Preferred Orientation in Quartz Ribbon Mylonites

C.J.L. WILSON School of Earth Sciences, Department of Geology, University of Melbourne, Parkville, Victoria 3052, Australia

ABSTRACT Australia, and an upper greenschist or lower amphibolite facies quartzite from Risfjallet, Sweden. Quartz mylonites composed of elongate ribbon quartz without appreciable recrystallization at grain boundaries were examined GEOLOGICAL SETTING and contrasted. One was from a lower greenschist fades environment at Mount Isa, Australia, in which the c-axis preferred orienta- The Mount Isa mylonite occurs as quartzite lenses within pelitic tion of the ribbons is either a pronounced orthorhombic distribu- schists, which are localized in a zone of intense deformation known tion or a small-circle distribution (with a small opening angle) as the Mount Isa fault zone (Wilson, 1973a). On the margin of the about the normal to the foliation and lineation. The other was an fault zone, the quartzites are generally massive with no obvious upper greenschist or lower amphibolite facies mylonite from layering, whereas in the center the quartzites have a well-developed Risfjallet in the Swedish Caledonides, in which the c-axis preferred planar layering within which is a prominent lineation. Other orientation of the ribbon is a maximum lying close to the foliation quartzite lenses are transitional between these two types, with only and normal to the lineation. Variation in preferred orientation can a poor foliation and lineation. Schist and quartzite in the fault zone be accounted for by temperature and (or) strain-rate differences, are not metamorphosed to grades higher than lower greenschist with basal-slip mechanisms predominant at lower temperatures facies, and the schist consists of white mica, chlorite, and deformed and prismatic slip (and possibly other slip systems), together with detrital quartz grains. The pelitic rocks overlying the fault are diffusion-controlled processes, predominant at higher tempera- characterized by white mica, quartz, and minor biotite, whereas the tures. Key words: quartz, mylonite, deformation, preferred orienta- underlying rocks are dolomitic, with occasional pure pelitic mem- tion, structural geology. bers containing white mica, chlorite, and quartz (Croxford and Jephcott, 1972). INTRODUCTION The Risfjallet mylonite comes from the allochthonous Seve-Koli complex in the eastern thrust belt of the central Swedish Quartz mylonites exhibiting similar microstructural features Caledonides. In the Marsfjallet area (Trouw, 1973; Glass, in from different metamorphic environments generally exhibit differ- prep.), the nappe complex can be divided internally into tectonic ent patterns of c-axis preferred orientations. If mylonitic rocks belts bounded by low-angle thrust faults at which mylonite zones composed only of ribbon (Spry, 1969, p. 294) quartz grains (al- are developed; the Risfjallet mylonite is one of these zones and also though there may be recrystallization at deformation band bound- coincides with a metamorphic boundary. The mylonite has a low- aries and grain boundaries) are considered, then three basic quartz dipping foliation bearing a strong quartz lineation, and it consists c-axis patterns consistently occur in different areas. These are (1) of mylonitized gneiss and schist (quartz, 90 percent; biotite, 3 per- an orthorhombic pattern with two maxima lying in a peripheral cent; muscovite, 6 percent; almandine, 2 percent; kyanite and girdle 30° to 60° from the foliation plane, or a crossed-girdle pat- minor apatite; rutile and opaque minerals; chlorite absent). The tern (Christie, 1963; Hara, 1971; Hietanen, 1938; Johnson, 1957, rocks that overlie the Risfjallet mylonite carry premylonitization 1960; Shelley, 1971); (2) a maximum or small-circle girdle normal assemblages belonging to the kyanite-almandine-orthoclase zone of to the foliation and lineation (Beavis, 1961; Christie, 1963; the amphibolite facies of this area; the rocks that underlie the Hobbs, 1966; Phillips, 1965a, 1965b); and (3) a maximum lying Risfjallet mylonite are lower grade, carrying premylonitization as- within or close to the foliation and normal to the lineation (Balk, semblages that belong to the staurolite-almandine zone. Myloniti- 1952; Behr, 1961; Christie, 1963; Crampton, 1963; Gan- zation postdates the metamorphic culmination, but during gopadhyay and Johnson, 1962; Hara and others, 1973; Hofmann mylonitization the metamorphic grade was still at least upper and Korcemagin, 1973; Ross, 1973; Sander, 1950; Wenk, 1973). greenschist facies. This is evidenced by the growth of biotite and Except for the papers cited above, and the very few that have some almandines during or just following the deformation that possibly been overlooked, literature on preferred orientation of produced the mylonite (Glass, in prep.); a slightly higher grade quartz in mylonites gives little insight into the relationship between cannot be excluded (that is, lower amphibolite facies). preferred orientation, microstructure, and tectonic conditions; the three main reasons for this are that (1) authors fail to describe the MICROSTRUCTURE OF THE MOUNT ISA MYLONITE microstructure of samples for which they present diagrams of preferred orientation; (2) many authors do not separate measurements A complete progression of microstructural changes from weakly of old deformed grains from measurements of new recrystallized deformed sedimentary strata to strongly deformed and recrystal- grains; and (3) some authors omit a description of the tectonic and lized quartz blastomylonites is displayed, and the quartz micro- (or) metamorphic environment in which the mylonites developed. structural changes are similar to those described from the Moine Quartz mylonite with different preferred orientations, but with thrust zone (Christie, 1963, p. 397). However, a progressive some similarities in microstructural features, are described and change from the margins to the center of the zone, as suggested by compared herein, and their patterns of c-axis preferred orientations McLaren and Hobbs (1972), cannot be demonstrated because of are interpreted in the light of some experimental studies of quartz. the lenticular and discontinuous nature of the quartzite bodies They are a lower greenschist facies quartzite from Mount Isa, within the fault zone.

