grain-size reduction of feldspars by fracturing and...
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Tectonophysics 407 (
Grain-size reduction of feldspars by fracturing and
neocrystallization in a low-grade granitic mylonite
and its rheological effect
Jin-Han Ree a,*, Hyeong Soo Kim a,b, Raehee Han a, Haemyeong Jung c
a Department of Earth and Environmental Sciences, Korea University, Seoul 136-701, South Koreab Department of Earth Science Education, Kyungpook National University, Daegu 702–701, South Korea
c Institute of Geophysics and Planetary Physics, University of California, Riverside, California 92521, USA
Received 29 March 2005; received in revised form 10 July 2005; accepted 31 July 2005
Available online 31 August 2005
Abstract
Feldspar grain-size reduction occurred due to the fracturing of plagioclase and K-feldspar, myrmekite formation and
neocrystallization of albitic plagioclase along shear fractures of K-feldspar porphyroclasts in the leucocratic granitic rocks
from the Yecheon shear zone of South Korea that was deformed under a middle greenschist-facies condition. The neocrys-
tallization of albitic plagioclase was induced by strain energy adjacent to the shear fractures and by chemical free energy due to
the compositional disequilibrium between infiltrating Na-rich fluid and host K-feldspar. With increasing deformation from
protomylonite to mylonite, alternating layers of feldspar, quartz and muscovite developed. The fine-grained feldspar-rich layers
were deformed dominantly by granular flow, while quartz ribbons were deformed by dislocation creep. With layer development
and a more distributed strain in the mylonite, lower stresses in the quartz-rich layers resulted in a larger size of dynamically
recrystallized quartz grains than that of the protomylonite.
D 2005 Elsevier B.V. All rights reserved.
Keywords: Granitic mylonite; Grain-size reduction; Granular flow; Dynamic recrystallization; Yecheon shear zone
1. Introduction
Grain-size reduction can trigger a change in rheol-
ogy of deforming rocks due to a switch in the domi-
0040-1951/$ - see front matter D 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.tecto.2005.07.010
* Corresponding author. Tel.: +82 2 3290 3175; fax: +82 2 3290
3189.
E-mail address: [email protected] (J.-H. Ree).
nant deformation mechanism (e.g., White et al.,
1980). This rheological change, in turn, may induce
strain localization in the lithosphere (e.g., Rutter and
Brodie, 1988; Braun et al., 1999), or soften the sub-
ducting slab as a whole causing a change in the mantle
convection mode (e.g., Karato et al., 1998; Cizkova et
al., 2002). Grain-size reduction also facilitates reac-
tions by increasing the surface area (e.g., Evans, 1988)
or may lead to change in magnetic properties (e.g.,
2005) 227–237
J.-H. Ree et al. / Tectonophysics 407 (2005) 227–237228
Halgedahl and Ye, 2000). Grain-size reduction under
non-coaxial deformation can also induce a strong
lattice preferred orientation, resulting in a significant
change in seismic wave propagation (Jung and Kar-
ato, 2001).
Grain-size reduction can be achieved by cataclasis,
reaction, dynamic recrystallization and phase transfor-
mation. In feldspar, the most abundant crustal mineral,
grain-size reduction may be produced by cataclasis
and neocrystallization of albitic plagioclase from feld-
spar porphyroclasts under greenschist facies condi-
tions (Fitz Gerald and Stunitz, 1993). Deformation-
induced myrmekite formation also incurs feldspar
grain-size reduction, but this has been known to be
effective in epidote–amphibolite facies and higher
grade conditions (Vernon et al., 1983; Simpson,
1985; Simpson and Wintsch, 1989; Pryer, 1993).
However, Tsurumi et al. (2003) recently reported
that myrmekite formation, together with fracturing,
can considerably reduce the feldspar grain size
under middle greenschist facies conditions. In this
paper, we report that all these processes of fracturing,
neocrystallization of albitic plagioclase along frac-
tures and myrmekite formation contribute to signifi-
cant grain-size reduction of feldspar in granitic
mylonites from a middle greenschist-facies shear
zone. We also show that feldspar grain-size reduction
not only leads to a switch in the dominant deformation
mechanism in feldspar-rich layers, but also to a
change in the size of dynamically recrystallized grains
in quartz-rich layers of the mylonites.
