The Lake Chatuge sill outlining the Brasstown Antiform [1974]

THE LAKE CHATUGE SILL OUTLINING THE BRASSTOWN ANTIFORM
by
M EUGENE HARrLEY ill and H MICHAEL PENLEY
Brautown Bold (478.4111
GUIDEBOOK 13
Georgia IRpu:tmmt of Natural Resources Joe D. 'Thnner, ComaDssimer Earth and Wctta- Divisim
'lbe Geologiad Survey of Georgia amtM Pickering, Jr., State Geologist
ATLANTA 1974

l
i
1 I
. I

THE LAKE CHATUGE SILL OUTLINING THE BRASSTOWN ANTIFORM
by
M EUGENE HARrLEY m and R MICHAEL PENLEY
GUIDEBOOK 13
Georgia IRpa:rtmmt of Natural Resources Joe D. 'Thnner, OJmnissi.mer FBrth and Water Divisim
1he Geologiml Survey d. Gett-gia Sun M Pickering, Jr., State Geologist
ATLANI'A
1974

CONTENTS

page

Introduction

1

Acknowledgments

1

General Geology . .

1

Ultramafic and Related Rocks .

1

Country Rocks

7

Topographic Expression

7

Stratigraphy . . . .

9

Brasstown Antiform

9

Origin of the Lake Chatuge Sill

10

Discussion

14

References Cited

17

Road Log

19

ILLUSTRATIONS

Figure

page

1. Generalized geologic map

2

2. Photomicrograph of dunite

4

3. Photomicrograph of a corona

4

4. Banded coronite troctolite .

5

5a. Photomicrograph of olivine gabbro

5

5b. Photomicrograph of olivine gabbro

5

6. Photomicrograph of olivine gabbro in transition to orthoamphibolite .

6

7a. Photomicrograph of green orthoamphibolite

6

7b. Photomicrograph of black orthoamphibolite

6

8. A mafic vein

6

9. Photomicrograph of garnet-pyroxene gneiss

8

10. Biotite gneiss . . . . . . . . . . . .

8

11. Garnet-mica schist . . . . . . . . . .

8

12. Generalized geologic map of the Brasstown Antiform .

11

13. Block diagram of the Brasstown Antiform . . . . .

12

14. Origin of the Lake Chatuge Sill . . . . . . . . .

13

15a. Generalized section of part of the Lake Chatuge Sill before metamorphism.

15

15b. Generalized section of part of the Lake Chatuge Sill after metamorphism but before local alteration . . . . . . . . . . . . . . . . . . . . . . 15

THE LAKE CHATUGE SILL OUTLINING THE BRASSTOWN ANTIFORM
by
M. Eugene Hartley 1 and H. Michael Penley 2
INTRODUCTION
The purpose of this field trip is to show the Lake Chatuge alpine-type ultramafic sill, present its origin and deformation history, show how it can be used to define the limbs of the Brasstown Antiform, and demonstrate that it can be used as a stratigraphic marker over 100 square miles (259 sq. km.) in this part of the otherwise complexly folded Blue Ridge. Hartley (1973) discussed the geology of the northern 40 square mile (104 sq. km) part of the field trip area (Figure 1). Later reconnaissance work by Penley (1974) for the new geologic map of Georgia showed the sill to continue southward and wrap around Brasstown Bald, Georgia's highest mountain. The authors think that new information obtained in these studies may have a bearing on the origin of other alpine-type intrusions in the Blue Ridge and contribute to the stratigraphy and structure of the Blue Ridge.
ACKNOWLEDGEMENTS
The authors wish to express their appreciation to the people who helped with this study. Thanks go to Lynda Stafford and David E. Lawton for making editorial comments and to William Clark for drafting some of the illustrations. Thanks are also given to Robert H. Carpenter and John M. Kellberg who initially recommended that the study presented in Georgia Geological Survey Bulletin 85 be made.
GENERAL GEOLOGY
The field trip area lies within the Blue Ridge thrust sheet. The area is located about 20 miles (32 km) east of the Murphy Syncline and about 15 miles (24 km) west of the Ashland-Dahlonega shear zone. Rocks in the area were metamorphosed to the kyanite and sillimanite grade by the main Paleozoic (probably middle to upper Ordovician) metamorphism and were complexly folded. The two main groups of rocks encountered in the field trip area are the ultramafic and related rocks of the Lake Chatuge sill, and the country rocks, which are subdivided into units of garnet-mica schist and biotite gneiss.
ULTRAMAFIC AND RELATED ROCKS
The study area is part of the 1800-mile (2880 km) long belt of alpine-type ultramafic rocks in the Appalachians. Locally, the ultramafic and related rocks occur as a concentrically zoned sill about 1000 feet (305m) thick, extending tens of miles in length. Best exposures of the unit occur along the shore of Lake Chatuge and can best be seen in the winter when the water level is low. The rock units present in the sill include dunite, coronite troctolite, olivine gabbro, mafic and metamafic veins and pods, orthoamphibolite, and garnet-pyroxene gneiss. Small amounts of other mafic rocks have been found, but only as float. Many of the rock types are gradational into others. The following paragraphs are a summary of the description of the main rock unit. A more complete petrographic description can be found in Hartley (1973).
1 University of Georgia 2 Georgia Geological Survey

DUNITE

CORONITE TROCTOLITE

-~ ~

OLIVINE
GABBRO

~ PYROXENE
~ GRANULITE

~ ORTHO-
~ AMPHIBOLITE

~ COUNTRY ROCK
~ (GNEISS/SCHIST)

