Middle and Upper Paleozoic Granitic 

Rocks in the Piedmont Near 

Fredericksburg, Virginia: 

Geochronology

GEOLOGICAL SURVEY PROFESSIONAL PAPER
Middle and Upper Paleozoic Granitic 

Rocks in the Piedmont Near 

Fredericksburg, Virginia: 

Geochronology

By LOUIS PAVLIDES, THOMAS W. STERN, JOSEPH G. ARTH, 

KATHLEEN G. MUTH, and MARCIA F. NEWELL

CONTRIBUTIONS TO THE GEOLOGY OF THE VIRGINIA PIEDMONT

GEOLOGICAL SURVEY PROFESSIONAL PAPER 1231-B

Several suites of plutonic rocks have been 

identified by geologic mapping and 

isotopic age dating in the polydeformed 

and metamorphosed Piedmont near 

Fredericksburg, Virginia

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1982
UNITED STATES DEPARTMENT OF THE INTERIOR 

JAMES G. WATT, Secretary

GEOLOGICAL SURVEY 

Dallas L. Peck, Director

Library of Congress Cataloging in Publication Data

Main entry under title:

Middle and upper Paleozoic granitic rocks in the Piedmont near Fredericksburg, Virginia.

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

Bibliography: p.

Supt of Docs, no.: I 19.16:1231-8

1. Granite-Virginia-Fredericksburg. 2. Geology, Stratigraphic- Paleozoic. I. Pavlides, Louis, 1921- . II. Series:

Geological Survey professional paper; 1231-B. III. Series: Contributions to the geology of the Virginia Piedmont. 

QE462.G7M48 552'.3 81-607861

AACR2

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

604 South Pickett Street, Alexandria, VA 22304
CONTENTS

Page

Abstract______________________________________________ 1

Introduction ___________________________________________ 1

Acknowledgments _____________________________________ 1

Geologic setting ____________________________________________ 1

Description of granitic intrusive rocks _______________________________ 3

Falls Run Granite Gneiss __________________________________ 3

Falmouth Intrusive Suite ____________________________________ 3

Analytical methods _____________________________ _________ 4

U-Th-Pb method ____________________________I_________ 4

Rb-Sr isochron method ___________________________________ 4

Decay constants _________________________________________ 7

Age of intrusive granites _____________________________________ 7

Falls Run Granite Gneiss ____________________________________ 7

Falmouth Intrusive Suite __________________________________ 7

Timing of metamorphism __________________________________ 8

Comparison with other Piedmont terranes ___________________________ 8

Conclusions ______________________________________________ 9

References cited 9

ILLUSTRATIONS

FIGURE 1. Geologic map of the Fredericksburg area, Virginia ______________________________________ 2 

2,3. Rb-Sr isochron diagrams for:

2. The Fails Run Granite Gneiss _____________________________________________ 6

3. The Falmouth Intrusive Suite _______________________________________________ 6 

4. Rb-Sr plot for the Falmouth Intrusive Suite, indicating the separation of these rocks into groups A, B, and C _______ 8

TABLES

TABLE 1. U-Th-Pb analytical data on zircons from the Falls Run Granite Gneiss and the Falmouth Intrusive Suite of the

Fredericksburg area, Virginia ________________________________________________ 5 

2. Rb-Sr analytical data for the Falls Run Granite Gneiss and Falmouth Intrusive Suite of the Fredericksburg area, Virginia _ 7

ill

CONTRIBUTIONS TO THE GEOLOGY OF THE VIRGINIA PIEDMONT

MIDDLE AND UPPER PALEOZOIC GRANITIC ROCKS IN THE PIEDMONT 

NEAR FREDERICKSBURG, VIRGINIA: GEOCHRONOLOGY

By Louis PAVLIDES, THOMAS W. STERN, JOSEPH G. ARTH, 

KATHLEEN G. MUTH, and MARCIA F. NEWELL

ABSTRACT

Several suites of plutonic rocks have been identified by geologic mapping in the polydeformed and metamorphosed (amphibolite-grade) 

Piedmont near Fredericksburg, Va. Two of these suites were dated by 

U/Pb (zircons) and Rb/Sr (whole-rock) methods. The oldest suite is the 

Falls Run Granite Gneiss, a coarse-grained, strongly foliated, and 

highly metamorphosed rock that ranges in composition from granite to 

monzonite. The chief mass of Falls Run Granite Gneiss (formerly called 

the Berea pluton) is intrusive into the Holly Corner Gneiss of Early 

Cambrian(?) age. Both these gneisses are allochthonous remnants of 

the inverted limb of a recumbent fold; subsequent deformation formed 

a type-2 interference fold. U/Pb and Rb/Sr studies indicate that the 

Falls Run is 410 million years old and has an initial 87 Sr/86Sr of 0.7070.

Younger granitoid plutons, dikes, and sills, assigned to the Falmouth 

Intrusive Suite, are widespread in the area. These plutons are abundant in the eastern part of the area but are rare west of the Quantico 

Formation. Rocks of the Falmouth consist of strongly to weakly 

foliated: A, biotite adamellite and granodiorite having Rb/Sr less than 

0.2, and B, muscovite-biotite adamellite and granite having Rb/Sr 

greater than 0.4. Concordant zircon ages and two whole-rock isochrons 

indicate that both groups are 300-325 million years old. The initial 

87Sr/86Sr of group A is 0.704, which suggests a lower crust or mantle 

source, whereas that of group B is 0.7088, similar to the 0.7070 for the 

Falls Run, and suggests crustal involvement in the magma generation. 

