Geological, geochemical, and geophysical studies of the Elberton batholith, eastern Georgia [1980]

Guidebook 19
GEOLOGICAL, GEOCHEMICAL, AND GEOPHYSICAL STUDIES OF THE ELBERTON BATHOLITH, EASTERN GEORGIA
A Guidebook to Accompany the 15th Annual Georgia Geological Society Field Trip
Edited by John C. Stormer, Jr. and James A. Whitney Department of Geology, University of Georgia Athens, Georgia 30602
1980
Georgia Department of Natural Resources Joe D. Tanner, Commissioner Environmental Protection Division J. Leonard Ledbetter, Director Georgia Geologic Survey William H. McLemore, State Geologist

For convenience in selecting our reports from your bookshelves, they are color-keyed across the spine by subject as follows.

Red

Valley and Ridge mapping and structural geology

Dk. Purple Piedmont and Blue Ridge mapping and

~ructural geology

Maroon

Coastal Plain mapping and stratigraphy

Lt. Green

Paleontology

Lt. Blue

Coastal Zone studies

.I

Dk. Green Dk. Blue

Geochemical and Geophysical studies Hydrology

Olive

Economic geology

Mining directory

Yellow

Environmental studies

Engineering studies

Dk. Orange Bibliographies and lists of publications

Brown

Petroleum and natural gas

Dk. Brown Collections of papers

Black

Field trip guidebooks

Guidebook 19
GEOLOGICAL, GEOCHEMICAL, AND GEOPHYSICAL STUDIES OF THE ELBERTON BATHOLITH, EASTERN GEORGIA
A Guidebook to Accompany the 15th Annual Georgia Geological Society Field Trip
Edited by John C. Stormer, Jr. and James A. Whitney Department of Geology, University of Georgia Athens, Georgia 30602
1980
Georgia Department of Natural Resources Joe D. Tanner, Commissioner Environmental Protection Division J. Leonard Ledbetter, Director Georgia Geologic Survey William H. McLemore, State Geologist

FIELD TRIP ADDENDUM AND ERRATA FOR THE GEORGIA GEOLOGICAL SOCIETY 1980 Guidebook

NOTE: All stops except 1,4,6,7, and 12 are on private commercial (quarry) property. Permission has been obtained for this trip, and must be obtained again should you want to visit these sites at a later date. Quarries can be dangerous BE CAREFUL.
Addendum .!2_ the Road Log First Day (start at mile 58.3 page 117)

Cumulative :t-fileage

Incremental Mileage

58.3

2.7

Intersection with paved road. Continue straight ahead

(rather than turn as printed in the guidebook).

59.7

1.4

Tum left (west) on paved road at intersection near

microwave tower.

60.1

0.4

Tum right (north) on GA Route 77

62.6

2.5

Turn right (east) on dirt road

62.7

0.1

Park near gate to pasture

STOP #4A THE GEORGIA GUIDESTONES

The Guidestones are five 21 ton granite monoliths 19 feet high engraved with humanistic messages or guides in several languages. The stones are rectangular slabs set on end. The central slab is oriented N-S with a hole drilled parallel to the earths axis through which Polaris can be viewed and a slot aligned with the rising and setting sun at summer and winter solstices. The other four with the messages engraved in English, Spanish, Russian, Chinese, Arabic, Hebret-1, Swahili, and Hindi are oriented radially NE-St.J and SE-NW. A sixth 12 ton square tabular stone caps the others and has a message in Sanskrit, Babylonian cuneiform, Egyptian hieroglyphics, and Classical Greek on the four sides.
These stones, quarried from the Pyramid Quarry and produced by the Elberton Granite Finishing Co., were ordered by a man using the alias "R. C. Christian". His identity and the identity of his backers remains a secret. The present configuration was completed and dedicated in March 1980. Apparently another outer ring of stones may be added at a later date. (references: The Athens Observer, v. 7, no. 35, p. 1 (August 28, 1980); and Lewis, B.: Mystery in Stone, Brown's Guide to Georgia, v. 8, no. 8, p. 59-63 (Aug. 1980).

62.8

0.1

Return to GA Route 77. Turn left (south) toward

Elberton.

68.6 Errata

5.8

Intersection with GA Route 368. Continue south on GA

Route 77 toward Elberton picking up the road log

as printed in the Guidebook at mile 62.4.

p. 63, 3rd line of authors: "Georgia Mason" should read "George Mason".

p. 64, Figure 1 caption: Add "from Ellwood, et al. (1980)"

p. 67, Figure 2 caption: Add "from Ellwood, et al. (1980)"

p. 78, References Cited: Add Ellwood,,B.B., Whitney, J.A., Wenner, D.B., Mose, D., and Amerigan, C. 1980, Age, paleomagnetism, and tectonic significance of the Elberton granite Northeast Georgia Piedmont: J. Geophys. Res. , in press.

Contents

Page PREFACE . - , . . iii

STRUCTURAL AND TECTONIC SETTING OF THE ELBERTON BATIIOLITK, EASTERN

GEORGIA, PIEDMONT. J. A. Whitney, D. E. Wells, and R. W. Rozen

1

PETROLOGY AND GEOCHEMISTRY OF 'l1lE E~ERTON GRANITE. J. C. Stormer, Jr., J. A. Wh.itney, and J. R. Kess ................................... , 10

OXYGEN ISOTOPE VARIATIONS WITHIN THE ELBERTON GRANITE PLUTON.
D. B. Wertner ........................... , . . . . . . . . . . . . . . . . . . . . . . . . . 31

U AND TH GEOCHE1>1ISTRY OF THE ELBERT~ PLUTON.

o. D. B. Wenner and J.

Spauld:I,.Ilg .................................... ~

THE U-PB AGE OF ZIRCONS FROM THE ELBERTON GRANITE, fiEDMONT OF GEORGIA.
C. R. Ross, II and M. E. Bickford , 52

GEOCHRONOLOGY AND COOLING HISTORY OF THE ELBERTON GRAi'iiTE.
J. A. Wh.itney, J. R. Hess, and D. Mose 63

MAGNETIZATION OF THE ELBERTON GRANITE.
B. B. Ellwood ..... , .............................. , . . . . . . . . .. . . . . . . 80

TECTONIC DEVELOPMENT OF THE ELBERTON AREA: INCLUDING AN lN'l'E:RPRETATION OF THE COCORP REFLECTION PROFILE. J. A. Whitney, B. B. Ellwood, J. C.
Stormer, Jr., and D. B. Wenner . 98

ROAD LOG FOR THE GEORGIA GEOLOGICAL SOCIETY FIELD TRIP. J. A. Wh.itney
and J. C. Stormer, Jr. . ........... , ... , , ...... , ............ ~ .... . 108

ii

. '
I'
1-

... r :- -

,

Preface

About 1975, several faculty members at the University

of Georgia became interested in the Elberton area as part

of a more extensive study of late orogenic plutonism in the

southern Appalachians. The Elberton batholith is one of the

largest plutons in Georgia, and yet as of 1975 there was very

little known about its nature. Early studies proved it to be

unique for it was extremely homogeneous and of fine grain size

throughout its extent. It also became obvious that earlier

mapping had included many unrelated lithologies within the unit

mapped as Elberton granite. We therefore started a mapping

program involving students with ~1asters thesis projects to

systematically explore the granite and its environs. Since

that time, theses by R. W. Rozen, J. R. Hess, J. Davidson, G.

Davis, and G. E. Wells have greatly expanded our knowledge of

the geology and geochemistry. In addition, other fields of

research include stable isotope geochemistry, geochronology,

paleomagnetism, and other geophysical studies have been

incorporated. Portions of this work have been funded by

several ~ational Science Foundation grants

including

DES75-14217 (Whitney and Stormer), EAR-7818127 (Henner,

Whitney, and Stormer), and EAR-791991 (Ellwood and Nhitney).

Now, with the publication of a deep seismic reflection profile across the Elberton area by the Consortium for Continental Reflection Profiling (COCORP) , the understanding of the geoloqic history of the Elberton area becomes crucial to the interpretation of southern Appalachian tectonic development. For this reason, we have attempted to bring together all current studies of this area in this volume. Although much of the material presented here has, or will soon be, published as separate papers scattered in several national or international journals. This volume is intended as a single source of recent work to aid in the synthesis of an overall picture of the geological history of the Elberton area, as well as providing background data for the Georgia Geological Society Field Trip.

We want to acknowledge the generous provision of facilities and personnel by the Department of Geology, University of Georgia for the preparation of this volume. Helen Pilcher persevered dheerf11lly in the difficult job of typing this typescript. F. D. Eckelmann, Steve Nagel, Karen Obenshain, Edna Parham and Sylvia Young all made very significant contributions to the successful production of this volume.

Jay Stormer and Jim Whitney Athens, GA August 4, 1980

iii

Structural and Tectonic Setting of the Elberton
Batholith, Eastern Georgia Piedmont
James A. Whitney, D. Edward Wells, and Robert w. Rozen,
Department of Geology, University of Georgia, Athens, Georgia 30602
ABSTRACT
The Elberton Batholith intrudes the eastern flank of the Inner Piedmont province, northeastern Georgia. The surrounding rocks were first deformed between 400 and 480 m.y. ago, with peak metamorphic conditions following the development of northeast trending isoclinal and recumbent folds. Peak metamorphic conoitions were of sillimanite grade with extensive migmatization occurring in both the Inner Piedmont Core and Inner Piedmont Flank. A second regional tectonic fabric is developed in a northwesterly direction following peak metamorphism. This fabric appears as crenulation of biotite and a mineral alignment in meta-intrusive lithologies which were intruded after the first (Dl) deformation. Subsequently, a localized ductile fabric was developed along certain zones of shear which border the Inner Piedmont. The Inner Piedmont lithologies were apparently still at upper greenschist to lower amphibolite grade at the time of this ductile deformation. The Elberton. granite was intruded about 320 to 350 m.y. ago following the development of these ductile shear zones. Subsequently, the Inner Piedmont was uplifted along near vertical normal faults, some of which followed zones of earlier ductile fabric development. Since the intrusion of the Elberton, the Inner Piedmont Flank has been uplifted by about 12 to 15 km or so. The brittle faults along which this uplift occurred are silicified in-certain areas, with massive quartz and occasionally zeolites being deposited in the brecciated zone.
INTRODUCTION
The southern Appalachian orogen has long been subdivided into a series of geomorphic provinces, one of which is the Piedmont, a crystalline terrain bounded by the regionally extensive Brevard Zone on the northwest and younger Coastal Plain sediments on the southeast. The Piedmont in turn has been divided into a series of belts which are characterized by differing structure, lithologies and metamorphic grade (see for example, Crickmay, 1952~ King, 1955~ Hatcher, 1972). In Georgia and South Carolina, these include the Chauga, Inner
1

ASHLAND-WEDOWEE BELT

MOBILIZED INNER PIEDMONT

//,_ EX
I
// /
/ ///

1\J

,,. ,..-,,

,----J I

/

;;,/

,..,. CHARLOTTE BELT r""/~ ~

.,.

Figure 1. Generalized geologic map o the Georgia Piedmont showing location of the Elberton Batholith.
Metamorphic Belts after Hatcher, 1972; Crickrna~ 1952; and King, 1955.

Piedmont, Kings Mountain, Charlotte, Pine Mountain, Carolina Slate, Kiokee, and Belair belts (Fig. 1).
These belts have been intruded by a series of late orogenic plutons which represent a diverse suite of granitic lithologies. In general, they show systematic correlations in age, geochemistry, isotopic composition, mineralogy and texture with their geographic location. During the field trip, plutonic and metamorphic rocks of the Elberton area within the Inner Piedmont and Charlotte belts will be visited.
INNER PIEDMONT
The Inner Piedmont of the Elberton area is composed of polydeforrned metasedimentary and metavolcanic rocks, many highly aluminous in composition. The age of these units is not known, but is generally considered to be late Precambrian to early Paleozoic. The age of peak metamorphism is generally thought to be younger than for the Blue Ridge, with ages of 380 (Dallmeyer, 1978) and 380 to 420 m.y. (Fullagar, 1971) being suggested. The core of the Inner Piedmont contains a . high percentage of rnigmatitic gneisses, with the flanks containing somewhat less.
According to Griffin (1978), the Inner Piedmont Flank (IPF) is a premetamorphic nappe which overthrust the Inner Piedmont Core (IPC). It is distinguished from the core by a greater abundance of amphibolites, less common sillimanite schists, and steeper dips of foliation. The flanks appear to be lower in grade on the northwest, and perhaps slightly lower on the southeast. The flanks contain steeply dipping isoclinal folds and foliations, while the core is more recumbent. The first deformation (Dl) is characterized by very plastic defonnational features, often accompanying anatexis. Peak metamorphism apparently occurred after the developmennt of Dl structures due to the growth of peak metamorphic minerals (for example sillimanite) across the early fabric. Later, northwest oriented structural features, part of a second (D2) deformation, deform peak metamorphic minerals. Granitic plutonism, now represented by granites and orthogneisses, apparently accompanied peak metamorphism and continued after metamorphism and D2 events.
Silicic plutonism probably continued with minor breaks from the first deformational cycle (420 to 450 m.y.?) to about 320 m.y. ago. All metamorphic fabrics had apparently formed and the area had begun cooling by the time the Elberton granite was intruded. This limits the age of regional deformation at 350 m.y. or before. Uplift and cooling did not end, however, until at least about 220 rn.y. ago, as there are argon retention ages as young as 240 m.y. reported from biotite from the eastern Inner Piedmont (Hess, 1979).
3

Uplift was probably complete by the Triassic, when 200 m.y. old diabase dikes intruded and were quenched against existing
units.

The surface we see now has risen considerably during the late Paleozoic since late orogenic granites (the Elberton and Stone Mountain plutons) were apparently intruded at depths of 12 to 15 km. The geothermal gradient was apparently much higher than for the Blue Ridge. -Calculations based on the depth of intrusion of the Elberton and temperature estimates from extrapolation of biotite and hornblende argon retention ages and metamorphic grade in surrounding ductile shear zones suggest gradients of at least 40 degrees C/km.

The Inner Piedmont probably started as a series of

aluminous sediments composed of a mixture of pyroclastic

debris, volcanic flows, and terriginous sediments deposited in

a basin near continental sources. The eastern portions may

have been more distal, as a more volcanic and pyroclastic

protolith is suggested. This section of crust was probably

deformed in an environment of rising geothermal gradients with

temperature continuing to rise 50 to 100 degrees C after Dl

deformation.

It remained hot with high heat flow and

continuing plutonism until uplift 350 to 2n0 m.y. ago.

The Elberton granite is the only major pluton studied in detail from the southeastern Inner Piedmont Flank. The Elberton crustal block, which contains both IPF and IPC units, is fault bounded on the northeast by the Hartwell extension of
the Towaliga fault zone and on the southeast by the Middleton-Lowndesville shear zone (Griffin, 19787 Rozen, 1978: Davis, 1980). The Hartwell extension shows no cataclastic fabric in this area, and although it can be traced on aeromagnetic maps, its exact location is hard to pinpoint. It appears to be a brittle fault, prohahly developed late in the tectonic history, during uplift of the Inner Piedmont. At several points along its extent it has been silicified with massive quartz deposited in a brecciated matrix.

The Middleton-Lowndesville zone has a significant (up to 1/2 km wide) zone of cataclasis and separates felsic gneisses of the IPF from more mafic lithologies of the Charlotte or Slate belts (Figure 1). It appears to have a complex history with at least two stages of movement (Rozen, 19787 Rozen and Whitney, 1978~ Davis, 1980). The first stage of movement is associated with the ductile fabric development and appears to be a reverse movement with the Charlotte and Slate belts overriding the Inner Piedmont. The second stage is a brittle deformation in which the Inner Piedmont rose along steeply dipping faults relative to the Charlotte and Slate belts. The ductile fabric development appears to predate the Elberton granite since no cataclastic fabric is seen within the granite even near the contact. Also aplites similar to those related to the Elberton cut the ductile fabric, and to the

4

south near Buckhead a granite identical to the Elberton cuts the ductile fabric as well. The brittle movement probably postdates the granite since the Elberton is never found to the east of the zone, and the same aplites are offset by brittle movement.
The depth of emplacement of the Elberton is not as well known as for the Stone Mountain granite, but it appears to be deep {13 to 15 km) and probably cooled slowly, based on K-Ar vs. Rb-Sr whole-rock dating {Hess, 19791 Whitney et al., this volumeJ Ellwood et al., 1930). The body follows regional structural trends, but is locally highly discordant with dikes and apophyses common. Throughout the northern part of the body, the contact is shallowly dipping and it would appear that just the top of the body is exposed. It is thought that the magma intruded upwards until it became vapor saturated and then froze in reponse to decreasing confining pressure. This model explains the homogeneous fine grain size of the body. The overall shape of the body is probably a laccolith, somewhat broader than shown at the surface today, with a depth of not more than 4 km or so.
KINGS MOUNTAIN BELT
The Kings Mountain belt pinches out at about the Georgia-South Carolina line and continues only as a zone of cataclasis, the Middleton- Lowndesville fault zone (Griffin 1978; Rozen, 1978; Rozen and Whitney, 1978; Davis, 1980). Further north, the Kings Mountain belt is formed by a series of metasedimentary units with minor amounts of carbonate and volcaniclastic material. It is of fairly low grade, greenschist to lower amphibolite, and is folded into a tight syncline. Its contacts with the Inner Piedmont and Charlotte belt are often poorly defined. Sometimes the boundary shown on geologic maps is only a rise in metamorphic grade, while in other areas it is a probable zone of faulting. Age of metamorphism and deformation are not known but structural style and metamorphic conditions are similar to those for the Slate belt to the east.
CHARLOTTE BELT
The Charlotte belt is composed of a series of mafic and granitic gneisses, originating probably as a series of volcanic, volcaniclastic, and volcanic sedimentary units. Deformation is severe, with most of the fabrics seen in the Inner Piedmont visible in the Charlotte belt. Age of peak metamorphism is not well known, but is probably similar to that of the Inner Piedmont. Maximum metamorphic conditions are in the upper amphibolite facies with some migmatization occuring. Thermal gradients were high, similar to those in the Inner Piedmont. Plutonism began as long ago as 550 m.y. in this
5

belt, and continued sporadically throughout the Paleozic. The late orogenic granites 1 which postdate all deformation except near Columbia, South Car.olina, are about 300 to 270 m.y. old and represent the last phase of granitic plutonism. Gabbroic intrusions are also common within the Charlotte belt, unlike the Inner Piedmont.
Within the area studied, the area indicated as Charlotte belt is dominantly a metagabbro complex composed of metagabbro, cumulate pyroxenites and norites, and related tonalites. The only vestiges of metamorphic country rock which may be used to name the belt are found in the southcentral portion of the Elberton East quadrangle. Here, the country rock appears to be a polydeformed mica schist, similar to that fo und in the upper part of the Charlotte belt in nearby South Carolina. This area is therefore termed Charlotte belt, although many of the gabbroic rocks which compose the majority of the terrain may be correlative with the Slate belt volcanism.
REGIONAL GEOPHYSICS Gravity
The regional gravity map of the southern Appalachians (Long, 1979) is dominated by the Appalachian gravity gradient and the change in texture of the gravity pattern on either side of this break (Figure 2). In the Carolinas, the Appalachian gravity gradient follows the edge of the Blue Ridge, and is partially caused by isostatic adjustment of the high altitude terrain. However, in Georgia, the gradient bifurcates with only one part following the elevation of the high Blue Ridge. The major part, a gradient from -40 to -60 mgal. to the northwest to ab0ut 0 mgal. to the southeast, cuts right across the Piedmont province. This Piedmont gravity gradient is totally unrelated to any topographic change. Long (1978 ) has modeled the regional gravity pattern as a change in the density of the lower crust. Models in which only the thickness of the crust changes seem to require too large a step in lower crustal thickness. Long suggests that this break represents a change in the lower crust from a granitic, probably Grenville-age, terrain to a More mafic, probably younger than Grenville, basement. The Piedmont gravity gradient follows the Hiddleton-Lowndesville fault zone until it reaches the Pine r.1.ountain helt, then swings south and follows the Goat Rock fault zone into Alabama. The Inner Piedmont appears to be over compensated with a substantial negative free-air anomaly present.
Regional Magnetic. Patterns
The same boundary s~en, in the gravity map is apparent from airborne magnetic compilations. The Inner Piedmont
6

76
7,.
rr
'-...!

0

100

2.
Bouguer gravity map of the southeast after Long, Note that the regional Piedmont gravity gradient, by the -10 mgal contour, follows the eastern edge of the Inner Piedmont from the Carolinas into central Georgia. There, it bends to the south and then southwest following the southern boundary of the Pine Mountains belt. Note also the area of positive anomalies near Columbia, S.C. which Long has interpreted as a buried fragment of Grenville Cru'st.

is characterized by a very flat, featureless aeromagnetic pattern, with only linarnents corresponding to fault zones showing much magnetic topography. The Charlotte and Slate belts however are characterized by short wavelength, high amplitude anomalies corresponding to highly magnetic mafic units. The regional field is also high which Zeitz (1979) has interpreted as a more mafic lower crust comming nearer the surface and portions of it cooling below its ~urrie point.
Paleomagnetic Data
Paleomagnetic studies of the late orogenic granites, mainly the Elberton pluton (Ellwood and Whitney, 1980: Ellwood et. al , 1980) suggest that little tectonic rotation has occurred since their intrusion. Vertical tectonics seems to be the main form of uplift dating from about 350 m.y. ago within the center of the orogen. This suggestion of little tectonic rotation is in contrast with thrusting accompanying thin-skin tectonics in the western part of the orogen and metamorphism and new fabric development along zones of shear to the east near Columbia, South Carolina. The amount of vertical movement within the., Inner Piedmont was extensive, since the e.astern part was uplifted at least 12 km be.tween 350 and 200 m.y. ago. Further, during the same period this area was rapidly cooling as demonstrated by argon retention ages (Dallmeyer 1980: Hess, 1979). Further, this period (350 to 200 m.y. ) was a time of extensive tectonism, but the tectonic style varied in terms of rotation in various parts of the orogen.
CONCLUSION
The Elberton area, therefore, seems to mark an important transition from areas of low density, low magnetite content lower crust, to ones o f higher density, more magnetic basement. Structurally, the area has undergone several periods of deformation. M.ost of the fabric development came before the intrusion of the Elberton granite, with vertical uplift dominating in the late Paleozoic. The history and development of this terrain may be the key to understanding the development of the southern Appalachian orogen. Especially in light of the recent COCORP seismic reflection profile done across this area.
8

REFERENCES CITED
Crickmay, G. w., 1952, Geology of the crystalline rocks of
Georgia. Georgia Dept. Mines, Mineralogy, and Geology Bull., v. 58, p. 1-59.
Davis, Gary J., 1980, The Southwestern Extension of the Middleton-Loudesville Cataclastic zone in the Greensboro,
Georgia area and its regional implications. M. s. Thesis,
Univ. of Georgia, Athens.
Dallmeyer, R. D., 1978, Ar/ Ar incremental-release ages of hornblende and biotite across the Georgia Inner Piedmont: their bearing on late Paleozoic Early Mesozoic tectonothermal history. Am. Jour. Sci., v. 278, p. 124-149.
Fullagar, P. D., 1971, Age and origin of plutonic intrusions in the Piedmont of the southeastern Appalachians. Geol. Soc. Amer. Dull., v. 82, p. 2845-2868.
Griffin, V. s., 1978, Detailed analysis of tectonic levels
in the Appalachian Piedmont. Geol. Rundschau, v. 67, p. 180-201.
Hatcher, R. D., Jr. , 1972, Developmental model for the Southern Appalachians. Geol. Soc. Am. Bull., v. 83, p. 2735-2760.
Hess, J. R., 1979, Geochemistry of the Elberton Granite and the Geology of the Elberton West Quadrangle, Georgia. M.S. thesis, Univ. of Georgia, 193 p.
King, P. B., 1955, A geologic section across the southern Appalachians: An outline of the geology in the segment in Tennessee, North Carolina, and South Carolina. in Russell, R. J., ed., Guides to southeastern geology, Geol. Soc. Amer., Boulder, Co. , p. 332-373.
Long, L. T., 1979, The Carolina Slate Belt- Evidence of a continental rift zone. Geology, v. 7, p. 180-184.
Rozen, R~ w., 1978, The Geology of the Elberton East
quadrangle, Georgia. M.s. thesis, Univ. of Georgia.
and J. A. Whitney, 1978, The Middleton cataclastic zone: A previously unrecognized fault zone within the eastern Georgia Piedmont (abs.). Geol. Soc. Amer. Abs. with program, v. 10, p. 196.
Zeitz, I., and Hatcher, R. D., Jr., 1979, Interpretation of regional aeromagnetic and gravity data from the southeastern United States. Part !--Crustal evaluation Geol. Soc. Amer. Abs. with program, v. 11, p. 219.
9

Petrology and Geochemistry of the,

Elberton Granite
J. c. Stormer, Jr., J. A. Whitney, and J. R. Hess
Department of Geology, University of Georgia, Athens, Georgia 30602

ABSTRACT

The Elberton granite is a large (500 sq. km) pluton of

fine

grained, homogeneous

granite composed of

approximately equal portions of alkali feldspar, plagioclase,

and quartz with 4-7% biotite. Ilmenite-hematite solid

solutions, magnetite, sphene, allanite, and zircon are common

accessories. Rare muscovite may be a late, primary mineral in

some localities. There is no metamorphic fabric nor other

evidence of metamorphism. The most striking geochemical

feature is the extreme homogeneity of major elements. The

trace elements, in contrast, are highly variable and show a

clear regional pattern. This pattern seems to be related to

variation in the protolith or the partial melting event. The

Elberton magma probably originiated by anatexis of

meta-volcanic rocks at depths around 18-20 km, rose until it

reached vapor saturation and its solidus at about 13-15 km

where it formed a laterally extensive, but relatively thin,

pluton. This pressure "quench" effect produced spontaneous

nucleation throughout the body, accounting for the fine

grained texture and lack of fractionation effects.

INTRODUCTION
The Elberton Granite is a fine to medium grained light grey (occasinally light pink) granite with typical hypidiomorphic texture. There is a very poorly defined foliation observed mainly in the alignment of biotite. This foliation is variable in direction and difficult to define except near the borders of the pluton where, in our experience, it always parallels the contact and crosscuts all regional metamorphic fabric elements. In a few places the foliation is very complexly contorted. This foliation is therefore interpreted as being a flow foliation related to intrusion. There is no evidence for metamorphic defamation or the development of a metamorphic fabric. This unmetamorphosed granite pluton intrudes ahigh grade metamorphic terrain which includes quartzofeldspathic gneisses, some of which are metagranites similar to the Elberton itself. Previous work
10

(Ramspott, 1964; Geologic Map of Georgia, 1976) of a preliminary or reconnaissance nature has included some of these
gneisses within the Elberton Granite.

