Bell, T.H. and Johnson, S.E., 1989b. The role of deformation partitioning in the deformation and...

J. metamorphic Geol, 1989, 7, 151-168

The role of deformation partitioning in the deformation and recrystallization of plagioclase and K-feldspar in the WoodrofiCe Thrust mylonite zone, central Australia T. H. BELL AND S . E. JOHNSON Department of Geology, lames Cook University, Townsville, Queensland 4871, Australia

ABSTRACT In the Woodroffe Thrust mylonite zone, central Australia, recrystallization in plagioclase and K-feldspar involved subgrain rotation, assisted by grain-boundary or kink band boundary bulging, without contribution from a change in the chemical composition from host grains to new grains. The size of subgrains and new grains changes across the mylonite zone, apparently as a function of the strain rate and the HzO content of the rock.

The partitioning of deformation into zones of progressive shearing and progressive shortening controls the sites of recovery and recrystallization in feldspar during mylonitization. The size of feldspar porphyroclasts in well developed mylonites is governed by the scale of deformation partitioning reached in the earlier stages of mylonitization, before the formation of a large proportion of tine-grained matrix that can accommodate the progressive shearing component of the deformation.

Recrystallization occurs in microcline, apparently without involving a translation to a monoclinic structure, as microcline-twinned new grains are common adjacent to microcline-twinned host grains. K-feldspar triclinicity values calculated from XRD traces increase from the margins to the interior of the mylonite zone, in conjunction with deformation intensity. K-feldspar host grains locally have cores of orthoclase or untwinned microcline, surrounded by mantles of twinned microcline, suggesting a relationship between the presence of microcline twinning and the degree of K-feldspar triclinicity.

Key words: deformation partitioning; K-feldspar triclinicity; mylonite; plagioclase; recrystallization.

I N T R O D U C T I O N

General agreement is lacking regarding deformation, recovery and recrystallization processes in feldspar, and the effects of physical and chemical conditions on these processes. Therefore, we decided to examine the role of deformation partitioning, as described by Bell (1981) and Bell, Fleming & Rubenach (1986), in controlling the physical and chemical aspects of these processes in the Woodroffe Thrust mylonite zone, central Australia (Fig. 1). This zone formed at the boundary between original granulite and amphibolite facies felsic gneisses that differ only in Rb/Sr ratios and H20 content (Bell & Etheridge, 1976). It thus provides an excellent opportunity to study plagioclase and K-feldspar ranging from an initially undeformed state on the margins of the mylonite zone, to a strongly deformed state in the interior of the zone where much, but not all, of the feldspar is totally recrystallized. The role of deformation partitioning in the deformation, recovery and recrystallization processes in these minerals can be compared and contrasted, and the effects of varying HzO content across the mylonite zone determined, enabling a comparison with experimental work (e.g. Tullis & Yund, 1980) that suggests a significant role for water in the behaviour of feldspar. In the Woodroffe Thrust mylonite zone, K-feldspar

triclinicity increases with progressive mylonitization. In

this context, we examine the conditions that might have facilitated interchange of A1 and Si during mylonitization. Microcline-twinned host grains, with a mantle of subgrains adjacent to microcline-twinned new grains, occur in the Woodroffe Thrust mylonite zone. We discuss the effects of high-angle boundary migration during recrystallization on ordering/disordering in K-feldspar.

Most studies of recrystallization in naturally deformed feldspar have documented a difference in chemical composition between the host grain and adjacent new grains (Vernon, 1975; White, 1975; Allison, Bamett & Kemch, 1979; Borges & White, 1980; Brown, Maca- udiere, Ohnenstetter & Ohnenstetter, 1980; Vidal, Kubin, Debat & Soula, 1980; Hanmer, 1982; Watts & Williams, 1983; Olsen & Kohlstedt, 1985). Comparative hosthew grain chemistry of feldspar in the Woodroffe Thrust mylonite zone is presented and discussed.

CEOLOC I CAL SETTI N C

The mylonites containing the feldspars described in this paper were obtained from the Woodroffe Thrust (Major, Johnson, Loeson & Mirams, 1967; Bell, 1978) north of Amata in the Musgrave Ranges of central Australia (Fig. 1). Granulite facies felsic gneisses outaop on the western side of the thrust, whereas amphibolite facies felsic

151

152 T. H. BELL & S. E. JOHNSON

Basic and ul l ra baamc - Woodrotlo Thr'usl

_c_ Davenport Shear E l intrusions

0 Granulite Iac8ea gn.iss

I)

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

I I-] Alluvium

I I . .

Plg. 1. (a) Location of Woodroffe Thrust, and regional geology of the Musgrave metamorphic belt in Central Australia. (b) Detailed map of the area outlined in (a), showing the traverse A-A' from which the specimens described in this paper were collected (modified from Bell & Etheridge, 1976).

gneisses outcrop on the eastern side. The mylonite zone formed along their boundary at upper amphibolite facies conditions

Bell & Etheridge (1976) divided the transition from country rock to the ultimate stage of mylonitization (slaty

T.Me 1. Seven microstructural stages in mylonite development across the Woodroffe Thrust mylonite zone (after Bell & Etheridge, 1976). The stage GA7 central slaty mylonite lies on the boundary between the granulite and amphibolite facies sides.

Granulite side Gl-least mylonitized granulite facies country rock G2-moderately mylonitized granulite facie country rock G3a-coarsc-grained quartz-feldspar mylonite G3hedium-grained quartz-feldspar mylonite GMne-gra ined quartz-feldspar mylonite G4-quartz completely recrystallized GS-mica crystallization widespread Gbmineralogical homogenization GA7-amtral slaty mylonite

A l 4 e a s t mylonitized amphibolite facies country rock A2-tnoderately mylonitized amphibolite faaes country rock A3-coame-grained quartz-feldspar mylonite A4-quartz completely recrystallized A5-mica crystallization widespread A6-cnineralogical homogenization

Amphibolite sidc

mylonite) on both sides of the Woodroffe Thrust mylonite zone into Seven stages, on the basis of the microstructural development of quartz (Table 1). They also made a qualitative estimate of strain in the mylonite zone, based on the degree of elongation in deformed quark (Fig. 2). This figure shows a marked variation in strain, and probably strain rate (discussed later) with distance away from the stage 7 slaty mylonite on either side. They attributed this to the effects of different H,O contents on either side of the mylonite zone (Fig. 2).

OPTICAL MICROSCOPY

The following descriptions apply to plagioclase and K-feldspar, except where otherwise noted. Though we worked with three orthogonal sections, we chose to show mainly N-sections (sections perpendicular to the stretching lineation on the mylonitic foliation). The effects of progressive shearing on the microstructure are still obvious in this section, even though the direction of shearing is perpendicular to the slide, because the partitioning occurs in three dimensions. However, the apparent strain is significantly lower, as the stretching lineation is generally intensely developed. Hence, this section allows better

I\ / \

observation of deformation and recrystallization microstructures, as well as the effects of deformation partitioning.

HP %

- 1.2

Deformation microstructures

The initial effects of mylonitization on feldspars in stage 1 (Table 1) are strain shadowing, deformation twinning (in plagioclase) and development of narrow zones of undulose extinction (Fig. 3), most of which have a kink-like or kink-band-like geometry when rotated under crossed polars at high magmlication; (these are referred to as kinks or kink bands in the remainder of the paper). These effects inensify up to stage 4 (Fig. 4), where all the quartz

becomes recrystallized (Table 1) and spectacular examples of plastically deformed feldspar can be seen (Fig. 5) . However, from stage 4 to stage 7, the number of kink bands preserved in remnants of feldspar host grains decreases considerably (Fig. 6). The strength of this trend varies from one side to the other in the Woodroffe Thrust mylonite zone, and possible reasons for this will be discussed later.

Host size reduction

In country rock that is only slightly affected by mylonitization (stage l), large lenticular feldspar grains tend to be aligned in the schistosity. With increasing

m. 3. Narrow zone of progeSaive shear cutting K-feldspar. (a) Amphibolite facies country rock. The zone is oriented E-W and is defined by a narrow region of u n d h extinction, which has a kink band like geometry when rotated under crossed polan. along which subgrains and new grains haw formed. (b) Granulite facies country rock. The zones are oriented E-W and are defined by narrow regions of undulose extiactioll. "he kft boundary of the central grain of feldspar is displaced sinistrally by these zones. (a) Section parallel to the stretching lineation and perpendicular to foliation; crossed polars; base 0.5 mm. (b) Section perpendicular to stretching lineation; crossed polars; base 0.875 mm.