Recently reported ages from North and South Carolina are similar to 

those of the Falmouth. Thus, an extensive belt of 300-325 million-yearold plutons is present in the eastern Piedmont.

INTRODUCTION

The Piedmont terrane of northern Virginia is of particular interest to Appalachian geology because it 

bridges the area between the extensively studied Piedmont of Pennsylvania and Maryland and the Piedmont 

of the Southeastern United States. Some of the best 

fresh bedrock exposures in the northern Virginia Piedmont are found along the Rappahannock River near 

Fredericksburg. Geologic mapping (Pavlides, 1976, 

1980) shows that this area is composed mainly of 

metasedimentary and metavolcanic rocks, but also includes several suites of intrusive granitoid rocks. The 

age of these suites can be used to place limits on the age 

of some deformational and metamorphic events which 

affected the terrane. This report summarizes the 

geochronology of two of these granitoid suites: Falls 

Run Granite Gneiss and the Falmouth Intrusive Suite. 

The petrography and modal composition of these rocks 

have been described elsewhere (Pavlides, 1980), and, 

therefore, they will be only briefly summarized in this 

paper.

ACKNOWLEDGMENTS

We wish to thank John Mangum for zircon separations, A. R. Bobyarchick for preparing the whole-rock 

splits, and S. Linda Cranford for numerous modal 

analyses. We are grateful to M. W. Higgins and J. F. 

Sutter for reviewing the manuscript.

GEOLOGIC SETTING

The Fredericksburg area (fig. 1) consists primarily of 

Proterozoic Z(?) and Paleozoic metasedimentary and 

meta-igneous rocks in the amphibolite facies of 

metamorphism that have been polydeformed (Pavlides, 

1976). The oldest rocks are the Po River Metamorphic 

Suite of Proterozoic Z(?) and (or) early Paleozoic age. 

The others are of Paleozoic age and include the

Bl
B2

77°37'30"

CONTRIBUTIONS TO THE GEOLOGY OF THE VIRGINIA PIEDMONT

77°30'00"

38°22'30"

38°15'00"

Sk - Storck F - Fredencksburg 

St - Stafford Sa - Spotsylvania 

S.C. - Salem Church

FIGURE 1.-Geologic map of the Fredencksburg area, Virginia.
PALEOZOIC GRANITIC ROCKS IN THE PIEDMONT NEAR FREDERICKSBURG, VA: GEOCHRONOLOGY B3

EXPLANATION

i-TtfiL'#>;||

 cfr:

|p

1| *

Wr\

iKI

?0qq

t I; Ch

> CRETACEOUS

 CARBONIFEROUS

 SILURIAN

 ORDOVICIAN

 CAMBRIAN(?I

 CENOZOIC

 MESOZOIC

PALEOZOIC 

PROTEROZC

GzKu Coastal Plain sediments Unconsolidated. 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 monzogranite to granodionte and less commonly tonalite.

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

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

biotite-muscovrte garnetiferous schist in areas of increased metamorpihism 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 amphtbolite and amphibolitic gneiss with lesser 

amounts of biotitic gneiss and schist In southwest corner of Stafford Quadrangle it 

includes a large mass of granodionte gneiss

Plagiogranitic metatonatite Medium- to coarse-grained leucocratic to mesocratic 

quartz-rich meta-intrusive granitoid.

Trondhjemitic metatonahte Fine-grained, allotnomorphic granular, leucocratic 

granitoid, locally having granophync texture

Holly Corner Gneiss fine- to medium-grained, well foliated, hornblende-biotite 

gneiss.

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

gneiss and schist

Contact Commonly inferred, clotted where concealed 

Faults Commonly inferred, dotted where concealed

Overturned Upright

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

P-19. S-47 

Zircon sample

P-19-1 

Rb/Sr sample

Note

Except for sample P-17-1, which is from a Falmouth Intrusive Suite (FIS) pluton large 

enough to be shown on the map, all other samples of the FIS are from dikes or 

mtrusives too small to be shown at the scale of the map and that intrude a variety of 

geologic units

FIGURE 1.-Geologic map of the Fredericksburg area, Virginia - 

Continued.

metavolcanic Chopawamsic Formation and its eastern, 

more mafic facies, the Ta River Metamorphic Suite (amphibolites with granitoid intrusions), both of Early Cambrian^) age. The Holly Corner Gneiss (biotitehornblende gneiss) is also considered to be an eastern 

facies of the Chopawamsic. Schists of the Quantico Formation of Late Ordovician age lie unconformably above 

the Chopawamsic. The Chopawamsic Formation is intruded by tonalite plutons of Cambrian to pre-Late Ordovician age. The Falls Run Granite Gneiss was intruded into the Holly Corner Gneiss, probably after the 

Quantico was deposited but before the major deformation that affected the region.

Regional deformation consisted of a pre-Quantico 

local deformation (Ft) followed by a phase of upright 

folding (F2) that locally produced recumbent folds. During this episode of recumbent folding, the Holly Corner 

Gneiss and its sill-like mass of Falls Run Granite Gneiss 

were transported westward during the forming of a 

recumbent fold or nappe and now are allochthonous 

remnants of the inverted limb of the fold. Subsequent 

deformation (F3) produced the type-2 interference fold 

within which the gneisses are found; earlier folds (F2) in 

the Ta River terrane were also refolded by this late (F3) 

episode. Metamorphism proceeded with the polydeformation of the area.

