The Central Virginia

Volcanic-Plutonic Belt:

An Island Arc of Cambrian(?) Age
The Central Virginia

Volcanic-Plutonic Belt:

An Island Arc of Cambrian(?) Age

By LOUIS PAVLIDES

CONTRIBUTIONS TO THE GEOLOGY OF THE VIRGINIA PIEDMONT

GEOLOGICAL SURVEY PROFESSIONAL PAPER 1231-A

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1981
UNITED STATES DEPARTMENT OF THE INTERIOR 

JAMES G. WATT, Secretary

GEOLOGICAL SURVEY 

Dallas L. Peck, Director

Library of Congress Cataloging in Publication Data

Pavlides, Louis, 1921-

The central Virginia volcanic-plutonic belt.

(Geological Survey professional paper ; 1231-A. Contributions to the geology of the Virginia Piedmont) 

Bibliography: p.

Supt. of Docs, no.: I 19.16:1231-A

1. Rocks, Igneous. 2. Geology, Stratigraphic-Cambrian. 3. Island arcs-Virginia. 4. Geology-Virginia. I. 

Title. II. Series: Geological Survey professional paper ; 1231-A. III. Series: Contributions to the geology of the 

Virginia Piedmont.

QE461.P328 552M 81-607018

AACR2

For sale by the Distribution Branch, U.S. Geological Survey, 

604 South Pickett Street, Alexandria, VA 22304
CONTENTS

Abstract _______________. 

Introduction _____________. 

Acknowledgments _________. 

Areal distribution __________. 

Enclosing rocks ___________. 

Age __________________________

Metavolcanic rocks _________. 

Lithology and petrography __.

Chopawamsic Formation __.

Ta River Metamorphic Suite . Geochemistry __________.

General statement _____.

Analytical methods ____.

Chopawamsic Formation.

Page

Al

1

2

2

3

6

6

6

6

7

7

7

Ta River Metamorphic Suite_______________ 11

	Page

Meta-intrusive rocks________________________ A14

General statement _______________________ 14

Metamorphosed mafic and ultramafic rocks _________ 15

Trondhjemitic and plagiogranitic metatonalites ______ 15

Lithology and petrography __________________ 17

Trondhjemitic metatonalite suite ____________ 17

Plagiogranitic metatonalite suite ____________ 17

Geochemistry _________________________ 18

Trondhjemitic metatonalite suite ____________ 18

Plagiogranitic metatonalite suite ____________ 20

Regional relationships_______________________ 21

James Run Formation _____________________ 21

Carolina slate belt _______________________ 24

Origin and tectonic setting of the central Virginia volcanicplutonic belt: Discussion and conclusions ___________ 26

Selected references _______________________ 31

ILLUSTRATIONS

Page

FIGURE 1. Generalized geologic map showing the distribution of the Chopawamsic Formation and other rocks in central Virginia _ A2

2. Correlation chart of the Chopawamsic Formation and related metavolcanic rocks and their enclosing formations in Virginia __ 3

3. Generalized geologic map of the Fredericksburg area, Virginia, showing location of samples discussed in text ______ 4

4. Chondrite-normalized rare-earth element patterns for the Chopawamsic Formation and for amphibolites of the Ta River

Metamorphic Suite _______________________________________________________ 12

5. Normative Ab-Or-An diagram showing fields of siliceous plutonic rocks and plots of trondhjemitic metatonalite suite and

plagiogranitic metatonalite suite rocks from the Fredericksburg area, Virginia _________________________ 16

6. Modal plots of A (potassic feldspar)-P (plagioclase, including albite)-Q (quartz) for plagiogranitic metatonalite suite and trondhjemitic metatonalite rocks from the Fredericksburg area, Virginia ____________________________ 16

7. Plot of A12 03 against ytterbium as a discriminant diagram for oceanic in contrast with continental trondhjemite ____ 21

8. Chondrite-normalized rare-earth element plot of metatonalitic rocks of the Fredericksburg area _____________ 22

9. Generalized geologic map of the Piedmont showing distribution of the Carolina slate belt, Eastern slate belt, central Virginia 

volcanic-plutonic belt, South Boston-Danville area, James Run belt, and Wilmington complex; also shown are Mesozoic 

basins, gabbro of the Baltimore Complex of Maryland, melange zone, granitoid plutons, schist beneath the Chopawamsic 

Formation, and other features as identified on the map; isotopic ages reported for some of these metavolcanic rocks are 

also shown ____________________________________________________________ 23 

10-12. Plots of:

10. Amphibolites of the Ta River Metamorphic Suite on the discriminant diagrams of Floyd and Winchester (1975) and

Pearce and Cann (1973) _____________________________________________________ 28

11. Amphibolites of the Ta River Metamorphic Suite on the discriminant diagrams of Winchester and Floyd (1977) ____ 30

12. Metavolcanic rocks of the Chopawamsic Formation on the composition-field diagram of Winchester and Floyd

(1977) _______________________________________________________________ 31

ill
IV CONTENTS

TABLES

Page

TABLE 1. Element mobility during weathering, hydrothermal alteration, and metamorphism __ A8 

2,3. Chemical and normative composition of:

2. The Chopawamsic Formation ___________________________________ 10

3. Amphibolites from the Ta River Metamorphic Suite ____________________ 13 

4, 5. Chemical, normative, and modal composition of:

4. Trondhjemitic-metatonalite in the Fredericksburg area, Virginia _______________ 17

5. Plagiogranitic-metatonalite in the Fredericksburg area, Virginia ______ ____ 19

CONVERSION FACTORS

Metric unit Inch-Pound equivalent

Length

millimeter (mm) 

meter (m) 

kilometer (km)

= 0.03037 inch (in) 

= 3.28 feet (ft) 

= .62 mile (mi)

Area

square meter (m2 ) 

square kilometer (km2 ) 

hectare (ha)

= 10.76 square feet (ft2 ) 

= .386 square mile (mi2 ) 

= 2.47 acres

Volume

cubic centimeter (cm3 ) 

liter (L) 

cubic meter (ms) 

cubic meter 

cubic hectometer (hm3 ) 

liter 

liter 

liter 

cubic meter

cubic meter

= 0.061 cubic inch (in3 ) 

= 61.03 cubic inches 

= 35.31 cubic feet (ft3 ) 

= .00081 acre-foot (acre-ft) 

= 810.7 acre-feet 

= 2.113 pints (pt) 

= 1.06 quarts (qt) 

= .26 gallon (gal) 

= .00026 million gallons (Mgal or 

10" gal) 

= 6.290 barrels (bbl) (1 bbl = 42 gal)

Weight

gram (g) 

gram 

metric tons (t) 

metric tons

= 0.035 ounce, avoirdupois (ox avdp) 

= .0022 pound, avoirdupois (Ib avdp) 

= 1.102 tons, short (2,000 Ib) 

= 0.9842 ton, long (2,240 Ib)

Specific combinations

kilogram per square 

centimeter (kg/cm2 ) 

kilogram per square 

centimeter 

cubic meter per second 

1 m:l/s )

 0.96 atmosphere (atm)

 .98 bar (0.9869 atm) 

= 35.3 cubic feet per second (ft:! /s)

Metric unit

Specific

liter per second (L/s) 

cubic meter per second 

per square kilometer 

[(mVs)/km2 ] 

meter per day (m/d)

meter per kilometer 

(m/km) 

kilometer per hour 

(km/h) 

meter per second (m/s) 

meter squared per day 

(m2/d) 

cubic meter per second 

(m«/s) 

cubic meter per minute 

(m8/min)

liter per second (L/s) 

liter per second per 

meter [(L/s)/m] 

kilometer per hour 

(km/h) 

meter per second (m/s) 

gram per cubic 

centimeter (g/cm3 ) 

gram per square 

centimeter (g/cm2 ) 

gram per square 

centimeter

Inch-Pound equivalent

combinations Continued

= .0353 cubic foot per second 

= 91.47 cubic feet per second per 

square mile [(fts/s)/mi2 ]

= 3.28 feet per day (hydraulic 

conductivity) (ft/d) 

= 5.28 feet per mile (ft /mi)

 .9113 foot per second (ft/s)

 3.28 feet per second

= 10.764 feet squared per day (ft2/d) 

( transmissivity ) 

= 22.826 million gallons per day 

(Mgal/d) 

= 264.2 gallons per minute (gal/min)

= 15.85 gallons per minute 

= 4.83 gallons per minute per foot 

[(gal/min) /ft]

 .62 mile per hour (mi/h)

 2.237 miles per hour

= 02.43 pounds per cubic foot (lb/ft3 ) 

= 2.048 pounds per square foot (lb/ft2 ) 

= .0142 pound per square inch (lb/in2 )

Temperature

degree Celsius (°C) 

degrees Celsius 

(temperature)

= 1.8 degrees Fahrenheit (°F) 

= f(1.8x°C)H-32] degrees Fahrenheit
CONTRIBUTIONS TO THE GEOLOGY OF THE VIRGINIA PIEDMONT

THE CENTRAL VIRGINIA VOLCANIC-PLUTONIC BELT; 

AN ISLAND ARC OF CAMBRIAN(P) AGE

By Louis PAVLIDES

ABSTRACT

Pre-Ordovician, probable Early Cambrian metavolcanic and somewhat younger meta-intrusive rocks form a linear belt in the central 

Virginia Piedmont. On the basis of the geologic and geochemical 

features of these rocks in the Fredericksburg area, and insofar as this 

area is representative of the entire belt, it is concluded that this belt 

constitutes an ancient island-arc sequence. The westernmost 

Chopawamsic Formation consists of a tholeiitic island-arc suite and 

associated calcalkaline metavolcanic rocks. Prior to Upper Ordovician 

time, low-potassic tonalitic plutons of plagiogranitic (pretectonic) and 

trondhjemitic (late-tectonic) affinities were intruded into the 

Chopawamsic terrane. The tholeiitic amphibolites of the Ta River 

Metamorphic Suite are considered to be an eastern, oceanward facies 

of the Chopawamsic Formation.

The James Run Formation of Maryland is probably a northern 

lateral equivalent of the central Virginia volcanic-plutonic belt. It may 

have formed as part of the ancient Virginia belt or along a penecontemporaneous but independent volcanic arc. The metavolcanic rocks and 

associated plutons of the Carolina slate belt in the southeast Piedmont 

are ^considered to have formed independently of the central Virginia 

volcanic-plutonic belt and perhaps in a different tectonic environment.

INTRODUCTION

The central Virginia volcanic-plutonic belt of preOrdovician age consists of (1) irregularly interlayered 

metavolcanic and metasedimentary rocks that trend 

northeast in northeastern and central Virginia and (2) 

metamorphosed intrusive rocks believed to be closely 

associated temporally and tectonically with the 

metavolcanic rocks (fig. 1). The volcanic nature of the 

belt was first recognized by Lonsdale (1927, pi. 1), who 

distinguished amphibolite, greenstone schist, and 

crystalline schist immediately beneath the Quantico 

Slate of the usage of Watson and Powell (1911). The 

Quantico Slate of Watson and Powell (1911) has been 

renamed the Quantico Formation (Pavlides, 1980a). 

Most of the volcanic rocks now assigned to the 

Chopawamsic Formation (Southwick and others, 1971) 

were shown as Peters Creek quartzite on the 1928

geologic map of Virginia (Virginia Geological Survey, 

1928) but on the 1963 geologic map of Virginia (Virginia 

Division of Mineral Resources, 1963) a belt of rocks that 

includes the Chopawamsic Formation is shown as 

"metamorphosed volcanic and sedimentary rocks." The 

name Chopawamsic Formation was first applied to the 

belt of metavolcanic rocks immediately below the Quantico Formation in the Quantico Quadrangle (Mixon and 

others, 1972) by Southwick and others (1971), who 

designated Chopawamsic Creek, about 40 km south of 

Washington, B.C., as the type section.

The Chopawamsic Formation, and its more mafic 

eastern equivalent, the Ta River Metamorphic Suite 

(Pavlides, 1980a), and the rocks intrusive into and tectonically associated with these formations in the 

Fredericksburg area (fig. 1), are the only units within 

the central Virginia volcanic-plutonic belt for which extensive chemical and petrographic data are presently 

available. The interpretation presented here of the 

paleotectonic environment of the whole belt is thus 

based on the data of these rocks alone. The general 

geology of the Fredericksburg area has been described 

elsewhere (Pavlides, 1976, 1980a) and is not discussed 

further in this paper. Because of their particular 

relevance to the paleotectonic history, the discussion 

emphasizes the felsic meta-intrusive rocks of the belt. 

The associated mafic and ultramafic rocks briefly mentioned in this report will be discussed elsewhere.

The purpose of this paper is to describe briefly and 

characterize the geology of the central Virginia 

volcanic-plutonic belt and to interpret new geochemical 

data that bear on the origin, tectonic setting, and 

general regional geologic relations of these ancient 

metamorphosed volcanic and plutonic rocks. The James 

Run Formation and Carolina slate-belt metavolcanic 

rocks are also briefly described and evaluated as to their

Al
A2 CONTRIBUTIONS TO THE GEOLOGY OF THE VIRGINIA PIEDMONT

0 10 20 30 40 MILES 

t

OCCOQUAN PLUTON

FIGURE 1.-Generalized geologic map showing the distribution of the Chopawamsic Formation and other rocks in central Virginia. Arvonia 

Slate within the Arvonia syncline (AS), Charlotte belt metavolcanic rocks (CBM), Columbia syncline (CS), Carolina slate belt (CSB), Evington Group metavolcanic rocks (EGM), granitoid plutons (g), Hatcher Compex of Brown (1969) (HC), melange zone (m), Mesozoic basins 

(Mz), Quantico-Columbia synclinorium (QCS), Quantico syncline (QS), Quantico Formation (Oq), Ta River Metamorphic Suite (TRS), Rappahannock anticlinorium (RA)-includes terrane of HC and TRS, and schist (s) stratigraphically and structurally beneath the Chopawamsic Formation.

correlativeness with the central Virginia volcanicplutonic belt.

ACKNOWLEDGMENTS

I am particularly indebted to my former colleague S. 

Linda Cranford, who helped compile and prepare the illustrations and tables used in this report and also contributed modal analyses of some of the granitoid suites. 

Discussions with numerous geologists helped formulate 

some of the ideas expressed in this report. Especially 

helpful were David Gottfried and Joseph G. Arth, who 

freely gave of their time to discuss, as well as to share 

their knowledge of, the geochemistry of volcanic and 

associated plutonic rocks. The helpful reviews of this 

manuscript by N. L. Hatch, Jr., M. W. Higgins, and G. 

W. Leo are greatly appreciated.

AREAL DISTRIBUTION

The central Virginia volcanic-plutonic belt of the 

Virginia Piedmont includes metavolcanic rocks underlying the Quantico Formation and the Arvonia Slate 

within the Quantico-Columbia synclinorium and the 

Arvonia synclinorium. Presently available geologic mapping indicates that it extends from the Coastal Plain 

boundary in northeastern Virginia southwestward into 

the central Piedmont of Virginia at least as far as midway between the James and Roanoke Rivers (fig. 1). The 

exact nature and location of its southwest extent is not 

presently known.

The main characterizing unit of the central Virginia 

volcanic-plutonic belt is the Chopawamsic Formation. 

Different segments of the Chopawamsic have been mapped by Southwick and others (1971), Seiders and others
THE CENTRAL VIRGINIA VOLCANIC-PLUTONIC BELT: AN ISLAND ARC OF CAMBRIAN(?) AGE A3

(1975), Pavlides (1976, 1980a), Pavlides and others

(1974), Good and others (1977), and Conley and Johnson

(1975). Metavolcanic rocks of the Evington Group along 

the northwest limb of the Arvonia synclinorium stratigraphically beneath the Arvonia Slate and beneath the 

Arvonia Slate in the Long Island syncline (Brown, 1969, 

pi. 1; Smith and others, 1964, pi. 1) are now considered 

to belong to the Chopawamsic Formation (Brown, 1976, 

p. 142; Conley, 1978, fig. 1). Southwick and others 

(1971, p. D9) originally suggested that the Evington 

Group metavolcanic rocks were possibly coeval with the 

Chopawamsic Formation.

More controversial is the correlation of the 

Chopawamsic Formation with the amphibolites of the 

Hatcher Complex of Brown (1969) that are found along 

the southeast side of the Arvonia and Columbia 

synclines. Chopawamsic metavolcanic rocks have been 

mapped around the southwest end of the Columbia 

syncline and reported as merging into and being interlayered with some of the amphibolitic rocks of 

Brown's Hatcher Complex (Conley and Johnson, 1975; 

Good and others, 1977; Conley, 1978, p. 134). Pavlides 

(1980a) considered the amphibolites and amphibolite 

gneisses of the Ta River Metamorphic Suite, along the 

southeast limb of the Quantico-Columbia synclinorium 

mostly south of the Rappahannock River, as a coeval 

eastern, more mafic facies of the Chopawamsic Formation. The Ta River Metamorphic Suite is an apparent 

northeast continuation of Brown's Hatcher amphibolitic 

rocks, and both terranes have the same magnetic signature (Neuschel, 1970). Figure 2 summarizes the correlations of the metavolcanic units assigned to the central 

Virginia volcanic-plutonic belt.

