Field excursion: Stone Mountain-Lithonia district

field Excursion
Stone .Mountnin ,-;
Lithonia District
WILlARD H. GRANT
ANNUAL MEETING
Sotlthe.astern Section
The Geological Society
of America
APRIL14.. 19b2
GUIDEBOOK NUMBER 2
DepaTtment ofMines, .Mining and
Geology
GARLAND DEYTON, Dlh'ECTOR Atlanta, Georgia
COVER PAGE
Lithonia Granite Gneiss The dark bands are recrystallized biotite which marks folia of an original gneiss contorted by flowage.
Photograph courtesy Davidson Granite Company.

NOTICE
At the time of preparation of this guidebook, development of a new State Park at Stone Mountain is underway. It is anticipated that this may necessitate eventual changes in the road log and possibly in stops in the park area.

Table of Contents

page

Introduction

5

Origin of Stone Mountauin.___ _ -- - -- -- - - - - -- --

5

Metamorphism

5

Amphibolites

5

Lithonia gneiss

- - - - -- - - - - - 5

Geologic history

- -- - -- - - - - -- - - - - - 5

Nomenclature

---------- - - - -

6

Map units ----- - - -- - - - --

6

Modal analyses __ - ----- - - ----

6

List of stops:

1. Contact between the Stone Mountain granite and porphyroblastic biotite gneiss - - ---- -- - 6

2. Stone Mountain Granite Co., quarry-------------------------------------------------------------------------------------------------------------- 8

3. Biotite gneiss-------------------------------------------------------------------------------------------------------------------------------------------------- 10

4. Porphyroblastic biotite gneiss_

------

10

5. Contact with the Lithonia gneiss,_ _ _ __ _ __

10

6. Muscovite-quartz schist

_______

11

7. Porphyroblastic biotite gneiss and biotite-hornblende gneiss___________________________________________________________________________ 12

8. Amphibolites_____________________________________________________________________________________________________________________________________________ 14

9. Muscovite quartzite, garnet-mica schist and amphibolite_____________________________________________________________________________ 15

10. Mount Arabia, Lithonia gneiss_____________________________________________________________________________________________________________________ 16

List of Figures:

Figure 1. The probable contact relations between the porphyroblastic gneiss and Stone Mountain granite____ 7

2. Muscovite granite dike cutting Stone Mountain granite

9

3. Generalized section along Ga. 124

- - -- - - - - - - - - - - - - - - - ------- 12

4. Amphibolite boundins and a small pegmatite dike _ __

12

3

5. Pegmatite dikes faulted along the foliation in porphyroblastic biotite gneiss saprolite________ ____________ 12

6. Epidote pod, minor quartz-feldspar pods and quartz-feldspar dikes transecting biotite-

hornblende gneiss

14

7. Stone Mountain, taken from the south side__________________________________

- - - - - - - - 14

8. Geologic sketch map of a borrow pit near Lithonia, Georgia ____

16

9. Three phases of the Lithonia gneiss__________________________________________________ _ _ __ _ __ 17

10. A contorted biotite-rich band in the Lithonia gneiss _ _ __ _ - - - - - - -- - - - 18

11. Folded garnet-rich bands in the Lithonia gneiss 12. Photomicrograph of garnet-mica schist___________ ___ __

18 _ _ _ _ 19

13. Photomicrograph of quartzite_____________________________________________________________________________________________________ 19

14. Photomicrograph of quartz-muscovite schist___________________________________________________________________________________ 19

15. Photomicrograph of biotite-hornblende gneiss__________________________________________________________________________ 19

16. Photomicrograph of biotite gneiss-------------------------------------------------------------------------------------------------- 20 17. Photomicrograph of porphyroblastic biotite gneiss_ _ _ _ _ _ _ _ __ _ _ _ __ 20 18. Photomicrograph of amphibolite______________________________________________________________________________________________________ 20 19. Photomicrograph of Lithonia gneiss_______________________________________________________________________________________________ 20 20. Photomicrograph of xenolith from Stone Mountain__________________________________________________ -------------------- 21 21. Photomicrograph of Stone Mountain granite________________________________________________________________________________ 21

