Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Downloaded from gsabulletin.gsapubs.org on April 6, 2015 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 Downloaded from gsabulletin.gsapubs.org on April 6, 2015 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 Downloaded from gsabulletin.gsapubs.org on April 6, 2015 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 Downloaded from gsabulletin.gsapubs.org on April 6, 2015 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 Downloaded from gsabulletin.gsapubs.org on April 6, 2015 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). Downloaded from gsabulletin.gsapubs.org on April 6, 2015 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 Downloaded from gsabulletin.gsapubs.org on April 6, 2015 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 Email alerting services click www.gsapubs.org/cgi/alerts to receive free e-mail alerts when new articles cite this article Subscribe click www.gsapubs.org/subscriptions/ to subscribe to Geological Society of America Bulletin Permission request click http://www.geosociety.org/pubs/copyrt.htm#gsa to contact GSA Copyright not claimed on content prepared wholly by U.S. government employees within scope of their employment. Individual scientists are hereby granted permission, without fees or further requests to GSA, to use a single figure, a single table, and/or a brief paragraph of text in subsequent works and to make unlimited copies of items in GSA's journals for noncommercial use in classrooms to further education and science. This file may not be posted to any Web site, but authors may post the abstracts only of their articles on their own or their organization's Web site providing the posting includes a reference to the article's full citation. GSA provides this and other forums for the presentation of diverse opinions and positions by scientists worldwide, regardless of their race, citizenship, gender, religion, or political viewpoint. Opinions presented in this publication do not reflect official positions of the Society. Notes Copyright © 1964, The Geological Society of America, Inc. Copyright is not claimed on any material prepared by U.S. government employees within the scope of their employment.