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Home , Deformation mechanism, Mylonite, Porphyroclast, Rock mechanics

Tectonophysics, 140 (1987) 297-305 297 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

Deformation mechanisms in a high- quartz- : 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. mechanisms in a high-temperature quartz-feldspar mylonite: evidence for superplastic flow in the lower continental crust. , 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 . Interpretation of the microstructures in terms of deformation mechanisms provides evidence for millimetre scale partitioning of and . Strain incompatibilities during grain sliding in the superplastic quartz-feldspar bands are mainly accommodated by boundary of potassic feldspar, the rate of which probably controls the rate of superplastic deformation. There is evidence for equal flow 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 . The deformation mechanics of monomineralic Crystal plasticity has become known as a com- quartzite has become reasonably well understood paratively “hard” . This is in both, experimental (e.g., Tullis et al., 1973; underlined for quartz by the deformation mecha- Koch et al., 1980) and natural (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 , 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 grade conditions feldspar 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 are given observed finite shape modifications of igneous by Hsti (1955) and Morton (1975, 1976). The . 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 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- and a strong stretching . 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 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 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 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 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 of tensional subgrain rotation process, but probably by nuclea- failure. This provides direct evi- tion and growth of new grains at sites of high dence that grain sliding deformation has taken strain energy. Both types of feldspar build up place, and was restricted to the fine grained bands. rims (Fig. 2a). The rims can be There is a conspicuous absence of in traced into the fine grained bands, suggesting that the fine-grained feldspars (Fig. 3d) indicating that recrystallization was a syntectonic process. Pres- dislocation glide was not active. Plagioclase grains sure shadow fillings are dominated by orthoclase are square shaped parallel to albite or pericline plus subordinate quartz (Figs. 2a, 2b). In the fine growth twins, indicating rapid crystallization. bands most quartz and plagioclase appears as convex-shaped subequant grains (Figs. 2c, 2d) Fabrics whereas alkali feldspar often forms interstitial films only a few microns thick between plagioclase Orientations of quartz-c-axes were measured in and quartz grains (Fig. 2d). the coarse-grained pure quartz ribbons (Fig. 5a) In the transmission electron microscope quartz and in the fine-grained bands (Fig. 5b). The coarse shows high unbound dislocation densities (Table quartz shows a strong preferred alignment along a 1, Figs. 3a, 3b) and occasional subgrain walls in girdle roughly orthogonal to the stretching linea- both, coarse-g&red quartz ribbons and fine- tion, and at a right angle to the foliation trace. On grained bands indicating active glide and climb geometrical grounds such fabrics can be related to mechanisms. Slip system analysis done on disloca- deformation by intracrystalline slip on first order tions in a few coarse quartz grains reveals domi- prism (lOjO), rhomb (loil), (Olil), and basal nant (liO1) (a) glide, as shown by the presence of (0001) planes in (a) directions (Lister et al., 1978; (2iiO) tilt walls (Fig. 3b), which correlates well Bouchez and Pecher, 1981; S&mid and Casey, with the information from the c-axis preferred 1986). The slip system interpretation of the orientation pattern (see next paragraph). Further- is similar to that of the dislocation geometries (see more many dislocations were found to be in the previous paragraph) suggesting that unbound dis- edge orientation, presumably left behind by more locations and preferred orientation are the prod- mobile screw dislocations which cross-slipped on ucts of the same deformation. Genesis of a strong glide planes co-zonal with the (a) glide direction. fabric requires large strains (shortenmg parallel Fig. 2. SEM backscattered electron images of microstructural details. a. Feldspar porphyroclasts with marginal recrystallization. Note composite alkali-feldspar-plagioclase rims. b. Tail end of a plagioclase porphyroclast. Massive alkali-feldspar precipitation is evident in the centre of the pressure fringe. The periphery of the fringes is made up of a very fine “eutectoid” mixture of quartz, plagioclase, and alkali feldspar. c. Close-up picture of a fine-grained band. Note convex quartz and plagioclase grains and concave interstitial alkali feldspar near the centre of the micrograph. d. Alkali feldspar films between quartz and plagicclase grains. Near the top of the micrograph alkali feldspar fills a pull-apart zone that has formed by unconstrained grain-boundary sliding. The horizontal stripes in the micrographs are artefacts due to inadequate image signal processing. All micrographs show sections perpendicular to the foliation, and parallel to the hneation.

