The development of slaty cleavage, fleurieu peninsula, south australia

Tectonophysics, 58 (1979) l-20 0 Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

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THE DEVELOPMENT OF SLATY CLEAVAGE, FLEURIEU PENINSULA, SOUTH AUSTRALIA

NEIL S. MANCKTELOW Department (A u&a&a)

of Geology

(Received December

and ~~~era~ogy,

University

of Adelaide,

Adelaide,

S.A. 5000

5, 1978)

ABSTRACT Mancktelow, N.S., 1979. The development of slaty cleavage, Fleurieu Peninsula, South Australia. In: T.H. Bell and R.H. Vernon (Editors), Microstructural Processes during Deformation and Metamorphism. Tectonophysics, 58: l-20. In low grade, biotite-rich metasiltatones and slates from the western side of Fleurieu Peninsula, the slaty cleavage is defined by elongate deformed old biotites and by the coincident elongate dimensions and (001) planes of thin, well aligned new biotites. Histograms of frequency versus (OO1)-S1 angle (Sl defined by aligned thin muscovite used as the reference plane) were determined in thin section for both populations. The old biotites show a symmetrical bimodal distribution of (001) about Si, with maxima at around 20’ either side of Si. This distribution, together with the preserved intracrystalline strain, indicates that these old grains deformed largely by slip on (001) with some modification of grain boundaries by diffusive transfer. The new biotite grains are very well aligned (standard deviation 3.8*), and show no sign of mechanical defo~ation. They have not been me~h~ically rotated into alignment but must have nucleated and grown in a specific orientation. The proportion of new to old grains increases with metamorphic grade, causing a rapid strengthening of the crystallographic alignment. In slat& and phyllites with similar microstructure, the mica fabric determined by X-ray texture goniometry cannot be used as a quantitative measure of the geometry and magnitude of the bulk strain, as the intensity of the crystallographic fabric will be strongly influenced by the proportion of new mica, which is itself greatly affected by the metamorphic grade during slaty cleavage formation.

The origin of slaty cleavage has been in debate since its recognition and description in the nineteenth century (Bakewell, 1815; Sedgwick, 1835; see Siddans, 1972, and Wood, 1974, for reviews). In particular the mechanism (or mechanisms) by which mica is oriented to produce a dimensional and crystallographic fabric is unresolved. Basically there are two broad mechanisms generally proposed : (1) Rotation of preexisting mica towards the XY-plane of the finite

strain ellipsoid, either by intrac~stalline slip or as rigid bodies in a viscous matrix (Tullis and Wood, 1975; Tullis, 1976); or (2) Growth of new mica in a specific orientation, generally suggested to be either (001) parallel to XY of the finite strain ellipsoid or (001) perpendicular to the direction of maximum deviatoric stress. Recently, Williams et al. (1977) have suggested that slaty cleavage and schistosity may form by host controlled recrys~llization and growth of new micas within old deformed grams. During deformation the old mica grains deform largely by kinking, the limbs of the kinks rotating towards the XYplane. These deformed grains recrystallize to produce new micas with (001) orientations parallel to the limbs and kink band boundaries in the old grains, thereby producing a well oriented new mica fabric. Etheridge and Lee (1975) and Holeywell and Tullis (1975) suggested that in some slates it is possible to differentiate between two populations of phyllosilicates (muscovite, biotite, chlorite), one of which has (001) closely parallel to the slaty cleavage orientation. Their observations are incompatable with a mechanism for slaty cleavage formation involving only rotation of preexisting grains. A similar two population grouping of biotites was observed in specimens from the FIeurieu Peninsula of South Australia. One of these specimens was selected for further study. GEOLOGICAL

SETTING

Specimen 474-67 (stored in the ~nive~ity of Adelaide Geology Department) was collected 30 m to the west of the mouth of Campbell Creek on the south coast of Fleurieu Peninsula, South Australia (Fig. I). The specimen

Fig. 1. Location diagram. C.C.‘= Campbell Creek; N.H. = Newland Head.

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comes from the hinge area of a minor anticline within the Talisker Formation of the Cambrian Kanmantoo Group, which in this area is a well laminated to massive, variably calcareous, grey metasiltstone with occasional arkosic and Fe-oxide rich members. One of the massive, essentially unbedded units was sampled in an attempt to minimize the effects of any sedimentary anisotropy. However, bedding and bedding cleavage intersection could be determined from enclosing, better laminated units. The dominant feature within the metasiltstone is a strong penetrative slaty cleavage which is axial plane to the common mesoscopic folds along this coastal section. A weak biotite lineation plunges steeply south at a high angle (>60”) to the variable, but generally shallowly plunging fold axes. The locality is within the biotite zone of the regional metamorphism, as indicated by abundant biotite, but with the preservation of chlorite + muscovite + quartz to the exclusion of andalusite and staurolite. The grade gradually increases east of this locality, until in the vicinity of Newland Head 35 km away, there occurs the first cordierite, together with andalusite, garnet, rare staurolite, and rare fibrolitic sillimanite. PETROGRAPHY

In thin section the rock consists of approximately 35% biotite, 30% muscovite, 25% quartz and 5% plagioclase, with accessory amounts of apatite, opaques (?pyrrhotite), tourmaline and zircon. All biotites have the same pleochroic colours, from light brown to straw, indicating a fairly homogeneous biotite composition throughout, and this was confirmed by electron microprobe analysis (Table I).

