Tectonophysics, 58 (1979) 201-219 201 0 Elsevier Scientific Publishing Company, Amsterdam - Printed in the Netherlands

PETROFABRIC ANALYSIS OF SAXONY GRANULITES BY OPTICAL AND X-RAY DIFFRACTION METHODS

JOHN STARKEY

Geology Department, University of Western Ontario, London, Ont. X6.4 587 (Canada)

(Received December 5, 1978)

ABSTRACT

Starkey, J., 1979. Petrofabric analysis of Saxony Granulites by optical and X-ray diffrac- tion methods. In: T.H. Bell and R.H Vernon (Editors), Microstructural Processes during Deformation and Metamorphism. Tectonophysics, 58: 201-219.

The regional development of distinct patterns of preferred orientation of quart,z c-axrs in the Saxony Granulites has been well documented in the literature. A suite of specimens representative of these fabrics has been examined both by optical universal stage. to determine quartz c-axis orientation, and by X-ray diffraction, to obtain orientation data from r, z, m and a. The data are combined to yield inverse pole figures of schistosity and lineation.

The finite strain of the Saxony Granulites is thought to be essentially a flattening and there is no evidence that the deformation path is other than one of continuous flattening. Elongation in the plane of the schistosity is local and not extreme. Because of this apparently simple deformation picture, and because preliminary transmission electron microscopy reveals the presence of dislocation structures similar to those found in deformed metals, an attempt is made to interpret the quartz orientation in terms of dis- location slip mechanisms. There is some evidence that the activation of different mechanisms is perhaps primarily controlled by temperature. At least some of the patterns of preferred orientation of quartz were probably produced by deformation in the field of stability of (Y-quartz.

INTRODUCTION

The Saxony Granulites outcrop over a lenticular area of approximately 40 km by 15 km. They occur as a dome surrounded by phyllites of Palaeo- zoic age. The petrology and structure of the area has been studied in con- siderable detail by Behr (1961). The Saxony Granulites, with their well known ribbon quartz texture, are high temperature mylonites developed under amphibolite facies metamorphic conditions from pre-existing granulite facies rocks (Scheumann and Behr, 1963; Watznauer et al., 1964). With the optical microscope in a typical specimen one can identify rounded grains of kyanite, garnet, alkali feldspar, usually perthitic, and rutile surviving from

202

the granulite facies assemblages, surrounded by the mylonite matrix which consists of ribbons of quartz, usually one grain thick, interlayered with bands of potassium and plagioclase feldspars. The relict grains range up to more than 1 mm in diameter while the matrix grains are generally 100 by 500 microns, elongated parallel to the schistosity.

In the field the mylonite foliation is well developed but the rocks often lack a lineation and small, isoclinal folds, with axial planes parallel to the foliation, are rare. The Saxony Granulites contain no features which permit determination of the finite strain. However, the deviation of the foliation around the relict clasts as seen .in thin section, particularly around garnet, suggests over 50% shortening perpendicu!ar to the foliation. Further, the aggregates of quartz grains which constitute the ribbons are usually more than 20 times longer than they are thick, in some rocks the ribbons extend throughout the thin section and their length to thickness ratio is indetermi- nate. The elongation is approximately the same in mutually perpendicular sections cut normal to the foliation and thus the quartz aggregates have the form of oblate spheroids. In lineated specimens the quartz aggregates are slightly elongated in the plane of the foliation, the ratio of major to minor axis is usually approximately 3 : 1, rarely 5 : 1. Thus the finite strain appears to have been dominantly a flattening perpendicular to the foliation with only local extension in the direction of the lineation. This style of deforma- tion is consistent with the axial and orthorhombic symmetries of the large number of orientation patterns of quartz c-axes which have been determined in these rocks (Behr, 1961,1964 and 1965; Scheumann and Behr, 1963).

