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 = 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 the micrographs, quartz appears dark grey, plagioclase is medium grey, and potassic feldspar is whitish grey, as verified by the qualitative EDAX analyses in Fig. lb. Going into more detail, fine- grained non-isochemical marginal recrystallization of the feldspar porphyroclasts is evident. This indicates that feldspar recrystallization is not by a subgrain rotation process, but probably by nuclea- tion and growth of new grains at sites of high strain energy. Both types of feldspar build up porphyroclast rims (Fig. 2a). The rims can be traced into the fine grained bands, suggesting that recrystallization was a syntectonic process. Pres- sure shadow fillings are dominated by orthoclase plus subordinate quartz (Figs. 2a, 2b). In the fine bands most quartz and plagioclase appears as convex-shaped subequant grains (Figs. 2c, 2d) whereas alkali feldspar often forms interstitial films only a few microns thick between plagioclase and quartz grains (Fig. 2d).

In the transmission electron microscope quartz shows high unbound dislocation densities (Table 1, Figs. 3a, 3b) and occasional subgrain walls in both, coarse-g&red quartz ribbons and fine- grained bands indicating active glide and climb mechanisms. Slip system analysis done on disloca- tions in a few coarse quartz grains reveals domi- nant (liO1) (a) glide, as shown by the presence of (2iiO) tilt walls (Fig. 3b), which correlates well with the information from the c-axis preferred orientation pattern (see next paragraph). Further- more many dislocations were found to be in the edge orientation, presumably left behind by more mobile screw dislocations which cross-slipped on glide planes co-zonal with the (a) glide direction.

Quartz-quartz grain boundaries in the coarse rib- bons show little or no creep damage (Ashby and Jones, 1980), whereas some grain boundaries in the fine bands are richly decorated with small elliptical voids (Fig. 3~). Void-bearing grain boundaries are predominantly oriented at high angles to the foliation and the stretching direction (Fig. 4), supporting an interpretation of tensional grain boundary failure. This provides direct evi- dence that grain sliding deformation has taken place, and was restricted to the fine grained bands. There is a conspicuous absence of dislocations in the fine-grained feldspars (Fig. 3d) indicating that dislocation glide was not active. Plagioclase grains are square shaped parallel to albite or pericline growth twins, indicating rapid crystallization.

Fabrics

Orientations of quartz-c-axes were measured in the coarse-grained pure quartz ribbons (Fig. 5a) and in the fine-grained bands (Fig. 5b). The coarse quartz shows a strong preferred alignment along a girdle roughly orthogonal to the stretching linea- tion, and at a right angle to the foliation trace. On geometrical grounds such fabrics can be related to deformation by intracrystalline slip on first order prism (lOjO), rhomb (loil), (Olil), and basal (0001) planes in (a) directions (Lister et al., 1978; Bouchez and Pecher, 1981; S&mid and Casey, 1986). The slip system interpretation of the fabric is similar to that of the dislocation geometries (see previous paragraph) suggesting that unbound dis- locations and preferred orientation are the prod- ucts of the same deformation. Genesis of a strong 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 can be made for a relation between dislocation substructure, fabric, and mylonitization, although the dislocations just contain a record of the last deformation increment (possibly about 2%). In the fine bands (Fig. 5b) there is no marked preferred orientation of quartz-c-axes.

Creep mechanism interpretation

The microstructure and crystallographic pre- ferred orientation data from the coarse-grained quartz ribbons make a convincing case for disloca- tion processes as dominant deformation mecha-

nism. Initially the ribbons may have been coarse single grains or aggregates of quartz. During de- formation to large strains they suffered dynamic recrystallization to build the observed microstruc- ture of subequant grains. The high density of free dislocations and the subgrain walls may be seen as remainders of this deformation.