The Holly Corner Gneiss (fig. 2) is also believed to be a 

more metamorphosed eastern facies of the Chopawamsic (Pavlides, 1980a). The Holly Corner and an intrusive 

sill-like pluton, the Falls Run Granite Gneiss, crop out 

northwest of Fredericksburg along the Rappahannock 

River (fig. 1). Because the Holly Corner is allochthonous 

(Pavlides, 1979, p. 51) and more tenuously correlated 

with the Chopawamsic Formation, it is not discussed in 

this report.

ENCLOSING ROCKS

The Chopawamsic Formation is underlain conformably by pebbly gneiss (diamictite) in the Quantico area of 

Virginia (Southwick and others, 1971). The diamictite of 

northern Virginia does not extend much farther south 

than the area studied by Southwick and others (1971, 

fig. 1). Rather, for a considerable distance along strike 

to the southwest the Chopawamsic has been intruded by 

plagiogranitic rocks and locally by mafic rocks of the 

mafic complex at Garrisonville (fig. 3; Pavlides, 1976) 

and related plutons. Farther southwest, beyond the belt 

of plagiogranitic intrusions, the Chopawamsic grades 

downward into schist and silty schist that locally may 

contain thin and sparse layers of metavolcanic rocks.

The substrate on which the Ta River Metamorphic 

Suite lies is not exposed. Locally, it is assumed to be in

ARVONIA SYNCLINE

QUANTICO-COLUMBIA SYNCLINORIUM

-COLUMBIA SYNCLINE- QUANTICO SYNCLINE

NORTHWEST 

LIMB

SOUTHEAST 

LIMB

ARVONIA SLATE

Conglomerate 

near CarysbrookNORTHWEST 

LIMB

SOUTHEAST 

LIMB

QUANTICO FORMATION

NORTHWEST 

LIMB

SOUTHEAST 

LIMB

^Quartz ite :r=-

QUANTICO FORMATION

- UNCONFORMITY - -UNCONFORM1TYTA RIVER 

METAMORPHIC 

SUITE

HOLLY 

CORNER 

GNEISS

PO RIVER METAMORPHIC SUITE

oEoL UJ-<

FIGURE 2.-Correlation chart of the Chopawamsic Formation and related metavolcanic rocks and their enclosing formations in Virginia.
CONTRIBUTIONS TO THE GEOLOGY OF THE VIRGINIA PIEDMONT

77°22'30"

38°30'00"

5 KILOMETERS

FIGURE 3.-Generalized geologic map of the Fredericksburg area, Virginia, showing location of samples discussed in text. Circles, 

Chopawamsic Formation; squares, Ta River Metamorphic Suite; triangles, plagiogranitic metatonalite; crosses, trondhjemitic 

metatonalite.
THE CENTRAL VIRGINIA VOLCANIC-PLUTONIC BELT: AN ISLAND ARC OF CAMBRIAN(?) AGE

EXPLANATION

A5

ftt 1-v-epv

CzKu

DSfr 

Oq

 Cu

------- - - -

ii^^g^j u-cchi

> Cretaceous

> Carboniferous 

Devonian 

and Silurian 

* Ordovician

 Cambrian!?)

^CENOZOIC 

- MESOZOIC

 PALEOZOIC

 - n PALEOZOIC AND(OR) 

_ _ _ JPROTEROZOIC Z(?)

Coastal Plain sediments: Unconsohdated, upland sand and gravel, terrace, and 

alluvium and swamp deposits

Falmouth Intrusive Suite: --Dikes and sills and small plutons of fine- to medium-grained 

granitoids, mainly monzogramte to granodionte and less commonly tonalite

Falls Run Granite Gneiss Coarse-grained, well-foliated, microclme granite gneiss 

(including Holly Corner Gneiss (Lower Cambrian?) Fine- to medium-grained, 

well-foliated, hornblende-biotite gneiss )

Quantico Formation: Gray to black graphitic slate and phyllite to staurolitic schist and 

biotite-muscovite garnetiferous schist in areas of increased metamorphism Lenses 

of micaceous quartzite occur within and locally at its base.

Chopawamsic Formation Chiefly metavolcanic rocks of felsic, intermediate, and 

mafic compositions mterlayered with lesser amounts of meta-epiclastic volcanic 

rocks and schist

Ta River Metamorphic Suite: Chiefly amphibolite and amphibolitic gneiss with lesser 

amounts of biotitic gneiss and schist.

Plagiogranitic metatonalite Medium- to coarse-grained leucocratic to mesocratic 

quartz-rich meta-intrusive granitoid.

Trondhjemitic metatonalite. Fine-grained, allotnomorphic granular, leucocratic 

granitoid, locally having granophync texture

Mafic complex at Garnsonville Metamorphosed mafic plutonic rocks, chiefly 

amphibolite and hornblendite

Diamictite Dark-gray granite like metasedimentary rocks characterized by quartz 

lumps and clasts of schist and gneiss randomly scattered in a micaceous quartz- and 

plagioclase-nch matrix.

Undifferentiated schists, metasiltstones, and graywackes

Po River Metamorphic Suite: Chiefly biotite gneiss with lesser amounts of hornblende 

gneiss and schist

CONTACT Commonly inferred, dotted where concealed. 

FAULT Commonly inferred, dotted where concealed

ANTIFORM Showing trace of crestal plane and direction of plunge Dotted where 

concealed

OVERTURNED UPRIGHT

SYNFORM Showing trace of trough plane and direction of plunge 'Dotted where 

concealed.

OVERTURNED UPRIGHT

FIGURE 3.-Continued.
A6 CONTRIBUTIONS TO THE GEOLOGY OF THE VIRGINIA PIEDMONT

stratigraphic contact with the Po River Metamorphic 

Suite of Paleozoic to Precambrian age, but mostly it is in 

fault contact with the Po River (Pavlides, 1980a, pi. 1). 

No direct information is available as to the nature of the 

rocks that may underlie the coeval parts of the Hatcher 

Complex of Brown (1969).

The Chopawamsic Formation is described by Southwick and others (1971, p. D3) as being conformably and 

gradationally overlain in the Quantico area by the Quantico Formation of Ordovician age. Within the area 

shown in figure 3, there is evidence for an unconformity 

between the Chopawamsic and the Quantico (Pavlides, 

1973). This unconformity is placed stratigraphically 

below an impure quartzitic unit that occurs discontinuously at the base of the Quantico. The unconformity 

is believed to be the same as that between the Arvonia 

Slate and subjacent metavolcanic rocks of the Evington 

Group (Chopawamsic Formation) in the Arvonia syncline (see fig. 2). This unconformity has now been 

recognized near Quantico (Pavlides and others, 1980), 

where heretofore the contact between the Chopawamsic 

and the Quantico was considered gradational and concordant (Seiders and others, 1975).

AGE

The rocks of the central Virginia volcanic-plutonic belt 

do not contain recognized fossils and thus can be dated, 

at present, only by isotopic methods. However, both the 

Quantico Formation and the Arvonia Slate that unconformably overlie the Chopawamsic Formation or its 

equivalents contain Late Ordovician fossils and thus 

establish a minimum age for these subjacent metavolcanic rocks. The Chopawamsic and the coeval James 

Run Formation were dated as about 550 m.y. (million 

years) old on the basis of zircons from felsic layers 

within these formations (Tilton and others, 1970; Higgins and others, 1971, p. 320). A controversy ensued, 

however, as to the meaning of such discordant zircon 

ages from the Piedmont (Higgins, 1976; Higgins and 

others, 1977; Seiders, 1978a; Zartman, 1978) following 

the report by Seiders and others (1975, p. 492-495) that 

the Dale City pluton near Quantico, which gave discordant zircon ages of about 560 m.y., intruded the Quantico Formation. Seiders and others concluded from 

these relations that the age of the Quantico had to be 

Early Cambrian or older. However, the age of the Quantico has been reestablished by fossils as post-Cambrian 

and most likely Late Ordovician, and the Dale City 

pluton has been demonstrated to be unconformable 

beneath the Quantico (Pavlides and others, 1980). 

Therefore, the zircon ages of the Dale City pluton, which 

I consider to be an intrusive into the Chopawamsic 

volcanic pile, are not in conflict with the known regional 

geologic relationships. In view of this, I conclude that

the discordant ages obtained from zircons of such rocks 

as the Dale City pluton (about 560 m.y.) and the 

Chopawamsic Formation (about 550 m.y.) are probably 

significant, and an Early Cambrian(?) age is provisionally accepted for the Chopawamsic Formation. However, the nature of the available data is such that an Ordovician, pre-Quantico age for the Chopawamsic cannot 

entirely be ruled out.

METAVOLCANIC ROCKS

LITHOLOGY AND PETROGRAPHY

CHOPAWAMSIC FORMATION

In the type area along Chopawamsic Creek 

(Southwick and others, 1971) the Chopawamsic Formation is a sequence of interbedded metavolcanic and 

metasedimentary rocks about 1,820-3,040 m thick. This 

estimate assumes that the section is free of structural 

complication and is essentially homoclinal. Southwick 

and others (1971) divided the Chopawamsic into three 

principal units, all of which are metamorphosed at least 

to greenschist facies: (1) Medium- to thick-bedded mafic 

to intermediate metavolcanic rocks derived from andesitic to basaltic flows, coarse breccias, and finer tuffaceous clastic rocks; (2) medium- to thick-bedded felsic 

metavolcanic rocks derived from flows and associated 

volcaniclastic sediments; and (3) thin- to medium-bedded 

metavolcaniclastic rocks of felsic to mafic composition, 

locally containing felsic to mafic flows and beds of nonvolcanic quartzose metagraywacke and green to gray 

phyllite. Units 1 and 2 grade vertically and laterally into 

unit 3 and appear to be tongues or lenses within a complex volcanic-sedimentary pile. A few thin beds of 

specular hematite, interlayered with felsic metavolcanic 

rocks, occur within the upper part of the Chopawamsic 

Formation in Chopawamsic Creek (A. R. Bobyarchick, 

oral commun., 1979). Immediately to the northeast of 

the Quantico area, Seiders and others (1975, p. 483) 

reported that the Chopawamsic consists of about 2,500 

m of metasedimentary and metavolcanic rocks.

The Fredericksburg area (fig. 3) in part overlaps and 

extends contiguously to the southwest of the area 

described by Southwick and others (1971). The 

Chopawamsic here also consists of a series of intertonguing lenses of metasedimentary and metavolcanic 

rocks, but the distribution of these lenses appears to be 

more random than in the type area of Southwick and 

others. The thickness varies but may be as much as 

3,000 m. The metavolcanic rocks of the Chopawamsic in 

the Fredericksburg area are mostly nonfragmental. 

Volcanic breccias and conglomerates are rare, although 

lithic and crystal tuffs or mixed tuffs have been 

recognized.
THE CENTRAL VIRGINIA VOLCANIC-PLUTONIC BELT: AN ISLAND ARC OF CAMBRIAN(?) AGE A7

Near Mineral, Va., coarse-grained fragmental metavolcanic rocks are present in diamond-drill cores 

(Gair, 1978, p. 32-52) but are seldom found in surface 

outcrops, suggesting that the fragmental volcanic rocks 

are more susceptible to weathering than are some of the 

other volcanic rocks in the section. Nonetheless, even in 

drill cores, which actually penetrate only narrow parts 

of the Chopawamsic belt, such coarse-grained fragmental volcanic rocks probably constitute only about 5 percent of the section, according to Gair (oral commun., 

Jan. 1979).

In the Fredericksburg area metavolcanic rocks of the 

Chopawamsic belt are mostly felsic and intermediate in 

composition. Felsic rocks are commonly light gray and 

have visible small phenocrysts of quartz and (or) 

feldspar. Some felsic rocks are highly sodic 

(keratophyres) and contain fine-grained feldspar and 

quartz insets in a finely crystalline quartzofeldspathic 

matrix. Epidote occurs as a minor alteration of some 

plagioclase phenocrysts in felsic rocks having 

plagioclase that was originally more calcic than albitic. 

Dark-green metavolcanic rocks are of intermediate composition, as indicated by their Si02 content, which 

ranges from about 55 to 65 percent. In thin section, they 

commonly have a nematoblastic groundmass texture 

formed by alined prismatic amphibole intergrown with 

fine-grained quartz and feldspar. Blue-green actinolitic 

amphibole locally forms fine-grained porphyroblasts 

that, in some rocks, are arranged in small bundles 

(fascicles). Epidote, chlorite, and accessory magnetite 

are common minor constituents of these amphibolitic 

rocks. Locally, carbonate- and (or) quartz-filled vesicles 

or amygdules are present. Pillow lavas have been 

observed only in two small outcrops (Pavlides, 1976). 

Greenstones composed of quartz, chlorite, generally 

albitic plagioclase, and blue-green amphibole, some with 

relict basaltic textures, are interpreted as metabasalt.

Metavolcanic tuffs and conglomerates are sparse. 

Where present, the clasts in tuffs commonly consist of 

both volcanic rocks and individual, generally wellformed feldspar crystals, suggesting that the rocks may 

have originated as mixed lithic and crystal tuffs. 

Epiclastic volcanic conglomerate is rare and consists of 

volcanic clasts and rounded quartz grains having slightly embayed margins set in a fine-grained quartz-feldspar 

groundmass.

Epivolcaniclastic rocks in the northern part of the 

area are generally sparse and consist of relatively 

amphibole-free quartzose and schist layers that interfinger or lie between amphibole-rich quartzose streaks 

or thin layers. Ironstone or ferruginous metasedimentary rock has not been recognized within the 

Chopawamsic Formation in the Fredericksburg area.

However, Good and others (1977, fig. 2) showed ferruginous quartzite as being locally present in 

Chopawamsic within tfre Columbia syncline.

TA RIVER METAMORPHIC SUITE

Amphibolite is the characterizing lithology of the Ta 

River Metamorphic Suite, described and named by 

Pavlides (1980a). These rocks are generally conformably 

layered with granitoid rocks and smaller amounts of 

biotite gneiss and schist in the northern part of the 

Fredericksburg region (fig. 3). To the southwest, along 

strike, the Ta River contains more biotite gneiss and 

schist and smaller amounts of amphibolite gneiss. 

Regional metamorphic grade also increases to the 

southwest, and the associated granitoid bodies are 

generally more felsic than are those in the northeast 

part of the Ta River Metamorphic Suite. Some of the 

granitoid rocks are of Carboniferous age (Pavlides and 

others, 1979) and are not part of the plutonic rocks temporally related to the central Virginia volcanic-plutonic 

belt.

The amphibolites of the Ta River contain green hornblende (commonly poikiloblastic) and plagioclase, both 

twinned and untwinned, ranging from andesine to 

bytownite. In general, textures in thin section range 

from granoblastic to foliated. Epidote normally is present in subhedral grains and is very abundant in some 

rocks. Sphene is commonly a fine-grained anhedral 

accessory. Quartz is locally abundant.

The Ta River, like the Chopawamsic Formation within 

the Fredericksburg area, also appears to lack ironstone 

beds. However, ironstone layers are locally present to 

the southwest in the amphibolitic gneisses (Gair, 1978, 

p. 32-52) on the southwest side of the Arvonia syncline, 

which are considered to be coeval with the Ta River 

Metamorphic Suite and the Chopawamsic Formation.

GEOCHEMISTRY

GENERAL STATEMENT

In geologically old volcanic terranes, particularly 

those that have been structurally deformed and metamorphosed, the original chemical composition of the 

volcanic protoliths has generally been modified. The central Virginia volcanic-plutonic belt is such a terrane. 

Diagenesis, hydrothermal alteration, weathering, metamorphism, and metasomatism are among the processes 

through which the bulk chemical composition of the 

original rock may be modified.

Table 1 is a partial list of the relative mobility of 

elements during some of these processes. This compilation shows that most major and many minor elements
A8 CONTRIBUTIONS TO THE GEOLOGY OF THE VIRGINIA PIEDMONT

TABLE 1.-Element mobility during weathering, hydrothermal alteration, and metamorphism

[+ , element gained; -, element lost; ( ), minor gain or loss; 0, element immobile; x, element mobile]

Element ,, . ., . Marme weathering Zeolite Amphibolite ",=- «SSi- * *«- , '«- , Hydrothermal

References

Si

Al

Fe

Mg

Ca

Na

K

Ti

P

Mn

Rb

Ba

Sr

Th

Zr

Hf

Nb

Ta

Cu

Li

Cr

Ga

B

Zn

V

LREE*

T T T> TTl T71 > 

_TT £Vl-J Hi

Y

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18'

+

- 0 (-) - -

+ + +

+ + + +- +0- -x (-)

+ 

+ + 

000 

00 

00

x

+ 0 - x

0

( + ) 00000 00 0

0 0

00 00 00 0

0

0

0

0 +

00 0

0 000 00

Amphibolite grade metamorphism considered isochemical and isovolumetric. 

! Light rare-earth elements: La, Ce, Pr, Nd, Pm, Sm. 

1 Heavy rare-earth elements: Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu.

References- Sources of data:

1. Hart (1970).

2. Mathews (1971).

3. Thompson and Rivers (1976).

4. Henricks and Thompson (1976).

5. Ludden and Thompson (1978).

6. Ludden and Thompson (1979).

7. 14, 17. Cann (1970).

8. Gottfried and others (1977).

9. Humphris and Thompson (1978). 

10. Wood and others (1976).

11. 15. Morrison (1978).