Geological Society of America Bulletin, v. 86, p. 968-974, 7 figs., July 1975, Doc. no. 50713.

968

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Two areas of microstructural development are found in these absence of recrystallization. The shape and grain size of these rela- quartzite lenses: (1) from the margins, and (2) from the center of tively undeformed quartz grains suggest that the original grains the fault zone, where there is a variable amount of recrystallization. were probably similar to those immediately outside the fault zone The relationship between amount of strain and microstructure (see Wilson, 1973b, Fig. 5), although now these grains are elongate cannot be established with certainty because there are no strain and reduced to as little as one-half of their former diameter in markers except detrital quartz grains. In quartzite that has under- sections normal to the foliation and lineation. There is also evi- gone intense deformation (and recrystallization), outlines of origi- dence that water-assisted diffusive or transport processes are re- nal grains cannot be identified; therefore, any quantitive estimate sponsible for some modification of grain shape in the mylonite after of strain is unobtainable. deformation. Deformed detrital grains in quartzite from the eastern margin of In the central part of the fault zone, the quartzites have a foliated the Mount Isa fault zone show undulatory extinction (Fig. 1A), appearance defined by the grain shape of the ribbon quartzes and off-basal deformation lamellae (Fig. IB), and quartz grains sepa- show varying degrees of recrystallization. In regions with little or rated by well-defined surfaces across which there are small dis- no recrystallization, it is often difficult to distinguish boundaries of placements (now healed) of deformation lamellae and deformation the original quartz grains from deformation band boundaries, be- bands (displacements of as much as 20 /jl); there is a noticeable cause the quartz occurs as distinct ribbons, commonly with ser-

0*2 mm i

Figure 1. A. Quartzite from eastern margin of Mount Isa fault zone, with detrital quartz grains that contain abundant banded extinction features (White, 1973); these include areas of undulose extinction and deformation bands. Old grains are noticeably elongate and illustrate beginnings of foliation development in these quartzites. B. Deformation lamellae and undulose extinction in quartzite from eastern margin of Mount Isa fault zone.