The most striking feature of the Elberton Pluton is its extreme homogeneity in both major element chemistry and its major mineralogy. Over the entire 500 sq. km we have not observed any major variation other than slight and gradual variations in grain size. No internal contacts are noted. The entire pluton s eems to be the product of a single intrusive event.

Modal analyses as published by Rozen (1978), Ramspott (1964), and Chayes (1951) shm'l that it is composed of about 30 % quartz, 30-35% oligoclase, and 30-35% microcline. Biotite is the only other significant primary mineral with abundances from about 4-7 %. Sphene, zircon, allanite, Magnetite, and ilmenite-hematite are common accessories. Muscovite and chlorite are locally important, but appear to be the product of secondary, suhsolidus alteration. A few
occurrences of muscovite in relatively large euhedral crystals may represent primary, late magmatic crystallization.

Pegmatites and aplites occur as both irregular

magmatic segregations and as late stage crosscutting

fracture fillings. The mineralogy of these is generally

sinilar to the ~ranite. However, at one location the mineral

polycrase, (Ce, u, Th)
(S tormer & Merzbacher,

(nb, work

Ti, in

pTroa)g2reos6s, )

occurs

in

pegmatites

MINERALOGY

Plagioclase

As cledetermined by microprobe (Hess,

1979, Table 2) and x-ray diffraction (Ramspott, 1968) the

plagioclase ranges although there is a

sfrloimghAtn1n6o-rAmna2l7

It is zonation

essentially unzoned, in some grains.

Microcline

Microprobe analyses show the microcline

to be ahout from considera

or ble

95su

This bsolidu

s

composition, exsolution of

of course, results albite component as

perthite and interstitial material during the slow cooling

accompanying regional uplift.

Biotite

Biotite is the major mafic phase. It

varies sl~ghtly in size from about O.Smm to 3mm, and has a

yellow green to dark olive green pleochroism. Some small

zircon inclusions can he found hut generally the biotites are

free of inclusions. A typical analyses is given in Table 1.

Huscovite - f\1uscovite is not a major mineral in the Elberton Granite and is rarely identifiable in hand speci!:len. HmV"ever, due to the petrological significance of this Mineral Hess (1979) made a relatively detailed study of

11

Tible 1. BioHte Apelysis From Semple EH-14 Of HeSS (19J9)

5.57} Si02

36.1

Si

8.0

Ti02

2.5

Al (IV) 2.43

= Mg/~oct. .33

Al2o3
FeO* MnO MgO CaO K20 H20*

16.2 23.5 0.01
8.1
0~03
9.8 3.7

Al (VI) 0.51

Ti

0.29

Fe

3.03

Mg

1.87

Ca

0.01

5.70

K

1.92

Fe/Eoct. = .53

Total

99.9

FeO* = total Fe as FeO. H20* "" H20 est. from stoichiometry

Charge balance indicates that the iron is probably largely Fe:2
Mg/Mg + Fe is about 0.4.

12

......., ~... :,. ,,

.,. ";t ...J" '. r

~

INNER PIEDMONT

Elberton



ELBERTON
BATHOLITH. ,

G) A
z

z



(I)

<J>z

A
sOz

<J>z

0

0

2

I

I

KM

.....---

/

I
(
\



0

""--

Figure 1. Simplified geologic map of the Elberton West Quadrangle, showing accessory mineral distributions. Symbols: o, no appreciable muscovite or calcite; G, muscovite without calcite; e, muscovite with calcite; A, substantial allanite present; Z, substantial zircon present. Dashed lines indicate areas of substantial chloritization.
13

the distribution of muscovite and an evaluation of its texture and association. It generally occurs in one of three modes: (1) very small interstitial grains in trace amounts~ (2) larger, poorly crystallized grains associated with calcite~ and (3) large (l-2mrn) euhedral grains with quartz inclusions. (1) and (2) are interpreted as being most likely secondary subsolidus phenomena, however (3) probably represents primary magmatic muscovite. The areal distribution of these types in the Elberton West Quadrangle is shown in Figure 1. The
=essentially metalurninous character of the Elberton Granite (Al
Na + K + 1/2 Ca) controls the relative scarcity of this mineral, but local variations in the fugacity of water probably control its presence or absence in a specific location.

Zircon, Sphene, and Allanite These are locally important accessories and the1r d1stribution appears to be systematic at least within the Elberton West Quadrangle as shown in Figure 1. The allanite occurs as small (1 mrn) euhedral crystals either metamict or complexly altered {Silver & Gruenenfelder, 1957). Typically it is zoned with an epidote rim. Sphene is found as small dispersed euhedral crystals up
to about lrnrn. If the experimental zircon saturation data of Watson (1979) can be applied to the Elberton magma it would indicate that the Elberton magma would have been saturated (140 ppm) to somewhat over saturated {280 ppm) and that zircon crystals must have been present in the magma when it was emplaced. However, the zircons are small, clear, euhedral prismatic crystals with no zoning or overgrowth apparent.

Fe-Ti Oxide Minerals

Small amounts of 0.2 to l.Omrn

grainsor an ilmen1te-r1ch solid solution, a hematite-rich

solid solution, and magnetite are all found. Variation in the

abundance of magnetite appears to be responsible for much of

the variation in magnetic susceptability {Ellwood, et al.,

1980). The ilmenite and hematite solid solutions are now seen

as composite exsolved grains with lamellae of nearly pure

hematite and ilmenite. The original bulk composition of the

high-temperature, coexisting ilmenite-hematite solutions may be

approximately determined by estimating the proportions of the

ilmenite and hematite lamellae. Preliminary analyses indicate

relatively high oxygen fugacities (1 to 2 log units below the

hematite - magnetite buffer) with temperatures between 650 and
7oo 0 c.

A very fine-grained (lprn) opaque phase is probably magnetite and gives the rock its blue to grey color. In some locations on the southeastern side of the pluton a pink variety of the granite occurs, which is identical to the grey except that the fine-grained oxide is hematite. Both fine-grained iron oxides are probably a result of subsolidus exsolution of Fe from the feldspars. The hematite could be produced by exsolution in regions with a higher fugacity of oxygen at the time of exsolution, or, as discussed by Ellwood, et al. (1980) due to post-exsolution oxidation of the fine

14

grained magnetite in volatile rich zones of tensional fracturing during the latest, most viscous movement of the emplacement.

MAJOR ELEMENT CHErUSTRY

The striking homogenP-ity of the Elberton Granite is

well displayed in Figure 2. Hess (1979) analyzed 15

samples from a single quarry complex, an area roughly 100

x SOm. The variation in the analyses of these samples

(standard deviation 6) can be used as a Measure of the sample

to sample variation, at the outcrop scale as well as sampling

and analytical uncertainty. Taking the analyses for 28 other

samples from quarries spread throughout the Elberton l'J'est

Quadrangle we find that even the extreme high to low ranges for

oxides, limit

Si0 of

23 ,

Al203, CaO, MgO, standard deviat

i oKn2 0s .,

and P20s fall within a None or the analyses are

significantly different from the average at a 95% confidence

level. FeO, TiO'- and Na,o show slightly greater ranges but are

close to the J ~ limi~ and probably do not represent a

significant variation. This may be contrasted with extreme

variability in Sr and Ba as seen in Fig. 2 and discussed later.

This general pattern of homogeneity in major elements has been

confirmed by subsequent analyses of samples from other parts of

the pluton.

The composition of the Elberton magma, then, is well
represented by the average of the analyses of Hess (1979) as shown in Table 2. This analysis compares very closely with analyses of "average granite" in compilations such as Nockolds (1954) and LeMaitre (1976) and with the average post-metamorphic granite of the southeastern Piedmont (Butler and Ragland, 1969). There is, on the average, only a small amount of normative corundum, and the normative quartz and feldspar components are in reasonable agreement with the mode.

TRACE ELEMENT CHEMISTRY
In contrast with the extreme homogeneity of the major elements, the trace elements, especially Sr, Rb, Ba and Zn show variations between sample localities that are far greater than the variation in a single quarry. This very significant variation shows a clear regional pattern as shown in Figure 3. The pattern is very similar for all of these elements and parallels the pattern of variation seen in oxygen isotopes (Whitney & Wenner, 1980) and the direction of anisotropy of magnetic susceptability (Ellwood, et al. 1980). The pattern does not correspond with the outcrop pattern of the granite, nor does it appear to closely parallel the northeast trending regional metamorphic fabric.
The origin of the trace element variation is curious

15

Si02 71.5
Al2~ 15.1
FeO 1.80 MgO 0.58 CoO 1.65 No~ 4.0 K20 4.82 P205 0.10 Ti02 0.38
Sr 211 ppm

1%

I

I

70.0

73.6

c....,,...:-;.;.~4(:f({(f(1(<1ff&..t

14.5 16.0
~r

1.~.2

0.33 0.80
1.25 1.96
1:.:- :<-;1
3.6 4.6
~
4.10 5.24
f ( ( .,:J
0.05.0.14

0.25,0.50

145

410

W.U/H&/~4

100ppm
Figure 2. Major and trace element variations in the Elberton Granite. Symbol and average analysis given in the column to the left. The total range of analyses is indicated by the diagonally ruled bar (the center line showing
the average value). The lower, shaded bar indicates 3" as determined by
multiple measurements in a single quarry.
16

Table 2. Analysis and C.I.P.W. Norm of the Elberton Granite

Average of 29 Analyses

Range of 29 analyses

Weight Percent

Si02 Ti02 Al 2o3
Fe2o3*
FeO*

71.50 0.38
15.06 0.79 1.09

MnO

0.03

MgO

0.58

CaO
Na2o
KzO P2o5 L. 0. I.

1.65 3.98 4.82 0.10 0.41

C.I.P.W. Norm

qz

25.43

co

0.54

or

28.48

ab

33.68

an

7.53

en

1.44

fs

0.78

mt

1.15

i1

0.72

ap

0.24

Trace elements (ppm)

Nb

14 - 31

Zr 143 - 287

y

13 - 24

Sr 145 - 410

Rb 165 - 284

Th

31 - 70

Zn

38 - 76

Mn 137 - 310

Ba 505 - 1117

La

29 - 144

Ti 1290 - 2720

Total 100.30

*Calculated from total Fe as FeO = 1.80 assuming Fe+3 /Fe+3 +Fe+2 .40
corresponding to Average Granite of Lemaitre (1976)

17

Rb/Sr

Ba

0

Mn

Zn

N

0

5

10

0

Kilometers

@

Figure 3.

Regional Variations In Trace Elements in the Elberton Granite. Solid line indicates outcrop area, solid dots indicate quarries for which data was obtai.ned, dashed lines contours of trace element concentration (ppm).

18

because it is not related to any variation in major element chemistry or mineralogy, nor to evidence of wall rock or xenolithic contamination. The most consistent pattern is given by Rb, Sr and Ba. Fractionation of the observable parts of the pluton must involve the major minerals in approximately their modal proportions, since there is no v~riation in the mineralogy or major element chemistry. Using these modal proportions to calculate solid/melt distribution coefficients
(Hanson 1978, Arth 1976) gives: DRb = 0.4: Dsr = 2.7; and DBa =
2.4. Applying the Rayleigh equation for crystal fractionation (see Cox, et al. 1979,Fiqure 14.1) shows that strontium and barium shoold be rlepleted while rubidium will be relatively quickly enriched. Depending upon details of the calculation
= the observed enrichment or depletion factors (Rb = 1.7, Sr
0.45, Ba = 0.50) are in general agreement with this model.
However, the percentge of crystallizati~n would. haveto be high (40-60%) and the agreeMent between Rb and Sr or Ba is not close. 'fhe distribution of Sr bet,.,.een plagioclase and granitic
melt is not at all well known (K = 1.5 to about 30?; Hanson,
1978: Irving, 1978) and could aff~ct our model (using K = 4.4).
Perhaps more significantly, the single rare earth pattern we have for the Elberton Granite, Figure 4, shows little or no europium anomaly which precludes feldspar as a significant part of the solid residue in any fractionation process.

An especially interesting feature of the trace element data presented hy Hess (1979) is the fact that the Sr/r.a ratio is quite consistent at about 0.25 to 0.35. It is oifficult to model a feldspar-free residual mineral assemblage that will concentrate Rb by nearly a factor of 2 while depleting Sr and Da equally by about 0.5. A further constraint is the apparent honogeneity of C7sr;86sr at an initial ratio of
0.7037 giving a very tiqht isochron of 350 m.y- 11 (Ellwood,et al., 1980) approxiMately concordant with a 320 M.y - 20 zircon U-Pb date. (Hoss and Bickford, 1980).

A possibility which must be considered is the

fractionation of trace elements into an evolving vapor

phase.

This probl~l is mathematically irlentical to

fractionation into a crystaline phase. Unfortunately though,

there is only v~ry limited data on the vapor/liquid or

vapor/solid distribution coefficients. These are likely to be

influenced very strongly by not only P, T, but also by

complexing ,.,.i th ligands such .as chloride, or fluoride, or

carbonate (Flynn and Burnham, 1978). Beswick (1973) found that
for Rb D(v/1) = 1 for aqueous vapor in equilibrium with

granitic liquids, ,.,.hereas the data discussed by Shaw (1978) and

Flynn and Burnhar.1 (1978) indicate that REE are excluded from

the vapor and D(v/1) are generally 0.1 or less. If Sr and Ba

are similarly excluded from the vapor we might guess that the

Rb/Ba and Rb/Sr patterns could be produced by vapor

fractionation. It seems unlikely though that the nearly

constant Ba/Sr ratio would be produced, and very large

quantities of water, 25% of the magma mass, would have to be

19

1000
ci
~100
:I:
u
<' z J
u 0 10

Elberton
-0- Equigranular Granite ~-Aplite Dike
~---
--,;-------~

YbLu

Figure 4.

Rare Earth Element Paterns for the Elberton Granite and an Aplite Dike. Note the lack of a significant negative europium anomally for the granite, but the pronounced anomaly for the aplite, a fractionated product of the granite.

20

I
4 M}
I
I
ol-~-o~
-~-z,
? i~.v
I 2 '2

10

700

Wt.

Figure 5. P-T and P-HzO content projections for a synthetic granite composition

similar in composition to the Elberton (Rl, Fig. 11, Whitney, 1972. See also

Whitney, 1975). This summarizes the phase assemblage data and indicates the

limits of conditions under which certain assemblages can exist. Invariant point

A indicates the lowest pressure at which Alkali feldspar can be a liquidus

phase. The dash-dot curve is the upper temperature limit of muscovite stability

at PHzO = P total from Kerrick (1972) according to the indicated breakdown

reactions. Point M and the lower curve migrate to higher pressures along the
upper part of the curve at lower partial pressures of H2o. The shaded area
indicates the possible range of conditions for the existence of primary

muscovite in the Elberton probably best represented

Granite. by the Pl

In the
+ Af +

HL20(Qc,Vo)ntelinnted, iaignrdaimcatthinegEtlhbeertbounlkis

H20 content of a magma containing those phases at the indicated pressure. Af =

alkali feldspar, As = Aluminosilicate (Sillimanite) Kf = potassium feldspar,

L = silicate liquid (melt) M = Muscovite, Pl = plagioclase, a.Q and eQ = alpha

and beta quartz, V =vapor (hydrous) 21

evolved to produce the observed variation.
Since no simple fractionation scheme can fit the data, we suggest that the regional pattern of trace element distribution as shown in figure 2. must represent heterogeneities developed during a partial melting episode in the source region and preserved by laminar flow during emplacement of the pluton. Major element chemistry would have been controlled by melting at a mineralogically fixed invariant point, but trace element concentrations controlled by degree of melting or heterogeneity in trace mineralogy.

PETROGENESIS
Emplacement - Whitney (1975 and 1972) studied the effects of P, T and H20 content on the phase relationships in four synthetic granitic and granodioritic compositions. Of these Rl (which is an Fe and Mg free equivalent of Nockold's (1954) Average hornblende- biotite granite) is 'virtually identical to the composition of the Elberton Granite (less Fe, Mg, Ti, etc.). The phase relationships for this composition, presented in Figure 5, should serve as a model for the Elberton Magma. The addition of ferromagnesian components could perhaps result in a slight lowering of the solidus temperature, but would effect no major change.
If any of the muscovite found in the Elberton Granite can be interpreted as a primary magmatic phase, the reactions:

Muscovite + quartz = K-feldspar + Al 2Si0 3 + H2 0 (vapor) muscovite + quartz + plagioclase = K-feldspar + Al2Si05
+ Liquid

must represent the upper temperature/lmV"er pressure limit on

final solidification of the Elberton Granite. Allowing for

some lowering of the solidus due to the effect of Fe and Mg in

the liquid, final solidification must have taken place at or

above 4 Kbar pressure near 6S0c. Application of Wanes' (1972)

equation for estimating the fugacity of water in biotite,

K-feldspar, magnetite assemblages at 4 Kbar and 650C with

estimated oxygen fugacites about 10~1 5 gives a partial pressure

of up

onHt2hoe

above 6 Kbar. This calculation is extremely dependent fugacity of oxygen and the result can only be used to

suggest vapor

that the magma at pressures

should have less than

become sa 6 Kbar.

turated w Locally

i

t

h an abun

dHa2n0t

pegmatite and aplite dikes also suggest vapor saturation. Fig.

6 shows the normative composition of the Elberton qranite

relative to the bivariant curves representing liquids in
equilibrium with plagioclase, K-feldspar, quartz and H1o vapor.

22

Quartz Saturated
Figure 6. The normative composition of the Elberton Granite (black dot)
in relation to liquids in equilibrium with quartz, plagioclase, alkali feldspar, and vapor as projected onto the f'eldspar ternary system. Liquids with insufficient water to produce a separate vapor phase at the given pressure will lie on surfaces sloping toward the KAlSiJPB corner as shown by the hachures on the curves.
23

The composition of the Elberton Granite would indicate that the mineral assemblage (Qz + K-fsp. + plag.) in the Elberton Granite would coexist with a liquid and water rich vapor at about 5 Kbar. Refering to FigureS such a magma would contain about 6-7 weight percent H20 and be at a temperature of about 660-c. This conclusion is in general agreement with relationships found by Hess (1979) using data of Winkler, et al. (1975). In a few localities sparse, small, phenocrysts of K-feldspar suggest that K-feldspar was the liquidus phase. Referring again to Whitney's (1972, 1975) Rl composition (Fig.
S) this could only have been the case at pressures above 4 kilobars.

The relatively fine grain size and uniform texture

throughout a single intrusive body as large as the

Elberton Granite suggests that cooling due to heat transfer to

the outside of the body was not a significant factor in

initiating nucleation and crystallization. It is more likely

that the magma originated at d~pth and intruded upward

approximately adiabaticly (isothermally) until it intersected

its solidus or, more likely, the point where crystallization and

vapor generation sharply increased its viscosity, preventing

further upward movement. Since the magma would have a tendency

to rise until it reached this limit, which we would suggest was

about 13-15 km depth (4.5

5 Kbar pressure), it seems

reasonable to suggest that it may have spread out to assume a

more or less sill-like shape only a few kilometers thick at

this depth.

Origin

The large volume of homogeneous low

temperature magma, and lack of any associated mafic

igneous activity, certainly suggests that the Elberton magma

was generated by anatexis within the lower crust.

The .source materials, or protolith, from which the Elberton magma was derived are not clearly indicated. The xenoliths found in the Elberton Granite all seem to be related
to common rock types in the surrounding metamorphic country rocks. There is little evidence of assimilation, and apparently no material that is a remnant of the protolith or "restite". Chappell and White (1974) established geochemical criteria for two contrasting granite types in the Ne\'l England Batholith: I, derived from an igneous protolith, and S, derived by anatexis of sedimentary materials. The chemical and
isotopic characteristics of the Elberton Granite (Table 3) are intermediate or tend towards Chappell and White's I type. The details of geochemical discrimination between granitic rocks derived by anatexis of "igneous" vs. "sedimentary" protoliths will almost certainly vary from one region to another. However, even when compared with other granites of the southern Piedmont, the Elberton has characteristics, particularly ~18o, which still indicate an . intermediate relationship between granitoids assigned to I and S categories (Wenner, 1980).

24

Table 3. Comparison of the Elberton Granite and the I and S types of Chappel and White (1974)

I type
>3.2 wt % in felsic rocks

Al (Na+K+O. 5Ca)

< 1.1

S type
< 3.2 wt % in rocks with 5 % 5% k20
>1.1

Elberton
3.98 with 4. 82 l'o K20
1.02

C. P. I.W. Norm corundum

< 1%

>1 %

0.54 %

Mineralogy

hornblende, sphene muscovite, monazite brown-green biotite "red" biotite

sphene, allanite green biotite (muscovite)

+7.7 to+ 9.9

+ 10.4 to + 12.5

+ 6.9 to 8.7

0.704 - 0.706

>0.708

0.7037

Oxygen isotopic data - O'Neil and Chappell (1977); Wenner, (1980) Strontium isotopic data - O'Neil and Chappell (1977); Ellwood, et al. (1980)

25

The low initial lt7srj86sr requires a protolith which was relatively rubidium-poor or young (relative to formation of the Elberton). In view of the fact that trace element and oxygen isotopic heterogenities in the source are apparently preserved in the pluton, it seems probable that Sr isotopes were also not homogenized. The possibility then exists that the isochron reported by Ellwood, et al. (1980) may be interpreted as a "pseudoisochron". However, the low initial ratio, and the fact that the apparent age is not significantly older than U-Pb zircon ages, still requires that the source materials be relatively "primitive" with respect to their Sr isotopic composition.

This evidence suggests a protolith largely composed

of mafic to intermediate metavolcanics, with perhaps, some

mixture of metasediments of early Paleozoic age. Suitable

analogues are found in the Carolina State belt and Charlotte

Belt, and are inferred to be present in the southeast flank of

the Inner Piedmont belt. (Whitney et al., 1978~ Whitney, et

al., 1980).

----

--

Assuming that the Elberton was emplaced into

metamorphic rocks at a temperature of about 550 C (Whitney and Wenner, 1980) at a depth of 15 km, extrapolation of this

I ,

geothermal gradient would generate a temperature of 700 C at

about 19 km (7 Kbar). Experimental work (Whitney, 1975, see

figure 10, p. 21.) shows that a "granodioritic" composition

with 6-7% H20 would partially melt to plagioclase and a

"granitic" liquid at 7000C and 8 Kbar. This suggests that the

Elberton magma could have been produced by partial melting of

more mafic, "meta-igneous" rocks leaving plagioclase,

amphibole, and, perhaps, biotite in the residum a few

kilometers below its emplacement depth. More detailed

mathematical modeling can, of course be done, (Whitney &

Stormer, 1978) but is of very limited value since the chemical

and mineralogical nature of the protolith is entirely

unconstrained by data.

SUMMARY
The geochemical and mineralogical data suggest that Elberton Granite was produced by anatexis of mafic to intermediate meta-igenous rocks of early Paleozoic age. The anatectic event took place at depths of about 18-20 km and temperature of about 700-7500C. This thermal event produced large quantities of very homogeneous magma, which, however, contained significant variations in trace elements and isotopic characteristics as a result of minor variations in the protolith or the melting process. The magma rose until the pressure drop produced vapor saturation and crystallization at a depth of about 13-15 km. It seems likely that this pressure "lid" has resulted in lateral spreading and emplacement as a relatively thin, "laccolithic;', pluton. The pressure "quench"
26

produced spontaneous nucleation and crystal growth throughout the pluton resulting in its fine-grained, even texture. The emplacement age, betwen 320 and 350 rn.y., post-dates peak metamorphisim and no evidence of metamorphic deformation is seen within the Elberton granite itself.
ACKNOWLEDGEMENTS This research was supported by National Science Foundation grant number EAR-7818127 and the use of various facilities of the Geology Dept. of the University of Georgia.
27

REFERENCES

Arth, J. G., 1976, Behavior of trace elements during magmatic processes--a summary of theoretical models and their
applications: Jour. Research u. s. G. s., v. 4, p. 41-47.

Beswick, A. E., 1973, An ~xperimental study of alkali metal distributions in feldspars and micas: Geochim. Cosmochim. Acta. , v. 37, p. 183-208.
Butler, Jr. and Ragland, P. c., 1969, A petrochemical survey
of plutonic intrusions in the Piedmont, Southeastern
Appalachians, u. s. A. : Contr. Mineral. and Petrol. ,
v. 24, p. 164-190.

Chappel, B. W. and White, A. J. R., 1974, Two contrasting granite types: Pacific Geol., v. 8, p. 173-174.

Chayes, F. , 1951, diagram: Annual Report of the Director of the Geophysical Laboratory for 1950-1951. p. 44.

Cox, K. G., Bell, J. D., and Pankhurst, R. J., 1979, The Interpretation of Igneous Rocks: George Allen and Unwin,
London, 450 p.

Ellwood, B. B., tihitney, J. A., Wenner, D. B., Mose, D.,
and Amerigan, c. , 1980, Age, paleomagnetism, and tectonic
significance of the Elberton Granite, northeastern Georgia Piedmont: J. Geophys. Res., in press.
Flynn, R. T. and Burnham, c. w., 1978, An experimental
determination of rare earth partition coeficients between a chloride containing vapor phase and silicate melts: Geochim. Cosmochim. Acta., v. 42, p. 685-701.

Hanson, G. N., 197R, The application of trace elements to the petrogenesis of igneous rocks of granitic composition: Earth Planet. Sci. Letters, v. 3R, p. 26-43.

Hess, J. R., 1979, Geochemistry of the Elberton granite and the geology of the Elberton West quadrangle, Georgia: M.S. Thesis, University of Georgia, 193 p.

Irving, A. J., 1978, A review of experimental studies of crystal/liquid trace element partitioning: Geochim. Cosmochim. Acta., v. 42, p. 743-770.

Kerrick, D. M., 1972, Experimental determination of muscovite

- quartz stability with PH o Ptotal= Amer. Jour. of

272, p. 946-958.

2

28

Lemaitre, R. w., 1976, The chemical variability of some common
igneous rocks: Jour. of Petrology, v. 17, p. 589-637.

Nockolds, S. R., 1954, Average chemical composition of some
igneous rocks: Geol. Soc. Am. Bull. v. 65, p. 1007-1032.
O'Neil, J. R. and Chappell, B. w., 1977, Oxygen and hydrogen
isotope relations in the Berridale batholith: Jour. Geol. Soc. Lend. , v. 33, p. 559-571.

Ramspott, L. D., 1964, The Elberton batholith: Southeastern Geology, v. 5, p. 223-230.