The granitoids of the Falmouth Intrusive Suite were 

emplaced during and at the close of Hercynian deformation. They represent the last Paleozoic plutonic event 

now recognized in the Fredericksburg region.

DESCRIPTION OF GRANITIC INTRUSIVE 

ROCKS

FALLS RUN GRANITE GNEISS

The Falls Run Granite Gneiss occupies about 25 km2 ; 

it crops out primarily in the type-2 interference fold (fig. 

1) near Berea but also forms a small pluton within the Po 

River Metamorphic Suite near Spotsylvania. The Falls 

Run is a pale-pink to nearly white, coarse-grained, 

strongly foliated hornblende and biotite granite gneiss 

ranging in composition from monzonite to granite, as 

classified by the IUGS (Streckeisen, 1973). Microcline 

forms elongate generally poikiloblastic grains as much 

as several centimeters long that are oriented within the 

plane of foliation defined by biotite. The habit of 

microcline, biotite, and hornblende demonstrate the 

recrystallized nature of the Falls Run. Myrmekite, 

which is also present, is thought to be of metamorphic 

origin; it is found in plagioclase that is in contact with 

potassium feldspar. Accessory minerals include apatite, 

epidote, sphene, and opaque minerals.

FALMOUTH INTRUSIVE SUITE

The Falmouth Intrusive Suite consists of fine-grained 

and pegmatitic granite, fine-grained adamellite, 

granodiorite, and tonalite (less common) that intrude 

the Fredericksburg terrane from the western edge of 

the Coastal Plain to the eastern margin of the 

Chopawamsic Formation (fig. 1). Both fine- and coarsegrained rocks occur as small plutons, tabular bodies concordant to the host-rock foliation, or cross-cutting dikes 

and are generally too small to be shown on the geologic 

map. Cross-cutting relationships suggest that pegmatites are the youngest of several generations of these intrusive rocks. Some of the dikes are folded and in turn
B4 CONTRIBUTIONS TO THE GEOLOGY OF THE VIRGINIA PIEDMONT

are cut by nonfolded dikes. Foliation, generally defined 

by dimensionally alined biotite, is slightly to well formed 

and is commonly at an angle to wall-rock contacts of 

dikes. Locally the foliation is highly folded, as in the 

pluton at the northeast edge of the Salem Church 

Quadrangle (fig. 1). These relationsips indicate that the 

Falmouth rocks were emplaced during deformation 

(folded granitoids), as well as near the end of or after 

deformation (nonfolded and (or) poorly foliated 

granitoids).

ANALYTICAL METHODS

U-Th-Pb METHOD

Uranium, thorium, and lead were determined on zircon separates by isotope-dilution mass spectrometry using a partly automated 12-inch 90°-sector NBS-type 

mass spectrometer. Zircons were separated from 

samples weighing about 70 kg by concentrating by 

means of a Wilfley table, 1 heavy liquids, and a magnetic 

separator. The zircon separates were washed with hot 

HN03 and HCL to remove surface contamination, and 

20 to 50 mg were digested with HF in teflon bombs. One 

aliquot was spiked for concentration measurement of 

uranium, thorium, and lead with a combined 235U-230Th208Pb-enriched solution prepared by Mitsunobu Tatsumoto of the U.S. Geological Survey. A second aliquot 

for lead-isotope composition was not spiked. Lead was 

separated from uranium and thorium in a 5-cc column 

using Dowex 1 anion-exchange resin and was then electrode deposited. Uranium and thorium were further 

purified on a 2-cc column using Dowex 50 anionexchange resin. Lead blanks ranged from 0.3 to 1.9 ng 

(nanograms). Isotopic measurements of lead were made 

by the silica-gel technique. Fractionation resulting in 

depletion of heavy isotopes by less than 0.05 percent was 

measured by using NBS SRM 982. No correction is applied. Accuracy of the concentration determinations is 

estimated at 1.0 percent at the 67 percent confidence 

level.

The analytical data for lead, uranium, and thorium, as 

well as the calculated ages, are given in table 1. The 

238U-206Pb ages are the most dependable because 238U 

constitutes more than 99 percent of natural uranium 

and the age has a laboratory analytical error of 2 percent at the 95 percent confidence level. The necessity of 

correcting for common lead causes a larger uncertainty 

in the 235U-207Pb, 207Pb-206Pb, and 232Th-2ospb ages.

'Any trade names are used for descriptive purposes only and do not constitute endorsement 

by the U.S. Geological Survey.

Graphical solutions developed by Wetherill (1956) and 

Tilton (1960) are of limited use for samples of Paleozoic 

age because the concordia curve is nearly linear in that 

age range and intersections to the curve by chords defined by data points cannot be accurately determined, particularly when the zircons have undergone several thermal events after initial crystallization. Several of the 

Falmouth Intrusive Suite zircons have 207Pb/206Pb ages 

that are much older than the ages determined by the 

lead-uranium method. These older ages may be the 

result of the presence of xenocrystic zircon, which would 

also increase the lead-uranium ages.