12. Melson and Van Andel (1966).

13. Cann (1969).

16, 18. Kerrich and others (1977).

are mobile under different alteration processes. Only a 

few elements such as zirconium, hafnium, niobium, tantalum, the heavy rare-earth elements (HREE), and, in 

part, A1203 , titanium, and phosphorus appear to be immobile or relatively immobile. Various diagrams have 

been constructed using combinations of elements, some 

stable and some mobile, to characterize the tectonic environments in which various relatively unaltered and 

generally basaltic volcanic assemblages formed (see, for 

example, those of Pearce and Cann, 1973, and Floyd and 

Winchester, 1975, 1978). However, the use of such 

paleotectonic discrimination diagrams may sometimes 

be inconclusive or misleading (Morrison, 1978; Smith 

and Smith, 1976; Gottfried and others, 1977). In this 

paper a combination of "immobile" minor elements in 

conjunction with rare-earth element (REE) patterns 

that have internally consistent distribution characteristics are used to identify magma types involved in 

the formation of the Chopawamsic and the Ta River 

volcanic rocks and some of the associated intrusive 

granitoids characterized by a low potassium content.

Paleotectonic environments are also deduced within the 

limitations of the analytical data base. Such limitations 

are severe in that only a small number of analyses are 

available as a consequence of the extensive weathering 

and generally poor exposure that characterize this part 

of the Virginia Piedmont. Also the stratigraphic position 

of samples for such analyses is not aljvays certain 

because of the structural complexity of the region. 

Therefore, the results and conclusions presented below 

are of necessity preliminary and subject to revision as 

more data become available. Finally, in the discussions 

of the ge.ochemistry of the metavolcanic and metaintrusive rocks of the Fredericksburg area, only those 

elements are emphasized that are generally considered 

to have little or no mobility during alteration processes.

ANALYTICAL METHODS

All chemical analyses cited in this report were made in 

the analytical laboratories of the U.S. Geological Survey 

in Reston, Va. Major element oxides were determined
THE CENTRAL VIRGINIA VOLCANIC-PLUTONIC BELT: AN ISLAND ARC OF CAMBRIAN(?) AGE A9

either by rapid rock methods described by Shapiro 

(1975) or by X-ray spectroscopy. The rapid-rock method 

is supplemented by atomic-absorption spectrometry. 

Abundances of 10 major constituents were determined 

from a single solution obtained by a nitric acid dissolution of a sample fused with lithium metaborate-lithium 

tetraborate. CaO, MgO, and Na20 contents were determined by atomic-absorption spectrometry; Si02, A1203 , 

Fe203, Ti02, P205, and MnO contents were determined 

spectrophotometrically. Separate sample portions were 

used to determine FeO, H20 (+ and -), and C02 contents.

Where major elements were determined by X-ray 

spectroscopy, 700 mg of ground (<200 mesh) sample 

was mixed with 5.600 g Li2B407 flux and 700 mg La203 

heavy absorber and fused in a platinum gold crucible at 

1,100°C. The molten flux was then poured into a discshaped mold and cooled. Sample discs were analysed on 

a diano model 8600 fully automated X-ray fluorescence 

spectometer using discs prepared from U.S. Geological 

Survey (USGS) standard rocks as calibration standards.

Niobium and nickel content were determined by a 

spectrophotometric method (Greenland and Campbell, 

1974). After decomposition by hydrofluoric acid and 

evaporation to volatilize silica, the samples were fused 

with pyrosulfate and dissolved in hydrochloric acidtartaric acid. After separation by a thiocyanate extraction with amyl alcohol and back-extraction with dilute 

hydrofluoric acid, the niobium was reacted with 

4-(2-pyridylazo)-resorcinol. Analytical error, calculated 

on the basis of replicate anaylses of eight USGS standard rocks, ranges from 2.9 to 6.4 percent in the concentration range 10-27 ppm (parts per million).

Synthetic standards and USGS standard rocks 

BCR-1, W-l, and AGV-1 (Flanagan, 1973) were used 

to establish the analytical curve for the element concentrations determined spectrographically.

Instrumental neutron activation was used to determine concentrations of large cation trace elements, 

minor ferromagnesian elements, and some of the rareearth elements (La, Ce, Nd, Sm, Eu, Tb, Yb, and Lu). 

Three 0.15-g replicate samples packed in polyethylene 

vials were irradiated for two hours at a flux of 5x 1013 

neutrons cm-^sec-- 1 at the National Bureau of Standards reactor, Gaithersburg, Md. A standard was synthesized from an analyzed obsidian that was doped with 

solutions of selected trace elements, dried, reground, 

and calibrated relative to seven USGS standard rocks: 

BCR-1, G-2, AGF-1, GSP-1, PCC-1, DTS-1, and 

W-l (Flanagan, 1973). The samples and standards were 

counted on a Ge(Li) detector one week and six weeks 

after irradiation. The tantalum content was determined

by counting on a low-energy photon detector five months following irradiation. The spectral data were processed on an IBM 370 * computer by means of the program SPECTRA (Baedecker, 1976).

CHOPAWAMSIC FORMATION

Metavolcanic rocks of a wide composition range that 

includes silicic, intermediate, and mafic types make up 

the Chopawamsic Formation. Table 2 lists chemical 

analyses of samples from the Chopawamsic, whose geographic distribution is shown in figure 3. The analyses 

are grouped into three suites based on the striking 

distribution differences of high-valence cations, particularly niobium and the rare-earth elements (REE). 

The suites also correspond to geographic groupings, in 

that most of the rocks of suite A are from the northern 

part of the Chopawamsic belt, suite C is from the 

southern part of the belt, and the two samples that constitute suite B lie between suites A and C (fig. 3). Some 

of the rocks in the southern part of the belt (samples 13 

and 14) have chemical similarities to suite A and are included with that suite (table 2). The small number of 

samples that make up suites B and C, as compared with 

suite A, reflect the differences in degree of saprolitization and depth of stream incision through the saprolite 

in the geographic areas of the various suites. Suite A 

samples come mostly from streams near the PiedmontCoastal Plain contact or from the lower part of streams 

tributary to the Rappahannock River that mostly flow 

on bedrock within the area of figure 3. On the other 

hand, suites B and C are from areas of more pervasive 

weathering, where fewer fresh-rock exposures are available. Furthermore, the fact that suites B and C appear 

to contain only silicic rocks (table 2) is, in part, an artifact of sampling and may not truly represent the compositional makeup of these suites. Intermediate and 

mafic metavolcanic rocks, too weathered to be analyzed 

for meaningful results, also occur in the geographic 

areas of suites B and C. In general, however, the central 

and southern parts of the Chopawamsic belt in the 

Fredericksburg area (fig. 3) are composed mostly of 

silicic rocks.

Suite A is highly sodic and locally contains 

keratophyres (table 2), which may result from alteration 

rather than primary magmatic composition. However, 

Coleman and Donato (1979, p. 161-162) suggested that 

keratophyres may be the effusive equivalents of plagiogranite and hence of magmatic and not metasomatic 

origin. In any event, mobility of K20 in suites B and C

1 Any use of trade names in this publication is for descriptive purposes only and does not 

constitute endorsement by the U.S. Geological Survey.
A10 CONTRIBUTIONS TO THE GEOLOGY OF THE VIRGINIA PIEDMONT

TABLE 2.-Chemical and normative composition of the Chopawamsic Formation

Suites

Sample No

Field No

SiO,

A12O3

Fe;03

FeO

MgO __ _ _ _ .

CaO

Na:0

K2O

H,O' H,OTiO,

P.O,

MnO

CO,

Total

Q __ _ - ___ _____

c

or

ab

an

wo

en

fs

fo

fa

mt

hm

il

ru

ap

cc

Total

di

di-wo

di-en

di-fs

hy ____________________

hy-en

hy-fs ____

ol

ol-fo

ol-fa

DI

Rb

Ba

Sr

K/Rb

Ba/Rb

Th

U

Zr

Hf Nb4

Ta

Th/U

Zr/Hf

NbxlOO/Ti

Nb/Ta

Co

Ni4

Zn

Cr

Sc

Ni/Co

1

P-71-

9

50.8

19.2

1.4

8.3

7.1

- _ 4.1

4.9

.12

3.4

.07

.36

. _ .06

.25

.05

100.11

3.8

.7

42.9

20.7

10.3

8.1

5.6

4.9

2.1

.7

.2

100.0

18.4

10.3

8.1

10.5

5.6

4.9

43.7

113

3

P-70-

73

51.5

17.0

.50

10.2

7.0

4.7

5.0

.25

2.5

.14

.87

.10

.25

.08

100.09

0.4

1.5

43.4

22.7

5.3

5.3

8.9

9.7

.7

1.7

.2

.2

100.0

10.6

5.3

5.3

18.5

8.8

9.7

44.9

48

110

1.7

30

1.0

2.7

76

.1

42.8

81

166

258.5

44.25

1.9

A

13 10 8 4 5 14 6 7

142A 13 150 67 64 124 63 61

Major oxide composition (weight percent)

51.69 53.4 55.0 56.5 59.0 64.40 64.5 76.0

15.20 15.6 15.3 14.6 13.8 14.72 11.4 12.7

8.2 2.8 5.5 3.6 5.2 3.2 3.5 2.4

7.25 3.8 6.8 6.2 2.5 2.85 3.8 1.2

3.40 4.8 4.1 4.4 5.7 3.26 4.6 5.6

.23 .08 .19 .15 .15 .64 .15 .80

1.3 2.1 .86 1.0 1.6 1.7 .80 .34

.39 .16 .11 .07 .22 .22 .03 .03

2.06 1.6 1.5 1.5 1.6 1.30 1.5 .21

.12 .25 .22 .26 .23 .18 .24 .07

.19 .24 .11 .22 .30 .14 .22 .03

.01 .04 .02 .05 .05 .01 .05 .02

99.32 100.97 101.11 99.75 100.0 98.93 99.79 99.94

Normative mineral composition (weight percent)

[Based on analyses recalculated to 100 percent water-free oxides]

12.9 2.9 9.6 10.7 13.6 33.8 24.6 37.9

1.4 .3 4.1 .7

29.4 41.2 34.6 37.8 49.1 28.4 39.3 47.6

26.1 17.2 22.7 19.9 11.1 13.3 10.1 5.4

4.1 3.9 4.0 3.1

8.1 13.6 9.7 8.8 7.6 5.4 6.5 .4

1.4 15.3 7.0 9.1 6.0 3.3 6.9

12.2 4.1 8.0 5.3 7.7 4.8 5.1 .7

1.9

4.0 3.1 2.8 2.9 3.1 2.5 2.9 .4

.3 .6 .5 .6 .6 .4 .6 .2

.1 .1 .1 .1 .1

100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0

7.8 7.7 7.9 6.1

4.1 3.9 4.0 3.1

3.1 2.2 1.9 1.5

.6 1.6 2.0 1.5

5.9 29.0 12.9 14.0 13.6 8.7 10.4 0.4

5.0 13.6 7.5 6.9 7.6 5.4 5.1 .4

.9 15.4 5.4 7.1 6.0 3.3 5.3

43.7 44.6 45.3 49.4 63.7 66.2 64.8 90.3

Trace-element abundances (parts per million) 

Large cations2

12

42 43 77 46 24 132 172

159 77 147 201 67 311 114

537

14

High-valence cations3

0.9 1.5 1.1 1.4 1.6 3.0 2.5

1.0 .4 .6 .3 1.5 1.0 .8

60 62 63 55 120 70 150

2.1 1.7 1.7 1.5 3.3 2.2 3.8

6.7 4.7 4.7 6.7 6.0 7.1 4.5

.12 .21 .32 .44 .41

.9 3.8 1.8 4.7 1.1 3.0 3.1

29 36 37 37 36 32 40

.1 .1 .1 .1 .1 .1 .4

55.8 31.9 18.8 16.1 11.0

Ferromagnesian elements5

28.9 28.6 29.7 33.6 65.2 7.4 2.3

12 6 1 12 5 1

124 175 63 120 172 174 44

14.4 7.2 5.7

38.83 36.56 37.85 35.48 32.78 23.87 9.78

.4 .2 .4 .1 .4

2

7

78.3

11.6

.61

1.9

.43

.27

5.2

.13

1.2

.14

.14

.04

.03

.05

100.04

45.8

2.5

.8

44.6

1.1

1.1

2.8

.9

.3

.1

100.0

3.9

1.1

2.8

91.2

3

80

29

324

24

1.6

.3

76

2.6

4.4

.20

5.3

12

.6

22.0

1.4

1

24

7.80

.7

B

11

37

72.2

13.17

1.3

2.0

.61

.19

1.43

6.68

.87

.30

.55

.07

.06

.01

99.44

37.0

3.5

40.2

12.3

.4

1.5

1.8

1.9

1.1

.2

.1

100.0

3.4

1.6

1.8

89.5

197

803

43

281

4

21.9

5.0

500

12.8

34

2.31

4.4

39

1

15

1.7

54

3.0

11.34

9

P-70-

128

73.6

12.7

1.1

2.3

1.4

.50

5.7

.85

1.0

.05

.59

.19

.04

.02

100.04

33.9

2.0

5.1

48.7

1.1

3.5

2.4

1.6

1.1

.5

.1

100.0

6.0

3.5

2.5

87.7

26

222

65

270

8

20.6

4.3

480

13.2

27

2.12

4.8

36

.8

13

4.5

2

35

4.4

12.80

.4

17

P-76-

117

61.72

15.90

2.3

5.8

2.15

2.32

2.41

2.31

1.6

.37

.87

.18

.22

.01

98.16

28.8

5.9

14.2

21.2

10.7

5.5

8.0

3.5

1.7

.4

.1

100.0

13.6

5.6

8.0

64.2

138

262

119

139

2

12.0

4.6

160

4.2

17

1.18

2.6

38

.3

14

12.0

27

104

15.35

2.2

C

16

P-76-

139

73.54

13.38

1.6

.16

.00

.17

.04

8.91

.74

.07

.24

.04

.02

.00

98.91

39.1

3.3

53.6

1.2

.6

1.6

.4

.1

.1

100.0

93.9

143

863

56

517

6

10.7

2.4

200

5.5

13

1.06

4.5

36

.9

12

0.2

90

11.83

15

P-76-

141

73.86

13.56

1.5

1.1

.01

.67

3.07

3.88

.58

.10

.29

.06

.05

.01

98.74

40.5

3.3

23.4

26.5

2.9

.1

.4

2.2

.5

.1

.1

100.0

0.5

.1

.4

90.4

79

534

67

408

7

9.4

1.8

200

5.5

11

.88

5.2

36

.6

12

2.5

52

7.60

12

P 76-

145

76.75

12.40

1.8

1.0

.00

.34

4.20

1.22

.56

.09

.20

.01

.06

.02

98.65

47.9

3.7

7.3

36.2

1.5

.1

2.7

.4

.1

.1

100.0

0.1

.1

91.5

27

362

71

374

13

6.6

1.4

150

 3.7

11

.76

4.7

40

.9

14

1.0

37

9.43
THE CENTRAL VIRGINIA VOLCANIC-PLUTONIC BELT: AN ISLAND ARC OF CAMBRIAN(?) AGE 

TABLE 2.-Chemical and normative composition of the Chopawamsic Formation- Continued

All

Suites

Sample No

Field No

1

P-71-

9

3

P-70-

73

13

P-76-

142A

10

P-73-

13

8

P-72-

150

A

4

P-70-

67

5

P-70-

64

14

P-76-

124

6

P 70

63

7

P-70-

61

2

P-71-

7

I

11

P-77-

37

i

9

P-70-

128

17

P-76-

117

(

16

P-76-

139

15

P-76-

141

12

P-76-

145

Rare-earth elements5

La

Ce

Nd

Sm

Eu

Tb

Yb

Lu

Y7

La/Yb

Nb/Y

3

7

5

1.9

.62

2.2

.30

29

1.4

.09

7

12

11

4.2

1.61

1.05

3.3

.49

60

2.1

.11

5

10

9

3.2

1.05

.86

3.6

.57

42

1.4

.11

5 7(2)

12 16(2)

9 10(2)

3.5 3.2(2)

.96 1.10(2)

.84 .70(2)

3.1 2.9(2)

.60 .40(2)

43 44

1.6 2.4

.11 .15

9(2)

17(2)

16(2)

5.3(2)

1.48(2)

1.25(2)

5.4(2)

.80(2)

84

1.7

.07

12

22

12

3.5

1.01

.76

2.2

.33

31

5.5

.23

11

28

16

5.3

1.05

1.16

5.2

.81

87 61

2.1

.07

8(2)

14(2)

14(2)

4.8(2)

1.13(2)

1.16(2)

5.4(2)

.80(2)

45

1.5

.10

66

138

67

13.9

1.88

1.89

7.0

1.00

84

9.4

.40

56 42(2) 31(2) 27(2)

124 63(2) 54(2) 58(2)

55 41(2) 28(2) 24(2)

12.3 7.6(2) 5.8(2) 5.8(2)

1.75 1.40(2) 1.08(2) 1.02(2)

1.85 .88(2) .78(2) .98(2)

7.5 1.8(2) 3.1(2) 4.2(2)

1.14 .50(2) .42(2) .59(2)

85 45 36 38

7.4 24 10.0 6.5

.32 .38 .36 .29

15

42

14

4.4

.79

.85

4.1

.67

44

3.6

.25

' Major elements determined by

1. X-ray spectroscopy: P-77-37, P-76-124, P-76-172A, P-76-145, P-76-117, P-76-139, P-76-141; P. Hearn and S. Wargo, analysts.

