evans, N.H., 2020, 

Geochronologic age constraints on tectonostratigraphic units of the central Virginia Piedmont, USA: U.S. Geological 

Survey Professional Paper 1861, 28 p., https://doi.org/10.3133/pp1861.

ISSN 2330-7102 (online)
iii

Acknowledgments

The authors would like to thank the many landowners who gave us access to their properties 

during geologic mapping and sample collection, especially Pembroke Pettit, Christian Goodwin, 

and Selene Deike. We would also like to thank Arthur Merschat, Wright Horton, Greg Walsh, 

and Randy Orndorff (U.S. Geological Survey) and Brent Owens (College of William and Mary) for 

their thorough reviews, which greatly improved an earlier version of this manuscript.
iv

Contents

Acknowledgments........................................................................................................................................iii

Abstract...........................................................................................................................................................1

Introduction.....................................................................................................................................................1

Geologic Setting....................................................................................................................................3

Sample Descriptions ............................................................................................................................6

Analytical Methods........................................................................................................................................9

Results ...........................................................................................................................................................11

Discussion.....................................................................................................................................................18

Age of the Quantico Formation.........................................................................................................18

Age of Long Island Syncline Rocks .................................................................................................18

Chopawamsic Formation Stratigraphy............................................................................................21

Age of Granodioritic-tonalitic Sheet Intrusions.............................................................................21

Metamorphic and Structural Considerations Along the East Flank of the Chopawamsic 

Terrane ....................................................................................................................................21

Conclusions...................................................................................................................................................22

References Cited..........................................................................................................................................23

Appendix 1. Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS) 

Methodology ...................................................................................................................................27

Appendix 2. Secondary Ionization Mass Spectrometry Methodology............................................27
v

Figures

1. Map showing location of study area in A, the Appalachian orogen and B, 

a generalized geologic map of Virginia.....................................................................................2

2. Map showing regional bedrock geology map of the central Virginia Piedmont................5

3. Photographs of rocks and structures from the central Virginia Piedmont.........................8

4. Plots of laser ablation-inductively coupled plasma-mass spectrometry detrital 

zircon data and sensitive high-resolution ion microprobe detrital zircon data 

from central Virginia Piedmont samples collected during this study ................................10

5. Scanning electron microscopy–back-scatter electron detector and scanning 

electron microscopy–cathodoluminescence images of representative zircon 

grains and plots of isotopic data from the samples analyzed on the Stanford/

U.S. Geological Survey sensitive high-resolution ion microprobe-reverse 

geometry ......................................................................................................................................13

6. Scanning electron microscopy–back-scatter electron detector and scanning 

electron microscopy–cathodoluminescence images of representative zircon 

grains and plots of isotopic data from the samples analyzed on the Stanford/

U.S. Geological Survey sensitive high-resolution ion microprobe-reverse 

geometry ......................................................................................................................................15

7. Plane polarized light, scanning electron microscopy–back-scatter electron 

detector and photoluminescence images of representative zircon grains from 

sample BK601 analyzed on the Stanford/U.S. Geological Survey sensitive 

high-resolution ion microprobe-reverse geometry...............................................................16

8. Plots of isotopic data from sample BK601 analyzed on the Stanford/U.S. Geological 

Survey sensitive high-resolution ion microprobe-reverse geometry ................................17

9. Comparative relative probability plots of detrital zircon data from samples dated 

by laser ablation-inductively coupled plasma-mass spectrometry and sensitive 

high-resolution ion microprobe-reverse geometry during this study, with published 

data from the central Piedmont in Virginia and Maryland...................................................18

10. Schematic diagram of Chopawamsic Formation stratigraphy in relation to 

overlying units and intrusive rocks in the central Virginia Piedmont, based 

on field relations and geochronology......................................................................................20

Tables

1. Summary of sample locations, dating methods, and age results of this study. ................7

2. Isotopic data for all analyses by laser ablation-inductively coupled plasmamass spectrometry at the U.S. Geological Survey Central Mineral and 

Environmental Resources Science Center Isotope Laboratory in Denver, 

Colorado .............................................................................................................. available online

3. Isotopic data for all analyses by secondary ionization mass spectrometry 

on the U.S. Geological Survey/Stanford sensitive high-resolution ion 

microprobe-reverse geometry ........................................................................ available online
vi

Conversion Factors

International System of Units to U.S. customary units

Multiply By To obtain

Length

micrometer (μm) 0.00003937 inch (in.)

centimeter (cm) 0.3937 inch (in.)

millimeter (mm) 0.03937 inch (in.)

meter (m) 3.281 foot (ft) 

kilometer (km) 0.6214 mile (mi)

Area

square centimeter (cm2) 0.001076 square foot (ft2)

square meter (m2) 10.76 square foot (ft2) 

square centimeter (cm2) 0.1550 square inch (in2) 

Volume

cubic centimeter (cm3) 0.06102 cubic inch (in3) 

Flow rate

liter per second (L/s) 15.85 gallon per minute (gal/min) 

liter per minute (L/min) 0.2642 gallon per minute (gal/min)

Density

gram per cubic centimeter (g/cm3) 62.4220 pound per cubic foot (lb/ft3) 

Energy

joule (J) 0.738 foot-pound (ft·lb)

Abbreviations

3DEP Three-Dimensional Elevation Program

BSE back-scatter electron

CL cathodoluminescence

CVSZ Central Virginia Seismic Zone

Ga billion years old

ID isotope dilution

M magnitude

Ma million years old

MSWD mean square of weighted deviates

LA-ICP-MS laser ablation-inductively coupled plasma-mass spectrometry

LST lithium heteropolytungstate

PPL plane polarized light

ppm parts per million

SHRIMP-RG sensitive high-resolution ion microprobe-reverse geometry

TIMS thermal ionization mass spectrometry

USGS U.S. Geological Survey

UV ultraviolet light

< less than

> greater than
Geochronologic Age Constraints on Tectonostratigraphic 

Units of the Central Virginia Piedmont, USA

By Mark W. Carter1, Ryan J. McAleer1, Christopher S. Holm-Denoma1, David B. Spears2, Sean P. Regan3, 

William C. Burton1, and Nick H. Evans4

Abstract

New geologic mapping coupled with uranium-lead 

(U-Pb) zircon geochronology (sensitive high-resolution 

ion microprobe-reverse geometry [SHRIMP-RG] and laser 

ablation-inductively coupled plasma-mass spectrometry 

[LA-ICP-MS]) analyses of 10 samples, provides new constraints on the tectonostratigraphic framework of the central 

Virginia Piedmont. Detrital zircon analysis confirms that the 

Silurian-Devonian Quantico Formation is a postorogenic 

successor basin, with zircons derived primarily from Ordovician Chopawamsic Formation volcanic rocks. Detrital zircons 

from strata of the Long Island syncline, previously mapped as 

a separate successor basin, have a peri-Gondwanan component distinct from Laurentian-sourced rocks of the Potomac 

terrane to the west. Volcanism of the Chopawamsic Formation 

spanned at least 14 million years during the Ordovician. The 

Chopawamsic Formation contains sheet-like Late OrdovicianSilurian granodioritic and tonalitic intrusions that were once 

mapped as Carboniferous. Biotite-muscovite migmatitic 

paragneiss, which borders the Chopawamsic Formation on 

its southeast side and also occurs east of the Lakeside fault, 

preserves evidence of Silurian deformation and metamorphism, with a Carboniferous (Alleghanian) overprint. Limited 

SHRIMP-RG analysis of detrital zircons from this paragneiss 

yields a Laurentian (Mesoproterozoic) signature, which suggests that the structurally concordant contact between volcanic 

rocks of the Chopawamsic Formation and paragneiss is either 

a pre-Alleghanian fault or an unconformity.

Introduction

Precision uranium-lead (U-Pb) zircon geochronology is 

a critical tool in conjunction with detailed geologic mapping 

to provide ages of rock units and their mineral constituents to 

bracket regional tectonic events. This is particularly true in the 

Piedmont Province of central Virginia (figs. 1 and 2), where 

metamorphic rocks in the Appalachian hinterland record 

tectonic events that are separated by just a few million years 

(for example, Sinha and others, 2012; Hughes and others, 

2013). The regional structure is of special interest because of 

its relation to the historically active Central Virginia Seismic 

Zone (CVSZ), including the epicentral region of the 2011 

magnitude (M)5.8 Mineral, Virginia, earthquake (Horton and 

Williams, 2012).

To better understand the regional framework for seismic 

risk assessment, cooperative detailed geologic mapping across 

nine 7.5-minute quadrangles in the central Virginia Piedmont 

was conducted by the U.S. Geological Survey and Virginia 

Department of Mines, Minerals and Energy, Division of 

Geology and Mineral Resources following the 2011 Mineral, 

Virginia, earthquake. This study builds on earlier mapping by 

Pavlides (1981, 1989, 1990) in the northern Virginia Piedmont, Spears (2011) along the James River to the south, and 

Hopkins (1960) and Hughes (2014) in the epicentral area of 

the 2011 earthquake.

We used both sensitive high-resolution ion microprobereverse geometry (SHRIMP-RG) and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) U-Pb 

zircon geochronology to help answer three outstanding questions in this geologically complex region. (1) Can stratigraphic 

succession be established within the Ordovician Chopawamsic Formation? (2) When was the onset of the postorogenic 

successor basin development above or near the volcanic arc 

represented by the Chopawamsic Formation? (3) What was the 

timing of deformation and metamorphism in the easternmost 

1

U.S. Geological Survey.

2

Virginia Department of Mines, Minerals and Energy.

3

University of Alaska-Fairbanks.

4

Center for Sustainable Groundwater.
2 Geochronologic Age Constraints on Tectonostratigraphic Units of the Central Virginia Piedmont, USA

Smith River allochthon

Blue Ridge

Eastern

Milton

belt

CT

CP

CSB

RRT

GT

PT

Raleigh belt

EXPLANATION

Extent of figure 2

Richmond

Storck

Palmyra

Mineral

Washington, D.C.