N
1
0

GNEISS

I
/
SCHIST
~-;-/

f f
Figure 1. Generalized geologic map showing Stops 1 and 2. 2

Dunite (Figure 2) is the dominant ultramafic unit and is similar to other dunites in the Blue Ridge. Electron microprobe analyses indicate a composition of Fo 9 0 2 The dunite is partially altered to serpentine, with serpentine constituting a probable average of 20 percent of the rock. Chloritization is also common. Alteration products such as anthophyllite, chrysotile, and talc are locally common. Hydrothermal alteration is more pronounced to the south, as can be seen at Trackrock and Soapstone Gaps.
Coronite troctolite, similar to that found at the Buck Creek intrusion in North Carolina, is commonly associated with the dunite. Troctolite is an olivine-plagioclase rock and the prefix coronite is added to explain the texture and account for the coronas (Figure 3) which are composed of concentric layers of pyroxene and amphibole-spinel. The appearance of troctolite varies considerably due to the arrangement of the coronas. They may be isolated, connected to form a network arrangement, or arranged in bands. On weathered surfaces, the coronas impart a knobby appearance to the rock. Rhythmically banded coronite troctolite (Figure 4) occurs locally as lenses within the dunite.
The coronas probably were not formed during metamorphism, as some geologists argue, since near the margin of the sill they are altered to amphibole and aligned to the regional foliation. The coronas were probably formed by a magmatic or peritectic reaction of olivine and plagioclase. Toward the margin of the sill the troctolite is gradational into orthamphibolite similar to the previously termed Roan Gneiss found elsewhere in the Blue Ridge.
Olivine gabbro (Figures 5a, b) is also common and is similar to the troctolite except that it contains over 10 percent diallage, the primary pyroxene, which appears completely unaltered in many places. The olivine in both rocks is Fo 9 0 and the plagioclase is labradorite-bytownite (An 6 8 to An8 3 ). Leucocratic or anorthositic olivine gabbro is locally common. Chloritization is locally common in both olivine gabbro and troctolite. Toward the margin of the sill the olivine gabbro grades into orthoamphibolite similar to the orthoamphibolite derived from troctolite. Most of the transitional rocks are difficult to identify in hand specimen but are generally obvious in thin section. Figure 6 shows a transitional rock between an olivine gabbro, that was probably similar to the one shown in Figure 5b, and orthoamphibolite.
Orthoamphibolite (Figures 7a, b) generally occurs at the margin of the sill, in contact with the schist and gneiss country rock units. The rock consists mainly of amphibole, plagioclase, and clinozoisite. Corundum is present locally. Texture, low 8 7 Sr/8 6 Sr ratios, and gradational contacts indicate that the amphibolite is the metamorphosed equivalent of parts of the mafic units present. Therefore, since corundum is not found in the mafic units it is proposed that the corundum was formed during metamorphism.
Mafic veins, common in alpine intrusions, (Figure 8) occur throughout the sill. In the ultramafic and mafic units the veins are composed of diallage and bytownite. Some diallage crystals are several inches long. Metamorphosed equivalents of the veins occur in the amphibolite. These metamafic veins are composed of hornblende, andesine, clinozoisite and corundum. Much of the corundum sought as rubies and sapphires in Georgia and North Carolina comes from similar veins.
The most unusual and probably the most controversial rock in the area is a garnetpyroxene-plagioclase-amphibole rock (Figure 9) that occurs locally as lenses within the dunite and orthoamphibolite units. Although somewhat similar to a coarse garnethornblende gneiss, outcrops of the unit are distinctive. The rock has a poor to welldeveloped gneissic structure, but more commonly it appears streaky. Red garnets ranging in diameter from 1 to 50 mm are characteristic of the unit. Pyroxene is the dominant dark mineral and andesine composes the white portions of the rock.
3

Figure 2. Photomicrograph of dunite in reflected light. Serpenti.ne (sp) is shown replacing olivine (ol). Secondary magnetite (mt) formed during the partial serpentinization.

Figure 3.

Photomicrograph of a corona. The typical reaction rim consists of two layers - an orthopyroxene (opx) adjacent to olivine (ol), and a layer of spinel-amphibole intergrowths adjacent to bytownite plagioclase (pi).

4

Figure 4.

Banded coronite troctolite. Distinct layers of bytownite plagioclase (pi) are separated from olivine (ol) by a reaction layer of orthopyroxene and spinel-amphibole intergrowths similar to the corona shown in Figure 2.

Figure 5a.

Olivine gabbro. This specimen is similar to unhanded troctolite containing coronas (cor) and bytownite plagioclase (pi), except that it contains over 10 percent diallage pyroxene (di).

Figure 5b. O,livine gabbro. This specimen shows the typical plagioclase (pi) and coronas (cor) as well as relatively large crystals of diallage (di) . No effects of metamorphism are evident.

5

Figure 6.

Olivine gabbro in transition to orthoamphibolite. This rock is similar to the one shown in Figure 5b, except that the diallage (di) is altered along its margins and cleavages to amphibole (am). The plagioclase (pi) is less calcic than that in the olivine gabbro shown in Figure 5b.

Figure 7a. Green orthoamphibolite. This foliated orthoamphibolite shows green amphibole (dark mineral) and andesine plagioclase (light mineral).
c.c

Figure 8. Mafic vein. This coarse-grained vein con-
sists of diallage (di) and plagioclase (pi) with an actinolite selvage (ac) at the margin.

Figure 7b. Black orthoamphibolite. This typical orthoamphibolite occurs near the margin of the sill, and is composed of hornblende (dark mineral) and andesine plagioclase (light mineral).