154 T. H. BELL & S . E. JOHNSON

pig. 4. Anastomosing zones of shearing in A4 (Table 1) plagioclase (a) and G4 (Table 1) K-feldspar (b) at a more advanced stage of mylonitization (stage 4), showing kink and deformation bands, along which subgrains and new grains have formed. (a) Section parallel to stretching lineation and perpendicular to foliation; crossed polars; base 1.59 mm. (b) Section uemndicular to stretching lineation; crossed polars; base 0.5 mm.

Fq. 5. Folded kink bands in orthoclase (G4). Subgrains and new grains have formed along the kink bands and other zones of undulose extinction. Section perpendicular to stretching lineation. Crossed polars. Base 0.645 mm.

degree of rnylonitization, feldspar remnants become elliptical and sub-parallel to the mylonitic schistosity (e.g. Fig. 6); they decrease in length from around 900pm to 600 pm on the amphibolite facies side, and around 500 pm to 200pm on the granulite facies side by stage 4. With further mylonitization, they continue to decrease in size, though at a much slower rate on the granulite facies side, until stage 6, where they are about 200pm on the amphibolite facies side, and about 150 pm on the granulite facies side. Host grain-sizes then converge to stage 7, where they are between 125pm and 150pm long in sections perpendicular to the stretching lineation.

The number of host remnants also decreases, though the average length-to-width ratio remains constant at about 1.5 up to stage 7, where relatively few host remnants remain (Fig. 6b). This is in contrast to quartz, which is completely recrystallized by stage 4 (Table 1; Bell & Etheridge, 1976).

Fig. 6. Some remains of the original country-rock feldspar grains at stages G5 (a) and GA7 (b) as porphyroclasts in a recrystallized matrix of smaller grains. By stage GA7, most of the original country-rock feldspar grains have been converted to an aggregate of small recrystallized grains. (a) Section parallel to the stretching lineation and perpendicular to the foliation; plane-polarized light; base 6.4 mrn. (b) Section perpendicular to the stretching lineation; crossed polars; base 2 mm.

DEFORMATION, WOODROFFE THRUST ZONE 155 ~

Fig. 7. (a) A kink or deformation band in plagiodase on the amphibolite facies side, oriented to show subgrains and oew @IIS (Al). Note the bulge of the lower kink band boundary (which shows up best at this orientation) around the subgrains and new grains, particularly along the bottom edge of the kink band. (b) A kink band in plagioclase on the granulite facics side (G3). Note the bulge of the kink band boundaries around subgrains and new gains, and the isolation of portions of the kink band as islands, due to migration of the kink band boundary entirely across the kink band. Note also the difference in size of subgrains and new grains with respcct to the amphibolite facies side shown in Fig. 7a. (a) Section parallel to the stretching lineation and perpendicular to foliation; crossed polars; base 0.5 mm. (b) Seaion perpendicular to the stretching lineation; crossed polam; base 0.055 rnm.

pinned on either side of the bulge by subgrains, is common in these areas. Recovery and recrystallization microstructures

The first signs of recovery and recrystallization occur in feldspar porphyroclasts that are most deformed in stage 1 on the amphibolite side (Al; Table l), and stage 3 on the granulite side (G3; Table 1). Very small subgrains (3 pm in A1 and 1.5pm in G3) and new grains (5pm in A1 and 2 pm in G3) occur first along kink band boundaries, which mmmonly bulge around them (Fig. 7).

In the more mylonitized portions of stages A1 and G3, the host grains develop a thin rim, which shows strong birefringence variation. Small subgrains (5-7pm in A1 and 2.5-3.5 pm in G3) occur within these rims, and merge with inithately associated new grains of similar size (Fig. 8). Grain boundary bulging between adjoining feldspar grains of similar composition, where the boundary is

With increasing mylonitization, the average subgrain size on the amphibolite facie side decreases to 2-3 pm by stage 5, where it remains through to stage 7. However, on the granulite facies side the subgrain size decreases slightly within stage 3 to 2-3 pm, and remains at that size through to stage 7. New grains directly adjacent to host grains follow this trend. However, the size of new grains increases rapidly away from the host grain remains on the amphibolite facies side, leading to average new grain-sizes of XI pm in stage 3, and 50 pm in stage 7. On the granulite facies side, the size increase is much slower up to stage 5, where the average new grain-size is 6pm. The average new grain-size then increases sharply to 30pm and 50pm at stages 6 and 7, respectively.

TRANSMISSION ELECTRON MICROSCOPY Figure 9 contains dark- and bright-field transmission electron micrographs showing the typical relationship between hosts, subgrains and new grains. The figures show a clear transition from high dislocation densities in the host grain (H), to progressive polygonization or formation of subgrains (S), to relatively dislocation-free new grains (N), to new grains containing dislocations (D). The appearance of dislocations, combined with optically observed strain shadowing and deformation twinning in new grains, as well as new-grain elongation parallel to the mylonitic schistosity, strongly suggest dynamic recrystallization.

New grains are similar in size to adjacent subgrains, and their dimensions correspond to those of new grains observed optically at this stage of mylonitization (stage 4). Subgrain boundaries are marked by dislocation networks OT chaotic tangles of dislocations. New-grain boundaria are sharp and clear of inclusions.

Fig. 8. Grain boundary between two plagioclase grains at stage A1 on the amphibolite facies side. Note the considerable degree of bulge of !kction perpendicular to stretching lineation; crossed polam; base 0.055 mm.

grain boundary around subgrains and new grains.

1s T. H. BELL 81 S. E. JOHNSON

Fig. 9. Transmission electron micrographs of a host-- to new-gram transition in orthoclase on the granulite facies side (G3.1) in (a) dark-field and (b) bright-field. Note the tangled mass of dislocations in the host grain (H, left side); the transition into dislocation arrays on subgrain walls and cleanog of dislocations from the subgrain antres (let3 of centre, e.g. S); the formation of new grains of the same sue with sharp boundaries (nght of centre, e.g N); and the formation of a few dislocations in oew grains on the right side (e.g. D). Basc of both photos 0.01 mm.

INTERPRETATION A N D DISCUSSION

Deformation partitioning and mylonitization Figure 10 is a sequence of diagrams representing the progressive development of a mylonite from an essentially undeformed rock. This diagrammatic sequence was generated by applying the strain field diagram of Fig. l a of Bell et al. (1986), to produce a mylonite by highly noncoaxial, progressive, bulk, inhomogeneous shortening.

At the earliest stages of mylonitization, the country rock quartz and feldspar grains are not recrystallized, and commonly meet at 120" triple-point grain-boundary contacts (Fig. 10a). In Fig. 10b (cf. Fig. 3), shearing strain has partitioned along grain boundaries and right through a number of grains forming kinks, and recrystallization has

begun along these zones. In Fig. lOc, the partitioning of the deformation shifts, further kinking some of the host grains, and thus leading to quicker reduction of the sue of these remnants relative to those with fewer kinks. This early partitioning stage is critical in determining

those host grains that will sumve to the later stages of mylonitization. If a host grain is sufficiently split up by kinks at this early stage, it will totally recrystallize by the later stages of mylonitization, leaving behind an aggregate of new grains in its place. Those host grains that receive relatively few kinks are likely to remain large and intact right through to stage 7, because once they are surrounded by an aggregate of new grains they will tend not to deform. Instead, the progressive shearing component of the deformation will partition around them in a similar manner

Flg. 10. !kquencc of equal-area sketches showing the progressive effects on country rock of deformation partitioning generated by mylonitization, and its transition into mylonitc. Dotted grains are original country-rock feldspar grains or their remains. Black grains arc quartz or phflasilicate and dear patches are rtcry~talliztd feldspar. (a) Country rock. (b) Anastornosing zones of shearing along Which r c c q m l h ' tion has begun. (c) and (d) show progressive development of mylonitization and reduction in the sizc of feldspar host grains. (e) and ( f ) show the increasing dominance of the matrix and its rok in accommodating deformation partitioning, such that remains of feldspar host grains arc no longer effectively reduad in sizc or number.

to porphyroblasts (Bell et ul., 1986), leaving them unstrained.

In Fig. 1Od (cf. Fig. 4), the quartz is completely recrystallized and begins to stretch out into elongate stringers. This continues to'Fig. 1Of (cf. Figs 6 and ll), which represents stage 6 of mylonitization. At this stage quartz and feldspar grain aggregates are drawn out into long stringers, which anastomose around remaining feldspar host grains. The homogenization of the matrix that occurs at stage 6 (Table 1) is not shown here.

lack of dissolution and volume loss

Lack of a siBnificant change in bulk chemistry across the mylonite zone (Table 2 and fig. 12) suggests that there has

been little or no volume loss, and therefore little or no dissolution and solution transfer resulting from the effects of deformation partitioning during mylonitization. We consider this to be a result of wry high struin rates associated with mylonitization, as explained below.

