Deformation mechanisms in a high-temperature quartz ... millimetre scale partitioning of crystal...
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Tectonophysics, 140 (1987) 297-305
Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
Deformation mechanisms in a high-temperature quartz-feldspar mylonite: evidence for superplastic
flow in the lower continental crust
J.H. BEHRMANN ’ and D. MAINPRICE 2
’ Institut fti Geowissenschaften und Lithosphiirenforschung, Universitiit Giessen, Senckenbergstr. 3, D-6300 Giessen (West Germany)
2 Luboratoire de Tectonophysique, Universite des Sciences et Techniques du Lmguedoc, Place Eug&te Bqtaillon,
F-34060 Montpellier cedex (France)
(Received March 24,1986; revised version accepted January 13,1987)
Behnnann, J.H. and Mainprice, D., 1987. Deformation mechanisms in a high-temperature quartz-feldspar mylonite:
evidence for superplastic flow in the lower continental crust. Tectonophysics, 140: 297-305.
Microstructures and crystallographic preferred orientations in a fme-grained banded quartz-feldspar mylonite were
studied by optical microscopy, SEM, and TEM. Mylonite formation occurred in retrograde amphibohte facies
metamorphism. Interpretation of the microstructures in terms of deformation mechanisms provides evidence for
millimetre scale partitioning of crystal plasticity and superplasticity. Strain incompatibilities during grain sliding in the
superplastic quartz-feldspar bands are mainly accommodated by boundary diffusion of potassic feldspar, the rate of
which probably controls the rate of superplastic deformation.
There is evidence for equal flow stress levels in the superplastic and crystal-plastic domains. In this case mechanism
partitioning results in strain-rate partitioning. Fast deformation in the superplastic bands therefore dominates flow,
and deformation is probably best modelled by a superplastic law.
If this deformational behaviour is typical, shearing in mylonite zones of the lower continental crust may proceed at
exceptionally high rates for a given differential stress, or at low differential stresses in case of fixed strain rates.
Introduction elongate quartz-ribbons anastomosing around un- deformed or only slightly deformed feldspars.
The deformation mechanics of monomineralic Crystal plasticity has become known as a com- quartzite has become reasonably well understood paratively “hard” deformation mechanism. This is in both, experimental (e.g., Tullis et al., 1973; underlined for quartz by the deformation mecha- Koch et al., 1980) and natural creep (e.g., Mitra, nism map of Rutter (1976, figs. 7, 9) as well as by 1976; White, 1976; Bouchez, 1977; Behrmann, palaeostress indicators and their calibrations (e.g., 1985). Crystal plasticity has been identified as an Mercier et al., 1977; Christie and Ord, 1980; Ord important mechanism, and there is evidence in the and Christie, 1984). If crystal plasticity of quartz literature (Bossiere and Vauchez, 1978; Berth6 et is a dominant mechanism in granitoid rocks at al., 1979; Watts and Williams, 1979) that crystal high temperatures, a considerable flow strength plasticity of quartz is one of the main factors must be assigned io the quartzo-feldspathic lower controlling the deformation of quartzo-feldspathic continental crust. For geologically reasonable granitoid rocks. Deformation leads to the familiar strain rates (lo-l3 to lo-l4 s-i) this may be in mesoscopic augen-gneiss structure formed by the order of 1 kbar (e.g., Parrish et al., 1976). In
0040-1951/87/$03.50 0 1987 Elsevier Science Publishers B.V.
amphibolite grade conditions feldspar ductility
seems to be present (see discussion by Simpson,
1985) but appears to be limited as shown by the
observed finite shape modifications of igneous
Crystal plasticity usually results in dynamic
grain-size reduction. In greenschist grade defor-
mation of quartzite, dynamic recrystallization to
grain sizes smaller than 10 microns results in a
deformation mechanism switch from crystal plas-
ticity to superplasticity (Behrmann, 1985) pro-
vided that the volume fraction of recrystallized
grains is large enough (70-80%) (Mainprice, 1981).
This mechanism change (see S&mid, 1982 for
review) has the consequence of reducing flow
strength. Medium- to high-grade gneisses rarely
show evidence of extensive deformation induced
grain refinement. At first sight this makes super-
plasticity somewhat hard to conceive as a defor-
mation mechanism at high-metamorphic grades.