---=--,1968, The nature of the Elberton batholith, Georgia: Geol. Soc. Am. Spec. Paper, 101, p. 372.
Ross, c. R.,II and Bickford, M. E., 1980, The u-Pb age
of zircons from the Elberton Granite, Piedmont of Georgia: Georgia Geol. Soc.
Rozen, R. w., 1978, The geology of the Elberton East
quadrangle, Georgia: ~~. s. thesis, Univ. of Georgia.

Shaw, D. M., 1978, Trace element behavior during anatexis in the presence of a fluid phase: Geochem. Cosmochim. Acta. v. 42, p. 933-943.

Silver, L. T. ann Grunenfelder, M., 1957, Alterations of accessory allanite in granites of the Elberton area, Georgia (abstract): Geol. Soc. Am. Bull., v. 68, p. 1796.

Watson, E. B.,l979, Zircon saturation in felsic liquids:

Experimental results and applications to trace element

geochemistry: Contrib. MinP.ral. Petrol.,

v. 70,

p.

407-419.

Wenner, D. B., 1980, Oxygen isotope relations in the Elberton Granite: Georgia Geol. Soc. Guidebook. (This volume)

~'lhi tney, J. A., 1972, History of granodiori tic and related magma systems: An experimental study: Ph. D. Thesis, Standford University, 192 p.

Whitney, J. A., 1972, The effects of pressure, temperature
and XH o on phase assemblage in four synthetic rock
composftions: J. Geol., v. 83, p. 1-31.

Whitney, J. A., Paris, T. A., Carpenter, R. H., and Hartley, H. T., III, 1978, Volcanic evolution of the southern slate belt of Georgia and South Carolina: a primative oceanic islann arc: J. Geology, v. 86, p. 173-192.

Whitney, J. A., \'lells, D. E., annd Rozen, R., 1980, Structural and tectonic setting of the Elberton batholith,

29

eastern Georgia Piedmonnt: Georgia Geol. Soc. Guidebook. (This volume)
Whitney, J. A. and Stormer, J. c., Jr., 1978, Rare earth
distribution within post-metamorphic granites of the southern Applachian Piedmont: EOS, v. 57, p. 1221. Whitney, J. A. and Wenner, D. B., 1980, Petrology and structural setting of post-metamorphic granites of Georgia: Geol. Soc. Amer. Field Trip Guide 1980 Ann. Mtg., Amer. Geol. Inst. (in press) Winkler, H. G. F., 1976, Petrogenesis of metamorphic rocks: New York, Springer-Verlag, Inc. , 334 p. Winkler,H. G. F. , Boese, M. and Marcopoulos, T., 1975, Low temperature granite melts: Neues Jahrbuch fur Mineralogie, Monatsh., p. 245-268. Wones, D. R. ,1972, Stability of biotite: A reply: Am. Mineral., v. 57, p. 316-317.
I ,
I '
30

Oxygen Iso~ope Variation~ within the
Elberton Granite Pluton 1
David B. Wenner, Department of Geology, University of Georgia, Athens, Georgia 30602
ABSTRACT
Oxygen isotope data from 42 sites within the Elberton pluton exhibit an unusually broad range of 618o values,
from 6.1 to 9.3 per mil with a mean of 7.9 0.7, compared to
most of the other late orogenic, post metamorphic granitic plutons of the Southern Piedmont. 18o;l6o isocontours constructed from these data exhibit an overall north-south alignment that is virtually identical to certain trace element patterns (most notably manifested by Rb/Sr) and foliation and flow direction estimates based on paleomagnetic data. These relationships demonstrate that the oxygen isotopic contour patterns reflect primary isotopic heterogeneities of the original magma caused by viscous laminar flow. The isotope data can be directly correlated with: (1) certain accessory mineral contents in which samples containing accessory sphene are invariably lao-depleted (<7.5), whereas sites containing possible primary muscovite are restricted to the 18 a-enriched (>7.8} areas~ and (2) initial strontium isotopic compositions. The granite at two quarry sites or approximately 8000m2 is isotopically homogeneous, to nearly within experimental measurement. One quarry containing numerous xenoliths and country rock exposures shows no isotopic variation within the granite, d~monstrating that little or no isotopic interchange occurred between thP. original magma and the country rock due to exchange and/or assimilation processes. This suggests that the primary 1Bo;l6o heterogeneities of the Elberton pluton are probably domin~ntly derived from a lithologically variable protolith in which partial melting has generated a grar.itic magma that is exceedingly uniform in its major element chemistry, but is highly varable in its oxygen and strontium isotopic and trace element concentrations. Such. a protolith . may exist within the lithologically variable subcrustal rocks that are reflected by the regional gravity gradient that transects the Appalachians.
INTRODUCTION
The Elberton pluton represP.nts an integral part of a suite of late orogenic, post metamorphic granitic plutons
31

in the Southern Piedmont for which oxygen isotopic data have been obtained. To date, some twenty one plutons have been studied, principally in Georgia and South Carolina.

These data show an overall regional isotopic trend in which plutons of the Inner Piedmont are lSo-enriched (olSo
values range from 11.4 to 7.9) compared to those within the Charlotte- Caroline Slate belts (8.3 to 5.5). Granites within the Kiokee belt exhibit a broad range from 8.9 to 5.5. This overall regional isotopic pattern, in which lBo-depleted plutons are largely confined to regions lying adjacent to present day ocean basins is similar to what has been observed in Australia (O'Neil, et al., 1978: O'Neil and Chappel, 1978) and in the Peninsular Ranges of Southern and Baja California (Taylor and Silver, 1978).

The regional oxygen isotopic patterns correlate

exceptionally well with certain geophysical data that

delineate subcrustal lithologies.

In particular, the

18o-enriched plutons within the Inner Piedmont and portions of

the Kiokee belt that lie adjacent to the Coastal Plain in South

Carolina are restricted to regions characterized by negative

gravity anomalies (less than the -10 mgal contours of Long,

1979), whereas the low 18o;l6o plutons invariably occur in

areas of positive gravity anomalies. Thus, if the gravity data

indicate that less dense, sialic type rocks underlie much of

the Inner Piedmont and portions of the Kiokee belt and more

mafic lithologies occur beneath the Charlotte-carolina Slate

and portions of the Kiokee belts at depths at which the

granites formed (estinaten to range from 10 krn deep in the

Inner Piedmont, Whitney et al. 1976, to 7-12 km deep in the

Carolina Slate belt, ~fuitney and Stormer, 1977), then the

isotopic patterns reflected by the granites of the Southern

Piedmont suggest that they may essentially be rooted to their

protoliths. This relationship in turn suggests that the

thrusting event inferred to have occurrerl along a pervasive

seismic discontinuity observed in the COCORP d,ata (Cook, et

al., 1980) must have happeneo prior to intrusion of the late

orogenic granite approxi~ately 350 m.y! ago (also se~ 1ihitney,

et al., this _volume)

The Elberton pluton is uniqne because it lies adjacent to ~ fundamental boundary separating the Inner Piedmont f rom the Charlotte-Carolina Slate belts, and upon a regionally extensive major qravity qradient that transects the Appalachinns. Because this pluton is extremely uniform in its textural, modal, and najor element che~ical composition, (Stormer, et al., this volume), it provides an ideal site for ascertaining the relationship between the isotopic composition of granites ann possihle compositionally variable protoliths. Additionally, hecause of this unique setting, the isotopic data may potentially place inportant constraints on the extent of late Paleozoic thrustinq in the Piedmont. Finally the existence of other supporting petrographic, geochemical, and

32

geophysical data provide a unique opportunity for achieving a better understanding of some of the factors that control primary oxygen isotopic variations within a pluton.

WHOLE ROCK OXYGEN ISOTOPIC VARIATIONS

The range and distribution of whole rock oxygen

isotopic compositions of the sample sites from the

Elberton Pluton are p~esented in Fiqure 1. The data are

contoured to display the regional isotopic patterns within the

body.

As is readily apparent, an approximate ove~all

north-south orientation of 61~0 isocontours exists, which is

most apparent in the center of the body where the major portion

of data exist.

The alignment of the oxygen isotopic isocontours
shown in Figure 1 is consistent with the paleomagnetic data, as revealed by the magnetic foliation and magmatic flow directi0n estimates (see Ellwood, this volume) and certain dispersed trace element data, illustrated by the Rb/Sr isocontours(Stormer, et. al., this volume); the intercomparison of all these data iare-shown in Figure 2. These similarities
suggest that the oxygen isotopic contours must essentially reflect primary magmatic flow patterns in \"'hich fundamentally distinct isotopic heterogeneities are preserved as the result of viscous laminar fluid flow of the magma.

Although systematic variations in whole rock oxygen isotopic compositions can he caused by many factors, a number of considerations described below indicate that the whole rock 18o;l6o variations must fundamentally reflect primary magmatic variations.

1. The major mineral modal composition is ext~emely uniform throughout the body (Stormer, et al., this volume)
indicating that little or no oxi%en isotopiC variation can be attributed to varying amounts of 0-enriched (ie. quartz) and more 18o-depleted (ie., feldspar and biotite) phases.

2. Measurements of isotopic fractionations between coexisting quartz, fEffdspar, and biotite from three representative samples (Wenner, 1980) do not reveal any evidence of the overprinting of some secondary hydrothermal or metamorphic process. Such events would be manifested by isotopic disequilibrium between th.e more resistant (ie., quartz) and easily exchangeable (ie., feldspar) minerals. In fact, virtually all of the late orogenic, post metamorphic granitic plutons of the Southern Piedmont exhibit a close approach to internal oxygen isotopic equilibrium (Wenner and Whi tncy, 1979).

3. Additionally, the general geologic setting of the Elberton indicates that no metamorphic event affected this

33

INNER PIEDMONT

CHARLOTTE AND SLATE BELTS

@

.0

0.5 contour interval

0

5

10

KM

818 0 CONTOURS

Figure 1. Map of the Elberton Pluton showing whole rock oxygen isotope isocontours, expressed as olBo values relative to SMOW (in which the NBS-28 standard is defined as+ 9~61), and the sample localities. The enclosed rectangular region within the center of the body corresponds to the locality of the Elberton \~est 7~ quadrangle.

34

D

tl24''
341~'

o~nw;)

LJ

INNER PIEDMONT

;)4"001

s aG~~
3... .5~
3400'

CHARLOTTE AND SLATE BELTS

~32

0

10

MAGNETIC FOLIATION

CHARLOTTE AND SLATE BELTS

0

O

FLOW DIRECTION ESTIMATES

INNER PIEDMONT

INNER PIEDMONT

CHARLOTTE AND SLATE BELTS

0.5 contour interval

0

10

818 0 CONTOURS

CHARLOTTE AND SLATE BELTS
Rb/Sr RATIO

Figure 2. Relationship between the oxygen isotopic data presented in Figure 1, the Rb/Sr data presented by Stormer, et ~- (this volume) and the paleomagnetic data given by Ellwood .(this volume).

35

body (see Stormer,~ al., this volume).

4. Petrographic data generally reveal the absence of

any significant degree of secondary alteration.

Only

minor small scale deuteric alteration in the plagioclase cores

of some samples have been noted (Hess, 1979), a feature that

could not in any way account for the observed broad range of

whole rock oxygen isotopic compositions.

RELATIONSHIP TO ACCESSORY MINERAL ASSEMBLAGES

A comparison was made between the whole rock 18o;l6o variations and accessory mineral assemblages observed within the center of the Elberton Pluton (Elberton West 7 1/2' quadrangle of Hess, 1979). Certain characteristic mineral assemblages of granites have for example been employed to distinguish between the "S"- and "I"-type granites (Chappell and White, 1974; Miller, 1980). In particular, the presence of sphene (an I-type indicator) and primary muscovite (an S-type indicator) appear to be most revealing for the Elberton Pluton.

A comparison of the whole rock oxygen isotopic data

and these unique IJ,'S" and "I~ accessory mineral indica tors is

shown in Figure 3. The occurrence of s~~ene in the granite

appears to be uniquely confined to the low 0 (<7.5) regions,

whereas invariab

l

s y

i

tes res

t

containing
ricted to

1~ossible
a-enrich

pri ed

mary (>7.8

m )

uscovit areas.

e

are almost Thus, the

oxygen isotopic data and the distribution of accessory minerals

suggest tha t t he Elbe rton granite has both an "S"- and "I" - type

character, in keepi~~ wi th its uniq u e position along the

boundary betwe en the n-enriche 1fl largely S-type plutons of

the Inner Pie dmont and the low 0, I-g rantites characteristic

of the Charlotte-Carolina Slate belt.

OXYGEN ISOTOPIC VARIATIONS WITHIN OUTCROP DIMENSIONS
Detailed sampling for whole rock oxygen isotopic analys"es were ma.de at two quarry sites in order to ascertain the extent to which isotopic heterogeneities exist on an outcrop scale. Such an assessment is important in understanding the various factors that control the isotopic variations throughout the body as a whole.
Ten whole rock samples from the Dawn Grey and Sunset Pink Quarries (shown in Figure 4) display an exceedingly
narrow range in ~18o values from 7.2 to 7.6, with a mean
variation of 7.4 : 0.2. This isotopic variation is nearly within experimental error and serves to demonstrate that the Elberton granite is iso2opicatlly homogeneous over an area of approximately 8,000 meters As: is also apparent from Figure 4, the whole rock values for both grey and pink varieties of
36

I I
I /
/
/
q,9
/

CX)

0 .

cO

CX)

I

I 0

KM

Accessory Mineral Distributions (from Hess, 1979)
o Sample locality

Primary Muscovite

Muscovite with Calcite
S Sphene present

- - - Chloritized Zone

_

76 _

818 0 Whole Rock lsocontours
0.4 Contour Interval

Figure 3. Map of the Elberton West 7~ quadrangle showing the relationship

between the whole rock the accessory mineral

oxygen contents

isotopic of the

sadmatpalessho(wfrnomby Haels8s0,

isocontours, 1979). Note

and that

the contour interval presented in this figure differs from that presented

in Figure 1.

37

S18 Whole Rock Data
Dawn Grey and Sunset Pink Quarries
Elberton W., Ga.
71;2 Quadrangle

G~4

~;J;"'"' 4 7.4 G

--~ -~~7.3"-1'-'-s'~'~
7.2 ~~ /.Qo-1--

-9 t: r

G' -9.q"'

~,

~

/.,..~ 7 ~

N

w co

I

0

25 50

>t.ET

7.3

VEAL QUARRY
Elberton West, Georgia 7 ~2 ' Quadrangle
~
8.2

~
N
""0 30 60 n.ET

D holnlAll --ol

granite

- biotite schist

gneiss

.---
1
bs

biotite schist cs

xenoliths exposed 'on quarry face -

~ leucocratic phase ~ biotite schist

(]

0

5

FEET

Figure 4.Map view of the Dawn Grey and Sunset Pink Quarries, and Figure 5. Map view of the Veal Quarry.
Both quarries are located in the Elberton West quadrangle. Sample locations and whole rock oxygen isotopic compositions are shown. The location and nature of xenoliths exposed in the Veal Quarry and details of one
quarry face are shown in Figure 5.

granite are virtually identical1 the latter type is due to the presence of very fine scale, dust sized inclusions of hematite in feldspar (Hess, 1979), presumably indicating slightly more oxidizing conditions in this portion of the granite. Detailed Rb/Sr data acquired for these same samples also reveal extremely uniform ratios (Hess and Stormer, 1980 and Stormer, ~ al., this volume).

In contrast to the lithologic uniformity of the Sunset Pink - Dawn Grey Quarry, the Veal Quarry represents a site containing numerous and varied types of xenoliths and adjacent exposures of biotite schist country rock. In the
context of this study, it is important to assess the possible effects of any oxygen isotopic contamination or exchange processes that may have occurred het\-Teen country rock and/or xenoliths and the granite magma. Significant isotopic exchange effects have been documented in instances where magmas come in contact with country rock that is isotopically dissimilar; such effects are especially pronounced in environments where the country rock is either unmetamorphosed or had previously been only slightly metamorphosed. (Shieh and Taylor, 1969).

The data in Figure 5 indicate that the granite within

the Veal Quarry is isotopically quite uniform, with six

samples exhibiting a range from 8. 2 to A. 5 ,.,i th a mean of 8. 3 -

0.1.

This isotopic uniformity is apparent, despite the

deliberate pattern of sampling at the biotite schist country

rock contact along the eastern edge of the quarry.

In summary, detailed sampling within two quarry sites
reveals extreme isotopic homogeneity on a moderate sized outcrop scale. This pattern thus appears to be consistent with data presented in Figure 1 in which oxygen isotopic qradients of approximately 0.4 per mil/km exist throuqhout much of the body. It should be noted that a steeper isotopic gradient occurs in at least one area in the south central portion of the pluton. Evidence from this study also indicates that sites containing numerous xenoliths in close proximity to country rock, such as at the Veal Quarry, do not exhibit any measurable isotopic gradients in the granite due to contamination or exchange effects between the granite magma and the adjacent country rock. This observation is consistent with other studies (Shieh and Taylor, 1969) that reveal only minimal isotopic exchange effects in instances where magmas come in contact with country rocks that were previously metamorphosed at moderate to high grades. The Elberton is thus entirely consistent with these other studies, since the country rock was metamorphosed to amphibolite facies prior to intrusion of the granite magma.

RELATIONSHIP TO STRONTIUM ISOTOPIC DATA Rb/Sr whole rock geochronological studies (Ellwood,
et -al., 1980) reveal that the Elberton Granite may be
39

heterogeneous in its initial strontium isotopic composition. This heterogeneity is manifested by two parallel isochrons with differing initial B7s-;86sr. Oxygen isotopic analyses of these same samples reveal that whole rock samples defining the lower
initial 87Sr/86sr intercept isochron of 0.7037 .0005 have a
mean oxygen isotopic composition of 7.45 .33, whereas those defining a pseudoisochron with a higher intercept of 0.7054 .0024 are !So-enriched, with a mean 618o of 8.35 .34 (Ellwood, et al., 1980). These data indicate that a positive correlation between strontium and oxygen isotopic compositions exists internally within the Elberton Pluton, a feature that is similar to what is observed in other granitic plutons (Hose and Wenner, 1980) and is seen in general in many other calcalkaline igneous suites (see Taylor, 1980).

SUMMARY AND CONCLUSIONS

The Elberton granite occupies a critical position

within the Southern Piedmont in (1) occurring adjacent to

two fundamentally different terrains , the Inner Piedmont to

a the northwest and the Charlotte-Carolina Slate belt to the

southeast, and (2) lying upon

major, regionally extensive

gravity gradient that transects the Appalachians. The

unusually broad range of lB.o;lo 0 (up to 3 per mil) for this

body compared to most of the other late orogenic granitic

plutons within the Southern Piedmont (~1 per mil) may reflect a

derivation from gradational sequences of mafic to somewhat

more lBo-enriched, less dense (metasedimentary?) rocks; such

lithologies occurring at depth may in fact be responsible for

producing the regional gravity gradient that subdivides the

Inner Piedmont from the Charlotte-Carolina Slate belt. This observation, combim~d with the fact that the lBo-enriched

plutons (11.4-7.9) are invariably confined to the Inner

Piedmont, whereas low 18o plutons (5.5-7.6) are restricted to

the Charlotte-Carolina Slate belts, serves to suggest that the

late orogenic plutons may be linked to their protoliths within

the subcrust; this relationship essentially suggests that the

Southern Piedmont may have been largely autochthonous during

the past 350 m.y.

Whole rock oxyqen isotope isocontours .based upon nata from 43 sites exhibit an approximate north-south orientation that is most pronounced within ~he center of the body (Elberton West 71/2'Quadrangle). These isocontours are quite similar to cbntours defining certain dispersed trace
element concentrations (most notably illustrated by the Rb/Sr isocontours of Figure 2) and to paleomagnetic data. These relationships indicate that the 1 Bo;l6o isocontours must
reflect an isotopically heterogeneous magma that effectively preserves these heterogeneities as the result o viscous laminar magmatic flow during enplacement. These isotopic contour "stripes" appear to be essentially homogeneous on a quarry sized outcrop scale (-8,000 m2). Within the center of

40

the body where most data are exist, lateral oxygen isotopic gradients of approximately 0.4 per mil/km exist.
The oxygen isotopic isocontour bands within the Elberton appear to be uniquely characterized by distinct accessory mineral contents. The low lao isocontours invariably contain sphene, a mineral often associated with !-type granitoids, whereas the !So-enriched regions commonly contain abundant primary (?) muscovite, a mineral often observed in S-type granites. Thus the Elberton appears to have certain accessory mineral characteristics and 18o;l6o variations suggestive of a granite intermediate between S-and !-types.
Although some subtle form of contamination and/or isotopic exchange occurring between the magma and the country rock cannot be completely discounted in producing the isotopic heterogeneities, it seems more likely that the 18o;l6o variations of the granite must largely reflect its derivation from a compositionally variabl~ protolith. Such variations could be produced as the result of some anatectic process operating within a terrain composed ~f mixed lithological content (for example, l~wer crustal, low 1 0 mafic amphibolites intermixed with a more 0-enriched metasedimentary material). Such a process would be expected to generate a minimum melt granitic magma, accounting for the extreme major element uniformity of the Elberton pluton, with a variable 18ojl6o and dispersed trace element content that essentially reflects a compositionally variable protolith. Such a magma would be expected to preserve whatev~r heterogeneities were acquired from the protolith, from its ascent to the final stages of crystallization, because silicic magmas behave as highly viscous laminar fluids.
ACKNOWLEDGEMENTS
This research was supported by National Science Foundation grant number EAR-7818127. Special recognition is extended to my collegue Brooks Ellwood for numerous comments on an earlier version of this paper.
41

REFERENCES
Chappell, B. w. and White, A. J. R., 1974. Two contrasting
granite types, Pacific Geology, 8, 173-174.

Cook, F. A., Albaugh, D. s., Brown, L. D., Kaufman, s., Oliver, J. E., and Hatcher, R. D., Jr., 1979. Thin skinned tectonics in the crystalline Southern
Appalachians: COCORP seismic-reflection profiling of the
Blue Ridge and Piedmont, Geology, I, 563-567.

Ellwood, B. B., Whitney, J. A., Wenner, D. B., Mose, D. ,
and Amerigian, c., 1980. Age, paleomagnetism, and
tectonic significance of the Elberton granite, Northeast
Georgia Piedmont, Jour. Geophys. ~ (in press).

Hess, J. R., 1979. Geology of the Elberton West Quadrangle, M. s. thesis Univ. of Georgia~

z, Long, L. T., 1979. The Carolina slate belt - evidence of a

continental rift zone, Geology,

180-184.

Hiller, F. K., 1980. Two- mica granites and two- mica granites, (abst.), Geol. Soc.~., Abst. with Programs (in press).

O'Neil, J. R. and Chappell, B. W., 1977. Oxygen and hydrogen isotope relations in the Berridale batholith, Jour. Geol. Soc. (London), 133, 559-571.

O'Neil, J. R., Shaw, s. E., and Flood, R. H., 1977. Oxygen and hydrogen isotope compositions as indicators of granite genesis in the New Englanrl batholith, Australia, Contr. Mineral. Petrol., ~' 313-328.

Taylor, H. P., Jr. , 1980. The effects of assimilation of country rocks by magmas on 1Bo;l6o and B7srj8(lsr systematics in igneous rocks, Earth. Planet. Sci. ~.,
47, 243-254.

Taylor, H. P., Jr. anrl Silver, L. T., 1978. Oxygen isotope
relationships in plutonic rocks of the Peninsular Ranges
Batholith, Southern and Baja California, u. s. Geol.
Survey Open-File Rept., ~-7091, 423-426.

Shieh, Y. N. and Taylor, H. P., Jr., 1969. O"ygen and carbon isotope studies of contact metamorphism of carbonate rocks, J. Petrol., 10, 307-331.

Wenner, D. B., 1980. Use of oxygen isotope data for

delineation of magmatic flow in the Elberton Pluton of the

N. E. Georgia Piedmont, Geol. Soc.~., Abst.

with

42

Prog., 12, 211-212. Wenner, D. B. and l~itney, J. A., 1979. Oxygen isotope
compositions of Hercynian age granites in the Southern Piedmont and their relationship to subcrustal lithologies and st~uctures, Geol. Soc. Amer., ~ with Programs, 11, 538.
Whitney, J. A. and Stormer, J. c., Jr., 1977. Two feldspar
geothermometry, geobarometry in mesozonal granitic intrusions: three examples from the Piedmont of Georgia, Contrib. Mineral. Petrol., 63, 51-64. Whitney, J. A., Jones, L. M., and Walker, R. L., 1976. Age and origin of the Stone Mt. Granite, Lithonia District, Georgia,Geol. Soc. Arner. Bull., 87, 1067-1077.
43

U-Th Geochemistry of the Elberton Pluton
David B. Wenner, Department of Geology, University of Georgia, Athens, Georgia 30602, James D. Spaulding, Center for Applied Isotope Studies, University of Georgia, Athens, Georgia 30602
INTRODUCTION
Preliminary uranium and thorium data have been acquired from ten sites within the central part of the Elberton pluton. This study was undertaken in order to: (1) determine how these two elements . vary throughout a moderate sized pluton of extreme major element chemical and textural homogeneity: (2) compare the U and Th contents and the U/Th of the Elberton pluton with other granite bodies: (3) ascertain how the U and Th contents vary between the granite and late stage pegmatites: (4) assess whether any relationship exists between the U and Th data and the systematic dispersed trace element and 18ojl6o patterns observed in this pluton.
METHOD OF ANALYSES
Approximately 1-2 kg samples of granite and up to 10 kg of pegmatite were selected from extremely fresh outcrops within quarry sites. These samples were crushed to about 30 mesh, and representative 600grn. aliquots were sealed for 21 days in 600 ml. Marinelli beakers for gamma ray spectrometry. Measurements were made e111ploying an 18% Ge (Li) detector with a 4096 channel microprocessor based multichannel analyzer. Calibrations were made with standards supplied by
the u. s. Geological Survey.
In this type of analyses, radium equivalent uranium (RaeU) is repor~e%, since this technique actually giv~s a measurement of 2 Ra: it yields the actual uranium concentration only if secular equilibrium exists within the upper part of the 238u chain. Although RaeU and actual U contents are commonly not identical in granites, numerous intercomparisons show that where differences do exist, most RaeU values are lower by 15-20 percent (Stuckless et al., 1977). Assuming that the reported U contents of samples -are within this range of the actual concentrations, then all of the tentative arguments advanced in this paper are valid. ?he data presented in this study are thought to have an uncertainty of 10-15%.
44

PRESENTATION OF DATA

Preliminary data from ten separate sites within the

central part of the Elberton pluton are reported in Fig.