GENERAL DISCUSSION OF OBSERVATION TECHNIQUES AND MICROSTRUC-

TURES

The quartz shows evidence of dynamic recrystallization. In thin section the quartz ribbons are one grain thick and the grain boundaries are perpen- dicular to the layer so that the grain shapes are rectangular in sections cut perpendicular to the foliation. In sections parallel to the foliation the grains are equidimensional. Undulose extinction is not pronounced. Preliminary transmission electron microscopy on two of the specimens, 68/125 and 68/133, shows the quartz grains to be largely recovered, with well developed subgrains. Dislocation loops and interactions are common, indicative of the operation of several slip systems.

The orientation of quartz has been analyzed in 30 specimens. Twelve of these samples have been used by Behr in his published analyses (Behr, 1961, 1965) and for the sake of comparison the results presented here will be illustrated primarily by reference to some of these samples, the results are duplicated in the other samples. The orientation of c-axes has been deter- mined optically using a universal stage and thin sections stained for both plagioclase and alkali feldspar (Hutchison, 1974). The orientation patterns have been prepared by contouring the data on a lower hemisphere, equal

203

area projection using a counting circle on the surface of the sphere with an area of 100/n% of the hemisphere, where n is the sample size. The contours are therefore in terms of number of points per 100/n% area which is equiva- lent to multiples of uniform. An indication of the degree of preferred orientation of the patterns can be obtained from Table I which lists the mean areas occupied by the different point concentrations and their stan- dard deviation, in parentheses. These data were obtained by sampling the data at intervals of n/10, where IZ is the total sample size (Starkey, 1977). X-ray diffraction has been used to determine the orientation of the lattice planes r + 2 = {loll} + (01111 ( since the d-spacing of these two forms is the same they cannot be isolated), m = {lOiO} and a = (1120) (Starkey, 1964b, 1974). Interference from feldspar and mica was eliminated by dissolving these minerals out of the thin sections using hydrofluor silicic acid.

For each specimen the orientation of the c-axes was calculated from the measured, r + z, m and a orientation pattern. Comparison between the measured and calculated c-axis orientation patterns verifies the consistency of the data. Inverse pole figures of the schistosity and the lineation, where present, have been derived from the measured crystallographic directions. The inverse pole figures express the orientation of the schistosity or lineation in terms of the crystallographic co-ordinates, and are represented in standard crystallographic orientation on an upper hemisphere equal area projection. They are derived by calculating inverse pole figures for each measured orientation pattern separately based on a 50 X 50 matrix. The final pattern is obtained by multiplying corresponding elements in the matrices. Because it is not possible to differentiate between r and z the inverse pole figures possess a pseudo hexagonal symmetry.

Behr (1961, 1965) has demonstrated that the patterns of preferred c-axis orientation of quartz in the Saxony Granulites vary systematically over the area of outcrop. In the centre the c-axes are oriented in small circle girdles which are parallel to the schistosity, Figs. 8a, 9a, 10a and lla. Around the periphery the orientation patterns consist of crossed, great circle girdles sym- metrically related to the schistosity and intersecting in the direction lying in the schistosity and normal to the lineation, Figs. 5a, 6a and 7a. On the southern margin of the granulites are small areas in which the quartz c-axes are concentrated in a single maximum lying in the schistosity, at right angles to the lineation, Figs. la and 2a, or paired maxima symmetrically displaced on either side of the schistosity and normal to the lineation, Figs. 3a and 4a. This distribution of pattern types has been confirmed from specimens of a drill core from a borehole at Tirschheim at the southwestern end of the Granulitgebirge (Behr, 1965). The borehole passed through a few metres of phyllite and then penetrated 500 m into the granulites. To a depth of 110 m the quartz in the granulites exhibits c-axis orientation patterns with a single maximum, the patterns change abruptly to ones with paired maxima which continue to a depth of 380 m, beyond which crossed girdle c-axis orienta- tion patterns are found. The borehole is presumably not deep enough to intersect rocks with a small circle distribution of quartz c-axes.