List of modal analyses Tables:

Table I. Porphyroblastic biotite gneiss and amphibolite___________________________________________________________________________ 7

Table lA. Stone Mountain granite_____________________________________________________________________________________________________________________ 7

Table II. Stone Mountain granite___________________________________________________________________________________________________ 8

Table IIA. Xenolith in Stone Mountain granite________________________________________________________________________________________________ 9

Table III. Biotite gneiss _ _ __

- 10

Table Table Table

IV. Grahite dike, Lithonia gneiss, and muscovite quartzite_________________________________________________________________ 11
v. Muscovite-quartz schist ------------------------------------------------------------------------------------------------------------------ 11
VI. Porphyroblastic biotite gneiss, biotite gneiss, and granite dike_______________________________________________________ 13

Table VIA. Biotite-hornblende gneiss and amphibolite______________________________________________________________________________________ 13

Table VII. Amphibolite ----------------------------------------------------------------------------------------------------------------------------------- 14 Table VIII. Garnet-mica schist, mica quartzite, and sillimanite quartzite__________________________________________________________ 15

Table IX. Lithonia gneiss ------------------------------------------------------------------------------------------------------------------- 17 Table IXA. Gamet layers in the Lithonia gneiss____________________________________________________________________________________________ 18

4

INTRODUCTION
This guidebook is for a one day field trip through the Stone Mountain-Lithonia District. The assistance of Dr. J. G. Lester and others in the Emory University Geology Department is gratefully acknowledged.
The area has been the subject of investigation for many years. One of the earliest studies by C. W. Purington (1894) described Stone Mountain as a laccolith. T. L. Watson (1902) gives chemical analyses and petrographic descriptions of both the Stone Mountain and Lithonia rocks. Detailed petrographic work was done by J. G. Lester (1938). Reconnaissance mapping of the entire area was completed by Crickmay (1939). Crickmay (1952) discusses the Lithonia gneiss and Stone Mountain granite. The first detailed map and most complete study was made by Herrmann (1954). Other workers who have contributed include: H. E. Cofer (1948) who studied the petrography and mineralogy of Mt. Arabia; K. G. Walter (1958) made studies of pegmatites of the district and H. S. Hanson (1958) did detailed petrographic work particularly on the twinning of feldspars. The latest work was done on weathering and exfoliation at Stone Mountain by Hopson (1959).
ORIGIN OF STONE MOUNTAIN
Most workers agree that Stone Mountain granite was of igneous origin. Herrmann (1958, p. 75) based his conclusions on the uniformity of mineral composition, the formation of discordant dikes and the conformity of flow structures to discordant contacts. To this may be added the sharpness of the contacts, and the presence of chill zones in some Xenoliths. Hanson (1958) studied the character and frequency of plagioclase twins using the method of Gorai (1950). His conclusions also favor an igneous origin.
METAMORPHISM
Mineral assemblages (garnet-sillimanite-muscovite-plagioclase-biotite and quartz-garnet-muscovite-biotite in the schists and andesine-hornblende in the amphibolites) indicate the metamorphic grade was the sillimanite-almandine-muscovite subfacies. Local epidotization which notably affects the amphibolites suggests a later stage of alteration. The presence of the mineral assemblage hornblende-plagioclase-diopside reported by Herrmann ( 1954, p. 24) indicates the highest metamorphic facies was sillimanite-almandine-orthoclase subfacies. The local presence of the low grade assemblage talc-actinolite-chlorite which is associated with magnesian schists further suggests hydrothermal alteration in small areas. All facies referred to are those of Turner and Verhoogen (1960). All of these assemblages point to a long complex metamorphic history, the details of which are yet to be completely unraveled.
AMPHIBOLITES
The origin of amphibolites is a subject for debate. There are some of these rocks whose mineral assemblage indicate a sedimentary origin and others whose origin is still unknown. However, one of the most common amphibolites which has a mineral composition of about two-thirds hornblende and one-third plagioclase is very widespread in the central Piedmont. Chemical analyses of these rocks are very similar to that of basalt. Their field relations suggest they might have been either sills or flows. There is the possibility that some might be metamorphosed gabbros, but gabbros are not common enough.
LITHONIA GNEISS
Herrmann 1954 (p. 68-72) calls the Lithonia gneiss a migmatite. The evidence to be seen on this excursion will entirely support his conclusions.
GEOLOGIC HISTORY
The following is abstracted from Herrmann (1954, p. 78-79). The original rocks were sandy to argillaceous sediments with possible volcanic flows or ash deposits.
Deformation and increase in temperature metamorphosed the rocks. Also during deformation the Lithonia gneiss was formed by injection and replacement with a syntectonic magma.
5