Fig. 1. Photomicrographs of granitic (a) protomylonite and (b)
mylonite from the Yecheon shear zone. Cross-polarized light.
2. Microfabrics
Samples of the granitic mylonites were collected
from an outcrop within the Yecheon shear zone which
is a branch of the right-lateral Honam shear zone
system of the Jurassic in South Korea (Yanai et al.,
1985; Park et al., in press). There is a transition from a
weakly deformed to highly deformed granitic rocks in
the outcrop, referred to as protomylonite and mylo-
nite, respectively, in this paper.
The protomylonite consists of porphyroclasts (0.5 to
14 mm in length) of K-feldspar (38%), plagioclase
(5%), and matrix of quartz (25%), plagioclase (18%),
K-feldspar (9%) and muscovite (4%) with a minor
content of biotite, garnet, chlorite, zircon and opaque
minerals (Fig. 1a). The modal percentage of the con-
stituent minerals was obtained by point counting on
thin-sections stained with Na-cobaltinitrite (for K-feld-
spar) and K-rhodizonate (for plagioclase). On the other
hand, the mylonite is composed of porphyroclasts (0.5
to 6 mm in length) of plagioclase (17%) and K-feldspar
(11%), and matrix of quartz (33%), muscovite (18%),
plagioclase (11%), K-feldspar (8%) and calcite (1%)
with a minor content of chlorite, zircon and opaque
minerals (Fig. 1b). There is a significant increase in the
muscovite, plagioclase and quartz contents, and a
remarkable decrease in K-feldspar content compared
with the protomylonite. Also, the feldspar porphyro-
clasts of the mylonite decrease both in size and amount.
It can be seen in the mylonite that matrices of feldspar,
quartz and muscovite form interconnected layers.
2.1. Protomylonite
2.1.1. K-feldspar
K-feldspar grains are deformed dominantly by
fracturing, although many grains show patchy undu-
J.-H. Ree et al. / Tectonophysics 407 (2005) 227–237 229
latory extinction. Most of the intragranular/transgra-
nular shear fractures are subparallel to the bulk shear
plane, while extension fractures tend to be at a high
angle to the bulk shear plane (Fig. 2). Occasionally,
antithetic shear fractures utilizing cleavage planes
within K-feldspar porphyroclasts occur at a high
angle to the bulk shear plane (Fig. 2a). The extension
fractures usually follow the cleavage planes of K-
feldspar, and are filled by quartz, muscovite or chlor-
ite. The precipitation of quartz with some muscovite
and K-feldspar also occurs in the pressure shadow
areas of K-feldspar porphyroclasts.
Along many of the shear fractures of K-feldspar
porphyroclasts, fine-grained plagioclase grains (5 to
40 Am) occur as a band with a width of two- to five-
grains (Fig. 2b, c and d). These plagioclases are albitic
(An5–15), as shown in a later section of this paper. The
phase boundaries between the host K-feldspar and the
neocrystallized albitic plagioclases are wavy or lobate,
suggesting phase boundary migration, and thus a
reaction. In the backscattered SEM micrograph of
Fig. 2d, the neocrystallized plagioclase grains appear
Fig. 2. Microstructures of K-feldspar porphyroclasts in protomylonite. (a)
oclast along antithetic shears. The difference in gray level of the fractured K
feldspar. Pl: plagioclase. Qtz: quartz. (b) Neocrystallization of albitic plagio
polarized light. Inset: c. (c) Enlargement of neocrystallized albitic plagioc
to have some zoning patterns. However, no composi-
tional variation was observed in X-ray intensity maps
of Na, K, Ca and Al.
On K-feldspar porphyroclast faces subparallel to
the mylonitic foliation, myrmekitic intergrowth of
plagioclase and quartz occurs (Fig. 3). These myrme-
kitic plagioclases are albite (An1–12), as shown later.
The myrmekite lobes commonly embay the margins
of K-feldspar porphyroclasts. The myrmekite forma-
tion together with neocrystallization of albitic plagio-
clase and fracturing significantly reduces the K-
feldspar grain size.