Rb-Sr ISOCHRON METHOD

Strontium, rubidium, and 87Sr/86Sr were determined 

on whole-rock splits by isotope-dilution mass spectrometry using a partly automated 6-inch 60°-sector 

NBS-type mass spectrometer. Sample powders were 

produced from unweathered 3,000- to 5,000-g rock 

samples by crushing, grinding, mixing, and splitting to 

200-mesh size. Rubidium and strontium spikes (99.9 percent 84Sr and 98.0 percent 87Rb) were added to a 300 to 

500 mg split of each sample. The samples were dissolved 

in HF in teflon beakers. Rubidium and strontium were 

completely separated using 3N HCL and Dowex 

50WX8,100/200 mesh, cation-exchange resin on a 26 by 

1 cm quartz column. Rubidium and strontium were loaded as chlorides onto a triple rhenium-filament assembly 

for mass analysis. No corrections for 87Rb were required 

in the strontium-isotope measurement. Strontiumisotope ratios were corrected for fractionation on the 

basis of an 86Sr/88Sr ratio of 0.11940. Blank determinations were less than 2 ng for strontium and 0.2 ng for 

rubidium.

Accuracy of the strontium-isotope measurements is 

based on 14 analyses of NBS SRM 987, which give a 

mean 87Sr/86Sr value of ttm± 0.00003 (67 percent C 

confidence level) compared with 0.71015±0.00003 obtained at the National Bureau of Standards (I. Lynus 

Barnes, oral commun.). Precision of individual rock 

determinations, based on eight complete replicate 

whole-rock determinations, is 0.011 percent for 

87Sr/86Sr, 0.6 percent for rubidium, and 1.0 percent for 

strontium at the 67 percent confidence level.

The analytical data for rubidium and strontium are 

given in table 2 and plotted on isochron diagrams in 

figures 2 and 3. The size of individual data points 

reflects the 95 percent confidence level. The regression 

and uncertainty calculation method of York (1969) was 

used to determine ages and their uncertainty at the 67 

percent confidence level.
PALEOZOIC GRANITIC ROCKS IN THE PIEDMONT NEAR FREDERICKSBURG, VA: GEOCHRONOLOGY B5

TABLE 1. - U-Th-Pb analytical data on zircons from the Falls Run Granite Gneiss and the Falmouth Intrusive Suite, of the Fredericksburg area,

Virginia

[n.d., no data]

Sample

No.

Concentration in ppm

Pb u Th 204Pb

Atom percent

2(,6Ph 207Pb 2C)Spl, 2U6 pb/23SU

Millions o

207pb/235U :

f years

*>7Pb/206Pb :!08 pb/232Th

Falls Run Granite Gneiss

P-5

P-5A

P-8

P-19

P-26

P-27

S-47

83.9

88.3

53.1

83.8

95.2

79.7

91.2

1,481.0 

1,473.4 

885.6 

1,205.0 

1,468.0 

1,282.0 

1,398.0

1,185.0 

1,134.0 

583.6 

687 

1,202.0 

n.d. 

2,163.0

0.0319 

.0380 

.0244 

.0114 

.0568 

.1318 

.1167

84.54 

84.53 

81.76 

81.60 

82.38 

79.86 

79.38

5.01 

4.94 

4.92 

4.62 

5.15 

5.53 

6.14

10.42 

10.48 

13.29 

13.77 

12.41 

14.48 

14.36

347 

366 

356 

411 

384 

351 

367

349 

355 

368 

409 

372 

302 

378

360 

280 

450 

400 

300 

n.d. 

450

147 

159 

255 

366 

183 

n.d. 

94

Falmouth Intrusive Suite

P-10

P-14

P-16

P-20

P-22

55.9

34.3

21.6

9.8

63.0

1,205.0 

533.4

372.1 

164.0

557.7

438.6 

646.7 

197.5 

187.2 

360.9

0.1693 

.0740 

.2068 

.0617 

.6926

78.72 

67.90 

76.58 

70.15 

50.16

6.70 

5.26 

7.06 

4.42 

12.95

14.42 

26.77 

16.14 

25.37 

36.19

258 

313 

309 

302 

307

268 

359 

310 

291 

331

360 

660 

310 

200 

500

227 

286 

204 

272 

370

Description of specimens

Falls Run Granite Gneiss:

P-5 and P-5A: Coarse-grained, foliated granite gneiss. Dark-colored streaks of biotite define foliation along which large dimensionally oriented microcline (34 percent) is also alined. 

Microcline is locally perthitic; coarse-grained quartz (30 percent) is also abundant. Twinned and untwinned plagioclase (32 percent) commonly is myrmekitic where in contact with or 

partly enclosed by microcline. Coarse-grained green biotite encloses well-formed sphene and forms clot-like aggregates with epidote and opaque minerals. Symplectic intergrowths of 

vermicular quartz with epidote and biotite are locally present. Salem Church Quadrangle, Va., at lat 38°22'03" N. and long 77°30'22" W.

P-8: Coarse-grained, foliated granite gneiss. Streaky foliation defined by biotite along which large dimensionally oriented pink microcline is alined. Salem Church Quadrangle, Va., at lat 

38°22'03" N. and long 77°30'24" W.

P-19 and P-19-1: Coarse-grained, weakly foliated granite gneiss. Green biotite (1 percent) is crudely dimensionally alined and imparts weak foliation to the rock. Quartz (30 percent), 

plagioclase, both twinned and untwinned (32 percent), and microcline, locally perthitic (32 percent), have allotriomorphic granular texture. Myrmekite (5 percent) occurs in plagioclase 

that is in contact with or partially enclosed by microcline. Salem Church Quadrangle, Va., at lat 38°21'34" N. and long 77°33'32" W.

