Journal of Structural Geology 26 (2004) 793–805
www.elsevier.com/locate/jsg

Development of shape- and lattice-preferred orientations of amphibole
grains during initial cataclastic deformation and subsequent deformation
by dissolution –precipitation creep in amphibolites from the Ryoke
metamorphic belt, SW Japan
Reiko Imona, Takamoto Okudairaa,*, Kyuichi Kanagawaba

Department of Geosciences, Osaka City University, Osaka 558-8585, Japanb
Department of Earth Sciences, Chiba University, Chiba 263-8522, Japan

Received 5 February 2003; received in revised form 30 July 2003; accepted 4 September 2003

Abstract
Amphibolites from the Ryoke metamorphic belt, SW Japan were deformed initially by cataclasis and subsequently by dissolution –precipitation creep. Initial cataclastic deformation produced a rather weak shape-preferred orientation (SPO) of brown amphibole grains withsmall aspect ratios as well as a poorly developed amphibole lattice-preferred orientation (LPO) with na (< a[100]) axes scattered subnormal
to the foliation and ng or c[001] axes scattered around the lineation. During later deformation by dissolution – precipitation creep, preferential
dissolution at grain boundaries subparallel to the foliation and simultaneous compaction normal to the foliation have likely produced adistinct SPO of elongate brown amphibole grains subparallel to the foliation as well as their LPO such that their ng or c axes are scatteredaround the lineation, while na (< a) and nb ( ¼ b[010]) are spread along a girdle normal to the lineation. Also during this deformation green
amphibole precipitated as isolated grains or in pressure shadow regions around brown amphibole grains. Nucleation and anisotropic growthof isolated green amphibole grains according to the orientations of the principal stress directions produced an LPO of these grains such that
their na (< a) are oriented normal to foliation, nb ( ¼ b) within the foliation normal to the lineation and ng (or c) axes are parallel to thelineation. In addition, there is an associated SPO. Growth of green amphibole in pressure shadow regions around brown amphibole grains
occurs either syntaxially or anisotropically according to the orientations of the principal stress directions.
q 2003 Elsevier Ltd. All rights reserved.
Keywords: Amphibole; Cataclasis; Dissolution–precipitation creep; Lattice-preferred orientation; Shape-preferred orientation

1. Introduction
Deformation of the continental crust is thought to be
dominated by flow of quartz-rich rocks in the upper crustand flow of feldspar- or hornblende-rich rocks in the lower
crust (e.g. Kirby and Kronenberg, 1987; Ranalli and
Murphy, 1987). The transition temperature of cataclastic
to plastic deformation of quartz is much lower than that ofplagioclase or hornblende (Tullis and Yund, 1987, 1992),
and plagioclase and hornblende are unlikely to deform
plastically in upper crustal conditions. However, such atransition of deformation mechanism with respect to T isapplicable only for monomineralic rocks or minerals* Corresponding author.
E-mail address: oku@sci.osaka-cu.ac.jp (T. Okudaira).
0191-8141/$ - see front matter q 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jsg.2003.09.004

forming a load-bearing framework (e.g. Handy, 1994). As
rocks constituting the upper and lower crusts are polymineralic, their dominant deformation mechanism can be
expected to differ from that of monomineralic rocks (e.g.
Hacker and Christie, 1990).
Since amphibole may comprise a significant componentof the lower crust, its rheological properties are importantwhen considering lower crustal deformation. Latticepreferred orientations (LPOs) and shape-preferred orientations (SPOs) of amphibole grains hold much informationon the kinematics and dynamics of deformed lower crustal
rocks. Previous studies of experimentally and naturally
deformed amphiboles have suggested that amphibole is oneof the strongest minerals, behaving as a rigid inclusion or
fracturing during deformation (e.g. Shelley, 1994). Clinoamphibole typically deforms experimentally by twinning on