2.1.2. Plagioclase
For the deformation of plagioclase porphyroclasts,
fracturing is dominant as in K-feldspar, although some
grains show mechanical twinning and kinking. Most
grains show patchy undulatory extinction. Cleavage
fractures within plagioclase porphyroclasts are filled
mostly by K-feldspar and occasionally by muscovite
or quartz (Fig. 4a). In the pressure shadow areas and
boudin necks of plagioclase porphyroclasts, K-feld-
Backscattered SEM micrograph of a fractured K-feldspar porphyr-
-feldspar porphyroclast is due to an irregular carbon coating. Kfs: K-
clase along a shear fracture within a K-feldspar porphyroclast. Cross-
lase in (b). (d) Backscattered SEM micrograph of (c).
Fig. 3. (a) Photomicrograph of myrmekite in protomylonite. Cross-
polarized light. (b) Backscattered SEM micrograph of myrmekite.
Kfs: K-feldspar. Pl: plagioclase. Qtz: quartz. Ms: muscovite.
J.-H. Ree et al. / Tectonophysics 407 (2005) 227–237230
spar precipitates with some quartz and muscovite, and
some fractured fragments of plagioclase are present
(Fig. 4).
Incipient development of alternating layers of fine-
grained feldspar, quartz and muscovite occurs in the
protomylonite (Fig. 4d). The feldspar-rich layer is
composed mainly of subrounded to subangular albitic
plagioclase (10 to 100 Am in length) with interstitial
K-feldspar and subordinate quartz filling the spaces
between the plagioclase fragments.
2.1.3. Quartz
The quartz grains in the protomylonite commonly
show patchy or sweeping undulatory extinction and
deformation bands. When quartz grains occur
between feldspar porphyroclasts, they are highly
stretched parallel to the mylonitic foliation. Most of
quartz grains show core-and-mantle structures with
diffuse grain boundaries (Fig. 5a). Recrystallized
grains occur at the boundaries of old grains with
their size (2–6 Am) similar to that of the subgrains
at the old grain mantles, indicating subgrain rotation
recrystallization although the recrystallization appears
to be somewhat incomplete (Hirth and Tullis, 1992;
Stipp et al., 2002).
We measured orientation of quartz c-axes of the
protomylonite using an automated fabric analyzer
apparatus developed at the University of Melbourne
(Russell-Head and Wilson, 2001; Wilson et al., 2003).
The quartz c-axis fabric shows a single girdle pattern
at a high angle to the mylonitic foliation with a
maximum in the Y-axis (i.e., 908 from the mylonitic
lineation on the foliation plane; Fig. 5b). All these
microfabrics of the protomylonite indicate that the
quartz grains were deformed by dislocation creep.
2.2. Mylonite
The most remarkable differences between the
mylonite and the protomylonite are compositional
layering, feldspar grain-size reduction (induced by
fracturing, neocrystallization of albitic plagioclase
and myrmekite formation), increase in the muscovite,
quartz and plagioclase contents, and decrease in the
K-feldspar content in the mylonite. Also, post-mylo-
nitization calcite micro-veins occasionally occur in the
mylonite. The feldspar-, quartz- and muscovite-rich
layers that define the mylonitic foliation are 30–300,
20–500 and 10–50 Am thick, respectively (Fig. 6a).
The feldspar-rich layer consists mostly of albitic
plagioclase and interstitial K-feldspar, with some mus-
covite, quartz and opaques (Fig. 6b and c). The inter-
stitial K-feldspars appear to fill the spaces between
monocrystalline or polycrystalline plagioclase frag-
ments. The spaces filled by K-feldspar tend to be at
a high angle to the mylonitic foliation. The albitic
plagioclase grains in the layer, many of them showing
patchy undulatory extinction, are subangular to sub-
rounded in shape and mostly 10–30 Am in length
although remnants of plagioclase porphyroclasts (up
to 100 Am long) occasionally occur in the layer (Fig.
6d). The remnants of plagioclase porphyroclasts show
some zoning patterns in the backscattered SEM
micrograph. However, no chemical zoning was iden-
tified in compositional profiles from electron microp-
robe analysis data. The pressure shadow areas and
extension fractures of the plagioclase porphyroclast
remnants are filled by precipitated K-feldspar.
