Syndeformational recrystallization ± dynamic or compositionally ...

Contrib Mineral Petrol (1998) 131: 219±236

Ó Springer-Verlag 1998

Holger StuÈnitz

Syndeformational recrystallization ± dynamic or compositionally induced?

Received: 8 July 1996 / Accepted: 17 November 1997

Abstract Dynamic recrystallization in the strict sense of the term is the reconstitution of crystalline material without a change in chemical composition, driven by strain energy in the form of dislocations. Driving potentials additional to internal strain energy may contribute to the recrystallization of naturally deformed minerals, which form solid solutions such as feldspar, amphiboles and pyroxenes, if they change their composition during recrystallization. To estimate the relative importance of these driving potentials, the chemical composition of porphyroclasts and recrystallized grains of plagioclase, clinopyroxene and hornblende have been investigated in samples from a high grade shear zone of the Ivrea Zone, Italy. The plagioclases show two different recrystallization microstructures: bulging recrystallization at grain boundaries and discrete zones of recrystallized grains across porphyroclasts probably involving fracturing. Deformation took place under amphibolite facies conditions on a retrograde P,T-path. Porphyroclast and recrystallized compositions from bulging recrystallization microstructures di€er only in their Or-content and yield a DG between mean host grain and mean recrystallized grain composition at ®xed P,T-conditions of approximately 5 Joules/10)4 m3. Extreme compositional variations yield approximately 60 J/10)4 m3. The increase of free energy due to dislocations calculated for common glide systems in plagioclase are on the order of 100 Joules/10)4 m3 for high values of dislocation densities of 1014 m)2. Thus, the e€ect of chemically induced driving energies on grain boundary velocity appears small for mean compositions but may be as great as that of deformational energies for larger chemical di€erences. In the other type of microstructure, porphyroclasts and recrystallized grains in discrete zones

H. StuÈnitz Geologisches Institut, UniversitaÈt Basel, Bernoullistr. 32, CH-4056 Basel, Switzerland; Fax: 0041/612673613, E-mail: [email protected] Editorial responsibility: V. Trommsdor€

di€er in their anorthite content. The maximum DG induced by the compositional disequilibrium is on the order of 100 J/10)4 m3. This maximum value is of the same magnitude as the DG derived from high dislocation densities of 1014 m)2. The resulting combined DG is approximately twice as high as for deformational DG alone, and heterogeneous nucleation may become a feasible recrystallization mechanism which is evident from the microstructures. The recrystallization mechanism depends on the nature of the driving potential. Grain boundary migration (GBM) and heterogeneous nucleation can release Gibbs free energy induced by compositional disequilibrium, whereas this is not likely for subgrain rotation. Therefore, only GBM and heterogeneous nucleation may link metamorphism and deformation, so that syndeformational recrystallization may represent a transitional process ranging from dynamic recrystallization to metamorphic reaction.

Introduction Recrystallization is the process of reconstituting an existing crystalline material and represents a structural transformation (Spry 1969, p. 47). One or several characteristics of the starting material usually change during this process, such as crystal structure, grain size and shape, crystallographic orientation, and chemical composition. A change of crystal structure represents a phase transformation (e.g. Kirby and Stern 1993, and references therein). A change of crystallographic orientation and/or grain size alone represents dynamic recrystallization (Poirier and Guillope 1979; Drury and Urai 1990), or static annealing. Recrystallization involving changes in chemical composition is usually considered as neocrystallization, neomineralization (Tullis 1983; Yund and Tullis 1991), or metamorphic reaction (Spry 1969, p. 114). Thus, the process of recrystallization of geological materials includes processes ranging from metamorphic reactions to dynamic re-

220

crystallization. Dynamic recrystallization is a process accompanying crystal plastic deformation, whereas metamorphic reactions may occur without deformation. However, in many cases, metamorphic reactions and deformation occur more or less simultaneously. In such a situation, the exact nature of recrystallization may be dicult to determine in solid solution minerals, and a general term such as syndeformational, syntectonic, or synkinematic should be preferred. The nature of the recrystallization (dynamic recrystallization, phase transformation, neomineralization, etc.) depends on the physical nature of the driving thermodynamic potential. The two most important mechanisms contributing to the dynamic recrystallization of materials during deformation are progressive subgrain rotation and grain boundary migration (Hobbs 1968; Poirier and Guillope 1979; Tullis and Yund 1985; Drury et al. 1985; Urai et al. 1986; Cumbest et al. 1989; Drury and Urai 1990; Yund and Tullis 1991). On the other hand, the most important recrystallization mechanism for metamorphic reactions is generally heterogeneous nucleation of phases, where a nucleus forms ab initio at non-equilibrium defects such as grain boundaries, inclusions, dislocations, etc. (Porter and Easterling 1981, p. 271; Spry 1969, p. 119±121). Grain boundary migration (GBM) is also important during metamorphic reactions, when a stable phase has nucleated and then grows (if the boundary separates di€erent minerals, the term ``phase boundary migration'' is more appropriate). Thus, grain (or phase) boundary migration is a recrystallization mechanism, which may be operative during dynamic recrystallization and during metamorphic reactions. For dynamic recrystallization (by strain induced grain boundary migration, SIGM) the driving potential is stored strain energy in the form of dislocations (Nicolas and Poirier 1976; Poirier and Guillope 1979), whereas for metamorphic reactions and chemically induced grain boundary migration (CIGM) it is the free enthalpy of the material (Hay and Evans 1987a, b). Most common rock forming minerals, except for quartz, are solid solutions and may show some compositional adjustment due to equilibrium with other phases, even if the recrystallization only occurs in deformed portions of the rock. Therefore, it appears likely that for solid solution minerals, SIGM and CIGM occur together during syndeformational recrystallization of naturally deformed rocks. For several reasons, plagioclase is one of the most suitable solid solution minerals to study the relationships between compositional and deformational e€ects during syndeformational recrystallization. The crystallographic and compositional properties of feldspars have been studied in depth. The largest data set of compositional changes during recrystallization in naturally deformed geological materials exists for plagioclase (White 1975; Brown et al. 1980; Sodre Borges and White 1980; Brodie 1981; Hanmer 1982; Watts and Williams 1983; Olsen and Kohlstedt 1985; Obee and White 1985; Olesen 1987; Molli 1994; Altenberger 1995a). However, most of these studies do not include enough details of

the mineral assemblage, the P,T-conditions of deformation or recrystallization microstructures to use them for thermodynamic calculations of phase equilibria. In order to obtain estimates of free energies, several microstructures from a high temperature Ivrea shear zone have been investigated in detail. This study tries to calculate order-of-magnitude-values of the di€erent driving potential for recrystallization and relate them to possible recrystallization mechanisms.