Coastal Plain

Mesozoic basins

Piedmont Terranes

and belts

Blue Ridge

subprovinces

Valley and Ridge

Cumberland Plateau

Geologic provinces, terranes, and belts

Location of city and name

Abbreviations:

PT

CT

GT

CP

CSB

RRT

Potomac terrane

Chopawamsic terrane

Goochland terrane

Central Piedmont

Carolina Slate Belt

Roanoke Rapids terrane 

Roanoke

Roanoke

Staunton

Winchester

Laurentian Realm

Iapetan Realm

Piedmont/Goochland

Peri-Laurentian arc/Peri-Gondwana arc

Peri-Gondwanan Realm

Carolina

Ganderia

Avalonia

Meguma

EXPLANATION

80°

70°

60°

50°

40°

VIRGINIA

A. MAP OF THE APPALACHIAN OROGEN

B. GEOLOGIC MAP OF VIRGINIA

0 100 200 KILOMETERS

0 50 100 MILES

0 200 400 KILOMETERS

0 200 MILES 100

Figure 1. Location of study area in A, the Appalachian orogen (modified from Hibbard and others, 2006) and B, a generalized 

geologic map of Virginia (modified from Virginia Division of Mineral Resources, 1993). Inset box on the geologic map of Virginia 

denotes area in figure 2.
Introduction 3

extent of the Chopawamsic Formation, and how did these 

events relate to the emplacement and juxtaposition of rocks of 

the Elk Hill Complex and Goochland terrane along the Lakeside and Spotsylvania fault zones?

Geologic Setting

A west-to-east traverse across the central Virginia 

Piedmont intersects several major rock units of varying age 

and crustal affinity (fig. 2). In the west, rocks of the early 

Paleozoic Potomac terrane (Horton and others, 1989) consist 

mostly of phyllitic metaclastic rocks (Mine Run Complex of 

Pavlides, 1989) that locally contain exotic blocks of metaplutonic, metavolcanic, metamafic, and metaultramafic rock 

types several meters and larger in size. These rocks occur to 

the east of the Mountain Run fault, the boundary between the 

Blue Ridge and Piedmont Provinces in this part of the central 

Appalachians (Pavlides, 1994). Burton and others (2015a) and 

Evans (2017) redefined lithostratigraphic units in this part of 

the Potomac terrane by replacing the fault-bounded Mine Run 

Complex units of Pavlides (1989) with (1) the informal Shores 

complex of Brown (1986) in the footwall of the Chopawamsic 

fault, which includes a narrow belt with mafic and ultramafic 

bodies bounded to the west by the Byrd Mill fault and the 

informal Byrd Mill formation of Burton and others (2014) 

in the footwall of the Byrd Mill fault; and (2) the informal 

Hardware formation of Evans (1984), located structurally 

beneath the Byrd Mill formation and in the hanging wall of the 

Mountain Run fault. In this schema, only rocks of the Shores 

complex between the Byrd Mill and Chopawamsic faults 

contain exotic mélange blocks of mafic and ultramafic rocks; 

Potomac terrane metaclastic rocks to the west are interpreted 

to be Laurentian slope-rise strata that were intruded by granitoid plugs, stocks, and small plutons.

Granodiorite of the approximately 444–436 million years 

old (Ma) Ellisville pluton (Hopkins, 1960; Pavlides and others, 1994; Hughes and others, 2013) intrudes both the Potomac 

terrane and the Chopawamsic Formation (fig. 2) and provides 

a temporal constraint for when the volcanic arc represented by 

the Chopawamsic Formation docked with Laurentia (Hughes 

and others, 2013). Other regional plutonic rocks of similar 

age include the Lahore, Green Springs, Carysbrook, and 

Columbia plutons.

The Chopawamsic Formation (Southwick and others, 

1971) is a heterogeneous unit consisting of interlayered felsic, 

intermediate, and mafic metavolcanic and metasedimentary 

rocks of island-arc affinity (Pavlides, 1981). Rocks of this 

formation occur in separate eastern and western belts that 

are separated by Quantico Formation rocks in the core of the 

Quantico synclinorium (fig. 2). Rocks of felsic and intermediate composition (leucocratic felsic gneiss and biotite gneiss, 

schist, and rare quartzite) dominate the western belt, whereas 

rocks of mostly mafic composition (amphibolite) are interlayered with Ordovician-Silurian sheet-intrusive granitoids in 

the eastern belt. For this reason, Pavlides (1980) interpreted 

rocks in the eastern belt to be a separate unit he termed the 

Ta River Metamorphic Suite. Sparse altered ultramafic rocks 

(amphibole schist and talc-chlorite schist) are associated with 

amphibolite in the eastern belt. Granodioritic to tonalitic 

sheet intrusions and associated pegmatites in the eastern belt 

were interpreted as the Carboniferous (325–300 Ma) Falmouth Intrusive Suite (Pavlides, 1980) by Pavlides and others 

(1982b) from concordant U-Pb zircon ages and two rubidiumstrontium (Rb-Sr) whole-rock isochrons. Mixon and others 

(2000) named one of these intrusive bodies near Lake Anna as 

the Elk Creek pluton, with no supporting geochronologic data.

In the western belt, a through-going and nearly continuous amphibolite body near Contrary Creek (Mineral quadrangle, fig. 2) separates Chopawamsic Formation rocks into a 

lower section of felsic gneiss and biotite gneiss, and an upper 

section of mostly felsic meta-volcanic gneiss and schist. The 

top of this variably thick mafic unit, which has been the target 

zone for extensive iron, precious metal, and sulfide mining 

since the 1800s (for example, Luttrell, 1966; Pavlides and 

others, 1982a; Sauer, 1984), is interpreted as an unconformity. 

Elsewhere, rocks in the cores of regional synclines define 

major stratigraphic horizons. Ferruginous quartzite, metasiltstone, and graphitic schist in the Long Island and subsidiary 

synclines cap the western belt of the Chopawamsic Formation, whereas in the eastern belt a syncline of Chopawamsic 

Formation felsic gneiss flanks a larger anticlinorium cored by 

amphibolite and sheet intrusions (fig. 2).

Coler and others (2000) reported a thermal ionization 

mass spectrometry (TIMS) U-Pb age of 471.4±1.3 Ma for 

a sample of Chopawamsic Formation rhyolite from near 

Palmyra, Virginia (fig. 2). Coler and others (2000) also dated a 

nearby metabasalt in the western belt of felsic gneiss (see their 

fig. 2, p. 365) at 470.0+1.3/−1.5 Ma and concluded that the Ta 

River Metamorphic Suite is equivalent in age and correlative 

with the Chopawamsic Formation. Bailey and others (2005) 

mapped Chopawamsic Formation around the polydeformed 

south terminus of the Quantico synclinorium (fig. 2) near 

Columbia, Virginia, just north of the James River, but Pavlides 

and others (1994) considered the Ta River Metamorphic Suite 

to be Cambrian in age. Hughes and others (2014a,b) reported a 

TIMS U-Pb zircon age of approximately 468 Ma for metavolcanic rocks (metafelsite) of the Chopawamsic Formation, and 

a strongly unimodal age of approximately 467 Ma for detrital 

zircons from interlayered metasedimentary rocks. Hughes 

and others (2014a) concluded that the Chopawamsic Formation metasedimentary rocks were mostly recycled from its 

syndepositional volcanic pile, but they did record a scattering 

of Cambrian (approximately 537–510 Ma), Neoproterozoic 

(approximately 640–575 Ma) and Mesoproterozoic (approximately 1.17–1.01 billion years old [Ga]) detrital zircons. 

Horton and others (2010) reported a younger SHRIMP-RG 

U-Pb zircon age of approximately 453 Ma for Chopawamsic 

Formation metavolcanic rocks in northern Virginia.

The Quantico Formation (Pavlides, 1980) consists predominantly of garnet-mica schist, with local occurrences of 
4 Geochronologic Age Constraints on Tectonostratigraphic Units of the Central Virginia Piedmont, USA

Mineral Lake Anna

West

Lahore

South

Anna

Pendleton Buckner

Louisa

Zion

Crossroads

Ferncliff

Ygg

Ysf

PzYm

PzYm

PzYm

Pzg

Pzg

QNK

QNK

Ysa

Yma

Zl

Zl

Zl

Zl

Zc

SOg

SOg

SOg

SOg

SOg

SOg

DSq

DSa

lis

Zc

Mountain

Run

fault

Ocm

Ocf

Ocf

Ocf

pb

ehc

sc

sc

CZm

CZm

CZm

ZYp

Byrd

Mill

fault

5.8

Hylas

fault

Spotsylvania

shear

Lakeside

fault

Chopawamsic

fault

zone

Ellisville

pluton

Lp

GSp

Cbp

2

3

6

1

4

5

8

7

10

9

Geology from Smith and others, 1964; Brown, 1969; Goodwin, 1970; Bobyarchick and others, 1981; 

Pavlides, 1990, 1995; Rossman, 1991; Virginia Division of Mineral Resources, 1993; Marr, 2002; 

Bailey and others, 2005; Spears, 2011; Weems, 2012, 2013; Burton and others, 2015a; 

Evans and Farrar, 2015; Hughes and others, 2015; and unpublished geologic maps

Base from U.S. Geological Survey 3DEP National Elevation Dataset

James

River

South

Ann a River

Riva nna River

Lake

Anna

Rapidan River

Rapidan Rive r

North Anna River

38 ̊

38 ̊15'

37 ̊45'

78 ̊15' 78 ̊ 77 ̊45' 77 ̊30'

Cp

Quantico

synclinorium

0 5 10 15 KILOMETERS

0 2 4 6 8 MILES

Louisa

Palmyra

Columbia
Introduction 5

EXPLANATION

QNK

Pzg

ehc

pb

DSq

DSa

SOg

Coastal Plain sediments

Mesozoic diabase dikes

Mesozoic sedimentary basins

Alleghanian granitoids

Pegmatite belt rocks

Rocks of the Elk Hill Complex 

(including paragneiss)

Successor basin synclinoriums

DSq—Quantico Formation

DSa—Arvonia Formation

Silurian-Ordovician granitoids

PzYm

ZYp

sc

lis

Ocf

Ocm

CZm

Sedimentary rocks of the Long Island 

syncline

Chopawamsic Formation

Ocf—Felsic

OCm—Mafic

Melange—Rocks of the Shores complex

and Byrd Mill formation

Laurentian slope-rise rocks of the 

Potomac terrane (Mine Run Complex)

Maidens Gneiss of the Goochland terrane

Carolina terrane rocks

Yma

Ysa

Ysf

Zl

Ygg

Zc

5.8

1

Blue Ridge rocks

Zc—Catoctin Formation

Zl—Lynchburg Formation

Ygg—Mesoproterozoic basement

Goochland terrane rocks

Yma—Montpelier Anorthosite

Ysa—Sabot Amphibolite

Ysf—State Farm Gneiss

North Anna Power Plant

Epicenter of magnitude 5.8 Mineral, 

Virginia earthquake

Location of city or town and name

Sample site discussed in text—

Number is site identifier 

Palmyra

Figure 2. (Left) Regional bedrock geology map of the central Virginia Piedmont. Quadrangles mapped in detail and referenced in 

the text are outlined with red boxes. Cp, Columbia pluton; Cbp, Carysbrook pluton; GSp, Green Springs pluton; Lp, Lahore pluton. 

