Grain-size reduction of feldspars by fracturing and neocrystallization

Tectonophysics 407 (2005) 227 – 237
www.elsevier.com/locate/tecto
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 Korea
Department of Earth Science Education, Kyungpook National University, Daegu 702–701, South Korea
Institute of Geophysics and Planetary Physics, University of California, Riverside, California 92521, USA
b
c
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 neocrystallization 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 rheology of deforming rocks due to a switch in the domi-
* Corresponding author. Tel.: +82 2 3290 3175; fax: +82 2 3290
3189.
E-mail address: [email protected] (J.-H. Ree).
0040-1951/$ - see front matter D 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.tecto.2005.07.010
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 subducting slab as a whole causing a change in the mantle
convection mode (e.g., Karato et al., 1998; Čižková et
al., 2002). Grain-size reduction also facilitates reactions by increasing the surface area (e.g., Evans, 1988)
or may lead to change in magnetic properties (e.g.,
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J.-H. Ree et al. / Tectonophysics 407 (2005) 227–237
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 Karato, 2001).
Grain-size reduction can be achieved by cataclasis,
reaction, dynamic recrystallization and phase transformation. In feldspar, the most abundant crustal mineral,
grain-size reduction may be produced by cataclasis
and neocrystallization of albitic plagioclase from feldspar porphyroclasts under greenschist facies conditions (Fitz Gerald and Stünitz, 1993). Deformationinduced 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 fractures and myrmekite formation contribute to significant 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.
minerals (Fig. 1a). The modal percentage of the constituent minerals was obtained by point counting on
thin-sections stained with Na-cobaltinitrite (for K-feldspar) 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 porphyroclasts 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-
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 mylonite, 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
Fig. 1. Photomicrographs of granitic (a) protomylonite and (b)
mylonite from the Yecheon shear zone. Cross-polarized light.
J.-H. Ree et al. / Tectonophysics 407 (2005) 227–237
latory extinction. Most of the intragranular/transgranular 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 Kfeldspar, and are filled by quartz, muscovite or chlorite. 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 fivegrains (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
229
to have some zoning patterns. However, no compositional 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 myrmekitic plagioclases are albite (An1–12), as shown later.
The myrmekite lobes commonly embay the margins
of K-feldspar porphyroclasts. The myrmekite formation together with neocrystallization of albitic plagioclase and fracturing significantly reduces the Kfeldspar 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-
Fig. 2. Microstructures of K-feldspar porphyroclasts in protomylonite. (a) Backscattered SEM micrograph of a fractured K-feldspar porphyroclast along antithetic shears. The difference in gray level of the fractured K-feldspar porphyroclast is due to an irregular carbon coating. Kfs: Kfeldspar. Pl: plagioclase. Qtz: quartz. (b) Neocrystallization of albitic plagioclase along a shear fracture within a K-feldspar porphyroclast. Crosspolarized light. Inset: c. (c) Enlargement of neocrystallized albitic plagioclase in (b). (d) Backscattered SEM micrograph of (c).
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J.-H. Ree et al. / Tectonophysics 407 (2005) 227–237
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
Fig. 3. (a) Photomicrograph of myrmekite in protomylonite. Crosspolarized light. (b) Backscattered SEM micrograph of myrmekite.
Kfs: K-feldspar. Pl: plagioclase. Qtz: quartz. Ms: muscovite.
spar precipitates with some quartz and muscovite, and
some fractured fragments of plagioclase are present
(Fig. 4).
Incipient development of alternating layers of finegrained 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
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-mylonitization 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 muscovite, quartz and opaques (Fig. 6b and c). The interstitial K-feldspars appear to fill the spaces between
monocrystalline or polycrystalline plagioclase fragments. 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 subrounded 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 identified in compositional profiles from electron microprobe 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 orientation of albitic plagioclase in the feldspar-rich layers
J.-H. Ree et al. / Tectonophysics 407 (2005) 227–237
231
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.
of the protomylonite and mylonite, electron backscattered 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
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 figures of Fig. 7, it can be seen that the albitic plagioclase grains in the feldspar-rich layer in both the
protomylonite and mylonite do not show any strong
lattice preferred orientation.
Fig. 5. (a) Photomicrograph of quartz grains showing subgrain rotation recrystallization in protomylonite. (b) Pole figure of quartz c-axes in
protomylonite. Quartz c-axes were measured in the surrounding areas as well as in the area of (a). Lower-hemisphere, equal-area projection.