Observations and data Shear zones of the Ivrea Zone The analysed minerals (plagioclase, clinopyroxene, and hornblende) have been sampled in a high temperature shear zone in the quarry of Anzola (Val d'Ossola crosssection of the Ivrea Zone, Italy). In a shear zone of the same locality, Brodie (1981) has reported amphibole and plagioclase compositions of porphyroclasts and recrystallized grains. Numerous, similar high temperature shear zones of the Ivrea Zone have been described by Brodie and Rutter (1987), Rutter and Brodie (1990) and Rutter et al. (1993). The sampled shear zone is approximately 10 metres wide and shows a vertical foliation, which strikes north± south and is discordant to the metamorphic layering of the host rocks (Fig. 1). The boundaries between the sheared and undeformed host rock are sharp. The shear zone itself displays various degrees of shear deformation, including almost undeformed layers or elongated boudins at the margins. The main portion of the shear zone is formed by a dark mylonite to ultramylonite, which is several metres wide. Ten metres west of the main shear zone, a di€erent type of shear zone occurs, which consists of narrow, ®ne grained layers (approxi-

Fig. 1 Field relationships of the large ma®c shear zone in a vertical face of the Anzola quarry. The mylonitic foliation is vertical, striking north±south. The pre-existing, discordant metamorphic foliation of the granoblastic event is indicated on the eastern side. (Dotted layer metapelitic layer within the ma®c shear zone, white no outcrop, mostly concrete for quarry installations and door)

221

mately 5 millimetres to several centimetres wide) of equigranular, almost unfoliated plagioclase + clinopyroxene + hornblende mixtures that crosscut virtually undeformed, coarser grained material. The two types of shear zones can clearly be distinguished by their microstructure (Fig. 2). The narrow, discrete shear zones are completely recrystallized (grainsize about 30 to 200 lm) and consist of plagioclase (plag) + hornblende (hbl) + clinopyroxene (cpx). In these narrow, discrete shear zones, clinopyroxene is more abundant than hornblende (Fig. 2A), whereas in the undeformed host rock, hornblende is more abundant. The main mylonitic part of the large shear zone consists of a mixture of porphyroclasts and recrystallized grains (plag, cpx, hbl). The recrystallized grains are

smaller than in the narrow, discrete shear zones (5 to about 70 lm) and are usually concentrated in tails extending from relict porphyroclasts (Fig. 2B). The formation of narrow, discrete shear zones with more abundant clinopyroxene than hornblende may indicate a prograde change of P,T-conditions with respect to the predeformational assemblage at the transition from the amphibolite to granulite facies (consistent with the data and conclusions of Brodie, 1981). Deformation then has probably continued under retrograding conditions into the amphibolite facies and has produced the main shear zone (recrystallization of clinopyroxene to clinopyroxene + hornblende or only sometimes hornblende at clinopyroxene clasts). Thus, it appears that both types of shear zones record P,T-conditions from the transition between the amphibolite and granulite facies retrograding to the amphibolite facies, probably with a prograde initiation. This interpretation of the development of the structures may reconcile some contrasting interpretations of high grade shear zones in the Ivrea Zone: Brodie (1981), Brodie and Rutter (1985, 1987) and Rutter et al. (1993) have interpreted the recrystallization in most of the high grade shear zones to have occurred during a prograde P,T path. Kruhl and Voll (1978), Zingg et al. (1990) and Altenberger (1995b) have found evidence for retrograde amphibolite facies shear zones in the Ivrea Zone. This study is con®ned to the large retrograde amphibolite facies parts of the shear zone. Recrystallization microstructures of the sheared rocks The recrystallized matrix of the shear zone consists of alternating layers of mainly monophase material (plagioclase, hornblende, clinopyroxene) and mixtures of plagioclase + clinopyroxene and plagioclase + hornblende (Fig. 2B). Plagioclase, hornblende and clinopyroxene show di€erent degrees of recrystallization. Clinopyroxene

Fig. 2A,B Microstructures of the two di€erent types of shear zones. Note the di€erent grainsize of the recrystallized grains in both shear zones. A Boundary between a small scale shear zone (upper half of the image) and undeformed host rock (lower half). The main phases clinopyroxene (cpx), hornblende (hbl) and plagioclase (pl) are well mixed in the shear zone. Cpx is more abundant than hbl in the shear zone, whereas hbl is more abundant in the host rock. Crossed polarizers. B Overview of a typical microstructure from the large shear zone in the quarry of Anzola. (Dark grains hbl, colourless with high relief cpx, colourless with low relief pl). Recrystallized grains of hbl and cpx always form tails extending from porphyroclasts. Plane polarized light

Clinopyroxene is the least recrystallized phase. Clinopyroxene porphyroclasts have tails of recrystallized clinopyroxene (Fig. 3A), occasionally containing small amounts of hornblende (Fig. 3B), which also occurs in fractures within the porphyroclasts or as equant grains adjacent to clinopyroxene grains (Fig. 4A, B). This recrystallization (reaction) of clinopyroxene to clinopyroxene + hornblende may be of very limited extent, or more pervasive, as some clinopyroxene porphyroclasts show tails of only hornblende. The di€erent degree of reaction progress may be due to the duration and timing of the reaction with respect to the retrograde deformation history. No corona structures, which would demonstrate a reaction of clinopyroxene to hornblende after recrystallization, are visible (Fig. 4B).

222

Hornblende For hornblende, the ratio of clasts to recrystallized grains is approximately 1:1. The hornblende porphyroclasts have their long axes orientated parallel to the lineation (Fig. 2B, 3A). Usually, there are beardlike overgrowths on the porphyroclasts. From these overgrowths, tails of recrystallized hornblende grains extend parallel to the foliation (Fig. 3C). Some plagioclase and clinopyroxene occur between recrystallized hornblende grains (Fig. 3C). These microstructures suggest that recrystallization of hornblende may pro-

duce well dispersed aggregates of hornblende + plagioclase (Fig. 3C) and, less commonly, hornblende + pyroxene grains. Later, retrograde alteration of hornblende to chlorite + actinolite is con®ned to reaction rims of 1 to 5 lm along the grain boundaries and cleavage fractures (Fig. 4C). Plagioclase Most of the plagioclase is recrystallized so that only a few porphyroclasts are left in the matrix. Two di€erent