2 > 30%) (Lister and Hobbs, 1980) so that a case nism. Initially the ribbons may have been coarse can be made for a relation between dislocation single grains or aggregates of quartz. During de- substructure, fabric, and mylonitization, although formation to large strains they suffered dynamic the dislocations just contain a record of the last recrystallization to build the observed microstruc- deformation increment (possibly about 2%). In the ture of subequant grains. The high density of free fine bands (Fig. 5b) there is no marked preferred dislocations and the subgrain walls may be seen as orientation of quartz-c-axes. remainders of this deformation. The microstructures in the very-fine-grained Creep mechanism interpretation quartz-feldspar bands appear to reflect a more complex situation. The high defect density (Table The microstructure and crystallographic pre- 1) in quartz indicates that dislocation processes ferred orientation data from the coarse-grained within the quartz grains were an important de- quartz ribbons make a convincing case for disloca- formation mechanism. However, here intracrystal- tion processes as dominant deformation mecha- line slip does not result in a preferred orientation. 302

Fig. 3. TEM micrographs, all taken in bright field at 120 kV. a. Typical dislocation substructure of the coarse-grained quartz. Most dislocations are viewed line-on, and are parallel to the trace of a rhomb plane (= Tr (loll)), which in turn is sub-parallel to the foliation trace (Tr. Fol). b. Prismatic (2iiO) subboundaries in coarse-grained quartz indicating the operation of dynamic recovery and r-rhomb slip in the (a) direction. Note that dislocations in tilt walls are parallel to [Oi12]. c. Gram boundary in fine quartz decorated with sub-micron size voids. d. Albite growth twin in a plagioclase of albitic composition. (An&n,,). The fine lines are spinoidal exsolution. Diffracting vector g = 402. AU micrographs were taken from thin foils oriented perpendicular to the foliation, and parallel to the lineation.

One way to explain this is to allow external grain (- 10 pm) grain size and the absence of a shape rotations to overprint any rotation of crystal axes, fabric (see Edington et al., 1976) support this as would be the case in grain-boundary sliding. interpretation. The defect densities in the coarse kbsence of preferred orientation has been in- and fine quartz are comparable. This can be in- terpreted in this way before (Starkey and Cutforth, terpreted as reflecting equal levels of late flow 1978; Boullier and Gueguen, 1985) but note that stresses. In this case, strain is concentrated in the this testifies only to the absence of an orienting fine-grained domains, and description of flow in mechanism, and is not proof of grain-boundary the whole specimen as superplastic may be valid sliding. Conclusive evidence for grain sliding comes as a first order approximation. from widespread grain boundary failure (see para- The intracrystalline substructure of the fine graph on TEM microstructure). Grain boundary grained feldspars shows no widespread evidence sliding is the dominant strain producing mecha- of dislocation processes, but there are indications nism in superplastic flow, suggesting that the fine for chemical mobility of both types of feldspar bands have deformed by this mechanism. The fine during deformation. Thus any shape changes the 303

f Discussion and conclusion

4 In the introductory section we have hinted at the apparent difficulty for quartz-rich rocks to 2 acquire superplasticity at medium or high meta- lineation morphic grades. This is true for rocks that owe 0 - their ultra-fine-grained syntectonic microstructure to recrystallization operated by subgrain rotation 2 (e.g., Poirier and GuillopC, 1979). In this case recrystallized grain size is inversely proportional 4 to differential stress magnitude. Clearly this argu- n=20 ment does not hold for syntectonic recrystalliza- fW tion by nucleation and growth of entirely new Fig. 4. Rose diagram to show the orientations of void-bearing crystals, as is the case in the ultra-fine-grained grain boundaries in the ultra-fine-grained bands, relative to the bands. Here the controlling variables for recrys- orientations of foliation and stretching lineation. f is the tallization are provided by reaction rates and the orientation frequency in a 10 o sector. Twenty grain boundaries were surveyed. diffusivity of the deforming aggregate. Conse- quently this type of “transformational” superplas- ticity can be acquired by deforming rocks irre- feldspars have to undergo when sliding past each spective of the stress or temperature regime. The other are likely to be overcome by boundary presence of the thin intergranular films of potassic mechanisms (Langdon, 1985). The predominance feldspar is likely to prevent dynamic grain coars- of potassic feldspar in pressure shadows around ening and serves to maintain a stable ultra-fine- both types of feldspar porphyroclasts suggests that grained microstructure. Superplasticity may then syntectonic boundary diffusion of potassic feld- be considered as a steady state deformation pro- spar must be the most efficient and therefore cess. Note that the rate limit is not provided by fastest mechanism to correct strain incompatibili- the viscous resistance to grain boundary sliding, ties in the deforming . The same is indicated but by the rate at which the fastest strain accom- by the alkali feldspar “films” between quartz and modation process (boundary diffusion of alkali plagioclase grains in the fine-grained bands.. At feldspar) can operate. least in a qualitative sense this answers the ques- Strain localization in the lower continental crust tion concerning the rate limiting step in superplas- has been recognized by geophysical means (Zoback tic deformation of the fine-grained bands. et al., 1985), and is evident from the existence of