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(101)
in the C2/m setting up to 600 8C (Rooney et al., 1975;
Morrison-Smith, 1976). However, (101)twinning has notbeen reported in naturally deformed clinoamphiboles(Dollinger and Blacic, 1975; Cumbest et al., 1989).
Naturally deformed clinoamphiboles exhibit fracturing,bending and kinking on (100)[001] and (100) twinning,whereas plastic strain features such as subgrains anddynamically recrystallized grains are rarely observed(Allison and La Tour, 1977; Biermann and van Roermund,1983; Cumbest et al., 1989; Nyman et al., 1992). Theseobservations suggest that a dominant deformation mechanism of amphibole grains under crustal conditions is not
mainly dominated by plastic intracrystalline process (e.g.
Shelley, 1994). On the other hand, Imon et al. (2002)
proposed that amphibole as well as plagioclase deformed bydissolution –precipitation creep under upper greenschist –lower amphibolite facies conditions. Wintsch and Yi (2002)
also suggested that dissolution –precipitation creep is theimportant deformation mechanism in wet polyphase rocks
under middle crustal conditions.
Deformed amphibolites in a ductile shear zone in the
Ryoke metamorphic belt, SW Japan, contain abundantamphibole grains. The deformed amphibolites occur as thinlayers (several centimeters thick) within granitic mylonites.
In the amphibolites, amphibole grains exhibit a strong
shape-preferred orientation parallel to the foliation, which isparallel to the mylonitic foliation of the surrounding granitic
mylonites, and to the lithologic contacts. In this study, theshape- and lattice-preferred orientations of amphibole
grains are analyzed and discussed in terms of their likely
deformation processes.

2. Geological setting and petrography of theamphibolites
2.1. Geological setting
The Cretaceous Ryoke metamorphic belt (Fig. 1a)consists of abundant granitoids and an associated low-P/
high-T metamorphic complex (e.g. Banno and Nakajima,
1992). The EW-trending Ryoke metamorphic belt is
juxtaposed against the EW-trending high-P Sambagawa
metamorphic belt and/or Izumi Group along the Median
Tectonic Line (MTL) in SW Japan.
In the northern part of the Kishiwada district (Fig. 1b),
foliated granitoids with E –W striking foliation and steep
dips are widely distributed. The granitoids are widely
mylonitized, and an area of particularly strong mylonitization in the southern part of Kawai is called the Kawai
mylonite zone, representing a narrow ENE-trending shear

zone (Itihara et al., 1986; Takagi et al., 1988; Fig. 1b). The
Kawai mylonite zone and the surrounding highly deformed
zone are dominated by granitic mylonites formed undersinistral shear (Takagi et al., 1988). The mylonitic foliation
strikes ENE to E and dips 50– 708 to the north, and ahorizontal stretching lineation trends ENE (Itihara et al.,1986). Along the Tsuda-gawa river section (Fig. 1c), thegrain size of minerals constituting the granitic rocks
gradually decreases towards the center of the mylonite
zone (Takagi et al., 1988; Imon et al., 2002). At the center ofthe mylonite zone, the granitic ultramylonites exhibit welldeveloped compositional layering (millimeter scale) definedby alternating quartz-rich and feldspar-rich layers.
2.2. Petrography
Deformed amphibolites occur as thin layers (several
centimeters thick) within the granitic mylonites. Theseamphibolites have been observed throughout the study areafrom low-strain zones to high-strain zones within and near
the Kawai mylonite zone. The deformed amphibolites do
not exhibit well-developed compositional layering, yet doshow foliation and lineation defined by alignment ofplagioclase and amphibole grains, parallel to the foliationand lineation in the surrounding granitic rocks. The
amphibolites do not exhibit pinch-and-swell or boudinage
structures, even within the high-strain mylonite zone,
suggesting that they were internally deformed to the same
extent as the surrounding granitic rocks. They may haveoriginally been mafic enclaves, as evidenced by thenumerous ellipsoidal inclusions of gabbroic – dioritic composition with granoblastic microstructure in weakly
deformed granitic rocks peripheral to the high-strain zone
(Itihara et al., 1986).
Amphibolite samples were collected from localities inand near the Kawai mylonite zone along the Tsuda-gawariver section, as shown in Fig. 1c. In this study, four samples(C, H, I and M) were selected for texture analyses. The
deformed amphibolite is composed mainly of plagioclase,brown amphibole, green amphibole, chlorite, quartz, titanitewith minor K-feldspar, biotite, muscovite, ilmenite, apatite
and allanite. Based on the degree of foliation development,
the deformed amphibolites can be divided into two types;
strongly foliated amphibolites (samples C and M) andweakly foliated amphibolites (samples H and I). The
mineral assemblages of the strongly foliated amphibolites
do not differ systematically from the weakly foliated ones,
although modal abundances of minerals are different
between them. Amphibole and plagioclase make up about
60– 80 modal percent of all samples. The modal abundancesof green amphibole, chlorite and titanite in the strongly

Fig. 1. (a) Index map of Ryoke metamorphic belt, SW Japan. MTL: Median Tectonic Line, ISTL: Itoigawa– Shizuoka Tectonic Line. (b) Geological map ofKishiwada district, Kinki province (modified after Itihara et al., 1986). (c) Route map along Tsuda-gawa river section, showing amphibolite sampling localities.