223 b Fig. 3A±E Recrystallization microstructures in the light microscope. A Clinopyroxene (cpx) clast (centre) with tails of recrystallized grains. Dark porphyroclasts and their recrystallized tails consist of hornblende (hbl). Some small hbl grains occur in the cpx tails that extend from the large cpx clast. Plane polarized light. B Cpx porphyroclast embedded in a matrix of recrystallized cpx with only very minor amounts of hbl. Plane polarized light. C Hbl porphyroclast (dark) and recrystallized aggregates. Beards of hbl-overgrowths occur between the porphyroclast and the tails or recrystallized grains. The beards and the tails have the same composition. Some mixing of hbl with plagioclase (pl) can be observed within the tails of recrystallized hbl. Plane polarized light. D Plagioclase porphyroclast embedded in a matrix of predominantly plagioclase. Bulging recrystallization microstructures are well developed at the margins of the clast. Subgrain formation (arrows) is present as well as abundant bulging of grain boundaries. Crossed polarizers. E Plagioclase porphyroclast embedded in a matrix of hbl + cpx + pl. The zones of recrystallized grains are discrete and in some cases appear to be microfractures. Some clinozoisite + hornblende occur in the discrete zones that crosscut the porphyroclasts

recrystallization microstructures can be distinguished in di€erent layers of the shear zone, only a few centimetres apart. Porphyroclasts embedded in a plagioclase matrix have recrystallized at their margins by subgrain rotation combined with local GBM (Fig. 3D). The combination of local GBM and some subgrain rotation may be termed ``bulging recrystallization'' (Drury et al. 1985) and corresponds to ``regime D'' of Drury and Urai 1990. In contrast to this type of microstructure, the porphyroclasts surrounded by a hornblende or mixed clinopyroxene + hornblende + plagioclase matrix have a more angular shape, show bent twin planes and undulatory extinction (Fig. 3E). Recrystallized grains occur in discrete zones, together with some clinozoisite or hornblende. Locally, some of these zones of recrystallized grains appear to be fracture ®lls in the plagioclase porphyroclasts (Fig. 3E). The microstructural di€erences are due to di€erent compositions of the porphyroclasts (see below). Small white mica ¯akes, very small amounts of calcite (Fig. 4D), and some clinozoisite grains occur in the plagioclase layers formed by bulging recrystallization (Fig. 4E). Two di€erent microstructures of clinozoisite can be distinguished: some clinozoisite grains are rimmed by a corona of albite and have a random orientation within the plagioclase matrix formed by bulging recrystallization (Fig. 4E). The other type of clinozoisite microstructure occurs in the discrete zones of recrystallized plagioclase and shows elongated clinozoisite grains, which are surrounded by intermediate plagioclase without coronas (Fig. 4F). These clinozoisite grains have a preferred orientation parallel to lineation. The di€erent microstructural relationships are interpreted as two generations of clinozoisite, one of which has formed under greenschist facies conditions after the deformation (random orientation and corona structures of albite). The other, earlier generation has formed syndeformationally in equilibrium with intermediate plagioclase under amphibolite facies conditions.

Chemical analyses of porphyroclasts and recrystallized grains Four plagioclase microstructures, one hornblende and one clinopyroxene microstructure have been analysed for their porphyroclast and recrystallized matrix compositions with a JEOL 8600 microprobe ®tted with four WDS spectrometers and a Voyager control system at 15 kV, 10 nA, using silicate standards, and correcting with the PROZA routine. Representative compositions of all examples are listed in Tables 1 and 2. The recrystallized grains of clinopyroxene (diopside) show less Tschermak component than the porphyroclasts at approximately constant Ca-contents (Fig. 5). The Mg/(Mg + Fe)-ratios of the recrystallized grains are higher compared to the porphyroclast (Fig. 5). Some analyses of recrystallized grains are similar to clast compositions. In such cases, it is not clear whether the analyses have been made in relict fragments of clasts in the recrystallized matrix or in new grains. The di€erences in composition between porphyroclasts and recrystallized grains are all consistent with lower recrystallization temperatures compared to porphyroclast formation (Raase et al. 1986; Schumacher 1991). The recrystallized grains of hornblende tend to have lower Al- and Ti-contents and higher Mg/(Mg + Fe)ratios than the porphyroclasts (Fig. 6). These chemical changes are consistent with retrograding P,T-conditions during deformation causing Ti- and Tschermak-substitutions (e.g. Raase et al. 1986; Schumacher et al. 1990; Schumacher 1991). The overgrowth beards have the same compositional range as recrystallized grains. Locally, there is no gradation in composition but always a discrete step between porphyroclasts and overgrowth/ recrystallized grains. The overgrowth beards and absence of transitional compositions between porphyroclasts and recrystallized grains indicate that di€usional exchange is not the cause of the compositional di€erence between recrystallized grains and porphyroclasts. It is important to note that the Na A-content is not systematically related to the Si-content of porphyroclasts and recrystallized grains of hornblende (edenite exchange vector, Fig. 6C). Thus, the hornblendes do not seem to follow a systematic relationship of decreasing edenite content in equilibrium with plagioclase under decreasing temperature conditions (Spear 1980, 1981). In plagioclase, the two di€erent types of microstructures show di€erent chemical characteristics for recrystallized grains and porphyroclasts (Fig. 7). Two microstructural sites of porphyroclasts embedded in a plagioclase matrix formed by bulging recrystallization (Fig. 3E) have a completely overlapping compositional range of clasts and matrix grains between An 30±35 and An 35±39 (Fig. 7A, B). However, the recrystallized grains are distinguished by higher Or-contents compared to the porphyroclasts (Fig. 7A, B). Two examples of a microstructural site showing discrete zones of recrystallized grains (Fig. 3F) have compositional ranges for the clasts of An 68±74 and An

224

81±83 (Fig. 7C, D). The corresponding recrystallized grains range between An 69 and 59 (Fig. 7C) and An 47 and 83. Both examples have their lowest An-compositions around An 60 (disregarding the single data point at An 47 in Fig. 7D). All compositions of recrystallized grains are consistent with retrograding P,T-conditions of deformation compared to the porphyroclasts. Some recrystallized grains and porphyroclasts have approximately 1 to 2 lm wide rims of albite (around plagioclase) or chlorite or actinolite (around hornblende and pyroxene) at their grain boundaries (Fig. 4A, E, C,

D). These rims are interpreted to have formed during a late, greenschist facies alteration, which has a€ected the rocks locally. The composition of all recrystallized grains is rather homogeneous, without zonation, except for these clearly de®ned rims. The alteration is limited to grain boundaries (and cleavage fractures in hornblende) in the investigated samples and indicates that later diffusional exchange in plagioclase, hornblende and pyroxene is limited to a discrete zone of 1 to 5 lm in the grain boundary region.