Figure 2. Mount Isa mylonite from center of fault zone, with quartz ribbons in section normal to foliation and lineation. Ribbons occur as optically distinct elongate areas separated from one another by curved or serrated boundaries or by regions composed of numerous fine recrystallized grains. Individual ribbons generally consist of numerous elongate and rectangular subgrains and fine new grains. Most of narrow ribbons are probably highly complex deformation bands that closely parallel mylonitic foliation. Lines show variation in c-axis orientation. Most c axes lie normal to lineation and at high angle to foliation (that is, in plane of section). However, c-axis orientation between adjacent bands is not symmetrically disposed about foliation, suggesting that deformation within grain is complex and may not be on single slip plane; nor are deformation bands simple kinklike structures.

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rated boundaries (Fig. 2), and if recrystallized grains are present lineation. In sections parallel to the prominent lineation, the length: they occur along these boundaries. The ribbons are generally un- breadth ratio of the ribbons is >100:1 (Fig. 3). These must be dulóse along their lengths, and the boundaries between undulatory considered to be maximum values, as in the case of the Risfjallet zones generally lie at a high angle to the dimensional orientation of mylonite, because deformation bands and new grain boundaries the ribbons. Some quartz grains are entirely composed of small that appear to be formed by heterogeneous deformation split the equant subgrains with ill-defined boundaries. Optically visible de- older grains or ribbons into narrower units separated by recrystal- formation lamellae (as described by Carter and others, 1964; lized aggregates. A similar feature has also been described in ex- McLaren and others, 1970) are generally absent in the rocks with perimentally produced quartz mylonites (Tullis and others, 1973, these elongate quartz ribbons. Length:breadth ratios s20:1 of the p. 300) and in metals (see Chin, 1973). ribbons have been observed in sections normal to the foliation and Many ribbons are completely enclosed by an envelope of recrystallized quartz grains (Fig. 4A); the amount of recrystallization varies from 10 percent to 90 percent of the quartz volume. The recrystallized grains are generally extremely small (3 to 7 /x; compare McLaren and Hobbs, 1972, Fig. lb); they are always less than the thickness of a normal optical section, so that clusters of these recrystallized grains commonly appear as small, irregularly shaped diffuse grains whose orientation cannot be measured optically.

MICROSTRUCTURE OF THE RISFJALLET MYLONITE

The foliation is defined by elongate quartz ribbons (Fig. 5A) or as alternating elongate quartz ribbons and biotite-muscovite—rich laminae (Fig. 5B). This foliation is strongly deflected around almandine and kyanite porphyroclasts, which acted during mylonitization as relatively strong materials (Fig. 6). Individual ribbons are commonly necked or parted as they bend around the porphyroclasts. The mylonite is strongly lineated, and the quartz ribbons are as much as 40 times longer parallel to the lineation than they are normal to the lineation. The ribbons, without exception, contain deformation bands and Figure 3. Mount Isa quartz ribbons in section parallel to lineation and subgrains, but no deformation lamellae are visible. Subgrains are normal to foliation. Individual ribbons are composed of banded extinction very common and are most clearly observed in sections normal to features, some of which are deformation bands with 60° to 90° rotations of the lineation. All the subgrains in these sections are elongate paral- c axis across them, whereas others appear to be separate individuals, with lel to the quartz c axis and in many cases form fanning aggregates similar orientations, separated by serrated boundary that sometimes con- about an axis that lies in the foliation and is normal to the lineation tains very fine recrystallized quartz grains. Same specimen as Figure 2. (Fig. 5B). Optically visible subgrain boundaries are generally straight and exhibit misorientations of as much as 6°. Such subgrains may change along their length into more misoriented bands, which exhibit irregular or serrated boundaries. The bands have

Figure 5. Risfjallet mylonite in sections normal to foliation and lineation. A. Quartz ribbons that define mylonite layering. B. Subgrains in ribbons. Misorientation across these straight and slightly curved subgrains Figure 4. Mount Isa mylonite, showing relics of old quartz ribbons varies from 3° to 6°. Larger misorientations are always associated with surrounded by newly recrystallized grains. No deformation lamellae are serrate or highly irregular outlines, strongly suggesting that at angles present. A. Section normal to foliation and lineation. B. Section parallel to greater than 6° such boundaries become mobile. Recrystallized grains occur lineation and normal to foliation. only on old grain margins.