TABLE

I

Microprobe

analysis

of biotite

Mineral

Old biotite

SiOz TiOz

36.93 1.80 20.16 16.39 10.73 0.47 8.89 0.34 95.71

A1203 ZFeO MgO CaO KzO NazO Total

* Figures in %. Analyst: G. Teale, energy and Cl below detection.

grains * New biotite

grains 37.70 1.60 21.79 14.38 9.90 0.21 9.64 0.34 95.56

dispersive

36.18 1.86 19.16 17.19 9.67 0.43 9.59 0.00 94.07

36.04 1.91 18.78 17.49 11.20 0.11 9.60 0.21 95.34

microprobe,

Australian

National

grains 36.56 1.92 18.59 17.58 11.07 0.00 9.80 0.18 95.69

University.

MnO

MICROSTRUCTURE

Three orthogonal sections were cut, one perpendicular to the slaty cleavage (8,) and weak biotite lineation (N), one perpendicular to SE and parallel to the lineation (P), and one parallel to S1 (S). In these sections it can be seen that muscovite and biotite form elongate platelets with their thinnest direction perpendicular to (001)) the preferential alignment of (001) of these grains about one plane producing the slaty cleavage. The steeply plunging mineral lineation within the slaty cleavage is defined by both a dimensional and a crystallographic alignment of the mica. The elongate direction of the mica platelets is preferentially aligned parallel to the lineation and the poles to (001) of the mica show maximum variation of orientation within the plane pe~endi~ular to this lineation (cf. Fig. 11). This accentuates the dimensional mica lineation by producing a parallel intersection lineation between the platelets of slightly varying orientation and the slaty cleavage plane. The measurements and descriptions which follow all refer to the P section. This section was the most useful for two reasons: (a) The mica fabric is most strongly developed in this section and the differentiation between the two biotite populations is therefore most marked. Detailed strain measurements in slates by previous workers (most recently Wood, 1973, 1974; Tullis and Wood, 1975) have shown that the slaty cleavage parallels the plane of flattening (XY) and the grain or mineral lineation parallels the elongation direction (X) within the accuracy limits of the measurements. If this correspondence between fabric and strain geometry applies in the specimen studied, the P section would correspond to X2, the plane of show the greatest two-dimensional strain. This section should therefore effects of strain geometry and magnitude on the mica orientation most clearly. (b) Because the biotite lineation is at a large angle to the bedding-cleavage intersection at this locality, the P section should be approximately perpendicular to the ~dd~~cleavage inte~ection. Therefore any effects due to original sedimentary layering or fabric should be most apparent in this section.

In thin section the biotite grains can be divided into two distinguishable populations: {a) Old grains. There are two subgroupings of old biotite grains. The majority show abundant evidence of intragranular deformation, with markedly undulose extinction and often a fine scale waviness of the (001) cleavage planes (Figs. 2-4). Well developed kinks with sharp kink band boundaries are uncommon. The grains are generally of rough partielogram ra&her than rectangular shape in section, with rather ragged and ill defined grain bound-

Fig. 2. Two old biotite grains (0), showing the characteristic rough parallelogram shape and wavy (001). Under crossed polars these grains display marked undulose extinction. Several new biotite grains also nresent (N). Width of field 0.4 mm.

Fig. 3. Highly deformed old biotite grains (0) with irregular grain boundaries, and new biotite grains (N) with typical elongate rectangular shapes, well defined grain boundaries, rounded ends, and no optical evidence of intracrystalline strain. Width of field 0.4 mm.

Fig. 4. Close-up of the central portion of Fig. 3, showing the marked difference in physical appearance between old (0) and new (IV) biotite grams. Width of field 0.1 mm,

Fig. 5. Old biotite grain with (001) at a high angle to slaty cleavage. Such grains show only limited optical effects of intracrystalline strain and develop well defined quartz fibre pressure shadows. Width of field 0.25 mm.