The microstructures in the very-fine-grained quartz-feldspar bands appear to reflect a more complex situation. The high defect density (Table 1) in quartz indicates that dislocation processes within the quartz grains were an important de- formation mechanism. However, here intracrystal- 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 rotations to overprint any rotation of crystal axes, as would be the case in grain-boundary sliding. kbsence of preferred orientation has been in- terpreted in this way before (Starkey and Cutforth, 1978; Boullier and Gueguen, 1985) but note that this testifies only to the absence of an orienting mechanism, and is not proof of grain-boundary sliding. Conclusive evidence for grain sliding comes from widespread grain boundary failure (see para- graph on TEM microstructure). Grain boundary sliding is the dominant strain producing mecha- nism in superplastic flow, suggesting that the fine bands have deformed by this mechanism. The fine

(- 10 pm) grain size and the absence of a shape fabric (see Edington et al., 1976) support this interpretation. The defect densities in the coarse and fine quartz are comparable. This can be in- terpreted as reflecting equal levels of late flow stresses. In this case, strain is concentrated in the fine-grained domains, and description of flow in the whole specimen as superplastic may be valid as a first order approximation.

The intracrystalline substructure of the fine grained feldspars shows no widespread evidence of dislocation processes, but there are indications for chemical mobility of both types of feldspar during deformation. Thus any shape changes the

303

f

4

2

0

2

4

lineation -

fW n=20

Fig. 4. Rose diagram to show the orientations of void-bearing

grain boundaries in the ultra-fine-grained bands, relative to the

orientations of foliation and stretching lineation. f is the

orientation frequency in a 10 o sector. Twenty grain boundaries

were surveyed.

feldspars have to undergo when sliding past each other are likely to be overcome by boundary mechanisms (Langdon, 1985). The predominance of potassic feldspar in pressure shadows around both types of feldspar porphyroclasts suggests that syntectonic boundary diffusion of potassic feld- spar must be the most efficient and therefore fastest mechanism to correct strain incompatibili- ties in the deforming rock. The same is indicated by the alkali feldspar “films” between quartz and plagioclase grains in the fine-grained bands.. At least in a qualitative sense this answers the ques- tion concerning the rate limiting step in superplas- tic deformation of the fine-grained bands.

Discussion and conclusion

In the introductory section we have hinted at the apparent difficulty for quartz-rich rocks to acquire superplasticity at medium or high meta- morphic grades. This is true for rocks that owe their ultra-fine-grained syntectonic microstructure to recrystallization operated by subgrain rotation (e.g., Poirier and GuillopC, 1979). In this case recrystallized grain size is inversely proportional to differential stress magnitude. Clearly this argu- ment does not hold for syntectonic recrystalliza- tion by nucleation and growth of entirely new crystals, as is the case in the ultra-fine-grained bands. Here the controlling variables for recrys- tallization are provided by reaction rates and the diffusivity of the deforming aggregate. Conse- quently this type of “transformational” superplas- ticity can be acquired by deforming rocks irre- spective of the stress or temperature regime. The presence of the thin intergranular films of potassic feldspar is likely to prevent dynamic grain coars- ening and serves to maintain a stable ultra-fine- grained microstructure. Superplasticity may then be considered as a steady state deformation pro- cess. Note that the rate limit is not provided by the viscous resistance to grain boundary sliding, but by the rate at which the fastest strain accom- modation process (boundary diffusion of alkali feldspar) can operate.

Strain localization in the lower continental crust has been recognized by geophysical means (Zoback 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 shear zones in exhumed high- grade metamorphic terranes. Kirby (1985) advo- cates dynamic recrystallization and reaction softening as major causes for this. We are now in a position to add transformational superplasticity as a third possibility. As a consequence, flow in lower crustal mylonites may be governed by non- linear creep with a very low stress exponent, and may be highly sensitive to grain size. If pre-super- plasticity stresses are dynamically maintained, creep rates will be increased, or alternatively deformation at geologically reasonable rates can be maintained at comparatively low differential stresses. The potential for drastic local changes in stress exponents also rules out uniform power-law creep models as adequate descriptions of plate- scale deformation (see England, 1982). The data presented here are observational, and cannot con- strain quantitative flow models for superplasticity in the lower crust, but they point out two indis- pensable variables: grain size and potassic feld- spar diffusivity as a rate limiting process.

Acknowledgements

J.H.B. thanks Art Snoke, Vicky Todd, and Jan Tullis for a most inspiring field trip during the GSA Penrose conference on mylonites at San Diego, and B. Stoeckhert for a stimulating debate on superplasticity. Travelling support was pro- vided by The Queen’s College, Oxford. TEM mi- croscopy by D.M. was funded by the Centre Na- tionale de la Recherche Scientifique.

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