0 68.07 (1.70) 59.61 (3.89) 48.35 (4.88) 48.13 (5.50) 46.73 (3.18)

1 14.77 (1.74) 19.72 (2.29) 25.71 (5.68) 26.91 (4.47) 26.18 (4.38)

2 4.26 (0.55) 10.92 (2.82) 12.42 (1.92) 11.71 (3.74) 14.07 (2.24)

3 2.51 (0.81) 3.76 (0.88) 7.28 (0.82) 6.85 (1.40) 8.10 (0.78)

4 1.96 (0.53) 1.89 (1.60) 4.24 (0.77) 3.78 (1.21) 3.21 (0.72)

5 1.97 (0.56) 0.80 (0.10) 1.50 (0.25) 1.71 (0.70) 1.48 (0.44)

6 1.64 (0.65) 0.58 (0.36) 0.49 (0.15) 1.15 (0.53) 0.23 (0.16)

7 1.48 (0.52) 0.45 (0.33) 0.24 (0.15) 0.25 (0.18) 0.10 (0.09)

8 1.00 (0.49) 0.37 (0.14) 0.05 (0.0) 0.14 (0.05)

9 0.96 (0.28) 0.35 (0.25) 0.10 (0.05)

10 0.71 (0.24) 0.27 (0.17) 11 0.45 (0.30) 0.24 (0.16) 12 0.25 (0.16) 0.27 (0.14) 13 0.29 (0.21) 0.24 (0.14) 14 0.22 (0.11)

15 0.16 (0.10)

16 0.20 (0.06) 0.10 (0.07

0.14 (0.13 ; 0.09 (0.07 1 0.13 (0.05 ) 0.05 (0.0)

0.08 (0.03

0.07 ‘; (0.08

0.20 (0.14 1 0.0 (0.0)

0.10 (0.0)

17 18 19 20 21 22

23 24 25 26

204

TABLE I

Quartz c-axis orientation data. The mean areas of the orientation patterns and their standard deviation, in parentheses, baaed on data obtained at sampling intervals of n/10. where n is the total sample size

-- -.. ___

Point 68/136a 681133a 68/132a 68/134a 68/135a cone.

ORIENTATION DATA

Five specimens from the Tirschheim borehole were analysed in the present study. Specimens 68/136 (from a depth of 10 m), Fig. 1, and 68/133 (20 m), Fig. 2, are from the zone characterized by a single quartz c-axis maximum. The X-ray data indicate that the quartz grains in these rocks have a unique crystallographic orientation. The inverse pole figures, indicate that m tends to lie in the plane of the schistosity while the a crystallographic axis parallels the lineation. In specimen 68/136 the orientations of the schistosity and lineation depart significantly from being parallel to m and a respectively. This tendency is still more pronounced in specimens 68/132 (280 m), Fig. 3, and 68/134 (unspecified depth), Fig. 4. In those specimens, which possess

205

68/129b 68/131b 681002 68/127b 68/125a 68/126a

37.77 (3.50) 36.52(3.45) 17.96(4.27) 4.86 (0.94) 1.78 (0.77) 0.74 (0.23) 0.38 (0.23) 0.22(0.11) 0.12(0.08) 0.15(0.05) 0.03 (0.04) 0.05 (0.0)

29.02 (2.85) 25.30 (3.55) 12.57 (2.09) 6.49 (0.41) 4.50(1.02) 1.53 (0.24) 0.63 (0.35) 0.40 (0.20) 0.14 (0.13)

59.46 (5.06) 16.96 (4.82) 8.76(1.68) 6.96 (1.47) 2.46 (0.62) 2.04 (0.82) 1.35 (0.45) 0.72(0.38) 0.59 (0.24) 0.53 (0.13) 0.53 (0.12) 0.11 (0.08) 0.17 (0.06) 0.08 (0.05)