P-26 and P-26-1: Coarse-grained, allotriomorphic-granular granite gneiss. Major constituents are quartz (39 percent), microcline (30 percent), and plagioclase (26 percent). Microcline is 

locally composed of thread and interpenetrant perthite. Large microcline grains are poikiloblastic and enclose smaller grains of microcline, quartz, and epidote. Myrmekite (2 percent) 

typically occurs along plagioclase that is in direct contact with microcline. Green biotite (2 percent) forms irregular clots and streaks that are crudely alined and that impart foliation to 

the rock. Green hornblende is a minor accessory (0.5 percent), as are epidote and opaque minerals that together make up about 1 percent of the rock. Salem Church Quadrangle, Va., at 

lat 38°22'06" N. and long 77°30'25" W.

P-27 and P-27-1: Coarse-grained, foliated granite gneiss. Major constituents in allotriomorphic granular texture are microcline (35 percent), quartz (30 percent), and plagioclase (25 percent). Microcline is locally perthitic (band and thread) and, in places, poikiloblastic. Plagioclase, in contact with microcline, has formed rim and lobate myrmekite (5 percent). Green biotite 

(3 percent) in local dimensional alinement imparts foliation to the rock. Locally it encloses sphene (rare). Other minor minerals are muscovite, epidote, and opaque minerals. Salem 

Church Quadrangle, Va., at lat 38°22'07" N. and long 77°30'24" W.

P-73-71A: Coarse-grained, foliated granite gneiss. Major constituents are microcline and orthoclase (37 percent), plagioclase (29 percent), and quartz (23 percent). Salem Church 

Quadrangle, Va., at lat 38°20'22" N. and long 77°32'52" W.

S-47: Coarse-grained, foliated granite gneiss. Streaky foliation is defined by biotite and elongate microcline grains. Spotsylvania Quadrangle, Va., at lat 38° 13'04" N. and long 77°34'14" W. 

Falmouth Intrusive Suite:

P-10 and P-10-1: Medium-grained granite in dike intruded into Falls Run Granite Gneiss. Fine-grained, allotriomorphic-granular rock composed mostly of microcline (32 percent), 

plagioclase (31 percent), and quartz (30 percent). Myrmekite (3 percent) forms in plagioclase bounded by and in contact with microcline. Greenish-brown biotite (3 percent) alinement imparts crude foliation to the rock. Other sparse minerals include muscovite, opaque minerals, and zircon. Salem Church Quadrangle, Va., at lat 38°22'03" N. and long 77°30'24" W.

P-14 and P-14-1: Foliated adamellite in small pluton intruded into Po River Metamorphic Suite. Major constituents are quartz (36 percent), plagioclase (29 percent), and microcline (24 percent), which have a general dimensional alinement in the foliation plane of the rock. Foliation is defined by fine-grained, reddish-brown biotite (7 percent). Muscovite (2 percent) is locally 

coarse grained and poikiloblastic. Minor assessory minerals include apatite, epidote, opaque minerals, chlorite, and myrmekite. Spotsylvania Quadrangle, Va., at lat 38°14'42" N. and 

long 77°32'29" W.

P-16 and P-16-1: Weakly foliated, fine-grained granodiorite in small pluton intruded into gneisses of the Po River Metamorphic Suite. Major constituents in allotriomorphic-granular texture are plagioclase (46 percent), quartz (30 percent), and microcline (14 percent). The plagioclase is both twinned and untwinned and clouded with fine-grained alteration. Microcline is 

locally poikilitic. Myrmekite (2 percent) formed within plagioclase that is in contact with microcline. Greenish-brown fine-grained biotite (5 percent) along with muscovite (2 percent) imparts a crude foliation to the rock. Other minor minerals are apatite and epidote. Fredericksburg Quadrangle, Va., at lat 38°19'28" N. and long 77°29'24" W.

P-20 and P-20-1: Fine-grained biotite granodiorite in a dike cutting granitic gneiss of the Po River Metamorphic Suite. Generally massive and allotriomorphic-granular rock composed 

mostly of plagioclase (51 percent), quartz (23 percent), and microcline (14 percent). Plagioclase is locally clouded with fine-grained sericitic alteration. Greenish-brown biotite (10 percent) 

and muscovite (1 percent) are chief accessory minerals. Minor minerals include myrmekite, epidote, and apatite. Salem Church Quadrangle, Va., at lat 38°19'31" N. and long 77°30'16" W.

P-22 and P-22-1: Strongly foliated biotite granodiorite in dike intruded into granite gneisses of the Po River Metamorphic Suite. Major constituents include plagioclase (41 percent), quartz 

(29 percent), and microcline (19 percent). Reddish-brown biotite (10 percent) in strongly alined habit defines the foliation of the rock. Leucocratic grains are dimensionally alined and also 

define the foliation. Accessory minerals include myrmekite, muscovite, apatite, zircon, epidote, and opaque minerals. Fredericksburg Quadrangle, Va., at lat 38°19'25" N. and long 

77°28'10" W.
B6 CONTRIBUTIONS TO THE GEOLOGY OF THE VIRGINIA PIEDMONT

O 

00

hs

CO

0.7300 r

0.7250

0.7200

0.7150

0.7100

0.7050

0.7000

P-26-1 cS3 P-27-1

P-19-1

P-73-71A

AGE = 408±5 M.Y. 