Kellberg (1943) first noted the rock and termed it eclogite, noting that the hornblende and plagioclase components were formed in an equilibration process after emplacement. The rock, however, might be termed a pyroxene granulite, due to its mmeralogical similarity to granulites described in Turner and Verhoogen (1960, p. 554-556), and because of the secondary nature of the garnet indicated by inclusions of amphibolite, pyroxene, and plagioclase. However, because of local gradational contacts with orthoamphibolite (metagabbro) and a lack of evidence indicating granulite facies metamorphism in other rocks, the descriptive term garnet-pyroxene gneiss was adopted by Hartley (1973).
Dallmeyer (1974), in a detailed chemical, mineralogical, and petrographic study, stated that the rock had eclogitic and some granulitic affinities. He noted, however, that the origin of the rock could best be described by a complex history in four stages. These include: 1) formation of a primitive clinopyroxene cumulate in the initial basaltic melt; 2) exsolution of garnet, omphacite and plagioclase in upper mantle conditions; 3) cation diffusion between contiguous garnet and omphacite during partial reequilibration at the lower pressures present during emplacement of the sill; and 4) retrograde effects of metamorphism. He noted also that such an eclogitic inclusion may have a bearing on any proposed model for the origin of the Appalachian ultramafic belt.
COUNTRY ROCKS
The two main country rock units, previously termed Carolina Gneiss (Keith, 1907), are garnet-mica schist and biotite gneiss. Both show high 8 7 Sr/8 6 Sr ratios indicating a crustal origin. Although the authors consider these country rocks as part of the Grenville basement, Hadley and Nelson (1971) show them as "Upper"(?) Precambrian with an unconformity separating them from the Ocoee Series shown as Upper Precambrian. The biotite gneiss unit (Figure 10) is extensively exposed along the shore of Lake Chatuge. It is typically a well-banded biotite gneiss with anhedral garnet porphyblasts locally common. The garnet-mica schist (Figure 11) is distinctive. Outcrops are highly foliated and crinkled and are characterized by garnet idioblasts ranging in diameter from 1 to 10 mm. Some interlayering of schist and gneiss is found in both units. The marked lithologic contrasts of these country rocks with the sill aid greatly in geologic mapping.
TOPOGRAPHIC EXPRESSION
The rock units of the sill show a direct relationship to the topography. The ultramafic rocks, especially dunite, have low resistance to weathering and erosion, as reflected by the valleys and gaps that outline the limbs of the structure. These include parts of the basin of Lake Chatuge, the Young Harris Valley, Track Rock Gap, and Soapstone Gap. Coronite troctolite is resistant and locally supports small hills and ridges on the sill. Amphibolite and olivine gabbro do not show a constant relation to the topography but are generally marginal to the lowest parts of the valleys and gaps. Garnet-pyroxene gneiss is a fairly resistant unit and locally supports several small hills in the valleys underlain by the sill. The best examples of these are around Lower Bell Creek Church.
The country rock units, garnet-mica schist and biotite gneiss, are more resistant than those of the sill and, therefore, support higher hills. The garnet-mica schist unit supports the area's highest mountains including Brasstown Bald, the highest mountain in Georgia.
7

Figure 9. Garnet-pyroxene gneiss. The

Figure 10. Biotite gneiss. Typical, well

Figure 11. Garnet-mica schist. The distinc-

dark areas (di-am) are mostly

banded biotite gneiss. Boudins

tive crinkling is typical of the

00

diallage with some hornblende.

occur locally.

unit.

The lighter areas are andesine

(pl). The outlined garnet (ga)

contains inclusions of plagio-

clase and diallage.

STRATIGRAPHY
One of the critical parts of the field trip will be to demonstrate the stratigraphy that can be followed in this complexly folded part of the Blue Ridge. Hartley (1971, 1973) and Penley (1974) have shown that the ultramafic rocks in the vicinity of Lake Chatuge and Brasstown Bald occur between a biotite gneiss unit and a garnet-mica schist unit. This indicates that the tabular and conformable intrusion can be termed a sill. The occurrence of a conformable rutile zone, previously termed the Shooting Creek rutile zone, in the biotite gneiss unit which parallels the sill is further evidence for the sequence below, which occurs in an area covering over 100 square miles (259 sq. km):
biotite gneiss --rutile zone-biotite gneiss
Lake Chatuge Sill
garnet-mica schist
Since the attitude of the limbs of the major folds and the dip slope on Ramey Mountain indicate anticlinal structures, the schist would seem to be the oldest unit. Locally, however, some dunites appear to be shifted in position toward the biotite gneiss sequence. This may be due to gravity settling. On the other hand, magmatic squeezing or irregular flow could account for this. Complex folding during metamorphism and refolding of the Blue Ridge thrust sheet or local nappes could have overturned the stratigraphic sequence. Thus, the folds in this report are called antiformal.
THE BRASSTOWN ANTIFORM
Previous detailed mapping by Hartley (1973) has shown the presence of the Lake Chatuge Sill in the nose of a northeast-plunging antiform located about 5.6 miles (9 km) north of Brasstown Bald. More recent reconnaissance mapping by Penley (1974) has traced this zone of ultramafic and related rocks along both limbs of the antiform for a distance of about 10 miles (16 km) to the south, where it closes, encircling Brasstown Bald, and indicating a major doubly plunging antiform. This is the most prominent structural feature in Towns and Union Counties and is referred to as the Brasstown Antiform, since Brasstown Bald is central to its recognition and delineation (Penley, 1974). The sill outlines the Brasstown Antiform but does not necessarily define its boundaries.
About 150 structural attitudes were measured on lamination planes which are roughly parallel to the compositional banding. The foliation of the sill is generally the same as that of the country rocks indicating contemporaneous deformation. The prevailing strike of both the east limb and the west limb of the structure is roughly N 20 E. Dip values for the east limb are fairly consistent, with an average of all dips being 52 SE. Values for the dip of the west limb vary greatly, yielding an average dip of 44 in the same direction. Rocks in the northern nose of the structure plunge to the north and rocks in the southern nose plunge to the south.
Local variations in strike and dip are common in the western limb of the structure with foliation being most divergent in the immediate vicinity of large pegmatites and alaskaite bodies. Apparently these deviations were caused by the intrusions.
9