Fig. 6. Close-up of a typical new biotite grain showing characteristic the rounded end truncating the cleavage and the long grain boundary with no evidence of any bending of (001). Width of field 0.1 mm.

grain shape. Note parallel to (OOI),

aries. In comparison with the new grains described below, the old grains are less elongate, with dimensional elongation in the P section seldom exceeding 5 : 1, and their alignment produces only a moderate dimensional fabric. The second sub-grouping involves occasional grains which have (001) at a high angle to SE (Fig. 5). These grams are more rectangular and equant, commonly develop quartz fibre pressure shadows, and either show little intragranular deformation or a weak symmetrical kinking of (001). The grain boundary truncating (001) is always nearly parallel to St. Microprobe analyses of the old biotite grains show only limited variation in composition (Table I), suggesting they have equilibrated during metamorphism and that they are not original detrital micas. (b) New grains. The new biotite grains show little evidence of intracrystalline deformation. They form well crystallized, thin, rectangular grains with sharply defined, low energy grain boundaries (Figs. 3, 4). The grains are always bounded on the two longest sides by (OOl), and develop characteristic rounded ends where the grain boundary truncates the cleavage plane (Fig. 6). The thinness of the grains perpendicular to (001) results in higher length to width ratios than in the old grains, ratios commonly being greater than 10 : 1. The composition of the new grains is not readily distinguishable from that of the earlier biotite grains on the limited analyses presently

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available (Table I), although the new grains do appear to be consistently a little lower in Na20. It is an important point that the two populations of new and old biotites can be readily d~ffe~ntiated on the basis of shape, intr~r~ul~ deformation, and grain boundary morphology independent of any orientation parameter.

15%

10%

5%

0’

90”

(b)

New Grains

Fig. 7. Histograms of frequency versus angle between (001) and SE (slaty cleavage reference plane as deflned by we11 allled muacovlte) for old and new biotite &. Measurements made in P section, 250 old grains, 150 new grains. Old grains summed over IO” intervats, new grains over 4’. VerticaI scale expanded for old grabs Aative to new grains.

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Orientation The angle between the trace of (001) and S1 in the P section was measured for biotite grains from the two populations at 800 times magnification, using a mechanical stage to conduct a series of equally spaced traverses. The orientation of S, as defined by well aligned muscovite was used as a reference plane. Such a two dimensional analysis appears to be a reasonable approximation and has been used by several previous workers (e.g., Etheridge and Lee, 1975; Marlow and Etheridge, 1977; Beutner, 1978), though the angle measured will always be the apparent angle in the P section and not the true angle. This apparent angle will of course be less than the true angle, and the two-dimensional analysis must introduce some bias towards lower angles. However, the total mica fabric for this specimen determined by X-ray texture goniometer shows a tight point maximum of poles to (OOl), with only limited variation within the plane perpendicular to the lineation (474-67, Fig. 11). Few biotite grains will have (001) more than 30” away from the slaty cleavage orientation and the cut effect should be small, although these higher angle grains may be of specific interest. This generalization is confirmed in thin section, where for the great majority of grains slight change in focus does not significantly displace the trace of (001). A total of 250 old grains and 150 new grams were measured. Although new grains predominate in this specimen (approximately 60/40 new to old grains), fewer were measured as there was little variation in their orientation. The resultant histograms are presented in Fig. 7. The new grains are obviously very well crystallographically aligned with (001) nearly parallel to S, and a standard deviation of only 3.8 degrees. Few old grams, on the other hand, have (001) parallel to S,. The old gram fabric is bimodal, with most grains having (001) at an angle of between 10 and 30 degrees to S,. There is a minimum parallel to S1, minima between 50 and 80 degrees from S1, and a weak maximum representing the second subgrouping of old grains with (001) at a high angle to S1. The old gram fabric is essentially symmetrical about S,. DISCUSSION

The presence of two biotite populations is particularly significant in determining the mechanism of formation of slaty cleavage but unfortunately not all the deformation history may be preserved. The biotite grains now present in the specimen may not necessarily be a representative sample of all the biotites which were present at various stages during the history. In a closed system, considerable preexisting biotite would need to have been consumed to provide components for the new biotite growth, and no direct evidence remains as to the orientation or physical characteristics of this material. However it seems most likely that these components were provided

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---.

./ ‘\_ jj>

,’

,./’

(4

_‘\\ _

,,,,’

A/’

,‘,’

’

,’

. ..-

A

l3

Fig. 8. A. Shape and orientation of old biotite gram before and after deformation by slip on (001). Lines external to the grain represent slaty cleavage as defined by new aligned muscovite and biotite grains. B. Shape and orientation of old grain before and after shortening by pressure solution (cf. Beutner, 1978).

Fig. 9. Old biotite grain (0) in central area with geometry similar to Fig. 8A, indicating the gram shape was largely modified by intracrystalline slip on (OOl), with only limited effects due to diffusive transfer. Field of view 0.5 mm.