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misorientations of between 6° and 13° and also contain local inter- Risfjallet Mylonite. In all specimens there is an extremely nal subgrains with misorientations of less than 6°. strong preferred orientation. The c axes are concentrated at a small Fine recrystallized grains occur at the boundaries between origi- angle to the foliation and normal to the lineation, with a pro- nal grains and also between relatively highly misoriented elongate nounced minimum at the pole to the mylonitic foliation. A typical subgrains or deformation bands, but approximately 80 percent of pattern is shown in Figure 7D. the quartz volume can still be considered as deformed original grains. DISCUSSION

PREFERRED ORIENTATION Tullis and others (1973) have shown that the orienting process operative in experimentally produced ribbon mylonites is probably The quartz c-axis orientations given in Figure 7 reflect the vol- the mechanical reorientation of the older quartz grains by intra- ume distributions of deformed old grains; the fine recrystallized crystalline slip. The slip within any one grain would take place on grains have not been measured. rational crystallographic planes and in rational directions within Mount Isa Mylonite. Three specimens representative of changes these planes. By this means, grain shape is progressively distorted observed in microstructure have been illustrated. Figure 7A repre- with increasing strain, and a preferred orientation of crystallo- sents deformed detrital quartz grains from the margin of the fault graphic axes is produced (compare Barrett and Massalski, 1966; zone. The pattern of c-axis preferred orientations is almost or- Honeycombe, 1968; Schmid and Boas, 1950). thorhombic, with a girdle normal to the lineation and two high In order to understand the preferred orientation developed in an concentrations lying almost 90° apart, symmetrically disposed aggregate of quartz grains because of intracrystalline slip, it is also about the foliation. Figure 7B represents a rock composed of 80 necessary to know the slip systems capable of producing a defor- percent ribbon quartz and 20 percent recrystallized quartz and mation within a grain. The slip systems so far established in quartz shows an orthorhombic pattern but with a higher concentration of have been described by Ave Lallemant and Carter, 1971; Baeta and c axes at 60° to the foliation plane than in Figure 7A. This c-axis Ashbee, 1970; Carter and others, 1964; Christie and others, 1964, pattern in the ribbons is also reproducible in other rocks with as 1966; Christie and Green, 1964; Heard and Carter, 1968. Griggs much as 40 percent ribbon quartz and 60 percent recrystallized and Blacic (1964) were able to demonstrate that the critical re- quartz. Figure 7C represents a rock composed of 40 percent ribbon solved shear stress for slip on the_basal plane (0001) is less than for quartz and 60 percent recrystallized quartz and shows two strong slip on the first-order prism (1010) at a temperature of less than c-axis maxima that lie in a small-circle girdle about the pole to the approximately 700°C and at a strain rate of 10"5 sec-1; the reverse foliation and lineation. The opening angle of this small circle is is true above 700°C. Similar results have also been obtained ex- approximately 25°. perimentally by Ave Lallemant and Carter (1971), Heard and Car- ter (1968), Hobbs and others (1972), and Tullis and others (1973). These workers have also been able to demonstrate that two other factors, namely strain rate and water content, affect the temperature dependence of the different orienting processes in quartz (see also Griggs and Blacic, 1964; Green and others, 1970; Griggs 1967, 1974). The quartz preferred orientations obtained in experimentally produced ribbon mylonites (Tullis and others, 1973) have been interpreted in terms of intracrystalline slip mechanisms. The c-axis patterns obtained are a maximum normal to the foliation (// to crj at lower temperatures or faster strain rates, which changes with a continuous gradation to small-circle girdles with small opening angles about the normal to the foliation (II to o^) to a girdle with larger opening angles at higher temperatures and slower strain rates. In these experiments, Tullis and others (1973) were able to iden- tify some of their major slip mechanisms before the onset of appreciable recovery and recrystallization at higher temperatures; this can rarely be done in nature because of slower strain rates and therefore a greater probability that recovery phenomena have modified earlier slip planes (White, 1973). Basal slip was predominant at the lower temperatures (faster strain rates) and low strains, with prismatic slip becoming more predominant at higher temperatures (slower strain rates) and at higher strains. Accompanying the increasing proportion of prismatic slip was a change in the opening angle of the c-axis small-circle girdle. Tullis and others (1973) have also calculated development of preferred orientation as a result of simultaneous basal and prismatic slip, and the results of these calculations are in agreement with their experimental results. In applying these experimental results directly to natural rocks, a number of limitations must be borne in mind: (1) Rhombohedral slip (not observed in the experiments of Tullis and others, 1973) or Figure 6. Risfjallet mylonite in sections parallel to lineation and normal other slip mechanisms, such as slip on the trigonal dipyramidal to foliation. A. General dimensional orientation of quartz ribbons parallel planes, may contribute to the final pattern of preferred orientation to lineation. B. Individual ribbons are necked or parted as they bend around developed under some natural deformation conditions. (2) The porphyroclasts of garnet, and muscovites occur as bent "fish." strain rates in natural environments are generally slower, thus low-