2. Rapid rock analysis:

a. P-70-64, P-70-67, P-70-63, P-71-7, and P-71-9; P. Elmore, H. Smith and J. Kelsey, analysts; 

b. P-70-61, P-70-128, P-70-73, P-72-150, and P-70-13; Lowell Artis, analyst.

3. H2 0*, H20-, and C02 ; N. Skinner, analyst.

2 Large cations determined by several methods listed below. Values listed in tables list arithmetric averages of elements determined by two different methods:

1. Rb and Sr: X-ray fluorescence, W. P. Doering, analyst; instrumental neutron activation analysis, L. J. Schwarz, analyst.

2. Ba and Sr: X-ray spectroscopy (Ba), J. Lindsay, B. McCalland, George Sellers, analysts; X-ray fluorescence (Sr), W. B. Doering, analyst.

3. Ba and Rb: Instrumental neutron activation analysis; L. J. Schwarz, analyst.

3 High-valence cations (excluding Nb) determined by instrumental neutron activation analysis; L. J. Schwarz, analyst.

' Nb: Spectrophotometric analysis; E. Campbell, analyst.

* Ferromagnesian and rare-earth elements: Instrumental neutron activation analysis; L. J. Schwarz, analyst.

6 Numbers in parentheses indicate number of analyses averaged to give cited value.

7 Y: Quantitative spectrographic analysis; N. Rait, analyst.

	DESCRIPTION OF SAMPLES

1. Metabasalt: quartz-chlorite-amphibole gneiss: Stafford Quadrangle at lat 38°25'57" N. and long 77°27'45" W.

2. Meta-quartz keratophyre: quartz-albite-chlorite-amphibole gneiss: Stafford Quadrangle at lat 38°25'58" N. and long 77°28'02" W.

3. Metabasalt: quartz-albite-chlorite-amphibole gneiss: Storck Quadrangle at lat 38°25'27" N. and long 77°30'02" W.

4. Meta-andesite: quartz-albite-amphibole gneiss: Stafford Quadrangle at lat 38°25'06" N. and long 77°29'51" W.

5. Metafelsite: quartz-albite-amphibole gneiss: Stafford Quadrangle at lat 38°24'58" N. and long 77°29'48" W.

6. Interior of metamorphosed pillow: Stafford Quadrangle at lat 38°24'57" N. and long 77°29'48" W.

7. Metafelsite: quartz-albite gneiss: Storck Quadrangle at lat 38°24'52" N. and long 77°29'44" W.

8. Meta-andesite: quartz-epidote-plagioclase-amphibole gneiss: Storck Quadrangle at lat 38°23'03" N. and long 77°36'22" W.

9. Metafelsite: quartz-albite-chlorite-mica porphyritic gneiss: Storck Quadrangle at lat 38°22'16" N. and long 77°37'13" W.

10. Meta-andesite: quartz-albite-amphibole gneiss: Salem Church Quadrangle at lat 38°21'57" N. and long 77°36'24" W.

11. Metafelsite: muscovite-biotite-plagioclase-quartz porphyritic gneiss: Brokenburg Quadrangle at lat 38°14'28" N. and long 77°44'55" W.

12. Metafelsite: muscovite-plagioclase-quartz gneiss: Belmont Quadrangle at lat 38°08'36" N. and long 77°51'18" W.

13. Metafelsite: amphibole-biotite-chlorite-quartz gneiss: Belmont Quadrangle at lat 38°07'49" N. and long 77°51'12" W.

14. Metabasalt: quartz-albite-amphibole gneiss: Belmont Quadrangle at lat 38°08'34" N. and long 77°50'00" W.

15. Metafelsite: muscovite-biotite-potassic feldspar-quartz gneiss: Lake Anna West Quadrangle at lat 38°05'47" N. and long 77°50'11" W.

16. Metafelsite: muscovite-potassic feldspar-quartz gneiss: Lake Anna West Quadrangle at lat 38°05'49" N. and long 77°51'28" W.

17. Meta-epivolcaniclastic rock: garnet-biotite-feldspar-quartz gneiss: Lake Anna West Quadrangle at lat 38°03'45" N. and long 77°50'46" W.

since the time these rocks were erupted is demonstrated 

by the marked reversals of K20 abundances in some of 

the metafelsites having about the same Si02 content 

(table 2, samples 9, 11, and 16). Within suite A, scandium varies inversely with respect to silica content, a 

relationship characteristic of fractionation, which suggests that silicon and scandium were not affected by major mobilization processes, as were some of the other 

elements within the Chopawamsic rocks.

Table 2 also shows that the high-valence cations have 

characteristic abundance groupings in each of the three 

suites. For example, suite A rocks, whose majorelement chemistries and Si02 content range from basalt 

to keratophyre, have niobium contents of only 2.7-7.1 

ppm. Suites B and C, although only represented by 

silicic rocks because of the sampling problems discussed, 

are also clearly separable from each other and suite A by 

their high-valence cation content, such, for example, as 

that of niobium in the range 27-34 ppm in suite B and 

11-17 ppm in suite C. Suites B and C are not considered 

to be more fractionated differentiates of suite A because <

all three suites have similar silicic rocks (see Jakes and 

White, 1972, p. 36), namely SiQ2 >70 percent and 

distinctive minor element and REE abundances.

The REE patterns of the three suites of table 2 are 

given in figure 4. The REE patterns of suite A rocks are 

generally flat and similar to chondritic or primitive patterns. Generally they have a 10-30 fold enrichment of 

rare-earth elements above those of chondrites. These 

patterns are similar to those of the tholeiitic island-arc 

suites of Jakes and Gill (1970) and Jakes and White 

(1969; 1971; 1972, p. 34). The La/Yb value of most of the 

suite A samples (table 2) is also within the 1-2 range 

described as characteristic of tholeiitic island-arc suites 

(Jakes and Gill, 1970, table 1). The REE patterns of 

suites B and C are characteristic of more highly fractionated rocks of calcalkaline affinities (Jakes and 

White, 1972, p. 36).

TA RIVER METAMORPHIC SUITE

The same limitations imposed by weathering and poor 

exposure as described for the Chopawamsic Formation
A12

500 r

CONTRIBUTIONS TO THE GEOLOGY OF THE VIRGINIA PIEDMONT

1000 r

CD 

< 

DC

Q 100 h

o 

o

QC

LA CE PR ND PM SM EU GD TB DY HO ER TM YB LU

LA CE PR ND PM SM EU GD TB DY HO ER TM YB LU

FIGURE 4.-Chondrite-normalized rare-earth element patterns for (A) 

rocks of suites A, B, and C of the Chopawamsic Formation and (B) 

amphibolites of the Ta River Metamorphic Suite.

also hamper the geochemical sampling and analysis of 

the Ta River amphibolites. Table 3 lists the chemical and 

normative analyses of seven amphibolites from the Ta 

River. Although considered an eastern mafic facies of 

the Chopawamsic Formation, the Ta River amphibolites 

show no chemical similarities to suites B and C of the 

Chopawamsic and have only some geochemical 

similarities to suite A (compare table 2 with 3). Except 

for sample 6 (table 3) the Si02 content of the Ta River 

amphibolites is within the basaltic range (< 53 percent). 

However, as noted in the sample description of table 3, 

sample 6 is cut by quartz-feldspar dikelets, which may 

have introduced Si02 into this rock. Also, analyses of 

samples 3 and 7 should be considered with caution 

because of the massive epidote replacement in sample 3, 

as refelcted in its high CaO content (21.6 percent) and 

the quartz veining present in sample 7 (see table 3). In 

contrast with suite A of the Chopawamsic Formation 

(table 2), the Ta River amphibolites are not markedly 

sodic (table 3). A1203 and Ti02 in table 3 have too wide a 

compositional range for meaningful comparison with 

standard ocean-ridge, island-arc, and continental-
THE CENTRAL VIRGINIA VOLCANIC-PLUTONIC BELT: AN ISLAND ARC OF CAMBRIAN(?) AGE 

TABLE 3. -Chemical and normative composition of ampkibolites from the Ta River Metamorphic Suite

A13

Sample No

Field No

3

P-76-53

5 

P-76-71

4 

P 76 61

i

P-70-51

7 

P-76-80

2 

P-70-50

6 

P-76-78

Major oxides composition (weight percent) 1

Si02

A12 0,

Fe203

FeO

MgO

CaO

Na2 0

K20

H20+

H20-

Ti02

P2 05

MnO

C02

Total

43.60

16.09

5.86

3.5

6.98

21.61

.46

.15

.57

.15

.81

.04

.23

.00

100.05

47.20 

16.63 

1.89 

4.9 

8.93 

17.00 

1.14 

.45 

.53 

.08 

.44 

.14 

.17 

.00

99.50

47.70 

21.30 

2.02 

4.6 

6.47 

14.53 

1.26 

.30 

.81 

.13 

.28 

.03 

.13 

.00

99.56

48.90 

14.84 

3.06 

8.8 

7.72 

9.96 

2.45 

.21 

.71 

.15 

1.50 

.09 

.22 

.01

98.62

49.00 

14.19 

6.87 

5.7 

6.80 

13.68 

.51 

.27 

.84 

.30 

.75 

.06 

.26 

.00

99.23

50.20 - 

14.49 

4.65 

6.4 

6.05 

11.65 

2.23 

.32 

.94 

.31 

1.30 

.10 

.24 

.00

98.88

55.17 

14.02 

3.36

4^1 

7.76 

3.48 

.11 

.70 

.15 

1.04 

.07 

.20 

.01

97.47

Normative mineral composition (weight percent)

[Based on analyses recalculated to 100 percent water-free oxides]

Q

or

ab

an

Ic

ne

wo

en

fs

fo

fa

cs

mt

il

cc

Total

di

di-wo

di-en

di-fs

hy

hy-en

hy-fs

ol ____________

ol-fo

ol-fa

wo

DI

41.7

.7

2.1

25.9

17.5

.7

1.2

8.6

1.5

.1

100

39.0

20.8

17.5

.7

5.1

2.8

2.7 

8.3 

39.4

.8 

18.8 

13.1 

4.1 

6.6 

2.3

2.8 

.8 

.3

100

36.0 

18.8 

13.1 

4.1

8.9 

6.6 

2.3

11.8

1.8 

10.8 

52.3

8.6 

16.1 

6.6 

.1

.1

3.0 

.5

.1

100

16.6 

8.6 

5.7 

2.3 

14.7 

10.5 

4.2 

.2 

.1 

.1

12.6

0.3 

1.3 

21.2 

29.5

8.5 

19.7 

11.8

 4~5 

2.9 

.2 

.1

100

16.6 

8.5 

5.1 

3.0 

23.4 

14.6 

8.8

22.8

11.0

1.6 

4.4 

36.3

13.5 

17.3

4.1

To~i

1.5

.1

100

25.8 

13.5 

9.9 

2.4 

9.1 

7.3 

1.8

 

17.0

5.8 

1.9 

19.3 

29.3

12.2 

15.4 

6.4

6.9 

2.5 

.3

100

23.6 

12.2 

8.0 

3.3

10.4 

7.4 

3.0

 

27.0

11.5

.7 

30.5 

23.1

6.8 

10.6 

9.6

 

5.0 

2.0 

.2

100

13.4 

6.8 

3.5 

3.1 

13.6 

7.1 

6.5

42.7

Trace-element abundances (parts per million) 

Large cations2

Rb

Ba

Sr

Ba/Rb

17

7

373

.4

18 

174 

928 

9.7

16.5 

35 

321 

2.1

21 

61 

125 

2.9

19.5 

16

228 

.8

16 

172 

185 

10.2

13 

18 

113 

1.4

High-valence cations3 ' 4

Th

Zr

Hf

Nb

Ta

Zr/Hf

NbxlOO/Ti

Nb/Ta

50

1.4(3)

3.1

37

.06

2.5(3) 

60 

1.6(3) 

3.6

37

.14

1.1(3) 

10 

.8(2) 

1.6

12

.09

0.4(2) 

80 

2.6(3) 

4.4 

.14(3) 

31 

.05 

44

40 

1.0(3) 

3.5

40 

.08

0.4 

70 

2.1(3) 

3.5 

.13(3) 

33 

.04 

27

1.0(3) 

70 

1.7(3) 

6.4 

.06(3) 

41 

.10 

107
A14 CONTRIBUTIONS TO THE GEOLOGY OF THE VIRGINIA PIEDMONT

TABLE 3. -Chemical and normative composition of amphibolites from the Ta River Metamorphic Suite-Continued

Sample No

Field No

3

P-76-53

5 

P-76-71

4 

P-76-61

i

P-70-51

7 

P 76 80

2 

P-70-50

6 

P-76-78

Ferromagnesian elements5

Co

Ni

Zn

Cr

Sc

Ni/Co

43.5(3)

250

100(3)

265.3(3)

40.4(3)

5.8

28.7(3) 

54 

104(3) 

103.6(3) 

61.5(3) 

1.9

29.9(3) 

37 

78(3) 

115.8(3) 

35.2(3) 

1.2

48(3) 

110 

138(3) 

239.7(3) 

45.3(3) 

2.3

27.6(3) 

220 

136(3) 

385.4(3) 

40.8(3) 

8.0

35.4(3) 

59 

117(3) 

149.2(3) 

36.6(3) 

1.7

28.8(3) 

2 

118(3)

33.3(3)

.07

Rare-earth elements5

La

Ce

Nd

Sm

Eu

Tb

Yb

Lu

Y

La/Yb

q/q\

. _ 6(3)

OQ/QI

2.6(3)

.99(3)

1.01(3)

2.3(3)

.35(3)

27

1.3

19(3) 

36(3) 

21(3) 

5.4(3) 

1.34(3) 

1.22(3) 

1.0(3) 

.18(3) 

12 

19

4(3) 

8(3)

1.2(3) 

.38(3)

.9(3) 

.16(3) 

14 

4.4

4(3) 

10(3) 

11 

3.5(3) 

1.01(3) 

.98(3) 

3.6(3) 

.55(3) 

48 

1

48(3) 

12(3) 

102(3) 

27.3(3) 

6.04(3) 

2.80(3) 

8.3(3) 

1.05(3) 

67 

5.8

3(3) 

10(3) 

10 

3.1(3) 

1.05(3) 

.92(3) 

3.0(3) 

.47(3) 

38 

1

6(3) 

10(3) 

12 

2.8(3) 

.72(3) 

.68(2) 

2.4(3) 

.39(3) 

31 

2.5

1 Major elements determined by X-ray spectroscopy: P. Hearn and S. Wargo, analysts; except for FeO, H20% H2 0', and C02 determined by rapid rock analysis, N. Skinner, analyst.

1 Large cations: Rb is average of values (2) determined by instrumental neutron activation analysis, L. J. Schwarz, analyst, and X-ray spectroscopy, George Sellers, B. McCall, and J. Lindsay, 

analysts. Ba and Sr determined by X-ray spectroscopy, George Sellers, B. McCall, and J. Lindsay, analysts.

3 High valence cations (excluding Mb) determined by instrumental neutron activation analysis, L. J. Schwarz, analyst. Nb determined by spectrophotometric analysis, E. Campbell, analyst.

' Numbers in parentheses indicate number of analyses averaged to give cited value.

! Ferromagnesian and rare-earth elements (except Ni) determined by instrumental neturon activation analysis, L. J. Schwarz, analyst. Ni determined by spectrographic analysis, S. Berman, 

analyst.

DESCRIPTION OF SAMPLES

1. Strongly foliated amphibolite with small-scale folds and with small segregation veins of quartz and quartz-feldspar. Layering consists of green amphibole-rich layers interleaved with quartzplagioclase-rich layers. Stafford Quadrangle at lat 38°23'39" N. and long 77°29'52" W.

2. Similar to P-70-51 above: Stafford Quadrangle at lat 38°23'37" N. and long 77°29'49" W.

3. Foliated amphibolite composed mostly of green amphibole, smaller amounts of untwinned plagioclase and accessory sphene granules. Epidote is abundant and locally forms massive replacement of the amphibolite: Brokenburg Quadrangle at lat 38°08'51" N. and long 77°39'26" W.

4. Foliated and layered (on a fine scale) amphibolite. Irregular laminae of green amphibole intergrown with calcic well-twinned plagioclase are interleaved with amphibolitic layers or are interspersed with them. Epidote and sphene are accessory: Lake Anna East Quadrangle at lat 38°05'08" N. and long 77°42'40" W.

5. Foliated amphibolite. Green amphibole and andesine with enclosed and partial (metamorphic) twinning. Amphibole poikilitically encloses small-size grains of plagioclase. Epidote is a minor 

accessory: Lake Anna East Quadrangle at lat 38°03'27" N. and long 77°43'51" W.

5. Foliated amphibolite. Green amphibole and andesine with enclosed and partial (metamorphic) twinning. Amphibole poikilitically encloses small-size grains of plagioclase. Epidote is a minor 

accessory: Lake Anna East Quadrangle at lat 38°03'27" N. and long 77°43'51" W.

6. Finely layered and strongly foliated amphibolite locally cut by quartz-feldspar dikelets and, in places, containing feldspathic segregation clots. Green amphibole locally encloses small grains of 

quartz and plagioclase. Plagioclase occurs as twinned and untwinned grains. Quartz is a minor constituent and fine-grained granular sphene is accessory: Lake Anna West Quadrangle at 

lat 38°02'38" N. and long 77°47'23" W.