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by partial or complete corrosion of grains identical to those old grains still preserved. The old grains provide the most information on the deformation history whereas the new grains can only preserve details of the history since their nucleation. The old grains have deformed both by slip on (001) and by diffusive transfer (pressure solution, Coble creep, Herring-Nabarro creep, cf. Rutter, 1976). Slip on (001) will produce characteristic parallelogram shapes (Fig. 8A) which are occasionally seen in some of the old grains (Fig. 9), but in most cases the accompanying diffusive transfer has considerably modified the grain shapes, producing grains with long grain boundaries roughly parallel to S,. Evidence of mechanical deformation is shown by the ubiquitous undulose extinction and wavy (001) of the old grains. However, the most striking effect of dislocation glide on (001) has been the establishment of the bimodal crystallographic alignment of Fig. 7, which is in marked contrast to the lack of crystallographic alignment in old phyllosilicates from the ~~tinsburg slate described by Beutner (1978), where deformation has been dominantly by diffusive rather than dislocation flow. The symmetrical pattern of Fig. 7 suggests that the stress--strain history was effectively coaxial. Any rotational component to the deformation would result in an asymmetric distribution, with rotation of biotite grams by slip on (001) being favoured in one direction over the other. Even stronger evidence suggesting an effectively coaxial history is provided by the second sub-grouping of old grains, namely those old biotites with (001) at a high angle to 8,. These grains show little evidence of mechanical deformation and appear to have acted as “hard” grains through the slaty cleavage event, forming well defined quartz fibre pressure shadows (Fig. 5). Any shape change appears to be entirely due to corrosion and chemical transfer similar to that of Fig. 8B. Such grains could only have survived if the resolved shear stress on (001) remained low throughout the strain history. It is difficult to see how this restraint could have been maint~ned throughout the slaty cleavage deformation unless the strain was non-rotational and coaxial. The specimen comes from the hinge region of a minor anticline (wavelength around 50 m) and as such a coaxial history is more readily envisaged than if the sample was from a limb area where bedding has obviously been rotated (cf. Williams, 1976). With such a non-rotational flattening history, the orientation pattern shown in Fig. 7 is readily explained as a result of passive rotation by slip on (001) of a pre-existing biotite fabric symmetrical about the shortening direction. As (001) rotates towards the flattening plane, the resolved shear stress on the slip plane will first increase and then decrease and the grains will eventually “lock-up” with (001) at a low angle to the flattening plane, producing a bimodal distribution of (001) about this plane in two dimensions. As discussed by Tullis (1976), in three dimensions such passive rotation would produce a small circle distribution of poles to (001) about the compression direction for axial deformation. For geometries eloser to plane strain, the (001) poles would form an elliptical ring about the diree-

_/

TEMP

Fig. 10. Schematic representation of the possible interplay of conditions and events with time which produced the observed slaty cleavage microstructure. Bl = growth of old biotite grains from lower grade constituents; I32 = strain induced recrystallization of biotite grains. Dislocation flow can only be the dominant deformation mechanism above a certain yield stress (dashed line). Above this yield stress diffusion flow will be subordinate but still significant. Below this yield etreas dislocation flow will be insignificant, and any further strain must be accomplished by diffusive transfer (pressure solution, Cable creep, Herring-Nabarro creep).

tion of maximum compression, with some elongation towards the intermediate stress axis. So is effectively perpendicular to the slaty cleavage S, in this specimen. In a coaxial deformation the shortening direction and So would thus maintain their parallelism, and the initial symmetry of the old biotite fabric about the shortening direction is most likely due to an original symmetric biotite fabric related to the sedimentary plane S,,. The original fabric could not have been random as this would have produced a greater concentration of old biotites with (001) at 0” than at 90” to S, in Fig. 7, due to the two dimensional cuteffect discussed earlier, whereas there are considerably more in the 90” orientation (parallel to the interpreted orientation of ~‘3,). The original old metamorphic biotite fabric must have been symmetrical about the bedding orientation, with few biotites having (001) at a high angle to this plane. The prograde growth of these old biotite grains from lower grade components was probably a relatively rapid event as a direct result of chemical disequilibrium established by rising temperature, the transformation apparently occurring prior to or early in the slaty cleavage deformation (Fig. 10). The subsequent deformation occurred only at slightly higher temperatures well within the biotite stability field, and any tendency to crystallize new biotite grains would have been entirely strain induced with little chemical change. This is confirmed by the consistency of composition of the old and new biotites (Table I). At the moderate strains produced in the slaty cleavage event, the kinetics of this strain induced new grain growth were obviously slow, as shown by the preservation of so many old, deformed grains.