Minor folds occur within the sill and the country rocks. They range from gentle flexures to isoclinal folds and their axial planes range from upright to recumbent. No slip cleavage lineations have been measured. Such lineations do exist, however, in the form of crinkles in the garnet-mica schist unit in the core of the structure. Several joints were observed penetrating sill-country rock contacts, indicating contemporaneous jointing.
Figure 12 shows the easterly bulge in the outcrop pattern near the center of the antiform indicating a folded hingeline.
In the northern part of the area an anticlinal saddle occurs between the northplunging Brasstown Antiform and a south-plunging antiform that extends into North Carolina. Figure 13 shows the offset of these noses. These departures from the prevailing trend indicate crossfolding of the antiformal structures.
ORIGIN OF THE LAKE CHATUGE SILL
The debate over the origin and emplacement of alpine-type ultramafic rocks has existed since Bowen's earliest works. Bowen argued that the dunite in the ultramafics was emplaced as an olivine crystal mush. Later works tended to emphasize a solid emplacement of dunite or serpentinite. Although Bowen acknowledged the possibility of solid emplacement near the end of his career, many geologists have retained the theory of a liquid emplacement of the intrusions. Evidence exists that indicates the intrusion at Lake Chatuge was of igneous origin.
The low 117 Sr/116 Sr ratios of the ultramafic, mafic, and metamafic rocks of the intrusion indicate a mantle origin (Hartley, 1973). Although it is possible that serpentinite can flow under high pressure, further evidence from the isotopic studies of Hartley, Walker, and Jones (1971) and Jones, Hartley, and Walker (1973) indicates in situ serpentinization. The direct relationships of the dunite and the surrounding mafic rocks have been shown by Hartley (1973). Therefore, because the movement of solid blocks of olivine gabbro and troctolite from the mantle is unlikely, an igneous mode of emplacement is required.
The well-ordered concentric zoning of the sill suggests that a model proposed by Bhattacharji (1967) may be applicable (Hartley, 1973). Simkin (1967) used a similar model to explain the origin of picrite sills at Skye, Scotland. In this model, olivine settles out of the magma as a crystal cumulate in a magma pipe after injection into the crust (Figure 14). This cumulate is similar to Bowen's olivine crystal mush. Further injection of the magma tends to concentrate the olivine in the center of the pipe. Simkin (1967, p. 68) describes how, in simple axial migration,"... particles move toward an equilibrium position at the tube axis during flow". Next the material is intruded higher in the crust. In the study area, the material spread out to form the sill while it retained the core of olivine crystal cumulate. Bhattacharji (1967, p. 70) noted that gravity settling of the olivine only becomes significant below certain flow rates and " ... chilling of the lower margin and increased crystallization, which produce an increase in the viscosity of the magma, tend to hinder crystal settling close to the margin". Locally, however, some settling may occur or magma may be squeezed away leaving parts of the dunite near or adjacent to the country rocks. This relationship was observed in some localities in the study area. After solidification, mafic veins were emplaced in the sill (Figure 15a).
10

CAROLINA

BG

CEDAR

N

MTN.

Scale 1:125,000

. Garnet Mica Schist

Biotite Gneiss

Ultramafic and Related Rocks .-~

Figure 12. Generalized geologic map of the Brasstown Antiform and adjacent area.

11

Generalized block diagram of the Brasstown Bald area
Figure 13. Block diagram of the Brasstown Antiform and a south-plunging antiform. The offset noses indicate crossfolds of the structures.
12

Step 1 - Differentiation of mantle-derived gabbroic magma and olivine perhaps in a magma chamber.
Step 2 - Injection of gabbroic magma transporting a central mass of olivine "crystal mush" based on Bhattacharji's model of magmatic flowage differentiation.
Step 3- Emplacement of the magma form
ing a sill within the Grenville Basement during the early stages of regional meta morphism.

CROSS SECTIONS
LOWU CRUST OR UPPER MANTLE
GABBROIC MAGMA
,......,....,.,.,-.-,=~~\...-"OLIV IN E "CRYSTAl MUSH "
t
1--1---oL iVINE
"CRYSTAl MUSH '1

Step 4 - Deformation and metamorphism of the sill during regional metamorphism. Olivine gabbro at the edge of the sill meta morphoses to amphibolite but due to the dry nature of the previously metamor phosed Grenville basement not enough water is present to react with the near central olivine gabbro and troctolite.
Figure 14. Origin of the Lake Chatuge Sill.
13