225 b Fig. 4A±F SEM backscatter images of recrystallization microstructures. A Clinopyroxene porphyroclast (right side of the image) and a tail of recrystallized grains. The outlined box is enlarged in B (medium grey clinopyroxene, light grey hornblende, dark plagioclase). B Enlarged part of A. Clinopyroxene (medium grey) and hornblende (light grey) grains occur adjacent to each other. Around the clinopyroxene grains, there are no hornblende coronas visible, which could indicate a later reaction of clinopyroxene to hornblende. Small, dark grains are plagioclase. C Chlorite and actinolite rims (darker grey) have formed along grain boundaries and cleavage fractures of hornblende. (white sphene, light grey hornblende). D Calcite (cc) and white mica (wm) in a recrystallized plagioclase matrix (medium grey). E Clinozoisite grain (cz) in a recrystallized intermediate plagioclase matrix (medium grey). Note the corona of albite (ab) around the clinozoisite grain. F Detail of plagioclase grains of a discrete zone of recrystallized plagioclase (see also Fig. 3E). The clinozoisite (cz) in the centre is orientated parallel to the lineation and is not rimmed by an albite corona. Narrow albite rims (approximately 1 lm) are visible at the grain boundaries of many recrystallized plagioclase grains

P,T-conditions of deformation The shear zones studied have developed after the granoblastic event at the end of the Variscan orogenesis (Brodie and Rutter 1987; Rutter and Brodie 1990; Zingg et al. 1990; Zingg 1990). A normal crustal thickness of approximately 30 km may have been established before the shear zone formation (Handy 1987; Schmid 1993), which initiated in an extensional geologic setting under Table 1 Average microprobe analyses of porphyroclasts and recrystallized grains of clinopyroxene, hornblende. Hornblende recalculation is based on cations = 13

the conditions of maximum burial of the Ivrea Zone ( ˆ lowest section of the crust; Brodie and Rutter 1987; Zingg et al. 1990; Rutter and Brodie 1990, 1992). Thus, it may be assumed that the P,T-conditions of the high temperature shear zones have not considerably increased above the values obtained from the granoblastic fabrics (the host rocks of the shear zones). The upper pressure limit for the granoblastic fabrics is given by Hunziker and Zingg (1980) as 9 to 11 kbar, derived from garnetplagioclase equilibria. Zingg (1983) estimates the pressure at around 8 kbar, which is in good agreement with the stability of sillimanite in the interlayered metapelites, constraining the maximum pressure to 7±8 kbar (at a temperature of 700 °C; Bohlen 1991; Bucher and Frey 1994, p. 196). Rutter et al. (1993) have estimated the deepest burial in the Ivrea Zone to be approximately 30 km, corresponding to pressures of 8 to 9 kbar. The temperature estimates vary from 700 to 940 °C (Zingg 1990; Rutter and Brodie 1990; Zingg 1983) for the temperature maximum of the granulite facies metamorphism. The syndeformationally recrystallized grains appear to form the assemblage Cpx + Hbl + Plag + Clizoisite ‹ white mica. Deformation occurred during a retrograde P,T-history, and recrystallization of plagioclase, clinopyroxene and hornblende might have occurred at di€erent episodes during the deformation history.

Clinopyroxene Porphyroclast

Hornblende Recrystallised grains

Porphyroclast

Recrystallised grains

SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O

49.57 0.34 2.12 14.77 0.34 10.10 22.20 0.39 0.01

51.34 0.13 0.97 14.67 0.37 10.58 22.25 0.23 0.02

40.27 2.22 13.52 19.61 0.29 7.31 11.50 1.91 1.67

40.47 1.89 12.77 19.58 0.28 7.58 11.54 1.71 1.67

Total

99.84

100.56

98.30

97.49

Si Aliv

1.92 0.08

1.96 0.04

6.13 1.87

6.20 1.80

Sum T

2.00

2.00

8.00

8.00

vi

Al Ti Fe3+ Mg Fe2+ Mn Ca NaM2 NaM4

0.01 0.01

0.005 0.004

0.58 0.48 0.01 0.92 0.03

0.60 0.47 0.01 0.91 0.02

0.55 0.25 0.17 1.66 2.33 0.04 1.88

0.50 0.22 0.24 1.70 2.27 0.04 1.89

0.12

0.11

Sum M1-M4

2.04

2.019

7.00

6.97

0.44 0.32 0.76

0.40 0.33 0.73

NaA K A-sum No. of analyses

10

17

17

20

226 Table 2 Average microprobe analyses of porphyroclasts and recrystallized grains of the two di€erent types of plagioclase recrystallization microstructures (one example for each microstructure)

Bulging recrystallised microstructure

Discrete zones of recrystallized grains

Porphyroclast

Porphyroclast

Recrystallised grains

Recrystallised grains

SiO2 Al2O3 CaO Na2O K2O

60.25 26.07 7.59 7.13 0.17

59.67 26.13 7.55 7.11 0.08

50.50 32.25 14.84 3.31 0.05

52.01 30.98 13.43 4.07 0.06

Total

101.21

100.54

100.95

100.55

Si Al Sum Si+Al Ca Na K

2.65 1.35 4.00 0.36 0.61 0.01

2.64 1.36 4.00 0.36 0.61 0.005

2.28 1.72 4.00 0.72 0.29 0.003

2.35 1.65 4.00 0.65 0.36 0.004

Sum Ca+Na+K

0.98

0.975

1.013

1.014

An Ab Or

36.65 62.35 1.00

No. of analyses

16

Fig. 5 Composition diagrams for clinopyroxene. Recrystallized grains have lower Tschermak components and higher Mg/Fe ratios than porphyroclasts

36.83 62.71 0.46 18

71.00 28.70 0.30

64.36 35.28 0.37

12

14

Therefore, the pyroxene, hornblende and plagioclase compositions probably do not represent an equilibrium assemblage. Furthermore, di€erences in recrystallization microstructures such as tails of recrystallized clinopy-

227

(Schumacher et al. 1990). These ®gures probably represent an upper temperature limit at the transition between amphibolite and granulite facies. The observed groups of plagioclase compositions (An 32±37 and approx. An 60) in recrystallized grains may coexist at temperatures (below the closure of the solvus) of 650±700 °C (Smith and Brown 1988; Carpenter 1995). Thermometry using plagioclase-hornblende equilibria after Holland and Blundy (1994) is impossible to apply, because there appears to be no exchange equilibrium between plagioclase and hornblende (Spear 1980, 1981). The presence of clinozoisite in the discrete zones of recrystallized grains suggests that the An-content of the plagioclase is controlled by the reaction: Plag1 ‡ H2 O ! Plag2 ‡ Zoi ‡ Ky ‡ qtz

…1†

where plag2 is more albitic than plag1. The presence of muscovite in the recrystallized matrix formed by bulging recrystallization is consistent with a lower Or-content in the recrystallized grains and suggests the reaction: Plag1 ‡ H2 O ! Muscovite ‡ Plag2