Stone Mountain was intruded after the major deformation was complete. This is based on the fact that it deflects regional lineation. This is correlated with late Appalachian deformation giving Stone Mountain a Permian age.
Folding in the mica schist, mica quartzite sequence shows two directions of deformation, an earlier one with a northeasterly trend and a later refolding with a northwesterly trend.

NOMENCLATURE
Stone Mountain granite is not a granite according to many well established classifications. It is a quartz monzonite or adamellite. However, long usage and the fact that such fine distinctions can not be made in the field has led the present writer to retain the word granite.

MODAL ANALYSES
Modal analyses are made by the point count method, Chayes (1956). Since the number of sections is small the data can be regarded as only a rough indication of the overall composition of the rocks.

MAP UNITS
The mapping units used by Herrmann (1954) are; Stone Mountain granite, Lithonia gneiss, porphyroblastic biotite gneiss, biotite-hornblende gneiss, amphibolite, muscovite-quartz schist, muscovite quartzite, and garnet-mica schist. Photomicrographs of selected samples of each of these rocks is shown in Figures 12-21.

ROAD LOG

Mileage:
0.0 0.3 4.3

Fork at the intersection of U.S. 29 and Ga. 236 in Tucker. DuPont Paint Plant on the left. Stop 1.

Bear right on Ga. 23~.

CONTACT BETWEEN THE STONE MOUNTAIN GRANITE AND THE PORPHYROBLASTIC BIOTITE GNEISS
The actual contact is not exposed but from the fragmentary evidence available, it must appear similar to that shown (Fig. 1).
The road cut slices obliquely to the fold axes which strike about N25E and plunge gently to the northeast. This accounts for the gentle undulation of the gneissic banding. A small steeply dipping fault is visible in the northwest end of the road cut.
The porphyroblastic biotite gneiss, dark gray when fresh and grayish brown in saprolite, contains bands ranging in width from Y.." to 2". Some bands are composed of quartz and feldspar others are rich in biotite. Amphibolites occur in pinching and swelling layers ranging in thickness from ~" up to a few feet. The saprolite is ochre colored and the fresh rock is black with scattered specks of white plagioclase. Modal analyses of some samples are given in Table I.
Approaching the contact between the granite and the gneiss, microcline porphyroblasts become abundant in the soil. They range in size from Y.. to 3 em. and are the result of potash metamomatism associated with the intrusion of Stone Mountain granite.
Under the power line immediately south of the porphyroblastic biotite gneiss is a badly weathered outcrop of Stone Mountain granite. Modal analyses of two granite samples are given in Table lA. It is noted that the amount of microcline exceeds that of plagioclase. The reverse of this is the rule in the main mass of the granite.
Foliation is virtually not present in this part of the granite. However, water dissected saprolite shows a very faint layering parallel to the foliation in the country rock. Pegmatites, which are usually well developed in many contact zones are scarce.
6

FIGURE 1- SKETCH SHOWING THE PROBABLE CONTACT RELATIONS BETWEEN THE STONE MOUNTAIN GRANITE ANO THE PORPHYROBLASTIC BIOTITE GNEISS.