87Sr/ 86Sr INITIAL=0.7070±0.0001

0.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50 4.00

87ni /86o Rb/ Sr

FIGURE 2.-Rb-Sr isochron diagram for the Falls Run Granite Gneiss. For analytical data on samples, see table 2. 

0.7500 r0.7400

r 0.7300

o 

oo

oo 0.7200

0.7100

.7000

P-9-1

GROUP C

P-22-1

P-16-1 

GROUP B 

P-14-1

GROUP A 

GROUP B

AGE IN M.Y. 

322±2

INITIAL 87Sr/ 86Sr 

0.7088 ±0.0001 

0.7040 ±0.0007

0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00

87n, ,86 C

Rb/ Sr

FIGURE 3.-Rb-Sr isochron diagram for the Falmouth Intrusive Suite. For analytical data on samples, see table 2.
PALEOZOIC GRANITIC ROCKS IN THE PIEDMONT NEAR FREDERICKSBURG, VA: GEOCHRONOLOGY B7

TABLE 2. -Rb-Sr analytical data for the Falls Run Granite Gneiss and 

Falmouth Intrusive Suite of the Fredericksburg area, Virginia

Sample 

No.

Concentration in ppm 

Rb Sr Rb/Sr 87Rb/86Sr 8 'Sr/86Sr

Falls Run Granite Gneiss

P-19-I __ 

P-26-1 __ 

P-27-1 

P-73-71A

178

263

250

167

310

236

219

514

0.57

1.11

1.14

.32

1.662

3.233

3.307

.9427

0.71673

.72584

.72602

.71241

Falmouth Intrusive Suite

Group A

P-6-1

P-9-1

P-10-1

P-14-1

P-16-1

P-20-1

P-22-1

P-17-1

127

210

242

64.9

93.2

53.9

99.2

133

216

92.6

164

Group B

800

624

957

779

Group C

438

0.59

2.27

1.48

0.08

.15

.06

.13

0.30

1.701

6.567

4.295

0.2347

.4322

.1631

.3685

0.8759

0.71657

.73896

.72838

0.70500

.70567

.70477

.70625

0.70989

Description of Specimens

Falls Run Granite Gneiss:

P-19-1: See table 1.

P-6-1: See table 1.

P-27-1: See table 1.

P-73-71A: See table 1. 

Falmouth Intrusive Suite:

P-6-1: Coarse-grained granite in a folded dike intruded into Holly Corner Gneiss. Major 

constituents are plagioclase (37 percent), quartz (27 percent), and microcline (21 percent) in allotriomorphic granular texture. Brown biotite (6 percent) has crude dimensional orientation. Chlorite (rare) locally replaces biotite. Other minor constituents include muscovite and epidote. Salem Church Quadrangle, Va., at lat 38°18'37" N. and 

long 77°32'27" W.

P-9-1: Crudely foliated, fine-grained granite in small intrusion emplaced in gneisses of 

the Po River Metamorphic Suite. Major constituents having allotriomorphic-granular 

texture are plagioclase (30 percent), mierocline (30 percent), and quartz (29 percent). 

Muscovite (8 percent) in coarse grains is dimensionally oriented and defines the rock 

foliation. Reddish-brown biotite (1 percent) is a minor, finer grained mica. Plagioclase 

is locally clouded by "dusty" alteration, and microcline is locally poikilitic. Myrmekite 

(2 percent) has formed in plagioclase where it is in contact with microcline. 

Fredericksburg Quadrangle, Va., at lat 38°17'17" N. and long 77°29'23" W.

P-10-1: See table 1.

P-14-1: See table 1.

P-16-1: See table 1.

P-17-1: Fine-grained, massive, allotriomorphic-granular adamellite from west margin of 

pluton. Major constituents include plagioclase (45 percent), quartz (26 percent), and 

microcline (21 percent). Plagioclase is generally clouded with sericitic alterations; 

microcline is locally poikilitic. Green biotite (5 percent) and muscovite (2 percent) are 

chief accessory minerals. Minor minerals include myrmekite, apatite, epidote, and 

opaque minerals. Salem Church Quadrangle, Va., at lat 38°21'10" N. and long 77°35'01" W.

P-20-1: See table 1.

P-22-1: See table 1.

DECAY CONSTANTS

The isotopic and decay constants used are those 

recommended by the IUGS Subcommission on 

Geochronology (Steiger and Jager, 1977). Ages quoted 

from the literature are recalculated to these constants.

AGE OF INTRUSIVE GRANITES 

FALLS RUN GRANITE GNEISS

The 238U-206Pb ages for zircons from six samples of the 

Falls Run Granite Gneiss (table 1) range from 411 to 347 

m.y. (million years). For two samples, the 238U-206Pb 

ages and 235U-207Pb ages are concordant (agree) within 

the analytical uncertainty. Sample P-19 is concordant 

at 410 m.y., and sample P-5 is concordant at 348 m.y. 

The remaining 5 samples are discordant.

Rb-Sr whole-rock data for four samples of Falls Run 

are plotted on an isochron diagram in figure 2. All 

samples plot on the regression line within analytical

uncertainty and indicate an age of 408 ± m.y. (uncertainty stated is the 67 percent confidence level). The initial 87Sr/86Sr is 0.7070 ±0.0001.

The whole-rock isochron age of 408 ± m.y. is, within 

analytical uncertainty, the same as the oldest and concordant zircon age of 410 m.y. (P-19, table 1). This age 

is thought to represent the time of magmatic crystallization of the Falls Run. The younger ages of five of the zircon samples may represent partial resetting of the zircon clocks by a younger event or events. A concordant 

age for sample P-5 (table 1) is probably fortuitous 

because of the large analytical uncertainty of w*Pb-23l>lJ 

ages. Another sample from the same locality, P-5A, is 

discordant.