7. Fine-grained foliate amphibolite laced by very thin quartz veinlets. Amphibole poikilitically encloses small grains of epidote, quartz, and untwinned plagioclase. Epidote is abundant, quartz 

occurs mostly in stringers and generally untwinned plagioclase is a minor constituent: Lake Anna West Quadrangle at lat 38°02'20" N. and long 77°46'12" W.

margin basalts described in some reports (Gottfried and 

others, 1977, table 4; Garcia, 1978, table 2; Jakes and 

White, 1971, table 2). However, the high-valence cations 

within the Ta River amphibolites show few differences 

from suite A of the Chopawamsic, except that the Ta 

River rocks are generally lower in the zirconiumhafnium pair (compare table 2 with 3). The Ta River 

rocks generally contain more of the ferromagnesian 

elements cobalt, nickel, and chromium than do the suite 

A Chopawamsic rocks. No major abundance differences 

are apparent in REE content between these two suites 

of rocks. The chondrite-normalized REE data for Ta 

River amphibolites is shown in figure 4B. The REE patterns of the Ta River amphibolites are not so definitive 

as those of the three suites of the Chopawamsic Formation. The unusual pattern for sample 7 (fig. 4B) may 

result from the altered nature of this rock (see description in table 3), as possibly reflected by its high Fe203

content (6.9 percent). Samples 7 and 5 have REE patterns with light rare-earth element (LREE) enrichment 

comparable with some calcalkaline rocks, whereas 1 and 

2 have trends comparable with ocean-ridge basalt, 

namely, flat distribution of heavy rare-earth elements 

(HREE) and depleted LREE. Samples 6 and 3 have patterns generally similar to those of 1 and 2, except that 

lanthanum shows relative enrichment rather than a continuing depletion (fig. 45).

META-INTRUSIVE ROCKS

GENERAL STATEMENT

Metamorphosed plutonic rocks of felsic, mafic, -and 

ultramafic composition intrude or occur within the belt 

of metavolcanic rocks of the Fredericksburg area. In 

this paper only the felsic intrusive rocks, believed to be 

temporally and tectonically related to the metavolcanic
THE CENTRAL VIRGINIA VOLCANIC-PLUTONIC BELT: AN ISLAND ARC OF CAMBRIAN(?) AGE A15

rocks of this part of the central Virginia volcanicplutonic belt, are discussed. The metamorphosed mafic 

and ultramafic rocks are only briefly summarized. Also, 

the Carboniferous felsic meta-igneous rocks of the 

Falmouth Intrusive Suite (Pavlides and others, 1979) 

are not discussed because they are tectonically and temporally unrelated to the early history of the central 

Virginia volcanic-plutonic belt.

METAMORPHOSED MAFIC AND ULTRAMAFIC ROCKS

The metavolcanic rocks of both the Chopawamsic Formation and the Ta River Metamorphic Suite shown in 

the southern part of figure 3 contain small masses of 

serpentinite and metapyroxenite(?) (not shown in fig. 3) 

that because of poor exposure are provisionally considered intrusive. Near Fredericksburg, the Chopawamsic is intruded by metagabbro (Pavlides, 1976, p. 6, p. 

34-36). The mafic complex at Garrisonville (fig. 3) is the 

largest metamafic mass in the area and is considered to 

be a sphenolith-like intrusion (Pavlides and others, 1974, 

p. 575). It consists of a variety of metamorphosed mafic 

rocks including hornblendite, amphibolite, metapyroxenite, metawebsterite(?), and metanorite (Pavlides, 

1976, p. 6, p. 12-14). However, because the contact of 

the Garrisonville mafic complex with its enclosing rocks 

is not exposed, the possibility that the Garrisonville was 

structurally emplaced into its present position cannot be 

ruled out.

TRONDHJEMITIC AND PLAGIOGRANITIC METATONALITES

Two petrographically dissimilar and metamorphosed 

granitoid rocks (here called metatonalites) occur in 

plutons and associated dikes of different ages within the 

Chopawamsic belt near Fredericksburg (fig. 3). Both 

plutons and their related dikes are of pre-Late Ordovician age, as they have not been observed to intrude the 

Quantico Formation. For reasons given in following 

paragraphs, a trondhjemitic protolith is suggested for 

the younger pluton and its associated dikes, whereas the 

older more metmorphosed and more deformed pluton is 

interpreted as having close compositional affinities to 

plagiogranite. However, because the granitoid rocks 

from both plutons and their related dikes now have color 

indices > 10 they cannot be classed as trondhjemite or 

plagiogranite under strict usage of these terms. 

Nonetheless, the overall mineral and chemical features 

of these granitoid rocks from the Fredericksburg area 

suggest affinities to such rock types, and they are, 

therefore, classified as belonging to trondhjemitic and 

plagiogranitic suites. Because trondhjemites and 

plagiogranites are defined in several ways by different 

petrologists, a brief discussion of the terminology used 

in this report is provided.

The general characterisitics of trondhjemite have 

been reviewed by Barker (1979b), and many papers 

describing trondhjemite are presented by other investigators in a volume edited by Barker (1979a). 

Petrographically, trondhjemite is a leucotonalite having 

a color index < 10 and contains sodic plagioclase in the 

range albite to oligoclase. Chemically, it is characterized 

by a low K20 content, which, under Barker's usage, may 

be as much as 2.5 percent. Si02 content is high, generally 68 percent or more. O'Connor (1965) published a 

very useful normative diagram (fig. 5) that differentiates trondhjemites from other granitoid rocks. All the 

rocks of the Fredericksburg area considered to have 

affinities to trondhjemite fall in the trondhjemite field of 

the O'Connor diagram (fig. 5). On the Coleman and 

Peterman modal diagram (fig. 6) they lie along the 

quartz-plagioclase tieline entirely within the 

trondhjemite-tonalite field. For these reasons and 

because of their other chemical features, described in 

the section Geochemistry (p. A18-A20), they are classed 

as trondhjemitic metatonalite in this report.

Plagiogranite has been used as a synonym with trondhjemite by some petrologists (Streckeissen, 1975, p. 

14). Plagiogranite, like trondhjemite, also generally has 

a color index <10 and contains sodic plagioclase. 

Chemically, however, it contains < 1 percent K20, and 

Si02 is generally 70 percent or more. Coleman and 

Peterman (1975) recognized a special class of 

plagiogranite closely associated with transported 

ophiolite sequences that they termed oceanic 

plagiogranite. Oceanic plagiogranite occurs in small 

masses believed to have been derived through fractionation from the associated ophiolitic rocks. In contrast, 

plagiogranite not genetically related to ophiolites has 

been reported in batholithic dimensions in the Sayan 

geosyncline of Siberia by Popolitov and others (1973). 

Here quartz-plagioclase rocks containing very low K20 

and Si02 of about 73 percent has been termed 

plagiogranite. These rocks cover several thousand 

square kilometers in area and have been emplaced into 

lower Paleozoic volcanic rocks formed on ocean crust. 

The plagiogranitic rocks of the Fredericksburg area are 

more closely similar to those of the Sayan belt than to 

oceanic plagiogranite.

Coleman and Peterman (1975, fig. 1) introduced a ternary diagram, modified from Streckeissen (1973, fig. 

1A) that is useful for classifying plagiogranites irrespective of their genetic relationships. This is reproduced in 

figure 6. The plagiogranite suite falls along the tieline 

Q-P and can range in composition from quartz-rich 

granitoid rock, to tonalite-trondhjemite, and to diorite 

in composition (see Coleman and Peterman, 1975, fig. 

1). In the Fredericksburg area, all the granitoid rocks 

considered to be of plagiogranitic composition lie along
A16 CONTRIBUTIONS TO THE GEOLOGY OF THE VIRGINIA PIEDMONT

50 -7 <-50

GRANO5»/ DIORITE '

20 30 35

FIGURE 5.-Normative Ab-Or-An diagram showing fields of siliceous plutonic rocks (O'Connor, 1965) and plots of 

rocks of the trondhjemitic metatonalite suite (triangles) and the plagiogranitic metatonalite suite (circles) from the 

Fredericksburg area, Virginia.

60

QUARTZ-RICH

GRANITOID

ROCKS

FIGURE 6.-Modal plots of A (potassic feldspar)-P (plagioclase, including albite)-Q (quartz) for rocks of the 

plagiogranitic metatonalite suite (circles) and the trondhjemitic metatonalite suite (triangles) from the 

Fredericksburg area, Virginia. Sample (4) is a trondhjemitic metatonalite that falls on two plagiogranitic 

metatonalite sample points. Diagram taken from Coleman and Peterman (1975, fig. 1).
THE CENTRAL VIRGINIA VOLCANIC-PLUTONIC BELT: AN ISLAND ARC OF CAMBRIAN(?) AGE A17

the Q-P tieline (plagiogranite suite), and all but one 

(sample 3) fall along the Q-P tieline of the tonalitetrondhjemite field. Sample 3, which falls along the 

tieline of the quartz-rich granitoid field, is still close to 

tonalite in composition. Therefore, these rocks of the 

older pluton are termed plagiogranitic metatonalite. 

The additional chemical features that characterize these 

rocks as plagiogranitic in composition are discussed in 

the section Geochemistry (p. A20-A21).

LITHOLOGY AND PETROGRAPHY

TRONDHJEMITIC METATONALITE SUITE

A small pluton of trondhjemitic metatonalite intrudes 

the Chopawamsic Formation immediately north of the 

Rappahannock River (fig. 3). It was originally assumed 

to be intrusive into both the Chopawamsic and the Quantico Formations (Pavlides, 1976), but new mapping indicates it intrudes only the Chopawamsic and is in fault 

contact with the Quantico. Similar rock is also common 

as plugs and dikes within the Chopawamsic Formation 

near this pluton. The pluton and its associated dikes and 

plugs are characterized by a weak to locally imperceptable foliation, by granophyric texture consisting of 

albitic plagioclase (An < 10 percent by flat stage 

measurements) graphically intergrown with quartz, and 

by blue-green amphibole that appears to be a sodic hornblende in the sodic edenite series (Malcom Ross, oral 

commun., 1978). This amphibole may form as much as 

36 percent of some rocks. Table 4 lists chemical, nor-, 

mative, and modal analyses of granitoid rocks of the 

trondhjemitic metatonalite suite from the pluton and 

from plugs and dikes intruding the Chopawamsic Formation. The normative analyses of table 4 are plotted on 

the normative Ab-Or-An diagram (fig. 5) of O'Connor 

(1965), and all fall in the trondhjemite field.

PLAGIOGRANITIC METATONALITE SUITE

Coarse-grained, quartz-rich metatonalites extensively 

intrude the Chopawamsic Formation in the Fredericksburg area (fig. 3). The granitoid rocks are highly 

foliated and were emplaced into the Chopawamsic and 

deformed with it, as they share the same foliation. In 

contrast the trondhjemitic metatonalite suite of rocks 

described, having weaker foliation, were emplaced later. 

The plagiogranitic-metatonalite-suite granitoid rocks of 

the Fredericksburg area have a modal quartz range of 

41-48 percent (table 5). Characteristically they contain 

granoblastic-textured aggregates of quartz, as well as 

single discrete grains of quartz and plagioclase, generally with chlorite, epidote, and, in places, white-mica 

alteration. Biotite and amphibole are common accessories; garnet is rare, and opaque minerals are 

minor. Some of the quartz is blue.

TABLE 4.- Chemical, normative, and modal composition of trondhjemitic metatonalite in the Fredericksburg area, Virginia

Sample No 

Field No

3451

P-73-1 P-73-5 P-73-6 P-72-41

2 

P-72-190

Major oxide composition (weight percent)1

Si02

A1.03

Fe.O,

FeO

MgO _ .

CaO

Na_0

K,0

H.O H.OTi02

P.O,

MnO

CO.

Total

69.7 74.1 75.1 75.8

13.2 12.1 11.9 12.0

3.9 3.7 2.4 1.3

3.5 2.6 1.1 2.0

.49 .33 .15 .33

2.4 2.4 1.7 2 1.5

5.2 4.2 5.5 2 4.6

.15 .13 .05 J .54

.66 .36 .37 .60

.20 .10 .06 .23

.46 .38 .29 .23

.14 .11 .05 .06

.16 .31 .06 .00

.04 .06 .02 .01

100.2 100.88 98.75 99.2

77.4 

11.5 

.78 

1.7 

.53 

2 2.9 

2 5.2 

'.02 

1.1 

.24 

.28 

.01 

.00 

.01

101.57

Normative mineral composition (weight percent)

[Based on analyses recalculated to 100 percent water-free oxides]

Q __ _ __.

C

or

ab

an

wo

en

fs

mt

hm

il

ap

cc

Total

di

di-wo

di-en

di-fs

hy _______

hy-en

hy-fs

DI

32.5 43.1 40.0 43.2

.5 1.1 1.3

.9 .8 .3 3.2

44.3 35.4 47.3 39.7

10.8 10.8 7.7 7.1

.1

1.2 .8 .4 .8

2.8 1.7 2.2

5.7 5.3 3.0 1.9

.4

.9 .7 .6 .4

.3 .2 .1 .1

.1 .1 .1 .1

100.0 100.0 100.0 100.0

0.2

.1

.1

4.0 2.5 .2 3.1

1.2 0.8 .2 .8

2.8 1.7 2.3

77.7 79.2 87.6 86.0

41.0

.1 

43.0 

8.4 

2.4 

1.3 

2.0 

1.1

"IT 

.1 

.1

100.0

4.9 

2.4 

1.0 

1.5 

.9 

.4 

0.5 

84.1

Modal analysis (percent)3

Quartz

Quartz (gra 

Plagioclase 

Plagioclase 

Biotite

Amphibole 

Chlorite

Garnet

Epidote

14.90 25.51 21.67

nophyric) 9.93 3.54 18.08 

49.86 23.96 27.04

(granophyric) 7.62 3.83 22.79 

.78

12.86 35.68 1.12

.99 .16

1.66 .32 .11

.61 1.77 6.77

____

 

Opaque minerals 1.75 5.22 1.62

Total

Color index

100.00 99.99 99.98

17.87 43.15 10.4

____

Trace-element abundances (parts per million) 

Large cations5 6

Rb

Ba

Sr

Ba/Rb

6 17 12(2)

45 229 138(2)

163(2) 100 98.3 153

8 13 11

2 

21 

36(2) 

10

High valence-cations7

Th

Zr«

Hf

Mb'

Ta

Zr/Hf

NbxlOO/Ti 

Nb/Ta

3.0 2.7 4.7

160 160 180

4.4 4.6 5.8

3.3 2.4 4.4

.32 .21 .38

36 35 31

.12 .10 .31

10 11 12

2.8(2) 

120 

3.6(2) 

2.4 

.22(2) 

33 

.14 

11

Ferromagnesian elements 10

Co

Zn

. Sc

2.2 1.9 1.3

66 49 31

16 13.15 9.94

2.2(2) 

58(2) 

9.89(2)

Rare-earth elements 1

La

Ce

Nd

Sm

10 11 9

25 27 45

17 18 17

5.7 5.1 5.4

4(2) 

11(2) 

8(2) 

2.6(2)
A18 CONTRIBUTIONS TO THE GEOLOGY OF THE VIRGINIA PIEDMONT

TABLE 4. - Chemical, normative, and modal composition of trondhjemitic metatonalite in the Fredericksburg area, Virginia - 

Continued

Sample No

Field No

3

P-73-1

4 5 1 

P-73-5 P-73-6 P-72-41

2

P 72 190

Rare-earth elements Continued

Eu

Tb

Yb

Lu

Y"

La/Yb

Nb/Y

1.6

1.16

6.3

.92

63

1.6

.05

1.49 

1.06 

12.3 

2.01 

82 

.89 

.03

1.08 

1.21 

7.4 

1.04 

81 

1.2 

.05

.68(2) 

.81(2) 

5.0(2) 

.74(2) 

52 

.8 

.05

1 Major elements determined by rapid rock analysis:

1. P-73-1, P-73-5, and P-73-6, Lowell Artis, analyst.

2. P-72-41 and P-72-190, H. Kirschenbaum and S. Botts, analysts.

2 Rapid rock analysis; Leonard Shapiro, analyst.

3 Modal analysis by S. L. Cranford.

4 Color index from Streckeisen, A., (1975).

* Large cations: Determined by several methods listed as follows. Values cited in tables list 

arithmetic averages for elements determined by two different methods:

1. Rb and Ba: P-73-1, P-73-5, P-72-190, X-ray spectroscopy, B. McCall, J. Lindsay, G. 

Sellers, and S. Wargo, analysts. P-72-41, instrumental neutron activation analysis, 

L. J. Schwarz, analyst; X-ray spectroscopy, B. McCall, J. Lindsay, G. Sellers, and S. 

Wargo, analysts.

2. Sr: P-73-1 and P-72-190, X-ray fluorescence, W. P. Doering, analyst; X-ray spectroscopy, B. McCall, J. Lindsay, G. Sellers, and S. Wargo, analysts. P-73-5 and 

P-72-41, X-ray spectroscopy, B. McCall, J. Lindsay, G. Sellers, and S. Wargo, 

analysts. P-73-6, X-ray fluorescence, W. P. Doering, analyst.

6 Numbers in parentheses indicate number of analyses averaged to give cited value.

7 High valence cations (excluding Zr and Nb) determined by instrumental neutron activation 

analysis, L. J. Schwarz, analyst.

 Zr: Emission spectroscopy, S. Berman, analyst.

' Nb: Spectrophotometric analysis, E. Campbell, analyst.