TABLE I

Porphyroblastic Biotite Gneiss and Amphibolite

A B

Quartz

23.5 15.1

Plagioclase An80 _______45.0 52.0 Microcline _________________19.6 15.6

Biotite ---------------------------10.9 14.3 Muscovite _ ______ 0.1 0.3

Epidote ----------------------- 0.7 2.7

c

Hornblende ,_ _ _ 63.4 Plagioclase An4o _____29.8

Quartz

2.9

Epidote ____

3.9

Biotite - - - - - - - Tr

Magnetite

Tr

A & B - Porphyroblastic biotite gneiss C- Amphibolite

*Sample extremely weathered

TABLE lA Stone Mountain Granite
1* 2 Quartz ________________________30.2 26.7 Plagioclase An18 _________30.8 28.3 Microcline ____________________33.7 29.8 Muscovite-------------------- 3.7 13.8 Biotite _ _________ 1.5 1.4
7

Mileage: In passing to Stop #2 the steep north face of Stone Mountain is visible. Continue on Ga. 236 till it intersects U. S. 78.
5.4 Intersection of U. S. 78 and Ga. 236 turn left. New railroad station for the Stone Mountain Railway and Memorial on the Mountain to your right.
6.0 Turn right on dirt road. 6.4 Stop #2.
STONE MOUNTAIN GRANITE CO. QUARRY ON THE EAST SIDE OF THE MOUNTAIN
The granite has a well-developed mica foliation which Herrmann (1954, p. 55) calls flowage foliation. Scattered phenocrysts of feldspar and more uniformly distributed biotite flakes occur in an almost white ground mass of quartz, feldspar and muscovite. A slightly more acidic dike (Fig. 2) bordered by pegmatite dikes cuts the main granite. Modal analyses of samples are given in Table II.
Tourmaline clusters surrounded by 2"-3" quartz-feldspar rims are scattered randomly in the granite. Herrmann ( 1954, p. 60) says these clusters are often concentrated in a zone parallel to the foliation.
Exploratory petrofabric studies show that mica has a strong maxima normal to the foliation and an incomplete girdle.
There are three joint-directions in the granite. The strongest strikes N60W, a weaker one strikes N25W and the weakest strikes N15E. Dips range from 70 to vertical. Related to the flow-fold axes on the geologic map, Herrmann (1954), the strongest joint is be the others ac and oblique respectively. Walter (1958, p. 49) shows that joints control the orientation of most pegrnatites. The sheet joints or exfoliation shells, are those which parallel the surface of the mountain. These are best explained by expansion of the mountain resulting from release of confining pressure caused by removal of the overlying rock (Hopson 1958, p. 70).

TABLE II

Modal Analyses of Stone Mountain Granite

A B c

Quartz ________________________27.2 28.6 29.5

Sadie Oligoclase __________39.3 33.9 33.6

Microcline

22.1 26.4 31.8

Muscovite ___________________ ! 0.3 9.2 4.6

Biotite -------- 1.1 1.6 0.4

Epidote

Tr 0.4 Tr

Apatite

Tr Tr Tr

A. Stone Mountain Granite, L. A. Herrmann (1954, p. 31) weight percent.
B. Stone Mountain Granite. plagioclase An1a
C. Granite Dike plagioclase Ann

8

A large xenolith occurs in the granite. In its original condition it was an irregular black-and white-banded gneissic block roughly 8 feet square. Modal analyses of both light and dark bands are given in Table IIA.

TABLE IIA

Modal Analyses of the Xenolith

Light Band Dark Band

Quartz

_52.8

21.4

0 ligoclase ______________43.5

31.9

Biotite - - - - - - 1.9

37.2

Muscovite -- 0.4

6.3

Microcline ----- 0.4

Tr

Calcite

2.2

Apatite .................. 1.0

Tr

Epidote ____ _

Tr

Zircon _____

Tr

Figure 2-Muscovite granite dike, flanked by pegmatite, cutting Stone Mountain ranite. See Table II for modal analysis.
Mileage: Turn around, return to U.S. 78.
6.9 Intersection, turn right on U.S. 78. 7.0 Note weathered diabase dike cutting Stone Mountain granite. 10.3 Cross Yellow River. 11.5 Intersection turn right on Ga. 264 heading toward Centerville. 12.3 Stop #3.
9

