Microstructure development in an experimentally sheared orthopyroxene granulite

l"mllllmlm~t~,~ IIII

ELSEVIER

Tectonophysics256 (1996) 83-100

Microstructure development in an experimentally sheared orthopyroxene granulite John V. Ross, Kenneth R. Wilks Department of Geological Sciences, University of British Columbia, 6339 Stores Road, Vancouver, B.C. V6T IZ4, Canada

Received 20 March 1995; accepted 14 August 1995

Abstract Rheologies of the lower continental crust and overall contrast with the underlying upper mantle likely control the evolution of first-order structures. Stress and strain partitioning between natural shear zones and wall rock(s), foliation development, flow mechanism(s) and constitutive relations of rocks that make up the lower crust are poorly known. We have undertaken an experimental study in which an orthopyroxene granulite is deformed in simple shear. The starting material is a three-phase (plagioclase-orthopyroxene-amphibole) equigranular granulite, previously used in coaxial experiments. Wafers cut at 45 ° to the long axis of cores were placed between Zircoa forcing blocks that have been cut obliquely, at 45°, at one end. All experiments were run at 1.2 GPa (Pc) and temperatures of 650°-950°C. Specimens have been strained up to a maximum of Y = 2.6, at shear strain rates of 7 X 10 -5 to 7 x 10 -7 s - 1. Most shear stress-shear strain curves have a similar form. They are characterised by an initial linear response that is followed by a work-hardening region that leads to a level of quasi-steady state. The quasi-steady-state behaviour is followed by a gradual decrease in flow stress to a second quasi-steady-state stress level, that very gradually decreases with increasing strain. Optical and TEM examination of deformed samples indicates that an amphibole foliation develops during the first quasi-steady-state stress level by fragmentation and alignment of amphibole; plagioclase and orthopyroxene undergo minimal crystal plastic strain. At the onset of the second quasi-steady-state stress level, plagioclase develops neoblasts, orthopyroxene develops clinopyroxene iamellae and the amphibole foliation is better developed. With increasing strain, compositional lamination develops, as in natural mylonites, with extensive neoblast development in plagioclase and orthopyroxene, and cataclastic grain refinement in amphibole. Strength data, measured at the onset of the second quasi-steady-state level, fit a power law with Q = 203 kJ/mole and n = 2.3. These Q and n values are significantly lower than those determined under similar conditions in previous coaxial experiments. With increasing shear strain, as under natural conditions, there is a significant grain-size reduction, that may lead to a switch in flow mechanism from grain-size insensitive flow, at low strain, to grain-size sensitive flow at higher natural strains.

1. Introduction Evolution of many first-order structures is probably influenced by the mechanical behaviour of the continental lower crust (CLC) which contrasts with the mechanical behaviour of the underlying upper

mantle across the Moho transition. Geophysical and geological observations and inferences accumulated to date indicate that the CLC is likely weak compared to the underlying mantle material. These data have most recently been summarised by Kirby and Kronenberg (1987) and Carter and Tsenn 0 9 8 7 ) .

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J.V. Ross, K.R. Wilks / Tectonophysics 256 (1996) 83-100

They both have taken experimental data for mantle rocks, CLC rocks and some mid-crustal lithologies, and extrapolated the respective steady-state flow data to natural strain rates along some specific temperature profiles in order to speculate upon rbeologic profiles through the continental lithosphere. However, most models that estimate the strength of the CLC as functions of depth and temperature in different thermal environments are based on materials such as granite, websterite etc., that are known not to be truly representative of this part of the lithosphere, rather than feldspar-dominated material believed characteristic of CLC regions (see discussion in Carter and Tsenn, 1987). Material more representative of the CLC recognised as granulite terranes have been described from the Pikwitonei and Kapuskasing regions of the Superior and Grenville provinces in Canada and from the Ivrea Zone, northern Italy (Brodie and Rutter, 1987; Rutter and Brodie, 1988). In all of these regions, and especially the latter, detailed field observations have shown that these rocks are frequently characterised by narrow (hundreds of meters) zones of ductile faulting involving dislocation creep, grain-size refinement and perhaps grain-size sensitive diffusional creep. To further understand and obtain rheological data for more realistic CLC material, Wilks and Carter (1990) carried out a series of coaxial high temperature/pressure experiments on samples of granulites. One of these, the Pikwitonei granulite (an amphibolite comprising 50:50 plagioclase feldspar and a magnesio-hornblende) displayed low strengths relative to other rocks comprising the CLC and detachment zones may develop within rocks of similar mineralogy in accordance with the field observations of Brodie and Rutter (1987). More recently, Ross and Wilks (1995) carried out a series of coaxial experiments on samples of an orthopyroxene granulite (35:35:30, plagioclase:orthopyroxene:magnesio-homblende) collected only 15 km from the Pikwitonei amphibolite sample used as the starting material by Wilks and Carter (1990). It is a three-phase granulite, similar in texture, grain-size and fabric to the Pikwitonei amphibolite. Experimental deformation of two-phase aggregates has shown that strain is always partitioned disproportionately within the weaker phase, even in aggregates comprising only 10-20% by volume of the weaker phase

(Price, 1982; Jordan, 1987; Ross et al., 1987; Ross and Bauer, 1992). While the stronger phase may initially form the load-bearing framework, the weaker phase tends to dominate the behaviour once a fabric is developed in which the weaker phase comprises an interconnected matrix. Similar conclusions may be drawn for the orthopyroxene granulites made up of three phases (Ross and Wilks, 1995). However, previous experiments on rocks of this CLC were performed in coaxial compression, while natural shear zones in this CLC material are believed to be produced under simple shear conditions (Brodie and Rutter, 1987). We have conducted further experiments on the same granulite, but under non-coaxial conditions. The purpose of this paper is to report the results of these experiments, since the value of direct shear experiments, under conditions of ductile flow, has already been demonstrated by Kunze and Av~ Lallemant (1976), Schmid et al. (1987) and Dell'Angelo and Tullis (1989) for olivine, calcite and quartz, respectively.