10 Ferromagnesian and rare-earth elements (excluding Y): instrumental neutron activation 

analysis, L. J. Schwarz, analyst.

11 Y: Emission spectroscopy, N. Rait, analyst.

DESCRIPTION OF SAMPLES

1. Mesocratic, granophyric garnet-amphibole bearing trondhjemitic metatonalite: Salem 

Church Quadrangle at lat 38°22'09" N. and long 77°35'05" W.

2. Mesocratic granophyric magnetite-amphibole bearing trondhjemitic metatonalite: Salem 

Church Quadrangle at lat 38°22'02" N. and long 77°36'48" W.

3. Mesocratic, granophyric amphibole-epidote bearing trondhjemitic metatonalite: Salem 

Church Quadrangle at lat 38°22'02" N. and long 77°36'48" W.

4. Cataclastic, muscovite-biotite-chlorite bearing trondhjemitic metatonalite dike: Storck 

Quadrangle at lat 38°23'58" N. and long 77°32'50" W.

5. Leucocratic, hypidiomorphic-granular chlorite-muscovite bearing trondhjemitic 

metatonalite dike: Storck Quadrangle at lat 38°23'47" N. and long 77°33'01" W.

Figure 6 is a modal plot of the metatonalites of the 

Fredericksburg area on the A-Q-P diagram of Coleman 

and Peterman (1975). As stated earlier, the rocks fall 

along the plagioclase suite tieline between Q and P. Normative analyses of these rocks, however, plot in both the 

tonalite and trondhjemite fields of O'Connor's Ab-An-Or 

diagram (fig. 5). Normally plagiogranitic rocks should 

plot close to the Ab-An tieline of the O'Connor diagram, 

as do samples 1, 2, and 3. The fact that samples 4, 5, and 

7 plot considerably away from this tieline is attributed to 

metamorphic alteration.

GEOCHEMISTRY

TRONDHJEMITIC METATONALITE SUITE

Although weakly foliated as well as metamorphosed, 

the trondhjemitic metatonalites of the Fredericksburg 

area have granophyric textures consisting of intergrowths of albitic plagioclase and quartz. Granophyric 

texture is generally considered to form from cotectic

precipitation of feldspar and quartz in magmatic rocks 

(Barker, 1970). Granophyric texture involving intergrowths of sodic plagioclase and quartz has also been 

interpreted as a late-stage crystallization producing 

plagiogranites from K2 0-poor magmas (Coleman and 

Donate, 1979, p. 153-154). The granophyric texture in 

the trondhjemitic metatonalite suite of the Fredericksburg area is, therefore, interpreted as indicative 

of a magmatic origin of these rocks. The inverse variation of scandium with Si02 (table 4) is consistent with the 

fractionation trend of this element and thus implies that 

mobility of Si02 in these rocks, if it did take place, was 

minor.

The weakly foliated and metamorphosed trondhjemitic metatonalite suite of rocks of the Fredericksburg 

area includes both corundum-normative and diopsidenormative samples regardless of its Si02 content (table 

4). This contrasts with the observation that in 

calcalkaline magmas there is a change from diopside 

normative to corundum normative where Si02 content 

is in the mid-60's (Cawthorn and others, 1976). The fact 

that both high-alumina (A12 03 >15 percent) and lowalumina (A1203 < 15 percent) trondhjemites are generally corundum normative (Barker, 1979b, p. 5) suggests 

that the corundum-normative trondhjemites are not 

dependent on an original high-aluminum bulk chemistry. 

The trondhjemitic metatonalites of the Fredericksburg 

area all contain substantially < 15 percent A12 03 and are 

thus similar to low-alumina trondhjemites, according to 

the definition of Barker and Arth (1976) modified by 

Barker (1979b, p. 6-7). Arth (1979) has subdivided trondhjemites into oceanic and continental varieties on the 

basis of their A1203 and ytterbium contents, as plotted 

on a semilogarithmic graph (fig. 7). On this diagram, in 

which Arth separates the high- and low-alumina fields at 

14.5 percent A12 03 , all the trondhjemitic metatonalites 

of the Fredericksburg area plot in the low A12 03 , oceanic 

field (fig. 7).

The REE patterns of the Fredericksburg area trondhjemitic metatonalites are shown in figure 8A. In general 

the patterns are flat except for HREE enrichment 

shown by sample 4 and the cerium enrichment of sample 

1. If we exclude these apparent local enrichments, the 

LREE show values about 15-33 fold above chondrite 

values, whereas the HREE show an enrichment of 

21-30 fold above chondrite values. The REE patterns of 

the Fredericksburg area trondhjemitic metatonalites 

are similar to patterns obtained from Cenozoic and 

Mesozoic trondhjemites found in island arcs and are 

strikingly dissimilar to patterns of trondhjemites in continental margins, continental interiors, and ophiolites 

(Arth, 1979, fig. 1). They differ from the patterns of the 

trondhjemite of the Mule Mountain stock that intrudes 

Devonian metavolcanic rocks of the West Shasta district 

in California, in that the Mule Mountain trondhjemite
THE CENTRAL VIRGINIA VOLCANIC-PLUTONIC BELT: AN ISLAND ARC OF CAMBRIAN(?) AGE A19 

TABLE 5. -Chemical, normative, and modal composition of plagiogranitic metatonalite in the Fredericksburg area, Virginia

Sample No

Field No

2 

P-71-55

i

P-71-38

5 3 

P-70 81 P-71 27

7 

P 70-240

4 

P 71 18

6 

P-70-179

Major oxide composition (weight percent)1

Si02

Al O

Fe203

FeO

MgO

CaO

Na,0

K20

H2 0*

H20-

Ti02

PA

MnO

C02

Total

70.4 

14.9 

1.6 

2.6 

1.0 

4.1 

3.1 

.20 

1.1 

.27 

.42 

.11 

.08

99.9

72.7 

12.2 

2.2 

2.6 

1.1 

3.8 

2.8 

.26 

1.5 

.04 

.49 

.00 

.13

99.8

73.1 75.1 

13.1 11.2 

1.3 1.1 

2.2 2.7 

.64 1.0 

3.3 3.5 

2.6 2.5 

1.5 .38 

1.4 1.3 

.07 .07 

.20 .52 

.00 .02 

.12 .11

99.5 99.5

75.2 

12.8 

.78 

1.2 

.45 

2.3 

3.7 

1.2 

1.2 

.05 

.13 

.00 

.08 

0.70

98.6

76.6 

12.6 

1.2 

1.3 

.72 

1.6 

3.2 

1.0 

1.2 

.12 

.33 

.06 

.07

100.0

77.7 

12.3 

1.3 

.60 

.30 

.48 

4.0 

2.0 

.61 

.08 

.16 

.00 

.11

99.6

Modal analysis (percent)2

Quartz

Plagioclase

Amphibole

Biotite

Muscovite

Chlorite

Epidote

Garnet

Calcite

Opaque minerals

Total

Color index3

41.0 

44.0 

4.6 

.7 

.3 

4.8 

3.6 

.1

.7

99.8 

14.8

41.7 

38.6

.4 

.1 

7.2 

10.1

1.8

99.9 

19.6

48.1 44.9 

33.9 26.4

5.3 1.2 

4.1 1.7 

.8 6.3 

7.3 19.0 

.3 __

.5

99.8 100.0 

17.8 28.7

41.8 

46.5

Tif

2.2 

.2

1.5

.1

99.8

11.5

47.2 

41.3

~2~2~ 

6.3 

.8 

.6

1.5

99.9 

11.4

46.2 

41.1

.3 

12.0

~~1~

.4

100.1 

12.8

Normative mineral composition (weight percent)

[Based on analyses recalculated to 100 percent water-free oxides]

Q _ ______ ___________

C

or 

ab

an

en

fs_

mt

hm

il

an

CC

Total

hy

hy-en

hy-fs

DI

41.0 

2.5 

1.2 

26.6 

19.6 

2.5 

3.0 

2.4

.8 

.3

.1

100.0

5.5

2.5 

3.0 

68.8

45.4 

.5 

1.6 

24.1 

18.9 

2.9 

2.4 

3.2

.9

.1

100.0

5.2 

2.8 

2.4 

71.1

43.9 49.7 

1.3 .5 

9.1 2.3 

22.4 21.5 

16.4 17.2 

1.6 2.5 

2.9 3.5 

1.9 1.6

.4 1.0 

__ .1 

.1 .1

45.4 

2.9 

7.2 

31.8 

7.1 

1.1 

1.5 

1.1

.3 Te~

100.0 100.0 100.0

4.5 6.0 

1.6 2.5 

2.9 3.5 

75.3 73.5

2.6 1.1

1.5 

84.3

50.2 

3.7 

6.0 

27.4 

7.3 

1.8 

1.0 

1.8

.6

.1 

.1

100.0

2.8 

1.8 

1.0 

83.6

45.9 

2.8 

11.9 

34.2 

2.1 

.8

1.8 

.1 

.3

"l~

100.0

0.8 

.8

92.0

Trace-element abundances (parts per million) 

Large cations4 *

Rb

Ba

Sr

Ba/Rb

6(2) 

47 

162 

8

8 

42 

119 

5

~\9(9\

____ 86 

105

7

 

32 

206(2) 

84 

6

 

High-valence cations6

ThZr7

Hf Nb8

Ta

Zr/Hf

NbxlOO/Ti _

Nb/Ta _

0.8

.6 

1.9 

.12

.08 

16

0.8(2) 

20 

.6(2) 

3.9 

.25 

33 

.13 

16

__ 1.3 

60

__ 1.8 

5.0

__ .16 

33

.16

31

 

 

2.1 

240 

5.5

~~27 

44

 

 

 

 
A20 CONTRIBUTIONS TO THE GEOLOGY OF THE VIRGINIA PIEDMONT

TABLE 5. -Chemical, normative and modal composition of plagiogranitic metatonalite in the Fredericksburg area, Virginia-Continued

Sample No

Field No

Co Ni7

Zn

Cr

Sc

Ni/Co

La

Ce

Nd

Sm

Eu

Tb

Yb

Lu

yio

La;Yb

Nb/Y

2

P-71-55

6.0

4

98

2.8

6.56

.7

5

8

6

1.2

.77

.26

.8

.09

13

6.25

.15

i

P-71-38

7 fi(9\

84

10.9(2)

21.08(2)

Z.(O\

~\9(9\

Q(O\ °(6)

2.6(2)

.84(2)

.45

1 7(9}

.26(2)

28

2.94

.14

5 3

P-70-81 P-71-27

Ferromagnesian elements9

6.8

4

52

10.1

10.93

.6

Rare-earth elements 10

6

11

6

1.4

.63

.23

.9

.15

14

6.67

.36

7 4

P 70 240 P 71 18

5.9

28

7.66

8

15

7

1.6

.54

.25

2.3

.38

18

3.48

6

P-70-179

___

1 Major elements determined by rapid rock analysis:

1. P-71-55, P. Elmore, H. Smith, J. Kelsey, and J. Glenn, analysts.

2. P-71-38, P-70-81, P-71-27, P-70-240, and P-70-179, P. Elmore, H. Smith, J. Kelsey, J.Glenn, and G. Chloe, analysts.

3. P-71-18, P. Elmore, H. Smith, and J. Kelsey, analysts. 

* Modal analysis by S. L. Cranford.

3 Color index from Streckeisen, A., 1975.

4 Large cations determined by several methods listed as follows. Values cited in tables list arithmetic averages for elements determined by two different methods:

1. Rb: P-71-55, X-ray spectroscopy, B. McCall, J. Lindsay, G. Sellers, and S. Wargo, analysts; X-ray spectroscopy, J. S. Wahlberg. P-71-38, X-ray spectroscopy, B. McCall, J. Lindsay, 

G. Sellers, and S. Wargo, analysts. P°71-27 and P-71-18, X-ray spectroscopy, B. McCall, J. Lindsay, G. Sellers, and S. Wargo, analysts; instrumental neutron activation analysis, 

L. J. Schwarz, analyst.

2. Ba: P-71-55 and P-71-38, X-ray spectroscopy, B. McCall, J. Lindsay, G. Sellers, and S. Wargo, analysts. P-71-27 and P-71-18, X-ray spectroscopy, B. McCall, J. Lindsay, G. Sellers, 

and S. Wargo, analysts; instrumental neutron activation analysis, L. J. Schwarz, analyst.

3. Sr: P-71-55, X-ray spectroscopy, B. McCall, J. Lindsay, G. Sellers, and S. Wargo, and J. S. Wahlberg, analysts. P-71-38, P-71-27, and P-71-18, X-ray spectroscopy, B. McCall, J. 

Lindsay, G. Sellers, and S. Wargo, analysts.

5 Numbers in parentheses indicate number of analyses averaged to give cited value.

6 High-valence cations (excluding Zr and Nb) determined by instrumental neutron activation analysis, L. J. Schwarz, analyst.

7 Zr and Ni: Emission spectroscopy, E. Silk, analyst. 

' Nb: Spectrophotometric analysis, E. Campbell, analyst.

9 Ferromagnesian and rare-earth elements: instrumental neutron activation analysis, L. J. Schwarz, analyst.

10 Y: Emission spectroscopy, N. Rait, analyst.

ird Quadrangle at lat 38°29'28" N. and long 77°26'02" W.

	DESCRIPTION OF SAMPLES

1. Quartz-feldspar-hornblende metatonalite: Stafford Quadrangle at lat 38°29'12" N. and long 77°26'26" W.

2. Chlorite-hornblende-quartz mesocratic tonalite, small intrusion in Garrisonville mafic complex: Stafford Q

3. Biotite-quartz granitoid: Storck Quadrangle at lat 38°25'58" N. and long 77°30'30" W.

4. Quartz-plagioclase-clinozoisite-chlorite-biotite plagiogranitic metatonalite: Stafford Quadrangle at lat 38°27'21" N. and long 77°27'08" W.

5. Plagioclase-quartz-muscovite-chlorite plagiogranitic metatonalite: Chancellorsville Quadrangle at lat 38°21'06" N. and long 77°3817" W.

6. Biotite-muscovite-chlorite-epidote-magnetite plagiogranitic metatonalite: Stafford Quadrangle at lat 38°26'17" N. and long 77°28'01" W.

7. Quartz-plagioclase-muscovite-biotite plagiogranitic metatonalite: Richardsville Quadrangle at lat 38°23'54" N. and long 77°38'05" W.

has a marked depletion of LREE (Barker and others, 

1979, fig. 7).

PLAGIOGRANITIC METATONALITE SUITE

The plagiogranitic rocks of the large intrusion in the 

Fredericksburg area have Si02 contents ranging approximately from 70 to 77 percent. Some of the silica of 

these rocks may have been introduced during alteration 

processes, particularly metamorphism. This interpretation may be supported by the nonsystematic variation of 

scandium with respect to Si02 (table 5), which contrasts 

with the systematic variation of these two components 

in the trondhjemitic tonalites described.

The plagiogranitic tonalites of the Fredericksburg 

area are all corundum normative even though they contain < 15 percent A1203 and in this respect are similar to 

the earlier described trondhjemitic tonalites of the area. 

They also contain from 0.20 percent to as much as 2.0 

percent K20 (table 5). However, the samples containing 

more than 1 percent K20 also contain the largest 

amount of metamorphic micas (for example, samples 4, 

5, 6, and 7, table 5), suggesting that these higher K20 

values probably result from postemplacement alteration 

processes. For this reason, samples 1, 2, and 3 (table 5), 

which contain 0.20-0.38 percent K20, may be more 

representative of the original K20 content of these 

rocks. The apparent absence of potassic feldspar in 

these metatonalites also suggests an original low K20 

content, because this mineral if formed magmatically in 

tonalites should still be present, even in relict form.
THE CENTRAL VIRGINIA VOLCANIC-PLUTONIC BELT: AN ISLAND ARC OF CAMBRIAN(?) AGE A21

->1

O

DC 

UJ

CO 

DC 

Q_

-Q

05

01

A3

A 2

A4

Low AI 2 03 (oceanic)

 3 

 2 4-

High AI 2 03 

(continental)

12 13 14 15 16 17 18

AI 2 03 , in WEIGHT PERCENT

FIGURE 7.-Plot of A1203 against ytterbium as a discriminant 

diagram for oceanic in contrast with continental trondhjemite. 

Rocks of the trondhjemitic metatonalite suite (triangles) and 

plagiogranitic metatonalite suite (circles) of the Fredericksburg 

area, with one exception, plot in the oceanic field. Diagram taken 

from Arth (1979, fig. 2).

The rare-earth elements of the plagiogranitic tonalites 

(table 5) are plotted in figure SB. Compared with chondrite they have a 15-25 fold enrichment of lanthanum 

and a 3-11 fold enrichment of lutetium. The 

plagiogranitic metatonalite REE patterns contrast with 

the trondhjemitic metatonalite patterns (compare fig. 

SA with SB) in two important aspects. Rather than the 

flat patterns of the trondhjemitic rocks with their 

negative europium anomaly, the plagiogranitic tonalites 

show a calcalkaline trend that has an enrichment of 

LREE with respect to the HREE. They also have a 

positive europium anomaly in contrast to the negative 

europium anomaly of the trondhjemitic rocks. Oceanic 

plagiogranites from Cyprus (Coleman and Peterman, 

1975, fig. 7) and Oman (Coleman and Donato, 1979, fig. 