Very large variations in finite strain, illustrated by change in the dimensions of quartz grain shapes (Bell & Etheridge, 1976). occur across the Woodroffe Thrust mylonite zone and within individual outcrops (Bell, 1978). Since the mylonites have formed along the Woodroffe Thrust, it is likely that the strain rate accompany mylonitization was also very high. High strain rates would have led to dominantly plastic strain and a corresponding lack of solution transfer. Bell (1981) and Bell et al. (1986) have argued that dissolution y d solution transfer are

Q. 11. Well foliated mylonite at stage 5 on the amphibolite facies side (A5) containing a few feldspar porphyroclasts and remains of quartz grain aggregates. The feldspar porphyroclasts, or rather, remains of original country-rock feldspars. are relatively unstrained. Section perpendicular to the stretching lineation; crossed polars; base 1.25 mm.

controlled by partitioning of deformation into zones of shearing and shortening, which results in strain and strain rate gradients across the boundaries between these zones. They argued that these gradients lead to chemical potential gradients that result in dissolution. However, strain rates during mylonitization are such that, although chemical potential gradients are established, dissolution cannot accommodate the strain imposed before more dislocations are generated. Consequently, the free energy increases, leading to recovery and recrystallization. This generates the characteristic host/new grain microstructures associated with mylonitization.

At lower strain rates and lower temperatures, mylonitic foliations form with a component of solution transfer, and hence volume loss. This results in a type of mylonite transitional between the mylonite described here, and non-mylonitic schists, in which solution transfer tends to be the dominant deformation mechanism (Bell et af., 1986).

Defonn;ltion/recrystlluation in feldspar4 brief review

Relationships between host grains, subgrains (where present), new grains, and kink and grain boundaries can be used to determine probable deformation and recrystallization mechanisms. Zeuch (1982), and Tullis & Yund (1985), have suggested that there are two types of dislocation creep, depending on the accommodation mechanism acting to prevent work hardening, namely dynamic recovery and dynamic recrystallization. In recovery-accommodated dislocation creep, disloca-

tion climb and cross-slip allows continued dislocation glide, and there are no pile-ups of pinned and tangled dislocations. Dislocations eventually arrange themselves into lowenergy walls forming subgrains and, as dislocations continue to glide from subgrains to the subgrain

DEFORMATION, WOODROFFE THRUST ZONE 153

Frequency

Q. l2. Cluster analysis of the chemical data from Table 2, showing that there is no distinctive difference in the chemistry of the rocks from either side of the mylonite zone, apart from water content and Rb/Sr ratio. However, the pseudotachylite is distinctly different in composition, presumably because of its melt origin.

walls, the wall angles increase, the subgrains rotate and a new grain is formed. In recrystallization-accommodated dislocation creep,

climb is difficult and the gliding of dislocations is halted by pinning and tangling. Dislocation gradients across grain and kink-band boundaries cause these boundaries to migrate into regions of higher dislocation density, and the boundaries commonly bulge between two points where they are pinned by subgrain walls and/or areas of low dislocation density, eventually pinching off new grains.

Which of the two above mechanisms occurs in rocks, or which one dominates when occurring together, appears to depend on a number of physical and chemical variables. The best documented variable is temperature (Lorimer, Champness & S p n e r , 1972; Vernon, 1975; White, 1975; Marshall & Wilson, 1976; Allison, et al., 1979; Borges & White, 1980; Brown et al., 1980; Vidal et al., 1980; Wilson, 1980; Hanmer, 1982; FitzGerald, Etheridge & Vernon, 1983; Watts & Williams, 1983; Olsen & Kohlstedt, 1985; White and Mawer, 1986). Evidence from the above workers suggests that recovery-accommodated dislocation creep becomes more active at higher temperatures.

Defomtion/recrystllization in the Woodroffe Thrust mylonite zone

Most feldspar in the Woodroffe Thrust mylonite zone appears to have undergone a combination of recovery-accommodated dislocation creep and recrystallization-accommodated dislocation creep, with recovery-accommodated

h Fig. 13. Kink bands, crossing K-feldspar (G4), that have undergone various degrees of subgrain formation and recrystallization. The bulging kink band boundaries on the upper kink bands disappear into a mass of new grains on the right. Section perpendicular to stretching lineation; crossed polars; base 0.5mm. ’

dislocation creep perhaps being the dominant mechanism. Subgrains appear on the edge of host grains adjacent to nearly all new grains, which is consistent with recovery- accommodated dislocation creep (e.g. Figs 7 and 8). In many places, however, a grain or kink band boundary bulge mechanism is active, even though subgrains are present (Figs 7 and 8). We think that the above mechanisms do not necessarily act separately, but that together they form a single mechanism, which we call ‘bulge-assisted rotation-recrystallization ’ ; this involves both migration and bulge of high-angle boundaries, and rotation of subgrains and material within the kink boundary bulges to form new grains.

Recrystallization of feldspar occurs, for the most part, along kinks and grain boundaries. Recrystallization along grain boundaries occurs at: (a) boundaries between feldspar grains with the same composition; (b) boundaries between feldspar grains with different composition, and (c) boundaries between feldspar grains and other grains, such as quartz or mica.

Where deformation partitioning results in a zone of shear strain cutting through a grain, the lattice is strained, commonly forming a kink band that can vary in width (Figs 3a, 4, 5, 7, 10, 13 and 14). Figure 14 shows a sequence of stages of recrystallization along a kink band, which originally has a width of around three subgrains. In the early stages, recovery-accommodated dislocation creep is active and dislocations within the kink band boundaries, where the strain is highest, begin to assemble into lowenergy subgrain walls. This massing of dislocations into increasingly localized zones causes dislocation-density gradients across the kink-band boundaries, which begin to migrate (bulge) into areas of high dislocation density, as shown in Fig. 14a (cf. Figs 7b and 13).

The bulging of the kink band boundaries constitutes a new heterogeneity in the grain that af€ects deformation partitioning and localizes strain around, and to a degree through, the bulge. This generates dislocations within and around the bulge that migrate to its rim and neck, causing

160 T. H. BELL & S. E. JOHNSON

(d)

a subgrain to form within (if one was not there already). Dislocations also migrate to the walls of adjacent subgrains in the kink band (and outside the kink band if present), causing them to rotate. With further deformation, this process continues and the subgrains rotate to form new grains within and adjacent to the bulge (Fig. 14). Eventually the kink band disappears in a mass of new grains (Fig. 14d).

Continuing deformation tends to affect the new grains and their immediate boundary with the host grain, rather than the core of host grain itself, because of the effects of grain-size on deformation partitioning (e.g. biotite porphyroblasts discussed in Bell et al., 1986). This

Fig. 14. !kquencc of diagrams showing how recrystallization occucs in a kink band. (a) A kink band (heavy tines) in feldspar that has formed by localized progressive shearing strain. Subgrains have formed within the kink band (light dashed lines). The kink band boundary has locally bulged into regions of high dislocation density within the adjacent kink band, such as subgrain walls. Thesc bulges have in turn been affected by continuing deformation generating dislocations within them. These dislocations have migrated towards the unstrained grain outside the kink band, forming subgrain walls. (b) Effects of further deformation, recovery and rccrystallization. Progressive shearing strain has now also affected the feldspar grain outside the kink band, leading to subgrain formation and bulge of the kink band bouadary in both directions. This has in turn localized strain within the bulged portions, leading to new grain formation (tight solid tines) through the migration of dislocations to subgrain walls and kink band boundaries. and consequent subgrain rotation. (c) and (d) show the progressive conversion of the remains of the kink band (still shown by heavy tines) and its immediate surrounding to a mass of new grains.

generates subgrains through dislocation migration into polygonal arrays on the rim of the host grain, which eventually become new grains if the deformation continues.

Bulge-assisted rotation-recrystallization also occurs along individual kinks, and along the boundaries between feldspar gmins (Figs 7 and 8). In an individual kink (cf. Fig. 15), after recrystallization has progressed sufficiently, the original boundary effectively splits into two boundaries and the process becomes almost identical to that in Fig. 14.

For a boundary between a feldspar grain and a grain of a different composition, such as quartz or mica, the initial stages are different. Because a grain boundary between

DEFORMATION, WOODROFFE THRUST ZONE 16l

I

Ftg. 15. !kquence of diagrams showing how recrystallization occurs on a grain boundary between two feldspar grains with the same composition or in an individual kink. (a) A grain boundary or kink.(heavy line) between two feldspar grains of similar composition, along which progressive shearing has localized generating dislocations. The grain boundary has bulged where pinned by dislocations into the grain with the highest dislocation density, or simply into high dislocation density areas such as subgrain walls (light dashed lines), witbout any pinning being involved. (b) Continued localization of progressive shearing along the grain boundary has generated new grains (light solid lines) through the migration of dislocations to subgrain walls and grain boundaries, and thus subgrain rotation. More subgrains also form and (c) and (d) show the progressive conversion of the original grain boundary into a mass of new grains. The heavy lines mark the boundary of the host grains with new grains forming on their rims, with previously recrystallized grains.

c.