FALMOUTH INTRUSIVE SUITE

Rocks of the Falmouth Intrusive Suite can be divided 

into at least three groups on the basis of petrographic 

and chemical criteria. Figure 4 is a plot of rubidium 

against strontium for the Falmouth. Samples of fine to 

medium-grained muscovite-bearing granites having 

high rubidium content, low strontium content, and 

Rb/Sr greater than 0.5 are designated as group A. 

Samples of fine-grained biotite granodiorite to granite 

having high strontium content, low rubidium content, 

and Rb/Sr less than 0.2 are designated as group B. A 

single sample of intermediate texture and composition 

represents a group designated as C.

For group A rocks, three samples are plotted on the 

whole-rock isochron diagram of figure 3. The samples 

plot on the regression line within analytical uncertainty 

and indicate an age of 322+ m.y. The initial ratio is 

0.7088 + 0.0001. Zircon sample P-10 of group A is 

discordant and has a 238U-206Pb age of 258 m.y. (table 1). 

This zircon contains more uranium plus thorium than do 

the other zircons of the Falmouth Intrusive Suite. It 

may have been more metamict and, therefore, more 

susceptible to loss of radiogenic lead than were the other 

zircons at the time of any younger events or during prolonged heating due to burial.

For group B rocks, 238U-206Pb ages for zircons range 

from 313 to 302 m.y. and have a mean of 309 m.y. (table 

1, P-14, P-16, P-20, P-22). One sample (P-16) gives a 

concordant age of 309 m.y. The same four samples of 

group B show a range of whole-rock 87Rb/86Sr ratios (fig. 

3) that is too limited to provide a precise linear regression, but a 309-m.y. age is not in conflict with the data. 

Because the points are close to the intercept, the initial 

87gr/86gr ratio is determined with reasonable certainty 

as 0.7040 + 0.0007 if a 309-m.y. age is assigned.

A single whole-rock sample was analyzed from group 

C. It has an isotopic composition intermediate between 

those of the other two groups (fig. 3).
B8 CONTRIBUTIONS TO THE GEOLOGY OF THE VIRGINIA PIEDMONT

300

Z

o

LU 

O.

CO

Q.

Z

200

TOO

200 400 600 800 1000

Sr, IN PARTS PER MILLION

FIGURE 4.-Rb-Sr plot for the Falmouth Intrusive Suite, indicating the separation of these rocks into groups A, B, and C.

The ages of crystallization of the Falmouth Intrusive 

Suite are in the range 300-325 m.y. before present. The 

time of magmatic crystallization of group B rocks is probably best represented by the zircon ages of 309 ± m.y. 

(table 1, P-16). The time of magmatic crystallization of 

group A rocks is probably best represented by the 

whole-rock isochron age of 322 + 2 m.y. The younger zir-. 

con age for one sample (table 1, P-10) may reflect the influence of a later event of Permian or younger age or 

may indicate slow cooling and uplift following intrusion.

TIMING OF METAMORPHISM

The deformation that caused recumbent folding, 

described earlier, was accompanied by metamorphism 

that produced sillimanite and kyanite in some of the 

metasedimentary rocks within the area (Pavlides, 1976). 

This deformation and metamorphism occurred after intrusion of the Falls Run Granite Gneiss at about 410 

m.y. ago and before the intrusion of the oldest Falmouth 

rocks at about 322 m.y. ago.

COMPARISON WITH OTHER PIEDMONT 

TERRANES

Paleozoic ages in the Piedmont north and south of the 

Fredericksburg area are similar to the ages reported 

here. In North Carolina and South Carolina, granitic 

plutons in the age range 410-380 m.y. constitute one of 

the major episodes recognized by Fullagar (1971). Welldetermined isochrons like those for the Salisbury pluton, 

North Carolina (402 + 4 m.y., Fullagar and others, 1971), 

and Lowrys pluton, South Carolina (398±4 m.y., 

Fullagar, 1971), show them to be very similar in age to 

the Falls Run Granite Gneiss (408 ±5 m.y.) of this 

report. In Maryland, swarms of large pegmatite dikes 

were dated by Wetherill and others (1966) as 416 ±20 

m.y. Thus, on the basis of the time scale in use by the 

U.S. Geological Survey in 1980 there is an apparent continuity of igneous activity along the length of the Piedmont, albeit in different tectonic belts, in Silurian to 

Early Devonian time. Pavlides (1976, p. 16) had con-
PALEOZOIC GRANITIC ROCKS IN THE PIEDMONT NEAR FREDERICKSBURG, VA: GEOCHRONOLOGY B9

sidered the Falls Run Granite Gneiss (the Berea pluton 

of former usage) to be correlative with the Petersburg 

Granite of Virginia on the basis of lithologic similarity 

and possible age equivalence. The data summarized in 

this report for the Falls Run and the age indicated for 

the Petersburg Granite by Wright and others (1975), invalidate a correlation between these rocks.