1. Although it is uncertain exactly what variations in U and

Th exist in an outcrop scale, two separate samples from a

quarry of about 8,000 m2 surface area have virtually identical

Th concentrations and almost marginally overlapping U contents,

considering a possible 15% error of uncertainty in measurement

for the u. Samples of pegmatites selected at two sites exhibit

U and Th concentrations that contrast markedly with the host

rock biotite granite~ in both instances, U is enormously

increased and Th somewhat decreased in the pegmatites as

compared to the granite. Although only a small portion of each

of the pegmatites were actually sampled, total scintillometer

gamma count measurements of the pegrnatites within these sites

indicate possible variable U and/or Th contents along strike of

a given pegmatite and among different pegmatites within a given

quarry site. Thus the U and Th concentrations reported in Fig.

1 may not represent the mean concentrations of the pegrnatites

as a whole, but do serve to demonstrate the enormous contrast

between the U and Th contents of the pegmatites and granite.

In one locality (site 9x), the mineral polycrase, (Ce, u, Th)

(Nb, Ti, (Stormer

Teat )2a l0. ,6

,

has this

been identified volume).

in

several

pegmatites

It is readily apparent that the Elberton pluton has a Th content that i~ considerably greater than the average for granites (SiO > 70%) worldwide. This body displays a relatively broad range in Th, 27 to 45 ppm, with a mean of 38.3
7.7(ai ppm, that repreSents greater than a two-fold increase in concentration of Th than the average worldwide range for granites of 10-20 ppm and mean of 18 ppm (Rogers and Adams,
1969a). In contrast, the U content of the Elberton exhibits a nearly normal range of values, varying from 2.4 to 10.6 ppm
with a mean of 4.24 2.55 (a), compared to the worldwide
average for granites of 4 ppm (Rogers and Adams, 1969b). This
discrepancy is of course reflected by the enormously varied Th/U of the Elberton that ranges from 3.0 to 19.1, with a mean for the ten sites of 10.3 ~ 5.0 (d)~ this contrasts markedly with the average Th/U of 3.5 to 4.0 for granites worldwide, (Rogers and Adams, l969a). The latter is well illustrated by Figure 2 which demonstrates that only two samples (9x and 15) have Th/U values similar to the worldwide average for granites. It should be noted that even if the true U concentrations were greater than the measured Raeu values by 15-20%, most Elberton
samples would still have quite anomalously high Th/U values.

DISCUSSION OF DATA
The data presented in Figure 2 clearly demonstrate that most of the sites within the Elberton pluton have anomalously high Th/U values compared to the worldwide
45

- (478.5.)

10.6 32.1

7.6 ( 153) 39.0 10.5
\.

0

2.8

~29.9

N

t

0

10

Kilometers

Figure 1. U. (reported as RaeU and Th concentrations, both reported
as ppm, are shown for samples from various localities in the central part of the Elberton pluton. Ratios shown in parentheses lying adjacent to these values are from pegmatities at the same sites. Note that the two sets of data for granite samples from the same location (eastern edge of p1u ton) represent two different samp1es from the same quarry.

46

o15

o9x
E
0. 0.
::::>

10



---,.,,.'/

10

20

30

40

50

60

Th ppm

Figure 2. Plot of U in ppm (reported as RaeU versus Th in ppm for various
granite samples (shown as closed circles) and pegmatites (shown as open
circles) from the Elberton pluton. Indentification numbers besides the data points indicate associated sample localities. Th/U values of 3 and 5, presented linearly, represent the mean range for granites worldwide
(Rogers and Adams, l969a). The two encircled datapoints are from two different samples from the same quarry site.

47

average for granites, suggesting that one element, namely U, has probably been lost from a major portion of the granite body. Although obviously either Th gain or U loss can account for these anomalous ratios, this interpretation is preferred, since it is well established that U can easily be mobilized in rocks exposed near the surface due to the influence of ground waters, despite the apparent freshness of the sample (Stuckless and Nkomo, 1978); in contrast Th is quite immobile under similar circumstances.
The U enrichment in the pegmatites however suggests that at least some, but an unknown amount of U loss may have occurred as a consequence of volatile phase transport from the crystallizing granite during formation of the pegmatites. It is interesting to note from Figure 2 that of the two pairs of pegmatite- granite samples examined, the most u-enriched pegmatite appears to be associated with a granite that is more U-depleted, suggesting that at least some portion of the apparent U loss observed within the Elberton granite may be due to u-removal as a consequence of magmatic differentiation during pegmatite formation. Such effects have been well documented in many plutonic bodies, where in some instances, ore grade deposits occur in pegmatites (Rogers~ al., 1978).
With the very limited data available, it is impossible to estimate what the original Th/U of the granite may have been prior to possible surficial leaching, but one can assume that the granite probably had a Th/U of at least around 3, the lowest such ratio measured at any of the sites. However, this value may have been even lower, since Th/U ratios are generally somewhat smaller for the two-mica granites than for those containing hornblende (Nash, 19~9). Some portions of the Elberton may contain primary muscovite in addition to biotite, making it somewhat similar to a 2 mica granite, (Stormer~ al., this volume).
Perhaps more importantly, the two-mica granites usually contain a more readily leachable form of uranium than do those containing hornblende and biotite; for the latter, U is dominantly restricted to minerals that are resistant to surficial leaching processes (eg., zircon, allanite, etc.) (Nash, 1979). Although no systematic study has as yet been made to find out which of the minerals the U and Th reside in, it seems likely, from the similarity to the two-mica granite suites noted elsewhere that at least some portion of the Elberton granite probably once contained some of its U in a readily leachable form.
The extremely limited U and Th data acquired to date do not show any obvious correlation with the systematic trace element and isotopic patterns recorded throughout the pluton (Stormer et al., this volume; Nenner, this volume), suggesting that--the variable Th and U concentrations observed in the Elberton granite (although a lowering of the primary U
48

concentration within the granite can of course occur, as noted
previously) do not appear to he directly or entirely controlled by primary variations within the protolith. It would appear that a number of c.ombined processes may he required to explain the variable Th (and probably the U as well) concentrations noted for a granite body that is essentially homogeneous in its major element chemistry. The obvious large partiting of U and Th between a granitic melt and a vapor saturated solution implied by the contrasting U and Th data recorded in the coexisting granite and pegmatites, suggest the possibility that perhaps variable amounts of vapor phase removal from the granite magma may be an important process in controlling the U
and Th concentrations throughout the Elberton Pluton.

CONCLUSIONS

Preliminary U and Th data acquired by gamma ray spectrometric measurements from 10 sites within the Elberton Pluton suggest the following tentative conclusions:

1. The Elberton granite contains more than twice the average Th content (mean of 38 ppm) compared to the mean worldwide average for granites (18 ppm).

2. 'rhe U content, however is nearly identical to the worldwide mean of 4 ppm:

3. The Th/U of 8 of the 10 sites examined are distinctly anomalous (mean of 11.9 4.3) compared to the range exhibited by most granites worldwide (3-5), although two sites have normal Th/U ratios:

4.

Samples from two pegmatites are distinctly

enriched in U (up to 20 times), compared to the host rock

biotite granite, illustrating the importance of possible vapor

phase transport of U during pegmatite formation:

5.

The anomalous Th/U values recorded by these

preliminary data, in all likelihood reflect, in part, the

loss of U due to groundwater leaching, although at least some

but indeterminable U loss from the granite may have occurred

during pegmatite formation:

6. These preliminary data do not show any obvious
correlation to the systenatic patterns displayed by the dispersed trace element and 18ojl6o data, suggesting that the variation in measured Th among different sites may not be
directly or entirely attributable to original variations within the protolith:

7. It seems probable that the broad range of Th data and the presumed initial variations in U concentrations may conceivably reflect varying amounts of vapor phase

49

transport of these elements from the granite
,..-...
ACKNOWLEDGEMENTS Par-tial support fqr this work was obtained from a
grant from the u. s. Geological Survey, Grant No.
14-08-0001-G-590.
50

REFERENCES
Nash, J. T., 1979. uranium ..and ~hor.ium in granitic rocks of
northeastern Washington and northern Idaho, with comments
on ur.aniurn ' . resource potential. u ~ S. Geol. survey
Open-File Report 79~2:33~ .:T;...3~f.
Rogers, J. J. w. and J. A. s. Adams, 1969a. "Thorium"
Chapter 90, in Wedepohl, K. H., ed., Handbook of Geochemistry: Berlin, Springer-Verlag, v. II-4.
Rogers, J. J. W. and J. A. s. Adams, 1969b. "Uranium" Chapter
92 in Wedepohl, K. H., ed., Handbook of Geochemistry: Berlin, Springer-Verlag, v. II-5.
Rogers, J. J. w., P. c. Ragland, R. K. Nishimori, J. K.
Greenberg, and S. A. Hauck, 1978. Varieties of granitic uranium deposits and favorable exploration areas in the eastern United States. Econ, Geol., v. 73, 1539-1555.
Stuckless, J. s. , H. T. Millarn, Jr. , c. M. Bunker, I. T. Nkorno, J. N. Rosholt, c. A. Bush, c. Huffman, Jr.,
and R. L. Keil, 1977. A comparison of some analytical techniques for determining uranium, thorium, and potassium
in granitic rocks. u. s. Geol. Survey, Jour. of Res., v.
5, !33-91.
Stuckless, J. s. and I. T. Nkomo, 1978. Uranium-lead isotope
systematics in uraniferous alkali-rich granites from the Granite Mountains, Wyoming: implications for uranium source rocks. Econ. Geol., v. 73, 427-441.
51

The U-Pb Age of Zircons from the Elberton
Granite, Piedmont of Georgia
Charles R. Ross, II and r1. E. Bickford, Department of Geology, University of Kansas, Lawrence, Kansas 66045
ABSTRACT
The Elberton Granite, which occurs as an intrusive body in high-grade metamorphic rocks in the Inner Piedmont region of eastern Georgia, was emplaced near the peak of tectonic and metamorphic events in that area. The age of the Elberton Granite is, therefore, important in determining the timing of these events in the formation of the southern Appalachian Mountains.
Previous determinations of the age of the Elberton Granite by the U-Pb, Rb-Sr, and K-Ar techniques, and an interpretation of its paleomagnetic pole age, have yielded values ranging from 235 to 490 m.y. The determinations reported here were done by the U-Pb method on zircons separated from fresh samples of the Elberton Granite, and have yielded an age of 320 20 m.y. This is in fair agreement with an independent 350 11 m.y. Rb-Sr whole-rock isochron and with paleomagnetic data. Ages determined by the K-Ar method are all younger than these values, and evidently record later events. The only previous U-Pb determination 'on zircons, which yielded an age of 415 to 490 m.y., may have been done on zircons with inherited older xenocrystic material.
INTRODUCTION
The Elberton Granite occurs in eastern Georgia where it intrudes high grade met&~orphic rocks and migmatite of the Inner Piedmont (Fiqure 1). Field relations indicate that the Elberton Granite \ITas formed at the peak of tectonic and metamorphic events affecting this portion of the southern Appalachian ~1ountains (Hess, 1979). Thus, the age of crystallization of the Elberton Granite will place limits upon the age of these events which were related to convergent plate motions attendant upon the closing of the Iapetus Ocean (Hatcher, 1972).
Previous determinations of the age of the Elberton Granite have utilized the U-Pb, K-Ar, and Rb-Sr techniques. These ages are in severe disagreement, ranging from about 450 m.y. (U-Pb on zircon; Gunenfelder and Silver,
52

__ ___ .. _____ ...
\
. I
\ VALLEY
. \ AND I \
..\ RIDGE

'84
--------------_rr" /

BLUE RIDGE .

/ !/:.l .~'

.., />.: . : , "' .".. .. ~ ..

0

50

II Ill I

J

KM

Figure 1.

Map of southern Appalachian metamorphic belts in Georgia and South Carolina; Elberton West Quadrangle, within which sample used in this study was collected, is outlined in black (after Griffin, 1971; Hatcher, 1972; Hess, 1979).

53

1958) to less than 250 m.y. (incremental 40Ar;39Ar method on
biotite~ Hess, 1979). Thus, for more precise understanding of this area, the need for more precise determination of the crystallization age of the Elberton Granite is apparent.

GEOLOGIC SETTING

The following description of the setting and petrography of the Elberton Granite is largely taken from
the thesis by Hess (1979).

The Elberton Batholith is located within the Inner Piedmont belt of the southern Appalachians (Fig. 1). The

I ~

Inner Piedmont is a zone of high-grade, migmatitic gneisses

which extends from Virginia to Alabama. The zone, which is a

maximum of 145 km wide in Georgia and South Carolina, has been

subdivided into a high-grade core and two lower-grade flanks

(Griffin, 1971~ Hatcher, 1972). The core is of sillimanite

grade, commonly migmatitic, and composed of sillimanitic

schists and gneisses, granitic gneisses, and amphibolite.

The flanks of the Inner Piedmont are characterized by greater abundance of amphibolite and amphibolitic gneiss and by lower metamorphic grade. In the area of this study, the Inner Piedmont consists of rnigmatitic gneiss, microcline gneiss, biotite and sillimanite schist, and minor amphibolite and calc-silicate rocks.

The Elberton Batholith intrudes the metamorphic rocks of the Inner Piedmont in an area approximately 60 krn long and 10 krn wide extending from north of Elberton, Georgia to south of Lexington, Georgia. The area of exposure of the batholith, which is composed of unfoliated granite, is generally parallel to the regional trend of the Inner Piedmont units. Ramspott (1968) regarded the Elberton Batholith to be miqmatitic biotite gneiss cut by fine- to medium-grained granite. He reported that the granite was intruded during the Early Paleozoic, either coincident with or after the peak of metamorphism, and had undergone further metamorphism during the Permian.

Within the Elberton West quadrangle, from which the sample studied here was taken, the Elberton Batholith exhibits extreme mineralogical and chemical homogeneity. The major phases, oligoclase, quartz, and microcline, are generally equal in abundance as noted by Chayes (1951), and biotite is the major mafic phase. The predominant variation among localities of the granite is in color~ the rock is typically
light to dark gray. Ramspott (1964) attributed the darker color to impurities in the feldspars. In the southeastern portion of the batholith a pink variety of the granite is exposed adjacent to the standard gray Elberton Granite. Where exposed, the contact between the two varieties is gradational

54

and, despite the color difference, the two varieties are virtually the same chemically and mineralogically. Ramspott (1966) hypothesized that the pink color is due to the presence of hematite, fo.rmed by the oxidation of primary magnetite, within fracture systems which occur in the pink variety, but not in the gray granite.
Petrologic and geochemical data (aess, 1979) suggest that the origin of the Elberton Granite is best explained by partial melting of felsic crustal material. The most likely source materials are the Inner Piedmont lithologies, as they are adjacent to the granite, similar to it in composition, and have experienced metamorphic conditions sufficient for granite magma to form (Hess, 1979).
PREVIOUS AGE DETERMINATIONS
Many previous age determinations have been reported for the Elberton Granite. Long and others (1959) reported
a K-Ar age of 247 9 m.y. from a biotite sample. Fairbairn
and others (1960) obtained a K-Ar age of 234 15 m.y. and a Rb-Sr mineral age of 254 f 13 m.y. from a biotite sample. They also reported a Rb-Sr mineral age of 24 5 13 rn.y. from an associated muscovite. Grunenfelder and Silver (1958), using U-Pb abundances in zircons, obtained an age of 415-490 m.y., and attributed the discrepancy between this value and ages obtained by Rb-Sr and K-Ar analyses to the occurrence of two metamorphic events. Ramspott (1964), however, suggested that the zircon sample used in the U-Pb analysis had included some fraction of zircons inherited in toto from the surrounding country rock during emplacement~ the magma.
Dallmeyer, as reported in Hess (1979), obtained a K-Ar age of 238 5 m.y. from a biotite sample, using the 40Arj39Ar incremental releasP. technique (Dalrymple and Lanphere, 1974)~ the sample appears to have experienced relatively little disturbance (Hess, 1979). Similar work on
amphiboles resulted in an age of 275 5 n.y. (Hess, 1979).
Ellwood and others (1980) obtained a Rb-Sr isochron indicating an age of 350 11 m.y. Further, the paleomagnetic directions in the Elberton Granite are consistent with an age of 350 m.y., by comparison of the tilt-corrected virtual geomagnetic pole with the standard polar wandering path for North America (Ellwood and others, 1980).
SOURCE AND UA'fURE OF SAMPLES
The sample used for this study was collected by J. A. Whitney and B. B. Ellwood of the University of Georgia during 1978. The sample was collected at the northeast corner of the Dawn Gray quarry, located in the southeastern part of the Elberton West Quadrangle (Location EW9-ll, Plate 1 Hess,
55

1979~ approximately 15m SW of analysed sample EW2-12 as shown on figure 7, p. 47, Hess, 1979; !attitude 340- 1' - 25" N longitude 820 53' - 38" W). The site was chosen to be remote from both xenoliths and dikes to insure a fresh sample free of contamination from extraneous material.
Upon arrival at the University of Kansas, the sample was mechanically reduced and the zircons extracted. The zircons are generally euhedral (FigurA 2) though those in the largest size fraction (-100 mesh) are commonly fractured. The zircons are clear and light red to lavender in color. There is no evidence of rounded zircons or zircons with rounded cores, either of which could indicate that some of the zircons had been inherited from older material.
The zircons were divided into two size fractions, -100 to -200 mesh and -200 to -400 mesh (few - zircons fell into the -100 mesh size fraction and these were not analysed). The -100 to -200 fraction was further split on the basis of magnetic susceptibility with a Frantz separator, yielding samples B, D, and E. A portion of sample E was leached for 45 minutes in cold, approximately 24N HF, and the residue (E Leach) was saved. All of these zircon samples were analysed for Pb isotopic composition and for U and Pb isotopic concentrations by standard mass spectrometric techniques.

ANALYTICAL METHODS

The U-Pb age determinations presented in this paper

were done in the Isotope Geochemistry Laboratory of the

University of Kansas using standard techniques of mass

spectrometry. ~he analytical methods, instrumentation, and

results of analysis of standards have been described by

Bickford and others (1969) and by Bickford and Mose (1975)~ the

samples were analysed on an automated 9-inch rnass spectrometer

which was placed in operation in 1978. For the U-Pb

mA2e3a8suur=em1e.n5t5s1

o x

n

l

o

zirc -10

ons, yrs.

-

1

the ~ A2

3fSoull=owi9n.8g4

c 8

onst x

ants w
10~10

ere used: yrs.-1;

atomic ratio 238u;236u = 137.88 (Jaffey and others, 1971;

Steiger and Jager, 1977). Common Pb corrections were made

using the two-stage growth model of Stacey and Kramers (1975).

As described by Bickford and Mose (1975), the U-Pb age of the zircon suite was determined by obtaining a best-fit line to the array of data points on a concordia diagram using the least-squares-cubic method of York (1966), and then solving for the value of the upper intercept of this line with the condordia curve. The least-squares-cubic method yields the 16 uncertainty in the slope and intercept of the line through the data points, and these are used to calculate maximum and minimum upper intercept ages. The difference between these and the mean age is taken as the lo uncertainty in the age.

56

~


.. .

: ')~,
:If,.
-

. .
:~..
i.

.~ ~~:..~#_.. .

. .- -

. ~..

~(

~

~K~~tah.aL--Aah~4a~~--__

Figure 2. Photomicrograph of typical zircons from the Elberton Granite, size: -100 to -200 mesh.

..

57

RESULTS
The analytical data for the zircon analyses are given in Table 1 and are plotted on a concordia diagram
(Wetherill, 1956) in Figure 3. The calculated age is 320 20
m.y. In the abs-ence of evidence for inherited zircons or disturbance of the U-Pb system in the zircons, we take this to be the time of crystallization of the Elberton Granite.
DISCUSSION
The age obtained is in fair agreement with the Rb-Sr age and paleomagnetism obtained by Ellwood and others, (1980). It is however, in serious disgreement with ages obtained by K-Ar and Rb-Sr methods on minerals.
We suggest, following Grunenfelder and Silver (1958), that the cluster of ages around 245 10 m.y. obtained by both K-Ar and Rb-Sr methods on single minerals represents a time of uplift or a later metamorphic event~ Hess {1979) has observed that this age -is widespread regionally. The age of 415-490 m~y. obtained by Grunenfelder and Silver {1958) may have resulted from analysis of a zircon population with an older, inherited component.
It is interesting to note that a recent study of zircons from felsic volcanic rocks in the North Carolina Piedmont (Wright and Seiders, 1980) yielded a U-Pb age of 586 10 m.y., with a lower intercept age of 340 m.y. If one interprets these data on an episodic lead-loss model, the lower intercept indicates the age of an event which caused lead loss from the zircon. This could be the metamorpic-tectonic event which culminated in the emplacement of the Elberton Granite. Wright and Seiners seem to prefer the episodic lead-loss model, but they make clear that their data do not preclude the possibility of a continuous-diffusion model.
ACKNOWLEDGEMENTS
This work constituted a senior honors thesis by the senior author at the University of Kansas, Lawrence, Kansas, and was supported by National Science Foundation grant EAR 7813687. The granite samples were kindly provided by James A. Whitney and David B. Wenner of the University of Georgia. Robert D. Schuster, Samuel A. Bowring, and Wendel J. Hoppe provided instruction and assistance to the senior author in many parts of the analytical work, and this is gratefully acknowledgerl.
58

'r

TABLE 1. ANALYTICAL DATA

Pb206

u

rad

Sample (ppm) (ppm)

B 1566.0 79.6

D . 184.6 88.9

E 1564.4 79.4

ELEACH 1010.2 49.5

-200 2010.9 85.4

Measured Ratios

208Pb

207Pb

206Pb

206Pb

0.23231 0.24913 0.23446 0.33617 0.31340

0.05543 0.05945 0.05759 0.08651 0.07891

204Pb 206Pb

Calculated Atomic Ratios

207Pb 206Pb 207Pb

206Pb 238u

235u

0.00018 0.00047 0.00030 0.00231 0.00179

0.0528 0.0525 0.0532 0.0526 0.0527

0.0462 0.0464 0.0462 0.0434 0.0469

0.3362 0.3356 0.3385 0.3148 0.3414

The analytical blank determined in parallel with these samples was 14.0 ng
total Pb, representing a small correction to these results. Analytical precision for measured isotope ratios is 0.1 percent or better for 208Pb/ 206Pb and 207Pb/ 206Pb and .00001 for 204Pb/ 206 Pb.

59

ELBERTON GRANITE GEORGIA
320 :t 20 m. y.
. 055

. 050
206
Pb
u 2~
. 045

. 040 .039 . 038

. 27 . 28 . 29 . 30

' 35 20ZPb /235U .40

.45

Figure 3. Concordia plot of U-Pb data for zircons from Elberton Granite.

60

REFERENCES CITED

Bickford, M. E., and Mose, D. G., 1975, Geochronology of

Precambrian rocks in the St.

Francois Mountains,

southeastern Missouri: Geol. Soc. America, Special Paper

165, 48 p.

Bickford, M. E., Wetherill, G. w., Barker, Fred, and Lee-Hu,
Chin-Nan, 1969, Precambrian Rb-Sr chronology in the Needle Mountains, southwestern Colorado: Jour. Geophys. Res., v. 74, p. 1660-1676.

Chayes, F., 1951, Annual Report of the Director of the Geophysical Laboratory for 1950-1951, p. 44.
Dalrymple, G. D., and Lanphere, M. A., 1974, 40 Ar/ 39Ar age spectra of some undisturbed terrestrial samples: Geochim. et Cosmochim. Acta., v. 38, p. 715-738.

Ellwood, B. B., Whitney, J. A., Wenner, D. B., Mose, D.,
and Amerigian, c., 1980. Age paleomagnetism, and tectonic
significance of the Elberton granite, northeast Georgia
Piedmont, J. Geophys. ~~., in press.

~
Fairbairn,

H.

W.,

Pinson,

w.

H.,

Hurley,

P.

M.,

and

Cormier,

R. F., 1960, A comparison of the ages of coexisting

biotite and muscovite in some Paleozoic granitic rocks:

Geochirn. et Cosmochim. Acta, v. 19, p. 7-9.

Griffin, V. s., 1971, Inner Piedmont Belt of the southern
crystalline Appalachians: Geol. Soc. Am. Bull., v. 82, p.
1885-1898.

Grunenfelder, M., and Silver, L. T., 1958, Radioactive age dating and its petrological implications for some Georgia granites (abstract): Geol. Soc. Am. Bull., v. 69, p. 1574.

Hatcher, R. D., Jr., 1972, Developmental model for the southern Appalachians: Geol. Soc. Am. Bull., v. 83, p. 2735-2760.

Hess, J. R., 1979, Geochemistry of the Elberton Granite and the geology of the Elberton West quadrangle, Georgia, Unpub.
r1. s. thesis, University of Georgia, Athens.
Jaffey, A. H., Flynn, K. F., Glendenin, L. E., Bentley, W. c.,
and Essling, A. M., 1971, Precision measurement of half-lives and specific activities of 235 U and 238 U: Phys.
Rev. c, 4, p. 1889.

Long, L. E., Kulp, J. L., and Eckelmann, F. D., 1959, Chronology of major metamorphic events in the southeasten United States: Am. Jour. Sci., v. 257, p. 585-603.

61

Ramspott, L. D., 1964, The Elberton Batholith: Southeastern Geology, v. 5, p. 223-230.
Ramspott, L. D., 1966, Tectonic origin of color in pink granites of the Elberton District (Piedmont), Georgia (abstract): Georgia Acad. Sci. Bull., v. 24, p. 75-76.
Stacey, J. s., and Kramers, J. D., 1975, Approximation of
terrestrial lead isotope evolution by a two-stage model: Earth Planet. Sci. Lett., v. 26, p. 207-221. Steiger, R. H., and Jager, E., 1977, Subcommission on Geochronology:. Convention on the use of decay constants in geo- and cosmochronology: Earth Planet. Sci. Lett., v. 36, p. 359.
Wetherill, G. w., 1956, Discordant uranium-lead ages, 1:
Trans. Amer. Geophys. Union, v. 37, p. 320-326. Wright, J. E., annd Seiders, V. M~, 1980, Age of zircon from
volcanic rocks of the central North Carolina Piedmont and tectonic implications for the Carolina volcanic slate belt: Geol. Soc. Am. Bull., v. 91, p. 287-294. York, D., 1966, Least-squares fitting of a straight line: Canadian Jour. Phys., v. 44, p. 1079-1086.
62

Geochronology and Cooling History of
the Elberton Granite
James A. Whitney, James R. Hess, Department of Geology, University of Georgia, Athens, GA. 30602, and Douglas Mose, Georgia Mason University, 4400 University Drive, Fairfax, VA 22030.
ABSTRACT
Various radiometric dating techniques give discordant ages for the Elberton granite which are interpreted as the result of diffusive loss of daughter products during a slow cooling and uplift history of the batholith. The U-Pb age of zircons from the Elberton of 320 20 m.y. is thought to be close to the age of magmatic crystallization. Previously reported older zircon ages are thought to be the result of inherited zircon. The Rb-Sr whole-rock isochron yields an age of 350 12 m.y., which overlaps with the u-Pb age for zircon. The true age of intrusion is thought to be close to these value~. 40Ar/39Ar release spectra from biotite give a tight clustering of ages from 235 to 238 m. y. A hornblende from a
xenolith gives an older 40Arj39Ar age of 275 5 m.y. Rb-Sr
mineral ages for biotite are considerably younger than the whole-rock age, being about 265 m.y. A thermal model for the Elberton has been constructed based on these age restrictions. Upon intrusion 320 to 350 m-. y. ago, the Elberton remained hot, probably above 450C, until after 300 m.y. B.P. or so. ny 235 rn.y. B.P. it had cooled below about 300 to 3500C, and had probably attained surface conditions by about 200 m.y. ago.
INTRODUCTION
Several previous studies have yielded confusing geochronological data for the Elberton. Gruenenfelder and Silver (1958) reported U-Pb ages for zircons of 450 m.y., which corresponds to Ordovician time. However, it appears that an Ordovician age is inconsistent with the structural relationships and uplift history of the Piedmont (Dallmeyer, 1978). We believe that these ages are older than the crystallization age, possibly due to inherited zircons.
Conventional K-Ar mineral ages from micas (biotite and one muscovite) have yielded ages of 235 to 245 m.y. (Long and others, 1959: Fairbairn and others, 1960) or late Permian. These ages are very close to argon retention ages obtained from the Inner Piedmont by Dallmeyer (1978).
63

CHARLOTTE
AND SLATE
BELTS

0

5

10

KM

L.------------------------~------------------------... 3345'

Figure 1.