. C

feldspar and, say, quartz cannot simply migrate into one or the other of the minerals, new grains must initially form along the feldspar host grain rims by recovery- accommodated dislocation creep only (Fig. 16). Once an outer mantle of new feldspar grains has formed against the adjacent quartz grain, the boundaries between these new grains and the feldspar host grain are free to migrate as discussed above.

Comparisons and contrasts between the two sides

In the Woodroffe Thrust mylonite zone, the H20 content is different on either side of the zone (Fig. 2), and we think

that this difference adequately explains the following contrasts between the two sides:

(1) the greatly increased degree of recrystallization relative to strain on the amphibolite facies side;

(2) the smaller subgrains and adjacent new grains through to stage 4 on the granulite facies side, followed by the convergence of subgrain and adjacent new-grain sizes on either side through to stage 7;

(3) the difference in strain, and probably strain-rate, versus distance from the slaty mylonite on either side (Fig. 2);

(4) a greater number of kinks in the feldspar on the granulite facies side through to stage 4, followed by a

162 T. H. B E L L & S. E . JOHNSON

Q. 16. A K-feldspar host grain (stage A3) shown at different orientations, (a) and (b), relative to the polars, to enhance the difference between subgrains and new grains adjacent to a quartz grain. The subgrains and new grains both show microcline twinning. The lowermost obvious new grain in the feldspar appears to have bulged into the adjacent quartz grain. However, what appears to be quartz below is a subgrain of feldspar; crossed polars; base 0.11 mm.

marked reduction in the number of kinks on both sides through to stage 7;

(5) the much greater rate of host grain-size reduction on the granulite facies side through to stage 4, followed by host grain-size convergence on either side through to stage 7, and

(6) the much smaller average new grain-size on the granulite facies side through to stage 5 , followed by a sharp increase in size, leading to average new grain-size convergence on either side through to stage 7.

As pointed out by Bell & Etheridge (1976), because of the simple structural pattern and small extent of the area studied, the above contrasts are unlikely to be caused by differences in external variables, such as pressure and temperature during mylonitization. Therefore, they are probably caused by differences in mechanical and/or chemical properties on either side. Despite original metamorphic grade differences, the mylonitic rocks have similar mineral assemblages, and the only bulk chemical differences are in H,O content and Rb/Sr ratios (Table 2, Figs 2 and 12).

The analysed H,O in these rocks is almost all tied up in hydrous minerals (Bell & Etheridge, 1976; Bell, 1979), and its availability to the deforming feldspar may be questioned. However, the redistribution of hydrous minerals during mylonitization necessitates diffusion of (OH)-bearing species on at least the grain-size scale. This may be sufficient to promote hydrolytic weakening (e.g. Tullis & Yund, 1980; Yund & Tullis, 1980). H20 has an important influence on quartz deformation and recrystallization (Griggs, 1%7; Green, Griggs & Christie, 1970; Tullis, Christie & Griggs, 1973), as well as in biotite deformation and recrystallization (Bell, 1979), and promotes hydrolytic weakening. Also, Griggs (1974), and Hobbs, McLaren & Paterson (1972) have shown that HzO

results in an increase in dislocation mobility in quartz. The same results hold for plagioclase (Tullis & Yund, 1980). On the amphibolite facies side, we interpret increased

H,O content as causing hydrolytic weakening and hence increased dislocation mobility. This allows a faster rate of recovery and subgrain formation in quartz and feldspar, causing recrystallization at lower strains (point 1 above). This is supported by the larger sue of the subgrains and new grains on the amphibolite facies side. The lower amount of strain on this side before recrystallization, and a potentially faster rate of recovery, means that fewer dislocations need be accommodated on low-angle grain boundaries, resulting in the formation of larger subgrains and hence new grains. On the granulite facies side, significantly less H 2 0 is present (Fig. 2), causing less hydrolytic weakening. Much higher strains are needed to produce the same degree of recrystallization as that observed in corresponding stages on the amphibolite facies side. This suggests that dislocation mobility and thus the rate of recovery are reduced. Consequently, more dislocations are developed before subgrains can form, resulting in smaller subgrains and hence new grains (point 2 above).

Because hydrolytic weakening made rocks on the amphibolite facies side easier to deform than those on the granulite facies side, the deformation partitioned differently across the Woodroffe Thrust. Figure 2 shows the difference in strain, and p-obably strain rate, versus distance from the slaty mylonite on either side (point 3 above). The rocks on the amphibolite facies side have been affected by mylonitization over a distance of more than 5 km (Bell & Etheridge, 1976) from the stage 7 slaty mylonite (Fig. 2), compared with 0.25 km on the granulite facies side. However, the largest strains, and probably strain rates, are confined to a narrow zone adjacent to the slaty mylonite, although a local zone of high strain exists to the east (Fig. lb) where intensely developed mylonite (subarea 2) anastomoses around a large pod of country rock (subarea 3).

The greater rate of reduction in host grain-size on the granulite facies side in stages 1-4 is directly related to the greater number of kinks in these host grains (points 4 & 5 above). As shown in Fig. 10, partitioning of shearing strain along grain boundaries, and through host grains forming kinks, generates dislocations and initiates recovery and recrystallization in these areas, which leads to the separation and reduction in the size of host grain remains (Fig. 4). If recrystallization occurs too slowly to accommodate the deformation, strain energy builds up and strain hardening may occur. This causes the pattern of deformation partitioning to shift, which redistributes the shearing strain, and more kinks are generated in feldspar grains that have not hardened.

This process continues until there is enough recrystallized material in the matrix to allow partitioning of progressive shearing entirely around those feldspar host grains that still remain, at which point no more kinks are generated in feldspar hosts. However, if recrystallization occurs at lower strains, the point at which no more kinks


are generated is reached sooner, less kinks are formed, and host grain-& reduction rates are slower. Because of the lower H,O content on'the granulite facies side (Fig. 2), less hydrolytic weakening occurs, and dislocation mobility and recovery rates are reduced, which results in less recrystallization.

Thus, more of the deformation is taken up in the form of kinks leading to a greater rate of host grain-size reduction. On the granulite facies side, the lack of hydrolytic weakening means that the feldspar grains are harder to deform, and hence the deformation partitions into narrow zones of intense shearing strain with large zones of essentially unstrained feldspar in between. That is, more kinks will form in order to accommodate the total strain.

On the amphibolite facies side, the higher H20 content causes fewer kinks in feldspar host grains, and a lower rate of host grain-size reduction. Additionally, the low strains and probably strain rates in stages 1-4 lead to even fewer kinks, further lowering the rate of host grain-size reduction. By stage 5 on the granulite facies side, recrystallization is sufficient to allow further deformation to be accommodated by straining of surrounding grains, and movement on grain boundaries, rather than by further kinking.

The remaining kinks recrystallize, reducing in number, and host grain reduction rates decrease markedly. By stage 5 on the amphibolite facies side, although recrystallization is extensive, the relatively greater size and number of remaining host grains, combined with the now increasing strain rate, allows the rate of host grain-sue reduction to remain fairly constant, such that the host grain-size on either side converges through to stage 7.

On the granulite facies side., the average new grain-size increases from 6 p m at stage 5 to 30pm and 50pm at stages 6 and 7 respectively, without any signdicant difference in H,O content between stages 1-5 and stage 6 (point 6 above). At stage 6, monominerallic aggregates are being destroyed due to the nucleation and growth of other minerals within them (Fig. 6b) on both sides of the mylonite zone. From the work of Smith (1952), De Vore (1956 & 1959), Vernon (1968), White (1968) and Flinn (1%9), it appears that lower interfacial energy for unlike phases favours the separation of grains of the same phase, rather than their aggregation. "his indicates that, at stage 6, diffusion is considerable. 'Ihe increased contribution of &ion, indicated by the disaggregation and homogenization in stage 6 through crystallization of new pins in the matrix, appears to be due to the number of high-angle grain boundaries associated with the new grains of quartz, biotite and feldspar developed through recrystallirntion of the original country rock grains (e.g. Bell & Etheridge, 1976; Bell, 1979). As a mult of, but concurrent with, this homogenization

of the matrix, the deformation partitions on a h e r and h e r scale until the matrix is accommodating strain nearly homogeneously on the grain scale. This is aided by the ability of phyllosilicates to accommodate progressive shearing strain (e.g. Bell et ul., 1986) such that the quartz

and feldspar grains simply slide past one another on phyllosilicates, apparently with a significant component of grain-boundary sliding, as the preferred orientation of quartz c-axes decreases from one stage 5 to stage 7 [Fig. 14 (GS, G6, GA7 and AS, A6, GA7) in Bell & Etberidge, 19761. The anastomosing character of the matrix foliation greatly decreases as a reflection of this fine-scale distribution of the deformation partitioning. The result is that each grain in the matrix undergoes less strain than when the deformation partitioning was coarser, because more porphyroclasts and porphyroclast new-grain aggregates are preserved. Hence, grain growth can compete more readily with plastic deformation.