The occurrence of well-developed foliation parallel to that of the country rock within the orthoamphibolite margin around the sill indicates that a high grade metamorphic event, or the peak of such an event, occurred after emplacement (Figure 15b). The high grade metamorphism may have obliterated the texture of any contact metamorphism formed during emplacement. If emplacement occurred during the early stages of regional metamorphism, the country .rocks would have been warm. In such a situation a high thermal gradient may not have existed between the magma and country rocks, thereby reducing the contact metamorphic effects.
Hartley (1973) presents 8 lines of evidence relating the amphibolite, formed by metamorphism, to the mafic rocks nearer the center of the sill, which were almost unaffected by the metamorphism. This may be explained by the apparent lack of enough water from the "dry" Grenville basement to hydrate and react with all of the mafic rocks during the metamorphism. Therefore, those mafic rocks at the sill margin were highly affected while those nearer the center of the sill show little effect, perhaps only a crystal re-equilibration which destroyed any crystal zoning in the olivine, pyroxene, and plagioclase. Olivine, such as that in dunite near the sill margin, would be stable during the high grade metamorphism even in the presence of water. Had all of the rocks, however, been emplaced in sediments, rather than Grenville basement, the results probably would have not been the same.
There is one problem inherent if the country rocks in the area are considered to be Grenville basement. If these rocks represent complexly folded basement, it is difficult to imagine the sill intruding so conformably between two Grenville-age units that were to become a schist and gneiss during the major Ordovician metamorphism.
DISCUSSION
The authors feel that the information gained during the study of the Lake Chatuge intrusion may have a bearing on the origin of other alpine-type ultramafic intrusions, especially those in the Blue Ridge. We use the following criteria to defend the Lake Chatuge intrusion as an alpine-type ultramafic unit.
Alpine-type ultramafic intrusions are small to intermediate intrusions that occur in deformed mountain belts. Generally the intrusions were emplaced during or just prior to metamorphism. Some of the intrusions consist almost entirely of serpentinite while many others are composed of dunite commonly originally associated with other igneous rocks, especially gabbro. Other alpine ultramafites together with spilite (altered basalt) and chert are termed the Steinmann trinity and were probably emplaced higher in the eugeosynclinal sequence. Still other alpine ultramafites along with associated gabbro, basalt, and graywacke .define the ophiolite complexes. The term "alpine-type" is used by some geologists in a broad sense covering many types of intrusions while other geologists make it more restrictive. The reader is referred to Wyllie (1967, p. 4-5, 203-204), Hyndman (1972, p. 103-113), Thayer (1967) and Jackson and Thayer (1972) for additional general information on alpine intrusions.
At least one geologist (Thayer, 1974, personal communication) has indicated that the Lake Chatuge sill may not be an alpine-type intrusion. The Lake Chatuge intrusion is, however, similar to others in the Blue Ridge. They are confined to a relatively narrow belt noted by Carpenter (1970) as the thermal axis of mid-Paleozoic metamorphism. They are all, therefore, probably related to the metamorphic event. Since they are all deformed by the metamorphism they were probably emplaced prior to the peak of metamorphism. This indicates a similar time of emplacement. Also the Lake Chatuge intrusion and many of the others in North Carolina are enveloped by an amphibolite,
14

OLIVINE GABBRO and TROCTOLITE
OLIVINE GABBRO and TROCTOLITE GRENVILLE BASEMENT
Figure 15a. Generalized section of part of the Lake Chatuge Sill before metamorphism.

rs sss

2 ( S ~ SS coarse diallage.bytownite OLIVINE GABBRO and TROCTOLITE ) )

(. ( (h (bl( d(

:2. transition zone(limit of hydration)

coarse om en e +anaes1ne

+clinozoisite :t corundum ORTHOAMPHIBOLITE( MET AGABBR

GARNET MICA SCHIST

Figure 15b. Generalized section of part of the Lake Chatuge Sill after metamorphism but before local alteration.
15

perviously termed Roan Gneiss. In addition corundum is associated with most of these intrusions and otherwise the mineralogies and lithologies, with the exception of garnetpyroxene gneiss, are similar. Therefore, if the intrusion at Lake Chatuge is not an alpine-type intrusion, then many others in the Blue Ridge are not either. It should be noted, however, that the Appalachian ultramafic belt is considered to be an alpine-type belt by Hess (1955), Turner and Verhoogen (1960) and others. In addition, these intrusions meet the requirement of alpine intrusions in that they occur in a deformed mountain belt.
The monolithologic units of amphibolite, serpentinite, and dunite that occur locally in the Blue Ridge may be directly related to the other intrusions in the belt in a simple way. Since the olivine probably occurred as a crystal concentrate, gabbroic magma locally may have been squeezed away. This segregation would form separate bodies of dunites and gabbros and after high grade metamorphism, dunites and amphibolite. Later alteration could have changed dunite bodies to serpentinites or chlorite schists. Units of serpentinite and other altered ultramafic and mafic equivalents that occur in some localities in the Blue Ridge may represent bodies segregated in this way but emplaced in the sediments overlying the Grenville basement. Abundant waters necessary for metamorphic and alteration reactions would be available in such an environment as contrasted with the deeper, previously metamorphosed, and relatively "drier" rocks of the Grenville basement.
The Lake Chatuge sill is the most conspicuous unit in the metamorphic core of the Blue Ridge . Its marked lithologic contrast with the adjacent country rocks and its brilliant ocherous saprolite make it easily traceable in this complexley folded portion of the Blue Ridge thrust sheet. It has been used as a stratigraphic marker to define the limbs of the Brasstown Antiform, a major 100 square mile (259 sq. km) structure and it is hoped that it can be used in adjacent areas of Georgia and North Carolina to determine other structural features.
16