To check if there is any lattice preferred orienta-
tion of albitic plagioclase in the feldspar-rich layers
Fig. 4. Microstructures of plagioclase porphyroclasts in protomylonite. (a) Backscattered SEM micrograph of K-feldspar precipitation in
pressure shadow and cleavage fractures of a plagioclase porphyroclast. Abbreviations are the same as in Fig. 3. (b) Enlarged backscattered SEM
micrograph of pressure shadow in (a). (c) Backscattered SEM micrograph of K-feldspar precipitation in boudin neck of a plagioclase
porphyroclast. (d) Backscattered SEM micrograph of the feldspar-rich layer showing fine-grained albitic plagioclase with interstitial K-feldspar.
J.-H. Ree et al. / Tectonophysics 407 (2005) 227–237 231
of the protomylonite and mylonite, electron back-
scattered diffraction (EBSD; Dingley and Randle,
1992) analysis was performed at the University of
California at Riverside using a scanning electron
microscope FEG XL-30 equipped with an HKL’s
EBSD system. The spatial resolution of this system
is about 1 Am. We used an accelerating voltage of 20
kV and working distance of 15 mm. To remove
surface damages, specimens were polished using
Fig. 5. (a) Photomicrograph of quartz grains showing subgrain rotation re
protomylonite. Quartz c-axes were measured in the surrounding areas as
the SYTON (a colloidal silica) fluid for chemical–
mechanical polishing (Lloyd, 1987). The specimen
surface was inclined at 708 to the incident beam. All
EBSD patterns of plagioclase were manually indexed
using the HKL’s Channel software. In the pole fig-
ures of Fig. 7, it can be seen that the albitic plagi-
oclase grains in the feldspar-rich layer in both the
protomylonite and mylonite do not show any strong
lattice preferred orientation.
crystallization in protomylonite. (b) Pole figure of quartz c-axes in
well as in the area of (a). Lower-hemisphere, equal-area projection.
Fig. 6. Microstructures of mylonite. (a) Photomicrograph of the alternation of feldspar (F)-, quartz (Q)- and muscovite (M)-rich layers. Cross-
polarized light with both upper and lower polars rotated 458 counterclockwise. (b) Enlargement of feldspar-rich layer. (c) Backscattered SEM
micrograph of (b). (d) Enlargement of (c). Abbreviations are the same as in Fig. 3.
Fig. 7. Pole figures of albitic plagioclase in feldspar-rich layers from (a) mylonite and (b) protomylonite. The north pole corresponds to the
normal to the foliation. The east–west direction corresponds to the lineation. Lower-hemisphere, equal-area projection.
J.-H. Ree et al. / Tectonophysics 407 (2005) 227–237232
Fig. 8. (a) Photomicrograph of quartz grains showing core-and-mantle structure. Cross-polarized light. (b) Pole figure of quartz c-axes in
protomylonite. Lower-hemisphere, equal-area projection.
J.-H. Ree et al. / Tectonophysics 407 (2005) 227–237 233
On the other hand, the quartz grains of the quartz
layers commonly show sweeping undulatory extinc-
tion in the mylonite. The quartz grains of the mylonite
exhibit core-and-mantle structures with the size of
recrystallized grains similar to that of the subgrains
at the mantle of old grains, indicating subgrain rota-
tion recrystallization (Fig. 8a) as in the protomylonite.
The size of the recrystallized quartz grains (5–25 Am)
in the mylonite is, however, much larger than that (2–
6 Am) in the protomylonite. Some boundaries of
quartz are wavy or lobate, indicating local grain
boundary migration. The c-axis fabric of quartz grains
shows a single girdle pattern at a high angle to the
mylonitic foliation as in the protomylonite (Fig. 8b).
Table 1
Major element chemistry of protomylonite (ABY009-129) and
mylonite (PE001-5)
Sample No. ABY009-129 PE001-5
SiO2 71.65 74.63
TiO2 0.02 0.05
Al2O3 15.61 14.44
Fe2O3* 0.51 0.56
MgO 0.06 0.14
MnO 0.02 0.04
CaO 0.55 1.24
K2O 7.00 3.67
Na2O 3.18 3.22
P2O5 0.06 0.08
L.O.I** 0.42 1.25
Total 99.07 99.32
N.B. *: total iron, **: loss of ignition.