The remains of feldspar porphyroclasts accommodate none of the strain, and hence no new subgrains form on their margins. Most of those subgrains generated at earlier stages in the rnylonitization have been converted into new grains. Some may still be partially preserved on the porphyroclast rim, where they failed to be consumed by the coarsening of the new grains.

Variations in the degree of K-feldspar tidinicity

On the margins of the mylonite zone, on the granulite facies side, K-feldspar grains show no microcline twinning. Towards the interior of the mylonite zone, K-feldspar porphyroclasts occasionally show a 'tweed' microstructure, which becomes more common with increasing distance from the margins of the zone. McLaren & FikGerald (1987) suggested that the tweed microstructure develops in response to an average degree of Al-order intermediate between that of sanidme and low microcline. At stage 6, microcline twinning appears and becomes more abundant as stage 7 is approached, where microcline-twinned grains predominate. On the amphibolite facies side, microcline twinning is seen in all stages, but becomes more abundant relative to porphyroclasts that are clear or show tweed microstructure as mylonitization progresses.

On the basis of the above trends, we suspected that K-feldspar triclinicity increased with increasing mylonitization; this has been recognized by other workers (e.g. Eggleton, 1979; Eggleton & Buseck, 1980). XRD traces were generated for K-feldspar at all stages across the mylonite zone to determine the degree of triclinicity by the method of Goldsmith and Laves (1954; Fig. 17). The degree of triclinicity shows an overall increase through to stage 6. At stage 7, which is on the boundary between the amphibolite and granulite facies sides, there is a scattered distribution of triclinicity values, the average lying between stage 6 triclinicity values on either side (Fig. 17). A similar trend bas been observed approaching the Main Central Thrust in the Himalaya Mountains (C. Cuff, personal

From the above relationships, it appears that the trend from clear K-feldspar, to tweed microstructure, to microcline twinning may be a-direct result of increasing K-feldspar triclinicity, as suggested by other workers (e.g. Tiing, 1968; Smith, 1974, pp. 381-388; McLaren & FikGerald, 1987). As well, it appears that a critical value

communication, 1988).

164 T. H. B E L L & S. E. JOHNSON

1.0 T

0.0 1 2 3 4 5 6 7

Microstructural Stages

Fig. 17. Graph showing the relationship between K-fel@par tricliniaty and the microstructural stage of development. Note the overall increase in triclinity with progressive mylonitization through to stage 6 on either side. At stage 7, which lies at the boundary between the amphibolite and granulite facies side, the average tridinicity value lies between the stage 6 values for either side. Less than 0.1 is generally considered as orthoclase, and greater than 0.9 is generally considered maximum microcline. Between 0.1 and 0.9 is generally called intermediate microcline. Solid line - amphibolite facies side; dashed Line - granulite facies side.

of triclinicity must be reached before true microcline twinning appears. This critical value lies between 0.37 (the lowest triclinicity calculated on the amphibolite facies side)

and 0.55 (first appearance of microcline twinning on the granulite facies side).

In several of the stages on the granulite facies side, a sharp orthoclase (131) peak occurs between split peaks representing the (131) and (131) microcline peaks. This suggests that orthoclase and microcline, with varying degrees of triclinicity, coexist at these stages (e.g. Steiger & Hart, 1%7; Wones, Tatlock & von Limbach, 1%7; Wright 1967; Tilling, 1968; Eggleton & Buseck, 1980; Bambauer & Bernotat, 1982). As well, in several stages on both the granulite and amphibolite facies sides, two sets of (131) and (131) microcline peaks can be seen, one giving a much lower triclinicity value than the other. This suggests that microcline grains with varying degrees of triclinicity coexist at these stages (e.g. Steiger & Hart, 1967; Tilling, 1968; Eggleton & Buseck, 1980).

For convenience in the following discussion, we will refer to microcline with or without tweed microstructure, but free of microcline twinning, as ‘untwinned microcline’, and we will refer to microcline with microcline twinning as ‘twinned microcline’. In stages 1-6 on the amphibolite facies side, and in stage 6 on the granulite facies side, K-feldspar hosts locally have cores of untwinned microcline surrounded by twinned microcline with twinned microcline new grains (Fig. 18a). More rapid Si/AI interchange rates have been suggested with increased strain and H 2 0 content, as well as when mobile dislocations are active (e.g. Martin, 1974; Eggleton & Buseck, 1980; Yund & Tullis, 1980). Because H,O access, strain energy and dislocation mobility are greater on the edges of host grains than in their interiors, it seems likely that the degree of triclinicity would increase from the interior to the rim of host grains.

Most host grains in the mylonite zone would probably have similar triclinicity variations from the core to the rim, but only those with rims that are triclinic enough to show microcline twinning would be obvious. There could even

ph. 18. (a) K-feldspar host grain remnant (A3) with an untwinned core surrounded by a microcline-twinned rim. Both the subgrains, and most of the new grains, surrounding the host remnant show microcline twinning. (b) Subgrains and new grains of a similar size that have formed in microcline-twinned K-feldspar. Both the subgains, and most of the new grains, show microcline twinning; crossed p l a n ; base of (a) 1.66 mm, base of (b) 0.21 mm.

T* 3.

Com

para

tive c

ompo

sitio

ns of

hos

t and

new

pla

gida

se grains a

t the

various

stag

es of

myl

onih

tion

.

Hos

t Si

O,

TiO

, Fe

O

MIlO

g

o

GO

N

a,O

K2O

cr203

NiO

Ba

O

Total

or

sa,$i

e N

CW

~~

s

f s

f s

f s

f s

f s

f s

f s

f s

f s

f s

f s

f 5

1G

Hos

t 3

60.6

6 1.

58

0.03

0.

06

24.5

0 0.

44

0.05

0.

06

0.04

0.

04

0.03

0.

03

6.61

0.

03

7.64

0.

03

0.30

0.

10

0.05

0.

06

0.01

0.

01

0.03

0.

04

99.9

5 1.

39

2.1G

H

ost

3 59

.62

0.33

0.

03

0.03

24

.81

0.27

0.

07

0.01

0.

01

0.02

0.

01

7.30

0.

23

7.22

0.

27

0.29

0.

03

0.04

0.

04

0.06

0.

04

0.03

0.

09

99.5

4 0.

43

2.2G

H

ost

3 59

.36

0.33

24

.87

0.21

0.

16

0.13

0.

02

0.02

0.

02

0.02

7.

41

0.17

7.

19

0.12

0.

26 0.u)

0.01

0.

01

0.06

0.

06

99.3

6 0.

24

2.2G

N

ew

4 59

.68

0.90

0.

02

0.02

24

.84

0.10

0.

18

0.20

0.

02

0.02

0.

02

0.02

7.

23

0.31

7.

14

0.31

0.

52

0.36

0.

02

0.02

0.

04

0.03

0.

05

0.07

99

.75

0.57

3.

1G

Hos

t 7

59.3

2 0.

65

0.02

0.

03

24.7

0 0.

50

0.16

0.

90

0.01

0.

02

0.01

0.

01

6.56

0.

31

8.03

0.

28

0.14

0.

06

0.05

0.

03

0.01

0.

02

0.03

0.

04

99.0

5 0.

53

3.1G

N

ew

4 58

.71

0.94

0.

04

0.04

25

.52

1.09

0.

25

0.11

0.

03

0.03

6.

13

0.65

7.

83

0.36

0.

11

0.04

0.

04

0.04

0.

04

0.03

0.

01

0.02

98

.70

0.62

3.

2G

Hos

t 4

59.3

5 0.

29

0.02

0.

03

24.5

3 0.

28

0.10

0.

07

0.02

0.

01

6.75

0.

17

7.65

0.

09

0.14

0.

02

0.07

0.

05

0.04

0.

02

0.05

0.

03

98.7

0 0.

52

3.2G

N

ew

2 59

.91

1.14

0.

10

0.01

24

.35

1.10

0.

25

0.07

0.