REFERENCES CITED
Bhattacharji, S., 1967, Scale model experiments on flow differentiation in sills, in P. J. Wyllie, ed. Ultramafic and related rocks: New York, John Wiley and Sons, p. 69-70.
Carpenter, R. H., 1970, Metamorphic history of the Blue Ridge province of Tennessee and North Carolina: Geol. Soc. America Bull., v. 81, p. 749-762.
Dallmeyer, R. D., 1974, Eclogite intrusions in an alpine peridotite sill, Georgia-North Carolina: their chemistry and petrogenetic evolution: Am. Jour. Sci., v. 274, p. 356-377.
Hadley, J. B., and Nelson, A. E., 1971, Geologic Map of the Knoxville Quadrangle, North Carolina, Tennessee, and South Carolina: U.S. Geological Survey Misc. Geol. Inv. Map, I-654.
Hartley, M. E., III, 1971, Ultramafic and related rocks in the vicinity of Lake Chatuge, Clay County, N. C. and Towns County, Ga.: Geol. Soc. America Abstracts, 5th Annual Meeting, Southeastern Section, Blacksburg, Va., 1971, p. 317.
_ _ _ 1973, Ultramafic and related rocks in the vicinity of Lake Chatuge: Ga. Geol. Survey Bull. 85, 61 p.
Hartley, M. E., III, Walker, R. L., and Jones, L. M., 1971, Strontium isotope composition of the ultramafic and related rocks in the vicinity of Lake Chatuge, Clay County, N.C. and Towns County, Ga.: Geol. Soc. America Abstracts, 5th Annual Meeting, Southeastern Section, Blacksburg, Va., 1971, p. 316-317.
Hess, H. H., 1955, Serpentinites, orogeny, and epeirogeny: Geol. Soc. America Spec. Paper 62, p. 391-408.
Hyndman, D. W., 1972, Petrology of igneous and metamorphic rocks: New York, McGraw-Hill, p. 103-113.
Jackson, E. D., and Thayer, T. P., 1972, Some criteria for distinguishing between stratiform, concentric, and alpine peridotite-gabbro complexes: 24th International Geological Congress, section 2, p. 289-296.
Jones, L. M., Hartley, M. E., III, and Walker, R. L., 1973, Strontium isotope composition of alpine-type ultramafic rocks in the Lake Chatuge district, Georgia-North Carolina: Conf. Mineralogy and Petrology, v. 38, p. 321-327.
Keith, A., 1907, Nantahala Folio: U.S. Geol. Survey Atlas no. 143, 12 p.
Kellberg, J. M., 1943, Basic intrusives in the Chatuge Reservoir: Manuscript Report, T.V.A. Files, Geology Branch, Knoxville.
Penley, H. M., 1974, The Brasstown Antiform, Towns and Union Counties, Georgia: Ga. Acad. Sci. Abstracts, v. 32, p. 15.
17

Simkin T. 1967 Flow differentiation in the picritic sill of North Skye, in P. J. Wyllie,



-~

, .

' r

.

.

, ed: Ultra,mafic and related rocks: New York, John Wiley and Sons, p. 64-69.

Thayer, T. P, 1967, Chemical and structural relations in ultramafic and feldspathic rocks in alpine intrusjve complexes: in.Ultramafic and related rocks (P. J. Wyllie, ed.): New York, John Wiley and Sons, p. 222-239.

Turner, F~ t,-arid V.erhoogeri, J., 1960, Igneous and metamorphic petrology: New York,

McGraw-Hill, 694 p.

-

-

-

Wyllie, P. J. (ed.), 1967, Ultramafic and related rocks: New York, John Wiley and Sons, 464p. -: ..

18

Mileage from Clayton
miles kilometers

0

0

0.2

.3

ROAD LOG - SUNDAY, OCTOBER 27, 1974

Interval mi. km .

0

0

0.2

. 3

Heart of Rabun Motel. Clayton, Georgia. Intersection of Highway 76 and 44. GO WEST on Highway 76.
Downtown Clayton, Main at Savannah Street.

9.2 14.7 11.3 18.1 16.8 26.9 24.5 39.2
24.8 39.7 24.9 38.8 26.2 41.9 27.2 43.5

.. . ~ ) :... '.: ,. ~.~:..---

9.0 14.4 2.1 3.3 5.5 8.8 7.7 12.3

0.3

.4

0.1

.1

1.3 2.1 1.0 1.6

Tallulah River Bridge at the north end of Lake Rabun.
Intersection of Highways 197 and 76. KEEP RIGHT (WEST) on Highway 76.
Towns-Habersham County boundary.
Intersection of Highways 17, 75 and 76. STAY RIGHT (NORTH) on Highway 76. The next 4.6 miles will be on biotite gneiss.
Intersection of Highways 288 and 76. CONTINUE on Highway 76.
Bridge on the Hiawassee River at the southern end of Lake Chatuge.
City limits of Hiawassee, Georgia.
Downtown Hiawassee, Main at Bell Street.

19

Mileage from Clayton
mi. km.

Mileage from Hwy.

76-75 Intersection

in Hiawassee

Interval

mi. km.

mi. km.

27.7 44.3

0

0

0.5 0.8

Intersections of Highways 76 and 75 in Hiawassee. TURN RIGHT (NORTHEAST) onto Highway 75. Lake Chatuge is visible on the left.

28.0 44.8

0.3

.4

0.3

.4

The mountain now visible ahead is Bell Mountain,

which is supported by a quartzite lense and sur-

rounded by the biotite gneiss unit. The white

quartzite is exposed in the large cut on the top of

the mountain.

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1 cnile ,, ..

Country rock: GMS = garnet mica schist; BG = biotite gneiss.
Sill rocks: D = dunite; T =troctolite; OG = olivine gabbro
W = wherlite; A= orthoamphibolite; GP = garnet pyroxene gneiss (metaaeclogite)