BIOTITE GNEISS
A strongly foliated, light-gray biotite gneiss occurs interlayered with biotite-bearing amphibolites which range in thickness from 5" at this outcrop to 3 feet at the next outcrop to the north.
Foliation varies from thin, 1-3 mm, parallel bands of slightly varying mineral composition, to an almost slatelike cleavage which may be caused by weathering. Sparse lineation occurs as a streaking whose strike is essentially the same as the local fold axes on the geologic map (Herrmann, 1954). The cause of the streaking seems to be a grain alignment which bas been accentuated by weathering.
Modal analyses in Table III show mineral proportions quite different from the Lithonia gneiss, some phases of which the biotite gneiss superficially resembles.

TABLE III

Modal Analysis of Biotite Gneiss

Quartz

46.2

Oligoclase _

37.8

Microcline

_ 11.2

Biotite - - - - -- - - - - - -- 4.6

Epidote

Tr

Muscovite

Tr

Opaque

Tr

Mileage: Continue southward on Ga. 264.
14.3 Intersection, turn right on Ga. 124. 15.0 Centerville 18.1 Yellow River 18.8 Stop #4.

PORPHYROBLASTIC BIOTITE GNEISS These exposures show the lithologic variability of this gneiss. Within ten feet vertically; granitic layers, biotite amphibolite layers, amphibolite layers and porpbyroblastic biotite gneiss may be seen.
The folding is typical of the gentle undulations found in the Stone Mountain area.
Mileage: Continue on Ga. 124.
20.0 Stop #5

CONTACT OF THE LITHONIA GNEISS The general relations between the Lithonia gneiss and the country rocks is shown (Fig. 3). The most interesting feature is the intrusive dike of Stone Mountain granite. The dike is contacted on either side by amphibolites, both of which turn up. The amphibolite on the left shows boudinage. Its mineral composition is mainly hornblende and clinozoisite with lesser amounts of diopside and plagioclase.
Modal analyses of the various rocks are given in Table IV.
10

TABLE IV Modal Analyses

Granite Dike

Quartz ~----

22.1

0 ligoclase ----------------------------43 .1

Microcline

8.0

Muscovite*

5.2

Biotite _

1.6

Garnet - - - - -- - - -- Tr

Lithonia Gneiss

Quartz _

36.8

Oligoclase _ _ __ _ _.J.-3.0

Microcline

31.9

Muscovite

2.5

Biotite _

5.8

Epidote

Tr

Muscovite Quartzite

Quartz . Muscovite Biotite

90.6 _____ 7.6
1.8

*includes sericite

Proceed on Ga. 124 noting small quarries in the Lithonia Gneiss on either side of the road.

Mileage: 20.2 Turn around in the intersection by the school and return on Ga. 124. 21.3 Turn left off Ga. 124 on to Rockbridge Road. 22.2 Stop #6

MUSCOVITE-QUARTZ SCHIST
This medium grained, light-colored schist is of limited geographic distribution. It is not associated with the other schistose rocks and is probably not related to them. Modal analyses of two specimens are given in Table V.

TABLE V

Modal Analyses of Muscovite-Quartz Schist

Quartz ------------------------60.6

7 0.1

Plagioclase ______ 1.1

Microcline _____ 9.5

Muscovite

28.8

29.9

Opaque

Tr

Tr

Mileage: Continue west on Rockbridge Rd.
23.2 Stop #7 Intersection of Rockbridge and Deshon Roads.

11

PORPHYROBLASTIC BIOTITE GNEISS AND BIOTITE HORNBLENDE GNEISS The biotite-hornblende gneiss is on the east side of the cross-roads and the porphyroblastic biotite gneiss is on the west side. The porphyroblastic biotite gneiss shows the common undulating structure. Amphibolite layers pinch and swell sometimes showing boudinage (Fig. 4).
GRANI'rE

opproximofe

scale

FIGURE 3- GENERALIZED SECTION ALONG GA.I24 NORTH OF Ll THONIA , GEORGIA

J

Boudinage seems best explained as a result of uplift in the later stages of deformation. The minor faults which occur parallel and transverse to the foliation (Fig. 5) are probably later than the boudinage and post-granite since they commonly offset pegmatites.