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ering the temperature at which prismatic and rhombohedral (or zation is strongly influenced by temperature. (5) In nature, recovery other slip mechanisms) are predominant over basal slip. (3) The mechanisms exist that do not operate in the experiments (or that deformation history can involve a nonconstant deformation gra- are more strongly time dependent), and these compete with recrys- dient or change in the set of activated slip mechanisms. (4) Not all tallization. Therefore the amount of recrystallization found in na- mechanisms are equally influenced by time. For example, it is pos- ture (for example, the Risfjallet mylonite) may be less than that sible that with reduced strain rate, rhombohedral or prismatic slip found in the experiments. (6) Strain in nature is generally nonaxial, quickly becomes more important than recrystallization (this is unlike that in most of the present experimental studies, especially true for the Risfjallet mylonite — see discussion below), In the Mount Isa mylonite, there is always an orthorhombic

Figure 7. Pole figure patterns of c axes. A. Quartzite from eastern margin of Mount Isa fault zone (specimen corresponds to Fig, 1A); contours 1 and 2 percent per 1 percent area; 300 grains. B. Mount Isa ribbon quartzite (specimen corresponds to Fig. 2); contours 1,2, and 5 percent per 1 percent area; 250 grains. C. Mount Isa ribbon quartzite with grain boundary recrystallization (specimen corresponds to Fig. 4); contours 1,2,3, and 5 percent per 1 percent area; 2S0 quartz grains. D. Pole figure patterns of c axes for Risfjallet mylonite; contours 1,3,5,8, and 13 percent per 1 percent area; 250 quartz grains.