Fig. 6 Composition diagrams for hornblende. Recrystallized grains tend to have lower Ti-contents and higher Mg/Fe ratios

roxene-matrix and tails of recrystallized hornblendematrix extending from clinopyroxene porphyroclasts suggest recrystallization under varying P, T, PH2 O conditions, producing only local equilibrium. From the compositions of recrystallized grains, one temperature estimate for the deformation may be derived from the Ti-content of the hornblende. The lowest Ti-values indicate temperatures between 650 and 700 °C

…2†

where plag2 contains less Or-component than plag1. The univariant reaction curves for the measured plagioclase compositions in the di€erent assemblages have been calculated using the program DOMINO by de Capitani (1994). The database used is JUNE 92, an update of Berman (1988), including the solution models for feldspar by Fuhrman and Lindsley (1988) and for mica by Chatterjee and Froese (1975). For reaction (2), the An-content of the plagioclase was controlled for the reaction by the presence of zoisite. The results are in good agreement with experimental results by Goldsmith (1982). The pressures of the calculated assemblages are near the maximum given in the literature for the Ivrea Zone. Using the constraint of an upper pressure limit of approximately 8.5 kbar (i.e. similar to the 8 kbar estimated by Zingg 1983), yields deformation temperatures for the plagioclase compositions of 570, 578 and 604 °C (Fig. 8). These temperature estimates are within a rather narrow range, are lower than those derived from the Ticontent in hornblende, and are more in accord with retrograde deformation conditions in the amphibolite facies. Realistic con®ning pressure values for the deformation probably have been lower than 8.5 kbar, considering clinozoisite is used as a pure phase in the data base. Deformation temperatures between 550 and 650 °C appear likely. The lower end of this temperature estimate is probably too low for the coexistence of clinopyroxene and hornblende, but, as mentioned above, plagioclase and clinopyroxene recrystallization may have occurred at di€erent episodes of deformation. Estimation of the driving thermodynamic potential for recrystallization Recrystallization is a process of a structural transformation by which a crystalline material releases Gibbs

228

Fig. 7A±D Composition diagrams for plagioclase. A and B Composition diagrams of microstructures showing bulging recrystallization. C and D Composition diagrams of microstructures showing discrete zones of recrystallized grains

dynamic potential for a structural transformation depends on the total energy of the system, this energy may be considered as (Urai et al. 1986; Cumbest et al. 1989): DGtot ˆ DGchem ‡ DGdef ‡ DGsurf ‡ DGel

free energy to reach an equilibrium state. The probability of a transformation depends on a ``driving force''. The driving force in this sense is strongly dependent on DG as a rate-determining variable: there is an exponential relationship for nucleation rate dependence (Ridley 1985) and a linear dependence of grain boundary velocity on DG (Poirier 1985, p. 69±71). However, the term ``driving force'' will not be used here in this rather general sense, because the driving force F for a migrating boundary moving a certain distance x through a grain is de®ned by Poirier (1985, Eq. 2.75) as: F ˆ ÿdG=dx

and has the units of a mechanical force (N). The driving force de®ned in this way applies to migrating boundaries only and is a meaningless term in the context of nucleation. In order to avoid ambiguity when considering recrystallization involving migrating boundaries but to keep the discussion general with respect to the physical nature of DG and to include nucleation processes, the term ``driving thermodynamic potential'' will be used in this text to denote a driving force in the general sense, following Korzhinsky (1959). Since the driving thermo-

where the DGchem-term describes the Gibbs free energy as a di€erence in potential and kinetic energies of the atoms (chemical bonding and thus the composition and crystal structure of the material in the mineral paragenesis), DGdef is a measure of the deformational energy stored in a crystal, usually in the form of dislocations, DGsurf is the surface energy, and DGel is the energy term of the elastic lattice distortions due to externally applied stresses. The DGel is usually neglected in plastic deformation (Paterson 1973; Poirier 1985, p. 70). The surface energy becomes an important term at grainsizes of £10 lm, where Gsurf is approximately ³500 kJ/m3, using a speci®c surface energy of 0.5 to 1 J/m2 for silicates (Parks 1984). However, at grain sizes of 100 lm, Gsurf is only about 50 kJ/m3, and these small energy contributions will be neglected. Thus, only the DGchem and DGdef will be considered, so that the driving thermodynamic potential for the compositional change of a crystal during recrystallization is the DGchem-term, and for dynamic recrystallization, it is the DGdef-term. For amphibole and pyroxene, it is dicult to estimate the contribution of the DGchem-term, because at present, there are no mixing models, that would allow the free

229 b Fig. 8A±C Reaction curves for plagioclase of measured compositions (recrystallized grains). Solid triangles mark the temperatures at 800 MPa (570, 578 °C), and 850 MPa (604 °C). A Diagram for the parageneses Mus + Plag + Qtz + Zoi + H2O (An 32.5, corresponding to recrystallized grains' compositions of Fig. 7A) in the bulging recrystallization microstructures (one example: Fig.3D). B Same as A but for An 37.5, corresponding to recrystallized grains' compositions of Fig. 7B. C Diagram for the paragenesis Plag + Zoi + Ky + Qtz + H2O (An 60, corresponding to lower bound of recrystallized grains' compositions in Fig. 7C,D) in discrete zones of recrystallized grains (one example: Fig. 3E)

energy of the solid solution to be calculated accurately. However, for plagioclase some mixing models describing the non-ideal solution exist (e.g. Salje et al. 1985; Fuhrman and Lindsley 1988) and the contribution to the driving potential of a compositional change at given P and T may be estimated. The estimate is based upon the assumption that the recrystallized plagioclase grains form in equilibrium with the mineral assemblage and ambient P and T at the time of recrystallization. The reaction curve has been calculated for the measured mean composition of the recrystallized grains and their assemblage (Fig. 8). For a given pressure and temperature on the reaction curve (using reasonable estimates of deformation P and T established in the previous section), the DG-curve for compositional variation of plagioclase has been calculated (Fig. 9) with the program THERIAK by de Capitani and Brown (1987), using the same database mentioned above. Two types of compositional variation have been calculated: a variation in orthoclase content (Fig. 9A, B) and a variation in anorthite content (Fig. 9C). The DG is calculated as the total DG of the system divided by the mole fraction of plagioclase. The DG of the plagioclase porphyroclast composition with respect to the equilibrium composition of the recrystallized grains can be obtained from the free energy curve (Fig. 9) and represents the DGchem-part of DGtot. There is a compositional variation of recrystallized grains and of porphyroclasts (Fig. 7A±D), so that there is quite some uncertainty in the estimation of DG. Adding the compositional variation of recrystallized grains to that of porphyroclasts, the di€erence in DG for di€erent orthoclase contents ranges from 0 to 60 J/ 10)4 m3 (Fig. 9A, B), for di€erent anorthite contents from 0 to 100 J/10)4 m3. This variation should be considered, when mean compositions are discussed. The calculated isopleths are all parallel to the reaction curves of endmember compositions, because only one component in the plagioclase changes during the reaction. Thus, the exact location of the P-T-point is not critical for the DG-estimate, as long as it lies on the calculated reaction curve. The deformational energy contribution (DGdef) may be estimated for screw dislocations by the relationship: E ˆ …lb2 =4p† ln…R=r0 † ‡ alb2

where b is the Burger's vector, l the shear modulus, R the outer radius of the volume around the dislocation