Sample localities discussed in text and listed in table 1. Original 1/3 arc-second resolution elevation data from the U.S. Geological 

Survey 3DEP National Elevation Dataset available at https://viewer.nationalmap.gov. Base map image constructed from these data 

using Global Mapper version 18.1.0.

Site 

identifier Sample site

1 White Walnut

2 ZXR-1

3 BBF-220

4 M16-05-31A

5 MN-183

6 BBF-019

7 BK234

8 BK650

9 BK544

10 BK601
6 Geochronologic Age Constraints on Tectonostratigraphic Units of the Central Virginia Piedmont, USA

graphite, chloritoid, and staurolite. These rocks occur in the 

core of the Quantico synclinorium (Mixon and others, 2000; 

fig. 2), which extends nearly 140 kilometers from the Occoquan River, in Fairfax County, northern Virginia, nearly to the 

James River in Fluvanna County, central Virginia. Preserved 

faunal assemblages (Watson and Powell, 1911; Pavlides and 

others, 1980) point to a latest Ordovician to Devonian age 

for the unit. Along the James River, the Arvonia Formation 

(Watson and Powell, 1911) is biostratigraphically equivalent 

to the Quantico Formation (Kolata and Pavlides, 1986), and 

demonstrably overlies Columbia Granite (459.1±7.3 Ma; 

Sinha and others, 2012) along a regional unconformity (Smith 

and others, 1964; Glover, 1989; Bailey and others, 2005). The 

Quantico Formation, and the equivalent Arvonia Formation 

to the southwest (fig. 2), contain detrital zircons ranging in 

age from 430 Ma (Hughes and others, 2014a) to as young as 

390 Ma (Bailey and others, 2008); these units have been interpreted as postorogenic successor basin deposits (for example, 

Hughes and others, 2015). Similar rocks occur in a syncline 

west of the Quantico and Arvonia Formations (Long Island 

syncline of Smith and others, 1964), but Bailey and others 

(2005) assigned these rocks (graphitic schist, metagraywacke, 

and interlayered metarhyolite) to the Chopawamsic Formation.

East of the Chopawamsic Formation are metamorphic 

rocks of the Elk Hill Complex (Taber, 1913), which include 

biotite-muscovite gneiss and strongly layered granitic gneiss 

with layers and boudins of amphibolite (Spears and others, 

2004). The Lakeside fault (Bourland and others, 1979; Brown, 

1986; Spears and others, 2004, 2013; Spears, 2011) separates 

the Chopawamsic Formation from the Elk Hill Complex. 

Lakeside fault exposures reveal high-grade (amphibolite 

facies) ductile-deformed rocks with retrograde mineral assemblages and S-C mylonitic fabrics in a variably thick zone, 

and brittle-deformed silicified cataclasite that is interpreted 

to represent Mesozoic overprinting of the Paleozoic structure (Spears and others, 2004). At the north terminus of the 

Elk Hill Complex near Lake Anna (fig. 2), biotite-muscovite 

migmatitic paragneiss structurally overlies the Chopawamsic 

Formation along a premetamorphic fault or an overturned 

unconformity, and S-C mylonitic rocks of the Lakeside fault 

terminate in the core of a map-scale fold of paragneiss.

Roig and others (2017) dated granitic gneiss at the type 

locality of the Elk Hill Complex using isotope dilution (ID)-

TIMS (U-Pb zircon) at 331±10 Ma. Granitic gneisses from the 

pegmatite belt adjacent to the west (Taber, 1913; fig. 2) were 

also dated; granitic gneisses from two localities yielded IDTIMS U-Pb zircon ages of 427.9±4.3 Ma and 423.7±1.5 Ma, 

and monazite from one of these localities yielded an age of 

325.2±1.3 Ma (Roig and others, 2017). The Elk Hill Complex 

is bordered on the east by the Maidens Gneiss of the Goochland terrane along the Spotsylvania fault zone (Bailey and 

others, 2004; Spears and others, 2004).

Metamorphic grade increases from west to east across 

the transect, but argon (Ar) geochronology indicates that the 

gradient is likely a result of several Paleozoic events (Burton 

and others, 2015b; McAleer and others, 2017). In the Potomac 

terrane and westernmost edge of the Chopawamsic terrane, 

primary bedding is preserved at greenschist-facies; published 

preliminary 40Ar/39Ar plateau and correlation ages indicate 

Late Ordovician metamorphism (Burton and others, 2015b). 

Eastward, Quantico Formation and underlying Chopawamsic 

Formation rocks are deformed by multiple ductile foliations 

and brittle cleavage and metamorphosed to upper greenschistto amphibolite-facies (kyanite zone); published preliminary 

data are indicative of cooling from a tectonothermal event 

during the Alleghanian orogeny (Burton and others, 2015b). 

Amphibolite and biotite-granitoid sheet intrusions east of 

the Quantico synclinorium are also metamorphosed at upper 

greenschist- to amphibolite-facies, and highly deformed, 

but compositions do not allow isograds to be easily traced. 

Biotite-muscovite paragneiss preserves migmatitic textures 

that suggest partial melting.

Sample Descriptions

Detailed 1:24,000-scale geologic mapping across nine 

7.5-minute quadrangles (fig. 2) identified the base of the 

Quantico Formation, refined the stratigraphy within the 

Chopawamsic Formation, and delineated a broad belt of 

granodioritic to tonalitic sheet intrusions in the eastern belt of 

the Chopawamsic Formation. Additionally, mapping confirmed the presence of the Elk Hill Complex, distinct from 

the eastern belt of Chopawamsic Formation belt of rocks. The 

confirmed and revised field relations allowed us to pinpoint 

sampling localities (fig. 2).

We collected three samples of metasedimentary rocks 

for LA-ICP-MS U-Pb detrital zircon geochronology and 

seven samples of metaigneous rocks for SHRIMP-RG U-Pb 

zircon crystallization geochronology (table 1). To determine 

a maximum depositional age of the Quantico Formation, a 

sample of quartzite (sample White Walnut) from the base of 

the formation was collected for LA-ICP-MS analysis, and a 

sample of quartz phenocryst-bearing felsic gneiss (sample 

M16-05-31A; figure 3A) was collected from near the base of 

the formation for SHRIMP-RG analysis. In addition, a sample 

of thinly bedded and cleaved metasiltstone in the core of the 

Long Island syncline (sample ZXR-1) was collected for LAICP-MS analysis to resolve the maximum depositional age of 

those rocks. A sample of fine-grained, well-foliated muscovitebiotite-chlorite-quartz schist (sample BBF-220) from the informal Byrd Mill formation (Shores complex, Potomac terrane) 

was collected to provide provenance data for comparison with 

previously published detrital zircon geochronology from the 

Potomac terrane (for example, Hughes and others, 2014a).
Introduction 7

Table 1. Summary of sample locations, dating methods, and age results of this study. 

[Map locations shown in figure 2. Datum is World Geodetic System of 1984. dd, decimal degrees; LA-ICP-MS, laser ablation inductively-coupled plasma-mass 

spectrometer; SHRIMP-RG, sensitive high-resolution ion microprobe-reverse geometry; USGS, U.S. Geological Survey; Ma, million years]

Sample 

identifier

Sample 

location Unit Latitude(dd)

Longitude

(dd)

USGS 

quadrangle Method Age

White Walnut 1 Quantico Formation 

quartzite 37.95006 77.95361 Pendleton LA-ICP-MS U-Pb detrital zircon

Youngest grain 

427±6 Ma

ZXR-1 2 Long Island syncline 

metasiltstone 37.88657 78.18687 Zion Crossroads LA-ICP-MS U-Pb detrital zircon

Youngest grain

467.7±8.9 Ma

BBF-220 3 Byrd Mill formation 

(Potomac terrane) 37.97501 78.0944 Ferncliff LA-ICP-MS U-Pb detrital zircon

Youngest grain

971 Ma

M16-05-31A 4 Felsic gneiss 37.91578 77.92860 Pendleton SHRIMP U-Pb 

zircon 448±2.2 Ma

MN-183 5 Chopawamsic Formation 

felsic schist 38.04192 77.89632 Mineral SHRIMP U-Pb zircon 470.9±4.2 Ma

BBF-019 6 Chopawamsic Formation 

metafelsite 37.97988 78.03678 Ferncliff SHRIMP U-Pb zircon 459.5±5.3 Ma

BK234 7 Granodiorite intruding 

Chopawamsic Formation 37.97679 77.82513 Buckner SHRIMP U-Pb zircon 456.2±3.8 Ma

BK650 8 Granodiorite intruding 

Chopawamsic Formation 37.99292 77.82754 Buckner SHRIMP U-Pb zircon 441.5±6.2 Ma

BK544 9 Tonalite intruding Elk Hill 

Complex 37.94335 77.81307 Buckner SHRIMP U-Pb zircon 451.5±5.6 Ma

BK601 10 Biotite-muscovite 

migmatitic paragneiss 37.93955 77.82457 Buckner SHRIMP U-Pb zircon

441.9±6.8 Ma pyramidal 

grains

BK601 10 Biotite-muscovite 

migmatitic paragneiss 37.93955 77.82457 Buckner SHRIMP U-Pb detrital zircon

Core of youngest 

rounded grain 926 Ma
8 Geochronologic Age Constraints on Tectonostratigraphic Units of the Central Virginia Piedmont, USA

1,000 μm 1,000 μm

500 μm 1,000 μm

a

e

a

a

a

a

g

g

g

a

e

a

a

a

a

g

g

g

A B

C D

E F

Figure 3. Photographs of rocks and structures from the central Virginia Piedmont. A, Photomicrograph of monocrystalline quartz 

phenocryst (porphyroclast) in felsic gneiss (sample M16-05-31A) from near the base of the Quantico Formation, plane-polarized light, 

2x magnification. B, Photomicrograph of monocrystalline quartz phenocryst (porphyroclast) with deformation tails of fine-grained 

quartz from sample MN-183, crossed-polarized light, 2x magnification. C, Photomicrograph of magmatic epidote (e) cored by allanite 

(a) from metagranodiorite (sample BK650), plane-polarized light, 5x magnification. D, Photomicrograph of bluish-green amphibole 

in Elk Hill Complex tonalitic gneiss from sample BK544, plane-polarized light, 2x magnification. E, An outcrop of layered amphibolite 

xenoliths (a) in strongly foliated approximately 452 million year old hornblende-biotite metagranodiorite (g) at location of sample BK544. 