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margin of the fault zone). A similar pattern is also found in ribbon with slip, the shape change and associated crystallographic pre- areas where there is as much as 60 percent recrystallization. This ferred orientation could then be produced (Groves and Kelly, suggests that in these areas, the deformation mechanism producing 1969). Therefore, it is possible that such a process involving both the c-axis preferred orientation may be similar, but differences in prismatic slip and climb (dynamic recovery) could have operated in the amount of recrystallization may be dependent on the total the Risfjallet mylonite. strain or on a temperature difference — both variables that cannot A predominance of basal slip (0001)_at lower temperatures ver- be estimated in these rocks. The c-axis small-circle girdle maxima sus slip on the prism (for example 1010) at higher temperatures, (Fig. 7C), although almost identical to the experimental results, dif- accompanied by recovery (both situations may be combined with fer from the experiments in that these mylonites developed under other slip systems, such as the rhombohedral planes r and z, conditions in which the strain was nonaxial (indicated by the pres- observed experimentally by Christie and Green, 1964), can explain ence of isoclinal folds, transposition, and strong lineation develop- the contrasting patterns of c-axis preferred orientations observed in ment), and therefore the strain path is not the same, the strain rates these two quartz ribbon mylonites. The perfection of any pattern were probably slower, and the temperature was lower. The speci- will also depend upon the strain path and the total strain; therefore mens that contain the c-axis small-circle pattern and the orthorhom- there may be a variation in the crystallographic orientation, such as bic pattern have similar microstructures and were collected near one a strengthening of the orthorhombic pattern, going from the mar- another. It should also be noted that the small-circle girdle pattern gin of a mylonite zone to the central regions. This variation is shows two maxima (Fig. 7C), making it strongly orthorhombic and observed at Mount Isa and in other mylonite zones (Balk, 1952; consistent with other aspects of the microfabric. Although recovery Christie, 1963; Hara and others, 1973). The polymorphic transi- and recrystallization processes may produce similar microstruc- tion between a (low) and /3 (high) quartz will have no appreciable tures in these specimens, the deformation mechanism on the scale effect on quartz c-axis patterns if the primary slip mechanisms are of the individual quartz grains may not be identical. This could the same in the two quartz polymorphs (Green and others, 1970). then explain the differences in the patterns of preferred orientation. In rocks where syntectonic recrystallization and grain growth are The presence of off-basal deformation lamellae in rocks with an occurring simultaneously, the preferred orientation of a recrystal- orthorhombic c-axis pattern from the margin of the fault zone lized quartz aggregate would probably be a result of the interplay suggests that basal slip was probably operative during the deforma- between slip mechanisms, recovery, and recrystallization, as is tion (compare Christie and others, 1964); the effect of competing shown and discussed in the experimental work of Green and others recovery (and recrystallization) cannot be estimated, although the (1970), Hobbs (1968), and Tullis and others (1973). Metamorphic existence of off-basal deformation lamellae suggests that recovery grade and different strain paths and strain rates in rocks of similar processes were important. Also, extensive recovery and recrystalli- composition probably can account for the different behavior in any zation have modified the quartz ribbons in the main body of the aggregate of quartz grains and, therefore, contribute greatly to the mylonite, and deformation lamellae are no longer visible; it is, three patterns of c-axis preferred orientations so commonly de- therefore, not possible to state whether lamellae were subparallel to scribed in the literature. Orthorhombic patterns and maxima or the base, prism, or rhomb planes. Because the c axes are concen- small circles normal to the grain-shape foliation appear to be trated at a high angle to the foliation, slip on the basal plane in the characteristic of greenschist-facies rocks, and strong maxima close a direction must be important (see the calculations of Tullis and to the foliation are characteristic of upper greenschist or higher others, 1973, and Hobbs and others, 1972). Variations in the grade rocks. Green and others (1970, p. 330) and Green (1966) c-axis patterns, such as the orthorhombic versus the small-circle have suggested that in some high-grade granulitic rocks, a point girdle, may well be a combination of different strain conditions, an maximum of c axes develops normal to the foliation; however, in initial preferred orientation, differences in the deformation path, the literature there is little evidence that such patterns are common and the operation of rhombohedral or prismatic slip. (compare Sander, 1950, Diagrams D 28, 29, 30, 31, 32; Behr, In the Risfjallet mylonite, the pronounced minimum at the pole 1961). The examples quoted by Green and others (1970) are the to the mylonite layering is in strong contrast to the situation in the specimens described by Hietanen (1938, Diagrams DS 46, 9). The Mount Isa mylonite, where the same position is occupied by an microstructure of these rocks (compare Hietanen, 1938, PL VI) orthorhombic distribution or small-circle girdle concentration of c suggests that the preferred orientation development may not be axes. The tendency of most c axes in the Risfjallet mylonite to lie at contemporaneous with the high-grade metamorphic assemblages small angles to the foliation suggests that basal slip was unimpor- but is a result of a later lower grade deformation involving intra- tant. The slip must be in a zone containing c, and the slip direction crystalline slip processes. appears to be a large angle to c. If the slip direction was parallel to c, we would expect a maximum parallel to the lineation (assuming CONCLUSIONS that the lineation is the direction of maximum extension). So in this mylonite, prismatic slip probably was predominant, and the slip In order to interpret development of preferred orientation in direction appears to be a and not c. Other evidence in the Risfjallet ribbon mylonites, it is necessary to understand the mechanisms of mylonite, such as deformed lineations and rotated garnets (Trouw, deformation or combinations under a particular set of conditions, 1973) suggests that the deformation path and principal strain di- with some inference from experimental work. In areas of intense rection did not remain constant throughout the deformation. deformation, such as mylonite zones, the deformation is probably Therefore, the direction of maximum extension in the late stages of great enough for many preferred orientations to be representative the deformation history may not correspond with the previously of an ultimate steady-state pattern; the type of pattern developed formed mineral lineation, and it is equally possible that the pris- will again depend on the physical conditions, which in turn will matic slip was a combination of the c, a, and c + a directions, with influence the behavior of individual minerals. In the two examples some contribution from rhombohedral slip. described, the contrasting c-axis patterns probably represent differ- However, in the Risfjallet mylonite, there is no optical evidence ences in the dislocation glide behavior of quartz under different that prismatic slip operated in the higher temperature conditions metamorphic conditions. The effect of different strain paths and under which mylonitization took place. Instead there is abundant strain rate on the development of preferred orientation could not evidence of recovery processes. If, on the other hand, the shape be estimated. change in the quartz grains was produced solely by climb of dislo- The Mount Isa quartz mylonite is characterized by orthorhombic cations (Nabarro Creep; Nabarro, 1967), then there would be no c-axis distributions, together with a small-circle distribution lying preferred orientation change. However, if climb was associated near normal to the grain-shape foliation of the ribbon quartzes. It is