04

0.04

5.

85

0.70

7.

60

0.04

0.

19

0.11

0.

12

0.01

0.

03

0.03

0.

01

0.01

98

.51

0.51

3.

3G

Hos

t 3

60.8

4 0.

07

0.02

0.

03

25.2

9 0.

27

0.24

0.

19

0.03

0.

03

0.02

0.

01

6.68

0.

82

7.39

0.

32

0.06

0.

02

0.03

0.

03

0.04

0.

02

0.06

0.

08

100.

70

0.26

3.

3G

New

1

60.69

24.3

6 0.

46

0.13

6.

56

7.62

0.

23

0.01

0.

11

100.

17

40

H

ost

2 61

.08

0.70

24

.21

0.13

0.

24

0.02

0.

02

0.02

6.

52

0.14

7.

71

0.08

0.

14

0.05

0.

02

0.02

0.

03

0.03

0.

09

0.05

10

0.05

0.

39

4G

New

3

61.8

4 0.

82

0.01

0.

01

23.3

2 1.

14

0.29

0.

26

0.03

0.

05

0.04

0.

02

5.08

1.

04

8.49

0.

22

0.10

0.

03

0.02

0.

01

0.01

0.

02

0.05

0.

08

99.2

9 1.

13

5G

Hos

t 4

59.4

0 0.

95

0.03

0.

02

25.1

6 0.

27

0.16

0.

19

0.02

0.

02

6.%

0.

37

7.92

0.

31

0.33

0.

27

0.02

0.

03

0.01

0.

01

0.05

0.

04

100.

06

0.77

5G

N

ew

4 59

.54

1.04

25

.04

0.43

0.

15

0.10

0.

01

0.01

0.

02

0.02

6.

82

0.69

7.

84

0.57

0.

16

0.08

0.

01

0.01

0.

07

0.07

99

.66

0.94

6G

Hos

t 2

61.5

1 0.

32

0.01

0.

01

23.2

3 0.

08

0.40

0.

26

0.01

0.

01

4.36

0.

04

9.14

0.

01

0.29

0.

02

o.oi

0.01

0.

04

0.02

0.

12

0.02

99

.12

0.08

6A

Hos

t 3

66.5

2 2.

17

0.03

0.

03

20.2

2 0.

92

0.08

0.

05

0.02

0.

01

0.01

0.

01

1.71

1.

07

10.6

8 0.

72

0.11

0.

02

0.02

0.

02

99.4

0 0.

89

5A

Hos

t 2

59.8

1 0.

05

0.02

0.

02

24.2

7 0.

19

6.60

0.

30

8.63

0.

11

0.07

0.

01

0.03

0.

01

0.01

0.

02

99.4

4 0.

54

6G

New

3

62.7

8 0.

10

0.03

0.

04

22.6

3 0.

43

0.10

0.

03

0.03

0.

02

3.91

0.

39

9.64

0.

23

0.12

0.

04

0.05

0.

04

0.04

0.

03

0.08

0.

09

99.4

1 0.

38

7GA

N

ew

1 62

.44

0.07

22

.11

0.35

0.

06

3.95

9.

06

0.27

0.

03

0.09

98

.44

6A

New

4

67.9

7 0.

82

19.8

9 0.

15

0.09

0.

06

0.02

0.

02

1.00

0.

29

11.0

8 0.

17

0.13

0.

04

0.02

0.

02

0.03

0.

03

0.02

0.

03

100.

24

0.49

5A

New

4

61.1

3 1.

38

23.9

0 0.

67

0.07

0.

12

0.01

0.

01

0.01

0.

01

6.01

1.

15

8.90

0.

31

0.10

0.

06

0.01

0.

01

0.01

0.

01

0.02

0.

02

100.

17

0.63

10

0.25

0.

22

4A

Hos

t 2

67.5

4 0.

38

0.01

0.

01

20.1

0 0.

15

0.04

0.

04

0.02

0.

02

0.01

0.

01

1.56

0.

33

10.8

5 0.

42

0.08

0.

01

4A

New

2

66.0

2 0.

82

0.02

0.

01

20.6

3 0.

20

0.32

0.

17

0.02

0.

09

0.09

1.

94

0.43

10

.53

0.34

0.

15

0.06

0.

03

0.02

0.

04

0.04

99

.61

0.15

3A

H

ost

3 60

.61

0.73

0.

01

0.02

24

.06

0.24

0.

08

0.03

0.

01

6.26

0.

06

7.93

0.

05

0.14

0.

04

0.06

0.

06

0.01

0.

01

0.04

0.

03

99.2

1 0.

94

3A

New

5

62.1

3 0.

68

0.04

0.

05

22.6

3 0.

37

0.14

0.

10

0.01

0.

01

4.09

0.

27

8.86

0.

34

0.15

0.

07

0.03

0.

03

0.04

0.

05

0.03

0.

03

98.1

5 1.

05

2A

Hos

t 3

60.4

0 1.

06

0.01

0.

01

25.0

6 0.

36

0.04

0.

04

0.02

0.

02

6.61

0.

22

8.20

0.

10

0.10

0.

01

0.01

0.

01

0.01

0.

02

0.92

0.

09

100.

59

0.11

7

A

New

4

60.1

6 1.

07

0.03

0.

04

24.9

7 0.

50

0.08

0.

02

0.02

0.

03

0.02

0.

02

6.23

0.

89

8.20

0.

51

0.15

0.

14

0.06

0.

06

0.03

0.

03

0.62

0.

03

99.9

5 0.

04

0.04

0.

03

T.M

C 4

. C

ompa

rativ

e com

posit

ions

of h

ost a

nd n

ew K

-feld

spar

gra

ins

at th

e va

riou

s sta

gca

of mylonitization. O

r is u

ntw

inne

d m

icro

clin

e an

d Mi is

twin

ned

mic

rocl

ine.

Hos

t O

r.

SiO

, Ti

O*

%O

, Fe

O

MnO

Me0

C

aO

Naz

O

&O

C

rzO

, N

iO

BaO

T

otal

------

or

or

Sam

ple

Ne

wn

Mi.

f

s f

s f

s f

s f

s f

s f

s f

s i

s f

s i

s f

s I

s

1G

2.1G

2.

2G

3.1G

3.

2G

3.3G

3.

3G

4G

4G

5G

5G

6G

6G

7GA

7G

A

6A

6A

5A

5A

4A

4A

3A

3A

2A

2A

Hos

t H

ost

Hos

t H

ost

Hos

t H

ost

New

H

ost

New

H

ost

New

H

ost

New

H

ost

New

H

ost

New

H

ost

New

H

ost

New

H

ost

New

H

ost

New

40

64

.20

0.84

18

.69

0.24

0.

16

5 0

63.7

8 0.

99

0.03

0.

04

18.2

3 0.

14

0.03

3 0

63.5

0 0.

55

0.02

0.

03

17.9

6 0.

33

0.13

1

0

63.8

2 0.

07

19.2

4 0.

48

3 0

64.0

3 0.

68

0.04

0.

03

18.5

6 0.

33

0.07

3 0

66.1

0 0.

77

0.02

0.

04

18.0

9 0.

23

0.09

2 0

64.5

9 0.

64

18.8

4 1.

48

0.39

2 0

65.7

5 0.

04

0.02

0.

01

18.0

1 0.

02

0.01

1 0

66.0

3 0.

07

18.0

5 0.

03

50

+M

64.

58

0.53

0.

03

0.03

18

.41

0.11

0.

03

50

+M

63.

94

0.92

0.

02

0.02

18

.75

0.73

0.

14

3 0

63.8

7 0.

17

0.04

0.

04

18.4

2 0.

20

0.05

3 0

63.7

1 0.

72

18.4

3 0.

15

0.09

3 0

63.4

5 0.

15

0.05

0.

01

17.8

8 0.

15

0.02

1 0

63.1

5 0.

08

18.2

1 0.

10

2 0

64.2

2 0.

13

0.02

0.

02

17.8

0 0.

06

0.01

2 0

64.1

6 0.

35

17.9

4 0.

20

0.09

2

0+

M 6

4.17

0.23

18.3

7 0.

08

40

+M

63.

93

0.51

0.

03

0.03

18

.32

0.17

0.

03

3 O

+ M

64.

32

0.59

17

.82

0.08

0.

09

4 O

+M

64.

18

0.36

17.8

8 0.

17

0.11

6

0+

M 6

3.60

1.

20

0.02

0.

03

18.2

4 0.

15

0.03

3

O+

M 6

4.06

0.

97

18.1

8 0.

18

0.04

3

0

63.5

0 0.

70

0.02

0.

03

18.4

9 0.

12

0.02

2 0

63.4

6 O.%

18

.20

0.40

0.