230

loys (Schoeck 1995). In the absence of more precise calculations, r0 ˆ 5b is used here for the size of the dislocation core, according to the approximation by Weertman and Weertman 1992, p. 47). The core energy is approximated by the second term in the equation, choosing a ˆ 0.3, following the arguments of Hirth and Lothe (1992) and Hull and Bacon (1984). The energy is given per unit length of a screw dislocation. For edge dislocations, the relationship is E ˆ ‰lb2 =4p…1 ÿ m†Š ln…R=r0 † ‡ alb2

where m is the Poisson ratio. The resulting values are multiplied by the dislocation density and calculated for 10)4 m3 (corresponding approximately to the molar volume of anorthite; Fig. 10), using data from Heinisch et al. (1975). Dissociation of dislocations in plagioclase (Montardi and Mainprice 1987; Olsen and Kohlstedt 1984) is not considered; all dislocations are treated as perfect dislocations with [001] and [100] Burger's vectors in the I 1 structure. A similar way of estimating the stored strain energy of crystals has been employed by Green (1972), Wintsch and Dunning (1985), Lasaga and Blum (1986), and Blum et al. (1990). For plagioclase in bulging recrystallization microstructures (Fig. 3E), the free energy di€erence between the recrystallized grains and porphyroclasts results from the di€erence in orthoclase content between recrystallized grains (mean: 0.48 Or and 0.71 Or), and porphyroclasts (mean: Or 1.46 and Or 0.97), Fig. 7A, B. The DGs for these mean compositions are 4 and 7 J/ 10 m)4 m3, respectively (Fig. 9A, B), although some compositional variations result in DGs up to 60 J/ 10)4 m3. For the microstructural sites showing discrete zones of recrystallized grains (Fig. 3F), the porphyroclasts show little compositional di€erence in anorthite content (mean: An 83 and An 71, Fig. 7C, D). The compositional variation of anorthite content in recrys-

Fig. 9A-C Gibbs free energy curves for compositional variation in plagioclase at inferred P,T-conditions (indicated on the isopleths of Fig. 8). G is given in J/10)4 m3, a volume, which corresponds approximately to the molar volume of anorthite (ˆ 100.79 cm3). DG ˆ 0 corresponds to the mean composition of recrystallized grains. DG for the porphyroclast composition can be obtained from the curve. A For the microstructures showing bulging recrystallization (Fig. 3D, data in Fig. 7A). B For another microstructure of the same type (data in Fig. 7B). C For the microstructure of discrete zones of recrystallized grains (Fig. 3E) and a second microstructure of the same type (data in Fig. 7C,D)

considered, r0 the radius of the dislocation core, and a a factor between 0.3 and 0.5 (Hull and Bacon 1984). One problem lies in the determination of r0 and the core energy of dislocations. So far, atomistic treatments of the core energy are limited to fcc crystals and some al-

Fig. 10 DGdef of plagioclase crystals for various dislocation densities and di€erent Burgers vectors. Details of the calculation are given in the text

231

tallized grains is large. However, the anorthite content of recrystallized grains converges to minimum values around An 60 (Fig. 7C, D) for both examples, so that An 60 will be used as the equilibrium composition of recrystallized grains. The resulting DG for the mean compositions of the porphyroclasts is 21 and 98 J/ 10)4 m3, respectively (Fig. 9C). These are the maximum values of DG for the compositional variation in anorthite content because there are compositions of recrystallized grains similar to porphyroclasts with DG near zero (Fig. 7C, D). The energy contribution from dislocations is more dicult to estimate in naturally deformed crystals, because TEM-measurements usually do not re¯ect the dislocation density at the time of deformation (White 1976; Christie and Ardell 1976). Observed high dislocation densities in naturally and experimentally deformed minerals are usually on the order of 1013±14 m)2 for most minerals (e.g. Griggs and Blacic 1965; McLaren and Hobbs 1972; Phakey et al. 1972; White 1976, 1977; Christie and Ardell 1976; Fitz Gerald et al. 1991), although higher densities (>1014 m)2) have been observed in the grain boundary regions of plagioclase (Sodre Borges and White 1980) and in quartz (Christie and Ardell 1976). Thus, it appears that 1014 m)2 is a reasonable estimate for high dislocation densities in naturally deforming crystals (at least transiently), and locally, e.g. at grain boundaries, even higher densities may occur. Recrystallization occurs predominantly in the grain boundary region, where dislocation densities are high. If a high value of 1014 m)2 is assumed, then the increase in free energy compared to annealed grains (dislocation density on the order of 1012 m)2; Christie and Ardell 1976; Phakey et al. 1972; Spry 1969, p. 18, corresponding to a DG-value of 1±2 J/mole; Fig. 10) is on the order of 100 J/10)4 m3 for plagioclase. Thus, for high dislocation densities, the increase in DGdef is on the same order of magnitude as DGchem for the highest compositional disequilibrium observed (Fig. 10, 9A, B). For the mean compositions of recrystallized grains and porphyroclasts in the bulging recrystallization microstructures (Fig. 3E), the DGchem is considerably lower than DGdef for densities of 1014 m)2.