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Etheridge and Hobbs (1974), and Tullis (1976) have both found experimentally that even at temperatures close to the melting point the kinetics of strain induced recrystallization of phlogopite and biotite are very slow. The new biotite has almost invariably nucleated and grown away from the old biotite grains, so diffusion may well be the rate controlling step in the natural example. The crystallization of new biotite is temperature dependent as shown by the increasing proportion of new to old deformed grains in specimens from the higher grade regions east of this locality. The old grains with the most intrac~stalline strain are the first to be consumed. The timing of growth and the orienting mechanism of the new grains are problematical. The significant characteristics of these new grains are their elongate thin shape, low energy grain boundaries, strong alignment, and lack of optical evidence of significant mechanical deformation. Some rare new grains display very weak undulosity of extinction, but grain boundaries are straight and there is no evidence of bending of new grains about variably deformed old grains (Figs. 3, 4, 9), even though such thin elongate grains should be quite susceptible to bending moments. The range of minerals with varying physical properties, the old grains in “locked” orientations, and the mechanically “hard” grains with (001) at a high angle to the cleavage all suggest that the strain on a granular scale was quite heterogeneous in the specimen studied. Si~ific~t mechanical strain of the rock aggregate should have introduced marked bending of the thin elongate new grains due to interference from variably straining neighbouring grains, even if (001) of these biotite grains was perpendicular to the shortening direction. The lowbintracrystalline strain in the new grains therefore indicates that both they and the rock aggregate have suffered minimal mechanical strain since the crystallization of the new grains and that they could not have achieved their strong crystallographic alignment either by physical rotation as “‘rigid bodies” (which would produce considerable bending in such elongate, thin grains through grain-grain interference), or through passive rotation by intracrystalline slip. It could possibly be argued that the new grains observed in thin section are only the last of a long series of new grains which grew and were deformed during the deformation, continued deformation producing continuing rotation and recrystallization to eventually produce the preserved strong crystallographic alignment of the new grains. However, in this case we might expect at least some of the intermediate stage grains to be preserved, and besides, this process does not explain the preservation of the highly strained old grains. At any stage during the deformation history these old grains would be more highly strained than intermediate grains which had suffered only a portion of the deformation, and yet these more recently formed grains are supposed to be driven towards new grain growth by the stored energy of mechanical deformation, while the old more highly strained grains are preserved. It seems much more likely that the presently preserved two biotite populations are a reasonably representative sample of those biotite grains which were present during the deformation history, and that

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the new well aligned biotite grains are growing directly at the expense of the old grains and not as the result of intermediate steps of which no evidence remains. As discussed earlier, the lack of significant intracrystalline strain within the new grains suggests there has been little mechanical deformation of the rock aggregate since their crystallization. The new biotite grains have possibly grown during a period of stress relaxation in the waning stages of the slaty cleavage deformation, largely at stress levels below the yield point for mechanical deformation by dislocation flow. This may reflect the experimental observation of Tullis (1976) and Etheridge and Hobbs (1974) that the kinetics of recrystallization of biotite and phlogopite are very slow, even when close to the melting temperature. The schematic time relations are presented in Fig. 10. Conditions promoting the growth of new biotite grains were instigated with the first strain increments following the rise in stress that initiated the deformation. The stored strain energy within the mechanically deforming old biotites would have immediately made them unstable relative to new biotite grains in which there was little distortion of the lattice, independent of any chemical driving force. New biotites may thereby be nucleated and grown at the expense of old biotites without any change in composition, the process driven solely by a tendency to reduce the strain energy within the biotite lattice. This reduction in stored strain energy may be achieved either by the generation and movement of high angle boundaries within the old deformed grains or by the nucleation of new biotite grains remote from the old grains, components for new grain growth being provided by corrosion of the old grains and diffusion of components to the site of new biotite growth. The kinetics of the two processes under differing conditions will determine which mechanism dominates in a particular example. In the sample studied the sites of new biotite nucleation have generally been remote from the old grains. The kinetics of new grain formation may be very slow when the sole driving force is the stored strain energy within the old grains, with little difference in chemistry between old and new grains. Certainly the experimental work of Tullis (1976) and Etheridge and Hobbs (1974) showed that the formation of new biotite and phlogopite grains from deforming grains was very slow, even at high temperatures where the kinetics would be accelerated. In the natural example studied, the slowness of the kinetics may have determined that appreciable new biotite growth did not occur till late in the deformation history when the rate may have been enhanced by a steady increase in temperature with time (Fig. 10). The peak of metamorphism in the Fleurieu Peninsula can be shown to occur immediately prior to and during the second deformation, effects of which are not developed in the area of this study. However, the grain size and proportion of new grams in specimens sh,owing only slaty cleavage increases towards the syn-D2 andalusite-staurolite isograd near Newland Head (Fig. l), suggesting that the temperature had already begun its upward rise during the first deformation (Fig. 10).