Marker is 13.6 centimeters (cm) (5.3 inches [in.]) long, for scale. F, An outcrop of biotite-muscovite migmatitic paragneiss at the north 

terminus of the Elk Hill Complex on the Buckner 7.5-minute quadrangle (fig. 2); hammerhead, approximately 17 cm (6.7 in.) long, for scale. 

μm, micrometers.
Analytical Methods 9

To establish stratigraphic succession within the 

Chopawamsic Formation based on relative stratigraphic relations from detailed mapping, two samples were collected from 

near the top and the bottom of the formation, respectively. Felsic schist (sample MN-183) occurs stratigraphically approximately 280 meters below a regional unconformity within the 

Chopawamsic Formation that was recognized through mineral 

exploration drilling (Sauer, 1984). This rock is multiply 

deformed and preserves both millimeter-thick isoclinally 

folded bands of coarser- and finer-grained quartz and feldspar, and penetrative axial-planar foliation defined by aligned 

muscovite. Significantly larger monocrystalline quartz phenocrysts are mantled by asymmetrical strain shadows composed 

of fine-grained recrystallized quartz (fig. 3B). Metafelsite 

(sample BBF-019) occurs stratigraphically beneath ferruginous quartzite in the core of a tight syncline that is along strike 

to the northeast from the Long Island syncline. The metafelsite 

is light gray and fine-grained, and contains relict phenocrysts 

of quartz and plagioclase, as well as porphyroblasts of biotite, 

in a matrix of fine-grained quartz, plagioclase, and potassium 

feldspar. A schistosity defined by fine-grained muscovite flattens or rotates the phenocrysts and biotite.

Four samples were collected for SHRIMP-RG analysis 

to determine the timing of deformation in the Chopawamsic 

Formation east of the Quantico synclinorium and the Elk Hill 

Complex. Two of these samples were collected from strongly 

foliated hornblende-biotite granodioritic and tonalitic sheet 

intrusions that are interlayered at map-scale with predominantly amphibolite in the eastern belt of the Chopawamsic 

Formation. The first sample (BK234) is a strongly foliated 

metagranodiorite that contains quartz, plagioclase, myrmekite, and potassium feldspar; the second sample (BK650) is a 

strongly foliated metagranodiorite that contains quartz and plagioclase, with potassium feldspar porphyroclasts (interpreted 

to be relict phenocrysts) as much as 1 centimeter in length. 

Biotite and epidote compose the mafic mineral phases in both 

samples, and muscovite is an accessory metamorphic phase 

(garnet is also an accessory phase in sample BK234). Coarse 

epidote is commonly cored by allanite and is interpreted to 

be magmatic (fig. 3C). Aligned biotite defines the dominant 

foliation in outcrop and samples, with a weak secondary foliation defined by aligned muscovite at a slight angle. Outcrops 

at the BK234 sample location contain several decimeterthick muscovite-bearing pegmatite dikes that are concordant 

with the dominant foliation. The third sample (BK544) was 

collected from strongly foliated hornblende-biotite tonalitic gneiss that contains xenoliths of amphibolite within the 

Elk Hill Complex between the Lakeside and Spotsylvania 

faults. Mafic phases include both bluish-green amphibole 

(fig. 3D) and biotite. Epidote typically has an aspect ratio of 

3:1, is elongate in the plane of the foliation, and is interpreted 

to be metamorphic in origin. Nearby granodioritic gneiss 

contains sparse garnet. Centimeter-thick bands of elongated 

quartz and feldspar, alternating with centimeter-thick bands 

of biotite-amphibole constitute the dominant foliation, but a 

weak oblique secondary foliation of muscovite is preserved in 

nearby granodioritic gneiss. In outcrop, tonalitic and granodioritic gneisses contain decimeter- to meter-scale xenoliths of 

amphibolite, which preserve an earlier foliation that is oblique 

to foliation in the enclosing metaigneous rock (fig. 3E). The 

fourth sample (BK601) was collected from biotite-muscovite 

migmatitic paragneiss at the north terminus of the Elk Hill 

Complex (fig. 2). It consists of quartz, plagioclase, muscovite, and biotite, with epidote, garnet, and apatite. In outcrop, 

biotite-muscovite gneiss is polydeformed; centimeter-thick 

layers and stringers of quartz and feldspar define migmatitic layering (fig. 3F), which is tight to isoclinally folded. 

Aligned biotite and muscovite define penetrative foliation that 

is axial-planar to isoclinal folds. Both migmatitic layers and 

phyllosilicate-rich foliation planes are broadly to tightly folded 

in several orientations.

Analytical Methods

LA-ICP-MS U-Pb detrital zircon analysis was conducted 

at the U.S. Geological Survey (USGS) Laser Ablation-ICPMS 

Isotope Laboratory, housed at the Geology, Geophysics, and 

Geochemistry Science Center in Denver, Colorado. Zircon 

was ablated with a Photon Machines Excite 193 nanometer argon fluoride (ArF) excimer laser in spot mode. The 

laser spot sizes for zircon were ~25 micrometers (μm); pit 

depths are typically less than 20 μm. Analytical procedures 

are described in appendix 1; data are presented in table 2 

(available at https://doi.org/10.3133/pp1861) and in standard 

graphical form in figure 4. 206Pb/238U ages are reported for 

zircons younger than approximately 1,300 Ma and 207Pb/206Pb 

ages are used for zircons older than 1,300 Ma following the 

recommendations of Gehrels (2012). Zircon grains from igneous samples and from biotite-muscovite migmatitic paragneiss were analyzed during two sessions in 2016 and 2017 

by secondary ionization mass spectrometry on the USGS/

Stanford SHRIMP-RG, using a spot diameter of ~20 μm. 

Samples BK234, BK544, BK650, and M16-05-31A were 

analyzed in 2016, and samples BBF-019, MN183, and BK601 

were analyzed in 2017. Analytical procedures are described 

in appendix 2. Weighted average ages determined from the 206Pb/238U spot ages are reported in table 1 and the uncertainty 

is reported at 2σ. Isotopic data for individual spots are reported 

in table 2 and are presented in graphical form in figures 5–8. 

For spot ages older than 1,300 Ma, the 207Pb/206Pb age is plotted (Gehrels, 2012). Samples analyzed with the SHRIMP-RG 

were imaged with scanning electron microscopy-cathodoluminescence (CL), a back-scatter electron (BSE) detector, and 

ultraviolet light (UV).
10 Geochronologic Age Constraints on Tectonostratigraphic Units of the Central Virginia Piedmont, USA

2,600

2,400

2,200

2,000

1,800

1,400

1,600

1,000

1,200

600

800

400

INSET

n=25

BK601 (biotite-muscovite migmatitic paragneiss)

800

1,000

1,200

0.100.8 1.0 2.0 3.0

0.25

0.15

0.20

N=52

1,000

800

600

1,400

1,800

2,200

2,000

2,800

2,400

2,600

1,023

1,020

1,170

1,401

1,152

1,368

n=120 1,456

BBF-220 (Byrd Mill formation)

640

1,038

1,195

1,488

1,760

2,500

3,000

2,000

1,500

1,000

500

n=119

ZXR-1 (Long Island syncline rocks)

2,200

1,800

1,400

INSET

1,000

600

200

450

1,038

1,179

1,479

n=120

White Walnut (basal Quantico Formation)

490

420

N=52

0.068

0.0640.46 0.50 0.54 0.58 0.62 0.66

0.072

0.076

0.080

0.6

0.4

0.7

0 0

0

5

10

15

20

10

20

30

40

0.5 50

0.1

0.2

0.3

0.4

0.2

0.5

0.3

0.1

0

0.6

0.4

0.7

0.2

0.5

0.3

0.1

0

0.55

0.40

0.20

0.50

0.30

0.10

0.05

0 1 2 3 4 5 6 7 8 9 10 11 12 13

0.45

0.25

0.35

0.15

0 2 4 6 8 10 12 14 16 18

0

0

10

8

6

4

2

5

10

15

20

0

0 1 2 3 4 5 6 7 8

2 4 6 8 10 12 14 16 18 20 22 24

300 600 900 1,200 1,500 1,800 2,100 2,400 2,700 3,000

300 600 900 1,200 1,500 1,800 2,100 2,400 2,700 3,000

300 600 900 1,200 1,500 1,800 2,100 2,400 2,700 3,000

300 600 900 1,200 1,500 1,800 2,100 2,400 2,700 3,000

Age, in millions of years

Age, in millions of years

Age, in millions of years

Age, in millions of years

207Pb/235U

207Pb/235U

207Pb/235U

207Pb/235U

206Pb/238U

Number

206Pb/238U

Number

206Pb/238U

Number

206Pb/238U

Number

A B

C D

E F

G H

Figure 4. Plots of laser ablation-inductively coupled plasma-mass spectrometry detrital zircon data (samples White Walnut, 

ZXR-1, and BBF-220) and sensitive high-resolution ion microprobe detrital zircon data (sample BK601) from central Virginia 

Piedmont samples collected during this study. Concordia and relative probability plots from A and B, basal quartzite of the 

Quantico Formation (sample White Walnut); C and D, Long Island syncline metasiltstone (sample ZXR-1); E and F, Byrd Mill 

formation mica-quartz schist (sample BBF-220); and G and H, rounded zircon cores from migmatitic paragneiss (sample BK601).
Results 11

Results

LA-ICP-MS analysis of quartzite (sample White Walnut) 

from the base of the Quantico Formation revealed the youngest grain in the sample to be 427±6 Ma, but this age may just 

be the youngest part of the normal distribution of a peak age 

population at approximately 450 Ma (fig. 4A). Other populations are at 1,479 Ma, 1,179 Ma, and 1,038 Ma (n=115). 