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foliated amphibolites are greater than those in the weakly
foliated samples.
Application of the amphibole – plagioclase exchange
thermometry of Spear (1980) to the adjacent Ca-poor
plagioclase and green amphibole yields temperatures of ca.
510 8C during deformation of the strongly foliated amphibolites (Imon et al., 2002). Although the pressures during
deformation cannot be estimated precisely, the lack ofmetamorphic epidote in the amphibolites suggests that
deformation occurred at pressures below 300 MPa (c.f.
Spear, 1993).

3. Microstructures and microfabrics
3.1. Microstructures
In the highly deformed zone, strongly foliated amphibolites contain elongated Ti-rich brown amphibole grains
rimmed by Ti-poor green amphibole and chlorite (Fig. 2aand b). The foliation is parallel to the mylonitic foliation in
the surrounding granite mylonites. C0 -type shear bands
defined by preferential distribution of the green amphibole,chlorite and titanite occur locally. Amphibole grains rarely
show microstructures indicating crystal plasticity such as
undulose extinction, deformation lamella, subgrains andgrain-boundary migration features (Fig. 2b). Amphibole
grains are divided into two types: chemically zoned
amphiboles (brown core þ green rim) and unzoned green
amphiboles. The green rims of zoned amphiboles are
preferentially developed in pressure shadow regions aroundthe brown cores, although some green rims are also
developed along C0 -type shear bands (Fig. 2a). Fig. 3 andTable 1 show the mineral chemistry of amphibole grains inthe strongly foliated amphibolite (sample C).
Plagioclase in the strongly foliated amphibolites also
consists of two components: a Ca-rich core and a Ca-poor
rim. The anorthite-poor plagioclase rims developed inpressure shadow regions around the anorthite-rich cores.
Based on these observations, the mass-balance reaction forthe deformation event is defined (Imon et al., 2002) as:
Ca-rich plagioclase þ Ti-rich brown amphibole þilmenite þ quartz þ H2 O ! Ca-poor plagioclase þ
Ti-poor green amphibole þ chlorite þ titanite

ð1Þ

As this reaction is a hydration reaction whose products
are preferentially distributed in pressure shadow regions, thereaction is considered to have occurred under hydrous, nonhydrostatic conditions.
In the weakly foliated amphibolites, the amphibole andplagioclase grains have angular outlines and straightboundaries (Fig. 2c and d). A few grain-scale faults,
as indicated by discrete but small offsets, are observed
within some amphibole and plagioclase grains (Fig. 2d).

Table 1
Representative chemical analyses of amphibole (Sample C)
Grain

Zoned

Unzoned

Brown

Green

Green

wt%
SiO2
TiO2
Al2O3
FeOa
MnO
MgO
CaO
Na2O
K2 O
Total

43.85
1.07
10.30
21.90
0.58
8.83
10.20
0.79
0.49
98.01

46.43
0.77
8.21
20.02
0.69
9.11
10.87
0.79
0.36
97.25

49.32
0.20
4.91
19.57
0.51
10.54
12.24
0.50
0.15
97.94

O ¼ 23
Si
Ti
[4]
Al
[6]
Al
Fe2þ
Fe3þ
Mn
Mg
Ca
Na
K

6.60
0.12
1.40
0.42
2.35
0.41
0.07
1.98
1.64
0.23
0.09

7.02
0.09
0.98
0.48
2.51
0.02
0.09
2.05
1.76
0.23
0.07

7.33
0.02
0.67
0.19
2.18
0.25
0.06
2.34
1.95
0.14
0.03

Mg#b

0.46

0.45

0.52

Note: Mineral chemistry was determined using an electron-probe
microanalyzer (Shimadzu, EPMA-8705) at Osaka City University. The
operating conditions were an accelerating voltage of 15 kV, a beam currentof 4 nA and a beam width of 5 mm. X-ray intensities were converted tooxide-weight percentages based on the method of Bence and Albee (1968).
Natural and synthetic oxides and silicate minerals were used as standards.
Cation contents are assigned using 15 eNK after Leake et al. (1997).
aTotal Fe as FeO.
b
Mg#, atomic ratio of Mg/(Fe2þ þ Mg).