52.33 (3.79) 21.29 (4.92) 12.91(1.67) 6.63 (1.27) 3.39 (0.73) 1.93 (0.99) 0.83 (0.41) 0.39 (0.29) 0.25 (0.24) 0.09 (0.08) 0.08 (0.09) 0.10(0.07) 0.05 (0.0) 0.05 (0.0)

62.28 (5.78) 16.90 (3.15) 6.37 (1.56) 4.64 (1.52) 4.07 (2.11) 2.19 (0.95) 1.36 (0.53) 0.84 (0.48) 0.41(0.18) 0.35 (0.18) 0.25 (0.13) 0.18 (0.09) 0.31 (0.13) 0.18 (0.10) 0.21 (0.09) 0.13 (0.08) 0.25 (0.08) 0.10 (0.04) 0.10 (0.07)

55.63 (2.72) 24.90 (2.83) 7.96 (1.20) 3.75 (0.61) 2.21 (0.91) 1.51(0.47) 1.15 (0.39) 0.81(0.30) 0.53 (0.16) 0.63 (0.13) 0.38 (0.14) 0.41 (0.15) 0.24 (0.07) 0.15 (0.10) 0.13 (0.09) 0.50 (0.0)

paired maxima distribution of c-axes, the schistosity departs progressively from parallelism with m and its pole becomes distributed in a small circle girdle which, in specimen 68/134 contains maxima centred on the positions 2 of the normals to (2245) + (4225). Specimen 68/134 is lineated and the inverse pole figure of the lineation shows maxima at {lOTi> + [Olll} within a small circle concentration.

Specimen 68/135 (454 m), Fig. 5, has a crossed girdle pattern of preferred orientation of c-axes. The inverse pole figure of the schistosity, Fig. 5b, is complex and consists of a small circle concentration which encompasses such low index forms as (20313 + (02%) and (11%) but there are maxima within this girdle around (3034) + (0334) and (lOi3) + {Oli3). The inverse pole figure of the lineation, Fig. 5c, consists of a broad concentration

206

WART2 C-AXES. SPECIMEN 6W136A.

150 DATA

CONTOUAECJ AT1 2 3 + 5

POINTS PER 0.7 x AREA

C~TOUA VALES 1.0 2.0 CONTOU3 VAl_@S 1.0 2.0 3.0 4.0 5.0

Fig, 1. Specimen 68/136a. a. c-axis orientation pattern, lower hemisphere equal area pro- jection, areas of lere than 1 point concentration are stippled, areaa of highest concentra- tion are cross hatched. The schistoeity is indicated by a great circle, and the l&&ion is indicated by X. b. Inveme pole figure of the achistosity baeed on orientation data for c. r, t, m and O. The contours are in multiples of uniform density, the area of less than uniform density is etippled, high concentmtiont are croon hatched. Upper hemhphere equal area projection. c. I.nv&ae pole figure of the lineation based on c, r, z. m and a. C-on- toured as in b.

207

QUARTZ C-AXES. SPECIMEN 6W133A.

100 DATA

CONTOURED AT 1 2 3 + 5 10

POINTS PER 1.0 % AREA

i

“.’ : : “.. :.

.:..:: .:ct : .:: .: k.r .:.. b I:& CONTOUR VALUES 1.0 2.0 3.0 4.0 5.0 CONTOUR VALUES 1.0 2.0 3.0 +.O 5.0

6.0 6.0 7.0 8.0 9.0

Fig. 2. Specimen 68/133a. a. c-axis orientation pattern. b. Inverse pole figure of the schistosity. C. Inverse pole figure of the lineation. For an explanation see Fig. 1.

in the zone about the c-axis. Specimen 68/129 (Behr’s specimen 350, from the quarry by the convent at Geringswalde) has been described as having a crossed girdle orientation pattern of the quartz c-axes, this is not immediately apparent from Fig. 6a. However, the inverse pole figures, Figs. 6b and 6c.