Location of the Elberton granite (stippled), Georgia, in relationship to the Inner Piedmont, Charlotte-Slate belts, and shear zones (diagonals). Dashed line outlines the Hutchins granite. Sample sites for Rb-Sr study are shown.

64

In order to determine the true magmatic age and cooling history of the Elberton we conducted a cooperative geochronological study involving several different isotopic
methods. The results of the U-Pb study of Zircons by c. R.
Ross, II and M. E. Bickford are summarized in a preceeding article. This article summarized the results of Rb-Sr whole-rock isochron studies, Rb-Sr biotite-whole rock mineral ages, and 40Ar/39Ar retention ages for biotite and hornblende
from the Elberton.

RB-SR STUDY

For Rb-Sr whole rock investigations fifteen samples

of about 10 kg each were collected from the Elberton

pluton. Each s~ple was crushed and split to a 10 q portion

which was subsequently powdered. The powder was split into a

0.2 to 0.3 g portion for Rb and Sr isotopic analysis. Each

analysis was done using B4sr and . 87Rb spikes, ultrapure HF,

HthCr1o0u4gh

and HCl, pyrex ca

and tion

Teflon beakers. The solutions were exchange resin columns to obtain Rb

passed and Sr

fractions.

The Sr isotopic analyses were made using a Nier-type, 6-inch radius mass spectrometer (S.I.M.A.) with a programable automatic data aquisition system at the Department of Terrestrial Magnetism of the Carnegie Institute. The Rb isotopic analyses were made using a ~ier-type 12-inch radius mass spectrometer at Florida State University.

All the Sr isotopic compositions were calculated from analyses of sample plus spike mixtures. The 85Rbj87Rb ratio was taken to be 2.593 and the decay constant used for B7Rb is 1.42 x lo-llyr-1 (Steiger and Jager, 1977). The Rb/Sr
age and initial 87srj86sr ratio on the whole rock isochron diagrams were calculated using the York (1966) regression treatment. The one-standard-deviation experimental error in 87Rb/86sr was derived from an examination of duplicate analyses
done over the past six years and was calculated to be 2 percent. The one-standard-deviation experimental error in 8 7sr;86sr was derived from an examination of multiple analyses of the Eimer and Amend standard CaC03 made during the time that these Sr analyses were performed and from an examination of the errors obtained from the mass spectrometer data. The one-standard-deviation experimental error of the ten E&A analyses was 0.005 percent; the experimental error of the ten
E&A analyses was OA005 percent; the experimental error in the normalized 87sr/0 6sr ratio for each sample analysis was
acceptable when less than 0.02 - percent. The one-standarddeviation experimental error for 87srj86sr used to calculate the Rb/Sr isochron age was 0.05 percent. The data points on the whole rock isochron diagrams are shown with error boxes whose dimensions correspond to two-standard-deviations. The errors assigned to the isochron age and the calculated initial

65

87 sr;86 sr ratio on the isochron diagrams are given at the 68 percent confidence level (1 sigma). The analytical data are presented in Table 1.
Discussion. The samples collected for the Rb/Sr whole rock study are listed in Table 2. Nine of the samples (no. 4, 5, 6, 7, 8, 10, 12, 13 and 14) came from rock quarry sites that showed no apparent problems in the form of near by xenoliths, hydrothermal alteration or cataclastic zones. These nine samples yield an isochron age of 350 11 m. y. and an initial 87srj86sr ratio of 0.7037 0.0005 (solid line in Figure 2).
Three of the collected samples (no. 1, 2 and 9) came from quarry sites in which there were abundant xenoliths, though almost all the xenoliths appeared to have sharp angular boundaries and exhibited no obvious evidence of assimilation. One sample (no. 3) came from a pegmatite dike within the Elberton. Another sample (no. 11) came from a site near an apparent hydrothermally formed vug 1 em in diameter containing copper (?) mineralization. Still another sample (no. 15) came from a relatively distant locality in the Vesta 7 1/2' quadrangle, south of the Elberton West 7 1/2' quadrangle in which all the other samples were collected.
The samples with possible problems (no. 1, 2, 3, 9, 11 and 15} were not usen to determine the Rb/Sr whole-rock age of the Elberton. However, it proved interesting to note that analyses of these samples all fall on an isochron with a calculated age of 376 45 m.y. and an initial 87Sr/~6Sr of 0.7054 0.0024 (dashed line in Figure 2). Although this isochron age and initial 87srj86sr ratio are essentially the same within the analytical errors as the age and initial H7 Srj86 Sr ratio derived from the samples which have no apparent problems, a composite of these data shows that, at the 95 percent confidence level, the sample analyses are better fit to two different isochrons than to a single isochron. We now believe that the 350 11 m.y. isochron derived from the samples without apparent problems is more meaningful in terms of determining the age of the Elberton. The fact that the isochron derived from the disturbed samples (dashed line in Figure 2) lies slightly above the isochron derived from the main body of the Elberton may be due either to 87Sr enrichment caused by some subtle form of contamination due to the incorporation of radiogenic 87sr from the country rock (with no obvious evidence of assimilation of the xenoliths by the magma except at one locality, i.e. location 9} or to heterogeneities in the initial 87srj86Sr of the magma. In any case, plutonic rocks from the Appalachians (and elsewhere) that yield what have been called parallel isochrons have been reported by others (Fullagar and Butler, 1976; Butler and Fullagar, 1978: Mose and Wenner, 19AO).
Sample no. 9 was arbitrarily selected for Rb/Sr
66

0.74
B

I

//

AGE= 375 45 M.Y.

//
/

I. R. = 0. 7054 .0024

0'\
~

'(J) (.0
00
~ 0.72
r-(J) 00

,.,., 8180mean = 8.35 .34 9 /

AGE= 350 11 M.Y.
I. R. = 0.7037 .0005
818 Omean=7.45 .33

ELBERTON GRANITE

0.700.0

2.0

4.0

6.0

87Rb/86Sr

Figure 2. Composite Rb-Sr isochron diagram showing the Elberton sample analyses from problem free locations (open error boxes) and from possible problem locations (filled error boxes).

TABLE 1

Rb/Sr and 18o;16o Analyses of Elberton Samples

SAMPLE

87Rb/86Sr

87sr/86sr

(atomic ratio) (atomic ratio)

G 1

4.488

o. 7290

G la

4.500

ave 4.494

0. 7282 0. 7286

G 2

3. 730

o. 7263

G 3

3.815

0.7256

G 4

3. 736

0. 7232

G 5

4.904

0. 7277

G Sa G 6

4.874 ave 4.889
4.869

0. 7274
o. 7276 o. 7271

G 6a

4.898

ave 4.883

0. 72 75 0. 72 73

G 7

4.142

0.7314

G 8

4.169

0. 7253

Measured Error (%) 0.015 0.014

86sr (ppm) 15.866 15.813

0.019 0.015 0.005 0.016 0.015

17.788 12.974 19.010 15.446 15. 780

0.012 0.013

16.004 15.086

0.020 0.014

17.088 15.702

continued on next page

87Rb

0180*

(ppm) (SMOW)
72.036 7.9 + .10
71.983

67.126 50.069 71.846 76.620 77.798

8.5 8.5
7.4 .16**
7.7

78.828 7.6 78.313

71.597 7.6 66.210 7.6

68

Continued

G 9 G 10 G 11 G 12 G 13 G 14 G 15

2.984 1.438 3.498
/
1.475 2.531 1.516 4.305

G 9 biotite 137.7

o. 7215
0. 7107 0. 7233 0.7116
o. 7157
0.7110 0. 7289
1.2303

0.012 0.009 0.012 0.018 0.014 0.020 0.013
0.014

20.723 32.006 18.799 36.107 26.599 31.191 16.150

62.561 46.549 66.525 53.885 63.254 47.833 70.332

8.0 + .02 6.8 + .03 8.4 .04 7.7 7.7 + .05 7.0 8.8

2.001 278.66

*No indicate~ one analysis; indicates average deviation of two samples.
All samples' are reported relative to the SMOW standard in which NBS Standard #28 is regarded to be +9.61. **Represents average and standard deviation of ten separate samples collected over an area of ~8,000m2.
Sample locations given in Figure 1.

69

SAMPLE

TABLE 2 Rb/Sr Sample Site Descriptions

possible problems

comments

1

few xenoliths in area

possibly contaminated

2

very many xenoliths

possibly contaminated

3

pegmatite dike sample

possibly came from melt other than Elberton

4

none visible

* good sample

5

none visible

* good sample

6

none visible

* good sample

7

none visible

* good sample

8

aplite with "soft" margins probably coeval with Elberton magma

9

many xenoliths, some are

possibly contaminated

partially melted

10

none visible

* good sample

11

vug near sample site;

possibly contaminated

gneissic (plastic deform-

ation) zone about 100'

away

12

some gneissic rock in

* possibly contaminated

area, but located far

from sample site

13

none visible

* good sample

14

none visible

* good sample

15

collected some distance

possibly from a magma within only the

to south, in Vesta quad-

Vesta area

rangle

*used to indicate sample with no apparent problems

70

mineral isochron study. A biotite-whole rock isochron for
~9is ftgmple yields an age of 265 6 m.y. and an initial
Sr/ Sr ratio of 0. 7102 0. 0004 (errors calculated assuming an analytical error of 2 percent and 0.05 percent in 87 'Rb)36 Sr and 87sr;86sr, respectively). Earlier Rb/Sr studies using mica
extracted from Elberton granite samples yielded ages of 248 13 m.y. from biotite and 240. 13 m.y. from muscovite (ages
recalculated . from Pinson et al., 1958 and Fairbairn,et al., 1960, using the 87Rb decay--constant = 1.42 x lo-l~r~). Although these Rb/Sr mica ages and the earlier mentioned K/Ar mica ages of about 235 and 245 m.y. are about 100 m.y. younger than the Rb/Sr whole rock age (and about 200 m.y. younger than the U/Pb ages), th~se mica ages are not thought to represent a time of metamorphism, since the granite contains no metamorphic fabric. Instead, these mica ages are assumed to be related to the time at which . the pluton cooled during uplift to a temperature below which radiogenic argon and radiogenic strontium were retained in the dated micas.
The long interval between the time of granite crystallization and the time of cooling to the retention temperature of mica seen in the Elberton has an implication with regard to the interpretation of Rb/Sr data. In earlier studies, the absence of metamorphic fabric in a pluton has been taken to mean that a Rb/Sr mica-whole rock isochron will reveal the time of crystallization. This is based on the assumption that plutons cool quickly, and this assumption has been used to interpret Rb/Sr mica-whole rock isochron ages as representing the tirne of crystallization (Stone Mountain pluton in t.Vhitney et al., 1976~ Clouds Creek, Coronaca and Churchland plutons in Fullagar and Butler, 1979). Rb/Sr mineral-\'rhole rock isochrons clearly yield cooling ages younger than the time of crystallization, and, as . shown by the Elberton study, the crystallization age may be considerably greater than the cooling age. As pointed out by Fullagar and Butler (1979), the initial 87srj86sr ratio of the mineral isochron may be helpful in evaluating the difference between the cooling age and the crystallization age.
K-Ar STUDY
The 40Arj39Ar incremental release technique was utilized in determining age spectra .and total gas ages for two amphibole samples and five biotite samples from the Elberton West quadrangle and one biotite sample from the Sandy cross quadrangle. This technique is a variation of the K-Ar dating method wherein 39Ar is produced from 39K in the sample by irradiation w;i.th fast neutrons. Then the amounts of 40Ar, 39Ar, 37Ar and 36Ar released from the sample at each temperature increment are measured by mass spectrometry (Dalrymple and Lamphere, 1974). This eliminates the need for separate potassium analyses and enables "ages" to be calculated for gas released during several heating increments. The
71

analyses were provided by R. David Dallmeyer, University of Georgia,
The results of the analyses are presented in Figure 3 which shows sample locations and release spectra. The amphibole samples were from an Inner Piedmont amphibolite (EW-4) and from a xenolith (EW-lOA) within a quarry. Both samples appear to have undisturbed rP.lease spectra (Dalrymple and Lamphere, 1974) and the total-gas ages are coincident, within estimated error, at about 275~5 m.y. One biotite analyzed is from the xenolith EW-lOA also and its total gas age is 2465 m.y. The remaining biotites are from quarried granite samples and have ages that are amazingly consistent at about 2385 m.y. Like the amphibole samples, the release spectra appear to be undisturbed for the biotite samples. The biotite ages are reasonably consistent with biotite ages previously determined by the K-Ar method: 24 79 m. y. (Long and others, 1959) and 23515 m.y. (Fairbairn and others, 1960).
The amphibole ages are younger than any of the hornblende ages of Georgia's Inner Piedmont determined by Dallmeyer (1978). The sample localities are further east and nearer the Charlotte belt boundary than those studied by Dallmeyer (1978) and thus may reflect a continuation of the trend in the Inner Piedmont of younger ages towards the southeast (Figure 4). The biotite ages from the xenolith and the F.lberton granite are very similar to those obtained by Dallmeyer (1978) for samples 13 (2445 m.y.) and 14 (2365 m.y.). ~hese two sample localities are each within lOkm of the Charlotte belt, whereas the Elberton samples vary from about llkm to 3km in distance from the Charlotte belt. In the only sample with coexisting biotite and amphibole, the xenolith EW-lOA, the amphibole recor-ds an older aqe than the biotite which coincides with the relationship reported by Dallmeyer (1978) for the two coexisting minerals and reflects the higher temperature at which hornblende retains argon.
The amphibole ages of the amphibolite and xenolith apparently represent the younger limit for the time of intrusion of the Elberton batholith. Had the intrusion occurred after about 2755 m.y., the release spectrum for the xenolith amphibole should have appeared disturbed and/or have given a substantially younger age than the country rock. The alternative would be that the xenolith minerals' release spectra were not obviously affected by inclusion within the granite. This would allow intrusion at about 240 m.y. ago. The xenolith, however, does appear recrystallized and the amphibole is actinolitic, a col':l.position commonly associated with contact metamorphism. ro properly address the effects of inclusion within the Elberton granite on the xenoliths, a detailed 40Ar/39Ar study of xenoliths in varying stages of assimilation must be made. Until such studies are completed, it seems most reasonable to conclude that the intrusion occurred before the country rocks cooled to the temperature
72

f W-4, amphibole

EW-40, biotite 251111.,.

278m] fW-IOA,omphlbole
::~
260 LL I I I I..J

-...J
w

EF:-,.~. ...., EW-31,biotite ~ rtJ I

SC-I, biotite - - ----,

s-:.26
~ 2401f _ J _::L J I

q

<(

E W-IOA,biotitt

E W-10. biotite 231M.J,

0

5

10

I

I

I

KM

Figure 3.

age spectra of hornblende and biotite from the Elberton batholith and nearby Inner Piedmont. Data plotted as age versus accumulative percentage of ~Ar released. All
biotite spectra have coordinates shown in lower left. Hornblende spectra require older ages and are so labelled.
Estimated uncertainties indicated by width of bar. Totalgas ages listed on each spectrum. Data and diagram from Hess, 1979.

84

83

0

10

I KM I
34

Winder 0

A then 0

BATHOUT
I
/ / /
/ /

INNER PIEDMONT

~ ~ '-'7S JBTON MOE UNTAIN

.......

~

GRANITE

.;:..

.261m.y.-B

Monroe 0

/
/
I
~36m.y.-B /

14

/

/

/

/.J~
h_-<,0~
-<~~
/~Q

284m.y.-B
9308m.y.-A

10
3 0 0 m . y . - A
II

12250m.y.-B

/

/

/

/

/

/

/

/

/

/

CHARLOTTE

/

/

/

/

BELT

/

/

/

/

3330'

/ 244m.y.-B I

// /

84

13

/TOWALIGA /
1

83

/FAULT ZONE

333o'

Figure 4. Generalized geologic map of the Georgia Inner Piedmont showing selected ~0Arj39Ar total-gas ages from this study and Dallmeyer (1978). Compiled from: Geologic Map of Georgia, 1976; Griffin (1971) Hatcher (1972). Biotite ages indicated by B. Amphibole ages by A. Dates for numbered sample localities
and Stone Mountain from Dallmeyer (1978). Dates from the Elberton Granite and adjacent Inner Piedmont from Hess (1979).

required for argon retention in amphiboles since the amphibole ages are approximately the same, thus making the age of intrusion older than about 2755 m.y.

COOLING HISTORY OF THE ELBERTON AREA

From the geochronological data summarized here and in the preceeding article, we can conclude that the Elberton granite was intruded about 320 to 350 m.y. ago. The surrounding country rocks must have been at temperatures above 450 to 500 C at that time, and remained hot for some time.

Within the Elberton area, there are some retrograde metamorphic assemblages formed along shear zones which apparently predate the intrusion of th Elberton pluton (see first article in this volume). These assemblages are of the upper greenschist grade and suggest that temperatures around 550qc were obtained prior to, or ~t the approximate time of, intrusion. We therefore are able to reconstruct the cooling history of the Elberton area from the time of the Elberton
intrusion. Such a thermal model is summarized in Figure 5.

Although our time-temperature curve is quite closely

I ,

constrained by data from the intrusion of the Elberton and

younger, the earlier portions of the curve are only relative.

We know that the majority of the deformation preceeded the peak

of metamorphic conditions from cross cutting relationships

involving peak metamorphic minerals and anatectic veins.

Further, we know that the development of the northwest fabrics

(S2) post-dated peak conditions, and was in turn followed by

cuctile fabric development along regional faults.

Peak

metamorphic conditions may have occured any time between about

480 m.y. to about 380 m.y. ago. When the Elberton was intruded

the temperature had to have dropped to around ssooc or so where

it stayed until cooling began about 300 to 280 m.y. ago. The

argon retention ages for hornblende and biotite then fix the

cooling curve, which must reach low temperatures by about 200

to 220 m.y. ago when diabase dikes intruded and quenched

against the Elberton with some having extremely fine-grained,

almost glassy appearing, borders.

This curve suggests two periods of dramatic change in the thermal conditions following peak metamorphism. First, the drop from peak conditions to about 550C occured just preceeding the intrusion of the Elberton. If peak metamorphism was about 400 m.y. ago instead of the older date shown, the cooling rate must have been about 150C in 50 rn.y. If the older age shown is chosen for peak metamorphic conditions this rate is less. Such a rapid decrease in temperature is hard to modei by uplift alone, but could be accomplished by a decrease in thermal gradient of about 10C/km.

75

Elberton Area

700
600
T
(oC)
500

Dl

02

~g ,.., .c

I

\

(f) ~

c:
.Q
en
.::.:.:.J
+c-:

I
I
I

\ +0 ::::J

c:
+0...-.

Q)

\l\ Q

..0

w

!'

I

\

....
<(
I

I

~

I

400

I
I

I

I

300

I

too pC

0 sD c

600 soCZ\ge m.y.b.p.)300

0'
.5
+-

::::J
~

en

~

Q)

++...--.

~
Q

([J

l ceQn) .c.0

Q

~ '
~
<(
I
~..

R 200

Figure 5. Generalized time-temperature curve for the Elberton area. Note that the early part of the curve is only relative as ages of peak metamorphism are not well known for this areq.
76

The second dramatic drop in temperature occured between 300 and 220 m.y. ago as indicated by the argon retention ages_ of hornblende and biotite. The extreme homogeneity in age of argon from biotite across the body suggests the whole body was cooling together and fairly rapidly. This dramatic drop in temperature may mark not only the start of vertical tectonics which uplifted the area up to 15 km., but may mark the turning off of the heat flow from the underlying mantle, an effect of the cessation of subduction processes in this region.

SUMMARY

Our geochronologic studies of the Elberton thus

suggest that it was intruded about 320 to 350 m.y. ago but

remained above 500C until after 300 m.y. ago. We suggest that

the area experienced a rapid drop in tempe~ature between 300

and 220 m.y. ago, perhaps caused by the cessation of subduction

processes in the proto-atlantic ocean which followed closure of

this ocean basin. Since granitic intrusion in the Charlotte

and Slate belts began again around 300 to 270 m.y., it may be

that the zone of high heat flow moved eastward just prior to

I I

its termination.

I

Further geochronological studies are required on the

metamorphosed granites of the Inner Piedmont before the

earlier thermal history of this block can be substantiated.

The foliated granites which the Elberton intruded post-date the

Dl key

deformation, to fixing the

but ear

pre lier

-

dthaet erman~2

,

and histo

t r

he y.

r

e

f

o

r

e

are

a

valuable

77

REFERENCES CITED

Butler, J. R., J. R., 1972, Age of Paleozoic metamorphism in

the

Carolinas,

Georgia

and Tennessee southern

Appalachians. Amer. Jour. Sci., v. 272, p. 319-333.

and P. D. Fullagar, 1978, Petrochemical and geochronological studies in the southern Appalachians: II. Leucocratic adamellites of the Charlotte belt near Salisbury, North Carolina. Geol. Soc. Amer. Bull., v. 89, p. 460-466.
Dalrymple, G. B., and r-1. A. Lanphere, 1974, 40Ar/39Ar age
spectra of some undisturbd terrestrial samples: Geochim. et. Cosmochim. Acta. v. 38, p. 715-738.

Dallmeyer, R. D., 1978, 40Arj39Ar incremental release ages of

hornblende and biotite acros~ the Georgia Inner Piedmont:

Their

bearing

on late Paleozoic-early Mesozoic

tectonothermal history.

Amer. Jour. Sci., v.

278,

124-149.

Fairbairn, H. w., w. H. Pinson, P. M. Hurley, and R. F.
Cormier, 1960, A comparison of the ages of the coexisting
biotite and muscovite in some paleozoic granite rocks. Geochi~. et Cosmochim. Acta, v. 19, p. 7-9.

Fullagar, P. D., and J. R. Butler, 1976, Petrochemical and geochronological studies of plutonic rocks in the southern Appalachians: II. The Sparta granite complex, Georgia. Geol. Soc. Amer., Bull., v. 87, p. 53-56.

and

, 1979, 325 to 265 m.y. old granitic

plutons in the Piedmont of the southeastern Appalachians,

Amer. Jour. Sci., v. 279, p. 161-lRS.

Griffin, V. s., 1971, Inner Piedmont belt of the southern
crystalline Appalachians. Geol. Soc. Amer., Bull., v. 82, p. 1805-1898.

Grunenfelder, M., and L. T. Silver, 1958, Radioactive age dating and its petrologic implications for some Georgia granites (abs.), Geol. Soc. Arner. Bull., v. 69, p. 1574.

Hatcher, R. D., Jr., 1972,

Developmental model for

Southern Appalachians, Geol. Soc. Amer., Bull., v. 83, p.

2735-2760.

Hess, J., 1979, Geochemistry of the Elberton Granite and the
geology of the Elberton west quadrangle, Georgia. M.s.
thesis Univ. of Georgia, Athens, 193 p.

Long, L. E., J. L. Kulp, and F. D. Eckelmann, 1959, Chronology

78

of major metamorphic events in the southeastern United States. Amer. Jour. ~ci., v. 257, p. 585-603.

Fa Mose, D. G., and D.

wrgner, 1980, Parallel Rb-Sr whole-rock

isochrons and 0/ 0 data from selected Appalachian

plutons. Geol. Soc. Amer. Abs. with programs, v.l2, p.202.

Pinson, W. H., Jr., H. w. Fairbairn, P. M. Hurley, L. F.
Herzog, and R. F. Cormier, 1957, Age study of some
crystalline rocks of the Georgia Piedmont. (abs.), Geol.
Soc. Amer. Bull., v. 68, p. 1781.

Steiger, R. H., and E. Jager, 1977, Subcommission on geochronology: Convention on the use of decay constants in geo- and cosmochronology. Earth and Planetary Sci. Letters, v. 35, p. 359-362.

Whitney, J. A., L. M. Jones, and R. L. Walker, 1976, Age and origin of the Stone Mountain .granite, Lithonia district, Georgia. Geol. Soc. Amer., Bull., v. 87, p. 1067-1077.

York, D., 1966, Least-squares fitting of a straight line. Canadian Jour. Physics, v. 44, p. 1079-1086.

79

Magnetization of the Elberton Granite

Brooks B. Ellwood, Department of Geology, University of Georqia, Athens, Geor9ia 30602

ABSTRACT

The anisotropy of magnetic susceptibility,

bulk

susceptibility, and remanent magnetization of 25 sites

within the Elberton granite and the initial susceptibility from

17 southeastern granites have been determined. Estimates of

tectonic rotation of the Elberton granite, based on field

evidence, seismic nata, and the assumption that the magnetic

fabric was initially horizontal, indicate tectonic tilting of

the body about a N - NE strike iiown to the southeast by from

30 to 350. The data are consistent with the Elberton being

emplaced and acquirinq a reversed Magnetization at 350 m.y.

B.P.

PJTRODUCTION

The 350 m.y. old Elberton granit~ (Ellwood et al., 19RO) is a large, fine qrained pluton economTcaily
iMportant as monument stone. A number of studies have been performed on this body, including analysis of the induced (Ellwood and Whitney, 1980) and remanent (Rllwood et at., 1980) nagnetic properties in the granite. The results of this work are summarized helm.,r.

Magnetic Susceptibility. Magnetic susceptibility (K
or x ) is a symmetrical second-rank tensor relating an
induced magnetic moment to an innucinq maqnetic field (Nye,
19h9) and is quoted in ter~s of the susceptibility per unit
volume K, or the susceptibility per unit mass x in cgs or SI
units.