Figure 4. Amphibolite boudins and small pegmatite dike in porphyroblastic biotite gneiss saprolite.

Figure 5. Pegmatite dikes faulted along foliation in porphyroblastic biotite gneiss saprolite.

A small granite dike at the west end of the outcrop cuts the foliation of the enclosing gneiss at a high angle. Lithologic variation in the gneiss ranges from biotite-rich to biotite-poor, quartz-feldspar, layers. Porphyroblasts are common but do not occur everywhere. Biotite gneiss layers also occur here. Modal analyses of these rocks are given in Table VI.

12

TABLE VI Modal Analyses

Porphyroblastic Biotite Gneiss

Quartz _ _ __ _ _ _ _ _ _ _ _ __ 40.9

Sodic Andesine

45 .2

Biotite

11.3

Epidote

1.8

Muscovite

0.4

Magnetite

0.4

Biotite Gneiss

Quartz ------------------------------------------------------------46.6

Calcic 0 ligoclase --------------------------------------------42. 6

Microcline ___

3.5

Muscovite

0.3

!

Biotite

5.8

Magnetite - - - - -- -- - - - - - 0.8

Epidote ____ _ __

0.2

Stone Mountain Granite Dike

Quartz ------------------------------------41. 8

37. 8

Sodic Oligoclase ------

22.8

32.8

Microcline

17.9

16.9

Muscovite

17.5

12.5

The biotite-hornblende gneiss contains amphibolites whose compositions vary from 60% felsic and 30% mafic minerals to 30% felsic and 60% mafic minerals. However, the unit is dominated by the biotite-hornblende gneiss. Modal analyses are given in Table VIA.

TABLE VIA Modal Analyses

Biotite-Hornblende Gneiss*

Quartz ------------------------------------------------------------20.9

Sodic Andesine

39.8

Hornblende

2 4 .4

Biotite ------------------------------------------------------------11.8

Epidote

3.1

Opaque

Tr

Amphibolite

J
t

Hornblende ----------------------------------------------------63.7

Labradorite

3.1

Quartz

2.4

Epidote Titanite

f( 0.8

Magnetite

*Badly weathered

A photograph of fresh biotite-hornblende gneiss is shown as Fig. 6.

13

Figure 6. Epidote pod, minor quartz feldspar pods, and quartz-feldspar dikes transecting biotite-hornblende gneiss. Taken on the shoals of the creek 0.8 miles north of Stop 7.

Figure 7. Stone Mountain, taken from the south side.

Hermann (1954, p. 43) suggests that the biotite-hornblende gneiss may be a facies of the amphibolite, or it may have been a partial replacement by silica-rich solutions. The occurrence of these rocks exclusively in or close to areas occupied by Stone Mountain granite tends to support the latter view.

Another view would be that they are a distinct, unrelated unit.

Continue on Rockbridge Road.

Mileage:

25.9 Excellent view of the south side of Stone Mountain. See (Fig. 7).

26.9 Intersection- Tum left off Rockbridge Road on to the Stone Mountain-Lithonia Road. 27.2 Stop 8.

AMPHIBOLITES

These amphibolites occur in thin layers with other rocks as well as in units large enough to map. The outstanding feature is their mineralogical uniformity. The principal mineral variations are diopside and epidote. Table VII shows a typical modal analysis.

TABLE VII

Modal Analysis Amphibolite

Hornblende ______________________________________69 .4

Andesine Quartz Epidote _

29.2 1.0

Titanite

Biotite

14

Mileage : 30.4 31.4 34.1 34.3 34.7 34.9

Continue south on the Stone Mountain-Lithonia Road
Cross Georgia Railroad at Redan Tum left at the fork and continue into Lithonia Intersection- Tum right on Ga. 124 from the Stone Mountain-Lithonia Road Tum left from Ga. 124 onto Klondike Road Intersection- Klondike Road and U. S. 278; continue on Klondike Road Stop #9. Tum off just before the expressway bridge and park.

