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  • Tectonophysics, 140 (1987) 297-305

    Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

    297

    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)

    Abstract

    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.

  • 298

    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

    crystals.

    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

    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

    deformation.

    Observations

    Optical petrography

    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

  • 299

    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.

    Electron petrography

    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).

  • 300

    TABLE 1

    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 =