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As discussed previously, the new biotites could not have suffered significant rotation since nucleation and growth, and their strong alignment must be dominantly an oriented growth feature. The possible controls on this orientation include: (1) Mimetic growth. This broad mechanism is taken to include both host controlled recrystallization and control exercised by a pre-existing fabric. There appears to be no tendency for the new grains to preferentially nucleate in old deformed grains, the new grains occurring most commonly in juxtaposition with quartz and feldspar, and strong host grain control of the new grain orientation is difficult to justify. The alternative is that the old grain fabric controlled the new gram orientation. The pre-existing mica fabric could promote diffusion parallel to the fabric rather than across it, allowing transport of components to sites of growth elongating parallel to the anisotropy much more readily than to grains attempting to transect the old fabric (cf. Etheridge et al., 1974; Etheridge and Lee, 1975). The limited experimental work would indicate that although the pre-existing fabric exerts some control, the recrystallized new grains growing in a hydrostatic environment form a decussate fabric with weaker alignment than the old fabric, and strong alignment of the recrystallized fabric only results when the recrystallization occurs during the deformation (Tullis, 1976). (2) Oriented growth with (001) perpendicular to the maximum compressive stress. Etheridge et al. (1974) have discussed in some detail possible mechanisms by which a trend to the thermodynamically most stable orientation might produce crystallographic alignment of mica during crystallization or recrystallization in an applied stress field. Some of their proposed mechanisms rely heavily on the geometry of pore spaces, and may be quite valid in experimental samples. However, the metasiltstones from Fleurieu Peninsula were metamorphosed to biotite grade and in some cases extensively veined by quartz prior to the slaty cleavage deformation. The porosity of the original sediment would have been reduced to. near zero by diffusive transfer in this metamorphic environment well before the deformation which produced the first fabric, and any mechanism dependent on pore space could not have played a significant part in the orientation of the new biotite. However, two other mechanisms discussed by Etheridge et al. (1974) may have played a dominant role: (a) Oriented crystallization or recrystallization under homogeneous stress, taking into account only the elastic strain energy in the grains due to the applied stress. Kamb (1959) applied the thermodynamic theory of equilibrium under non-hydrostatic stress, developed by Gibbs (1906), to infinitesimally strained linearly elastic crystals. He showed that the preferred orientation for a given mineral is the one for which the chemical potential of the crystal component within the intergranular film is a minimum. This chemical potential is that required for equilibrium across a surface perpendicular to the axis of greatest compressive stress. Kamb (1959) found from his calculations that the preferred orientation for a given mineral is in most

16

cases that in which the most compliant direction is aligned parallel to the axis of greatest compressive stress. Using the elastic constants of Alexandrov and Rhyzova (1961), Schwerdtner (1964) demonstrated that this criterion predicted the thermodynamically most stable orientation for biotite to be with (001) normal to the axis of greatest compressive stress, As discussed by Etheridge et al, (1974), the driving force for this reorientation is very small, and the kinetics of the transformation would have to be very fast for such small driving forces to produce significant rates of oriented crystallization. However, this is only the case if these forces are the only ones driving towards new grain growth, whereas in the natural example the dominant driving force will be the stored energy in the plastically deformed old grains. The old grains would tend to recrystallize anyway, the effects of the elastic anisotropy of biotite in an applied stress field simply favouring one orientation of new grain growth as being thermodynamically slightly more stable. One problem with the analysis of Kamb (1959) is the simplifying assumption of homogeneous stress, which may be unrealistic for a polycrystalline aggregate of different minerals (cf. Calnan and Clews, 1950). (b) Interaction of orientation-dependent pressure solution and anisotropic growth rate. This mechanism proposed by Etheridge et al. (1974) can be considered complementary to that proposed by Kamb (1959) in that it considers two variables, stress variation and grain shape, which were excluded from his analysis. As such the mechanism may be more realistic. Thermodynamic considerations of the equilibrium between a stressed solid and its solution (Gibbs, 1906) leads to the important prediction that the solubility of the solid increases appreciably with increasing normal stress on a particular face. This forms the basis of all pressure solution mechanisms of deformation, material tending to go into solution on faces of high compressive stress and precipitate in areas of low stress, solution and transport occurring within an adsorbed, water-rich intergranular film (cf. Rutter, 1976). Mica grains have a marked anisotropic growth rate, tending to consistently elongate parallel to (001). Any micas which commence growth with (001) inclined to the plane of compression will also tend to elongate in that direction. Such elongate grains nearer to the compressive loading direction will tend to concentrate stress, particularly at their ends, promoting the pressure solution effect and thereby favouring the orientation with (001) and elongation parallel to the compressive plane. Mica grains will tend to grow preferentially with (001) perpendicular to the compression direction, as this is the only orientation in which the anisotropic growth of mica would not produce a stress concentration on the grain. (3) Oriented growth related to the flattening plane of the strain ellipsoid. Effects due to stress and strain geometry and magnitude are not readily differentiated in this specimen, as the microfabric would indicate that stress and strain were coaxial However, the passive rotation of old grains by dislocation glide on (001) is directly related to the bulk strain, and the contribution this old grain fabric makes to the slaty cleavage, together with any