06

0.26

0.

01

0.12

0.07

0.

05

0.39

0.

01

0.02

0.

08

0.05

0.

08

0.03

0.01

0.

02

0.01

0.

03

0.07

0.

02

0.04

0.

02

0.08

0.01

0.

02

0.04

0.

02

0.01

0.

01

0.02

0.

03

0.01

0.0

2 0.

01

0.07

0.

11

0.08

0.

01

0.03

0.

01

0.01

0.

02

0.05

0.

03

0.01

0.

01

0.02

0.

02

0.01

0.

02

0.02

0.

01

0.01

0.

01

0.01

0.

01

0.01

0.

02

0.05

0.

02

0.01

0.

02

0.02

0.

02

0.01

0.

01

0.02

0.

01

0.01

0.

01

0.01

0.04

0.0

2 0.

01

0.02

0.

01

0.08

0.02

0.

03

0.03

0.

02

0.01

0.01

0.

01

0.03

0.

02

0.02

0.01

0.

01

0.02

0.08

0.

13

1.32

0.

52

14.7

0 0.

79

0.05

0.

79

0.10

15

.19

0.14

0.

02

0.20

0.

25

0.81

0.

06

15.2

1 0.

33

0.04

0.

70

16.0

7 0.

03

0.06

0.

10

1.01

0.

14

15.1

2 0.

25

0.02

1.

58

0.69

14

.04

1.16

1.

26 0.44

14.2

6 1.

51

1.09

0.

07

15.4

1 0.

04

0.02

0.

04

1.37

14

.49

0.02

0.

74

0.17

15

.60

0.43

0.

04

0.19

0.

31

1.00

0.

38

15.1

9 0.94

0.05

0.

91

0.42

16

.17

0.80

0.

01

0.61

0.

17

16.5

3 0.

41

0.05

0.

83

0.03

15

.73

0.20

0.

05

0.49

15

.89

0.03

0.

47

0.06

15

.90

0.08

0.

83

0.40

15

.38

0.65

0.

01

1.04

0.

50

14.9

5 1.

20

0.69

0.

18

15.2

6 0.

57

0.20

0.

58

0.57

15

.42

0.22

0.

03

0.45

0.

05

15.8

9 0.

19

0.02

0.

77

0.19

15

.98

0.12

0.

01

0.66

0.

13

16.2

0 0.40

0.02

0.

83

0.29

15

.96

0.41

0.

02

0.75

0.

05

15.9

6 0.

48

0.02

0.05

0.

02

0.04

0.03

0.01

0.06

0.

03

0.01

0.

04

0.09

0.02

0.

03

0.04

0.

01

0. 02

0.

03

0.01

0.03

0.

02

0.43

0.

11

99.7

1 1.

00

0.04

0.

05

0.70

0.

06

98.8

4 0.

79

0.02

0.

03

0.71

0.

16

98.6

3 0.

24

0.04

0.

35

100.

83

0.04

0.

04 0.44

0.03

99

.40

1.33

0.

02

0.03

0.

32

0.06

10

0.33

0.

78

0.07

0.

07

0.26

0.

06

99.7

5 1.

66

0.01

0.

01

0.22

0.

01

100.

55

0.02

0.

03

0.28

10

0. 44

0.

04

0.04

0.

49

0.06

99

.99

0.72

0.

02

0.03

0.

50

0.06

99

.85

0.99

0.

02

0.03

0.46

0.11

99

.98

0.69

0.

03

0.04

0.

59

0.09

10

0.05

0.

81

0.07

0.

02

0.21

0.

11

98.3

0 0.

28

0.03

0.

01

0.37

0.

08

98.8

4 0.

04

0.02

0.

01

0.35

0.

08

98.8

2 0.

49

0.03

0.

04

0.34

0.

06

98.9

0 0.

54

0.27

0.07

98

.72

0.89

0.

01

0.01

0.

35

0.06

98

.68

0.63

0.

05

0.05

0.

35

0.12

98

.87

0.77

0.

03

0.03

0.

32

0.09

99

.01

1.16

0.

03

0.05

0.36

0.03

99

.56

0.94

0.

04

0.04

0.

23

0.14

99

.14

0.71

0.

24

0.05

98.7

1 0.

04

0.32

98

.27


be a population of host grains on the granulite facies side with cores of orthoclase, and rims of twinned or untwinned microcline, which would explain the coexisting orthoclase and microcline peaks discussed above. This coexistence of K-feldspar with varying degrees of triclinicity causes local variation in calculated triclinicity. For example, a small area within the mylonite zone that has an unusual concentration of host grain remains may lead to a calculated triclinicity below the average for that stage. As well, an area that has totally recrystallized may result in an anomalously high calculated triclinicity. Apparently anomalous trends in Fig. 17. such as the drop in triclinicity from stage 1 to stage 3 on the amphibolite facies side, may be due to such local variation in triclinicity.

We believe the reason that twinned microcline does not form from untwinned microcline on the granulite facies side until stage 6, is the low initial triclinicity relative to the amphibolite facies side (Fig. 17). This difference in initial triclinicity values is probably a function of the different deformation histories, cooling histories and H,O contents (Fig. 2) in the country rock.

Microdine new grains and recrystallhation mechanisms

The Occurrence of twinned microcline host grains with a mantle of subgrains and adjacent twinned microcline new grains has been used as evidence for progressive subgrain rotation (recovery-accommodated dislocation creep) by White & Mawer (1986). However, in the Woodroffe Thrust mylonite zone this occurs where recrystallization- accommodated dislocation creep is active.

The migration of high-angle boundaries during recrystallization-accommodated dislocation creep should remove the various lattice defects caused by deformation, but it is unclear what af€ect these migrating high- angle boundaries should have on K-feldspar ordering/ disordering. Because we see twinned microcline new grains, apparently formed by bulge-assisted rotation- recrystalliration. adjacent to twinned microcline host grains (Fig. 18), it appears that migration of high-angle boundaries does not destroy the ordering of K-feldspar and, in fact, may help facilitate an increase in triclinicity through access of H20 via diffusion along the interface.

Comparative host grain and new grain chemistry, and driving forces for recystallition

Although most published work on naturally deformed feldspar recognizes a difference in chemical composition between host grains and new grains, there is widespread disagreement as to the cause (Vernon, 1975; White, 1975; Allison et al., 1979; Borges & White, 1980; Brown et al., 1980; Vidal et ul., 1980; Hanmer, 1982; Watts & Williams, 1983; Olsen & Kohlstedt, 1985). Two main reasons have been suggested to explain these chemical differences:

(a) The Gibbs free energy is reduced when new grains form with a lower An content, and may provide an

additional dnving force for recrystallization (Vernon, 1975; White, 1975; Allison et al., 1979).

(b) Increased An content in the new grains could simply reflect recrystallization under prograde metamorphic conditions (Olsen & Kohlstedt, 1985). However, the completely opposite trends in comparative host ga in and new grain compositions in feldspars studied by Borges & White (1980) and Brown et al. (1980) suggest that the reasons are more complex than this.

Urai, Means & Lister (1986) listed four types of energy available as driving forces for dynamic recrystallization processes: (1) intragranular lattice defect energy (commonly referred to as stored strain energy); (2) grain (and kink) boundary energy; (3) chemical free energy, and (4) external load-supporting elastic strain energy. The first and last of these driving forces involve elastic distortional energy that is respectively locked into the material around defects, or maintained in the material by an imposed stress. Grain and. kink boundary energy is primarily a surface energy.

In the Woodroffe Thrust mylonite zone, no compositional differences occur between. host grains and new grains (Tables 3 and 4), and grain boundary inclusions are quite common. So, chemical contributions to free energy changes were not involved in recrystallization. As discussed earlier, recrystallization is accomplished by a bulge-assir fed rotation -recrystallization mechanism, and it appears that this mechanism is driven by elastic distortional energy and surface energy (Urai et ul., 1986).

A C K N O W L E D C E M E N T S

T. H. B. would like to acknowledge the support of the Australian Research Grants Scheme and its microprobe facitity at the University of Melbourne. S.E.J. would like to acknowledge support from an NSF Graduate Fellowship and the James Cook University. We would like to thank C. Cuff and P. Blevin for their help in obtaining and interpreting XRD data for K-feldspar in the Woodroffe Thrust mylonite zone. C. Cuff read the hal draught and we appreciated his comments. W. D. Means and R. H. Vernon made numerous critical comments, which im- proved the paper considerably.