The small plutons and dikes of the Falmouth Intrusive 

Suite are probably part of the extensive group of plutons 

325-265 m.y. old recognized to the south in Georgia and 

the Carolinas by Fullagar and Butler (1979) that possibly 

also includes the Petersburg Granite (Wright and 

others, 1975). To the north in Maryland, no plutons of 

this age are yet known, but Wetherill and others (1966) 

suggested that a thermal event may have occurred from 

325 to 275 m.y. ago on the basis of presumed reset Rb-Sr 

biotite ages and K-Ar ages in older rocks. Thus, a Pennsylvanian to Permian event or events is also suggested 

through much of the eastern Piedmont.

Determination of the extent and nature of events between about 410 and 325 m.y. is more subtle. We infer 

that the deformation that produced nappe structures in 

the Fredericksburg area began after about 410 m.y. and 

culminated before about 325 m.y. ago. To the south, 

Acadian deformation is evidenced in shearing, which is 

thought to have ended in the Charlotte belt by about 368 

m.y. ago (Butler and Fullagar, 1978). To the north, lamprophyre dikes were emplaced near Great Falls on the 

Potomac River about 360 m.y. ago (Reed and others, 

1970). In Maryland, small pegmatites intruded the 

Guilford quartz monzonite and Ellicott City granodiorite 

about 340 m.y. ago (Wetherill and others, 1966). Thus, 

tectonothermal events are reported throughout the 

Piedmont in the interval 370-340 m.y. ago, but the timing and nature of the activity were quite diverse.

CONCLUSIONS

The ages of two episodes of igneous intrusion and one 

deformation have been clarified in the Fredericksburg 

area by Rb-Sr and U-Pb age determinations. The Falls 

Run Granite Gneiss was intruded about 410 m.y. ago 

and then deformed to become part of a nappe before 

about 325 m.y. ago. The Falmouth Intrusive Suite was 

emplaced between 325 and 300 m.y. ago syntectonically 

and late tectonically during the Hercynian deformation. 

Both the 410-and the 325-300-m.y. intrusive events in 

the Fredericksburg area were part of similar granitic activity occurring throughout the Piedmont at those

times. The initial 87Sr/86Sr of group B of the Falmouth 

Intrusive Suite suggests a lower crust or mantle source. 

However, the initial 87Sr/86Sr of group A of the 

Falmouth Intrusive Suite is 0.709, similar to the 0.707 of 

the Falls Run, and suggests crustal involvement in the 

magma generation of these rocks.

REFERENCES CITED

Butler, J. R., and Fullagar, P. D., 1978, Petrochemical and 

geochronological studies of plutonic rocks in the southern Appalachians; III, Leucocratic adamellites of the Charlotte Belt near 

Salisbury, North Carolina: Geological Society of America Bulletin, 

v. 89, no. 3, p. 460-466.

Fullagar, P. D., 1971, Age and origin of plutonic intrusions in the Piedmont of the southeastern Appalachians: Geological Society of 

America Bulletin, v. 82, no. 10, p. 2845-2862.

Fullagar, P. D. and Butler, J. R., 1979, 325 to 265 m.y. old granitic 

plutons in the Piedmont of the southeastern Appalachians: 

American Journal of Science, v. 279, no. 2, p. 161-185.

Fullagar, P. D., Lemmon, R. E., and Ragland, P. C., 1971, 

Petrochemical and geochronological studies of plutonic rocks in the 

southern Appalachians; pt 1, The Salisbury pluton: Geological 

Society of America Bulletin, v. 82, no. 2, p. 409-415.

Pavlides, Louis, 1976, Piedmont geology of the Fredericksburg, 

Virginia, area and vicinity: Geological Society of America, 

Northeastern-Southeastern Section Meeting, Arlington, Va., 

Guidebook for field trips 1 and 4, 44 p.

 1980, Revised nomenclature and stratigraphic relationships of 

the Fredericksburg Complex and Quantico Formation of the 

Virginia Piedmont: U.S. Geological Survey Professional Paper 

1146, 29 p.

Reed, J. C., Jr., Marvin, R. F., and Mangum, J. H., 1970, K-Ar ages of 

lamprophyre dikes near Great Falls, Maryland-Virginia: U.S. 

Geological Survey Professional Paper 700-C, p. C145-C149.

Steiger, R. H. and Jager, E. 1977, Subcommission on geochronology: 

Convention on the use of decay constants in geo- and 

cosmochronology: Earth and Planetary Science Letters v. 36, no. 

3, p. 359-362.

Streckeisen, A. W., 1973, Plutonic rocks: Classification and 

nomenclature recommended by the IUGS subcommission on the 

systematics of igneous rocks: Geotimes, v. 18, no. 10, p. 26-30.

Tilton, G. R., 1960, Volume diffusion as a mechanism for discordant 

lead ages: Journal of Geophysical Research, v. 65, no. 9, p. 

2933-2945.

Wetherill, G. W., 1956, Discordant uranium-lead ages: American 

Geophysical Union Transactions, v. 37, p. 320-326.

Wetherill, G. W., Tilton, G. R., Davis, G. L., Hart, S. R. and Hopson, C. 

A., 1966, Age measurements in the Maryland Piedmont: Journal of 

Geophysical Research, v. 71, no. 8, p. 2139-2155.

Wright, J. E., Sinha, A. K. and Glover, Lynn, 1975, Age of zircons 

from the Petersburg Granite, Virginia; with comments on belts of 

plutons in the Piedmont: American Journal of Science, v. 275, no. 

7, p. 848-856.

York, Derek, 1969, Least squares fitting of a straight line with correlated errors: Earth and Planetary Science Letters, v. 5, no. 5, p. 

320-324.

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