Discussion Driving thermodynamic potential and the recrystallization mechanism Strain induced grain boundary migration (SIGM) occurs when a grain boundary moves from a region of low dislocation density to a region of high density (Bailey and Hirsch 1962; Nicolas and Poirier 1976; Poirier and Guillope 1979; Cahn 1983; Poirier 1985; Drury et al. 1985; Drury and Humphreys 1986; Urai et al. 1986). Dislocations are eliminated in the grain boundary region, and the migrating boundary leaves a ``strain-free'' crystal behind. Thus, the driving thermodynamic po-

tential for this process arises from the higher dislocation density of the more deformed material. If the driving potentials due to dislocations are combined with those due to compositional disequilibrium, then these extra driving potentials are expected to increase the grain boundary velocity, provided that the migrating boundary can release the free energy due to compositional disequilibrium. In order to achieve this, there has to be an ionic exchange along the migrating grain boundary, as has been demonstrated by Evans et al. (1986) and Hay and Evans (1987A, B) for calcite. The concentration of the solute along the grain boundary will also have a great in¯uence on boundary velocity driven by compositional disequilibrium (Hay and Evans 1987a). A general type of GBM involving driving potentials induced by strain and chemical energy would represent a combination of ``strain induced grain boundary migration'' (SIGM) and ``chemically induced grain boundary migration'' (CIGM). Such a situation can be implied for the microstructure dominated by local GBM (Fig. 3D, bulging recrystallization). The contribution of compositional disequilibrium to the total driving potential (Fig. 9A, B) is probably small compared to the driving potentials arising from a dislocation density on the order of 1014 m)2 (Fig. 10). In order to estimate the magnitude of DG necessary to drive GBM in naturally deformed rocks, quartz may serve as an example for SIGM, because the compositional variation of quartz is very limited, and GBM may be considered as pure SIGM. If the driving potential for GBM are estimated for a high dislocation density of 1014 m)2, the resulting maximum DGdef is on the order of 60 J/10)4 m3. Thus, the frequently observed SIGM in quartz can be induced by DGs of 60 J/ 10)4 m3 and smaller. The DGs due to di€erences in orthoclase content in bulging recrystallization microstructures (approximately 5 J/10)4 m3) are an order of magnitude smaller than those values and are not expected to drive recrystallization to a great extent. However, there is some variation to higher DGchemvalues, so that there may be a contribution of DGchem to the driving force for GBM. Furthermore, it should be emphasized here, that for the system investigated, the presence of an aqueous ¯uid is evident from the formation of hydrated phases (clinozoisite, mica). In general, the presence of water may strongly enhance the mobility of grain boundaries, so that a small free energy increase could still have some e€ect on the microstructure during ``liquid ®lm migration'' (Hay and Evans 1987a, b; Urai et al. 1986). The situation is di€erent for the microstructure containing discrete recrystallization zones, where there is a change in anorthite content, and possibly fracturing during deformation. The maximum DGchem is on the same order of magnitude as the potential maximum contribution from deformational energy. Thus, the total DG may be approximately twice as high for the combined driving potentials as for a high dislocation density or a compositional change alone, and the critical nucleus

232

size could be suciently small to allow heterogeneous nucleation. This is re¯ected in the microstructure, where discrete zones of recrystallized plagioclase between plagioclase fragments and isolated small grains within porphyroclasts occur (Fig. 3F). Similar microstructures have been described by Fitz Gerald and StuÈnitz (1993) and StuÈnitz (1993). Usually, the free energy derived from even high densities of dislocations alone is not sucient for heterogeneous nucleation (Cahn 1983) and other driving forces are necessary for this recrystallization mechanism (Etheridge and Hobbs 1974; White 1975; Bell 1978; Cumbest et al. 1989; Fitz Gerald and StuÈnitz 1993). From the observations and estimates above, a DG of approximately 200 J/mol (max. DGchem + max. DGdef) appears to be sucient to overcome kinetic barriers for heterogeneous nucleation in plagioclase. Thus, the increase in free energy derived from the compositional disequilibrium, combined with DG from dislocations in the material, could lead to a recrystallization by heterogeneous nucleation, a recrystallization mechanism that normally does not occur during dynamic recrystallization. The compositional variation of recrystallized grains in the microstructures (especially the variation in anorthite content) may be explained in two di€erent ways. The variation may be due to a di€erent timing of the nucleation of new grains, always adjusting the composition to changing P,T-conditions on a retrograde P,T-path. Alternatively, it may be due to grain boundaries sweeping the recrystallized grains several times. Each time a boundary sweeps a grain, the composition may change, a feature observed experimentally by Hay and Evans (1987a, b). Contrary to GBM and heterogeneous nucleation, progressive subgrain rotation does not facilitate ionic exchange in new grains except along the new planar defects. From the planar defects into the grain interior, ionic exchange has to occur by volume di€usion, a mechanism that is too slow to be signi®cant in plagioclase at temperatures between 500 and 600 °C (Yund 1986; Yund et al. 1989; Yund and Tullis 1991). Consequently, progressive subgrain rotation does not produce a di€erence in composition between recrystallized grains and porphyroclasts. Although subgrain formation was observed in these microstructures (Fig. 3E, F) it may be inferred from the compositional di€erence between the porphyroclast and the recrystallized grains, that the latter have mainly formed by boundary migration mechanisms. Subgrain rotation is only important for the initiation of new planar defects, which immediately after their formation begin to migrate and lead to dynamic recrystallization (Guillope and Pourier 1979; Drury et al. 1985; Buatier et al. 1991). Grain boundary migration is an ecient way to adjust a crystal composition to the equilibrium composition (Yund et al. 1989; Yund and Tullis 1991). The compositional di€erence between recrystallized grains and porphyroclasts may be used as evidence in this case (and probably in many others) to demonstrate that recrystallization of plagioclase mainly

takes place by boundary migration mechanisms and only to a minor extent by progressive subgrain rotation, as distinguished and described previously by Tullis and Yund (1985), Tullis et al. (1990), Yund and Tullis (1991), and Hirth and Tullis (1992).

Relationship between dynamic recrystallization and metamorphic reactions The combined contribution from compositional disequilibrium and stored strain energy to the driving forces for recrystallization may explain the observation that plagioclase often recrystallizes at small amounts of strain (Brodie and Rutter 1985, 1987; Fitz Gerald and StuÈnitz 1993), because grain boundary velocity may be increased or nucleation facilitated due to increased DG. Crystal plasticity of plagioclase is often used as a temperature indicator for deformation (T greater than 500 to 550 °C, e.g. Voll 1976; Tullis 1983). If recrystallization is used as evidence for crystal plasticity in such a case, the conclusion might be invalid, because the recrystallization is not only strain induced under these conditions (Fitz Gerald and StuÈnitz 1993). Depending on the amount of free energy increase due to compositional disequilibrium, the recrystallization may be related more to a metamorphic reaction. Even in such cases, the recrystallization may take place in deformed portions of the rock only. This situation could be explained by deformation induced processes such as aqueous ¯uid input, which enhances the kinetics of grain boundary migration or nucleation suciently. In either case, the recrystallization involving chemical change should be termed syndeformational, syntectonic, or synkinematic rather than dynamic.