Metasiltstone from the Long Island syncline (sample ZXR-1) 

contains a few grains with ages between approximately 

570 Ma and 470 Ma; the youngest grain in this sample is 

467.7±8.9 Ma. The peak age populations are at 1,195 Ma and 

1,038 Ma but are broadly distributed to approximately 2 Ga 

(fig. 4B), including population peaks at 1,760 Ma, 1,488 Ma, 

1,038 Ma, and 640 Ma (n=89). For comparison, schist (sample 

BBF-220) from the Byrd Mill formation (Potomac terrane) 

yielded a peak age population at 1,023 Ma (fig. 4C), with other 

populations at 1,456 Ma, 1,368 Ma, 1,152 Ma and 971 Ma 

(n=115). There were no detrital zircons younger than approximately 900 Ma, therefore a maximum depositional age is not 

well constrained by the Byrd Mill formation sample.

Zircon grains from felsic gneiss (sample M16-05-31A) 

from near the base of the Quantico Formation exhibit oscillatory zoning in CL and BSE images (fig. 5A). No texturally distinct rims are present. Analyses yielded a 206Pb/238U weighted 

average age of 448.1±2.2 Ma (fig. 5A).

Zircon grains from two samples (MN-183 and BBF-019) 

from different stratigraphic intervals in the Chopawamsic 

Formation west of the Quantico synclinorium have cores that 

exhibit oscillatory zoning in CL and BSE and 1–2-μm-thick 

patchy and irregular overgrowths. In sample MN-183 the overgrowths are zircon (fig. 5B), and in sample BBF-019 they are 

xenotime (fig. 5C). Analyses of grain cores in sample MN-183 

yielded a weighted mean age of 471.0±4.2 Ma (fig. 5B), and 

BBF-019 yielded a weighted mean age of 459.5±5.3 Ma 

(fig. 5C).

Zircon grains from two samples (BK234 and BK650) 

from biotite granodioritic sheet intrusions from the eastern 

portion of the Chopawamsic Formation have cores that exhibit 

oscillatory zoning in CL and BSE, dark mantles, and narrow 

rims that embay the zoning and are relatively bright in CL. 

SHRIMP-RG analyses of grain cores from sample BK234 

yielded a 206Pb/238U weighted average age of 456.2±3.8 Ma 

(fig. 6A) and analyses of grain cores from sample BK650 

yielded a 206Pb/238U weighted average age of 441.5±6.2 Ma 

(fig. 6B). In both samples the CL-bright rims were typically 

too narrow to analyze. However, a single spot analysis from 

a grain rim in each sample yielded anomalously low thorium (Th) concentrations and high U/Th ratios, and ages that 

suggest the narrow rims are of Alleghanian metamorphic 

origin (sample BK234: 306±6 Ma, U=79 parts per million 

[ppm], Th=0 ppm; sample BK650: 277±27 Ma, U=4 ppm, 

Th=0 ppm). Three inherited cores of ~1 Ga were also analyzed 

in sample BK234. Diffuse CL zoning, low uranium and thorium concentrations, and U/Th ratios of less than 5 chemically 

distinguish these inherited cores.

Zircon grains from tonalitic gneiss of the Elk Hill Complex (sample BK544) have cores that exhibit broad sector 

zoning in CL and relatively bright rims that embay the zoning. 

SHRIMP-RG analyses of cores yielded a 206Pb/238U weighted 

average age of 451.5±5.6 Ma (fig. 6C). Four analyses of the 

thickest CL-bright rims yielded anomalously low uranium 

(7–25 ppm) and thorium (0–1 ppm) concentrations and ages of 

331±9 Ma, 328±9 Ma, 314±6 Ma, and 253±9 Ma. The textural 

and compositional results indicate the rims are metamorphic in 

origin and the U/Pb ages suggest the rims are broadly Alleghanian in age.

Three morphologic populations of zircon were identified 

in migmatitic paragneiss at the north terminus of the Elk Hill 

Complex (sample BK601; figs. 2 and 7A): elongated pyramidal grains with aspect ratios of as much as 8:1 (fig. 7B), 

rounded zircons that appear to be detrital in origin (fig. 7C), 

and zircon aggregates composed of 2–8 elongate zircon grains 

(fig. 7C). Rounded and elongate grains were analyzed by 

SHRIMP-RG U-Pb geochronology methods (figs. 4G and H, 

and 8). All rounded grains yielded Neoproterozoic or older 

ages consistent with a detrital origin for these grains and a 

sedimentary protolith for the gneiss (fig. 4G and H). The 

youngest core of a rounded grain yielded an age of approximately 926 Ma; cores of other rounded grains yielded peak 

age populations of 1,170 Ma, 1,020 Ma, and approximately 

1,000 Ma. One grain also yielded an age of 1,401 Ma (n=20; 

fig. 4G and H). A single, discordant Archean to Paleoproterozoic grain core was also present (fig. 7B). Elongate pyramidal 

grains were dark in CL and bright in BSE, and zoning patterns varied from oscillatory to patchy. Several grains also had 

rims that were brighter in CL and embayed the grain cores. 

Cores of elongated pyramidal grains yielded a mean age of 

439.4±7.3 Ma (fig. 8A). Rims on the elongated grains yielded 

ages of 334±8 Ma, 322±5 Ma, and 318±13 Ma (fig. 8A). Both 

the cores and rims had U/Th ratios greater than 20 (fig. 8B), 

but grain cores were typically richer in uranium (>1,000 ppm).
12 Geochronologic Age Constraints on Tectonostratigraphic Units of the Central Virginia Piedmont, USA

400 

420 

440 

460 

480 

500 

Metafelsite

Igneous age=459.5±5.3 Ma

(n=10/12, MSWD=1.5)

0.062

0.066

0.070

0.074

0.078

0.082

0.064

0.068

0.072

0.076

0.080

0.084

0.46 0.48 0.50 0.52 0.54 0.56 0.58 0.60 0.62 0.64 0.66

207Pb/235U

206Pb/238U

460

500

420

Concordant cores

206Pb/238U age,

in Ma

50μm

463±12 Ma

460±7 Ma

xtm

50μm

BSE

CL BSE

CL

C. BBF-019

430 

450 

470 

490 

Felsic schist 510 

Igneous age=471.0±4.2 Ma

(n=14/14, MSWD=1.8)

0.066

0.070

0.074

0.078

0.082

0.068

0.072

0.076

0.080

0.084

0.46 0.48 0.50 0.52 0.54 0.56 0.58 0.60 0.62 0.64 0.66

207Pb/235U

206Pb/238U

470

500

440

Concordant cores

206Pb/238U age,

in Ma

B. MN-183

471±5 Ma 

470±12 Ma 

50 μm

50 μm

CL BSE

CL BSE

Zr2

Zr2

420 

440 

460 

480 

Metafelsite

Igneous age=448.1±2.2 Ma

(n=20/22, MSWD=0.87)

0.065

0.070

0.075

0.080

0.46 0.48 0.50 0.52 0.54 0.56 0.58 0.60 0.62 0.64 0.66 0.68

206Pb/238U

207Pb/235U

450

480

420

206Pb/238U age,

in Ma

Concordant cores

449±5 Ma 

447±5 Ma 

50 μm

50 μm

CL BSE

CL BSE

A. M16-05-31A

Concordant

Discordant

EXPLANATION

Concordant

Discordant

EXPLANATION

Concordant

Discordant

EXPLANATION
Results 13

Figure 5. (Left) Scanning electron microscopy–back-scatter 

electron (BSE) detector and scanning electron microscopy–

cathodoluminescence (CL) images of representative zircon 

grains and plots of isotopic data from the samples analyzed on 

the Stanford/U.S. Geological Survey sensitive high-resolution 

ion microprobe-reverse geometry. All concordant (green) 

and discordant (red) analyses of grain cores are plotted on 

the concordia diagrams. Only concordant data are plotted on 

weighted mean diagrams (inset). Data for all analyses can 

be found in table 3. A, CL and BSE images of prismatic zircon 

exhibiting oscillatory zoning in sample M16-05-31A (metafelsite 

near the base of the Quantico Formation). Concordia and 

weighted average diagrams are for igneous zircon cores from 

sample M16-05-31A; B, CL and BSE images of prismatic and 

elongate zircon exhibiting oscillatory zoning in sample MN-183 

(quartz phenocryst-bearing felsic schist of the lower part of the 

Chopawamsic Formation). Overgrowths of irregular zircon (Zr2) 

are present on grain margins. Concordia and weighted average 

diagrams are for igneous zircon cores from sample MN-183; C, CL 

and BSE images of prismatic zircon exhibiting oscillatory zoning 

in sample BBF-019 (metafelsite beneath ferruginous quartzite). 