2. Experimental methods 2.1. Starting material

The starting material used is an orthopyroxene granulite from the Archean Pikwitonei domain of the Superior Province, Manitoba. Samples were taken from the same block of granulite as those used in the previous suite of coaxial experiments (Ross and Wilks, 1995). It is a fine-grained, equigranular rock that attained equilibrium metamorphic conditions of approximately 650 MPa confining pressure and 650°-750°C, well within the granulite metamorphic facies (Mezger et al., 1986). This rock is an association of 35% orthopyroxene, 35% plagioclase and 30% amphibole. All grain boundaries show 120° equilibrium relations, especially between plagioclase and orthopyroxene. Plagioclase grains exhibit albite twin lamellae and orthopyroxene grains are clear. Both phases have approximately the same grain size, 0.2-0.8 mm, whereas the amphibole grains are slightly elongate with extended prism (110) faces with mean sizes for this longer dimension of 0.4-1.2 mm with equant basal sections. None of these phases exhibit optical evidence for crystal-plastic deforma-

J. V. Ross, K.R. Wilks / Tectonophysics 256 (1996) 83-100

tion (Fig. 1). Compositions of the starting mineralogy were determined from microprobe analyses yielding plagioclase compositions of An75_82 2+ Fe0.16 3+ (bytownite), the orthopyroxene (Mgl.2 Fe0.64 [Sil.89 A10.06106)and the amphibole ([Cal.84 Na0.61 K0.12] [AllV42 Ti0.23 Cro.01 Fel.71 Mno.03 Mg2.82][Si6.27 All,73] 022 [OH, F, C1]z; compositionally a magnesio-hornblende (Leake, 1978)) are all within the granulite metamorphic facies.

2.2. Sample assemblies Samples were prepared from cylindrical cores (6.4 mm diameter) taken in the same orientation, parallel to the weak foliation as were samples tested by Ross and Wilks (1995). Samples of Pikwitonei amphibolite tested by Wilks and Carter (1990) were cored in a similar orientation with respect to the weak foliation. Wafers, 2.4-2.7 mm thick, were cut at 45 ° to the long axis of the cores and were kept in a vacuum furnace at 110°C for a minimum of 24 h before being placed in the deformation apparatus. All experiments were conducted in a GriggsBlacic solid pressure medium apparatus (Green et al., 1970; Tullis and Tullis, 1986) with NaC1 as the confining medium. Fig. 2 shows the modification to the normal axial compression geometry, by using Zircoa forcing blocks cut obliquely, at 45 °, at one end and a sample wafer fit between. The forcing

85

blocks plus sample wafer, approximately 19.0-20.8 mm long, placed in a Pt jacket (0.25 mm thick), were used to 'isolate' the granulite chemically from the confining medium. The oblique surfaces of the samples and the Zircoa (ZrO 2) pistons were ground with 400 and 600 /zm grit in order to increase the friction between the sample and the piston face(s). This assembly was heated using a graphite resistance furnace that was supported on its outer diameter by soft-fired pyrophyllite. Temperature was measured using a compensated Ph00-Pt90Rhl0 thermocouple positioned at the centreline of samples, and was controlled to within 2°-3°C of the desired temperature; axial temperature gradients of 20°-40°C, at nominal temperature of 900°C, were determined from experiments where a second thermocouple was translated through a sample with a hollow core. The platinum-jacketed samples were not sealed. However, even though the wafers were all dried, the tests are not 'vapour absent' because air (N 2, CO 2, 0 2, H 20, etc) was trapped in the jacketed samples. Thus, any water vapour inside the jackets is diluted by the other gases and s o XH20 is < 1 and .fH,o is likely low. Because of this, amphibole stability is probably reduced (Valley et al., 1983) and together with the fact that H e can diffuse rapidly through Pt jackets (Smithells and Brandes, 1976) and Fe in mafic minerals can be leached by Pt jackets (Merrill and Wyllie, 1973), the magnesio-hornblende may un-

Fig. 1. Photomicrograph of thin section of undeformed orthopyroxene granulite starting material; crossed polarisers. Bar scale is 0.5 mm.

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J. v. Ross, K.R. Wilks / Tectonophysics 256 (1996) 83-100

dergo some oxidation or dehydrogenation during the experiments. Amphibole pleochroism is observed to change from pale-green in the starting material to red-brown in the highest temperature test, implying that some oxidation a n d / o r dehyrogenation did take place (see probe analyses of deformed and undeformed amphibole in Table 1). A more detailed discussion of the geochemical environment within granulite specimens during deformation experiments is given in Wilks and Carter (1990). Data presented here are from monotonic, constant strain rate experiments at confining pressures of 1.2 GPa, somewhat higher than pressures (1.0 Gpa) applied during the coaxial experiments of Ross and Wilks (1995) on the same material. Shear deformation within the wafer was facilitated by choosing a confining pressure sufficiently high that frictional resistance along the wafer/forcing block interface was well above the flow resistance within the wafer. The shear zone deformed in this way exhibited

(a)

macroscopic ductile simple shear. Shear strain has been calculated, as detailed in Dell'Angelo and Tullis (1989), from thin sections (cut parallel to the piston axis in the plane containing the thermocouple and the principal slip direction oriented along the wafer-piston interface) as determined by measurements of forcing-block offset, and wafer thickness before and after deformation. One of the major advantages of simple shear experiments is that very large strains may be achieved, much larger than in coaxial experiments, and that the strain across the shear zone is fairly homogeneous (Schmid et al., 1987). Shear strains reported here reach 2.2. As a modification of the above sample assembly for simple shear, four experiments were carried out using forcing blocks cored and cut at 45 ° from a block of Balsam Gap dunite (Fig. 2b). In these experiments, the wafer of orthopyroxene granulite was sandwiched between two surfaces of fine-grained (0.75-1.0 mm) polycrystalline Balsam Gap dunite in

(bl

SIMPLE SHEAR Load

T KEY:

Cu

sa NaCl

le Steel

Graphite

Soft-fired Pyrophyllite

alumina t.~u'd-fired

Pyrophyllite

25.4 mm

Fig. 2. (a) Line drawing of sample assembly, showingwafer between45° forcingblocksof Zircoa. The tl~rmocouple,sheathes in mullite tubing, locatedwithin the inner NaCI sleeve and against the Pt-jacketedsample, is not shown for clarity; Pb = lead spacer. (b) Assembly, after deformation,using Balsam Gap dunite as 45° forcingblocks. Note well developedfoliationwithin wafer of orthopyroxenegranulite.