stretching ~tion Y

coarse quartz fine quartz

Fig. 5. Quartz-c-axis fabric diagrams. 150 c-axes each, lower hemisphere, equal area projections. For discussion see text. 304 localized ductile zones in exhumed high- Bouchez, J.L., 1977. Plastic deformation of quartzites at low grade metamorphic . Kirby (1985) advo- temperature in an area of natural strain gradient. cates dynamic recrystallization and reaction Tectonophysics, 39: 25-50. Bouchez, J.L. and Pecher, A., 1981. The Himalayan Main softening as major causes for this. We are now in Central Thrust pile and its quartz-rich in central a position to add transformational superplasticity Nepal Tectonophysics, 78: 23-50. as a third possibility. As a consequence, flow in BouIIier, A.M. and Gueguen, Y., 1975. SP-. Origin of lower crustal mylonites may be governed by non- some mylonites by superplastic flow. Contrib. . linear creep with a very low stress exponent, and Petrol., 50: 93-105. Christie, J.M. and Ord, A., 1980. Flow stress from microstruc- may be highly sensitive to grain size. If pre-super- tures of mylonites: example and current assessment. J. plasticity stresses are dynamically maintained, Geophys. Res., 85: 6253-6262. creep rates will be increased, or alternatively Edington, J.W., Melton, K.N. and Cutler, C.P., 1976. Super- deformation at geologically reasonable rates can plasticity. Progr. Mater. Sci., 21: 63-170. be maintained at comparatively low differential England, P., 1982. Some numerical investigations of large scale stresses. The potential for drastic local changes in continental deformation. In: K.J. Hsii (Editor), Mountain BuiIding Processes. Academic Press, London, pp. 129-139. stress exponents also rules out uniform power-law Hsii, K.J., 1955. Gram&es and mylonites of the region about creep models as adequate descriptions of plate- Cucamonga and San Antonio Canyons, California. Univ. scale deformation (see England, 1982). The data Cahf., Publ. Geol. Sci., 30: 223-352. presented here are observational, and cannot con- Kirby, S.H., 1985. observations pertinent to strain quantitative flow models for superplasticity the of the continental lithosphere and the locahza- tion of strain along shear zones. Tectonophysics, 119: l-27. in the lower crust, but they point out two indis- Koch, P.S., Christie, J.M. and George, R.P., 1980. FIow law of pensable variables: grain size and potassic feld- “wet” quartzite in the alpha quartz field. Eos, Trans. Am. spar diffusivity as a rate limiting process. Geophys. Union, 61: 376. Langdon, T.G., 1985. Regimes of plastic deformation. In: Acknowledgements Wenk, H.R. (Editor), Preferred orientation in deformed metals and rocks: an introduction to modern texture anaIy- sis. Academic Press, London, pp. 219-232. J.H.B. thanks Art Snoke, Vicky Todd, and Jan Lister, G.S. and Hobbs, B.E., 1980. The simulation of fabric Tullis for a most inspiring field trip during the development during plastic deformation and its application GSA Penrose conference on mylonites at San to quartzite: the influence of deformation history. J. Struct. Diego, and B. Stoeckhert for a stimulating debate GeoI., 2: 355-370. on superplasticity. Travelling support was pro- Lister, G.S., Paterson, M.S. and Hobbs, B.E., 1978. The simu- vided by The Queen’s College, Oxford. TEM mi- lation of fabric development in plastic deformation, and its application to quartz&e: the model. Tectonophysics, 45: croscopy by D.M. was funded by the Centre Na- 107-158. tionale de la Recherche Scientifique. Mainprice, D.H., 1981. The experimental deformation of quartz polycrystals. Ph.D. Thesis, 171 pp., Aust. Nat]. Univ., References Canberra. Mercier, J.C., Anderson, D.A. and Carter, N.L., 1977. Stress in

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