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believed that this was achieved by slip predominantly on the basal regime [abs.]: Am. Geophys. Union Trans., v. 45, p. 102. plane (0001) in the a direction under lower greenschist facies con- Groves, G. W., and Kelly, A., 1969, Change of shape due to dislocation ditions. The c-axis distribution in the Risfjället mylonite lies at a climb: Philos. Mag., v. 19, p. 977-986. low angle to the grain-shape foliation. This pattern probably de- Hara, I., 1971, An ultimate steady-state pattern of c-axis fabric of quartz in veloped as a result of gliding predominantly on prism planes metamorphic tectonites: Geol. Rundschau, v. 60, p. 1142—1173. Hara, I., Takeda, K., and Kimura, T., 1973, Preferred lattice orientation of (1010), but it was also accompanied by diffusion controlled proc- quartz in shear deformation: Hiroshima Univ. Jour. Sei., v. 7c, esses under upper greenschist facies conditions. p. 1-10. Heard, H. C., and Carter, N. L., 1968, Experimentally induced "natural" ACKNOWLEDGMENTS intragranular flow in quartz and quartzite: Am. Jour. Sei., v. 266, p. 1-42. I thank J. Tullis and R. H. Vernon for critically reading the Hietanen, A., 1938, On the petrology of the Finnish quartzites: Finlande manuscript. This study was supported in part by the Geological Comm. Geol. Bull., v. 122, 118 p. and Mineralogical Institute, Leiden, Netherlands; the Australian Hobbs, B. E., 1966, Microfabric of tectonites from the Wyangala Dam National University, Canberra; and Mount Isa Mines, Australia. area, New South Wales, Australia: Geol. Soc. America Bull., v. 77, p. 685-706. W. C. Laurijssen and W.A.M. Devile assisted with the photo- 1968, Recrystallization of single crystals of quartz: Tectonophysics, v. graphs. I particularly thank J. Glass for indicating the existence of 6, p. 353-401. the Risfjället mylonite and the c-axis pattern found in the unrecrys- Hobbs, B. E., McLaren, A. C., and Paterson, M. 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Research, v. 79, p. 1653-1661. MANUSCRIPT RECEIVED BY THE SOCIETY MAY 28, 1974 Griggs, D. T., and Blacic, J., 1964, The strength of quartz in the ductile REVISED MANUSCRIPT RECEIVED OCTOBER 7, 1974

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