8) have REE patterns with LREE depletion and 

negative europium anomalies. The plagiogranitic 

metatonalites from the Fredericksburg area (fig. 8B), 

however, have calcalkaline patterns similar to the 

silicious (> 65 percent Si02 ) trondhjemites from Finland 

(Arth, 1979, figs. 4, 5), including a positive europium 

anomaly. Such fractionated REE patterns with positive 

europium anomalies generally characterize continental

trondhjemites, which are also rocks high in A1203 . Thus, 

although the Fredericksburg area plagiogranitic rocks 

have REE patterns similar to continental tonalitic 

rocks, their relatively low A1203 content resembles 

oceanic tonalitic rocks. They generally also plot in the 

oceanic field of trondhjemites on the basis of their 

ytterbium-A1203 contents (fig. 7). Possibly they are tran- 

. sitional between oceanic and continental trondhjemiticplagiogranitic tonalites, because the field boundaries 

between continental and oceanic types are also transitional, depending on the stage of tectonic development 

in which they formed (Arth, 1979, p. 125). LREE are 

reported to be somewhat mobile under certain rock 

alteration processes (table 1). Alternatively, the REE 

patterns of the plagiogranitic metatonalites of the 

Fredericksburg area may have been slightly modified 

during the shearing and metamorphism these rocks 

have undergone, which is greater than that present in 

the younger trondhjemitic metatonalites.

REGIONAL RELATIONSHIPS

The Chopawamsic Formation has been tentatively correlated with the volcanic rocks of the Carolina slate belt 

(Glover, 1974, p. 757), and Higgins (1972, p. 1020) 

assigned the volcanic rocks of the Chopawamsic, the 

James Run, and the Carolina slate belt to the "Atlantic 

Seaboard volcanic province." Neither the slate-belt correlation nor the Atlantic Seaboard volcanic province 

assignment are considered appropriate in light of recent 

work on the Chopawamsic Formation. Figure 9 shows 

the distribution of the central Virginia volcanic-plutonic 

belt with respect to other belts of volcanic rocks in the 

Piedmont.

JAMES RUN FORMATION

The James Run Formation as defined by Higgins 

(1972) includes a belt of metavolcanic and 

metavolcaniclastic rocks that are exposed near the Fall 

Line in the Maryland Piedmont (fig. 9). Higgins (1972) 

included within the James Run Formation the James 

Run Gneiss of Southwick and Fisher (1967), as well as 

felsic rocks considered by others to be intrusive. The 

Relay Quartz Diorite of Hopson (1964), originally considered a pluton differentiated from Hobson's Baltimore 

Gabbro and silicified and albitized by its own residual 

fluids, was reinterpreted (Higgins, 1972, p. 1007) as an 

assemblage of felsic metavolcanic and metavolcaniclastic rocks. He also suggested (p. 1000-1001) that 

parts of the Port Deposit Gneiss may be metavolcanic rocks of the James Run Formation, the rest 

being part of a pluton that intruded its own volcanic 

pile. Southwick (1979) divided the Port Deposit Gneiss, 

which he considered polygenetic, into five units, among
A22

1000 rCONTRIBUTIONS TO THE GEOLOGY OF THE VIRGINIA PIEDMONT

1000 r100

i i i i i i i

A

i i

CD

< 

DC

DC 

Q

CJ 

O 

DC

LA CE PR ND PM SM EU GD TB DY HO ER TM YB LU LA CE PR ND PM SM EU GD TB DY HO ER TM YB LU

FIGURE 8. -Chondrite-normalized plot of rare-earth elements in metatonalitic rocks of the Fredericksburg area. Data from table 4. A, trondhjemitic metatonalite; B, plagiogranitic metatonalite.

which are metasedimentary rocks, plutonic rocks in the 

form of sheets or thick sills, metavolcanic rocks composed of dacite flows and tuff sheets, and a leucocratic 

quartz augen gneiss interpreted to be a metamorphosed 

trondhjemitic pluton. Southwick assigned the 

metadacite and associated metavolcanic rocks to the 

James Run Formation. The presence of trondhjemitic 

rocks within the James Run terrane is analogous to the 

situation in the Chopawamsic terrane of the 

Fredericksburg area in Virginia.

Crowley (1976) assigned amphibolite and biotitequartz-plagioclase gneiss within tectonically 

transported rocks in the Baltimore area to the James 

Run Formation. He mapped the James Run Formation 

as conformably overlying amphibolite and ultramafic 

rocks of his Baltimore Mafic Complex (Crowley, 1976, 

Geologic Map of Baltimore County and City, in pocket). 

However, in his more generalized geologic map 

(Crowley, 1976, plate 2, in pocket), he suggested that 

the James Run and his Baltimore Mafic Complex may be

separated by a thrust fault, the James Run being on the 

upper plate. The fact that the youngest member of the 

James Run, Crowley's (1976) Relay Gneiss Member, is 

juxtaposed against Crowley's Baltimore Mafic Complex 

further supports the fault interpretation. Crowley divided the James Run of the Baltimore area into three 

members, the uppermost of which is his Relay Gneiss 

Member, thereby agreeing with Higgins (1972, 1976) 

that Hopsons' Relay Quartz Diorite is a metavolcanic 

rock.

Higgins (1972, p. 1001-1002) originally subdivided the 

James Run Formation in Cecil County, Md., into four informal units but has subsequently refined the subdivision of the James Run into six formal members and an 

unnamed member (Higgins, 1977). Three coeval or nearly coeval members occur at the base of the James Run 

and consist generally of felsic metavolcanic and metavolcaniclastic rocks and subordinate mafic metavolcanic 

rocks. Locally some of these units grade into probably 

metasedimentary biotite gneiss. Diamictite is inter-
THE CENTRAL VIRGINIA VOLCANIC-PLUTONIC BELT: AN ISLAND ARC OF CAMBRIAN(?) AGE

WC;

A23

SMITH RIVER 

ALLOCHTHON

150 MILES

50 100 150 KILOMETERS

FIGURE 9. - Generalized geologic map of the Piedmont showing the distribution of the Carolina slate belt (CSB), Eastern slate belt (ESB), 

central Virginia volcanic-plutonic belt (CVVB), South Boston-Danville area (SBD), James Run belt (JR), and Wilmington complex (WC). 

Also shown are Mesozoic basins (Mz), gabbro of the Baltimore Complex (GBC) of Maryland, melange zone (m), schist beneath the 

Chopawamsic Formation (s), and other features as identified on the map. Isotopic ages reported for some of these metavolcanic rocks 

are also shown. -Gf and p-Cf are the Early Cambrian and Precambrian fossil localities, respectively, from within CSB.
A24 CONTRIBUTIONS TO THE GEOLOGY OF THE VIRGINIA PIEDMONT

calated with some of the metavolcanic rocks, as also 

reported by Southwick (1979). The Gilpin Falls Member 

(Higgins, 1971, p. 322-323) overlies the three basal 

members of the James Run. It consists of metamorphosed basalt, which in its basal part is characterized by 

pillow lavas and aquagene tuff. The Gilpin Falls Member 

is a very distinctive unit that is widely distributed within 

the James Run terrane of Cecil County, Md. Overlying 

the Gilpin Falls Member are three other coeval to nearly 

coeval members containing metamorphosed mafic and 

felsic rocks that originally were probably volcanic and 

volcaniclastic accumulations interlayered with sediments. Higgins (1977, p. 126) stated that one of these 

(Big Elk Creek Member) appears gradational with metagraywackes of the Wissahickon Formation, a relationship that is difficult to understand because it would 

make one of the upper members of the James Run Formation gradational with the stratigraphically underlying Wissahickon. Structural or stratigraphic complications do not appear to be involved, as the Big Elk Creek 

Member is shown in its correct stratigraphic order and 

then in contact with the Wissahickon Formation (Higgins, 1977, fig. 3). The Principio Creek Member above 

the Gilpins Fall Member is in turn overlain by undescribed "unnamed felsite member." However, the Principio 

Creek Member is described as underlying pelitic schist 

of the Wissahickon Formation (Higgins, 1977, p. A127 

and fig. 3). Here again an anomalous situation occurs, in 

that rocks stratigraphically high in the James Run stratigraphic section are reported to underlie the rocks 

(Wissahickon Formation) that are stratigraphically 

beneath the James Run. In any event, the James Run 

Formation of Cecil County appears to be composed of a 

complex pile of laterally and vertically grading volcanic 

and volcaniclastic rocks that show marked lithologic 

changes along strike both locally within Cecil County, as 

well as regionally with the James Run of the Baltimore 

area described by Crowley (1976). Of particular interest, 

however, is the presence of the pillow lavas of the Gilpin 

Falls Member in Cecil County, described by Higgins as 

emplaced in water shallower than 0.5 km on the basis of 

the size and percentage of amygdules in the pillow lavas 

of this unit (Higgins, 1971). Pillow lavas are not 

recognized in the James Run Formation from elsewhere 

in Maryland and are extremely rare in the Chopawamsic 

Formation of Virginia, being confined to two localities 

of outcrop size (Pavlides, 1976).

Southwick and Fisher (1967) had originally described 

rocks of the James Run Formation as the James Run 

Gneiss, and Southwick (1969) proposed that metavolcanic rocks of the Baltimore area and parts of the Wilmington Complex described by Ward (1959) in Delaware 

were similar. Later Southwick and others (1971) correlated the James Run areas with the Chopawamsic Formation of Virginia, a correlation also accepted by Higgins (1972).

The correlation between the James Run and the 

Chopawamsic Formations was based largely on what 

appeared to be coeval but discordant Cambrian zircon 

ages. Because of the uncertainties introduced by the 

debate on the significance of zircon ages in the Piedmont (Higgins and others, 1977) this age correlation is 

not conclusive. However, the James Run and the Chopawamsic generally occur along strike, albeit in widely 

separated areas and probably belong to the same tectonic province. They may be coeval and possibly have 

been essentially coextensive at one time. Alternatively, 

they may have been parts of different volcanic chains of 

about the same age and within the same tectonic 

regime.

CAROLINA SLATE BELT

Correlation of the Carolina slate-belt rocks with what 

is now defined as the Chopawamsic has been proposed 

by Glover (1974, p. 757). Stratigraphic comparisons between the two districts, however, are handicapped by 

the controversies that exist among geologists as to the 

stratigraphic succession present within the Carolina 

slate belt. Briggs and others (1978, p. 511) suggested 

that two separate stratigraphic sequences are within the 

Carolina slate belt. The oldest of these is the sequence 

within the Virgilina synclinorium of the Virgilina 

district (figs. 1, 9), as originally defined by Laney (1917) 

and somewhat modified by later workers. It consists of, 

from top to bottom, Laney's Virgilina Greenstone, 

Aaron Slate, and Hyco Quartz Porphyry. These rocks 

generally give discordant zircon ages of 740-620 m.y. 

(Precambrian) and according to Briggs and others 

(1978, p. 511) are the older of two sequences that once 

occurred within the Virgilina area of Virginia and North 

Carolina. The younger sequence is now believed to be 

represented in the Albemarle area of North Carolina 

(Glover, 1974; Briggs and others, 1978, p. 511), where 

tuffaceous rocks determined as younger than 565 m.y. 

(Early Cambrian) by Fullagar (1971) are present (fig. 9). 

In the Virgilina area, the Roxboro Granodiorite of 

Glover and Sinha (1973, p. 242-244), which has discordant zircon ages of about 565 m.y., intrudes folded rocks 

of the Carolina slate belt and is assumed to be a plutonic 

root of the younger volcanic sequence, namely the temporal equivalent of the 565 m.y.-old metavolcanic rocks 

near Albemarle, N.C., that have since been removed by 

erosion in the Virgilina area. The two fossil localities 

from within the Carolina slate belt (fig. 9) are also consistent with an Early Cambrian to Late Precambrian 

age span for these rocks.

The nature of the contact between the Carolina slatebelt rocks in the Virgilina area of Virginia and the
THE CENTRAL VIRGINIA VOLCANIC-PLUTONIC BELT: AN ISLAND ARC OF CAMBRIAN(?) AGE A25

higher grade gneisses of the South Boston-Danville area 

immediately to their west (fig. 9) is also controversial. 

Laney (1917) considered the contact between these two 

terranes to be either a fault or an unconformity. Tobisch 

and Glover (1971) mapped recumbent nappes west of the 

Carolina slate belt in the South Boston area of Virginia, 

which they believed were rooted near the Carolina slatebelt contact, and considered the gneisses and schists in 

the nappes to be more highly metamorphosed equivalents of the Carolina slate belt. Henika (1977) has mapped gneisses and schists in the Danville area of Virginia 

as a sequence of polydeformed metavolcanic rocks that 

appear to structurally merge to the east (figs. 1, 9) with 

the nappes mapped by Tobisch and Glover (1971). 

Tobisch and Glover (1969) have considered the contact 

between the upper amphibolite-grade metavolcanic 

rocks of the nappes in the South Boston area and the 

greenschist-facies metavolcanic rocks of the Carolina 

slate-belt volcanic rocks in the Virgilina synclinorium to 

be essentially a closely compressed metamorphic gradient increasing in grade from east to west. Conley 

(1978) cited the unpublished data of R. D. Kreisa in the 

South Boston area of Virginia as supporting conformable contact between the Carolina slate belt and higher 

grade metavolcanic rocks to the west (gneisses of the 

nappes of Tobisch and Glover, 1971), which Conley considered to be coeval with the Chopawamsic, with the implication that the 620-740 m.y. zircon ages from the 

South Boston area of gneiss are anomalous and "might 

be the product of contamination of younger rocks by 

older zircons" (Conley, 1978, p. 137). Conley (1978, p. 

138, and Explanation, fig. 1) concluded that the slatebelt rocks of the Virgilina synclinorium are younger and 

stratigraphically overlie the higher grade metavolcanic 

rocks to their east. On this reasoning, the Chopawamsic 

Formation, which Conley considered coeval with these 

amphibolite-grade metavolcanic rocks, is hence considered to be older than the Carolina slate belt.

Obviously many stratigraphic and geochronologic 

problems remain to be resolved within the slate belt 

metavolcanic rocks and their presumed equivalents to 

the west, as well as in the Chopawamsic. However, 

Pavlides (1976, p. 24) has indicated that despite these 

problems sufficient stratigraphic, chemical, and geophysical differences exist between the Chopawamsic 

rocks and the Carolina slate-belt metavolcanic rocks to 

preclude their having been part of a single volcanic province or even coextensive. The Carolina slate belt has a 

distinctive stratigraphy at various places along its belt 

of outcrop. The tripartite stratigraphy for the Carolina 

slate belt in the Virgilina synclinorium as first established by Laney (1917) and generally adopted and refined by 

Glover and Sinha (1973) cannot be duplicated anywhere 

within the Chopawamsic belt.

The chemistry of the volcanic rocks in the Carolina 

slate belt is different from that of the rocks in the 

Chopawamsic Formation. Major element chemistry for 

Carolina slate-belt metavolcanic rocks in the Albemarle 

area of North Carolina (fig. 9) demonstrates that they 

are strongly bimodal, about 17 percent mafic, 80 percent 

felsic, and 3 percent intermediate, as well as being 

generally calcalkaline (Seiders, 1978b). This is in contrast to the Chopawamsic features described earlier. 

Whitney and others (1978) also concluded that the metavolcanic rocks of the "Little River Series" of the 

Carolina slate belt at the Georgia-South Carolina border 

area are bimodal and predominantly felsic. On the basis 

of major element chemistry they considered them to be 

comparable with a tholeiitic island-arc suite. They correlated the "Little River" sequence with the lower part 

of the metavolcanic sequences in the Albemarle area of 

North Carolina. The number of presently available 

chemical analyses from the Chopawamsic is small but 

contains minor element and REE analyses, which include elements generally stable under metamorphic conditions. In contrast, such stable-element data have not 

been published as of this writing for Carolina slate-belt 

rocks. Therefore, the conclusion by Whitney and others 

(1978) that the major-element chemistry of the "Little 

River Series" represents primitive island-arc volcanism 

needs additional verification by stable-element data. The 

large number of analyses used by Seiders in determining 

the Peacock index despite the use of mobile elements, 

makes it statistically reasonable that the Carolina slatebelt sequence near Albemarle is calcalkaline. Although 

the data have a large scatter, they nonetheless form 

statistical clusters that permit reasonable graphs to be 

extrapolated through them (Seiders, 1978b, fig. 8). 

Hence, the volumetrically predominantly calcalkaline 

character of the Carolina slate-belt metavolcanic rocks, 

at least in the Albemarle area, contrasts with the 

character of the Chopawamsic Formation and the coeval 

amphibolites of the Ta River Metamorphic Suite described earlier. Laney's Hyco Formation of the Carolina slate 

belt, or unit II of Glover and Sinha (1973), is also 

reported as showing evidence of welding in tuffs (Glover 

and Sinha, 1973, p. 238) that suggests subaerial deposition. Furthermore, Glover and Sinha (1973, p. 239) 

reported conglomerate beds in their unit III containing 

volcanic, as well as quartzite, feldspathic quartzite, and, 

rarely, granite pebbles, suggesting a continental source 

for these deposits. Such lithologic features have not 

been recognized in the Chopawamsic.

The Eastern slate belt (fig. 9) is generally considered 

to be similar to the Carolina slate belt but has not been 

studied extensively. It is not discussed in this report.