208

OUARTZ C-AXES. SPECltlfN M/1324

100 DATA

CONTOURED AT i 2 3 ‘) POINTS PCR I.0 X hl?EA

CoNTaR VALUES 1 .o

Fig. 3. Specimen 68/132a. a. c-axis orientation pattern. b. Inverse pole figure of the schistosity. For an explanation see Fig. 1.

are comparable with those obtained from specimen 68/135, Figs. 5b and 5c. In the inverse pole figures of the schistosity the small circle concentration is present and some of the maxima are similar. The inverse pole figures of the lineation also show the same concentration in a great circle normal to c but in Fig. 6c there are distinct maxima on either side of m.

Specimen 68/131 (Behr 275, Dittmansdorf near Penig) is similarly described as having a crossed girdle c-axis orientation pattern, Fig. 7a. Again, in the inverse pole figure of the schistosity, Fig. 7b, a small circle concentra- tion is present but it is broader than in the previously described crossed girdle types, Figs. 5b and 6b; and the isolated maxima are absent. Figure 7b appears to be transitional to the inverse pole figure of schistosity, Fig. 8b, obtained from specimen 68/002 (near Rohrsdorf) in which the c-axes are distributed in a small circle girdle, Fig. 8a. A further development is shown by specimen 68/127 (Behr, 21, quarry northwest of the gymnasium at Russdorf). Here the small circle distribution of quartz c-axes has separated into two maxima, Fig. 9a, and in the inverted schistosity pole figure distinct maxima occur at r + z, Fig. 9b.

Specimen 68/125 (Behr 228, near Penig), Fig. 10 shows c-axes distributed in a small circle girdle with a smaller radius than observed in specimens 68/002 and 68/127, Figs. 8a and 9a. The inverse pole figure of the schistos- ity, Fig. lob, is also distinct and shows a concentration in a small circle of lesser radius. In specimen 68/126 (B&r 197, quarry at Winkeln--Zschoppels- heim), the orientation patterns are comparable to those obtained from

209

WARTZ C-AXES. SPECIflEN 68/13+A.

200 DATA

CONTOURED AT 1 2 3 4 5

POINTS PER 0.5 % AREA

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

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

I ..................... ..................... ....... ............. ... ... ...... ..............

CONTOUR VALUES 1.0 2.0 CONTOUR VALUES 1.0 2.0

Fig. 4. Specimen 68/134a. a. c-axis orientation pattern. b. Inverse pole figure of the

schistosity. c. Inverse pole figure of the lineation. For an explanation see Fig. 1.

specimen 68/125 but the small circle girdle of c-axes has separated into two maxima, Fig. lla. The small circle concentration in the inverted pole figure of the schistosity is essentially complete but the inverse pole figure of the lineations shows a small circle concentration containing maxima centred at the rhombs (2023) + (0223).

210

OUARTZ C-AXES, SPECIMEN 68/135h.

!OO DATA

CONTOUt7ED AT 1 2 3 + POINTS PER 1.0 Y AREA

:: . . : . _

. . . _...:

CONTOUH VALUES 1.0

Fig. 5. Specimen 68/135a. a. c-axis orientation diagram. b. Inverse pole figure of the schistosity. c. Inverse pole figure of the lineation. See Fig. 1 for an explanation.

DISCUSSION

It was noted earlier that transmission electron microscopy of thetae rocks has revealed some of the same features observed in metals which have deformed by dislocation glide. Hence it seems appropriate to attempt to explain the observed patterns of preferred orientation in terms of the

211

QUARTZ C-AXES. SPECItlEN 68/1296.

100 DATA

CONTOURED AT 1 2 3 4 POINTS PER 1.0 X AREA

I --

CONTOlM VALUES 1.0 2.0 CONTOUR VALUES 1.0 2.0 3.0 4.0

Fig. 6. Specimen 68/129b. a. c-axis orientation diagram. b. Inverse pole figure of the

schistosity. c. Inverse pole figure of the lineation. See Fig. 1 for an explanation.