Amphibole grains do not exhibit appreciable chemicalzoning, and green amphibole is much less abundant than in
the strongly foliated amphibolites.
3.2. Shape-preferred orientation (SPO)
The lengths of the long axis (A) and short axis (B) of eachamphibole grain in sections of the selected samples cut
perpendicular to the foliation and parallel to the lineation(XZ sections) were measured under an optical microscope,
assuming that the grains can be treated as elliptical particles.
The grain size (d) of amphiboles is definedas the geometricpﬃﬃﬃﬃﬃﬃﬃﬃﬃ
mean of A and B, that is, d ¼ ðA £ BÞ. Grain size
distributions of amphiboles in samples C, H, I and M are
shown in Fig. 4. For the strongly foliated samples (C andM), the grain sizes of zoned and unzoned amphiboles havebeen measured separately (Fig. 4a and b). The averageamphibole grain sizes are 107 mm for zoned grains and
41 mm for unzoned green grains in sample C, 161 mm forunzoned brown grains in sample H, 161 mm for unzoned

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797

Fig. 2. Photomicrographs showing deformation microstructures of amphibolites (XZ sections). (a) and (b) Strongly foliated amphibolite (sample C; plane
polarized light). C0 -type shear bands indicating sinistral shear are developed, which are composed of green actinolitic ferrohornblende, chlorite and titanite.
Amphibole porphyroclasts have brown tschermakitic ferrohornblende cores rimmed by green actinolitic ferrohornblende. The internal asymmetry of sigmoidal
shape of the foliation between the C0 -planes indicates dextral sense of shear, and the segments between shear bands may have rotated antithetically. (c) and (d)Weakly foliated amphibolite (sample H; cross-polarized light) showing cataclastic features. Plagioclase and amphibole grains are broken and have subangularshapes. Locations of the microcracks and microfaults are indicated by arrows.

brown grains in sample I, and 173 mm for zoned grains and
62 mm for unzoned green grains in sample M. In thestrongly foliated samples, the unzoned (green) amphibole
grains are finer-grained than the zoned (brown þ green)
grains which are similar in size to the brown grains in the
weakly foliated samples. The brown grains in the weakly
foliated amphibolites have a wider range in grain size thanthe zoned grains in the strongly foliated samples. As in thecase of plagioclase grains (Imon et al., 2002), the brown
amphibole grains smaller than 100 mm are abundant in the
weakly foliated amphibolites, whereas the zoned amphibole
grains smaller than 100 mm are less abundant in the strongly
foliated amphibolites (see Figs. 2 and 4).
The SPOs of amphibole grains can be shown by theirlong-axis orientations with respect to the foliation direction(u), and aspect ratios (R). The long axis orientation of eachgrain was measured as the angle (anticlockwise positive)

from the foliation trace in the XZ section to the direction of
the long axis (Fig. 5). For the chemically zoned amphibole
grains, two sets of data were obtained: the long-axis
orientation and aspect ratio of a whole grain (browncore þ green rim), and those of its brown amphibole core.
Fig. 5 shows the relationships between u and R in thestrongly and weakly foliated amphibolites. In the strongly
foliated amphibolites, there is a difference in the R – urelation of zoned amphibole grains between samples C and
M (Fig. 5a and b). In sample C, the brown amphibole cores
have small aspect ratios and show a weak preferredorientation, whereas the whole amphibole grains are
preferentially aligned parallel to the foliation. In contrast
in sample M, both brown cores and whole grains are
preferentially aligned parallel to the foliation. In both
samples, grains with aspect ratios smaller than 3 havevariable orientations. Among amphibole grains with aspect

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Fig. 3. Chemistry of amphibole grains in the strongly foliated amphibolite(sample C). (a) Mg/(Fe2þ þ Mg) vs. Si diagram. (b) Frequency diagram of
Ti content. Cation proportions are shown for one formula unit. Amphibole
classification is according to that of Leake et al. (1997).