For any rock specimen, an orthogonal system of

maxi~um, intermediate, and minimum

eigenvectors

and

eigenvalues (directions and magnitudes) of the susceptibility

tensor coefficient is derived from the 3 x 3 susceptibility

tensor matrix of direct measurements which are generally

netermined for a coordinate system unrelated to the coordinates

defined by the principal axes. Anisotropy of magnetic

susceptibility (AHS) data are conventionally expressed as

triaxial ellipsoir1s whose Major, intermediate, and minor axes

represent the corresponding maximum, intermediate, and minimum

80

susceptibility directions, with magnitudes respectively _(magnitude ellipsoid of susceptibility is given as

Kb, and Kc, 1969). Bulk

For igneous rocks it has been experimentally and theoretically shown that the total susceptibility ellipsoid shape expresses the preferred alignment of ferrimagnetic mineral qrains within the sample (for example, Khan, 1962).

Remanent ~1agnetism.

Since the Elberton granite is

relatively fine gra~ned and uniform throughout the area in

which it outcrops, a large ntmher of widely spaced,

mineralogically similar and cogenetic samples are available.

This has proven to hP- an ideal setting for studying the RM of

this granite in terms of the genetic implications of the unit.

METHODS

Field. A total of 290 cores were taken with a gasol~ne-powered portable drill from 24 quarries and one blasted road cut in the Flherton Pluton and oriented using a Brunton Compass. Core sites were selected which avoiQed a) rocks within 50 em of surfaces cut hy a flame cutting tool during quarrying, b) \'Teathered surfaces, and c) any unusual features such as-xenoliths, peqrnatites, or sheared surfaces. Cores were separated laterally hy an average distance of greater than 1 meter.

Laboratory. ~h~ cores, of

2.5 ern diameter, were

s1iced into lenqths of

2.2 em. The natural remanent

magnetization (~RH) was measured for all samples using a

Schonstedt spinner maqnetometer.

To isolate and identify the characteristic or primary

magnetization in pilot samples,

a

step-wise

a.f.

demagnetization was performed on the first sample taken from

all but one site. Samples were demagnetized, after NID1

measurement, at lOmT steps to llOmT for the first six sites,

and at 20rnT steps for reMaining sites. The data were then

analyzed using stan~ard techniqt~s, including J/Jo plots, and

declination-inclination equal area. plots. The AHS was measured

for six samples fro~ each site usinq an automated low-field

torque magnetometer for which the precision has been determined

(F.llwood, 1978a).

Due to the wide apparent range of

coercivities in the granite, the remanent magnetization (RM) of

remaining samples \-lere measured after demagnetization to 40mT

and 80mT.

81

Several Curie temperatures were determined with a thermomaqnetic balance of chips placed in a 300mT magnetic fi~ld. - Measurements were made in vacuuo. Curie temperatues ranged from 564-SSOOC, indicating that magnetite is present. No evidence of ilrneno-hematite or ferrianilmenite Curie temperatures were apparent. This may be due to the nominance of the maqnetite saturationremanence. Examination of polished sections from several sites reveals that the dominant iron oxide is an ilmenite-hematite exsolution pair. Secondary to this is very fine grained magnetite, with a few larger euhedral magnetite grains present, some contained within clusters of mica grains.

DISCUSSIOI'l OF ELBERTON GRANITE ~-1AGNETIC RESULTS

Bulk S~ceetibility.

A distinct band of relative

susceptibility h1ghs occupies the central portion of the

granite (Figure 1). It has heen shown, for low K, thC!t K is

approximately linearly related to volume percent magnetite

content (Nettleton, 1976). Therefore, this relationship

indicates an .. approxi:mate order of magnitude difference in

magnetite content between the central portion of the body and

the NW and SE. sections of the granite (Figure 1).

Magnetic Fabric.

The

orientation of magnetic

foliation defined by within r;i te AMS distributions is given

in Figure 2. While a distinct macroscopic foliation exists in

the Elberton near .thE" margins of the pluton, it disappears

rapidly just a few meters into the body. Where this foliation

is observed, however, it mirrors the magnetic foliation.

Trends in the magnetic foliation are similar throughout the

pluton inoicatinq an extrene uniformity which was not expected

for a granite as large as the Elberton ( >500km2 outcrop).

Tectonic Rotation. Field and AMS data are consistent

with the hypothesis that the present exposure of the

Elberton Pluton represents the original top of the body

(Ellwood and ~fuitney, 1980). If this is true, it should follow

that the near-margin foliation in the hody will mirror the

boundary orientation: since the upper boundary is

sub-horizontal, the observed magnetic foliation is expected to

be nearly horizontal. For this to be the case, it is necessary

that the site AMS inclinations of each magnetic foliation plane

be similar in magnitude and direction over the entire extent of

the body. Systematic variability from one side to the other

would indicate a change in orientation of the contact, but such

a change is not observed.

Magnetic foliation plane

inclinations are not horizontal however, but nip to the east by

i2.4, on average, about a .strike of 12.10 (Figure 2) possibly

inr=!icatlve ofa physical rotation of the body at sometime after

emplacement. Further, sketchy field evidence in conjunction

with published studies showinq uplift of the Inner Piedmont

core (Dallmeyer, 1978), also suggests that the Elberton pluton

82

.-----------------~----~8~3Too~Io~-------------------------8,23445o115'
0

~~0\ ---~.

(J\ t:?

I

\

0

5

10

KM

BULK SUSCEPTIBILITY
Figure 1. Bulk susceptibility (K) variability within the Elberton pluton (from Ellwood and Whitney, 1980).

83

....- - - - - -...... ~ --~a~~- od--------~..,34a~41~ 51 N. E. Georgia 26b ELBERTON GRA ITE v. ~ 33
INNER PIEDMONT

CHARLOTTE AND SLATE BELTS

~32

0

5

10

KM

MAGNETIC FOLIATION

Figure 2. Magnetic foliation with the Elberton pluton (modified from Ellwood and Whitney, 1980).

84

may have experienced a sliqht amount of tectonic rotation about an approximate NE - SW strike. From these observations, it is inferred that the Elberton Pluton may have undergone a slight amount of tilting about a N to NF strike, having been rotated down to the east by possibly as much as 35.
COCORP seismic data (Cook et al., 1979) exhibit reflectons imrnediatel6 below the Elberton granite which are inclined at .. 30 to the SE (Figure 3: Dr. Donna Jurdy, personal communication). These data are consistent with the AMS data, and therefore reinforce the concept of a physical tilting of the pluton about a N - NE strike down to the SE by .. 30 - 35.
EMplacement. King (1966) has suggested that AHS Ka alignment results from flow. When the magma behaves as a viscous fluid, as is the case for basaltic magmas, a magnetic foliation is developed parallel to flow planes with K- axes containen within the plane and aligned normal to the flow direction. Such a result has been reported by Khan (1962) for basaltic lavas and dikes, and by Ellwood (1978b) for Icelandic dikes. However, King (lq6f;) suggests that deep seated intrusives (granitic rocks), durinq later stages of flow, behave as plastic fluids in which rigid particle (magnetic grain) alignments result from plastic strain, with long axes of such grains oriented parallel with the direction of maximum strain (the plastic flow directin). Therefore plastic flow or deformation during emplacement of granites (produces) magnetic grain lonq-axis clusters parallel to the flow direction. Mean K azimuths are plotted in Figure 4 for sites within the Ef~erton West 71/2' quadrangle. Also plotted (Fiqure 4) are ~ 0 contours which mirror AMS orientations. It is inferred that both the isotopic and magnetic data result from flow during emplacement of the granite.

Rt~ Polarities.

Hean

directions

of

Rr1,

and

correspond1nq poles for all sites before a slight tilt

correction has been applied, are reported elsewhere (Ellwood,

et al., 19RO). Several sites, in the ~~J portion of the body,

are normally magnetizer. but MOSt are reversed. Further, some

saMples within sites exhibit opposite polarities. Two

explanations can he given for the existence of both normal and

reversed polarities within the same body, namely: (1), the

geomagnetic field reversed during cooling of the pluton with

both polarities beina recorded, or (2), self-reversed samples

(possibly whole sit.es) are present. l-7e interpret the

non-symmetrical, within sit~ variations of polarity which exist

in the Elberton to indicate that, (a) a self-reversed weak ID1

is carried in the ilmeno-hematite, but (b) becomes apparent

only when the magnetite content is so low (in the NW) that the

R~ carried in the ilrneno-heMatite dominates. This suggests

that the geomagnetic field was reversed when the Elberton

granite acquired its magnetization and the normally magnetized

85

COCORP GEORGIA LINE 1

0

5

KM

1400 1450
I I

TOWALIGA-HARTWELL

FAULT ZONE

1500 1550 I 16!50 I

I

_I

I+ I ~o.o

ELBERTON GRANITE

MIDDLETON-
LOWDNESVILLE FAULT ZONE
l

1.0
2 .0
3.0
..;x... ::.~~~~~-z.--::;. .i:l~~~~f'~~"'"~~~~~~~:~~~~-;;~ co
(j'\ ~.:~~::~~.~~~~~.,:;~;-~~~~~~~;r--~~~~j:~
4.0
"5.0
6.0
7.0
8.0
9.0
Figure 3. COCORP seismic data (modified from Cook et al., 1979) in the region of the Elberton granite.

ELBERTON WEST QUADRANGLE, GEORGIA MAGNETIC, LINEATION

I
/
I /
/
/
/
/
(:/
~
/
/
/

<0.

Nt-

co
<0
}

7 I

0

2

.:...:.-....._.:........:....:.._..:: :::::~~\\\:.

34o~-------------------K_M_________________.:_/.0_)._.~;:. 34o

8300'

8354.5'

Figure 4. Magnetic lineation (Ka) directions in the E!erton West
Quadrangle. Also plotted are contours of o 0 data (Wenner,
this volume).

87

samples result from a self-reversing mechanism.

Baked Contact Stability Test.

Examination of one

quarry (site 4 of Ellwood et al., 1980) reveals the

presence of a diahasic intrusion into the granite and thus

provides an opportunity for a baked stability test. In

prin-ciple', an intrusion into the pluton should have reheated

the granite immediately adjacent to the dike/granite contact

and upon cooling, the reheated granite should have acquired the

geomagnetic field direction at the time the dike was intruded.

Thus, if the normal polarity component does not reside in the

baked zone of the granite, it would indicate that this

component was acquired before intrusion of the dike.

The diJce has acquired a reversed and stable m~
direction, as have two qranite samples in close proximity to the dike. Granite samples farther from the contact yield the normal nM direction, consistent \-dth the other non-reheated samples from site 4. These data indicate that the reheated granite has acqnired a remanence which appears to be stable over ~eologically long time priods. If the unheated granite polarity at site 4 results from a self-reversed magnetization, then the reversed polarity reheated granite samples would appear to he anomalous. It is inferred therefore, that either (1) the reheated qranite no longer exhibits a self-reversed magnetization, possibly due to a change in magnetic mineralogy durinq reheating, or (2) that the high intensity of magnetization of the high bl.ocking tPnperatnre rliabase, in close juxtaposition to the two qrani te samples, may dominate over the lower intensity, high blocking
temperature hematite rnaqnetization assumed to be responsible for the self-reversal.

It is interesting to speculate that while the dike has not been independently dated, it appears very fresh in thin section, and yields a single site paleomagnetic pole coincident with Irving's (1979) apparent polar wander path at about 200 m.y. This may inr'l.icate that the dike intruded the Elberton qranite in the late Triassic ano is one many such dikes found in rocks along the eastern marqin of North America.

Paleomagnetic Aqe Estimates. Figure 5 is a northern

hemisphere equal area projection on '-.rhich is plotted the

Apparent Polar Wander Path for North America of Irvinq (1979)

for the period 350 m.y. to 150 m.y. B.P. Also plotted are two

pole positions, with confioence limits, for the Elberton

pluton.

Point A renresents the Elberton pole before a

correction for physical rotation of the body.B is that pole

after a structural correction based on the N1S and the

assumption that the magnetic foliation plane was initially

horizontal and has since heen rotated into the present

orientation. Since confidence limits for A and B overlap and

given a 10- cone of confidence for the 350 m.y. paleopole of

Irving (1979), these data are consistent with the 350 rn.y.

88

180

270

90

0

Figure 5.

North American Polar Wander Path of Irving (1979) and the two magnetic poles for the Elberton; A, before the AMS tilt correction and B, after AMS tilt correction (from Ellwood et al., 1980).

89

Rb/Sr whole rock age. ~ and B also overlap with the confidence limits for a 245 m.y. paleopole. A 245 m.y. age, therefore, is not excluded by interpretation of the paleomagnetic data.

Point B (rotation based on the assumption of an initially horizontal magnetic foliation plane) is located farther from the polar path than is point A (no structural correction). Since Irving's (1979) 350 m.y. paleopole position is poorly defined, the increased separation of B from the polar path may only bP- fortuitous. However, if one assumes that the 350 m.y. paleopole position reported by Irving (1976) is correct, then a physical rotation of the Elberton of - 15 down to the NW about a E-NE strike is required to brinq A into coincidence with Irving's paleopole. While the axis of rotation is not too different fron that inferred from interpretation of the AMS and seismic data, the tilt direction is in the opposite sense. Both tilt magnitudes are relatively slight.

Elberton Genetic Model.

The 350 m.y. old Elberton

granite, s1.tuated along the COCORP seismic traverse, can

ideally provide an indication of the age of emplacement of the

Southern Appalachian allochthon.

If the pluton

is

autochthonous, then the age of the intrusion places a minimum

limit on the thrusting event of 350 m.y. n.P. An autochthonous

Elberton pluton is consistent with the apparent in situ

acquisition of a characteristic remanent magnetiZatiOn;

although a thrust model for the Elberton is not excluded by the

paleomagnetic data. An autochthonous model, however, is also

consistent with the structural relationship between the

Elberton and

the

age

of

thrusting

along

the

Hiddleton-Lowndesville fault as prf'viously discussed (Ellwood,

et al., 1980). Further, Wenner and Whitney (1979) that an excellent correlation exists between

havEJ.. the 8

shq_~
0/ "~

patterns exhibited by these late orogenic Piedmont plutons and

the regional gravity data of Lonq (1979) which define deep,

subcrustal lithologies. ~his consistency suggests that any

large scale thrusting involving the Piedmont probably predates

intrusion of the Elberton granite. Any tilting that may have

occurred has entailed only a very slight rotation about a

horizontal, generally NE-S~7 axis, anrl. is compatible with the

Elberton pluton being containe~ within a hinge block separating

a rising Inner Pie0mont (nallmeyer, 1978) from the relatively

stable Charlotte-Slate helts. In Figure f', a model is

presented which ilJ ustrates hm<~ the uplift of the Inner

Piedmont may be absorbed by slight rotation of the Elberton

hinge block and hy shP-arinq along the pre-existing

Middleton-Lowndesville zone and the concurrently developing

Hartwell extension of the Towaliga fault zone. l~ile sense of

rotation may have been either to the SF or NW with the

magnitude being very small, the preferred tilt, based on both

the seismic and N-1.8 data, is to the SJ':.

Significance for Southern Appalachian Tectonics.

It

90

NW

RELATIVE UPLIFT

SE

t

1t

0

1', - LATE PALEOZOIC SURFACE

~~ ,'I

:..~... :,. f::.

~-EROsioNAL

__.,.
suRFACE

r"U

\

~ llo... t~"~ ...... ~~..,.,c.
........__ . . . . . . 1 " ..

\

.1....0.

o~"<.--_.... / ~ t _P_R_E_.S__ E_.N_1..,_,

fT1

'

CJ) ~

................. .",,".' ..... ..,.............. ",.""' .c.....7. >

,.~

~-

~

ELBERTON

m :I:'\
I' )::>0 'I
N0 t~'
zm,,'
\
I
INNER PIEDMONT L)