3. Bulk and feldspar chemistries
Bulk rock compositions were measured from the
crushed fractions of rock samples using a Philips
PW2405 X-ray fluorescence spectrometer at the
Korea Basic Science Institute. The major element
data for the samples are shown in Table 1. The
chemical compositions of the plagioclase and K-feld-
spar in the two samples were determined by electron
microprobe analysis (EPMA) using a JEOL JX-8600
at Korea University.
When comparing the chemical data of the pro-
tomylonite with those of the mylonite, not much
difference is observed in the SiO2, Al2O3, Fe2O3,
MgO, MnO and Na2O contents. However, the CaO
content in the mylonite is two times higher than that in
the protomylonite, whereas that of K2O in the mylo-
nite is half of that in the protomylonite (Table 1). The
difference in the CaO content reflects the higher pla-
gioclase modal content and occasional post-myloniti-
zation calcite veins in the mylonite. The occurrence of
K-feldspar veins in adjacent outcrops within the shear
zone may account for the K2O escaped from the
studied outcrop system.
The mineral composition of porphyroclastic K-
feldspars both in the protomylonite and mylonite
ranges from Or92 to Or98 that is similar to that of
precipitated K-feldspars (Fig. 9). The chemical com-
position of plagioclases varies from An1 to An15.
Myrmekite plagioclase tends to be more albitic than
porphyroclastic plagioclase in the protomylonite. The
majority of the plagioclase compositions within feld-
spar-rich layers fall within An4 to An10 in the mylo-
nite. The plagioclase compositions in the mylonite
show no systematic difference in myrmekites, frag-
Fig. 9. Chemistry of (a) K-feldspar in protomylonite, (b) plagioclase in protomylonite, (c) K-feldspar in mylonite and (d) plagioclase in
mylonite.
J.-H. Ree et al. / Tectonophysics 407 (2005) 227–237234
ments, porphyroclasts and feldspar-rich layers. The
composition of the interstitial K-feldspars in the feld-
spar-rich layer ranges from Or93 to Or98 in the mylo-
nite (Fig. 9).
The deformation temperatures of the protomylonite
and mylonite are estimated to be 355F44 and
335F48 8C, respectively, with an assumption of 3
kbars pressure, employing the two-feldspar geother-
mometry on neocrystallized plagioclase and adjoining
K-feldspar (Stormer and Whitney, 1985; Green and
Usdansky, 1986), although the temperature estimation
using the two-feldspar geothermometry at these low
temperatures may be inaccurate. The pressure of 3
kbars is assumed because the deformation age of the
Yecheon shear zone is close to a late stage of the
emplacement of granite plutons in the margins of the
shear zone (Kwon and Ree, 1997; Otoh et al., 1999)
and also because the emplacement depth is about 10
to 15 km based on hornblende Al geobarometry (Cho
and Kwon, 1994). Also, the microfabrics of quartz
indicate a greenschist facies condition (see Stipp et al.,
2002) together with feldspar compositions.
4. Discussion
The myrmekite lobes at K-feldspar porphyroclast
faces parallel to the mylonitic foliation described
above are similar to those reported by Simpson and
Wintsch (1989) and Tsurumi et al. (2003), indicating
an interrelationship between deformation and the myr-
mekite-forming reaction. Deformation-induced K-
Fig. 10. Schematic diagram illustrating the reaction and deformation
processes of feldspars.
J.-H. Ree et al. / Tectonophysics 407 (2005) 227–237 235
feldspar replacement by myrmekite plagioclase and
quartz in the Yecheon granitic mylonites can be
expressed as follows:
1:05KAlSi3O8K�feldspar porph
þ 0:95Naþ þ 0:05Ca2þ
¼ Na0:95Ca0:05Al1:1Si2:9O8myrmekite plagioclase
þ 0:2SiO2quartz
þ 1:05Kþ:
ð1Þ
Simpson and Wintsch (1989) suggested that the
aqueous K+ precipitates as K-feldspar in pressure
shadows and fractures, as observed in the mylonites
studied (see Fig. 5). The aqueous Ca2+ and Na+ in
reaction (1) were presumably derived from the circu-
lating fluids. The K+, Na+ and Ca2+ in the circulating
fluids play key roles, not only in the precipitation of
K-feldspar in extensional sites, but also in the myr-
mekite formation and neocrystallization of fine-
grained albitic plagioclase along the shear fractures
of K-feldspar porphyroclasts.