20

28.7 45.9 29.8 47.7 29.9 47.8
30.1 48.2 30.5 48.2 30.6 48.9
31.0 49.6

1.0 1.6 2.1 3.4 2.2 3.5
2.4 3.8 2.8 4.5 2.9 4.6
3.3 5.3

0.7 1.1 1.1 1.8

0.1

.2

0.2

.3

0.4

. 6

0.5

.8

0.4

.6

The mountains now visible about a mile ahead are underlain by the garnet-mica schist unit and occur in the core of a south-plunging antiform outlined by the Lake Chatuge sill.
Small sideless bridge on Highway 75. Dunite is the least resistant rock in the area and underlies both the small stream valley that the bridge crosses and part of the lake visible on the left. The dunite continues up the valley on the right.
TURN LEFT (WEST) onto the unmarked paved road at the Lower Bell Creek Church sign.
The small hills along the road on the right (north) side and even the small bunker-sized knobs on the left are supported by garnet-pyroxene gneiss (metaeclogite) and surrounded by orthoamphibolite and dunite.
Orthoamphibolite saprolite is exposed in the road, bank on the right side of the road.
The curvature of the lake on the left follows the dunite in the nose of the south-plunging antiform.
Lower Bell Creek Church. Continue around the curve on the paved road.
TURN LEFT onto the small dirt road leading toward the lake. Brasstown Bald Mountain is visible about 9 miles ahead.
Orthoamphibolite saprolite is exposed in the roadbanks on the right side of the road. The orthoamphibolite grades into olivine gabbro and troctolite towards the lakeshore stop ahead.
STOP 1. The sill exposed along the shore of Lake Chatuge. Gene Hartley, University of Georgia (moderator).
Most of the rocks present in the sill can be seen at this stop when the water level is low during the late fall and winter. The dunite, which is the least resistant rock, underlies most of this part of the lake. The coronas in the troctolite are very resistant and impart on overall resistance to the rock. The thickest unit of troctolite in the area underlies the hill at this stop. It is gradational into amphibolites on both sides. Well banded troctolite can be seen here when the lake is low. Locally the troctolite grades into dunite along the strike of the sill, elsewhere the contacts are sharp. Olivine gabbro is also present with the troctolite. Several small knobs, some only a few feet high and a few yards long, occur in both the dunite and amphibolite here and are underlain by garnet-pyroxene gneiss (metaeclogite). Each of these is commonly surrounded

21

34.2 54.7 34.8 55.7

6.5 10.4 7.1 11.4
IIAMIY MOUNTAIN

3.2 5.1 0.6 1.0

Stop 1 (continued)
by a thin selvage of black amphibolite. The end of the small peninsula extending from the church is a good example of this. Wherlite, a peridotite composed of olivine and clinopyroxene, here represented by serpentine and clinopyroxene, crops out as a thin lense southeast of the church along the lake.
Care should be taken not to interpret each boulder as an outcrop. The shorelines were bulldozed during the lake construction.
The Lower Bell Creek Corundum Mine is also at this stop. Although once visited by collectors of corundum and hornblende, the private property has been generally off limits in the past few years. Hornblende crystals up to 8 inches long have been reported from the mine which is located in a metamorphosed mafic pegmatite consisting of hornblende, andesine plagioclase, clinozoisite, and corundum. The author has found masses of corundum weighing up to 2 pounds in this area. Most corundum is most easily found, however, by panning. The lakeshore is still a favorite collecting locality.
RETURN to Highway 76 and 75 intersection in Hiawassee.
TURN RIGHT (NORTHWEST) onto Highway 76. The next 2 miles will be on biotite gneiss.
Bridge on Highway 76. The mountain visible about 3 miles ahead is
Ramey Mountain. Its north slope is a dip slope of orthoamphiholite plunging northwards outlining the north nose of the Brasstown Antiform. The dip slope accounts for the wide apparent width on the geologic map. The cross section below shows the rock unit sequence.

.....

......

......

.............

.........
.....

..... .....

GARNET MICA SCHIST
Cross section of the north side of Ramey Mountain No vertical exaggeration Scale 1:24,000
Cross section of the north side of Ramey Mountain. No vertical exaggeration. Scale: 1:24,000
22

36.2 57.9

8.5 13.6

37.1 59.4

9.4 15.0

37.2 59.5

9.5 15.0

37.5 60.0

9.8 15.7

37.7 60.3

10.0 2.0

1.4 2.2

0.9 1.4

0.1

.2

0.3 0.5

0.2

.3

The mountains visible several miles to the left (south) including Brasstown Bald occur in the center of the Brasstown Antiform and are underlain by garnet mica schist.
Intersection of Highways 76 and 288 at the Gulf Station. TURN LEFT (South) onto Highway 288. The next 2.0 miles will be on orthoamphibolite and will parallel the margin of the sill . .RGadcuts Jliong the way show amphibolite saprolites.
Entrance to Lake Chatuge Recreation Area. The recreation area is underlain by the biotite
gneiss unit.
TURN RIGHT (west) onto paved road at Union Hill Church at the Hog Creek bridge.
The road cut shows orthoamphibolite which grades into olivine gabbro over the next 0.3 mile.
STOP 2. Sill exposed on the northeast flank of the Brasstown Antiform. Gene Hartley, University of Georgia (moderator): UNLOAD BUSES at the small unmarked dirt road and proceed on foot. We will reload 0.5 mile down the dirt road.
Olivine gabbro and troctolite outcrops on the small ridge we are crossing. These rocks are similar to those seen at Stop 1. The small dirt road on the right leads to the Hog Creek Corundum Mine. The old mine was investigated by the U. S. Bureau of Mines during World War II as part of an evaluation of natural abrasives resources. The valley ahead is underlain by dunite as was the lake at Stop 1. The mountains ahead (west) are underlain by the garnet mica schist unit. The schist unit contains a rutile zone which, similar to the sill, is a stratigraphic marker. The mountains to the right (north) are capped by the orthoamphibolite unit plunging northwards.