J. V. Ross, K.R. Wilks / Tectonophysics 256 (1996) 83-1 O0

87

a n a t t e m p t to m i m i c g e o m e t r i c c o n d i t i o n s o f t e n o b -

t h e g r a n u l i t e d e c r e a s e s w i t h d e c r e a s i n g s h e a r strain

s e r v e d at e x p o s u r e s o f t h e m a n t l e - C L C ( B r o d i e a n d Rutter, 1987).

rate at c o n s t a n t t e m p e r a t u r e a n d c o n f i n i n g p r e s s u r e , and (b) strength decreases with increasing temperature at c o n s t a n t s h e a r s t r a i n rate a n d c o n f i n i n g pressure. T h e g e o m e t r y o f all o f the s h e a r s t r e s s - s h e a r s t r a i n c u r v e s is v e r y similar. E a c h h a s a n initial l i n e a r elastic p o r t i o n t h a t is f o l l o w e d b y a t r a n s i e n t w o r k h a r d e n i n g r e g i o n , u n t i l a p e a k stress is a c h i e v e d . P e a k s h e a r stress l e v e l s w i t h i n e a c h r e g i o n d e c r e a s e w i t h i n c r e a s i n g t e m p e r a t u r e o r d e c r e a s i n g s h e a r strain

boundary

3. Mechanical results Most of the sample-forcing block arrangements w e r e s h o r t e n e d axially to 4 0 - 6 0 % so t h a t s h e a r strain(s), 3', d e v e l o p e d w i t h i n t h e w a f e r s ( N a d a i , 1963), r e a c h e d a b o u t 1.8 to 2.3. Fig. 3 d i s p l a y s the results o f s h e a r e x p e r i m e n t s o n s a m p l e s o f o r t h o p y r o x e n e granulite. T h e y i n d i c a t e t h a t (a) s t r e n g t h o f

rate. F o l l o w i n g this p e a k stress, strain s o f t e n i n g is o b s e r v e d until a q u a s i - s t e a d y state is r e a c h e d in

Table I Amphibole compositions; undeformed and deformed (expt. y317, 900°C, 10-5) Und.

Und.

SiO 2 41.92 43.15 TiO 2 2.03 1.99 A1203 12.20 12.16 Fe20 a 0.08 0.11 FeO 13.66 13.26 MnO 0.22 0.14 MgO 12.62 12.68 CaO 11.50 11.40 Na~_O 2.10 2.07 K 20 0.61 0.60 F 0.26 0.29 H:O 1.88 1.89 O=F -0.11 -0.12 Total 98.97 99.64 No. of ions based on 24(O,OH,F) Si 4+ 6.27 6.38 AI * 1.73 1.63 Sum 8.00 8.00 AI 3+ 0.42 0.50 Ti 4+ 0.23 0.22 Cr + 0.01 0.01 Fe ~+ 0.00 0.00 Fe :+ 1.71 1.64 Mn 2+ 0.03 0.02 Mg 2+ 2.82 2.79 5.21 5.18 Ca 2+ 1.84 1.81 Na ÷ 0.61 0.59 K+ 0.12 0.11 Sum 15.78 15.69 F0.12 0.14 H÷ 1.88 1.86 02 23.88 23.86 * Calculated from stoichiomery.

Und.

Und.

Und.

y317-1

y317-4

y317-5

y317-6

y317-10

41.38 2.08 12.27 0.09 13.16 0.16 12.64 11.41 2.09 0.60 0.23 1.88 -0.10 97.89

43.75 1.78 11.59 0.07 13.07 0.18 12.93 11.53 1.92 0.60 0.40 1.84 -0.17 99.52

43.16 2.06 12.05 0.12 13.00 0.19 12.71 11.65 2.04 0.58 0.30 1.89 -0.13 99.60

43.14 2.04 12.27 0.15 13.72 0.22 12.31 11.50 1.98 0.67 0.33 1.88 -0.14 100.07

38.76 1.99 12.32 0.20 13.08 0.16 12.66 11.36 2.01 0.68 0.31 1.77 -0.13 95.17

41.93 2.14 12.22 0.13 13.48 0.18 12.50 11.37 2.07 0.63 0.31 1.85 -0.13 98.68

40.54 1.87 11.67 0.11 12.83 0.12 13.31 11.50 1.95 0.59 0.34 1.79 -0.14 96.38

43.29 2.08 12.37 0.14 13.50 0.14 12.45 11.66 2.02 0.69 0.33 1.89 -0.14 100.42

6.25 1.75 8.00 0.43 0.24 0.01 0.00 1.66 0.02 2.85 5.21 1.85 0.61 0.12 15.78 0.11 1.89 23.89

6.46 1.54 8.00 0.48 0.20 0.01 0.00 1.61 0.02 2.85 5.17 1.82 0.56 0.11 15.66 0.19 1.81 23.81

6.38 1.62 8.00 0.48 0.23 0.01 0.00 1.61 0.02 2.80 5.15 1.84 0.58 0.11 15.68 0.14 1.86 23.86

6.37 1.64 8.00 0.50 0.23 0.02 0.00 1.69 0.03 2.71 5.17 1.82 0.57 0.13 15.68 0.15 1.85 23.85

6.06 1.94 8.00 0.33 0.23 0.03 0.00 1.71 0.02 2.95 5.28 1.90 0.61 0.14 15.93 0.15 1.85 23.85

6.28 1.72 8.00 0.44 0.24 0.02 0.00 1.69 0.02 2.79 5.20 1.83 0.60 0.12 15.75 0.15 1.85 23.85

6.23 1.77 8.00 0.34 0.22 0.01 0.00 1.65 0.02 3.02 5.26 1.89 0.58 0.12 15.84 0.17 1.84 23.84

6.36 1.64 8.00 0.50 0.23 0.00 0.00 1.66 0.02 2.73 5.15 1.84 0.58 0.13 15.69 0,15 1.85 23,85

