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SONIA MEHECH DE HILLS
JOSE CORVALAN
UNDULATORY EXTINCTION IN QUARTZ GRAINS OF
SOME CHILEAN GRANITIC ROCKS OF DIFFERENT AGES
Abstract: Microscopic study of primary quartz
grains from Chilean granitic rocks of known radiogenic ages shows a significant correlation between
the intensity of the undulatory extinction in this
mineral and the age of the rock. In the Paleozoic
rocks the undulatory extinction ranges from 31°
to 35°; in Jurassic rocks it ranges from 23° to
25°; in Cretaceous rocks it ranges from 15° to 17°;
and in Tertiary rocks it ranges from 12° to 13°.
The marked difference in the intensity of the
undulatory extinction in quartz grains is interpreted
as the result of the degree of tectonic deformation of
the granitic rocks of different ages present in a
single linear folded belt affected by successive
erogenic movements.
Introduction
In connection with the program of radiogenic
age determinations being conducted by the
Geochronology Labratory at the Institute de
Investigaciones Geologicas (IIG) Chile, and
with the purpose of determining the petrographic characteristics of the intrusive bodies
of known radiogenic ages, the IIG started to
make detailed microscopic studies of representative samples. Some of the results were
given by Ruiz and others (1960) and Levi and
others (1963); these include results on the
granitic rocks designated as Chile 1 through
Chile 35.
From observation of the primary quartz
grains in these rocks, it became evident that
this mineral had undulatory extinction of varying intensity in different samples. As undulatory extinction is usually developed in quartz
grains of rocks that have been mechanically deformed (S«?Frondel, 1962, p. 132), this study
was made to find out whether there is any correlation between the intensity of the undulatory extinction of quartz and the age of these
Chilean granitic rocks.
Several measurements of the quartz undulatory extinction were made in each of the
samples Chile 1 to Chile 35; these have shown
that this property is more strongly developed
in the older rocks, decreasing in intensity as
the rocks become younger (Fig. 1), and that
it remains fairly constant in the quartz grains of
each individual sample.
Two phenomena are classed as undulatory
extinction for our measurements: (1) continuous undulatory extinction, in which extinction is microscopically gradational within
crystals or sectors of crystals, and (2) discontinuous undulatory extinction, in which crystals consist of sharply bounded sectors or
bands of slightly different optical orientation.
The two phenomena are commonly combined,
so that sharply bounded sectors show internal
continuous undulatory extinction. Discontinuous undulatory extinction may result from
partial anealing of continuous undulatory
extinction (see Bailey and others, 1958, Turner
and Weiss, 1963, p. 212-213).
Granitic rocks of known radiogenic ages have
been collected from different localities in
northern and central Chile. The corresponding
batholitic intrusions are located in a relatively
narrow and elongate belt which represents the
areas where the Andean geosyncline as well as
the main Paleozoic basin were developed.
At least three main erogenic epochs are
recorded in this folded belt; they are a Late
Paleozoic (Hercynian) orogeny, a Late Jurassic
and a Cretaceous orogeny, the last possibly
having had three culminations (Ruiz and
others, 1961, p. 1557-1558). Each of these
successive orogenies affected the batholiths
previously emplaced, causing deformation on a
regional scale. It is proposed that undulatory
extinction in these Chilean granites is the
cumulative result of orogenic strain incurred
since their crystallization, and is, therefore,
correctable with their ages.
These conclusions are drawn from observa-
Geological Society of America Bulletin, v. 75, p. 363-366, 1 fig., 2 pis., April 1964
363
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364
DE HILLS AND CORVALAN-UNDULATORY EXTINCTION IN QUARTZ GRAINS
Rodiogenic oges
( m i l l i o n years)
O—i
O
O
Nl
O
Z
U
O
OCH-IB
—
O CH-17
—
-r
.•-
^
OCH-26
CH-28 y C H - 3 I O
100-
O CH-I
OCH-IO
-
QCH-20
C H - 2 4 , CH-34
^
O
0
4
_
|
~
-
^"O
QCH-2
OCH-23
CH-3
OCH-35
N
O
LJ
OCH"3°
OCH-6
_
OCH-18
OCH-I9
200-
-
CH-9
0
O CH-13
O
O
tsl
—
O CH-22
O
UJ
-1300—
a.