.AP~.L> :U._TAO,. 1N' ~
I

~ ~

~+~

\~
~ ~

~~~~

(:
0~~

0~ ~~

CHARLOTTE-

t UPLIFTED BLOCK 1 ELBERTON HINGE BLOCK 'SLABTLEOCBKELT

Figure 6. Elberton hinge block model (see text; modified after Ellwood, etal., 1980).

is inferred from these results that no major, post 350

m.y. compressional event has affected the pluton, either

through physical translation or chemical alteration. This

suggests that the Southern Appalachian region of the North

American continent has not been involved in a major continental

collision in the last 350 m.y. Hercynian deformation ( -270

m.y.) is present, however, in rocks farther to the southeast

(Snoke, et al., 1979). The lack of 350 m.y. and younger

deformation in the Inner Piedmont, and presence of 270 m.y.

deformatin to the southeast is more consistent with strike-slip

motion of the North American plate past some other continental

houn0ary during the Carboniferous and Permian periods than with

nirect

collision

during

the

Permian.

Continental-reconstructions which are consistent with such

motions have been presented by Irvinq (1979) and Smith et al.

(1979). Thus, if the Inner Piedmont is part of a major

allochthonous sheet (as suggested for example by Cook et al.,

1979), major episodes of movement along this sheet must have

ended during the late Devonian.

DISCUSSION OF REGIONAL HAGNETIC RESTTLTS

Hagnetic Susceptibility. The magnetic susceptibiJ.ity

in several late orogenic granites for which oxygen

isotopes have heen determined (Wenner, and Ellwood ann Henner,

manuscripts submitted to Earth. Planet. Sci. Lett.) has also

been evaluated. These data are platte~ in Fiqure 7 relative to

individual granite mean 018o values. It is apparent that there

is a siqnificant inverse relationship between susceptibility

and
in s-

0

1Bo. type

Since it or sialic,

has been shown that magnetite is depleted crystal-derived ~ranites and enriched in

I- type, or more mafic derivative rocks (Ishihara, 1979), and

since susceptibility can he related directly with magnetite

content (Nettleton, 197n), this magnetite- 01Bo relationship (Figure 7) is not too surprisinq. High magnetite rocks

(magnetite series granites) have susceptibilities which are > 1

x lo-4 whereas low magnetitP. content granites (ilmenite series

granites) have snsceptibilities '"hich are < 1 x 10'""4. Included

within the later group is the Elberton qranite (EB in Fiqure 7)

which, in general, has low magnetic susceptibilities, but has

intermediate in ol8o values.

ReManent ~~agn e tism. A useful result of this nagnetic susceptibility \vork relates to the ..,.lithin-site precision of remanent magnetism (R~~) dirPctions in th~ qrani tes. T"lliile no sites with mean susceptihilities > 1 x 10- have been found which exhibit a coherent within-site directional RM, several units with susceptibilities < 1 x lo-4 exhibit qood within-site ru1 clusters. The best example of these is the Elberton granite. This susceptibility relationship provides a useful and easily measured indicator of possibly more meaningful ru1 results when one is limited hy the number of samples which can be collected in the field or processed in the laboratory.

92

I ,. I"" - - J ., - '- 1 r ~: ...

.,

i. - r.. :

"t f" .J

~

1

r
"'< "'"""' ,..

~

,..

c.

t',.

~ "",ll~ '<~ ~

W 'f ,..

:

..,. .,

~ j 4 ,

.l

"' ",. r "' ..,

",. 4'11 .,

JoDB A ., "> " ,.
V,.

~...,

.,

,...to-.,, :. "' ..., "".,..., .,c , ,. ,. r; : ,. "r .,. ~ , ,.~v

t"
~,..

"'"'
~ ,.
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" '"

~ r

" , ,.. ~, .,. ,. _., .J "

c. .. 4 <
..i ,) ,..

v ,. , ~ '"r

.,

,. v 4- r 11o r ..._ ( ot ., .,

~
1

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.io ._. -..

>r-

,.. (" ~ " ,. <t "'

,.,..

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'3o

a.""r-.,a.. .,.,. v

..,

ol "'1 V
.. r

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ttl

:1- ,
...........
'I

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:> ,. r"' "" C
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1..0
w

- lo-4 (/)
-C" (..)
>-
:-cb-I:o-J: lo-5
w

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411
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;~,.~~..<.,..Al.L...I,.,'.T." .""<'c."-,-a~.~,~.t",oc.."rc-.....<".'",...c"_J'.,c".~~ .r,. '.,),l,o.".c....","~'f,. ,C.,'',.",."..,,.L""...

, r~ ",.
,.,. ... ", r""r,.,,.

.

) ~ <to. oJ <t"'r., ., v.l
.,r&.
.,J ..... "'

~.,_.. ,~.Jo

-~



,.tJt(

.,
>,'-:"'~c.

14L"1" ~t,'.":..,, .~.;)o

''
...

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"

r,..a

"'., c,. ..1&... >
,.
,..,..,
..1 .. r-
1:

'",Jo
.""' ,
",..., ;,""~
c "<
'(.,
.,.,,....~.

)o ..C.

... :

,"'o4C,) ::~"':.a."); 11"1->J,,'--,._~1'

C.~ W' L

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r.:===:f':::::::;tlr::;;:i.. ~ ~ ~ ~ or"' r- , "' )

.J. "'"" &. ..,.. < "'., "-' v .c. ., ""

.. "'".:t ,...,.. ~""~..A~; c~..,.,",.,.c,..,__ ~LHr.. """r "'~""

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1'"-tf"
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r

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,. ..1

<,~:'"~'c'."'

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(f)
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55

6.5

7.5

8.5

9.5

10.5

11.5

8180

Figure 7. Three cycle semi-log plot of initial magnetic susceptibility (in cgs mass units) versus ~18o
for 17 late orogenic granitic units in Georgia and South Carolina (from Ellwood and Wenner, manuscript submitted to Earth. Planet. Sci. Lett.). Precision bars represent one standard deviation. AP = Appling; BB = Batesburg; BN = Ben Hill; CG =Columbia; CT = Cuffytown; DB = Danburg; EB =Elberton; HB =Harbison; JS = Johnston; LH = Liberty Hill; NB =Newberry; PG = Panola; PM = Palmetto; SL = Siloam; SP = Sparta;
ST = Stone Mountain; and WN = Winnsboro. The granites are named after the closest town name. If more than 1 community lies within the granite, then the largest r-is chosed for the name.

Susceptibility measurements can easily be made on unoriented samples obtained during exploratory field surveys, or by using portable equipment and making measurements in the field. Such studies can serve as a reconnaissance method for evaluating favorable sites for later, more extensive field sampling.

SUMMARY

Based on induced and remanent analyses for samples from 25 sites within the Elberton Pluton, the following major points can he concluded.

1. It is assumed that the magnetic foliation in the body was initially horizontal and was later tilted downward by - 30-340 to the SE at about a N-NE strike.

2. The body acquired a stable magnetizaton in the

presence of an anoarP.ntly reversed geomagnetic field. This

I

field direction has been nccurately recorded within the granite and is recoverable.

3. Some parts of the pluton are normally magnetized. The preferred interpretation is that this is due to the granite exhibiting a self-reversed magnetization, which results from relatively low magnetite content, and a self~reversed magnetization, carried in . the dominant (by volume) ilmeno-hematite magnetic phase present in the rock.

4. Th~

Elberton

has a

t-'lississippian paleomagnetic acre

although, a late Permian magnetic

m.y. B.P.) also fits the data.

late Devonian-Early

( .. 350

m.y. B.P.),

age (approximately 245

5. While two possible tilt corrections might be inferred from the data, both are of slight magnitude and are not requireo to render the Elberton pole consistent with established apparent polar wander paths. Therefore, it is concluded that a major physical relocation of the granite is not necessary to produce a paleomagnetic age consistent with the available Rb/Sr whole rock age of 350 m.y. B.P.

6. It is concluded from these data that no major continental collisional event involving the Southern Appalachian region of North America has occurred in the last 350 m.y.

Additionally,

the maqnetic susceptibility in 17

granite bodies is inversely related to the ol8o

variability.

These r~sults are consistent with the

interpretation that s- type granites in the southern

Appalachian Piedmont .generally yield low magnetic

susceptibilities (susceptibilities < 1 x lo-4) while I- type

granites are generally characterized by high susce~tibilities

94

(> 1 x 10~4). In general, I- type granites appear to exhibit
low remanent magnetic nirectional prec1s1on, while some s- type
granites nispl~y good directional clusters.

ACKNOWLEDGF.MENTS
This research was supported by Foundation grant number EAR 79-19911.

~ational Science

95

REFERENCES
Cook, F. A., Albaugh, D. s., Brown, L. D., Kaufman, s., Oliver,
J. E., and Hatcher, R. D., Jr., 1979, Thin skinned tectonics in the crystalline southern Appalachians; COCORP seismic-reflection profiling of the Blue Ridge and Piedmont, Geology, 7, 563-567.
Dallmeyer, R. D., 1978, 40Arj39Ar Incremental Release Ages of Hornblende and Biotite Across the Georgia Inner Piedmont: Their Bearing on Late Paleozoic-Early Mesozoic Tectonothermal History: Amer. J. Sci., 278, 124-149.
Ellwood, B. B., 1978a, Measurements of anisotropy of magnetic susceptibility: A comparison of the precision of torque versus spinner magnetometer systems: J. Physics E, 11, 71-75.
Ellwood, B. B., 197Bb, Flow and emplacement direction determined for selected basaltic bodies using magnetic susceptibility anisotropy measurements: Earth Planet. Sci. Lett., 41, 25A-264.
Ellwood, B. B., and Whitney, J. A., 1980, Magnetic fabric of the Elberton granite, Northeast Georgia: J. Geophys. Res., 85, 1481-1486.
Ellwood, B. B., Whitney, J. A., Wenner, D. B., Hose, D., and
Amerigian, c., 1980, Aqe, paleomagnetism, and tectonic
significance of the Elberton granite, Northeast Georgia Piedmont: J. Geophys. Res., in press.
Irving, E., 1979, Paleopoles and Paleolatitudes of North America and speculations about displaced terrains: Can. J. Earth Sci., ln, 666-694.
Ishihara, s., 1977, The magnetite-series and ilmenite-series:
Mining Geology, 27, 293-105.
Kahn, ~. A., 1962, The anisotropy of magnetic susceptibility of some igneous and metamorphic rocks: J. Geophys. Res., 67, 2873-2895.
King, R. F., 1966, The magnetic fabric of some Irish granites: Geol. J. (Liverpool), 5, 43-66.
Long, L. T., 1979, The Carolina slate belt- Evidence of a continental rift zone: Geolo~y, 7, 180-184.
Nettleton, L. L., 1976, Elementary gravity and magnetics for geologists and seismologists: Hono. Soc. Explor. Geophys. 1, p.l21.
96

Nye, J. F., 1969, Physical Properties of Crystals, 322 p., Oxford University Press, London.
Smith, J. W., Wampler, J. M., and Green, M. A., 1969, Isotopic dating and metamorphic isograds of the crystalline rocks of Georgia: Georgia Dept. Mines, Mining, and Geol. Bull., 80, 121-139.
Snoke, A., Kish, s. A., and Secor, D. T., Jr., 1980, Deformed
Hercynian granitic rocks from the Piedmont of South carolina: Am. J. Sci., in press. Wenner, D. B. and ~'lhitney, J. A., 1979, Oxygen isotope compositions of Hercynian age granites in the southern Piedmont and their relationships to subcrustal lithologies and structures: Geol. Soc. Amer., Abstracts with program, 11, 538.
97

Tectonic Development of the Elberton Area:
Including an Interpretation of the COCORP
Reflection Profile
James A. Whitney, Brooks B. Ellwood, John c. Stormer, Jr., and
David B. Wenner, Department of Geology, University of Georgia, Athens, GA. 30602
ABSTRACT
The COCORP seismic reflection data from the Southern Appalachians is reconsidered from the point of view of tectonics in the central part of the orogen. Rather than building on the structure of the .Blue Ridge and Valley and Ridge, models are developed from tectonic information from the Piedmont province. Regional geology, structure, tectonics, geophysics, and geochemistry are considered. An internally consistent interpretation of the COCORP data is developed in which vertical movement along late brittle faults is considered. A paleo-reconstruction suggests that the Inner Piedmont may have been overlain by a continuation of the Charlotte-Slate-Kings Mountain belt at one time. These rocks have since been extensively eroded as a consequence of rapid exhumation of the Inner Piedmont during the period 350-220 m.y. B.P. Although the reconstruction is hypothetical, it does demonstrate that the COCORP data, like any other geophysical or geochemical information, is open to multiple interpretations which must be considered in conjunction with other available data.
INTRODUCTION
Interpretation of the COCORP seismic reflection profile across the southern Appalachians by Cook and others (1979) has led to a simple model for the Inner Piedmont and surrounding metamorphic belts in which they are all part of a single thrust sheet, some 200 to 300 km wide. The proposed age of this thrust is Hercynian, or about 250 m.y. ago (Cook and others, 1979). However, geologic information from the Piedmont province, including structural, geophysical, geochronological, and geochemical data is inconsistent with this simplistic interpretation of the COCORP data (see articles by Wenner, Ellwood, and Whitney and others, this volume). We propose an alternative interpretation of the reflection profile which incorporates the available geological information. The purpose of this re-evaluation is to demonstrate that the COCORP seismic data (Cook and others, 1979) is open to a multitude of interpretations, many of which may be more consistent with
98

..

available information than the single 250 m.y.

thrust

hypothesis, and to delineate one such possible interpretation

which we believe more realistic.

REGIONAL SETTING
The southern Appalachian orogen has long been subdivided into a series of metamorphic belts. These subdivisions contain metamorphic units of varying age, metamorphic grade, tectonic origins, and kinematic fabrics (for example, King, 1955; Hatcher, 1972; Crickmay, 1952). The COCORP seismic line begins in the Valley and Ridge, crosses the front of the Blue Ridge, and then skips all of the Blue Ridge province. The line starts again east of the high Blue Ridge in the continuation of the Ashland-Wedowee belt, crosses the Brevard zone into the Inner Piedmont, continues southeast with one important break in the Inner Piedmont and crosses the Carolina Slate belt and portions of the Charlotte belt to end at the Modoc line, a \vide zone of cataclasis (see Figure 1).
In this article, we will concentrate on the Piedmont section which crosses the Elberton area. Much of the regional setting has been summarized in this volume (Whitney and others, Structural Setting; Wenner, Geochemistry; Ellwood, Paleomagnetic data; Whitney and others and Ross and Bickford, Geochronology) and will not be repeated here. The approximate traverse within the Inner Piedmont is shown in Figure 2. Note that the Elberton batholith is crossed by the reflection profile within the region visited on the field trip.

COCORP SEISMIC PROFILE
The COCORP data is divided into two parts. Critical sections of both are shown by Ellwood (this volume). The first segment, a northwest line crossing the Blue Ridge front shows the Smoky Mountain thrust extP.nding below the Blue Ridge, with sediments below it. In addition, there is a sharp break below the parallel reflectors which are supposed to represent the sediments. This break is presumably the beginning of Grenville basement.
The second segment of the line starts east of the Blue Ridge, and due to the larqe separation we do not believe any direct correlation can be made across this break. Within the Ashland-Wedowee bPlt and into the Inner Piedmont there is some sort of discontinuity between 2 to 4 seconds. The nature of this break is not clear, but below it lies a featureless terrain which looks very similar to the response received from the Grenville basement further northwest. Under the first section of this profile, a series of repeated reflectors may be seen, but if these are sec'l.imentary units it must be remembered that the thrust would have to be above these

99

Figure 1. Tectonic position of the Piedmont COCORP traverse relative to the metamorphic belt of the southern Appalachians.
100

84

34

/

/

..,.,10 ...0,

10

20 miles

I

I

/ / /

0

/

/

/

/

/

/

/

/

84

Figure 2. COCORP Seismic traverse of the Inner Piedmont relative to Major lithologic units.

101

units. Under the Inner Piedmont there are few examples of

repeated reflectors, but some form of discontinuity continues

at 2 to 4 seconds. This horizon is discontinuous and

undulating and is underlain by the same featureless terrain

seen before.

Beyond the Middleton-Lowndesville zone of

cataclasis a totally different pattern is present. A seris of

reflectors, dipping to the southeast are seen, with reflectors

visible down to deeper depths. This series of features

flattens to the southeast and is unoulating with several

discontinuities where some form of vertical offset has

apparently occurred. Near the southeastern edge of the section

a weak reflector at ~ 10 sec can he seen which is presumably

the Hoho.

HTTERPRE':l:'ATIOH OF SEISMIC SECTION

Our interpretation of the seismic section (Figure 3) is based on a dowm.,rard extrapolation of the surface geology, and other regional geophysical studies such as those of Long, (1979) for gravity and Zeitz and Hatcher (1979) for aeronagnetic data.

First, we see a change in crustal structure near the southeastern edge of the Inner Piedmont corresponding to the regional gravity gradient and change in aeromagnetic pattern. ~lumerous studies have suggested that this marks a change in lower crustal structure and we believe we can see this change
in pattern within the COCORP data. The zone of demarkation appears to be nearly vertical. Therefore, the actual boundary probably follows one of the late brittle structures along this zone. The earlier cataclastic to ductile fabric has a shallower dip, and may correspond to one of the reflective horizons dipping to the southeast in the profile.

Since the Inner Piedmont contains several documented

late,

vertical,

brittle

faults, we interpret the

discontinuities in the 2 to 4 second reflector as offsets

caused by late brittle faulting. From the argon retention ages

and interpretations of Dallmeyer (1978), we know that the Inner

Piedmont rose substantially, up to 12 to 15 km, in the late

Paleozoic to early Hezozoic. Therefore, we interpret these

breaks as nonaal faults along which this vertical movement took

place. From the sharp change in argon retention ages along the

sotttheastern Inner Piedmont (Hurst, 1970); Dallmeyer, 1978) we

surmise that this bound.ary not only marks a change in lower

crustal structure, hut also a zone of substantial normal

displacement in th~ latest Paleozoic times. \le similarly

interpret the Brevard zone as a zone of multi-stage movement.

An early deformational event produced ductite to brittle

fabrics in the Brevard zone, and late brittle movement in the

same zone uplifted the Inner Piedmont relative to zones to the

northwest. The late vertical faulting we see as another break

in the 2 to 4 second reflector.

102

Figure 3. (Following page) Cross sections of the Piedmont through
the Elberton area approximately along the COCORP seismic line. A. Geologic cross section based on surface geology and the COCORP
seismic reflection profile, part of which is shown in Fig. 3 of Ellwood (this volume). Seismic data is from Cook and others (1979).
True horizontal and vertical scales. B. Reconstruction, to scale (palinspastic), at the time of intrusion of the Elberton Batholith (~350 m.y.). Note that the vertical uplift of the Inner Piedmont has been removed by depressing and rotating the blocks appropriately along the vertical, brittle, late faults. Note also that the Middleton-Lowndesville shear zone now projects over the Inner Piedmont. C. Reconstruction to scale at the time of Slate Belt volcanism (~550 m.y.). Note the change in both horizontal and vertical scales. Some distances are schematic due to uncertainties in the amount of crustal shortening involved. At this time the rocks of the Pine Mountain Belt would have been a fragment of Grenville age crust within the sedimentary-volcanic basin.
Symbols: BZ = Brevard Zone, CB = Charlotte Belt, IPC = Inner
Piedmont Core, IPF = Inner Piedmont Flank, MLZ = Middleton-
Loundesville shear zone.
103

A. Present

I 0

saI o

I

I 1500

COCORP Wegrgio Line

Inner Piedmont Belt

I 2000

d oo:...1.,.""'k"_m__,

15 km Slate Belt/Charlotte Belt

B. "'350 m. g.
Charlotte Belt/Inner Piedmont

Kings

C. ""550 m. g .

Inner Piedmont Basin IPC

Kings Mt. Belt IPF

104

TECTONIC DEVELOPMEr1T

In order to reconstruct the tectonic development of the Piedmont terrain it is necessary to remove the various stages of deformation one at a time. Since the Inner Piedmont was apparently uplifted vertically 12 to lS . km by late brittle faulting, it is necessary to move it downward a similar amount in reconstruction. When this is done, it appears that the dip of the reflectors east of the Inner Piedmont beyond the Middleton-Lowndesville zone may be due to the dragging upward of these units by vertical, late tectonic movement along this crustal discontinuity. This def.Qrmation is 9onsistent with higher metamorphic grades for Slate belt rocks next to the Middleton- Lowndesville zone. Removing other discontinuities in the subhorizontal reflectors by vertical movement of crustal blocks, it appears that the reflectors below the Inner Piedmont at 2 to 4 seconds must have been at a depth of at least 18 to 24 km prior to uplift. We prefer to interpret these Seismic reflectors as the boundary between granulite grade Grenville rocks and overlying Inner Piedmont units. We would futher hypothesize that these reflectors should be exposed in the Pine Mountain belt. It also becomes apparent that once the reflectors within the Slate and Charlotte belts are rotated to horziontal, these units project over the top of the Inner Piedmont, being separated from it by ductile to cataclastic shear zones now represented by shallow dippi.ng horizons under the Slate and Charlotte belts. The Ashland-l'ledowee belt similarly projects over the top of the inner Piedmont. Therefore, the Inner Piedmont as a whole could represent a window through a thrust of earlier age, just as the Pine Mountain belt may represent a window through the Inner Piedmont. Instead of a single fault plane, however, we would prefer to think of this thrusting in terms of several pre-existing zones of ductile shear. These, need not be of the same age, although both the Brevard and l-iiddleton...Lowndesville
zones were probably active around 350 to 400 m.y. age. Figure 3B is drawn assuming the multiple fault hypothesis. Whether, or not the thrusting is a single or multiple event some fragments of mafic material within the Inner Piedmont, such as Soapstone ridge, or the rectangular outcrop pattern of mafic rocks southeast of Atlanta (on the state geologic map of Georgia) may be klippen of the basement of the Slate and Charlotte belts~remaining on ~op of the Inner Piedmont.

Numerous

possibilities

for

alternative

reconstructions are posf;ible for the period prior to 350

m. y. ~'le prefer to hypothesi :;:e major compressional events at

about 375 m.y. corresponding the ,plate convergence suggested by

Irving (1979) juxtaposing Africa to this section of the

Appalachians. If we assume that the style of deformation was

one of thrust faulting with deep ductile zones being the

continuation of thrusts, and reconstruct the original position

of the plates, an original cross section might have looked

somewhat like Figure 3C at 500 rn.y. At this time, the

105

Grenville rocks of the Pine Mountain belt and the area underlying Columbia, S.C. would have been crustal fragments within the ba-sin in which Charlotte and Slate belt rocks were deposited. The Pine Mountain belt subsequently being joined to the main Grenville mass during later compression.
This model reconstruction explains most of the geologic, geochemical, geochronological, and geophysical data for the Piedmont province. What is not explained is the late paleozoic thin-skin thrusting in the Valley and Ridge province at the time the Inner Piedmont was undergoing uplift. There are two possibilities. First, the Blue Ridge might have originally overlapped the Inner Piedmont lying on a pre-existing thrust surface. This thrust surface may have been reactivated as a gravity slide. This model, however, does not explain why the thin-skin deformation extends so far northwestward from the Blue Ridge front. Another possibility which we currently prefer is the hypothesis that the driving force for Valley and Ridge deformation and some reactivation of the Blue Ridge movement came from the southwest and was not related to tectonics in the central Appalachian orogen. In this case, the thin-skin movement seen in the Tennessee seismic line would not be correlative with the discontinuities seen in the eastern orogen. The thin-skin thrusting could, however, be indirectly connected with deformation of the basemPn.t which could have helped drive the vertical tectonics in the Piedmont.

REFERENCES CITED
Cook, F. A., D. s. Albaugh, L. D. Brown, s. Kaufman, J. E.
Oliver, and R. D. Hn.tcher, Jr., 1979. Thin-skinned tectonics in the crystalline southern Appalachians: COCORP seismic reflection profiling of the Blue Ridge and Piedmont. Geology, v. 7., p. 5~3-567.

Crickmay, G. N., 1952, Geology of the crystalline rocks of Georgia: Georgia Dept. Hines, Mineralogy and Geology Bull., v. 58, p. 1-59.

Dallmeyer, R. D. , 1978, 4~r/39Ar incremental released gas

ages of hornblende and biotite across the Georgia Inner

Piedmont: their bearing on late Paleozoic-early N.esozoic

tectonothermal history.

Amer. J. Sci., v. 278, p.

124-149.

Hatcher, R. D., Jr., 1972. Developmental model for the southern Appalachians: Geol. Soc. Amer., Bull., v. 83, p. 2735-2760.
Hurst, v. J., 1970. The Piedmont in Georgia1 in Fisher, G. w.
Pettijohn, F. s., Reed, J. c., Jr., and Weaver, K. N.,
Studies of Appalachian Geology: Central and southern: Wiley- Intersc., New York, p. 383-396.

106

Irving, E., 1979, Paleopoles and paleolatitudes of North America and speculations about displaced terrains: Can. J. Earth Sci., v. 16, p. 666-694.
King, P. B., 1955, A geologic section across the southern Appalachians: An outline of the geology in the segment in Tennessee, North Carolina, and South Carolina: in Rinsell, R. J., ed., Guides to southeastern Geology: Geol. Soc. Amer., Boulder Co., p. 332-273.
Zeitz, I. and R. P. Hatcher, Jr., 1979, Interpretation of regional aeromagnetic and gravity data from the southeastern United States. Part !-crustal evaluation Geol. Soc. Arner. Abs. with program, v. 11, p. 219.
107

Road Log for the 1980 Geor.gia Geological Society Field :Trip
J ames A. Whitney and John c. Stormer, Jr., Department of Geology
University of Georgia., Athens GA 30602
10,8

GEORGIA FIELD GUIDE TO
THE ELBERTON GRANITE

------+ FIRST DAY ROUTE
----4>----<> SECOND DAY ROUTE
.. COCORP SEISMIC TRAVERSE
(g) STOPS

0

5

10 MILES

--=]0~i2115 f==~10 ::K::I:LJOMETERS

SILOAM

USGS 7-! 1 QUADRANGLES
BH a BUCKHEAD CA =CARLTON CR = CRAWFORD DR = DEWEY ROSE EE = ELBERTON EAST EW = ELBERTON WEST LE = LEXIN<3TON MA =MAXEYS RA =RAYLE RB = ROCK BRANCH SC =SANDY CROSS VE =VESTA
Fttl ELBERTON GRANITE

109 . ;~:.~.~- i

ROAD LOG FIRST DAY The field trip will leave at 8:00 A.M. from the parking lot just south of the Georgaphy, Geology and Speech building.

Cumulative Incremental

Mileage

Mileage

0.0

o.o

Leave GGS parking lot, turning right (Southeast) and immediately left (north) onto bridge past the end of Sanford Stadium on Sanford Drive.

0.1

0.1

Pass Sanford Stadium.

0.3

0.2

Turn right (east) onto Baldwin Street.

0.5

0.2

Pass under railroad trestle.

0.7

0.2

Turn right (southeast) onto U.S. 78.

1.8

1.1

Pass under Athens bypass, get into left hand lane.

2.0

0.2

Turn left (north) onto Winterville Road.

4.0

2.0

Pass Athens airport on right.

4.1

0.1

Bear left following Winterville Road.

6.5

2.4

Continue straight through five-way intersection in

Winterville. Road now changes its name to Athens Road.

6.9

0.4

Continue straight ahead. Road now changes its name

to Smithonia Road,

10.4

3.5

Turn left (east onto paved road at "T" intersection.

13.3

2.9

Turn left (northeast) on paved road at another "T"

intersection.

13.6

0,3

Remains of Smithonia plantation.

14.0

0.4

Turn right (east) on paved road just after Smithonia

Plantation.

17.3

3,3

Continue straight ahead across intersection with Georgia

Route 22, following sign to Watson Mill Bridge State

Park. You are now on Carlton 7~ quadrangle map.

20.5

3.2

Enter Watson Mill Bridge State Park.

20.7

0.2

Park just past covered bridge.

110

STOP # 1. WATSON MILL BRIDGE STATE PARK. Various lithologies of the Inner Piedmont-Flank are visible as pavements
below the covered bridge. Starting near the bridge, is a series of biotite gneisses showing a strong northeast foliation. Proceeding downstream a series of folds may be seen. These are somewhat anomalous for the Inner Piedmont in that they are rather planar structures in a chevron pattern. There may be two generations of northeast folds at this locality. In addition, a faint northwest foliation is sometimes visible formed by crenulation of biotite.
Two dikes cut this series of outcrops. The older is at the downstream end of the outcrop. This granitic dike contains a moderate foliation which appears to be metamorphicly derived as it is not parallel to the contacts of the dike and is parallel to the northwest fabric in the surrounding rocks. It therefore appears to have been intruded between the time of peak metamorphism and the generation of tre northwest fabrics.
The second dike may be seen just below the bridge, along the footpath descending to the river. This granitic unit is very similar to tre Elberton in mineralogy, fabric, and tectonic relationships. It appears to contain no foliations which may be related to metamorphic fabric and is therefore postkinematic. A large block of this dike has been moved downstream about 50 yards and may be seen on the north bank of the stream.
We therefore see the deformational and intrusive events of tre Inrer Piedmont in microcosm at this outcrop. The first deformation gives rise to northeast trending structures which may be complex. In general, the majority of this first deformation preceeded peak metamorphic conditions. Granitic intrusions followed this deformational period forming the older, foliated granite gneisses. A second deformation generated northwest trending structures and post-dated peak metamorphic conditions. The intrusion of the Elberton and Elberton-like granites post-dated this event.
111

We presently prefer to refer to the northeast structures as F1 structures caused by a o1 deformation, and to the northwest structures as F2 structures caused by a D2 event, although the possibility exists that the first deformation may eventually be subdivided as appears to be the case in
other parts of the Appalachian orogen.

20.8 21.4 23.7 23.9 24.0 24.6 25.3
26.6 26.9 27.6 29.1

0.1

Leave park continuing north.

0.6

Road becomes parallel to railroad tracks and Ga.

Rt. 72.

2.3

Good saprolite exposure of aluminous migmatite gneisses

in railroad cut to the left.

0.2

Turn left (north) at atop sign and cross bridge over

railroad tracks. Good saprolite of migmatitic

gneisses may be seen below.

0.1

Turn right (east) onto Ga. Rt. 72. at mile 17.

0.6

Good outcrop of biotite, migmatite gneisses with

granitic dikes visible on left side of road.

0.7

Road to Berkely Blue quarrie~ goes off to the right.

This crossroad is one used by the COCORP seismic

group. The break in data occurs at the end of the

road to the right where they jumped the quarries

and the Broad River. The Berkely Blue quarries are

some of the biggest in the Elberton area. The

quarries are producing a fine grade of "blue" stone

which is a slightly finer grained phase found near

the contact on the north and west side.

1.3

Cross the Broad River.

0.3

Saprolite exposure on the right. We are now entering

Elberton West 7~ sheet.

0.7

Apophysis of Elberton,granite exposed in boulders

and small quarry to the left.

1.5

Cross Dove Creek. We are now entering Elberton

batholith. The contact dips very shallowly so

remenants of gneiss filled with pegmatites may be

found capping some hills with granite being exposed

at lower elevations.

112

32.1 33.3

3.0

Entrance to Py~amid Quarries Ine . on the left.

1.2

Good saprolite exposure along railroad tracks on

the left composed of a mixed zone of granite with

gneiss xenoliths.

33.5

0.2

Turn left (north) across railroad tracks toward

buildings of Veal Granite Co. Drive back by buildings,

bearing first right then left and proceed toward

quarry.

34.5

1.0

Veal granite quarry.

STOP if 2. THE VEAL QUARRY.

Within this quarry the contact between the Elberton granite and a septa,

or large roof pendant, of amphibolite and biotite schist may be observed.

The granite is fine grained, polishing to a good blue stone. Near the

contact it has a moderate flow foliation defined by the orientation of biotite.

It has locally injected along the foliation of the biotite schists to form

an intimate lit-par-lit injection texture. The amphibolite has been largely

affected by potassium metasomatism converting the hornblende to biotite.

Many large xenoliths of varying lithologies including calc-silicate, biotite

schists, and amphibolites may be seen along the walls of tre quarries and in

the waste dumps. Oxygen isotope studies have been conducted across the margins

of these Zenoliths (Wenner, this volume) with no evidence of local contamination

effects being found.

The tectonic history may be summarized within the xenoliths. Regional

fabrics were developed before the xenoliths were included in the granite.

A period of pegmatite and aplite development also preceeded inclusion within

the granite as pegmatites within the xenoliths are terminated against the

granite. A moderate to weak flow foliation developed along the margins of

the body during Emplacement. The granite here at the contact is somewhat finer