Taylor-Bishop-Hill model of dislocation creep. In this model, the deforma- tion occurs by the movement of dislocations in a glide direction within a glide plane, the resulting plastic deformation tends to orient the glide plane perpendicular to the direction of maximum compression (Hobbs et al., 1976). Because the deformation of the Saxony Granulites appears to have been coaxial and in the flattening field the schistosity probably lies in the

212

OUAATZ C-AXES. SPECItIEN 68/1318. 200 DATA

CONTOURED AT 1 2 3 + 5 CONlOUR VALUES I.0

Fig. 7. Specimen 68/131b. a. c-axis orientation diagram. b. Inverse pole figure of the schitiosity. See Fig. 1 for an explanation.

OUARTZ C-AXES. SPECIMEN 6WOO2. 100 DATA

CONTolmED AT I 2 3 * POINTS PER I.0 x AREA CONTOUR VALUES 1.2 2.G

Fig. 8. specimen 68/002. a. c-axis orientation diagram. b. Inverse pole figure of the schistosity. See Fig. 1 for an explanation.

213

WART2 C-AXES. SPECIMEN 681127B.

100 DATA I

b ._.._...,_.._._.._____.. .

I CONTOURED hi 1 2 3 4 POINTS PER 1.0 % AREA I

CONTOUR VALUES 1.0 2.0 3.0 4.0

Fig. 9. Specimen 68/127b. a. c-axis orientation diagram. b. Inverse pole figure of the schistosity. See Fig. 1 for an explanation.

SPECIMEN 66/125A

200 DATA

I

CONTOURED AT 1 2 3 t 5 10

POINTS PER 0.5 % AREA CONTOUR VAIJES 1.0 2.0 3.0 4.0

Fig. 10. Specimen 68/125a. a. c-axis orientation diagram. b. Inverse pole figure of the schistosity. See Fig. 1 for an explanation.

214

OUAR’IZ C-AXES. SPECIHEN 6W126A. 100 DATA

CONTOWIED AT 1 2 3 + 5

POINTS PER 1.0 Z AMA

CONTWR VALUES I.0 2.0 ccwm VALUES 1.0 2.0 3.0

Fig. 11. Specimen 68/126a. a. c-axis orientation diagram. b. Inverse pole figure schistosity. See Fig. 1 for an explanation.

of the

XY plane of the finite strain ellipsoid (where X > Y > 2). The lineation is a stretching lineation parallel to the major axis of the strain ellipsoid, X. It is therefore tempting to correlate the maxima of the inverse pole figures of the schistosity with specific glide planes and the maxima of the inverse pole figures of the lineation with glide directions. However, such an interpreta-

215

tion is, in general, erroneous. Lister (1974) has calculated orientation patterns for quartz on the basis of the Taylor-Bishop-Hill model assuming a variety of glide mechanisms with different values of critical resolved shear stress and deformation conditions. The interaction of the several slip systems results in the glide planes and glide directions assuming compromise orienta- tions. Only where the strain is irrotational and where one slip mechanism is predominant over the others do the normal to this glide plane and the glide direction approach parallelism with the X and 2 axes of the finite strain ellipsoid respectively.

The simple and strong orientation patterns of Figs. 1 and 2 suggest that slip parallel to m in the direction of the a axis may have been the predomi- nant deformation mechanism. Limited operation of other slip systems was perhaps responsible for the deviation of the pattern from an ideal single crystal pattern and permitted slip on the dominant system to continue to operate.

Transition to the paired c-axis maximum patterns of Figs. 3 and 4 may reflect a reduction in the predominance of m slip and the increase in impor- tance of other systems, perhaps a rhomb or dipyramid, causing the apparent shift in the inverse pole figures compared with Figs. 1 and 2. Still further reduction in the importance of m slip is perhaps indicated in Figs. 5-7. Maybe an increase in basal slip is responsible for the development of the maxima inside the small circle concentration of the inverse pole figure of the schistosity. The inverse pole figures of the lineation indicate the possible operation of other glide directions, e.g., [lO‘iO].