ratios larger than 5, unzoned grains are more abundant thanzoned grains. Interestingly, the most common long-axis
orientation of grains with higher aspect ratios is slightly
oblique to the foliation (Fig. 5a and b). This may be due to amodification in SPO caused by the development of C0 -shear
bands. The maximum aspect ratio in these samples is < 9 forzoned grains, and < 14 for unzoned grains.
Although amphibole grains with higher aspect ratios tend
to be aligned parallel to the foliation in the weakly foliatedamphibolites (samples H and I), the long-axis orientations inthese samples are more scattered than in the strongly
foliated samples. In addition, the maximum aspect ratios(R < 6) are smaller, and typically less than 3. Furthermore,grains with low aspect ratios appear randomly oriented.
3.3. Lattice-preferred orientation (LPO)
LPO analyses of amphibole were carried out in XZ
sections using an optical microscope equipped with aconventional universal stage, and by a scanning electron
microscopy (SEM)-based electron backscatter diffraction(EBSD) technique. The optical indicatrix axes of clinoamphibole, na, nb and ng, determined by using a universal
stage are related to its crystallographic axes a[100], b[010]and c[001] such that na is close to a, nb coincides with b,and ng is at an angle of 128– 348 with c. Both of the twostrongly foliated samples (C and M) exhibit essentiallysimilar amphibole LPOs (Fig. 6a and b). The ng axes of both
zoned and unzoned grains are oriented close to the lineation(X). For the unzoned amphiboles, the na (< a) axes exhibit aweak concentration normal to the foliation (Z), and the nb( ¼ b) axes are plotted in the direction perpendicular to thelineation within the foliation (Y). The na and nb axes for thezoned amphiboles are scattered along a great girdle normal
to the X direction. The ng-axis concentration direction andthe girdles of na and nb are slightly anticlockwise obliqueto the X direction and the X-normal girdle, respectively (Fig.
6a and b), which may be due to a modification in LPO
caused by the development of C0 -shear bands which also
affected SPO.
Fig. 6 also shows the relationship between the opticalindicatrix axis orientations and aspect ratios in the strongly
foliated samples C and M. The ng axes of the moreelongated brown cores tend to be more concentrated in the X
direction than those of the less elongated cores, while the na
and nb axes of the more elongated brown cores have atendency to be more concentrated in the Z and Y directions,
respectively, than those of the less elongated cores. In
contrast, in the green amphibole rims and unzoned grains,no clear correlation is found between the optical indicatrix
axis orientations and the aspect ratios (Fig. 6a and b).
In the weakly foliated amphibolites (samples H and I),
the amphibole optical indicatrix axes exhibit rather weak
preferred orientations such that na axes are scattered around
the Z directions (Fig. 6c and d). In addition, nb and ng axes
tend to be scattered around the Y and X directions,
respectively, in sample H, while they tend to be variably
oriented subparallel to the XY plane in sample I.
Amphibole LPO data of sample C were also obtained by
the EBSD technique. This technique allows the complete
crystallographic orientation of grains on the surface of apolished sample to be determined with spatial resolution of
about 1 mm and an absolute angular resolution of 18 (Prioret al., 1996). This technique has become widely used for
textural analysis of deformed rocks (Prior et al., 1999). The
XZ section of sample C was polished using diamond paste
on a paper lap followed by chemical polishing using a
colloidal silica suspension in order to remove any surface
damage, and then coated with a thin carbon layer (about5 nm) to reduce specimen charging. EBSD patterns werecollected using a JEOL JSM-5600 scanning electron
microscope at Chiba University with an accelerating voltageof 17 kV and a beam current of approximately 3.5 –3.8 nA.
The sample was tilted to 738 in the microscope chamber.
The EBSD patterns were indexed using Channel 5 Softwareof HKL Technology. Amphibole cores and rims were
measured separately. The results are similar to thoseobtained by universal stage measurements (see Figs. 6aand 7). The crystallographic c[001] axes of brown cores andgreen rims in sample C are oriented in the X direction

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799

Fig. 4. Frequency diagram showing amphibole grain size distributions in two strongly foliated samples (C and M) and two weakly foliated samples (H and I).

(Fig. 7). The a[100] axes are spread along a great girdle
around the X direction, with a weak tendency to be scatteredaround the Z direction. The b[010] axes are oriented quite
variably, but have a weak tendency to be scattered aroundthe Y direction. Although there is a correlation between
crystallographic orientations and the aspect ratios in the
results obtained by universal stage measurements, thecorrelation is not so clear in the results of EBSD analysis.
This may be due to the smaller number of EBSD data,
especially for grains with higher aspect ratios.
The relationship in each zoned grain of sample C
between the brown core c axis with respect to the lineation X
and the internal angle between the c axes of the brown coreand green rim is shown in Fig. 8. There are zoned grains
plotted near abscissa in Fig. 8 with approximately the same
core and rim c-axis orientations irrespective of their corec-axis orientations, while in other zoned grains the internalangle between the core and rim c axes tends to increase withthe core c-axis orientation going away from X.