88

J. V. Ross, K.R. Wilks / Tectonophysics 256 (1996) 83-100

which strain is accumulated at nearly constant shear stress; the amount of strain at nearly constant shear stress decreases with increasing temperature throughout the range of approximately Y = 0.1-0.2. Beyond this region of near constant shear stress, the granulite strain softens, at first with a steeper rate of softening that is then followed by an almost constant rate of change, regardless of temperature or shear strain rate. This mechanical behaviour of orthopyroxene granulite under simple shear is very similar to that exhibited by the same material in coaxial experiments over a comparable range of conditions (Ross and Wilks, 1995).

boundary (SZB). Most of these grains exhibit undulose extinction and narrow kinks perhaps resulting from slip along mechanical twins. Healed microcracks are common at these grain boundaries that frequently pass into kink bands within the grain interior. Other plagioclase grains that do not exhibit a preferred shape orientation (porphyroclasts) exhibit patchy undulose extinction and narrow rims of very finely recrystallised plagioclase, with the latter appearing as poorly developed tails to these more rounded grains. The latter are subequant but with irregular grain boundaries, the inference from which is that they have formed by grain boundary migration recrystallisation. Similar microstructures have been described in experimentally deformed plagioclase felspar, under comparable conditions, by Tullis and Yund (1985) and Tullis et al. (1990). This type of microstructure related to dislocation creep with grain boundary migration as the strain-induced recovery process, as observed in experimentally deformed quartz aggregates, is defined as Regime-1 type creep (Hirth and Tullis, 1992). In contrast, the orthopyroxene does not appear to have undergone a shape change, and where grains are adjacent to plagioclase, orthopyroxene seems to behave passively and only develops very fine clinopyroxene lamellae (Kirby and Etheridge, 1981; Kirby and Stem, 1993) subparallel to the orthopyroxene cleav-

4. O b s e r v a t i o n a l results Most of the experiments were carried out to shear strains ranging from 1.7 to 2.2 (corresponding to axial strains of 50% or more), with a maximum shear strain of 2.6. Macroscopically, all of the wafers exhibit a foliation outlined by elongate amphibole grains. At low temperarures (650°-800°C) and axial strain rates of 10 -6 tO 10 -7 S -1, ductile strain is localised within plagioclase grains that have developed an elongate lozenge shape (ribbon grains) and outline a foliation that is nearly parallel with the shear zone ~, n -~

(a)

8.00 7

F---~

800 C

....

850 C .

G~) 0.00

.

.

.

.

.

.

.

.

.

900 C

.

950 C 0.0

0.5

1o

2.0

Shear Strain

.

.

.

.

.

_

800C

. . . . . . . . .

4.00 -,

800C 10"s

1 0 "5

4 ~

0.00

0.0

0.5

1.5

2.0

Shear Strain

Fig. 3. (a)Shear stress vs. shear strain data for orthopyroxene granulite at constant Pc = 1.2 GPa, T= 7.0 × 10-5 s- I and variable temperatures, as indicated. (b) Shear stress vs. shear strain data for orthopyroxene granulite at constant Pc = 1.2 GPa, and variable shear strain rates and temperatures, as indicated. Points 1, 2. 3 and 4 indicate positions on the curve where successive experiments were terminated to evaluate the substructure development.

J. v. Ross, K.R. Wilks / Tectonophysics 256 (1996) 83-100

age and microcracks. No sign of recrystallisation, either by optical or TEM examination, is observed. Amphibole grains define a foliation that makes variable (200-30 °) angles with the SZB. Another obvious optical feature is the marked colour change as compared with the starting material. Amphibole pleochroism in deformed samples changed to darkredbrown from that of light-dark green in the starting material. Electron microprobe data from samples deformed at 900°C for over 10 h, however, suggest that pleochroism changes cannot be attributed to major compositional changes in the amphibole; probe-determined compositions are essentially unchanged (Table 1). Thus, amphibole dehydration and associated nucleation of transiently fine-grained reaction products can be eliminated as contributing to the strain-softening observed in the mechanical data. Pleochroism changes are attributable, however, to the conversion of magnesio-hornblende to oxyhornblende during the deformation experiments. Studies show that amphibole rapidly loses hydrogen and converts to oxyhornblende under unbuffered conditions when no free H20 is present, and that charge balance in the lattice is maintained by conversion of Fe 2+ to Fe 3÷, thereby altering the pleochroism (Clowe and Popp, 1988; P o p p e t al., 1995). Amphibole grain shapes appear to have changed due to fragmentation of original grains that can be seen transected by small faults; these microfaults occur both across and parallel to cleavage planes. Thus, foliation development appears to involve mechanical rotation of amphibole fragments. Many of the amphi-

89

bole fragments that comprise the foliation exhibit an undulose mosaic extinction pattern, probably related to incipient cataclasis. Abundant microcracks offset grain boundaries and have formed by slip along the (110) prismatic cleavage. No sign of recrystallisation is observed, either by optical or TEM examination in these amphibole grains. Thus, at these low temperatures and strain rates, the total strain is accommodated dominantly by crystal plastic mechanisms of plagioclase grains. Ribbon grains result where the plagioclase grains, the apparent weakest phase, are caught between stronger orthopyroxene and amphibole grains. Orthopyroxene also accommodates some of the sample strain by crystal plastic mechanisms whereas amphibole responds mainly by fracturing and mechanical rotation, with only minor evidence of crystal plastic behaviour. The strain partitioning between these three phases, in which y-plag > y-opx > y-amph, where y-plag is the strain within plagioclase, involves grain shape changes with increasing strain. Most of these changes are likely accommodated by intragranular movements, but may also involve intergranular movements (grain boundary sliding) that combine with mechanical rotation to produce a foliated rock. At the higher temperatures (850°-950°C) and shear strain rates of 10 -5 to 10 - 6 S-I, an amphibole foliation is still the most obvious feature, perhaps at a wider variation of angles with respect to the SZB (10°-30°), having more of an S-shaped geometry than the more planar form observed at lower temperatures and higher strain rates (Fig. 4). This foliation

Fig. 4. Foliationin wafer outlinedby rotated fragmentsof amphibole(A) and elongate areas of plagioclase(P) and orthopyroxene(O); crossed polarisers. Shear sense is dextral.Widthof zone is 2.5 mm.