REFERENCES

AUison, I. , Barnett, R. L. & Kemch, R., 1979. Superplastic flow and changes in crystal chemistry of feldspars. Tectonophysicr,

Bambauer, H. H. & Bernotat, W. H. , 1982. The microcline/sanidine transformation isograd in metamorphic regions: I. Composition and structural state of alkali feldspars from granitoid rocks of two N-S traverses across the Aar Massif and Gotthard “massif‘. Swiss Alps. Schweirerische Minerdog- kche wrd Petrographkdu Miniclwrgen, 62, 185-2.3).

Bell, T. H., 1978. Progressive deformation and reorientation of fold axes in a ductile mylonite zone, central Australia. Tectonophysiu, 44, 285-320.

Bell, T. H., 1979: The deformation and recrystallization of biotite in the Woodroffe Thrust mylonite zone. Tectonophysks, 58,

53,4146.

139-158.

168 T. H. BELL 81 S . E . JOHNSON

Bell, T. H., 1981. Foliation development: the contribution, geometry and significance of progressive, bulk, inhomogeneous shortening. Tectonophysics, 75,273-296.

Bell, T. H. & Etheridge, M. A,, 1976. The deformation and recrystallization of quartz in a mylonite zone, central Australia. Tectonophysics, 32, 235-267.

Bell, T. H.. Fleming, P. D. & Rubenach, M. J., 19%. Porphyroblast nucleation, growth and dissolution in regional metamorphic rocks as a function of deformation partitioning during foliaton development. Journal of Metamovhic Geology,

Borges, F. S. & White, S. H., 1980. Microstructural and chemical studies of sheared anorthosites, Renoval, South Hams. Journal of Structural Geology, 2, 273-280.

Brown, W. L., Macaudiere, J., Ohnenstetter, D. & Ohnenstetter, M., 1980. Ductile shear zones in a meta-anorthosite from Harris, Scotland: textural and compositional changes in plagioclase. Journal of Structural Geology, 2,281-287.

De Vore, G. W., 1956. Surface chemistry as a chemical control on mineral associations. Journal of Geology, 64, 31-56.

De Vore, G. W., 1959. Role of minimum interfacial free energy in determining the macroscopic features of minimal assemblages. 1. The model. Journal of Geology, 67, 211-226.

Eggleton, R. A., 1979. The otdering path for igneous K-feldspar megacrysts. American Mineralogist, 64,906-911.

Eggleton, R. A. & Buseck, P. R., 1980. The orthoclase- microcline inversion: a high-resolution transmission electron microscope study and strain analysis. Contributions to Mineralogy and Petrology, 74, 123-133.

FitzGerald, J. D., Etheridge, M. A. & Vernon, R. H., 1983. Dynamic recrystallization in a naturally deformed albite. Textures and Microsmcchcres, 5, 219-237.

Flinn, D., 1969. Grain contacts in crystalline rocks. Lithas, 3,

Goldsmith, J. R. & Laves, F., 1954. The microcline-sanidine stability relations. Geochimica et Cosmochimica Acta, 5, 1-19.

Green, H. W., Griggs, D. T. & Christie, J. M., 1970. Syntectonic and annealing recrystallization of fine-grained quartz aggregates. In: Experimental and Natural Rock Deformation (ed. Paulitsch, P.) pp. 272-335. Springer Verlag, Berlin.

Griggs, D. T., 1%7. Hydrolytic weakening of quartz and other silicates. Geophysical Journal of the Royal Astronomical

Griggs, D. T . , 1974. A model of hydrolytic weakining in quartz. J o d of Geophysical Research, 79, 1653-1661.

Hanmer, S. K., 1982. Microstructure and geochemistry of plagioclase and microcline in naturally deformed granite. Journal of Structural Geology, 4, 197-213.

Hobbs, B. E.. McLaren, A. C. & Paterson, M. S., 1972. Plasticity of single crystals of synthetic quartz. American Geophysical Union Monograph Series, 16,29-53.

Lorimer, G. W., Champness, P. E. & Spooner, E. T. C., 1972. Dislocation distributions in naturally deformed omphacite and albite. Name (Physical Sciences), 239, 108-109.

Major, R. B., Johnson, J. E., Loeson, B. & Mirams, R. C., 1967. Woodroffe 1 : 250,OOO Geological Map. Geologic Survey, South Australian Mines Department.

Marshall, D. B. & Wilson, C. J. L., 1976. Recrystallization and peristerite formation in albite. Contribufions to Mineralogy and Petrology, S7, 55-69.

Martin. R. F., 1974. Controls of ordering and subsolidus phase relations in the alkali feldspars. In: The Feldrpars, eds Mackcnzie, W. S. & Zussrnan, J., pp. 313-336, NATO advanced study Institute, Manchester University Press.

McLaren, A. C. & FitzGerald, J. D., 1987. CBED and

4, 37-67.

361-370.

S o ~ i e t y , 14, 19-31.

ALCHEMI investigations of local symmetry and AI, Si ordering in K-feldspars. Physics and Chemistry of M i n e r d , 14,281-292.

Olsen. T. S. & Kohlstedt, D. L., 1985. Natural deformation and recrystalhition of some intermediate plagioclase feldspars. Tectonophysics, 111, 107-131.

Smith. C. S., 1952. Interphase interfaces. In: Impeflechm in Nearly Perfccl Crystals (ed. Shockley, W.), pp. 377-401. Wiley. New York.

Smith, J. V., 1974. Feldspar Miner&, vol. 2., Springer-Verlag, Heidelburg.

Steiger, R. H. & Hart, S. R., 1%7. The microdine-orthlase transition within a contact aureole. American Mineralogist, 52,

Tilling, R. I., 1968. Zonal distributions of variations in structural state of alkali feldspar within the Rader Creek Pluton, Boulder Batholith, Montana. Journal of Petrology, 9,331-357.

Tullis, J., Christie, J. M. & Griggs, D. T., 1973. Microstructures and preferred orientations of experimentally deformed quartz- ites. Buletin of the Geological Society of America, 84,297-314.

Tullis, J. & Yund. R. A.. 1980. Hydrolytic weakening of experimentally deformed Westerly granite and Hale albite rock. Journal of Structural Geology, 2,439-451.

TuUis. J. & Yund, R. A., 1985. Dynamic recrystallization of feldspar: a mechanism for ductile shear zone formation. Geological Magazine, W , 238-241.

Urai, J. L., Means, W. D. & Lister, G. S., 1986. Dynamic recrystallization of minerals. American Geophysical Union Monograph Series, 36, 161-200.

Vernon, R. H., 1968. Microstructures of high-rade metaniorphic rocks at Broken Hill, Australia. Journal of Petrology, 9, 1-22.

Vernon, R. H., 1975. Deformation and recrystallization of a plagioclase grain. American Mineralogist, 60,884-888.

Vidal, J. C.. Kubin, L., Debat. P. & Soda. J. C., 1980. Deformation and dynamic recrystallization of K-feldspar augen in orthogneiss from Montagne Noire. Occitania, southern France. Lithos, 13,247-255.

Watts, M. J. & Williams, G. D., 1983. Strain geometry, microstructure and mineral chemistry in metagabbro shear zones: a study of softening mechanisms during progressive mylonitization. l o d of Structural Geology. 5,507-517.

White, J., 1968. Phase distribution in ceramics. In: Ceramic Micraslr~ctiucS (eds Fulrath, R. M. & Pask, J. A.), pp. n&762. John Wiley & Sons, New York.

White, J. C. & Mawer, C. K., 1986. Extreme ductility of feldspars from a mylonite, Parry Sound, Canada. Journal of Structural

White, S. H., 1975. Tectonic deformation and recrystallization of oligoclase. Connibufions to Mineralogy and Petrology, 50, 287-305.

Wilson, C. J. L., 1980. Shear zones in a pegmatite: a study of albite-mica-quartz deformation. Journal of Structural Geology, 2,203-209.

Wones, D. R., Tatlock, D. B. & von Limbach, D., 1967. Coexisting orthoclase and microcline in altered volcanic rocks, West Humboldt Range, Pershing County, Nevada. Schweizerirche Mineralogische und Petrographische Mifteilungen, 47,169-177.

Yund, R. A. & Tullis, J., 1980. The effect of H,O, pressure and strain on AI/Si order/disorder kinetics in feldspar. Contributions to Mineralogy and Petrology, 72,297-302.

Zeuch, D. H.. 1982. Ductile faulting, dynamic recrystallization and grain-size-sensitive flow of olivine. Tectonophysics, 83,

87- 116.

Geology, 8, 133-143.

293-308.

Receiwd 9 June 1987; revision accepted 20 April 1988.

Bell, T.H. and Johnson, S.E., 1989b. The role of deformation partitioning in the deformation and...

Documents

Transcript of Bell, T.H. and Johnson, S.E., 1989b. The role of deformation partitioning in the deformation and...