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J.-H. Ree et al. / Tectonophysics 407 (2005) 227–237
Fig. 6. Microstructures of mylonite. (a) Photomicrograph of the alternation of feldspar (F)-, quartz (Q)- and muscovite (M)-rich layers. Crosspolarized 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) 227–237
233
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.
On the other hand, the quartz grains of the quartz
layers commonly show sweeping undulatory extinction 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 rotation 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).
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-feldspar 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 protomylonite 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 mylonite is half of that in the protomylonite (Table 1). The
difference in the CaO content reflects the higher plagioclase modal content and occasional post-mylonitization 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 Kfeldspars both in the protomylonite and mylonite
ranges from Or92 to Or98 that is similar to that of
precipitated K-feldspars (Fig. 9). The chemical composition 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 feldspar-rich layers fall within An4 to An10 in the mylonite. The plagioclase compositions in the mylonite
show no systematic difference in myrmekites, frag-
Table 1
Major element chemistry of protomylonite (ABY009-129) and
mylonite (PE001-5)
Sample No.
ABY009-129
PE001-5
SiO2
TiO2
Al2O3
Fe2O3*
MgO
MnO
CaO
K2O
Na2O
P2O5
L.O.I**
Total
71.65
0.02
15.61
0.51
0.06
0.02
0.55
7.00
3.18
0.06
0.42
99.07
74.63
0.05
14.44
0.56
0.14
0.04
1.24
3.67
3.22
0.08
1.25
99.32
N.B. *: total iron, **: loss of ignition.
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J.-H. Ree et al. / Tectonophysics 407 (2005) 227–237
Fig. 9. Chemistry of (a) K-feldspar in protomylonite, (b) plagioclase in protomylonite, (c) K-feldspar in mylonite and (d) plagioclase in
mylonite.
ments, porphyroclasts and feldspar-rich layers. The
composition of the interstitial K-feldspars in the feldspar-rich layer ranges from Or93 to Or98 in the mylonite (Fig. 9).
The deformation temperatures of the protomylonite
and mylonite are estimated to be 355 F 44 and
335 F 48 8C, respectively, with an assumption of 3
kbars pressure, employing the two-feldspar geothermometry 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 myrmekite-forming reaction. Deformation-induced K-
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:
1:05KAlSi3 O8 þ 0:95Naþ þ 0:05Ca2þ
Kfeldspar porph
¼ Na0:95 Ca0:05 Al1:1 Si2:9 O8 þ 0:2SiO2 þ 1:05Kþ :
myrmekite plagioclase
quartz
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 circulating 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 myrmekite formation and neocrystallization of finegrained 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 Kfeldspar 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 infiltrated sodium-rich fluid and host K-feldspar (Stünitz,
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 Stünitz (1993). The absence of plagioclase neocrystallization along the fractures of plagioclase 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 fracturing, myrmekite formation and neocrystallization of
albitic plagioclase. With grain-size reduction, finegrained feldspar grains constituted feldspar-rich layers
in the mylonite. The albitic plagioclase grains in the
layers show a nearly random lattice preferred orientation (Fig. 7). The intergranular spaces between the
fine-grained plagioclases filled by interstitial K-feldspar tend to be at a high angle to the mylonitic
Fig. 10. Schematic diagram illustrating the reaction and deformation
processes of feldspars.
foliation, resembling grain boundary voids filled by
vapor phase (thus fluid) of octachloropropane in polycrystals of octachloropropane deforming dominantly
by grain boundary sliding (Ree, 1994, 2000). Kruse
and Stünitz (1999) also reported similar microstructures 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 dominantly by granular flow (Stünitz 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 protomylonite. 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-
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J.-H. Ree et al. / Tectonophysics 407 (2005) 227–237
oclasts in granitic mylonites of the Yecheon
shear zone deformed under middle greenschistfacies 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 muscovite-rich layers developed in the mylonite. The
feldspar-rich layers, composed of fine-grained albitic 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 quartzrich 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 discussions on feldspar microstructures, and R. Guillemette
and S. Choi for initial electron microprobe analyses
on our samples. We appreciate thorough and constructive reviews by K. Kanagawa, an anonymous referee
and J.-P. Burg which improved the paper significantly.
This research was supported by KOSEF grant R022003-000-10039-0 to JHR.
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