In Figs. 8 and 9 a further change in the relative importance of the slip systems may be indicated, the well defined maxima in Fig. 8b may be due to dominant r and/or z slip. The patterns of Figs. 10 and 11, where the small circle concentration in the inverse pole figures of the schistosity are displaced towards the c-axis compared to Figs. 8 and 9, could indicate renewed importance of basal slip.

The reasons for such changes in the dominant deformation mechanisms giving rise to the different orientation patterns is not known. Within the 500 m depth of the Tirschheim borehole‘ three patterns of preferred quartz orientation are developed and the change from one to the other is abrupt (Behr, 1965). The total thickness of the Saxony Granulites is probably less than 5 km. Thus the quartz seems to be reacting in a very sensitive but regular way to some variable or combination of variables operating on this scale. There is no apparent, systematic difference in finite strain and the deformation appears to have been one of continuous flattening. Changes in confining pressure must have been small, especially over the distance represented by the depth of the borehole which is inclined to the schistosity and the concordant boundaries of the zones of different fabric types.

Some constraints on the conditions of deformation can be postulated from the orientation patterns of r and z, representative examples of which are illustrated in Fig. 12.

216

UJAI%! (R+Z). SPECIPEN 6&‘125A (FIi_fi 1054). QUARTZ (R+Z) . SPECIMEN 68/00;1. (Fii_M 11.?7;

CONTOUR VALUES 1.0 2.0 CONTOUR VhLUES , .0 2.0 3.b

QUARTZ (R+Z). SPECIREN 681135A (F1Lt-I 107% QUARTZ (R+Z). SPECIIIEN 68/133A (FILE1 If19

CONTOUR VALUES 1.0 2.0 3.11 CONTOUR VALUES 1.0 2.0 3.5

OUARTZ (R+Zl- SPECIMEN 68/136A (FILM 1074)

. .._ :, .~..._~~~~~;~~~~.~,. .::::.~:::;:..:;:: ..,,..

:. ::

CONTOUR VALUES 1-D 2.0 3.0 4.0

217

In Fig. 12a, there is a strong point maximum normal to a great circle con- centration. These must be due to the strongly diffracting r planes and indi- cate that the quartz has an r plane lying in the schistosity and the remaining r planes are approximately at right angles to it, the r to r interfacial angle is 86”. The poles to the more weakly diffracting z occur at 46” to r, and are thus distributed in a small circle between the r maximum and r girdle. Crystallites oriented with z in the position of the r maximum would have the poles to the r planes distributed in a 46” small circle around the maximum, such a girdle is not present in Fig. 12a. Thus most, if not all the quartz is oriented with r parallel to the schistosity and not z. If this orientation pattern developed in P-quartz a face of the hexagonal bipyramid {lOil} would have been parallel to the schistosity. Upon inversion to cr-quartz statistically half of the quartz would invert so that r would be parallel to the schistosity and the remainder would have z in this orientation (n.b. this could be accomplished by twinning and does not necessarily imply two orientations of grains). Since this is not the case the quartz must have developed the observed preferred orientation in the a-quartz stability field when the r and z planes were crystallographically distinct.

This type of r + z orientation pattern has been found in many rocks which, on the basis of metamorphic grade, were probably deformed in the stability field of a-quartz (e.g., Starkey, 1964a). It has also been produced experimentally (Green et al., 1970) and correlated with deformation in the a-quartz field (Green, 1967). Tullis et al. (1973) also produced this pattern of preferred r + z orientation but correlated it with deformation in the /3-quartz stability field.

