Deformation Mechanisms in a High-Temperature Quartz-Feldspar Mylonite: Evidence for Superplastic Flow in the Lower Continental Crust
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Tectonophysics, 140 (1987) 297-305 297 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) 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 Canyon. The canyon transects the eastern San seems to be present (see discussion by Simpson, Gabriel Mountains near Los Angeles, California. 1985) but appears to be limited as shown by the Detailed accounts of the local geology are given observed finite shape modifications of igneous by Hsti (1955) and Morton (1975, 1976). The crystals. granulites are exposed along the southern margin Crystal plasticity usually results in dynamic of the range, which has suffered a vigorous recent grain-size reduction. In greenschist grade defor- uplift along the E-W trending Cucamonga fault mation of quartzite, dynamic recrystallization to zone. The pre-uplift history of the Aurela Group grain sizes smaller than 10 microns results in a consists of granulite-grade metamorphism of a deformation mechanism switch from crystal plas- sequence of igneous and sedimentary rocks of ticity to superplasticity (Behrmann, 1985) pro- unknown age. This was followed by locally intense vided that the volume fraction of recrystallized retrograde shearing under amphibolite-grade con- grains is large enough (70-80%) (Mainprice, 1981). ditions (Hsi.i, 1955). The latter deformation is re- This mechanism change (see S&mid, 1982 for sponsible for pervasive mylonitization in the review) has the consequence of reducing flow southern San Gabriel Mountains. strength. Medium- to high-grade gneisses rarely Mesoscopically the sample is a banded quartz- show evidence of extensive deformation induced potassic feldspar-plagioclase mylonite with a platy grain refinement. At first sight this makes super- foliation and a strong stretching lineation. The plasticity somewhat hard to conceive as a defor- foliation dips north at a moderate angle, and the mation mechanism at high-metamorphic grades. stretching lineation plunges towards 290’ at a In fact microstructural evidence for superplasticity shallow angle. The foliation is defined by thin in quartz-rich rocks has so far exclusively been (< 2 mm) ribbons of quartz, and by slightly described from sub-greenschist to greenschist (Al- flattened feldspar porphyroclasts up to 1 cm in lison et al., 1979; S&mid et al., 1981; Behrmann, size. The stretching lineation is due to elongation 1985) or blueschist (Rubie, 1981) grade deforma- of quartz aggregates, and corresponds to the long tion. However, the question whether granitoid axes of pressure shadows around feldspar rocks can be superplastic or not in high-grade porphyroclasts. We interpret the foliation as ap- metamorphism is critical to our understanding of proximating to the XY plane, and the lineation as the mechanical state of the lower continental crust. representing the X direction of finite mylonitic This study expands on some observations made deformation. by Allison et al. (1979) on a granite deformed in nature at low (200”-300°C) temperature. The Observations authors inferred superplastic behaviour from a fine-grained albite microstructure formed in pres- Optical petrography sure shadows of feldspar porphyroclasts. We have found similar microstructures in a banded The mylonite is composed of approximately quartz-feldspar mylonite formed in amphibolite- equal proportions of quartz, potassic-feldspar, and grade natural shearing and wish to demonstrate: plagioclase. Subsidiary minerals are a few flakelets (1) millimetric layer-by-layer partitioning of crystal of brown biotite, and zircon. Most of the quartz is plasticity and superplasticity, and (2) the action of contained in discrete ribbons built up of equant to an unusual accommodation mechanism for grain subequant grains 40-100 pm in diameter (Table sliding in a very-fine-grained mixture of quartz, 1). The ribbons are between 0.1 and 2 mm thick, potassic feldspar, and plagioclase. and are separated by thin (typically 20-100 pm) continuous bands of very-fine-grained (< 10 pm) The specimen equiaxial quartz and feldspar (Fig. la). Most fine bands are connected with pressure shadows of The specimen is an acid orthomylonite from potassic feldspar or plagioclase porphyroclasts the Aurela Group granulites in Cucamonga (Fig. la). This suggests that the bands are created 299 at the porphyroclasts, and are simply rolled out by Electron petrography progressive deformation. This mechanism was ad- vocated by Boullier and Gueguen (1975) to create More instructive information on the petrogra- superplastic layers in high-temperature deforma- phy of the fine-grained bands is obtained by back- tion of peridotites and anorthosites. 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 = average grain diameter, o = standard deviation, p = free dislocation density, n = number of grains measured, M = number of thin foil subareas measured. using a scanning electron microscope (Fig. lb). On Quartz-quartz grain boundaries in the coarse rib- the micrographs, quartz appears dark grey, bons show little or no creep damage (Ashby and plagioclase is medium grey, and potassic feldspar Jones, 1980), whereas some grain boundaries in is whitish grey, as verified by the qualitative EDAX the fine bands are richly decorated with small analyses in Fig. lb. Going into more detail, fine- elliptical voids (Fig. 3~). Void-bearing grain grained non-isochemical marginal recrystallization boundaries are predominantly oriented at high of the feldspar porphyroclasts is evident. This angles to the foliation and the stretching direction indicates that feldspar recrystallization is not by a (Fig. 4), supporting an interpretation