grained, suggesting a more rapid crystallization then seen further into the

113

body. A later phase of pegmatite development after the granite was essentially solid is also demonstrated by the crosscutting dikes which are mainly dilatant joint filling structures.
This quarry has been economical in the past because of the high quality of the blue stone it yields, but its location near the contact of the body, which causes the good blue color due to the finer grain size, also causes the abundance of xenoliths which results in large percentage of waste. Currently, the quarry is flooded and some previously studied exposures may be underwater at the time of the trip.

35.5 36.7
37.0
37.1 39.1 39.2

1.0

Turn left (east) on Ga. Rt. 72 after returning

from quarry to main road.

1.2

Georgia Granite Co. is on the left. This is the

best source of polished stone for decorative

purposes (i.e. table tops, fireplaces, etc.).

0.3

Turn right (south then through cloverleaf to north)

over the overpass following Ga. Rt. 17 north toward

Royston.

0.1

Enter Dewey Rose 7~' map sheet.

2.0

Turn left on dirt road just past buildings labeled

"Rock Knob Granite Co.".

0.1

Turn left behind Rock Knob building through metal

gate and proceed to quarry. This is Cornmolli Bros.

quarry.

STOP :ff 3 COMOLLI BROTHERS QUARRY. This elongate, homogeneous quarry is typical of the more economic quarries
in the Elberton area. It has been active for a long time, as evidenced by the different methods of quarrying demonstrated by the walls. At the northern end of the quarry the vertical striations are vertical drill holes typical of the old quarrying techniques. A series of vertical, nearly tou~hing, drill holes would be made around a block. A series of short holes would then be
114

drilled below the block, packed with explosive, and fired to lift the block from its base. This quarrying method was used up until the mid-1950's or so, when it was largely replaced by flame-cutting, sometimes known as jet-piercing.
In jet-piercing a mixture of kerosene and compressed air is fed through a long pipe and burned to form an intense flame similar to that in a jet engine. This flame is run along a surface to be cut. The high temperature causes the rock to expand and flakl! off thus "cutting" the rock. This procedure is used to cut vertical troughs around a block, and it is then lifted by blasting as before. Flame-cutting is hazardous in that the noise is similar to that of a jet engine. The dust can also be intense and the heat generated is significant.
Now a new technique known as water-piercing is just being introduced in this quarry. This method uses a stream of high pressure water, 30,000 psi or so, to flake the granite. This method has many advantages. It is less noisy for the workers and will not cause ear damage. The water washes away the residue r~ducing the dust levels and the dangers of silicosis. The "fuel", namely water, is also cheaper. The energy of the stream of water dissipates in a short distance so it is less dangerous than the kerosene flame. It is also more useful on quartz-free rocks than the flame me~hod. The thermal expansion of granite is dominantly effected by the alpha-beta quartz transition at about 575C. This rapid expansion of about 3% in wholerock volume appears to be important in the flame method as the flame usually will not cut rocks which do not contain quartz. The water technique is not dependent on thermal expansion and therefore is more flexible.
This quarry has been the site of intensive paleomagnetic study as it contains a pod of reversely polarized granite within an otherwise normally polarized quarry (Ellwood and Whitney, 1979; Ellwood and others, 1980) see
115

article by Ellwood, this volume). In addition, chemical data (Stormer and others this volume). Oxygen isotope data (Wenner, this volume) is also available.

40.0 41.7 41.8
42.3 44.4 45.0
51.0 51.8
52.7 53.5 55.5

0.8

Turn right (south) on Ga. Rt. 17 after returning

to main road.

1.7

Cross railroad spur.

0.1

Turn left (east) across railroad track and continue

on paved road past Elberton Granite Finishing Co. ,

Apex, Comet, and Universal Granit~ Co. cutting and

polishing plants.

0.5

Turn left (north) at stop sign.

2.1

Cross river over old concrete bridge.

0.6

Cross Ga. Rt. 77 and continue on Ga. Rt. 368 toward

Iv~ S. C. You are now on Rock Branch 7~' topographic

sheet.

6.0

Cross Coldwater Creek.

0.8

Outcrop of Elberton granite on right. This is

the most northeasterly occurence of Elberton, here

thought to be cutting rocks of the Inner Piedmont

Core.

0.9

Turn left on paved road toward Rock Branch.

0.8

Go straight ahead at 4 way intersection at Rock Branch.

2.0

Bridge over river. Park just beyond bridge.

STOP # 4 At this stop typical migmatitic gneisses cut by a post-kinematic dike
may be seen. The migmatitic gneisses are typical for this part of the Inner Piedmont. The melt portion (leucosome) is granitic in composition, while the residual crystalline material (melanosome, or restite) is composed of biotite, plagioclase, and minor quartz. Veinlets of granitic material appear to crosscut metamorphic foliations thus suggesting that peak metamorphic conditions occurred after the first phase of deformation.
116

The melting relationships suggest that the controlling reaction was the breakdown of biotite to form hydrous silicate melts. A representative reaction would be of the form:
biotite + plagioclase + quartz = silicate melt + alkali feldspar
The degree of melting is controlled by the composition of the biotite and plagioclase and increases slowly with temperature. The composition of the melt and the residual phases is consistent with temperatures in the range
of 700C 50 (Wells, 1980).
The Elberton granite dike cuts across the migmatitic fabric in a very planar structure suggesting that the country rocks had cooled considerably before its intrusion.

I

I

55.6

0.1

Turn left (southwest) on paved road just past the

. I

bridge

I

58.3

2.7

Turn left (south) on paved road.

60.3

2.0

Bear left (southeast) on paved road after stop sign.

60.5

0.2

Stop sign. Turn right (southwest) on Ga. Rt. 368.

62.4

1.9

Turn left (south) onto Ga. Rt. 77 toward Elberton.

65.4

3.0

Cross railroad tracks.

You are now entering Elberton East 7~' of topographic

sheet.

65.5

0.1

Turn left at stop light onto Ga. Rt. 72 and 17.

67.5

2.0

Continue straight ahead on Ga. Rt. 17 south.

69.3

1.8

Turn right into Keystone Granite Co , .Hedquist Pink

quarry. Park and walk back past buildings to quarry.

STOP ifo 5. THE lillDQUIST PINK QUARRY OF THE KEYSTONE GRANI'L'.E COMPANY. This quarry is one of the few quarries cutting pink stone. It also has
a very interesting collection of xenoliths which give a cross-section of

117

lithologies exposed on the east side of the body.

This quarry is located on an outlier of the body, separated from the main

body by a septa of metamorphic rocks to the west. It has a definite pink cast

to it which is enhanced by polishing. Chemically and isotopically (Stormer, and

otbers, this volume; Wenner, this volume) it is identical to the gray phase.

The tectonic history of the eastern side of the body may be summarized from

the xenoliths. First of all is a porphyroblastic microcline gneiss which probably

origli1ated as a sedimentary or volcanic sedimentary rock. This metamorphic unit

sometimes demonstrates a weak cataclastic fabric, with some shearing of the

porphyroblasts and groundmass visible. A second metamorphic unit, a biotite schist,

is found interbedded with the porphyroblastic gneiss. Originally it was thought

that the porphyroblasts could be metasomatically related to the granite, but

their apparent degradation by latter ductile fabrics which did not develop in

the granite suggests that they are considerably older. In some areas up to a

mile away from the granite they can be found intimately interlayered with biotite

schist.

These units are cut by a lineated intrusion. This darker colored granitic

unit shows a strong lineation caused by its biotite. This unit is poorly exposed

except in the quarry and its tectonic significance is not well understood.

The biotite schist is also cut by a strongly foliated granitic intrusion.

One xenolith on the side of the quarry demonstrates the following chronology.

A biotite schist, with strong S schistosity, is cut by the foliated granite
1

gneiss. The granite gneiss does not exhibit an S schistosity, but does contain
1

another foliation at an acute angle to both S and the contact. This schistosity

-

1

corresponds to an s schistosity formed by a crenulation of the biotite in the
2

118

schist. The entire xenolith is then surrounded by pink Elberton granite which contains neither schistosity, but does show a very faint flow foliation at an angle to either of the two metamorphic fabrics. Thus, the biotite schist was metamorphosed, cut by a granitic intrusion, both being deformed by a second fabric forming event, and then incorporated into the granite as a xenolith.
The metamorphic rocks are also cut by pegmatites and aplites which are then dropped into the granite as large xenoliths. On the west side of the quarry numerous blocks containing pegmatite can be seen incorporated into the granite. The xenolithic nature of these blocks can be seen best from the east side of the quarry.
Therefore, the xenoliths in this quarry give the following chronology: 1. Biotite schist and porphyroblastic gneiss protoliths deposited.
2. Deformation forming s1 surfaces, probably preceeding peak metamorphism
slightly. 3. Intrusion of the meta-granite, now a granite gneiss.
4. Deformation forming s2 surfaces, post-dating peak metamorphism.
5. Intrusion of lineated dike. 6. Intrusion of pegmatites and aplites. 7. Intrusion of the Elberton granite. 8. Generation of late stage pegmatites and quartz veins related to cooling
and crystallization of the Elberton. On the way out from the quarry the methods of cutting and polishing may
be seen in the metal sheds near the busses. A wire is run across the surface of the block to be cut . A carborundum abrasive is fed as a slurry into the cutting channels and is carried by the wire across the surface. A series of cuts of a given width are made at one time to yield a series of slabs. These are then placed on a polishing lap where a large, circular lap is lowered onto
119

their surface and finer and finer abrasive used to polish the surface.

69.4

0.1

Turn left (north) on Ga. Rt. 17 upon leaving driveway.

71.2

1.8

Turn right (east) on Ga. Rt. 72.

72.0 74.1

0.8

Turn right (south) on paved road at mile 45 across

from Harold's furniture store.

2.1

Steep downhill just before Niddleton-Lowndesville

fault zone.

STOP# 6. EASTERN EDGE OF THE INNER PIEDMONT.

This stop is a saprolite exposure of porphyroblastic biotite gneisses and

related intrusive rocks at the eastern most edge of the Inner Piedmont. The

porphyroblastic gneiss is similar to that seen in the Hedquist Pink quarry of

stop 5, and is cut by dikes of the lineated granitic unit, pegmatites and

aplites. In some zones, the gneiss becomes cataclastic with the porphyroblasts

of microcline being rolled and sheared to form resistent remnants. Figure

shows representative occurences of these cataclastic fabrics. These zones of

cataclasis are sometimes silicifed or filled with pegmatitic material. In

extreme cases the rock becomes a phyllonite or blastomylonite. At this stop,

the gneiss is also cut by brittle faults which offset both the gneiss and

aplite dikes. These brittle faults have a normal sense of movement with the

western side up.

The outcrop is located on a break in slope which stretches in a long line

5 to 6 miles across the Elberton East quadrangle and is the most striking

topographic feature on the 7 ~ ' or 15' topographic sheets. This slope marks

the most eastern exposure of the Inner Piedmont lithologies. Toward the

eastern end of the outcrop cataclastic fabric becomes more common, although

still restricted to defined zones. At ~he base of the slope, in low swampy

120

areas, the nature of the rock, the residual soils, topographic relief, and agricultural utilization changes dramatically. First, the area is lower lying with less relief. The land is more often tilled with a smaller proportion in pine forests. Cotton in particular seems more abundant on the lowlands. The rock in this area is a series of metagabbros cut by tonalitic dikes. The structural fabric is in a different orientation than in the Inner Piedmont, and the grade of metamorphism appears somewhat lower, although still in the amphibolite field. The most striking feature is that all primary structures, such as porphyrytic textures, cross cutting dikes, layering of coarse gabbros, are excellently preservedwithoutbeing obliterated by metamorphism or deformation.
An excellent saprolite exposure of metagabbro cut by several different generations of granitic dikes used to be visible just beyond the base of the slope, but grading and seeding following logging has covered over most of the area. A residual patch or two may be seen where erosion has removed the cover.
The sharp change in terrain characterized by this zone is interpreted as the extension of the Lowndesville fault zone. It has been termed the MiddletonLowndesville fault zone as it is well defined just to the north near the town of Middleton (Rozen, 1978) Griffin, 1971, 197~. It has now been mapped continuously from the South Carolina border through the Elberton area (Rozen, 1978), the Vesta quadrangle (Ellwood and Whitney, 1980, and this publication), the Lexington and Rayle quadrangles (Davidson, 1980) and through the middle of Georgia to just short of the Towaliga fault zone (Davis, 1980). It appears to be one major splay of the Towaliga and has both components of movement reported in the Towaliga. In other outcrops in the East Elberton area small scale folds developed in the ductile fabric which forms during cataclasis suggest a reverse movement (or thrusting movement depending on the dip at the time) with the eastern metagabbro terrain overriding the Inner Piedmont. This sense of movement is
121

opposite to the movement along the latter brittle faults along which the Inner Piedmont was rising. Within the Elberton area, the ductile fabric development was accompanied by the development of greenschist minerals suggesting a temperature of 450 or so for the time of deformation. Further southwest, these conditions may have been somewhat hotter.
In any case, the fabric along this zone represents a later, local deformational fabric (s3 ,F3 , related to D3) which forms a ductile shear zone along which the eastern block overrode the Inner Piedmont at a time when the rocks were still at moderately high temperatures. These fabrics are cut by aplites, and perhaps by the Elberton itself (see stop 13 of second day) suggesting that igneous activity occurred after ductile fabric development ended.

74.6 75.0 76.1 77.0
78.2

0.5

Reload buses and continue on paved road.

0.4

Turn right (southwest) on paved road.

1.1

Turn left on paved road.

0.9

A hill of chloritized pyroxenite is visible on the

right. Here the ultramafic horizon within the

gabbroic sequence has been hydrated by solutions

coming up a small shear zone and has formed a very

dense, resilient rock of interlocking chlorite books.

1.2

Good outcrop of norities, pyroxenites and gabbros

on both sides of road.

STOP# 7. CUMULATE PYROXENITES, .NORITES AND COURSE GRAINED GABBROS. At this stop a series of metamorphosed mafic and ultramafic units are
exposed along the road cut. The ultramafic units appear to be layered and are thought to represent cumulates from gabbroic magmas. Some remnants of olivine, now reacted to serpentine, and norites are visible at the northern end of the outcrop. To the south, the dominant units were mainly clinopyroxenites and gabbros. This layering suggests that the stratigraphic top of the sequence may be to the south. The exposure, however, is too poor to
122

define fold axes. All of these rocks have been extensively metamorphosed. The grade of
metamorphism as determined from mineral assemblage and chemistry is rather high, generally inmid-amphibolite range. These rocks have therefore been through metamorphic conditions as severe as any in the surrounding rocks and were probably intruded either prior to metamorphism or during conditions close to those of peak metamorphism. The retention of primary structures during such intense conditions of metamorphism is partially due to the very dry nature of the parent rock. All water was apparently consumed in hydrating the outer portions of pyroxenes and olivine~ with the cores escaping such reaction. The retention of primary crystal habit also suggests that metamorphism in this area was dominantly thermal and not accompanied by strong shear stress. Such a pattern of deformation is totally different from that seen for early deformation in the Inner Piedmont.
One problem in this area is, in which metamorphic belt should these rocks be placed? Since they are metamorphosed intrusive rocks they should presumably be classified along with their host. Unfortunately, very little of the host which they intruded is now exposed as 90% of the area is metagabbro. Where it is exposed it is mica schist of at least amphibolite grade, deformed by both northeast and northwest fabrics. It is most similar to mica schists exposed to the east which are categorized as the upper portion of the Charlotte belt. The structural complexity of the country rocks suggests that this area should not be classified with the Slate belt or King's Mountain belt without some explanation of the deformational fabric.
It has been proposed that these gabbros, sheeted dikes, and related layered ultramafic rocks represent intrusion in a back arc basin in which crustal thickening by intrusion was occuring behind an active island arc
123

(Rozen, 1978). The major element chemistry, metamorphic history, and tectonic setting are consistent with this theory.

78.3
79.1 81.2 81.3 84.1 85.1
87.6 87.7
88.9
105.7 120.2 121.6
121.9 122.8 124.0 124.1 125.0 125.1 125.2

0.1
0.8 2.1 0.1 2.8 1.0
2.5 0.1
1.2
16.8 14.5
1.4
0.3 0.9 1.2 0.1 0.9 0.1 0.1

Reload buses and continue on paved road, returning to Athens.
Stop sign. Bear right on paved road.
Turn right (north) on Ga. Rt. 17 in Fortsonia.
Turn left (west) on paved road.
Turn right (north) on dirt road at "T" intersection.
Continue straight ahead at intersection with smaller dirt roads in criangle intersection.
Bear left about 100 yards to main road.
Turn left (south) on Ga. Rt. 77. Entrance to Pyramid Quarry (Stop 9 of second day) is about 1 mile northeast. Stop 10 is about 0.5 miles to east on paved road which is across from the point at which we join Ga. Rt. 77.
Cross Broad River. Stop 11 of second day will be by river at this point.
Turn right (northwest on U.S. 78 toward Athens.
Turn left at stop light onto Gaines School Road.
Turn right (north) on Barnett Shoals Road just past C&S bank.
Turn left onto Research Road.
Turn right onto College Station Road.
Cross East Campus Drive.
Turn right onto D. W. Brooks Drive (formerly Ag Drive).
Turn left on Cedar Street.
Turn right onto Sanford Drive.
Turn right into GGS parking lot.
END OF FIRST DAY

124

ROAD LOG SECOND DAY

Cumulative Incremental Mileage Mileage

0.0

o.o

0.1

0.1

0.4

0.3

0.9

0.5

1.2

0.3

1.7

0.5

1.8

0.1

3.6

1.8

4.0

0.4

4.2

0.2

4.4

0.2

6.0

1.6

7.9

1.9

8.0

0.1

9.8

1.8

15.3

5.5

19.5

4.2

19.8

0.3

24.3

4.5

25.2

0.9

26.~

1.6

28.5

1.7

Leave GGS parking lot, turning left onto Sanford Drive. Continue through stop light. Turn left (east) on Carlton Street. Turn right (southwest) on East Campus Drive. Turn left (east) on College Station Road. Pass under Athens bypass. Riverbend Research Laboratory on the right. Turn right (southwest) on Barnett Shoals Road. Turn left (southeast) following Barnett Shoals Road. Stop sign. Turn left following Barnett Shoals Road. Turn left (southeast) onto Old Lexington Road. Be careful, this is a blind turn. Stop sign. Continue through intersection on Old Lexington Road. Turn right on Morton Drive. Turn left on Old Lexington Road. Turn right (southeast) on Lexington Road (U.S. 78). Cross into Elberton Batholith. We remain in the batholith thru Crawford and Lexington. Pass county courthouse in Lexington. Turn left (northeast) on Ga. Rt. 77 toward Elberton. You are now entering Jackson 1 s Cross road 7 1/2 1 quadrangle map. Cross Big Indian Creek. Cross Little Indian Creek. Turn left (northwest) on paved county road 92164. Turn right (east) on paved road.

125

31.2
31.3
32.8 33.0 33.1

2.7

Pass Veribest grocery store on right.

You are now on the edge of the Vesta 7 1/2'

quadrangle map sheet.

0.1

Turn left (north) on dirt road.

You are on the east edge of the Jackson's Crossroads

quadrangle.

1.5

Bear slightly left of first quarry and ford the

stream.

0.2

Turn right onto dirt road.

0.1

Park in front of larger of two quarries.

STOP II 8. Within this quarry the contact between the Elberton granite and roof
pendants of older, foliated granite and biotite schists is exposed. The biotite schists have a pronounced foliation, and although the metagranite has intruded parallel to this foliation, it does not show this sl structure. The metagranite is foliated by an s 2 structure which correlates with a crenulation of the biotite in the schist. Blocks of both metagranite and schists can be seen within the Elberton as disconnected, floating blocks. There is little evidence of any assimilation of the metagranite or biotite schists.
The Elberton demonstrates several generations of pegmatites and possibly one internal contact within this quarry. On the back wall, a block of metagranite can be seen within the granite. This xenolith and the Elberton below it are cut by a pegmatite, which in turn is cut by the Elberton again. It appears on this wall that after the Elberton initially intruded, but while it was still plastic, the Elberton reintruded along a parallel fracture within the metagranite thus forming the xenolith. Where the second pulse of Elberton came in contact with the first no contact can be seen as they have completely flowed, or annealed together.
126

Three stages of pegmatite generation may be seen in this quarry. The first is only seen on top of the quarry, near the southeastern corner. Here, a round mass of pegmatite with large, homogeneous feldspars and very dark grey quartz can be seen. Within this pod was found a single crystal of a highly radioactive substance, tentatively identified as polycrase. The darkness of the quartz testifies to the highly radioactive nature of this early pegmatite generation. Apparently, from the shape of this body, a pegmatitic event occurred while the magma was still fluid. This event was probably the first generation of volatiles and occurred in response to decreasing confining pressure during intrusion. Vapor soluble trace elements were concentrated in these fluids as they left the body. Much of the uranium in the original magma may have been mobilized at this stage.
The second generation of pegmatites form dilatant dikes usually containing feldspar, grey quartz, and biotite with minor muscovite. These are still somewhat rich in trace elements such as uranium and thorium, but they do not seem to have the concentration found in the round pods.
A third generation, probably formed as the magma completed crystallization, is a more leucocratic phase composed dominantly of feldspar and quartz with only minor biotite. In this case, the quartz is much clearer testifying to the lower radioactive element content of this phase. These dikes are generally emplaced along joints suggesting that the host was brittle by this time.

33.2 33.3
34.9
37.4

Turn around and return on dirt road.

0.1

Turn left (south) on dirt road.

0.1

Re-ford the stream.

1.6

Turn left (east) on paved road.

You are now in the Vesta quadrangle.

2.5

Now passing out of batholith and into Inner

Piedmont Flank.

127

38.3 41.9
42.9
45.5 45.9 46.7

0.9

Turri left (northeast} on Ga. Rt. 77.

3.6

Saxon cross roads. The dirt road cross Ga. Rt. 77

is the one used by the COCORP seismic group for the

southern line across the Elberton area.

1.0

Cross the Broad River.

You have now enetered the Elberton East 7 1/2'

Mapsheet.

2.6

Turn left (west) toward the Blue Diamond Quarry.

A granite sign marks the entrance.

0.4

Metal gate at entrance to quarry.

0.3

Park by quarry.

STOP It 9. BLUE DIAMOND QUARRY, AND NEIGHBORING PINK QUARRY. The Blue Diamond quarry is unique in that there is a banding of pale
grey and pink layers visible in the main quarry. (Unfortunately new quarrying may remove the best exposures before the trip.) These red bands are approximately normal to the foliation defined by the anisotropy of magnetic susceptibility (Ellwood and Whitney, 1980); These features have been interpreted as tensional features along which volatiles escaped as the magma was cooling, and along which oxidation of fine grained magnetite within the feldspar occurred causing the change in color.
Behind the Blue Diamond Quarry is a companion quarry of pink granite. The two phases are nearly identical except for their color. The color is caused by fine grained iron oxides within the feldspars. Magnetite causes the granite to appear grey, while hematite causes the pink color. Chemically, isotopically, and magnetically there is no difference between these two phases. The relationship between these two will be further examined at the next stop.

Return to paved road.

128

46.8 48.1
48.8 49.2

0.6

Turn right (south) on Ga. Rt. 77.

1.3

Turn right on paved road at granite sign to Dawn

Gray and Sunset Pink quarries of Elberton Granite

Industries.

0.7

Turn left on dirt road.

0.4

Turn right into quarry entrance.

STOP If 10. DAWN GREY AND SUNSET PINK QUARRIES. These two quarries owned by the Elberton Granite Industries, Inc .': give
us an opportunity to examine the transition from pink to grey granite in detail. The contact is exposed on a quarried shelf between the two quarries. In detail, the color change is gradational over a few feet. Petrographically, there is no change, nor is there any variation in major or minor chemistry, oxygen isotope values, strontium isotopic ratios,, or magnetic characteristics. The zone within the body in which the pink granite is found i~ in genera~ an area of high magnetic susceptibility suggesting a slightly higher magnetite content, but within this zone there is no difference between the pink and grey phase.
This quarry is very important for chronological studies. A biotite 40Ar/39Ar age of 236 m.y. was obtained by Hess (1979). A sample from this quarry was also used for a Rb/Sr isochron (Ellwood et. al., 1980; and this
volume), which yielded an age of 350 + 11 m.y. The zircon sample reported by Bickford in this volume, yielding an age of 320 t 20 m.y. also came from
the Dawn Grey quarry. A great deal of chemical data is available for these two quarries as
they were used by Hess to evaluate small scale, local, chemical variations. In general, the variations in major element content are very small, but they are as large as seen throughout the body, thus proving what the eye tells us,
129

that this is a very homogeneous body in terms of major components. The trace element distribution is another matter, however, Within the two quarries studied in detail there is little within site variation in most trace elements. Between quarries there is a much larger variation, in fact sometimes as much as a factor of 4, in trace element concentration. These chemical relationships suggest that the magma generation process formed a product that was homogeneous in major elements, but variable in trace elements. It may be that trace element variations reflect differences in source material.

49.6 50.0 51.1 51.9

0.4

Return to dirt road and turn left.

0.4

Turn right (south) on paved road.

1.1

Turn right (southwest) on Ga. Rt. 77.

0.8

Cross Broad River. Park just beyond river.

STOP II 11. WEATHERED PAVEMENT ALONG BROAD RIVER, AND LUNCH.

Here in the river are exposed a series of migmatitic biotite gneisses to the east of the Elberton. We are within 1/2 mile or so of the eastern boundary of the Inner Piedmont. To the north, these migmatitic gneisses grade into microcline gneiss similar to those seen at stop 6. The peak metamorphic conditions of the rocks exposed along the flank of the Inner Piedmont may be increasing as we go southwest along this zone.

52.1
53.1 57.2 78.7

0.2
1.0 4.1 14.5

Continue down road to southeast. You are now crossing the Vesta quadrangle.
Saxon Cross road and intersection with COCORP line.
Cross back into Elberton granite.
Turn right (west) on U.S. 78. You are nmT in the Lexington 7 1/2' quadrangle.

130

79.3 79.7 80.0

0.6

Turn left (south) on Ga. Rt. 77 toward Union Point.

0.4

Elberton granite on right.

0.3

Park on side of road.

You are now on the western edge of the Lexington

quadrangle.

STOP II 12. SOUTHERN CONTACT OF THE ELBERTON. This outcrop is one of the few good exposures at the south end of the
body. The Elberton here may be a little coarser grained and the muscovite may be a little more prominent, but otherwise it is very similar to that seen further north. This is the southernmost exposure of the main body. Numerous small stocks are found further south, but these are small and not interconnected. Walking west along the road we come to another series of low outcrops which are believed to be an older, foliated granite. This body occurs to the south of the Elberton and is cut by small stocks and dikes of the Elberton. It is here termed the granite of Hutchins as it underlies the small town of Hutchins to the south.

80.1
82.2
100.8 101.3 107.3 113.3

0.1
2.1
18.6
0.5
6.0 6.0

Reload and proceed ahead on Ga. Rt. 7 7. Older foliated granite is visible on right side of road.
Continue ahead on Ga. Rt. 77 through intersection with smaller paved road. Those who wish to leave trip at this point may turn right on smaller paved road and follow it to U.S. 78. You may turn right (west) on U.S. 78 and return to Athens as you did on the first day.
Intersection with Georgia Route 44. Continue on
Ga. Rt. 77.
Intersection with U.S. 278. Continue on Ga. Rt. 77.
Intersection with Interstate 20. Get on Interstate 20 going west toward Madison and Atlanta.
Exit at Buckhead exit.

131

113.5 113.7

0.2

Turn left off of exit and cross over interstate.

0.2

Park on left and walk back to quarry.

STOP # 13. THE BUCKHEAD GRANITE AND THE AGE OF DUCTILE SHEAR ALONG THE MIDDLETON-LOWNDESVILLE FAULT ZONE.
The country rock in tlds old crushed rock quarry is a hornblende biotite
'
blastomylonite gneiss that has been crosscut by numerous aplite and pegmatite dikes (Davis, 1980). The ductile shear observed in this quarry is mapped as a continuation of the Middleton-Lowndesville zone, and the deformation of this ductile fabric is thought to be 1 corr~lative with that further northeast. The grade of metamorphism at the time of this latest deformation was higher here than in the Elberton area, being at least lower amphibolite facies. Aplitic and pegmatitic material which may have been formed by metamorphic and anatectic processes have been deformed by the ductile fabric development.
The Buckhead granite forms a dike and small elliptical loccolith-shaped intrus~on which clearly cross cuts the ductile fabric. This granite is petrographically, chemically, and magnetically identical to the Elberton and may be of the same age. It is clearly younger than the ductile fabric which is thought to be equivalent to that within the Elberton area. If this is so, it would indicate that the ductile fabric development, thought to be caused by a reverse or thrusting movement of the Charlotte-Slate belts over the Inner Piedmont material, would pre-date the Elberton and would thus be older than 350 m.y. In any case, these cross cutting relationships are clear evidence of the relative age of fabric and magma generation in this area and should be studied in more detail using all the tools of geochronology.

113.9

Turn around and return to I-20.

0.2

Turn left on Interstate 20 west toward Atlanta and

Madison.

132

120.9
153.0 153.3
153.4
'
153.8 153.9
153.8 154.9 155.0

7.0
32.1 0.3
0.1 0.4 0.1
0.9 0.1 0.1

Exit onto U.S. 441 and U.S. 129 north. through Madison. Follow U.S. 441 and U.S. 129 back to Athens.
Follow U.S. 441 and U.S. 129 onto Athens bypass.
Exit on College Station Road exit following sign for the University of Georgia.
Turn left onto College Station Road.
Cross East Campus Drive.
Turn right onto D. W. Brooks Drive (formerly Ag Drive).
Turn left on Cedar Street.
Turn right onto Sanford Drive.
Turn right into GGS parking lot.
END OF SECOND DAY

133

REFERENCES ~Davidson, J. W., 1980, The geology, petrology, and economic mineral
resources of Eas~-Central Oglethorpe County, Georgia, M.S. Thesis University of Georgia. (In preparation Aug. 1980). Davis, Gary J., 1980, The Southwestern Extension of the MiddletonLoundesville cataclastic zone in the Greensboro, Georgia area and its regional implications: M.S. Thesis, Univ. of Georgia, Athens. Ellwood, B. B., and Whitney, J. A., 1980. Magnetic fabric of the Elberton
granite; Northeast Georgia, 1 Geophys. Res., 85, 1481-1486.
Ellwood, B. B., Whitney, J. A. Wenner, D. B., Mose, D., and Amerigian, C., 1980. Age, paleomagnetism, and tectonic significance of the
Elberton granite, Northeast Georgia Piedmont: 1 Geophys. Res., in
press. Griffin, V. S., 1971, Inner Piedmont belt of the southern crystalline
Appalachians: Geol. Soc. Amer., Bull., v. 82, p. 1885-1898. -----------' 1978, Detailed analysis of tectonic levels in the Appalachian
Piedmont: Geol. Rundschau, v. 67, p. 180-201. Hess, J. R., 1979, Geochemistry of the Elberton Granite and the geology
of the Elberton west quadrangle, Georgia! M.S. thesis, University of Georgia, Athens, 193 p. Rozen, R. W., 1978, The Geology of the Elberton East Quadrangle, Georgia: M.S. Thesis, University of Georgia. Wells, D. E., 1980, The Geology and petrochemistry of Rock Branch Quadrangle, Elbert County, Georgia: M.S. Thesis, University of Georgia. (In preparation, Aug. 1980).
134