The type of r + z orientation pattern illustrated in Fig. 12a, where the r and z maxima can be differentiated, is associated with the smaller radius small circle girdle orientation patterns of c-axes. Similar patterns are asso- ciated with the larger radius small circle girdle c-axis orientation patterns, Fig. 12b. The r and z maxima are also distinct in the orientation patterns associated with the crossed girdle distribution of c-axes, Fig. 12c, thus these orientation patterns too must have developed by deformation in the stability field of cr-quartz.

The r and z orientation patterns corresponding to the single and paired point maxima c-axis distributions are ambiguous, Figs. 12d and e. In these patterns six maxima of approximately equal intensity are developed and each must cont.ain both r and z. This could reflect inversion from &quartz.

Fig. 12. Orientation diagrams for r and z. The diagrams are contoured in multiples of uni-

form density, areas of less than uniform density are stippled, areas of highest density are cross hatched, lower hemisphere equal area projection. The orientation of each diagram is

the same as that of the c-axis orientation diagram for that specimen. a. Specimen 68/ 125a, c.f. Fig. 10a. b. Specimen 68/002, c.f. Fig. 8a. c. Specimen 68/135a, c.f. Fig. 5a. d.

Specimen 68/133a, c.f. Fig. 2a. e. Specimen 68/136a, c.f. Fig. la.

218

However such an orientation pattern can be expected to develop as a result of deformation in the a-quartz field because two orientations of quartz, i.e. with r parallel to z, are necessary to impart a centre of symmetry to the fabric, unless one has equal quantities of right and left handed quartz (Starkey, 1964b); postulated deformation plans require that fabrics be centrosymmetric.

If the single and paired point maxima c-axis patterns have developed in the pquartz stability field, then, since they immediately overlie the aquartz fabrics in the Tirschheim borehole, this implies increasing temperature upwards. It further suggests that temperature might be the dominant control of the deformation mechanisms since the quartz a-j3 transition is very sensi- tive to temperature and the pressure changes over a few metres will be minimal. Behr (1965) records that single and paired point maxima quartz c-axis orientation patterns only occur in the vicinity of mafic igneous bodies in the rocks immediately above the Saxony Granulites. This suggests the possibility of local heating.

CONCLUSION

The regional development of small circle girdle and crossed girdle quartz c-axis orientation patterns in the Saxony Granulites suggests that the defor- mation mechanisms which produced them varied in response to some large scale influence rather than to local factors. The associated r + z orientation patterns indicate deformation in the stability field of e-quartz. The more locally developed single and paired maxima quartz c-axis orientation patterns may result from heating caused by nearby mafic igneous rocks, the r + z orientation patterns are consistent with deformation in either the Q- or p-quartz stability field. If the latter did develop in the /3-quartz field, and if the single and paired maxima patterns are gradational through the crossed girdle patterns to the small circle distributions, which is suggested by the inverse pole figures, then all of these patterns of preferred orientation may have developed close to and on either side of the cuj%quartz transition. In the absence of independent temperature and pressure determinations this must remain speculative.

The inverse pole figures obtained from the orientation of quartz in these rocks suggest a variety of dislocation slip deformation mechanisms. How- ever, only in two cases is it probable that specific, dominant slip systems are indicated. The data obtained from rocks with a single point concentration of quartz c-axes, Figs. 2b and c, are most readily explicable by postulating slip on m with (I as the slip direction. Figure 9b may indicate dominant slip on r and/or z.

ACKNOWLEDGMENT

This research was made possible by an operating grant, (A3555), from the National Research Council of Canada. I am indebted to A.C. McLaren of

219

Monash University, Melbourne, Australia for electron microscopy observa- tions, and M.A. Etheridge and B.E. Hobbs for providing the opportunity to visit Monash University where we had many useful discussions. I would like to thank H.J. Behr, now at the University of Gijttingen for his warm hospi- tality while visiting the Saxony Granulitgebirge and for his kindness in providing several specimens. The visit was financed in part by a travel grant from the National Research Council of Canada.

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