4. Discussion
4.1. Deformation processes in the deformed amphibolites
In the weakly foliated amphibolites, brown amphibole
and plagioclase grains have angular outlines and straightboundaries (Fig. 2c and d). Many microcracks andmicrofaults can also be recognized. In addition, theamphibole grain-size distributions with a small number oflarge grains and a large number of small grains (Fig. 4c andd) are consistent with grain-size reduction due to microcracking and comminution (e.g. Passchier and Trouw, 1996,
p. 25). All of these are suggestive of cataclastic deformation,
since the deformed amphibolites may have originally beenmafic enclaves having granoblastic microstructure. The
weakly foliated amphibolites are therefore likely deformed
predominantly by cataclasis.
In the strongly foliated amphibolites, amphibole grains
do not show such cataclastic microstructures as observed in
the weakly foliated amphibolites (Fig. 2a and b). They also
rarely show such microstructures as undulose extinction,
deformation lamella, subgrains and grain-boundary
migration features (Fig. 2b), and hence they are unlikely
to have been deformed by crystal plasticity. In fact, it isgenerally accepted that amphibole is the least plastic among
common rock-forming silicate minerals under crustal
conditions (e.g. Shelley, 1994). The deformation temperatures of ca. 510 8C (Imon et al., 2002) estimated for thestudied amphibolites may be too low for amphibole todeform by crystal plasticity.
Green amphibole in the strongly foliated amphibolites is
a product of the reaction (1), and occurs as either unzoned
grains or in pressure shadow regions around brown amphibole grains. The latter occurrence suggests precipitation ofgreen amphibole in low normal-stress regions, in the same

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Fig. 5. Amphibole aspect ratios (R) and long-axis orientations (u) in the XZ section. (a) and (b) Strongly foliated amphibolites (samples C and M). (c) and (d)Weakly foliated amphibolites (samples H and I). u is measured anticlockwise positive from the foliation trace (S). C0 denotes the orientation of C0 -type shear
bands.

way as that of Ca-poor plagioclase precipitation aroundCa-rich plagioclase grains in the same amphibolites (Imonet al., 2002). The reaction (1) may therefore be an
incongruent pressure solution by which Ti-rich brown
amphibole and Ca-rich plagioclase grains dissolve at theirhigh-normal stress grain boundaries parallel to the foliation,
and Ti-poor green amphibole and Ca-poor plagioclase
precipitate in pressure shadow regions around brown
amphibole and Ca-rich plagioclase grains, respectively.
Thus dissolution –precipitation creep probably plays animportant role in ductile deformation of the strongly foliatedamphibolites.
Because characteristic microstructures of the deformed
amphibolites change from cataclastic to ductile withincreasing strain (Fig. 2), it is likely that initial cataclastic
deformation has been followed by dissolution– precipitation
creep in the strongly foliated amphibolites (Imon et al.,2002), and that the brown amphibole cores in the strongly
foliated amphibolites have originated from cataclastically
deformed amphibole grains such as those in the weakly

Fig. 6. Lower-hemisphere, equal-area projections showing the orientations of amphibole optical indicatrix axes of na, nb and ng. (a) and (b) Strongly foliatedamphibolites. (c) and (d) Weakly foliated amphibolites. Grains with low (R # 3) and high (R . 3) aspect ratios are indicated by closed and open circles,
respectively. S and C0 denote the orientation of the foliation and C0 -type shear bands, respectively.

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801

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Fig. 8. Relationship in sample C between the orientation of brown corec[001] axis with respect to mineral lineation X and the internal angle
between c[001] axes of brown core and green rim.

significantly different from that in the former case (Wintsch
and Yi, 2002).
4.2. Development of shape- and lattice-preferredorientations of amphibole grains

Fig. 7. Lower-hemisphere, equal-area projections showing the orientationsof amphibole crystallographic axes of a[100], b[010] and c[001] in sample
C. Grains with low (R # 3) and high (R . 3) aspect ratios are indicated byclosed and open circles, respectively. S and C0 denote the orientation of thefoliation and C0 -type shear bands, respectively.

deformed amphibolites. Available data suggest that thestrongly foliated amphibolites contain less abundant finebrown-amphibole grains than those in the weakly foliatedamphibolites (cf. Figs. 2 and 4), and this is probably because
fine brown-amphibole grains in the former have been
reduced in number by dissolution. The initial cataclastic
deformation such as fracturing and comminution ofamphibole and plagioclase grains would draw fluid into
the cataclastic zone (e.g. Matthäi and Roberts, 1997), whichis required for the hydration reaction (1). Such deformationinduced fluid flux is also important in metamorphic
re-equilibration and mass transport required for dissolution–
precipitation of amphibole and plagioclase grains.
Thus fluid infiltration during the initial cataclastic
deformation may have promoted subsequent deformation
by dissolution –precipitation creep of the studied amphibolites at pressures below 300 MPa and temperatures of ca.
500 8C. The microstructural change from cataclastic toductile in these amphibolites probably represents a transition in dominant deformation mechanism from cataclasis
to dissolution –precipitation creep. Although brittle –ductile
transition from cataclasis to dislocation creep is generally
accepted (e.g. Kohlstedt et al., 1995), our study therefore
suggests another brittle –ductile transition from cataclasis
to dissolution – precipitation creep occurring in the middle
crust. In the latter case, the crustal strength would be