GCH-14
—
-
OCH-15
O CH-21
34°
32°
30°
28«
26°
24°
22°
20"
18°
16°
14°
12°
10"
MEASUREMENTS OF THE UNDULATORY EXTINCTION IN QUARTZ
Figure 1. Variation of the undulatory extinction in quartz in relation to the radiogenic age of different
granitic rocks
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Figure 1. Starting position; first appearance
of undulatory extinction bands
Figure 2. Intermediate position
Figure 3. Final position; last evidence of
undulatory extinction bands
MEASUREMENT OF THE UNDULATORY EXTINCTION IN A QUARTZ
GRAIN (60X)
(Diagram above indicates rotation of the state)
DE HILLS AND CORVALAN, PLATE 1
Geological Society of America Bulletin, volume 75
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Figure 1. Sample Chile 9, 265±30
m.y. (Paleozoic)
Figure 2. Sample Chile 24, 130
20 m.y. (Jurassic)
Figure 3. Sample Chile 28, 90 :
10 m.y. (Cretaceous)
Figure 4. Sample Chile 17, 50 :
20 m.y. (Tertiary)
PHOTOMICROGRAPHS SHOWING VARIATION OF THE UNDULATORY EXTINCTION IN QUARTZ GRAINS FROM GRANITIC ROCKS OF DIFFERENT AGES (60 X)
DE HILLS AND CORVALAN, PLATE 2
Geological Society of America Bulletin, volume 75
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365
SHORT NOTES
tions on this particular folded belt, without
assuming that the same conditions would be
found elsewhere.
Messrs. Carlos Ruiz, Kenneth Segerstrom,
Luis Aguirre, and Walter Danilchik and Mrs.
Beatriz Levi critically reviewed the manuscript
and made helpful suggestions; the authors are
indebted to them.
Method of Study
Each of the samples of granitic rocks Chile 1
through Chile 35 was studied under the microscope. Preliminary observations of primary
quartz grains showed that in each sample the
quartz grains had relatively constant intensity
of the undulatory extinction as well as fracturing. Subsequently a minimum of 10 optically oriented crystals was selected in each
thin section; the orientation selected was
parallel to the crystallographic axis c, which
is easily recognizable by a characteristic flash
figure. Measurements of the undulatory extinction in these crystals were made according
to the following procedure (PI. 1):
(1) The crystal was set in the position of
highest birefringence;
(2) The microscope stage was rotated until
the first clear evidence of undulatory extinction appeared; the reading on the stage was
recorded;
(3) The stage was rotated until it passed
through complete extinction and further, until
the undulatory extinction bands disappeared
(the extinction bands should be barely visible);
the reading on the stage was recorded;
(4) The angle measured was calculated.
Future plans include making more accurate
measurements with the universal stage and
obtaining more information on the distribution
and radiogenic ages of granitic rocks with a
different stratigraphic setting.
Conclusions
The results obtained from measurements of
the undulatory extinction angles in primary
quartz grains from the granitic rocks Chile 1
to Chile 35: are summarized as follows:
(1) In individual samples the undulatory extinction angles are fairly constant.
(2) The undulatory extinction angles in
quartz grains from the granitic rocks which on
the basis of their radiogenic ages (260 + 30 to
340 ± 40 million years) are assigned to the
Paleozoic, range from 31° to 35° with an aver1
Samples labeled Chile not included in Ruiz and
others (1960) and Levi and others (1963) are excluded.
age value of 32°. In the rocks assigned to the
Jurassic (120 + 15 to 170 + 20 million years)
the undulatory extinction angles are of 23° to
25°. In the Cretaceous rocks (90 + 10 to 105
± 10 million years) the angles range from 15°
to 17°, which clearly separates them from the
older rocks. The Tertiary granitic rocks (30 +
20 to 60 + 10 million years) have quartz crystals with undulatory extinction angles of 12°13°.
(3) The undulatory extinction bands in the
quartz grains of the older rocks become evident
when birefringence is still high; In the younger
rocks, they appear only when the crystals are
in a position close to total extinction (PI. 2).
Table 1 lists the undulatory extinction
angles measured in the samples studied.