Irregular overgrowths of xenotime (xtm) are present on most 

zircon grains. Concordia and weighted average diagrams are 

for igneous zircon cores from sample BBF-019. All plots are at 

2-sigma; Ma, million years old; MSWD, mean square of weighted 

deviates; μm, micrometers.
14 Geochronologic Age Constraints on Tectonostratigraphic Units of the Central Virginia Piedmont, USA

400 

420 

440 

460 

480 

500 

0.062

0.066

0.070

0.074

0.078

0.082

0.2 0.3 0.4 0.5 0.6 0.7 0.8

206Pb/238U

207Pb/235U

Elk Hill Complex granitoid

Igneous age=451.5±5.6 Ma

(n=19/19, MSWD=2.9)

490

420

455

Concordant cores

206Pb/238U age,

in Ma

50 μm

454±6 Ma

331±9 Ma

50 μm

50 μm

50 μm

328±9 Ma

450±6 Ma

CL CL

CL CL

C. BK544

390 

410 

430 

450 

470 

0.060

0.064

0.068

0.072

0.076

0.062

0.066

0.070

0.074

0.078

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

206Pb/238U

207Pb/235U

Granitoid sheet intrusion

Igneous age=441.5±6.2 Ma

(n=9/9, MSWD=1.7)

440

470

410

Concordant cores

206Pb/238U age,

in Ma

50 μm

425±8 Ma 

448±7 Ma

50 μm 50 μm

444±6 Ma

CL

CL CL

50 μm

277±27 Ma 

CL

B. BK650

420

430 

440 

450 

460 

470 

480 

490 

0.067

0.069

0.071

0.073

0.075

0.077

0.079

0.46 0.48 0.50 0.52 0.54 0.56 0.58 0.60 0.62 0.64

206Pb/238U

207Pb/235U

Granitoid sheet intrusion

Igneous age=456.2±3.8 Ma

(n=18/18, MSWD=1.8)

460

485

435

206Pb/238U age,

in Ma

Concordant cores

994±14 Ma 

462±6 Ma 

456±7 Ma 

462±6 Ma 

50 μm 50 μm

50 μm 50 μm

CL

CL

CL

CL

A. BK234

Concordant

Discordant

EXPLANATION

Concordant

Discordant

EXPLANATION

Concordant

Discordant

EXPLANATION
Results 15

Figure 6. (Left) Scanning electron microscopy–

cathodoluminescence (CL) images of representative zircon 

grains and plots of isotopic data from the samples analyzed on 

the Stanford/U.S. Geological Survey sensitive high-resolution ion 

microprobe-reverse geometry. For all CL image age labels: orange, 

inherited core; white, igneous crystallization; yellow, metamorphic 

rim. All concordant (green) and discordant (red) analyses of grain 

cores are plotted on the concordia diagrams. Only concordant 

data are plotted on weighted mean diagrams. Data for all analyses 

can be found in table 3. A, CL images of prismatic zircon showing 

oscillatory zoned cores, dark mantles, and bright rims in sample 

BK234 (metagranodioritic sheet intrusion in the eastern belt of 

the Chopawamsic Formation). Rims embay and truncate zoning 

in grain cores. Rare inherited grain cores (n=3) exhibit diffuse CL 

zoning with oscillatory outer rims, dark mantles, and bright CL 

rims. Concordia and weighted average diagrams are for igneous 

zircon cores from sample BK234. B, CL images of prismatic zircon 

showing oscillatory zoned cores, dark mantles, and bright rims in 

sample BK650 (metagranodioritic sheet intrusion in the eastern 

belt of the Chopawamsic Formation). Rims embay zoning in grain 

cores. Concordia and weighted average diagrams are for igneous 

zircon cores from sample BK650. C, CL images of prismatic zircon 

showing sector-zoned cores, dark mantles, and bright rims in 

sample BK544 (metatonalite in the Elk Hill Complex near its north 

terminus). Rims embay zoning in grain cores. Concordia and 

weighted average diagrams are for igneous zircon cores from 

sample BK544. All plots are at 2-sigma. Ma, million years old; 

MSWD, mean square of weighted deviates; μm, micrometer.
16 Geochronologic Age Constraints on Tectonostratigraphic Units of the Central Virginia Piedmont, USA

100 μm 100 μm

318±13 Ma

434±11 Ma

454±9 Ma

436±10 Ma

322±5 Ma

449±6 Ma

427±18 Ma

1,045±27 Ma

1,077±41 Ma

2,613±5 Ma

1,178±34 Ma

100 μm

100 μm

200 μm

λexc = 365 nm

100 μm 100 μm

100 μm 100 μm

A B

C D

Figure 7. Plane polarized light (PPL), scanning electron microscopy–back-scatter electron (BSE) detector and photoluminescence 

images of representative zircon grains from sample BK601 (biotite-muscovite migmatitic paragneiss at the northern terminus of the 

Elk Hill Complex) analyzed on the Stanford/U.S. Geological Survey sensitive high-resolution ion microprobe-reverse geometry. A, PPL 

image of zircon separate from sample BK601. Stubby ovoid grains are the dominant morphology but elongate dipyramidal grains are 

also present. B, BSE images of elongate grains with spot ages indicated (1σ). Two grains have inherited cores and one grain on the 

left is an aggregate. Age label coloring: orange, inherited core; blue, igneous crystallization age; yellow, metamorphic age. C, BSE 

images of ovoid grains. The morphology and ages of these grains are consistent with a detrital origin. Data from detrital grains plotted 

in figure 4G and H. D, BSE (upper) and photoluminescence (lower) images of rare zircon aggregates. There is significant variance in the 

photoluminescence response among grains within a single aggregate, suggesting the grains had a different initial crystallization history 

and then aggregated. Ma, million years old; nm, nanometers; μm, micrometers; λexc, excitation wavelength.
Results 17

200

400

600

800

1,000

1,200

1,400

1,600

0 10 20 30 40

Analysis number

Age, in Ma

Detrital (U/Th ratio<5)

Metamorphic (U/Th ratio>20)

B

0 

140 

EXPLANATION

U/Th ratio

206Pb/238U

207Pb/235U

280

320

360

400

440

480

520

0.04

0.05

0.06

0.07

0.08

0.09

0.25 0.30 0.35 0.40 0.45 0.50 0.55 0.60 0.65 0.70

A BK601

Migmatitic paragneiss

Core age=439.4±7.3 Ma (2σ)

(n=8/8, MSWD=0.95)

Rim age = 324.8±8.0 Ma (2σ)

(n=3/3, MSWD =1.02)

340

Concordant cores, included

in weighted mean

420

500

206

Pb/

238

U age,

in Ma

Concordant core

Concordant rim

Discordant

EXPLANATION

Plotted at 2σ

Plotted at 2σ

Figure 8. Plots of isotopic data from sample BK601 analyzed on the Stanford/U.S. Geological Survey sensitive 

high-resolution ion microprobe-reverse geometry. A, Concordia plot of analyses from elongate pyramidal grain rims 

and cores from BK601. Inset: Weighted average plot is for concordant analyses of cores. B, Age versus U/Th plot for 

analyses from sample BK601. Grains with ages older than 900 million years old (Ma) have U/Th ratios<10, which is 

typical of igneous zircon, and grains younger than 500 Ma have U/Th ratios>20, which is typical of metamorphic zircon 

(for example, Rubatto, 2017). Data for all analyses can be found in table 2. Ma, million years old; MSWD, mean square 

of weighted deviates; <, less than; >, greater than.
18 Geochronologic Age Constraints on Tectonostratigraphic Units of the Central Virginia Piedmont, USA

Discussion

Our SHRIMP-RG and LA-ICP-MS zircon geochronology, coupled with new geologic mapping and existing 

published data, provide new insight into the evolution of 

the Virginia Piedmont Province. These data assign absolute 

ages or distinct provenance signatures to previously undated 

but regionally recognized units, or to new units defined and 

delineated during recent geologic mapping investigations. This 

research provides new temporal and geologic constraints concerning the character and duration of Chopawamsic Formation 

arc volcanism and sedimentation, the onset of post-Taconian 

successor basin development, and the origin and significance 

of the Elk Hill Complex on the easternmost margin of the 

volcanic arc.

Age of the Quantico Formation

Detrital zircon geochronology supports the interpretation that the Quantico Formation is a successor basin, like 

the Arvonia Formation (as interpreted by Hughes and others, 

2014a). The youngest detrital zircon in the quartzite sample 

of the Quantico Formation (White Walnut) is 427 Ma. The 

peak age population in the Quantico, at approximately 450 Ma 

(fig. 4B), suggests a source from local, approximately 450 Ma 

granitoids and perhaps recycling of the youngest Chopawamsic Formation volcanic arc rocks during deposition of the 

Quantico Formation. The youngest relative probability peak 

also compares favorably to detrital zircon ages reported by 

Hughes and others (2014a) from the stratigraphically highest units in the Arvonia basin—the Bremo Quartzite Member 

of the Arvonia Formation and overlying Buffards Formation (fig. 9A). These data support the interpretation that the 

Quantico and Arvonia Formations represent different parts 

of the same regional successor basin and (or) formed at 

similar times.

Detailed geologic mapping demonstrates that a sequence 

of metasedimentary and metavolcanic rocks occurs beneath 

the typical garnet-mica schist of the Quantico Formation, 

including thinly interlayered quartzite, felsic gneiss, graphitic 

schist, and lesser mafic gneiss and greenstone. The quartzite 

in contact with the overlying garnet-mica schist has been 

interpreted as the base of the Quantico Formation (for example, Bailey and others, 2005, 2008; Spears and others, 2013) 

and a detrital zircon as young as approximately 427 Ma from 

the quartzite (sample White Walnut) supports this interpretation. Map relations show that the unconformity between the 

Quantico Formation and underlying rocks creates triple-point 

junctions at map-scale, and there is no evidence for ductile or 

brittle faulting along the contact. Rocks beneath the contact 

are also locally aluminum- and iron-enriched (Spears and others, 2013), leading Burton and others (2014) to speculate that 

they represent metamorphosed soil horizons. The sample of 

felsic gneiss (sample M16-05-31A) from the base of the Quantico Formation, as defined by these map relations, yielded a 

SHRIMP-RG age of 448±2.2 Ma. These data are consistent 

with the crystallization age of 448±4 Ma for a felsic lapilli 

metatuff layer (Horton and others, 2010) mapped in the lower 

part of the Quantico Formation by Seiders and Mixon (1981) 

in northern Virginia.

Assigning approximately 448 Ma metavolcanic rocks 

from our study area to the base of the Quantico Formation is 

incompatible with the interpretation that these rocks are postorogenic successor basin deposits rather than waning-stage 

volcanogenic or back-arc basin deposits; in other words, the 

basin should be no older than the youngest rocks of the previous orogenic cycle over which they are interpreted to have 

been deposited—in this case, the approximately 444–437 Ma 

Ellisville pluton. We thus assign these interlayered metasedimentary and metavolcanic rocks to a unit stratigraphically 

beneath the quartzite of the Quantico Formation but above 

Chopawamsic Formation rocks (fig. 10).

Age of Long Island Syncline Rocks

Mesoproterozoic peaks in the sample of the Long Island 

syncline (ZXR-1) compare well to those in the sample (White 

Walnut) of quartzite from the Quantico Formation (fig. 9A). 