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mimetic control on new gram growth, is effectively strain controlled. The oriented growth of the new grains may also be more directly related to the strain geometry. The growth of new thin biotites with their thinnest direction parallel to the direction of maximum shortening at the expense of old thicker grains will produce the maximum shortening possible by diffusive transfer in the closed system, and may therefore be the favoured process during defo~ation. CONCLUSION The orienting mechanisms for the two biotite populations have been quite different. The old biotite grams have attained their c~s~lo~aphic alignment largely by rotation due to dislocation glide on (001). This appears to be the only operative slip plane in biotite, with possibly three well defined slip directions (Etheridge et al., 1973). The single slip plane leads to extreme plastic anisotropy in biotite, the resolved shear stress on (001) (which will be orientation dependent) controlling the plastic behaviour of individual grams in the deforming aggregate. Diffusive transfer occurring co~cidentally with dislocation glide will modify gram shapes but will have no effect on the crystallographic reorientation (cf. Beutner, 1978). The March model (March, 1932; modified for non-random initial fabrics by Owens, 1973) would treat (001) as a passive plane in an isotropically deforming material, and is obviously based on conditions markedly different from those encountered in a deforming phyllosilicate-rich rock. The March model predicts that (001) of the old biotites would rotate towards parallelism with the XY-plane of,the strain ellipsoid, whereas a more realistic model based on a rotation mechanism due to intracrystalline slip on (001) would predict a small circle distribution of poles to (001) about the compression direction (Tullis, 1976). The two dimensional orientation diagram of Fig. 7 is more consistent with the latter model. The new biotite grains have not been rotated into pigment but have grown in a specific orientation related to the stress and/or strain geometry. This new biotite growth appears to have occurred during a period of little mechanical deformation, when diffusive processes were dominant. The strong alignment of new grains generates a tight point m~imum of poles to (001) of biotite perpendicular to the slaty cleavage, which would occupy the low intensity central region of the old biotite small circle distribution. The addition of the old and new biotite fabrics will therefore produce a total biotite fabric, as could be determined by X-ray texture goniometer, consisting of a broad point maximum of poles to (001) perpendicular to the slaty cleavage. The proportion of new to old grams will largely determine the maximum intensity and tightness of the maximum. Two major factors influence the recrystallization of new grams from old: the stored strain energy in the old grains, and temperature. Increasing the total finite strain will increase the strain energy in the deformed biotite grams and thereby

Fig. 11. Pole figures for the total mica fabrics of specimens 474-67 and 474-105, determined by X-ray texture goniometer in reflexion mode, for lattice plane (001). Slaty cleavage horizontal, mica lineation east-west. Spec. 47467 contours at 1, 2.5, 5, 7.5, 10, 12.5 times uniform; 474-105 at 1, 5, 10, 15, 20, 26 times uniform. Spec. 474-67 collected from within the biotite-hlorite zone near Campbell Creek, 474-105 from the andalusite-staurolite zone east of Newland Head. Note the marked strengthening of the crystallographic alignment in the higher grade specimen, reflecting the greatly increased proportion of new to old grains.

promote recrystallization, increasing the proportion of new to old grains and therefore strengthening the point maximum of poles to (001). This may explain the approximate correspondence between point maximum intensities predicted by the March model and the observed values in dynamically recrystallizing natural (Oertel and Wood, 1974; Tullis and Wood, 1975; Wood et al., 1976) and experimental (Tullis, 1976) examples. However, the mechanisms of mica orientation assumed for the March model and those in natural examples are very different, and the correspondence in results is largely fortuitous. Values predicted by the March model will often be in error (e.g., Siddans, 1978). This will particularly be the case when the second factor affecting recrystallization, namely temperature, is variable. As temperature during deformation increases along the south coast of Fleurieu Peninsula from west to east, the proportion of new to old biotite grains increases markedly, until in the coarser g-rained phyllites and schists almost all the old biotite grains have been consumed. This produces a concomitant strengthening of the mica fabric point maximum related to temperature during deformation and largely independent of variations in the magnitude of finite strain (Fig. 11). Temperature dependence varies with differing minerals, muscovite apparently recrystallizing more readily than biotite. In summary, the establishment of a strong mica fabric in phyllites of the southern Fleurieu Peninsula follows a two-stage process involving plastic deformation and rotation of old mica grams by intracrystalline slip on

19

(OOl), the stored strain energy in these grains promoting the growth of new mica grams with a strong preferred orientation. The two processes should overlap in time, recrystallization being possible from the first strain increment. However, in practice the kinetics of mica recrystallization may be so slow (Tullis, 19’76; Etheridge and Hobbs, 1974) that significant new mica growth does not occur till late in the deformation history, when the accumulated strain energy in the old grams is sufficient to drive the recrystallization at appreciable rates. ACKNOWLEDGEMENTS