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J.V. Ross, K,R. Wilks / Tectonophysics 256 (1996) 83-100

Fig. 5. Porphyroclastsof plagioclase with zones of plagioclase neoblasts; crossed polarisers. Sense of shear is sinistral. Bar scale is 0.5 mm.

is now marked, not only by aligned amphibole fragments, as at the lower conditions, but is also outlined by attenuated orthopyroxene grains and plagioclase with extensive regions of plagioclase neoblasts (Fig. 5). Locally within the wafer, phase segregation has produced a well-developed compositional lamination comparable to that of natural mylonites (Fig. 6). Within such high-strain regions between plagioclase

domains, adjacent to plagioclase ribbons, very thin slivers of amphibole are sandwiched, serving to enhance foliation parallel to the SZB, a C-foliation (Fig. 7). The ribbons of plagioclase have aspect ratios that vary from 5:1 to 10:1 and the traces of their (010) planes, the easy slip plane (Ji et al., 1988), make an angle of ~ 25 ° with the C-surfaces and the SZB. This is in contrast to the larger, more

Fig. 6. Composition lamination and shape foliation making angle 15°-20° with the shear zone boundary. Sense of shear is dextral. A = amphibole; O = orthopyroxene; P = plagioclase; plane light. Width of zone is 2.5 mm.

J. V. Ross, K.R. Wilks / Tectonophysics 256 (1996) 83-100

91

Fig. 7. Ribbon plagioclase(P) separated by thin slivers of amphibole(A) that are parallelto the shear zone boundary;C-foliation.Sense of shear is sinistral.Plane light; O = orthopyroxene; bar scale is 0.5 mm.

equant plagioclase porphyroclasts, whose (010) traces make very high angles with the SZB and are oriented unfavourably for easy slip. Microstructures in the three different phases are similar, but recrystallisation of plagioclase is more

marked than it is for the other phases, with long tails of small (0.01 mm) neoblasts strung-out and aligned parallel with the SZB (Fig. 5), and thus defining a C-foliation (Simpson and Schmid, 1983). TEM examination of the plagioclase porphyroclasts shows

Fig. 8. TEM photograph of orthopyroxene neoblasts in a host orthopyroxene. Neoblasts exhibit clinopyroxenelamellae and a few free dislocations. Host is characterised by tangles and high dislocationdensity. Bar scale is 5 /~m.

92

J.V. Ross, K.R. Wilks / Tectonophysics 256 (1996) 83-100

subgrain development, whose size is comparable to many of the neoblasts. At these higher temperatures and high shear strains, the mechanisms of recrystallisation, grain boundary migration and grain boundary rotation, are operating probably simultaneously, although the former mechanism is by far the dominant of the two. Orthopyroxene neoblasts around rims of highly strained orthopyroxene grains, although not well-developed optically, are extensive under TEM examination (Fig. 8). At the same time, amphibole exhibits better developed undulose extinction patches and they show optical-scale clusters of small ( < 0.01 mm) amphibole grains within them (Fig. 9). Difficulties with ion thinning TEM foils prevented us from obtaining TEM images of amphibole and thus determining whether these refined amphibole grains are neoblasts resulting from recrystallisation or are simply the result of brittle deformation and grain fragmentation. Wilks and Carter (1990) deformed coaxially a Pikwitonei amphibolite, whose composition is very similar to the amphibole in the orthopyroxene granulite, and observed extreme grain refinement of the amphibole at T = 900°C, Pc = 1.0 Gpa and ~ = 10 -5 s -1. At these conditions there was also abundant neoblast development in associated plagioclase. The refined amphibole grains were examined only by optical means and were inferred to be neoblasts

resulting from recrystallisation. Similar results were also reported by Ross and Wilks (1995) from coaxial experiments, under similar conditions, on orthopyroxene granulite. In contrast, Hacker and Christie (1990) deformed coaxially samples of a natural and a synthetic amphibolite and found their strengths to be markedly greater than those of the Pikwitonei amphibolite at comparable conditions, with no evidence of recrystallisation. The strength difference is explained by Wilks and Carter (1990) as the result of a chemical composition difference and the presence of a strong foliation in the starting material of that used by Hacker and Christie (1990). However, grain refinement in amphibole by natural cataclasis has been recognised optically and by TEM (Nyman et al., 1992). This naturally deformed amphibole has a similar composition and optical characteristics to that of the fragmented and grain-refined amphibole of the orthopyroxene granulite, whose texture we infer also results from extreme brittle failure. Grains of all three phases are transected by microcracks. Amphibole grain boundaries show offset along small microfaults that can occasionally be traced into adjacent mineral grains, especially where the matrix comprises neoblasts of plagioclase. The total mechanistic response within the shear zone is semi-brittle, with the bulk of the strain softening

Fig. 9. Small cataclastic amphibole grains in a highly strained (mosaic extinction patches) amphibole host. Bar scale is 1 mm.

J. V. Ross, K.R. Wilks / Tectonophysics 256 (1996) 83-100

taking place within plagioclase by development of neoblasts. Orthopyroxene also undergoes strain softening by neoblast development, but not to the same extent as in plagioclase. In contrast, amphibole, at these same conditions, accommodates the strain by slip along prismatic cleavage and internal crystal plastic mechanisms. The mechanical results, described above, show that all of the curves have a similar geometry. To characterise the sequential evolution of the substructure that is evident in all of the experiments at completion, three additional experiments at 900°C and shear strain rates of 7 × 10 -5 s -1 were run and stopped at critical points on the curve. These termination points are (1) the top of the initial linear portion of the curve, (2) immediately prior to the onset of shear stress reduction at the end of the first quasi-constant shear stress part of the curve, and (3) the end of this shear stress reduction immediately prior to the beginning of the second quasi-constant shear stress part of the curve. These points are labelled 1, 2, and 3 in Fig. 3b. Up to point 1, at small axial shear strains (3' = -0.1), the wafer in the shear zone essentially has undergone elastic loading during this almost linear initial part of the curve. The fabric at this point is identical to the starting material, except that some