23

37.8 60.4

10.1 16.2

38.4 61.4

10.7 17.1

39.2 62.7 40.2 64.3

11.5 18.4 12.5 20.0'

40.4 64.6 40.8 65.3

12.7 20.3 13.1 21.0

41.2 66.0

13.5 21.6

41.3 66.1

13.6 21.8

43.0 68.8

15.3 24.5

43.2 69.0

15.5 25.0

0.1

.2

0.6 1.0
0.8 1.3 1.0 2.0

0.2

.3

0.4

.6

0.4

.6

0.1

.2

1.7 2.7

0.2

.3

The trenches in the pasture are prospect trenches for nickel. Although little nickel was found some bright green garnierite occilrs in the dunite.
The hill in the pasture is underlain by a lense of near vertically dipping, banded troctolite surrounded by dunite. The lense is approximately 250 feet long and 50 feet wide near the center.
The ultramafics continue down the valley to the south. The amphibolites associated with them probably dominate to the south.
We will proceed over the alluvium in the valley to the garnet mica schist that borders the sill. This schist is part of the same unit that underlies the mountains previously mentioned.
LOAD BUSES. RETURN to Union Hill Church at Highway 288.
TURN LEFT (North) onto Highway 288.
Intersection of Highways 76 and 288. TURN LEFT (west) onto Highway 76 and proceed towards the base of Ramey Mountain shown in the previously shown cross-section.
Orthoamphibolites in large road cut.
The hill on the right !s underlain by the biotite gneiss unit.
Arm of lake on the right. The lake here is underlain by dunite similar to that at Stop 1.
LUNCH STOP. T .V .A. road side park on the lake near the Friendship Community sign and an electric substation. The previously shown cross section shows the lunch spot.
CONTINUE west on U.S. 76 toward Young Harris, Georgia.
Brasstown Gap Here a large road cut of intensely folded biotite
gneiss is cut by a granite intrusion that weathers to a near-white homogenous saprolite.
Young Harris Valley to the left. The valley is underlain by orthoamphibolite and
dunite. Dunite offers little resistance to weathering. This and the headward erosion of the surrounding orthoamphibolite and biotite gneiss formed the valley.

24

4~.7 73.1

18.0 28.8

2.5 4.0

The town of Young Harris at the intersection of U.S. 76 and Ga. 66.
The town is underlain by orthoamphibolite and dunite which mark the western limb of the Brasstown Antiform. The hills on the right (west) are underlain by the biotite gneiss unit while those on the left (east) are underlain by the garnet-mica schist sequence.

46.1 73.7

18.4 29.4

46.5 74.4

18.8 30.0

48.9 78.2

21.2 33.9

49.2 78.7 49.3 78.9

21.5 34.4 21.6 34.6

50.3 80.5

22.6 36.2

51.5 82.4

23.8 38.1

51.6 82.5

23.9 38.2

0.4

.6

0.4

.6

2.4 3.8

0.3

.5

0.1

.2

1.0 2.0

1.2 2.0

0.1

.2

25

Country Rock: GMS =garnet-mica schist Bgn = biotite gneiss
Sill Rocks: Cu = chloritized ultramafic A = orthoamphibolite
Stream alluvium = QAL
STOP 3. Western limb of the Brasstown Antiform. Mike Penley, Georgia Geological Survey (moderator).
The sill appears here on the western limb of the Brasstown Antiform. The sill has extended from Lake Chatuge under the town of Young Harris and is exposed here on Ga. 66. Orthoamphibolite saprolite occurs from the other side of U.S. 76 to this outcrop of chloritized ultramafics.
TURN AROUND and return to U.S. 76 in Young Harris.
View of Track Rock Gap in the distance on left. The gap is underlain by the Lake Chatuge Sill continuing southward .
Brasstown Creek at Union-Towns County line.
TURN LEFT (south) on unmarked paved county road at Track Rock Archeological Sign.
Ultramafic outcrop altered to chlorite and talc on right.
Track Rock Archeological Site. Petroglyphs of bird tracks, carved presumably by
Indians, are preserved here in chloritie soapstone.
Outcrops of chloritized olivine gabbro on right.

52.3 83.7

24.6 39.4

53.8 86.1

26.1 41.8

55.4 88.6

27.7 44.3

55.8 89.0

28.1 45.0

57.1 91.3 57.5 92.0

29.4 47.0 29.8 47.7

58.7 85.9

31.0 50.1

61.7 98.7

34.0 54.4

0.7 1.1

1.5 2.4

1.6 2.6

0.4

. 6

1.3 2.1 0.4 .6 1.2 1.9
3.0 4.8

TURN LEFT (east) on unmarked paved county road at the sign marked Track Rock Cottages.
For the next 4 miles the road crosses contacts of the sill with both biotite gneiss and garnet-mica schist several times. Through this section the units are complexly folded and cross cut by several large granitic pegmatites.
Hills on left are supported by the garnet-mica schist unit.
TURN LEFT (southeast) on unmarked paved county road.
Alexanders General Store. We are now located in the southern nose of the
Brasstown Antiform where the sill is trending approximately east-west. The contact between the sill and the garnet-mica schist can be seen in a 100' section of the dirt road to the left.
New Liberty Baptist Church on left.
Stop at junction with Ga. 180. Small isoclines in orthoamphibolite to the
immediate right. Turn left and continue to the northeast on Ga. 180.
Biotite gneiss on right. For the following 1.1 miles the road cuts diagonally through the sill into the garnet-mica schist unit. At Soapstone Gap, appropriately named and approximately in the center of the sill, outcrops of altered ultramafics can be seen.
Large outcrops of orthoamphibolite on right

Sill: M = metatroctolite A = orthoamphibolite
Country Rocks: GMS =garnet-mica schist
Bgn = biotite gneiss
26

62.4 100.0 34.7 55.5

0.7 1.1

STOP 4: Jacks Gap. Eastern limb of the Brasstown Antiform. Mike Penley, Georgia Geological Survey (moderator).
We have followed the trace of the Lake Chatuge sill around the southern nose of the Brasstown Antiform and then back to the northeast through Soapstone Gap to our present location at Jacks Gap. Here a complete stratigraphic sequence can be seen from the biotite gneiss through the sill to the contact between the sill and the garnet-mica schist. A zone metatroctolite is present near the biotite gneiss contact. The entire sequence dips steeply to the southeast.

27