MUSCOVITE QUARTZITE, GARNET-MICA SCHIST, AND AMPHIBOLITE
The garnet-mica schist shows limited variability in mineral composition and texture. Some layers contain only quartz and mica, while others contain quartz, mica, garnet, and plagioclase. Mica, in particular, shows grain-size variation from about 1 rom to over 10 rom. Small folds, crinkles, and mica elongations constitute lineations.
The amphibolite is similar to that described in Stop 8 except that considerable epidote is developed in some places. This rock is also intensely folded and foliated (fig. 8).
The quartzite shows a lithologic variation which originated as a primary sedimentary feature. The quartzite just above the amphibolite contact contains mica, sillimanite, and plagioclase and is distinctly schistose. This changes upward into more massive, less micaceous quartzite. Folding is similar to that in the amphibolite. Striations, mica elongations, and small folds constitute the lineations.
Modal analyses of the rocks are given in Table VIII.

TABLE VIII

Modal Analyses

Garnet-Mica Schist

Quartz ____________29.4 71.5 52.6

Biotite ____________ 31.2

10.1

Muscovite ______28.8 26.3 25.7 Garnet ____________ 4.8

Feldspar* ______ 5.8 Tr 10.1

Opaque** ______ Tr 2.2 Tr

Mica Quartzite

Quartz

77.5

Muscovite _______,__14.8

Biotite

3.1

Gamet - ------

4.3

Iron Oxides

0.3

Sillimanite Quartzite

Quartz

63.3

Oligoclase ________25.9

Sillimanite ____________________________ 10. 6

Zircon - Iron Oxides

-

-

-

-

-

-}

0 3

*Partly based on clay pseudomorphs in some thin sections; other sections show oligoclase. **Opaque material mostly iron oxides.

All of the above samples are weathered.

Figure 8 is a map showing the geologic relations at Stop 9.

15

\
'

'

'

'

.< ' '

FIGURE 8
X

\ \
' '
\
' '
' '

'
.. -- ~ ' '
'
.

Jo-
Quartzite

. ""

4! '

...

I'

:J h. \ ', \ '
L\ '
Garnet-Mica Schist

Foliation

Fold Axis

Mica Lineation

GEOLOGIC SKETCH MAP OF A BORROW PIT NEAR LITHONIA,GEORGIA

0

50ft.

~pproxlmat~

scale

WH.G. \962

Continue south on the Klondike Road Mileage:
37.5 Stop #10 Mount Arabia.
LITHONIA GNEISS AT MOUNT ARABIA The Lithonia gneiss is a migmatite composed of three main phases. These are: a gneissic biotite-rich phase which is relatively poor in microcline; a granitic phase which is very poorly or non-foliated, and a contorted gneiss phase. The latter two phases have similar compositions, (Figs. 9 and 10).
16

Figure 9. Three phases of the Lithonia gneiss; Gn is the dark, biotite-rich phase with a straight foliation; C is lighter in color with a well-defined highly contorted foliation; Gr is the granitic phase, still lighter in color and with no obvious foliation.

Hermann ( 1954, p. 14) shows the biotite-rich phase has the highest plagioclase content and is richest in anorthite (An17 ) while the aplitic phase contains the least amount of plagioclase and the lowest anothite content (Anto). The other phases, including the garnet phases, are intermediate. Hermann ( 1954, p. 72) shows that the biotite phase contains the lowest proportion of microcline whereas the aplite phase contains the most. Modal analyses of the Lithonia gneiss are given in Table IX.

TABLE IX

Modal analyses of Lithonia gneiss from about 1 mile north of Stop #10. Hermann's specimen L-4 (1954, p. 13) is given in weight percent.

Quartz -----------------------------------35.5

Oligoclase Microcline

29.9 An14 25 .4

Biotite

4.5

Muscovite

2.7

Epidote

Tr

Magnetite

Tr

Apatite

Tr

Zircon Calcite

Tr ___ ____ Tr

Sericite _ __ __ _ __ Tr

100.0

17

The garnet-rich phase (Fig. 11) has been discussed by Watson (1902, p. 259), Lester (1939, p. 841-847), Cofer (1948, p. 35-42), and Herrman (1954).