Both the Carolina slate belt and the central Virginia 

volcanic-plutonic belt are characterized by positive
A26 CONTRIBUTIONS TO THE GEOLOGY OF THE VIRGINIA PIEDMONT

magnetic anomalies. In general, however, the Chopawamsic magnetic signature is characterized by highamplitude strongly linear patterns, whereas the 

Carolina slate belt has a broader, less linear, pattern. 

The Carolina slate belt overlies a large, broad, gravity 

high, presumably related to mafic rocks at some depth 

beneath the surface. In contrast, the Chopawamsic belt 

lacks such a directly related mafic substrate.

The stratigraphic, geochemical, and geophysical contrasts between the central Virginia volcanic-plutonicbelt and the Carolina slate-belt metavolcanic rocks are 

believed to reflect differences related to their formation 

in different tectonic belts and settings. The Carolina 

slate belt is part of the Avalonian province of eastern 

North America, which is characterized by "relatively 

undeformed and unmetamorphosed, chiefly terrestrial 

rocks of mainly late Precambrian age" (Williams, 1978).

ORIGIN AND TECTONIC SETTING OF THE

CENTRAL VIRGINIA VOLCANIC-PLUTONIC

BELT: DISCUSSION AND CONCLUSIONS

The tectonic setting in which the Chopawamsic Formation and its related rocks formed is difficult to 

evaluate, as it is in most ancient volcanic terranes. The 

linearity of the present belt probably reflects to some extent the paleogeographic linearity of the belt of 

volcanism during the Early Cambrian, at which time the 

Chopawamsic and associated rocks of the central 

Virginia volcanic-plutonic belt are believed to have formed, but subsequent tectonism could also account for 

some of this linearity. Few unequivocal data in the 

presently exposed metavolcanic rocks of this belt indicate whether these rocks formed subaerially or in 

water. Pillow lavas are rare and at best indicate local 

deposition either in lacustrine or marine waters. Ignimbritic tectures, which would suggest subaerial deposition, have not been recognized in this belt. Some of the 

volcaniclastic rocks were obviously water laid, but again 

no criteria are available as to whether such deposition 

was marine or terrestrial. Because the Chopawamsic 

grades downward into diamictite near Quantico, Va. 

(Southwick and others, 1971) and because such diamictite has been interpreted as having formed as a submarine slide (Hopson, 1964), it is assumed that the basal 

Chopawamsic probably formed initially in a marine environment. This interpretation is not inconsistent with 

the stratigraphic relationships to the southwest in the 

Fredericksburg area, where the Chopawamsic grades 

downward into a thick sequence of schist. All in all, 

however, the field evidence is equivocal as to the precise 

tectonic regime in which the Chopawamsic and its 

related rocks formed.

The geochemical features of the rocks of the central 

Virginia volcanic-plutonic belt in the Fredericksburg 

area are more indicative of their paleotectonic setting 

than are the stratigraphic features of these rocks. For 

example, as described on page All, suite A of the 

Chopawamsic Formation has an REE composition (table 

2, fig. 4) that is characteristic of tholeiitic island-arc 

suites (Jakes and Gill, 1970; Jakes and White, 1969, 

1971, 1972). The closely associated calcalkaline rocks of 

the Chopawamsic suites B and C under the model of 

Jakes and White may represent stratigraphically 

younger rocks or represent volcanics that formed on the 

continentward side of an island arc (see also Miyashiro, 

1975, p. 185). However, the Chopawamsic tholeiitic and 

calcalkaline suites do not show a zonation parallel to the 

length of the volcanic belt, as do most examples cited by 

Jakes and White for younger volcanic arcs. Instead, the 

zonation from a tholeiitic suite to a calcalkaline volcanic 

suite takes place along the volcanic belt rather than 

across it in the Fredericksburg area (see figs. 3 and 4 

and table 2), comparable with the north to south increase in alkalinity of volcanic rocks in the Lesser Antilles island-arc chain (Arculus and Johnson, 1978, p. 

120-121, fig. 3). Indeed, Arculus and Johnson argued 

that the island-arc tholeiitic-calcalkaline-alkaline zonation across island arcs in which alkalinity increases continentward, as suggested by Jakes and White (1969, 

1972) and Jakes and Gill (1970), may not be a valid 

general model. Arculus and Johnson pointed out that 

the examples cited by Jakes and his coworkers from 

Papua, New Guinea, and Vite Levu in the Fiji chain 

were based on incorrect concepts of the local geology. 

The geologic relationships in these areas have since been 

shown to be both different and more complex (Johnson 

and others, 1978; Gill, 1976a, b) than originally envisioned by Jakes and his coworkers. Arculus and Johnson 

(1978) pointed out that the alkalic or shoshonitic 

volcanism in some of the cited examples of Jakes and 

White is generally a late event, unrelated to a trenchsubduction-zone process. Furthermore, calcalkaline and 

tholeiitic volcanic rocks on some volcanic islands, such 

as Martinique, are intercalated at several stratigraphic 

levels and do not fit a model of alkalinity increasing with 

time (Arculus and Johnson, 1978, p. 123). Indeed, 

Arculus and Johnson emphasized that tholeiitic, 

calcalkalic, and alkalic volcanism can take place essentially contemporaneously in different provinces of a 

volcanic terrane. Thus, it is also possible to consider 

suites A, B, and C of the Chopawamsic Formation as 

distinct tholeiitic and more alkalic suites that formed 

more or less contemporaneously but that were derived 

from chemically different magmas. These differences in
THE CENTRAL VIRGINIA VOLCANIC-PLUTONIC BELT: AN ISLAND ARC OF CAMBRIAN(?) AGE A27

source magmas are indicated by the differences in REE 

patterns (fig. 4) and high-valence cation abundances 

(table 2) among the three suites. The differences indicated by these elements are considered to be particularly relevant for the metamorphosed volcanic rocks 

of the Chopawamsic because of the general immobility, 

particularly of the high-valence cations, during 

postdepositional alteration. Regardless of the degree of 

applicability of the model of Jakes and his coworkers, 

the geochemical features of the island-arc tholeiite suite 

seem valid and are found within suite A of the 

Chopawamsic Formation.

The trondjemitic and plagiogranitic tonalites intrusive 

into the Chopawamsic are analogous to similar tonalitic 

rocks in other ancient island arcs, as well as in modern 

island-arc chains. For example, trondhjemite in the 

Mule Mountain stock of the West Shasta district, California, intrudes a Devonian island-arc suite (Barker and 

others, 1979), the Twillingate Trondhjemite intrudes a 

lower Paleozoic island-arc sequence in northern Newfoundland (Payne and Strong, 1979), and Popilitov and 

others (1973) reported plagiogranites of batholitic 

dimensions intruding Cambrian metavolcanic rocks in 

the Sayan geosyncline of Siberia. Similarly, the 

Miocene-age rocks of the Izu-Bonin arc in Japan are intruded by low-potash tonalites and trondhjemites of the 

Tanzawa plutonic complex (Ishizaka and Yanagi, 1977). 

Additional examples of trondhjemites and related 

tonalitic rocks intruding Tertiary island-arc-chain 

volcanic rocks have been reported in the Pacific, such as 

in the Fiji chain (Band, 1968; Gill and Stork, 1979). Lowpotash dacite, the effusive equivalent of trondhjemite, 

also occurs in the Tonga-Kermadec island-arc chain 

(Bryan, 1979), on Fiji (Gill and Stork, 1979), and on 

Saipan in the Marianas (Barker and others, 1969,1975). 

Thus, as emphasized by many others, low-potash granitoids such as trondhjemites and plagiogranites are 

characteristic, although generally minor, rock types in 

island-arc assemblages.

The eastern temporal equivalent of the Chopawamsic 

Formation, the Ta River Metamorphic Suite, is less well 

characterizable by its REE chemical features. However, 

it has a few amphibolites showing generally primitive 

patterns and LREE depletion (see fig. 4) suggestive of 

oceanic tholeiites.

Many discriminant diagrams using immobile elements 

have been constructed recently to evaluate tectonic setting, principally of basaltic rocks. Some of the discriminant diagrams of Floyd and Winchester (1975, figs. 2, 5) 

and Pearce and Cann (1973, fig. 3) were used to evaluate 

the tectonic setting of the amphibolites (generally 

metabasalts) of the Ta River Metamorphic Suite. Most 

of the amphibolites of the Ta River plot in the field of 

oceanic tholeiitic basalt on the Floyd and Winchester

diagrams (fig. 10A, B) and less convincingly within the 

ocean-floor-basalt field of Pearce and Cann (fig. 10C). 

Except for one analysis, they also plot in the 

subalkaline-basalt field (fig. 11) on the diagrams of Winchester and Floyd (1977, figs. 2, 4). All these are 

generally consistent with the proposed model that the 

Ta River amphibolites are an oceanward facies of the 

Chopawamsic Formation. (However, the proximity of 

the amphibolitic Ta River Metamorphic Suite to the 

lithologically dissimilar Chopawamsic Formation (fig. 

3), without an intervening lithologic transition zone, 

poses a problem in considering these two formations as 

coeval facies equivalents. Possibly, the Ta River was 

structurally juxtaposed against the Chopawamsic by 

northwestward directed thrusting in pre-Quantico Formation time. Such an interpretation would necessitate 

the additional speculation that the trace of such a thrust 

is buried beneath the Quantico Formation that now intervenes in a synform between the Chopawamsic and 

the Ta River.)

Discriminant diagrams purporting to show tectonic 

setting are as yet not constructed for volcanic suites 

consisting of felsic rocks, as well as mafic rocks, such 

as those in the Chopawamsic Formation. However, as 

indicated earlier, suite A of the Chopawamsic has 

characteristics of the tholeiitic island-arc suite of Jakes 

and Gill (1970), as indicated by some of its chemical 

features and particularly its REE pattern. The general 

protolithic character of these metavolcanic rocks of the 

Chopawamsic is suggested in part by their plot on the 

composition-field diagram (fig. 12) of Winchester and 

Floyd (1977, fig. 6). Suites B and C of the Chopawamsic 

Formation (table 2) plot in the rhyodacite-dacite field, 

whereas most of the suite A rocks plot as subalkaline 

basalt, although the Si02 content of some of these 

samples is greater than 53 percent (compare fig. 12 with 

table 2). The sodic nature of some of the felsic rocks, 

such as the keratophyres of suite A, poses a problem in 

the use of diagrams such as figure 12. If keratophyres 

can be derived from sodic magmas, as suggested by 

Coleman and Donate (1979, p. 150-151), then figure 12 

is not usable for these rocks because it lacks a 

keratophyre field.

Although the rocks of the central Virginia volcanicplutonic belt generally have features found in ensimatic 

island arcs, the possibility that they formed as chains 

along a continental margin having a thin sialic crust, 

perhaps as represented by the Po River Metamorphic 

Suite, cannot be excluded. However, irrespective of the 

precise tectonic environment within which these 

volcanic rocks formed, the oceanward side of the chain 

was apparently to the east.

The presence of a possible tectonic melange zone 

about 8 km northwest of the Chopawamsic Formation
A28 CONTRIBUTIONS TO THE GEOLOGY OF THE VIRGINIA PIEDMONT

CJ 

CC

I 

CD

LU

CM

o

567

Y/Nb

10 11

fi

0.05 0.10 0.15 0.20

Zr/P 2 O 5 x10'

FIGURE 10. -Plot of amphibolites of the Ta River Metamorphic Suite on the discriminant diagrams of (A,B) Floyd and Winchester (1975, figs. 2, 

5) and (C) Pearce and Cann (1973, fig. 3). Fields for A are A, oceanic alkali basalts; B, continental alkali basalts; C, continental tholeiitic 

basalts; and D, oceanic tholeiitic basalts. Fields for B are A, continental alkali basalts; B, oceanic alkali basalts; C, oceanic tholeiitic basalts; 

and D, continental tholeiitic basalts. Fields for C are D, ocean island or continental basalts; B, ocean-floor basalts; A-B, low-potassium 

tholeiites; and C-B, calc-alkali basalts. Numbered circles are of samples whose locations are shown on figure 3.
THE CENTRAL VIRGINIA VOLCANIC-PLUTONIC BELT: AN ISLAND ARC OF CAMBRIAN(?) AGE

Ti/100

A29

FIGURE 10.-Continued.

(fig. 1) in the Fredericksburg area suggests that the central Virginia volcanic-plutonic belt may be allochthonous 

and has been transported westward in early Paleozoic 

time (Pavlides, 1980b). I believe that the hypothesis of a 

Cambrian island-arc system composed of the central 

Virginia volcanic-plutonic belt and the James Run terrane of Maryland with a marginal basin on the west side 

is the model most compatible with the available data 

discussed in this paper. As stated under the discussion of 

the James Run Formation, the volcanic belt may not 

necessarily have had absolute linear continuity with the 

Virginia belt. Indeed, the proposed Cambrian island arc 

may have been an outer or even frontal arc system. This 

speculation finds some support in the nature of the 

melange northwest of the Chopawamsic Formation. The 

widely scattered blocks within this melange include 

serpentinite, metagabbro, metavolcanic rocks, and some 

metagraywacke and may be remnants of a tectonically 

disrupted island arc and marginal basin that lay to the 

west of the Chopawamsic Formation and its associated 

rocks. Mapping in progress may eventually provide data 

needed to evaluate this speculation. Brown (1976, p. 

142) has described a tectonic melange(?) beneath the

Chopawamsic near Shores, Va., along the James River. 

It may be a southwestward extension of the melange 

now recognized west of the Chopawamsic belt, which by 

convergence southeastward now lies beneath the Chopawamsic there. Bland and Brown (1977, p. 121-122) indicated that some of the greenstone blocks of the Shores 

melange plot as ocean-floor basalts on the titaniumzirconium-yttrium discriminant diagrams of Pearce and 

Cann (1973) and, also using other criteria, they suggested that the Shores melange may represent a collapsed and eastward subducted back-arc basin. Bland and 

Brown (1977) also suggested that the Chopawamsic 

volcanic rocks, which they characterize as calcalkaline 

there (Arvonia district), may have formed from this 

eastward subducting slab. I have no evidence with which 

to evaluate the polarity of a subduction zone, if one existed, with respect to the melange west of the 

Chopawamsic belt now recognized in northern Virginia. 

However, on the basis of the geochemical evidence cited 

that supports a model wherein the Ta River amphibolites are a mafic oceanward facies of the Chopawamsic, 

it can be assumed that a subduction zone, if present, lay 

east of this belt of rocks and dipped westward. The
A30 CONTRIBUTIONS TO THE GEOLOGY OF THE VIRGINIA PIEDMONT

001 0 10 1 00

Zr/TiO 2 x10'

72

DC

LU

Q_

LJJ

o

64

60

56

52

48

44

40

A

 2

 1

 4

 3

 5

D

\

fi

0 10 1 00 100

Nb/Y

FIGURE 11.-Plot of amphibolites of the Ta River Metamorphic Suite on the discriminant diagrams of Winchester and Floyd (1977, figs. 2, 4). Fields for A are A, rhyolite; B, rhyodacite and dacite; C, comendite 

and pantellerite; D, trachyte; E, andesite; F, trachyandesite; G, phonolite; H, subalkaline basalt; I, alkali 

basalt; and J, basanite, trachybasanite, and nephelinite. Fields for£ are A-l, same as for A; J,basanite 

and nephelinite. Numbered circles are of samples whose locations are shown on figure 3.
THE CENTRAL VIRGINIA VOLCANIC-PLUTONIC BELT: AN ISLAND ARC OF CAMBRIAN(?) AGE A31

1 00 r010 

o t001

001 0 10 1 0

Nb/Y

FIGURE 12. -Plot of metavolcanic rocks of the Chopawamsic Formation (table 2) on the composition-field diagram of Winchester 

and Floyd (1977, fig. 6). Fields are A, comendite and 

pantellerite; B, rhyolite; C.rhyodacite and dacite; D, andesite; 

E, andesite and basalt; F, subalkaline basalt; G, alkali basalt; H, 

trachyandesite; I, trachyte; and J, phonolite. Numbered circles 

are of samples whose locations are shown on figure 3.

difference in polarity of a subduction zone remains to be 

resolved between the model of Bland and Brown (1977) 

and the model suggested in this report.

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A34 CONTRIBUTIONS TO THE GEOLOGY OF THE VIRGINIA PIEDMONT

Whitney, J. A., Paris, T. A., Carpenter, R. H., and Hartley, M. E., Ill, 

1978, Volcanic evolution of the southern slate belt of Georgia and 

South Carolina: A primitive oceanic island arc: Journal of Geology, 

v. 86, no. 2, p. 173-192.

Williams, Harold, compiler, 1978, Tectonic lithofacies map of the 

Appalachian orogen: Newfoundland Memorial University Map 1, 2 

sheets, scale 1:1,000,000.

Winchester, J. A., and Floyd, P. A., 1977, Geochemical discrimination 

of different magma series and their differentiation products using

immobile elements: Chemical Geology, v. 20, no. 4, p. 325-343.

Wood, D. A., Gibson, I. L., and Thompson, R. N., 1976, Elemental 

mobility during zeolite facies metamorphism of the Tertiary 

basalts of Iceland: Contributions to Mineralogy and Petrology, v. 

55, p. 241-254.

Zartman, R. E., 1978, U-Pb zircon dates from the central Appalachian 

 Piedmont: A possible case of inherited radiogenic lead: Reply: 

Geological Society of America Bulletin, v. 89, no. 7, p. 1116-1117.

 d U.S. GOVERNMENT PRINTING OFFICE: 1981 361-614/265