93

plagioclase grains now exhibit undulose extinction; no foliation is developed. At position 2 on the curve, which is at slightly higher shear stresses than at point 1 and at shear strains 3' of 0.1 to 0.2, a foliation is developed, outlined by aligned amphibole grains and fragments that make an high angle ( ~ 30 °) with the SZB (Fig. 10). Microfaults and cracks intersect larger fragments, many of which are bent and exhibit undulose and mosaic extinction. Many of these cracks separating the fragments are oriented at high angles across the foliation and appear to accommodate extension of much larger grains. The new grain alignment is a shape orientation parallel with the local maximum extensional strain direction: an S-foliation. Plagioclase grains are much more interconnected, some are lozenge shaped, exhibit undulose extinction and contain well-developed mechanical twins. TEM examination shows mechanical twins with dislocation tangles (Fig. 11). Orthopyroxene grains show no shape change, but are transected by microcracks. No neoblast development is observed in any of the phases either optically or by TEM examination. At position 3 (at 3' = > 0.2) shear stresses are much lower than those characteristic of position 2, while the fabric is developed to about the same degree as at position 2 (i.e. obliquity with the SZB

iiii?ill

Fig. 10. Photomicrograph of weak amphibole foliation developed at stage 2. Foliation outlined by amphibole fragments ( A); orthopyroxene (O) and plagioclase ( P ) appear undeformed except for microcracks. Plane light. Sense of shear is dextral. Width of zone is 2.5 mm.

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Fig. 11. TEM photograph of dislocation tangles in plagioclase; stage 2. Bar scale is 1.0/zm.

has not changed). Amphibole fragments appear somewhat smaller and are transected by more microcracks than at lower strains. Plagioclase grains still show undulose extinction and deformation twins, but now show optical evidence for plagioclase neoblasts

around many of the larger plagioclase porphyroclasts apparently by grain boundary migration recrystallisation. Early phases of subgrain development are evident in TEM (Fig. 12). Orthopyroxene fragments, when adjacent to amphibole, develop very fine

Fig. 12. TEM photograph of early phase of subgrain in plagioclase; stage 3. Bar scale is 5 ~m.

J.V. Ross, K.R. Wilks/ Tectonophysics 256 (1996)83-100 clinopyroxene lamellae, but when adjacent to or surrounded by plagioclase appear undeformed. Hence, at this stage after shear stress reduction, the strain softening is accommodated mostly in the plagioclase by neoblast development, more crackino and rotation in the amphibole and fragmentation and clinopyroxene lamellae development in orthopyroxene. Thus, in comparison with all the other high-temperature experiments carried to completion, this fabric at stage 3 appears to evolve with increasing strain by increased contributions from crystal plastic mechanisms within all of the phases, together with increased volume development of neoblasts within each of the plagioclase and orthopyroxene, but mainly within plagioclase. As previously detailed, experiments carried to completion, stage 4, are characterised optically by well-developed S- and C-foliations defined by all phases leading to compositional lamination. Both plagioclase and orthopyroxene occur as elongate porphyroclasts, with the former also occurring as elongate ribbons. These foliations, together with the development of compositional lamination, are accompanied by a decrease in shear stress. Hence this softening, with increasing strain and progressive foliation development, results from the shape changes, redistribution and alignment of the phases as suggested recently by Shea and Kronenberg (1993) on experiments of schists deformed in various orientations with respect to the fabric of the starting material. Many plagioclase porphyroclasts have extensive zones of plagioclase neoblasts, whose overall orientation is at low angles with SZB. These S- and C-foliations, which are first observed to be poorly developed planar arrangements at stage 3, are now well marked, often exhibiting S-shaped mineral aggregates and a compositional lamination. Microstructures within plagioclase at stage 4 show the operation of both recrystallisation mechanisms: grain boundary migration and grain boundary rotation, with the former abundantly well developed. In contrast, at stage 3, optical evidence for grain boundary migration is abundant, but TEM examination indicates early development of subgrains in plagioclase, the pre-cursor(s) to grain boundary migration. Neoblasts of orthopyroxene are also extensive at stage 4, but are not apparent at stage 3 or earlier. Hence, at these high temperatures and lower strain rates, with in-

95

creasing strain, there is a gradual addition in recovery mechanisms, initially only grain boundary migration and then the development of grain boundary rotation as well. Stage 4, then, may represent a transition to the Regime-2 creep of Hlrth and Tullis (1992).

5. Steady-state deformation Steady-state mechanical data for various shear experiments on orthopyroxene granulite wafers, using Zircoa forcing blocks, are shown in log shearstress-log shear-strain rate space (Fig. 13). All stress data are taken between about y = 0.2 and 0.4 (corresponding to coaxial shortening strains of the sample column of ~ 10% and ~ 15%, respectively). These shear strains correspond to positions on the z / y curves where the stress becomes close to a steadystate value after the gradual stress drop (point 3 on Fig. 3). These data fit well, by the least squares technique described by Heard and Raleigh (1972), a power law relation (correlation coefficient = 0.83). The constants resulting from this regression are A = 3 . 9 × 10 3 MPa -~ s-I; Q c = 2 0 3 + 6 k J m o l 1; n = 2.3 _+ 0.4. 3/= 3.9 × 103exp[-203 _+ 5 . 9 / R T ] r 23±°4

(1)

Isotherms generated from the fit are also shown. A power law for this orthopyroxene granulite has already been estimated from conventional coaxial experiments run over a wide range of temperatures (650°-950°C) (Ross and Wilks, 1995). The equation determined from these coaxial experiments is: e = 2.5 ×

10 2 MPa-i

s-~exp[ - 2 6 5 _+ 6 / R T ]

× ~ 5.5 _+0.6

(2)

Before this coaxial result can be compared with the results of the shear experiments, this coaxial law must be generalised and then rederived as a shear stress-shear strain rate relation, as described by Nye (1953) and applied more recently to calcite by Schmid et al. (1987). After transformation, the coaxial law becomes: y = 8.9 × 103exp[- 265 _+ 6 / R T ] z 55+ 0.6

(3)

Both sets of experiments utilised the exact same confining medium and sample assembly components

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2AI

~

650

C

1.(1 to

o

0

~T o

-1.0

6. - L o g S h e a r Strain Rate

7

Fig. 13. Log shear-stress vs. log shear-strain rate plot of quasi-steady-state data taken between T = 0.2 and 0.4. Isotherms shown are generated from the power law given in the text. Stars correspond to strength data taken at similar T values but in composite assemblies with Balsam Gap dunite forcing blocks.