Figure 10. A contorted, biotite-rich band in the Lithonia gneiss; Gr is the granitic phase and C is the contorted phase.

Figure 11. Folded garnet-rich bands in the Lithonia gneiss.

Modal analysis of a garnet layer taken from the exposure shown (Fig. 11) is given in Table IXA.

TABLE IXA Modal Analysis Garnet Layers in Lithonia Gneiss

Quartz __

40.9

Oligoclase ------------------------------------------3 9. 8

Garnet

15 .2

Epidote

1.5

Carbonate

1.5

- -}o.s Muscovite
Green tourmaline Sphene

- - - - - - 0.7

Pyrite ___ _ _

Cofer (1948, p. 27) reports a variety of uncommon minerals which occur sparsely in pegmatites, along with the usual quartz, feldspar and mica. These minerals are: a thermoluminescent and fluorescent variety of fluorite, ankerite, samarskite and molybdenite.

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Photomicrographs of Selected Thin Section

Figure 12 - Garnet-mica schist showing garnet, muscovite, biotite and quartz. Crossed nicols, 32X

Figure 13 - Quartzite showing sillimanite, quartz and plagioclase. Crossed nicols. 32X

Figure 14 - Quartz-muscovite schist showing quartz and muscovite. Crossed nicols. 32X

Figure 15 Biotite-hornblende gneiss showing biotite, hornblende, quartz and plagioclase. Crossed nicols. 32X

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Figure 16 - Biotite gneiss showing biotite, quartz, plagioclase and microcline. Crossed nicols. 32X

Figure 17-Porphyroblastic biotite gneiss showing quartz, plagioclase, epidote, biotite, and microcline. Crossed nicols. 32X

Figure 18 - Amphibolite showing hornblende and plagioclase. Crossed nicols. 32X

Figure 19 - Lithonia gneiss showing microcline, quartz, and plagioclase. Crossed nicols. 32X

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Figure 20- Xenolith from Stone Mountain showing plagioclase containing calcite, and biotite and quartz. Crossed nicols. 32X

Figure 21 - Stone Mountain granite showing microcline quartz and plagioclase. Crossed nicols. 32X

REFERENCES
Chayes, F., (1956) Petrographic modal analysis: John Wiley, New York. Cofer, H. E. Jr., (1948) Petrology, petrography, mineralogy and structure of the Arabia Mountain gneiss, DeKalb County, Georgia:
Unpublished Master's thesis, Emory University. Crickmay, G. W. (1939) Geologic Map of Georgia: Division of Mines, Mining, and Geology. Crickmay, G. W., (1952) Geology of the crystalline rocks of Georgia: Ga. Geol. Surv. Bull. 58. Gorai, M., (1951) Petrological studies on plagioclase twins: Am. Min. v. 36, p. 884-901. Hanson, H. S. (1958) A study of the relative frequency of feldspar twin types in crystalline rocks: Unpublished Master's thesis,
Emory University. Herrmann, L. A. (1954) Geology of the Stone Mountain-Lithonia District, Georgia: Ga. Geol. Surv. Bull. 61. Hopson, C. A. (1958) Exfoliation and weathering at Stone Mountain, Georgia and their bearing on disfigurement of the Confed-
erate Memorial: Georgia Mineral Newsletter, v. 11, p. 65-79. Lester, J. G. (1938) The geology of the region around Stone Mountain, Georgia: Doctoral dissertation, University of Colorado. Lester, J . G. (1938) Garnet segregations in the granite gneiss of DeKalb County, Georgia: Jour. Geol., v. 47, p. 841-847. Purington, C. W. (1894) Geologic and topographic features of the region about Atlanta, Georgia: Am. Geol., v. 14, p. 106-108. Walter, K. G. (1958) A study of the pegmatites of the Stone Mountain-Lithonia-Panola Shoals area: Unpublished Master's thesis,
Emory University. Watson, T. L. (1902) A preliminary report on the granites and gneisses of Georgia: Ga. Geol. Surv. Bull. 9-A.
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