This paper is the result of post-graduate research carried out in the Geology Department of the University of Adelaide while the author was supported by an Australian Commonwealth Postgraduate Scholarship. Thanks are extended to Drs. P. James, M. Etheridge and T. Bell who read various drafts of the manuscript. REFERENCES Alexandrov, K.S. and Rhyzova, T.J., 1961. Elastic properties of the rock-forming minerals: layered silicates. Izv. Acad. Sei. U.S.S.R., Geophys. Ser., 1961: 1799-1804. Bakewell, R., 1815. An ~troduction to Geology, Harding, London, 2nd. ed., 492 pp. Beutner, E.G., 1978. Slaty cleavage and related strain ,in Martinsburg slate, Delaware Water Gap, New Jersey. Am. J. Sci., 278: l-23. Calnan, B.A. and Clews, C.J.B., 1950. Deformation textures in face centred cubic,metals. Philos. Mag., 41: 1085-1100. Etheridge, M.A. and Hobbs, B.E., 1974. Chemical and deformational controls on recrystallisation of mica. Contrib. Mineral. Petrol., 43: 111-124. Etheridge, M.A. and Lee, M.F., 1975. Microstructure of slates from Lady Loretta, Queensland, Australia. Geol. SOC. Am. Bull., 86: 13-22. Etheridge, M.A., Hobbs, B.E. and Paterson, M.S., 1973. Experimental deformation of single crystals of biotite. Contrib. Mineral. Petrol., 38: 21-36. Etheridge, M.A., Paterson, M.S. and Hobbs, B.E., 1974. Experimentally produced preferred orientation in synthetic mica aggregates. Contrib. Mineral. Petrol., 44: 275294. Gibbs, J.W., 1906. On the equilib~um of heterogeneous substances. In: The Scientific Papers of J. Willard Gibbs. Longmans, Green and Co., New York, N.Y., 1: 55-353. Holeywell, R.C. and T&is, T.E., 1975. Mineral reorientation and slaty cleavage in the Martinsburg Formation, Lehigh Gap, Pennsylvania. Geol. Sot. Am. Bull., 86: 12961304. Kamb, W.B., 1959. Theory of preferred crystal orientation developed by crystallisation under stress. J. Geol., 67: 153-170. March, A., 1932. Mathematische Theorie der Regelung nach der Korngestalt bei affiner Deformation. Z. Kristallogr., Kristallgeom., Kristallphys., Kristallchem., 81: 285298. Marlow, P.C. and Etheridge, M.A., 19’77. Development of a layered crenulation cleavage in mica schists of the Kanmantoo Group near Macclesfield, South Australia. Geol. Sot, Am. Bull., 88: 873-882. Oertel, G. and Wood, D.S., 1974. Finite strain measu~ment: a comparison of methods. Trans. Am Geophys. Union, 55: 695 (abstr.).

20 Owens, W.H., 1973. Strain modification of angular density distributions. Tectonophysics, 16: 249-262. Rutter, E.H., 1976. The kinetics of rock deformation by pressure solution. Philos. Trans. R. Sot. London, Ser. A., 283: 203-219. Schwerdtner, W.M., 1964. Preferred orientation of hornblende in a banded hornblende gneiss. Am. J. Sci., 262: 1212-1229. Sedgwick, A., 1835. Remarks on the structure of large mineral masses, and especially on the chemical changes produced in the aggregation of stratified rocks during different periods after their deposition. Trans. Geol. Sot. London, 2nd Ser., 3: 461-486. Siddans, A.W.B., 1972. Slaty cleavage - a review of research since 1815. Earth-Sci. Rev.. 8: 205-232. Siddans, A.W.B., 1978. The development of slaty cleavage in a part of the French Alps -reply. Tectonophysics, 47: 187-191. Tullis, T.E., 1976. Experiments on the origin of slaty cleavage and schistosity. Geol. Sot. Am. Bull., 87: 745-753. Tullis, T.E. and Wood, D.S., 1975. Correlation of finite strain from both reduction bodies and preferred orientation of mica in slate from Wales. Geol. Sot. Am. Bull., 86: 632-638. Williams, P.F., 1976. Relationships between axial plane foliations and strain. Tectonophysics, 30: 181-196. Williams, P.F., Means, W.D. and Hobbs, B.E., 1977. Development of axial-plane slaty cleavage and schistosity in experimental and natural materials. Tectonophysics, 42: 139-158. Wood, D.S., 1973. Patterns and magnitudes of natural strain in rocks. Philos. Trans. R. Sot. London, Ser. A., 274: 373-382. Wood, D.S., 1974. Current views of the development of slaty cleavage. Annu. Rev. Earth Planet. Sci., 2: 369-401. Wood, D.S., Oertel, G., Singh, J. and Bennett, H.F., 1976. Strain and anisotropy in rocks. Philos. Trans. R. Sot. London, Ser. A., 283: 27-42.