and the sample cores were taken from the same block of starting material and in the same orientation; for coaxial experiments the cores were compressed parallel to the core axes, whereas in the noncoaxial experiments shear was constrained to occur at 45 ° to the core axes (maximum compressive stress again parallel with the core axis). The difference between these two laws, Eqs. 2 and 3, is mostly in the activation energy for creep and the shear stress exponent: 265 and 210 kJ mo1-1, and 5.5 and 2.3, respectively. Data sets for both were taken over similar quasi-steady-state conditions and the micromechanical response in both is observed to be semi-brittle. Data for the shear experiments were collected after the development of a foliation within the shear zone, wherein amphibole has become fragmented and aligned, and the 'soft' phase, plagioclase, has become interconnected, with the latter undergoing strain softening associated with the development of plagioclase neoblasts by dynamic recrystallisation. With increasing shear strain, beyond

the level at which the quasi-steady state data were collected, the whole shear zone gradually softens with the shape-preferred orientation of the mineral phases becoming better developed (i.e. the angle between foliation defined by shape orientation and the SZB decreases). Hence, the changing anisotropy is likely to control the rheology with increasing strain (Takashita and Wenk, 1988; Wenk et al., 1989), and perhaps allow the extrapolation of this shear equation to geologic strain rates to give crustal strengths more reasonable than inferences drawn from the coaxial results. Four other experiments were run using wafers of orthopyroxene granulite sandwiched between 45 ° forcing blocks cut from cores of Balsam Gap dunite. Data from these experiments is also plotted in Fig. 13, and in each instance is weaker than the granulite deformed at the same conditions but with Zircoa forcing blocks. Textures within the wafers, after shear strain to similar values as the other experiments, are indicative of foliation development out-

J.v. Ross, K.R. Wilks / Tectonophysics 256 (1996) 83 - 1O0

lined by amphibole, and recrystallisation of plagioclase and orthopyroxene. However, some strain was accommodated within the dunite forcing blocks; the olivine is strongly kinked along (0kl):[100]--the medium- to low-temperature slip system (Carter and Av~ Lallemant, 1970) and recrystallisation is not observed. The degree of foliation development, involving both S- and C-surfaces and the microstructures observed are essentially the same as those observed in experiments using ZrO 2 forcing blocks.

6. Discussion

The mechanical results of these shear experiments indicate, that the granulite attains a quasi-steady state response at low strains (y of ~ 0.1 to '0.2) over the entire range of experimental conditions. However, by shear strains of ~ 0.25 strain softening occurs and a new, lower shear stress level is achieved that asymptotically decreases with increasing shear strain. This common geometric form of the mechanical results is similar to those described for two-phase aggregates, wherein, the 'stronger' phase comprises the loadbearing framework prior to deformation and the 'softer' phase fills non-connected spaces between this framework (Shelton and Tullis, 1981; Tharp, 1983; Jordan, 1987; Ross et al., 1987; Handy, 1990; Tullis et al., 1991; Ross and Bauer, 1992). The granulite used in these shear experiments comprises nearly equal volume fractions of plagioclase, orthopyroxene and amphibole, with plagioclase representing the 'soft' phase, amphibole making up the 'strong' phase and orthopyroxene having intermediate strength. The experiments stopped at critical positions in the mechanical evolution indicate that initially the 'strongest' phase controls the planar fabric development until the amphibole responds to increasing strain by essentially bending, cracking and mechanical rotation of fragments. Up to this point, the individual 'weak' phases have not been interconnected, as in their original configuration; plagioclase shows evidence for crystal plastic mechanisms and orthopyroxene exhibits microcracks and the overall response of the aggregate is semi-brittle. Only after interconnection of the phases, resulting from mechanical rotation and alignment of amphibole fragments is

97

there any detectable difference in response of the two 'softer' phases, plagioclase and orthopyroxene. Hence, to this stage of fabric evolution, the total response is dominated by the 'strong' phase (amphibole) that comprises the load-bearing framework. Stress concentrations in amphibole grains eventually lead to the mechanical break-up of the strong framework distribution of all three phases. The reduction of strength to the new quasi-steady state value takes place gradually and the microstructure, after strength reduction, is characterised by the development of small plagioclase neoblasts mantling some of the larger plagioclase grains, microcracks in orthopyroxene and refinement of the amphibole-fragment alignment. TEM examination shows that the plagioclase subgrains are of a smaller size ( < 5 p~) than the optically recognised neoblasts ( ~ 75 /z) and their origin may be associated with incipient dynamic recovery recrystallisation. However, their origin is probably not the classic 'cascade' recrystallisation as seen in metals (Sakai and Jonas, 1984) or in halite-anhydrite aggregates (Ross et al., 1987) that leads to rapid coarsening of the grains, even though the initiating stress instabilities are similar. More likely, since we are dealing with a three-phase aggregate, the strain is partitioned between all three phases and strain energy sufficient for recrystallisation of plagioclase, the weakest phase, was not large enough to produce large volumes of neoblasts at this stage. Increasing shear strain at the quasi steady-state level leads to further enhancement of the mylonitic fabric, extensive recrystallisation in plagioclase, clinopyroxene lamellae development and recrystallisation in orthopyroxene. At the highest strains amphibole exhibits grain refinement, probably by extreme fragmentation (cataclasis), resulting in an overall reduction in grain size of the three phases. Plagioclase ribbon grains are well-developed, especially where plagioclase is sandwiched between orthopyroxene(s) and/or amphibole(s), are bent or 'deflected' by the 'stiffer" grains and have their planar orientation nearly parallel with the SZB. Here the ribbons frequently display a narrow rim of plagioclase neoblasts, whose orientation is only a few degrees off that of the adjacent ribbon. Their size is much smaller than those neoblasts mantling the plagioclase porphyroclasts. These small neoblasts likely comprise part of a normal distribution of neoblast