TABLE 1. UNDULATORY EXTINCTION ANGLES IN
QUARTZ OF CHILEAN GRANITIC ROCKS
Age (in million years)
(Ruiz and others.' 1960,
table 1 ;
Undulatorv
Levi and others, 1963,
extinction
table 1)
angles in quartz
Sample no.
CHILE 1
CHILE 2
CHILE 3
CHILE 6
CHILE 9
CHILE 10
CHILE 13
CHILE 14
CHILE 15
CHILE 16
CHILE 17
CHILE 18
CHILE 19
CHILE 20
CHILE 21
CHILE 22
CHILE 23
CHILE 24
CHILE 25
CHILE 28
CHILE 30
CHILE 31
CHILE 34
CHILE 35
100 + 10 (Cretaceous)
120 + 15 (Jurassic)
125 + 15 (Jurassic)
95 + 10 (Cretaceous)
265 + 30 (Paleozoic)
105 + 10 (Cretaceous)
270 + 30 (Paleozoic)
310 + 35 (Paleozoic)
320 ± 35 (Paleozoic)
30 + 20 (Tertiarv)
50 + 20 (Tertiary)
160 + 20 (Jurassic)
170 ± 20 (Jurassic)
120 + 20 (Jurassic)
340 ± 40 (Paleozoic)
280 + 50 (Paleozoic)
130 + 20 (Jurassic)
130 + 20 (Jurassic)
60 + 10 (Tertiary)
9 0 + 1 0 (Cretaceous)
9 0 + 1 0 (Cretaceous)
90 + 10 (Cretaceous)
130 + 20 (Jurassic)
140 + 20 (Jurassic)
15°
17°
17°
15°
33°
17°
31°
32°
32°
13°
12°
23°
24°
25°
35°
31°
23°
24°
13°
17°
16°
17°
24°
25°
Samples Chile 2 and Chile 3, which Ruiz and
others (1960, p. 16) assigned to the Upper
Jurassic, have quartz crystals with undulatory
extinction angles of 17°. According to this,
they would fall within the group of the
Cretaceous intrusive masses. The plutons represented by samples Chile 2 and 3 are known
to intrude stratified sequences of Middle and
Late Jurassic age (Ruiz and others, 1960, p. 16).
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366
DE HILLS AND CORVALAN-UNDULATORY EXTINCTION IN QUARTZ GRAINS
References Cited
Bailey, S. W., Bell, R. A., and Peng, C. J., 1958, Plastic deformation of quartz in nature: Geol. Soc.
America Bull., v. 69, p. 1443-1466
Fronde!, Clifford, 1962, Dana's system of mineralogy, v. 3: New York, John Wiley and Sons, 334 p.
Levi, B., Mehech S., and Munizaga, F., 1963, Edades radiometricas y petrografia de granitos chilenos;
muestras Chile 13 a Chile 36: Inst. Invest. Geol., Bol. 12
Ruiz, C., Aguirre, L., Corvalan, J., Rose, H. J., Jr., Segerstrom, K., and Stern, T. W., 1961, Ages of
bathoh'thic intrusions of northern and central Chile: Geol. Soc. America Bull., v. 72, p. 1551-1559
Ruiz, C., Segerstrom, K., Aguirre, L., Corvalan, J., Rose, H. J., and Stern, T. W., 1960, Edades plomoalfa y marco estratigrafico de granitos chilenos; con una discusion acerca de su relacion con la orogenesis. Inst: Invest. Geol., Bol. 7
Turner, F. J., and Weiss, L. E., 1963, Structural analysis of metamorphic tectonites: New York, McGrawHill Book Co., 545 p.
INSTITUTO DE INVESTIGACIONES GEOLOGICAS, SANTIAGO, CHILE
MANUSCRIPT RECEIVED BY THE SOCIETY, AUGUST 8, 1963
PUBLISHED WITH THE PERMISSION OF THE DIRECTOR, INSTITUTO DE INVESTIGACIONES GEOLOGICAS, CHILE
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Geological Society of America Bulletin
UNDULATORY EXTINCTION IN QUARTZ GRAINS OF SOME
CHILEAN GRANITIC ROCKS OF DIFFERENT AGES
SONIA MEHECH De HILLS and JOSÉ CORVALÁN
Geological Society of America Bulletin 1964;75, no. 4;363-366
doi: 10.1130/0016-7606(1964)75[363:UEIQGO]2.0.CO;2
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