In the weakly foliated amphibolites predominantly
deformed by cataclasis, cataclastic deformation has probably produced a rather weak SPO and a poorly developed
LPO of brown amphibole grains (Figs. 5c and d and 6c andd). As shown in Fig. 6c and d, the observed LPO pattern issuch that na (< a[100]) axes are oriented subnormal to thefoliation (< Z). The nb ( ¼ b[010]) axes are scatteredaround the lineation-normal direction within the foliation(Y). The ng axes exhibit a weak concentration around thelineation (X) (Fig. 6c) or subparallel to the foliation (< XY)(Fig. 6d). Allison and La Tour (1977) have reported an LPO
of cataclastically deformed hornblende grains with theirc[001] axes away from the foliation due to rotation offragments bounded by planes subnormal to the c axis. But
this is not the case for amphibole grains in the weakly
foliated amphibolites studied, where their ng axes and hencec axes are oriented subparallel to the foliation. The reason
for the development of the observed LPO pattern duringcataclastic deformation of brown amphibole grains in the
weakly foliated amphibolites is unknown at present.
The unzoned green amphibole grains in the strongly
foliated amphibolites have nucleated and grown by thereaction (1), and their SPO and LPO can be produced byanisotropic growth during deformation by dissolution–
precipitation creep (c.f. Shelley, 1994; Bons and den Brok,
2000; Heidelbach et al., 2000). Schwerdtner (1964) andHara et al. (1983) have advocated a mechanism whereby
amphibole c axis preferentially grows in the direction of theminimum principal stress. The unzoned green amphibole
grains exhibit an LPO such that their na (< a), nb ( ¼ b)
and ng (or c) axes are oriented in the Z, Y and X directions,
respectively (Figs. 6a and b and 7).

R. Imon et al. / Journal of Structural Geology 26 (2004) 793–805

The X direction should be subparallel to the minimum
principal stress direction in the strongly foliated amphibolites, because this is the direction where green amphibole
precipitated around brown amphibole grains. This relationship is suggested by the other microstructural features. As
shown in Fig. 2a, the internal asymmetry of sigmoidal shapeof the foliation between the C0 -planes indicates dextral senseof shear, while the external asymmetry of angle between theenveloping surface of the foliation and the shear bands
indicates sinistral sense of shear. The segments between
shear bands may have rotated antithetically in the stretching
shear zones, whereas the bands must have rotatedsynthetically if flow in a shear zone is a simple shear(Passchier and Trouw, 1996, p. 113). The C0 -planes in thestrongly foliated amphibolites, therefore, are extensional
crenulation cleavage of Platt and Vissers (1980). In such
stretching shear zones, two sets of conjugate shear bands
would be expected around the shortening instantaneous
stretching axis (ISA), but only one, at a small angle to thefoliation is realized (Passchier, 1991). The shortening ISA
located at high angle to the foliation and the extensional ISA
oriented subparallel to the foliation ( ¼ lateral extension of
the shear zone). Therefore, in the strongly foliatedamphibolites, the stretching ISA is roughly parallel to thelineation, and may be parallel to the minimum principal
stress direction. The observed LPO of unzoned green
amphibole grains is therefore consistent with anisotropic
amphibole growth according to the orientations of theprincipal stress directions. The SPO of unzoned green
amphibole grains such that their long axes parallel to their caxes are preferentially oriented parallel to the X direction(Fig. 5a and b) is a necessary consequence of suchanisotropic growth. It should be noted that our data suggestthat amphibole a and b axes preferentially grow in the
maximum and intermediate principal stress directions,
respectively.
In principle, anisotropic growth should also apply to thegreen amphibole rims of zoned grains, because they also
have grown by the reaction (1). The LPO of green
amphibole rims is such that the c or ng axis is oriented in
the X direction and the a (, na) and b ( ¼ nb) axes areoriented along a girdle around the X direction, which isbasically the same as that of brown amphibole cores (Figs.
6a and b and 7). Unlike the green amphibole rims, the LPO
patterns of unzoned green amphiboles resemble pointmaximum patterns. This implies that the growth of green
amphibole rims is primarily controlled by crystallographic
orientations of brown amphibole cores rather than by theprincipal stress directions, i.e. syntaxial. Green amphibole
rims with approximately the same crystallographic orientations as brown amphibole cores irrespective of theirorientations, such as those plotted near abscissa in Fig. 8,
represent syntaxially grown rims. Fig. 8 also shows thepresence of green rims with their c axes away from those ofbrown cores oblique to the X direction. This implies that thec axes of these green rims keep close to the X direction