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sizes characteristically developed about the average stress level, but it is also possible, in this particular position of high strain, that they could indicate that the stress and strain gradient is higher in this orientation than it is around plagioclase having neoblast mantles, where only the extreme ends of the mantle tails are parallel with the SZB. The orientation of these tails are then in an orientation of maximum extensional strain and are incipient C-planes. These zones comprising recrystallised grains are zones of softening, similar to those described by Tullis and Yund (1985) in their coaxial experiments on sintered plagioclase, and under similar conditions as those detailed herein. These plagioclase neoblasts have been produced by grain boundary migration recrystallisation, which is a mechanism that promotes softening by the elimination of larger grains in an unfavourable orientation for slip and consequently enhancing lattice reorientation and easier glide. At the highest shear strains (stage 4), however, optical subgrains, indicative of grain boundary rotation mechanism(s), are of variable small size in regions adjacent to plagioclase ribbons. This would imply that, at this stage of the microstructural evolution, recovery is not only by grain boundary migration mechanism(s) but that climb, at least to a limited extent, must occur in felspar. In coaxial experiments on prefaulted specimens of felspathic rock, at similar conditions, Tullis et al. (1990) found that plagioclase undergoes recrystallisation-accommodated dislocation creep with plagioclase neoblasts forming only by grain boundary migration. In these experiments on orthopyroxene granulite, at the highest temperatures, 900 ° and 950°C, steady-state conditions are not achieved at experiment completion. The flow stress is gradually decreasing with increasing strain so that softening is also increasing. Apart from geometric softening, in that foliations are better developed with increasing strain, the volume fraction of plagioclase neoblasts is also increasing, with grain boundary mechanism(s) dominating their formation. It is only in regions of high strain that climb-accommodated recovery in plagioclase is inferred. This mechanism involves angular rotation and misorientation development of subgrains (White, 1976), and perhaps in these regions of high shear strain the necessary energy to overcome the barrier to climb is aided by high shear strain. Certainly, textural evidence suggests that, un-

der natural deformation at upper amphibolite to granulite grades of metamorphism, climb in felspar becomes sufficiently easy to accommodate dislocation creep (White, 1976; Pryer, 1993). It is suggested that, at localised situations of high shear strain and high temperature, attained at stage 4, incipient climb-accommodated creep is occurring. This may correspond to a transition regime, such as that described for quartz (Hirth and Tullis, 1992), where an early regime of recrystallisation-accommodated creep becomes associated with the gradual development of climb-accommodated creep. The flow law estimated from the quasi-steady state data, measured immediately after the stress drop, is different from that determined in the coaxial experiments on the same material; the activation energy for the operative process is lower (210 kJ/mol as opposed to 265 kJ/moi, respectively) and the stress exponent is less than half (2.3) the value determined in the coaxial experiments (5.5). These differences are probably related to the fact that data for the shear experiments were collected after the formation of a foliation wherein the whole of the shear zone was undergoing softening (geometric and grain-size reduction) and that, within the confines of the shear experiments, volume of plagioclase neoblasts increases within increasing strain. Perhaps at even higher strains than explored here, extreme grain-size reduction may become even more pronounced and a change of creep mechanism(s), as can be inferred from the change in the stress exponent, from grain-size insensitive flow to one of grain-size sensitive flow may be relevant in natural mylonites. Steady-state conditions imply a constant microstructure, which is unlikely at very large strains, unless flow is by diffusion-accommodated grain boundary sliding or by cyclic grain boundary migration recrystallisation plus intracrystalline glide. Hence, since these data were collected over only a narrow range of relatively low shear strain, extrapolation of this law to natural strain rates may result in predictions of CLC strengths that are too high. All the orthopyroxene samples deformed between dunite forcing blocks were consistently weaker than those in experiments with Zircoa forcing blocks. While some strain was taken up in the olivine, the wafers of granulite continued to develop the same microstructures with increasing strain as those de-

J. V. Ross, K.R. Wilks / Tectonophysics 256 (1996) 83-100

scribed above; the granulite is the weaker of the two rock types and increasing shear strain is partitioned preferentially within the granulite. This type of composite is frequently encountered under natural conditions at the CLC-mantle boundary, where granulite rocks occupy the cores of cuspate zones between lobate upwarps of mantle (Rutter and Brodie, 1988). It is within these weaker zones that strain is preferentially localised leading to ductile shear zones and large separation(s) that are dominated by recrystallisation-accommodated creep of feldspar.

7. Conclusions We have determined a flow law for shear deformation of an orthopyroxene granulite to high shear strains; the experimental data best fit a power law function with an apparent activation energy of 203 k J / m o l and a stress exponent of 2.3. The operative mechanical response is semi-brittle with the bulk of the strain partitioned within plagioclase. With increasing shear strain the granulite gradually weakens associated with the increasing volume fraction of recrystallised plagioclase, minor recrystallisation of orthopyroxene, and finally, cataclasis of amphibole. These mechanisms are all associated with eventual mechanical separation of the mineral phases and refinement of the foliation, leading to the formation of S- and C-foliations, as observed in natural mylonites. The increased development of neoblasts with increasing strain is associated with gradual softening of the material. Shear strains reached were very low compared with natural shear zones. However, the stress exponent calculated from these shear experiments is smaller than that determined from coaxial experiments on the same material. These higher strain results are associated with a gradual grain-size reduction of the phases, especially plagioclase that is the weakest. The bulk strength of the shear zone depends on the bulk volume percentage of the ' weak' phase(s) with the 'strong' phase acting solely to concentrate stress. Thus, progressive development of S - C fabrics within the shear zone reduces the grain size of the 'strong' phase which, together with redistribution and alignment of those fragments, results in interconnection and partitioning of still more strain within

99

the 'weaker' phases. As a result, the stress concentrations are reduced and the amount of strain energy required for continued strain is reduced.

Acknowledgements The authors are grateful to Dr. A. Kronenberg and an anonymous reviewer, whose written comments substantially improved an earlier version of this paper. The work was supported by NSEC grant A2134 to JVR. Finally, the authors take great pleasure in dedicating this paper to Neville Carter on the occasion of his 60th birthday. Over the years he has been many things, teacher, mentor and colleague, always ready to talk about experiments or a paper we were working on, and always insistent that any results be tested on 'the real rocks outside'. Behind his often gruff exterior there was a kind and patient man. Thanks for everything Neville, especially your friendship.

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