The neocrystallization of the fine-grained albitic
plagioclase only along the shear fractures of the K-
feldspar porphyroclasts suggests that the nucleation
was induced not only by strain energy adjacent to the
shear fractures, but also by chemical free energy due
to compositional disequilibrium between the infil-
trated sodium-rich fluid and host K-feldspar (Stunitz,
1998). The strain energy was presumably produced by
a high dislocation density localized along the shear
fractures as shown by Tullis and Yund (1987), and
Fitz Gerald and Stunitz (1993). The absence of plagi-
oclase neocrystallization along the fractures of plagi-
oclase porphyroclasts implies that the chemical free
energy was too low for nucleation due to only a little
compositional disequilibrium between the infiltrated
sodium-rich fluid and albitic plagioclase host.
Fig. 10 schematically illustrates the deformation
processes of feldspar grains in the mylonitic samples
deformed under middle greenschist-facies conditions.
Feldspar grain-size reduction was induced by fractur-
ing, myrmekite formation and neocrystallization of
albitic plagioclase. With grain-size reduction, fine-
grained feldspar grains constituted feldspar-rich layers
in the mylonite. The albitic plagioclase grains in the
layers show a nearly random lattice preferred orienta-
tion (Fig. 7). The intergranular spaces between the
fine-grained plagioclases filled by interstitial K-feld-
spar tend to be at a high angle to the mylonitic
foliation, resembling grain boundary voids filled by
vapor phase (thus fluid) of octachloropropane in poly-
crystals of octachloropropane deforming dominantly
by grain boundary sliding (Ree, 1994, 2000). Kruse
and Stunitz (1999) also reported similar microstruc-
tures in which hornblende grains nucleated at dilatant
sites between plagioclase grains deformed dominantly
by granular flow in anorthositic mylonites. All these
microfabrics of the feldspar-rich layers in the mylonite
indicate that feldspar grains were deformed domi-
nantly by granular flow (Stunitz and Fitz Gerald,
1993), although minor fracturing still occurred in the
layers.
With the development of compositional layering
and the dominant granular flow of feldspar-rich layers
in the mylonite, it is expected that the strain would be
more distributed in the thin-section scale, resulting in
a lower stress and/or lower strain rate in quartz-rich
layer of the mylonite relative to that of the protomy-
lonite. The lower stress led to the larger grain size of
recrystallized quartz in the mylonite than that in the
protomylonite.
5. Conclusions
1. A significant grain-size reduction of feldspar
occurred due to fracturing, myrmekite formation
and neocrystallization of albitic plagioclase
along the shear fractures of K-feldspar porphyr-
J.-H. Ree et al. / Tectonophysics 407 (2005) 227–237236
oclasts in granitic mylonites of the Yecheon
shear zone deformed under middle greenschist-
facies conditions.
2. Neocrystallization of albitic plagioclase along the
shear fractures of K-feldspar porphyroclasts was
induced by both strain energy adjacent to the
shear fractures and chemical free energy due to
the compositional disequilibrium between Na-rich
fluid and host K-feldspars.
3. As the deformation proceeded, compositional
layering consisting of feldspar-, quartz- and mus-
covite-rich layers developed in the mylonite. The
feldspar-rich layers, composed of fine-grained albi-
tic plagioclase and interstitial K-feldspar, were
deformed dominantly by granular flow, while the
quartz-rich layers were deformed by dislocation
creep.
4. With the layer development in the mylonite, strain
was more distributed among the layers, resulting in
a lower stress and/or lower strain rate in the quartz-
rich layers of the mylonite compared to those of
protomylonite. This led to a larger grain size of
recrystallized quartz in the mylonite than that in the
protomylonite.
Acknowledgements
We thank W.M. Lamb and J. Newman for discus-
sions on feldspar microstructures, and R. Guillemette
and S. Choi for initial electron microprobe analyses
on our samples. We appreciate thorough and construc-
tive reviews by K. Kanagawa, an anonymous referee
and J.-P. Burg which improved the paper significantly.
This research was supported by KOSEF grant R02-
2003-000-10039-0 to JHR.
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