803

irrespective of their brown core orientations. These green
amphibole rims have likely grown anisotropically with theirc axes parallel to the X direction as unzoned green
amphibole grains. The distinct SPO of zoned grains insample C may be due to this anisotropic growth of greenrims, because that of brown cores in this sample is rather
weak (Fig. 5a).
The brown amphibole cores in the strongly foliatedamphibolites probably have been inherited from cataclastically deformed amphibole grains such as those in the
weakly deformed amphibolites, as discussed above. The
brown amphibole cores in sample C have relatively small
aspect ratios and exhibit a weak SPO similar to those ofbrown amphibole grains in the weakly foliated amphibolites(Fig. 5a, c and d). Although the brown amphibole cores insample M exhibit a distinct SPO, those with small aspectratios are variably oriented (Fig. 5b). In addition, the brown
amphibole cores with small aspect ratios (R # 3) in both
samples C and M exhibit an LPO such that a (, na), b( ¼ nb) and c or ng axes are weakly oriented in the Z, Y and
X directions, respectively (Figs. 6a and b and 7), which isalso similar to that of the weakly foliated sample H (Fig. 6c).
Hence the brown amphibole cores with small aspect ratios
in the strongly foliated amphibolites may preserve the SPO
and LPO produced during initial cataclastic deformation,
which have not been largely modified by later deformation
by dissolution –precipitation creep.
In contrast, the brown amphibole cores with large aspectratios (R . 3) in the strongly foliated amphibolites exhibit a
distinct SPO and an LPO such that a (, na) and b ( ¼ nb)
axes are spread along a girdle around the X direction (Figs.
6a and b and 7), both of which are different from those of the
weakly foliated amphibolites. Such SPO and LPO ofelongate brown amphibole cores may have been produced
during initial cataclastic deformation by rigid-body rotationin cataclastically flowing fine-grained matrix produced byfracturing and comminution. The rigid-body rotation ofelongate brown amphibole clasts subparallel to their c axes
would have resulted in their preferential alignment subparallel to the lineation X around which their a and b axes
are variably oriented. A similar mechanism for preferentialalignment of elongate clasts in cataclastically flowing finegrained matrix has also been reported from experimentallyand naturally deformed rocks (e.g. Tullis and Yund, 1987,
1992; Tanaka, 1992). The difference in brown core SPO
between samples C and M (Fig. 5a and b) may then be due tothe difference in cataclastic strain, i.e. cataclastic strain ofsample M being larger than that of sample C. However, thisis inconsistent with the observation that zoned grains insample M are significantly coarser-grained than those insample C (Fig. 4), because a higher cataclastic strain should
result in a finer grain-size distribution. The SPO and LPO
development of elongate brown amphibole cores duringinitial cataclastic deformation is therefore problematic.
Alternatively, the SPO and LPO of the elongate brown
amphibole cores may have been produced during later

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R. Imon et al. / Journal of Structural Geology 26 (2004) 793–805

deformation by dissolution– precipitation creep. Preferential dissolution of brown amphibole clasts along highnormal-stress boundaries, i.e. along grain boundaries
subparallel to the foliation, possibly produced elongate
brown amphibole grains with their SPO subparallel to thefoliation, which may have been further enhanced bycompaction associated with dissolution – precipitation
creep. Although preferential dissolution alone does notnecessarily produce an LPO, the pre-existing weak LPO
with c or ng axis scattered around the X direction as those in
the weakly foliated amphibolites could have been modified
to more distinct point-maximum pattern, and a and b axes
could have been rotated around the c axis during theassociated compaction. The difference in brown core SPO
between samples C and M (Fig. 5a and b) may then be due totheir difference in strain during deformation by dissolution–
precipitation creep. In fact, the smaller aspect ratios andweaker SPO of unzoned green amphibole grains in sample C
than those in sample M (Fig. 5a and b) indicate that thestrain during deformation by dissolution – precipitation
creep is smaller in sample C than in sample M.

Acknowledgements
N. Aikawa and K. Ishii are appreciated for comments andencouragement. The paper benefited substantially fromdetailed reviews by T.E. La Tour and D. Shelley. J. Hipperttis thanked for the editorial handling of the paper. This study
was financially supported by the Grants-in-Aid for Scientific
Research (14740305) from the Japan Society for the
Promotion of Science to T. Okudaira.

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