The youngest detrital zircon is approximately 467 Ma (fig. 4C

and D), suggesting input from arc rocks of the Chopawamsic 

Formation, but not from Ordovician-Silurian granitoids (note 

that the approximately 244 Ma analysis is likely a result of 

lead-loss or contamination; table 2). The Paleoproterozoic and 

Neoproterozoic probability peaks in the Long Island syncline 

sample, however, compare better to pelitic metasedimentary 

rocks from near Storck, Virginia (fig. 9A), which Hughes and 

others (2014a) considered to have predominantly a periGondwanan provenance. Comparatively, the Potomac terrane 

sample (BBF-220; fig. 9B) has a strong Laurentian signature 

like other Potomac terrane samples from the Piedmont of 

Virginia (Hughes and others, 2014a; fig. 9B) and southern 

Figure 9. (right) Comparative relative probability plots of detrital 

zircon data from samples dated by laser ablation-inductively 

coupled plasma-mass spectrometry (samples White Walnut, 

ZXR-1, and BBF-220 in figure 4A–F) and sensitive high-resolution 

ion microprobe-reverse geometry (sample BK601 in figure 4G and 

H) during this study, with published data from the central Piedmont 

in Virginia and Maryland. A, Data from the Chopawamsic 

Formation and overlying successor basin deposits; B, Data from 

the Elk Hill Complex and Potomac terrane units in Virginia and 

Maryland. Shaded columns designate time periods of terranes 

and orogenic events that are possible sources of detrital zircon.
Discussion 19

Quantico Formation*

(White Walnut)

Arvonia Formation Buffards Formation1

(KSH-11-Buf)

Byrd Mill formation Potomac terrane*

(BBF-220)

Rocks of the Long Island syncline*

(ZXR-1)

Arvonia Formation Bremo Member1

(Bremo)

Chopawamsic Formation1

(KSH-11-28)

Chopawamsic Formation1

(KSH-11-39)

Chopawamsic Formation1

(KSH-11-19)

Chopawamsic Formation1

(KSH-11-70)

Storck, VA rocks1

(KSH-11-40)

Mine Run Complex Potomac terrane1

(KSH-11-01)

Mine Run Complex Potomac terrane1

(KSH-11-05)

Mine Run Complex Potomac terrane1

(KSH-11-08)

Mine Run Complex Potomac terrane1

(P310-4)

Potomac terrane1

(KSH-11-16)

Mine Run Complex Potomac terrane1

(KSH-11-18)

Maryland Piedmont composite2

Elk Hill Complex paragneiss*

(BK601)

n=115

n=94

n=84

n=17

n=27

n=9

n=89

n=107

n=14

n=115

n=78

n=94

n=88

n=36

n=106

n=75

n=1,247

n=20

300 600 900 1,200 1,500 1,800 2,100 2,400 2,700 3,000

A. Data from Chopawamsic Formation and 

overlying successor basin deposits

B. Data from the Elk Hill Complex and Potomac 

terrane units in Virginia and Maryland

300 600 900 1,200 1,500 1,800 2,100 2,400 2,700 3,000

Relative probabilityRelative probability

Age, in millions of years Age, in millions of years

EXPLANATION

Time period of terranes and orogenic events

Taconic orogeny

Gondwana–peri-Gondwana

Grenville orogeny

Granite rhyolite province

Data from this study

Published data from Hughes and others (2014a)

Published data from Martin and others (2015)

1

2

Source of rock samples

Quantico and Arvonia Formations

Rocks of the Long Island syncline and rocks at Stork, Virginia

Chopawamsic Formation rocks

Byrd Mill formation and Mine Run Complex rocks of the Potomac terrane

Other regional Potomac terrane rocks

Elk Hill Complex paragneiss

*
20 Geochronologic Age Constraints on Tectonostratigraphic Units of the Central Virginia Piedmont, USA

EXPLANATION

Lithology

Metasiltstone, phyllite, and slate

Garnet-mica schist

Pegmatite

Granodiorite and tonalite

Quartzite

Felsic schist and gneiss

Felsic gneiss and schist

Biotite gneiss

Amphibolite and greenstone

Sample location—Upper number

is age of sample. Lower label 

is sample identifier BK234

456 Ma

Correlation uncertain

Rocks of the 

Long Island

syncline

Chopawamsic

Formation

Chopawamsic

Formation

Quantico

Formation

Not to scale

MN-183 BK650

471 Ma 442 Ma

BK234

456 Ma

BBF-19

460 Ma

<468 Ma

<427 Ma

ZXR-1

M16-05-31A

White Walnut

448 Ma

MN-183 BK650

471 Ma 442 Ma

BK234

456 Ma

BBF-19

460 Ma

<468 Ma

<427 Ma

ZXR-1

M16-05-31A

White Walnut

448 Ma

Figure 10. Schematic diagram of Chopawamsic Formation stratigraphy in relation to overlying units and intrusive rocks in the central 

Virginia Piedmont, based on field relations and geochronology. Ma, million years old.
Discussion 21

Maryland (Martin and others, 2015; fig. 9B). Because the periGondwanan-sourced Long Island syncline and Storck rocks 

lie in the hanging wall of the Chopawamsic fault, and Laurentian-sourced Potomac terrane rocks compose its footwall, 

the detrital zircon data confirm that the Chopawamsic fault is 

a major tectonic boundary. Rocks of the Long Island syncline must have received their peri-Gondwanan zircons from 

sources currently not exposed at the surface; these peri-Gondwanan source rocks may have either been completely eroded 

away or were overridden by, and are now buried beneath, the 

Chopawamsic fault.

Chopawamsic Formation Stratigraphy

The Chopawamsic Formation is an Ordovician volcanic arc sequence punctuated by internal unconformities and 

capped by one or more regional unconformities at the top 

of the unit. Within this framework, the structurally lowest 

sample (MN-183: quartz phenocryst-bearing felsic schist 

beneath amphibolite that separates the western belt into lower 

and upper sections) yielded the oldest age of approximately 

471 Ma and the structurally highest sample (BBF-019: 

metafelsite beneath younger rocks in the core of a syncline 

subsidiary to the Long Island syncline) yielded an age of 

460±5 Ma. These new results are equivalent to previously 

published ages of 474–465 Ma (for example, Coler and others, 

2000; Hughes and others, 2013, 2014a), but extends volcanism 

in central Virginia by 5 million years and within analytical 

uncertainty of 453±4 Ma volcanism reported by Horton and 

others (2010) in northern Virginia. These data also provide 

a lower age bound at 460±5 Ma to interlayered metasedimentary and metavolcanics rocks that occur stratigraphically 

between the Chopawamsic Formation and Quantico Formation 

(fig. 10). The gap between the schist in the Quantico and this 

metavolcanic/metasedimentary sequence is supported by additional field relations noted during regional mapping: pegmatites associated with Late Ordovician-early Silurian metagranitoids locally intrude the approximately 448 Ma metavolcanic/

metasedimentary sequence but never the garnet-mica schist of 

the Quantico Formation.

Age of Granodioritic-tonalitic Sheet Intrusions

Our SHRIMP-RG U-Pb zircon crystallization ages 

from two samples of biotite granodiorite range from 456 to 

442 Ma. These data indicate that granodioritic intrusions east 

of the Quantico synclinorium are coeval with granodiorite of 

the Ellisville pluton (Pavlides and others, 1994) west of the 

synclinorium. The occurrence of magmatic epidote in both 

the Ellisville pluton granodiorite (Pavlides and others, 1994; 

Hughes and others, 2013) and metagranodiorite-metatonalite 

east of the Quantico synclinorium (fig. 4C) indicates similar 

depths of emplacement in the lower crust, under moderately 

high pressure and oxidizing conditions (Zen and Hammarstrom, 1984).

Metamorphic and Structural Considerations 

Along the East Flank of the Chopawamsic 

Terrane

Our SHRIMP-RG U-Pb zircon data provide evidence 

for Ordovician deformation east of the Quantico synclinorium. Within the Elk Hill Complex, foliation in amphibolite 

xenoliths is truncated by surrounding approximately 452 Ma 

granodioritic and tonalitic gneiss (fig. 4E), and therefore must 

have formed prior to 452 Ma. Further evidence for Ordovician metamorphism is provided by SHRIMP-RG ages of 

approximately 440 Ma for elongate zircon in biotite-muscovite 

migmatitic paragneiss (fig. 7B). These zircon grains have high 

U/Th ratios typical of zircon that is metamorphic in origin 

(fig. 8B). The elongate, dipyramidal morphology, however, is 

more typical of igneous zircon, and solid-state growth of these 

elongate zircon grains is difficult to envision. Furthermore, 

UV imaging of zircon aggregates demonstrates that aggregates 

are composed of individual grains with varying UV response 

(fig. 7D). In that the UV response is chiefly a function of 

radiation damage (in other words, U+Th concentration; Gaft, 

1992), this indicates high compositional variance and suggests 

that the individual grains did not form from the same melt, 

but rather had a different initial crystallization history and 

then aggregated—that is, they formed by synneusis (Jocelyn 

and Pidgeon, 1974). This process of postgrowth aggregation is inconsistent solid-state growth and strongly suggests 

the presence of melt (Prasad, 1975). We therefore suggest 

that the elongate grains grew in the migmatitic melt and 

record migmatization at approximately 440 Ma. The age also 

overlaps with the crystallization age of the youngest regional 

sheet intrusions. Rim ages of approximately 325 Ma (fig. 7B) 

on this sample (BK601) and sample BK544 from the Elk Hill 

Complex (fig. 6C) are consistent with regional Alleghanian 

metamorphism.

At the northernmost terminus of the Elk Hill Complex 

(fig. 2), the Alleghanian Lakeside fault is marked by a thin belt 

of S-C mylonitic phyllonite that tips out into a tight antiform 

of biotite-muscovite migmatitic paragneiss. Detailed geologic 

mapping demonstrates that biotite-muscovite paragneiss is 

structurally concordant above the easternmost Chopawamsic 

Formation (in other words, foliation is parallel to the contact 

in both paragneiss and Chopawamsic Formation rocks) on the 

flanks of the antiform. The paragneiss yields Paleoproterozoic 

to Neoproterozoic (Laurentian) peak age populations (figs. 3D

and 7C) that are mostly absent in Chopawamsic Formation 

rocks (fig. 9A), which suggests that the contact might be a 

premetamorphic (pre-Alleghanian) fault or an overturned 

unconformity. Juxtaposition of migmatitic paragneiss with 

penetratively deformed but nonmigmatitic interlayered felsic 

and mafic gneiss of the Chopawamsic Formation is the only 

field evidence for faulting along this contact. Ordovician 

displacement along this potential fault contact may have been 

significant to juxtapose rocks from differing crustal levels. 