In fact microstructural evidence for superplasticity
in quartz-rich rocks has so far exclusively been
described from sub-greenschist to greenschist (Al-
lison et al., 1979; S&mid et al., 1981; Behrmann,
1985) or blueschist (Rubie, 1981) grade deforma-
tion. However, the question whether granitoid
rocks can be superplastic or not in high-grade
metamorphism is critical to our understanding of
the mechanical state of the lower continental crust.
This study expands on some observations made
by Allison et al. (1979) on a granite deformed in
nature at low (200”-300°C) temperature. The
authors inferred superplastic behaviour from a
fine-grained albite microstructure formed in pres-
sure shadows of feldspar porphyroclasts. We have
found similar microstructures in a banded
quartz-feldspar mylonite formed in amphibolite-
grade natural shearing and wish to demonstrate:
(1) millimetric layer-by-layer partitioning of crystal
plasticity and superplasticity, and (2) the action of
an unusual accommodation mechanism for grain
sliding in a very-fine-grained mixture of quartz,
potassic feldspar, and plagioclase.
The specimen is an acid orthomylonite from
the Aurela Group granulites in Cucamonga
Canyon. The canyon transects the eastern San
Gabriel Mountains near Los Angeles, California.
Detailed accounts of the local geology are given
by Hsti (1955) and Morton (1975, 1976). The
granulites are exposed along the southern margin
of the range, which has suffered a vigorous recent
uplift along the E-W trending Cucamonga fault
zone. The pre-uplift history of the Aurela Group
consists of granulite-grade metamorphism of a
sequence of igneous and sedimentary rocks of
unknown age. This was followed by locally intense
retrograde shearing under amphibolite-grade con-
ditions (Hsi.i, 1955). The latter deformation is re-
sponsible for pervasive mylonitization in the
southern San Gabriel Mountains.
Mesoscopically the sample is a banded quartz-
potassic feldspar-plagioclase mylonite with a platy
foliation and a strong stretching lineation. The
foliation dips north at a moderate angle, and the
stretching lineation plunges towards 290’ at a
shallow angle. The foliation is defined by thin
(< 2 mm) ribbons of quartz, and by slightly
flattened feldspar porphyroclasts up to 1 cm in
size. The stretching lineation is due to elongation
of quartz aggregates, and corresponds to the long
axes of pressure shadows around feldspar
porphyroclasts. We interpret the foliation as ap-
proximating to the XY plane, and the lineation as
representing the X direction of finite mylonitic
The mylonite is composed of approximately
equal proportions of quartz, potassic-feldspar, and
plagioclase. Subsidiary minerals are a few flakelets
of brown biotite, and zircon. Most of the quartz is
contained in discrete ribbons built up of equant to
subequant grains 40-100 pm in diameter (Table
1). The ribbons are between 0.1 and 2 mm thick,
and are separated by thin (typically 20-100 pm)
continuous bands of very-fine-grained (< 10 pm)
equiaxial quartz and feldspar (Fig. la). Most fine
bands are connected with pressure shadows of
potassic feldspar or plagioclase porphyroclasts
(Fig. la). This suggests that the bands are created
at the porphyroclasts, and are simply rolled out by progressive deformation. This mechanism was ad- vocated by Boullier and Gueguen (1975) to create superplastic layers in high-temperature deforma- tion of peridotites and anorthosites.
More instructive information on the petrogra- phy of the fine-grained bands is obtained by back- scattered electron images of polished thin sections
Fig. 1. a. Microstructure of the specimen in polarized light. Nicols crossed, scale bar is 1 mm. For explanation see te xt. b. SEM back :sc tattered electron image of a polished petrographic thin section. Approximately same scale as Fig. la. Contrasts in a, lerage
elemental numbers allow to visualize plagioclase (medium grey), quartz (dark prey), and potassic feldspar (whitish prey).
Parameters of quartz microstructure
Recrystallized grain size Free dislocation density
(diameter in pm) (number of lines x 10 cm)
d 0 n P 0 m
Coarse bands 85.3 48.2 150 3.03 1.26 82
Fine bands 11.0 3.3 150 2.77 1.39 51
Legend: d =