These data also suggest that Alleghanian ductile folding during 
22 Geochronologic Age Constraints on Tectonostratigraphic Units of the Central Virginia Piedmont, USA

amphibolite-facies metamorphism within the Chopawamsic terrane was a more important contributor to Alleghanian 

shortening in this part of the central Virginia Piedmont than 

Alleghanian ductile fault displacement, and that the more 

significant Alleghanian fault east of the Taconic Chopawamsic 

fault is the Spotsylvania high-strain zone, which marks the 

contact between the Elk Hill Complex and the Goochland terrane to the east.

Conclusions

Our SHRIMP-RG and LA-ICP-MS U-Pb zircon geochronologic data allow us to draw the following five conclusions about the tectonostratigraphic composition of the central 

Virginia Piedmont:

1. The basal quartzite of the Quantico Formation has a 

maximum depositional age of approximately 427 Ma, 

indicating that the formation is a successor basin to the 

volcanic arc represented by the Chopawamsic Formation. The basal quartzite is composed of mostly detritus 

from the Chopawamsic terrane and the numerous intrusions within it.

2. Rocks of the Long Island syncline have a strong periGondwanan detrital zircon provenance signature, like 

the rocks near Storck, in the northern Virginia Piedmont. 

These rocks are likely not part of the Chopawamsic 

Formation but are part of a separate basin that developed 

above or near the arc.

3. Volcanism and arc development represented by the 

Chopawamsic Formation lasted from roughly 474 to 

at least 460 Ma. Felsic gneiss within a sequence of 

interlayered metasedimentary and metavolcanic rocks 

bounded by unconformities above the typical approximately 460 Ma Chopawamsic arc rocks and beneath 

approximately 427 Ma Quantico Formation yields an 

age of approximately 448 Ma. This unit consists of 

thinly interlayered and interbedded felsic volcanic rocks, 

quartzite, and graphitic schist, and likely represents the 

youngest part of the Chopawamsic arc.

4. Hornblende-biotite granodioritic and tonalitic sheet 

intrusions in the eastern belt of the Chopawamsic 

Formation are approximately 150 million years older 

than previously thought. The term “Elk Creek pluton” 

of Mixon and others (2000) may be retained, but its 

Ordovician to Silurian age means that the rocks can no 

longer be correlated with the Carboniferous Falmouth 

Intrusive Suite.

5. Biotite-muscovite migmatitic paragneiss preserves 

evidence for Ordovician high-grade metamorphism on 

the east flank of the Chopawamsic terrane. The age of 

this unit, based on limited detrital zircon geochronologic 

data, suggests that the structurally concordant contact 

between the paragneiss and layered mafic and felsic 

rocks of the Chopawamsic Formation is either a premetamorphic (pre-Alleghanian) fault or an overturned 

unconformable contact, which would place paragneiss 

as Laurentian-sourced basement beneath arc volcanic 

rocks of the Chopawamsic Formation. The Lakeside 

fault, which separates the Elk Hill Complex from the 

Chopawamsic Formation along its length, tips out into 

an antiform of biotite-muscovite migmatitic paragneiss 

at the northernmost terminus of the Elk Hill Complex.
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Appendix 1. Laser Ablation-Inductively 

Coupled Plasma-Mass Spectrometry 

(LA-ICP-MS) Methodology

Zircon was ablated with a Photon Machines Excite 193 nanometer argon-fluoride (ArF) 

excimer laser in spot mode. The laser spot sizes for zircon were approximately 25 micrometers 

(μm). Each analysis consisted of 150 total bursts with a repetition rate of 5 hertz, laser energy 

of approximately 3 millijoules, and an energy density of 4.11 Joules per square centimeter. Pit 

depths are typically less than 20 μm. The rate of helium carrier gas flow from the HelEx cell of 

the laser was ~0.6 liters per minute (L/min). Make-up argon gas (approximately 0.6 L/min) was 

added to the sample stream prior to its introduction into the plasma. Nitrogen with flow rate 

of 5.5 milliliters per minute was added to the sample stream to allow for significant reduction 

in ThO+/Th+ (<0.5 percent) and improved the ionization of refractory thorium (Hu and others, 

2008). With the magnet centered at a constant mass, the flat tops of the isotope peaks of 202Hg, 204(Hg+Pb), 206Pb, 207Pb, 208Pb, 232Th, 235U, and 238U were measured by rapidly deflecting the ion 

beam with a 30-second on-peak background measured prior to each 30-second analysis. Raw 

data were reduced off-line using the Iolite 2.5 program (Paton and others, 2011) to subtract onpeak background signals, correct for U-Pb downhole fractionation, and normalize the instrumental mass bias using external mineral reference materials, the ages of which had previously 

been determined by isotope dilution-thermal ionization mass spectrometry. Ages were corrected 

by standard sample bracketing with the primary zircon reference material Temora2 (417 Ma; 

Black and others, 2004) and secondary reference materials FC-1 (1,099 Ma; Paces and Miller, 

1993) and Plešovice (337 Ma, Sláma and others, 2008). Reduced data were compiled into probability density plots using Isoplot 4.15 (Ludwig, 2012). 206Pb/238U ages are reported for zircons 

younger than approximately 1,300 Ma and 207Pb/206Pb ages are used for older zircons following 

the recommendations of Gehrels (2012).

Appendix 2. Secondary Ionization Mass 

Spectrometry Methodology

Zircon grains from igneous samples and from biotite-muscovite migmatitic paragneiss 

were analyzed by secondary ionization mass spectrometry on the U.S. Geological Survey 

(USGS)/Stanford sensitive high-resolution ion microprobe-reverse geometry (SHRIMP-RG), 

using a spot diameter of approximately 20 micrometers (μm). Calculated ages are the weighted 

averages of concordant ages determined from the 207-corrected 206Pb/238U ratio and are reported 

at 2σ. Spot ages were considered concordant if the 2σ error ellipse overlapped concordia on a 

Wetherhill plot (Spencer and others, 2016). Uncertainties on single spot analyses, when stated, 

are at 1σ. The 204-corrected 207Pb/206Pb age was used for zircons older than 1,300 Ma in the 

probability density plot for detrital zircons from sample BK601 (n=2) and an analysis was considered concordant if it overlapped concordia on a Tera-Wasserburg plot for these grains. All 

zircon grains dated by SHRIMP-RG methods were imaged in backscatter electron (BSE) and 

panchromatic cathodoluminescence (CL) modes on a Hitachi SU5000 field emission scanning 

electron microscope after grain polishing but prior to isotopic analyses. The grains were then 

re-examined by the same methods after isotopic analysis. A subset of dated and undated grains 

from sample BK601 were imaged in ultraviolet light (365 nanometers) on a Zeiss AxioImager 
28 Geochronologic Age Constraints on Tectonostratigraphic Units of the Central Virginia Piedmont, USA

petrographic microscope prior to grain polishing (fig. 7D). Isotopic data for all analysis are 

given in table 3 (available online at https://doi.org/10.3133/pp1861).

Zircon grains were extracted from approximately 5-kilogram samples at the USGS in 

Reston, Virginia. Samples were crushed and ground in a Sturtevant jaw crusher and Bico 

direct-drive disk mill, respectively, and then sieved to less than 250 μm (60 mesh). Particles 

larger than 250 μm were given a second pass through the disk mill. The fraction smaller than 

250 μm was then passed over a Wilfley table. For samples with abundant heavy minerals, the 

heavy fraction was sent over the Wilfley table a second time to reduce the volume for further 

processing. The heavy minerals were dried immediately on a hotplate. Following removal of 

the most magnetic material with a hand magnet, the sample was sent through a Frantz L1 magnetic separator and lithium heteropolytungstate (LST) heavy liquid (ρ=2.85 grams per cubic 

centimeter [g/cm3]). The order of these two steps varied depending on the mineral assemblage 

of the sample. The nonmagnetic fraction heavier than LST was then put through methlyene 

iodide (ρ=3.3 g/cm3), and zircons of interest were hand-picked from the sinks on a binocular 

microscope under incident and transmitted light. Some samples with abundant apatite or pyrite 

remaining in the heavy fraction were also treated with nitric acid to remove these impurities 

and increase the concentration of zircon for easier grain picking.

Selected zircon grains were mounted on double-sided kapton tape and fixed with Struers EpoFix epoxy in a 1-inch round cylinder. The resulting 1-inch round mount was polished 

with 1500 or 2500 grit sand paper to expose grain interiors. The mount was then polished 

on a Struers LaboPol polisher, with 6 μm and then 1 μm diamond suspension with a goal of 

polishing halfway into the grains (for more information see https://shrimprg.stanford.edu/

sample-types-and-sample-preparation). 

SHRIMP-RG analyses were conducted during two separate analytical sessions in 2016 and 

2017. A similar analytical setup was used in each session and measured 9 (2017) or 10 (2016) 

peak locations in 5 cycles on the single collector SHRIMP-RG. The spot size for all analyses 

was approximately 20 μm in diameter and approximately 1 μm in depth. Zircon standard R33 

(419 Ma; Black and others, 2004) mounted with the unknowns was used to correct 206Pb/238U 

ages for elemental fractionation and was run after every fourth unknown analysis. Raw data 

were reduced using Squid 2 (Ludwig, 2009) and plotted using Isoplot 3.75 (Ludwig, 2012). 

Calculated ages are the weighted averages of ages determined from the 206Pb/238U ratio and are 

reported at 2σ. For ages older than 1,300 Ma (sample BK601), the 207Pb/206Pb age is used in 

plotting. Uranium and thorium concentrations are also reported for each analysis and are relative to analyses of the concentration standard MADDER (Barth and Wooden, 2010) that was 

mounted with each set of unknowns.
For additional information, contact:

Director, Florence Bascom Geoscience Center

U.S. Geological Survey

12201 Sunrise Valley Drive

Reston, VA 21092

or visit our website at: 

https://www.usgs.gov/centers/fbgc

Publishing support provided by the 

West Trenton Publishing Service Center
Carter and others—Geochronologic Age Constraints on Tectonostratigraphic Units of the Central Virginia Piedmont, USA—Professional Paper 1861

ISSN 2330-7102 (online)

https://doi.org/10.3133/pp1861