Other minerals The lack of non-ideal mixing models for hornblende and pyroxene precludes the calculation of the free energy increase from compositional disequilibrium for recrystallization of these porphyroclasts. However, the situation is the same as for plagioclase: progressive subgrain rotation cannot explain the observed compositional di€erences between clasts and new grains. Skrotzki (1992) and Dornbusch (1995) describe subgrain formation in ma®c Ivrea shear zones, but as in the case of plagioclase, dynamic recovery by subgrain rotation is probably the precursor stage to dynamic recrystallization by GBM. Furthermore, hornblende porphyroclasts (Fig. 3C) show compositionally di€erent overgrowth beards, which suggest heterogeneous nucleation of hornblende during deformation. The formation of the mixture of plagioclase + hornblende grains at the porphyroclast margins (Fig. 3C) may also well be explained by heterogeneous nucleation. Thus, GBM by bulging recrystallization and heterogeneous nucleation

233

appears to be the main mechanism for recrystallization of hornblende. There is little data on compositional e€ects during recrystallization of clinopyroxene, but the trend and magnitude of compositional di€erences observed in this example (Table 1) are the same as those of Molli (1994). The microstructures of the rather equant recrystallized grains of clinopyroxene do not allow a determination of the recrystallization mechanism from light microscope observations. However, the chemical di€erences between porphyroclasts and recrystallized grains suggest a mechanism of GBM or heterogeneous nucleation because progressive subgrain rotation or cataclasis do not lead to a change in composition. Heterogeneous nucleation and CIBM have been proposed for hornblende recrystallization by Cumbest et al. (1989), but the compositional di€erences between hornblende porphyroclasts and recrystallized grains of Ivrea shear zones are far more subtle (Brodie 1981; this study) than the example described by Cumbest et al. (1989). They extend the meaning of the term ``dynamic recrystallization'' to cases where substantial compositional changes are involved, based on the argument that it is more practical for rock-forming minerals, which usually display a large degree of chemical variation. It does not appear to be advantageous to abolish the more strictly de®ned concept for dynamic recrystallization formulated by Poirier and Guillope (1979), namely ``a . . . reworking . . . with little or no chemical change'', as done by Cumbest et al. (1989), since it is clear from the discussion above that chemical di€erences between porphyroclasts and recrystallized grains (even small di€erences) have consequences for the recrystallization mechanisms and may help to identify or infer these mechanisms. Most rock-forming minerals (being solid solutions) will show a compositional dependence of recrystallization, and therefore the strictly de®ned term ``dynamic recrystallization'' may only rarely be applied to these cases. The recognition of these compositional e€ects is a valuable tool for the understanding of the recrystallization processes. Implications for tectonics and deformation mechanisms The close relationship between metamorphic reaction and dynamic recrystallization for recrystallizing plagioclase is expected for deformation under most P,T-conditions except those of the initial plagioclase formation. Any P,T-path deviating from the reaction curves (or isopleths in this case) in Fig. 8 (plag + H2O ˆ ab + zoi + qtz + ky + H2O) will induce an increase in free energy in plagioclase due to compositional disequilibrium. Normal P,T-paths (prograde and retrograde) deviate from straight reaction curves such as in Fig. 8, and compositionally induced driving potential for recrystallization are expected in virtually all cases. Other examples of compositionally induced driving potentials for

recrystallization are the miscibility gaps in plagioclase, which close somewhere between 600 and 800 °C (Carpenter 1995; Smith and Brown 1988). Immiscibility is particularly important for retrograde P,T-paths of ma®c magmatic or granulite facies rocks, where the initial formation temperatures of plagioclase are above about 700 °C and exsolution is to be expected during cooling. Strain weakening through dynamic recrystallization and recovery is a common and well known process during crystal plastic creep (Poirier 1985, p. 188±189). Additional to the weakening in crystal plastic deformation, the recrystallization mechanism can have important consequences for the active deformation mechanism, as has been proposed for the transition from dislocation creep to grainsize sensitive deformation (Schmid 1982; Rutter and Brodie 1988; Handy 1989). While it is disputed that the transition in deformation mechanism can be achieved by dynamic recrystallization (Poirier 1985, p. 189) because the concept of the equilibrium grainsize is in contradiction with such a change in mechanism (Etheridge and Wilkie 1979), it is clear that heterogeneous nucleation is a very e€ective way to produce ®ne grained aggregates that may show grainsize-sensitive deformation mechanisms (Fitz Gerald and StuÈnitz 1993; StuÈnitz and Fitz Gerald 1993). As there is abundant evidence for grainsize-sensitive deformation in polyphase aggregates in nature (e.g. Boullier and Gueguen 1975; Kerrich et al. 1980; Rubie 1983; Behrmann and Mainprice 1987; StuÈnitz and Fitz Gerald 1993), recrystallization by a combination of heterogeneous nucleation, SIBM and CIBM appears to be one of the most important mechanisms through which ®ne grained aggregates can be produced, which are a prerequisite for grainsize-sensitive deformation mechanisms. Concluding remarks Deformation in a ma®c shear zone from the Ivrea Zone took place during a retrograde P,T-path under amphibolite facies conditions at approximately 550 to 650 °C and con®ning pressures of probably less than 8 kbar. The syndeformational recrystallization of clinopyroxene, hornblende and plagioclase has produced compositional di€erences between porphyroclasts and recrystallized grains. Two di€erent plagioclase microstructures show di€erent chemical variations between porphyroclasts and recrystallized grains: a di€erence in orthoclase content and a di€erence in anorthite content. The di€erences arise from the compositional adjustment to the equilibrium composition during recrystallization under the given P,T-conditions. The extreme values of compositional di€erences between recrystallized grains and porphyroclasts induce approximately the same driving thermodynamic potential for recrystallization as high dislocation densities for both types of microstructures. The relative magnitude of driving potentials appears to have an e€ect on the recrystallization mechanisms: if

234

the compositional driving potentials are small compared to the deformational ones, the e€ect on grain boundary velocity during GBM is small (small contribution of CIGM compared to SIGM) and the recrystallization may more or less be termed ``dynamic''. This is probably the case for mean compositional changes in orthoclase content of bulging recrystallization microstructures (Fig. 3D). If the compositional driving energies are on the same order of magnitude as the deformational ones, the recrystallization may take place by heterogeneous nucleation or the chemical e€ect to increase grain boundary velocity is great. Such a process may probably be termed metamorphic reaction as much as dynamic recrystallization. The release of compositional DG is necessary, if the driving potentials are expected to have a signi®cance in terms of a€ecting a physical process. The only realistic way to change the chemical composition and release energy from compositional disequilibrium during recrystallization of geologic materials is by ionic exchange through a migrating boundary (CIBM) or by heterogeneous nucleation. Progressive subgrain rotation is a recovery feature and probably does not produce chemical di€erences between porphyroclasts and new grains. Chemical di€erences have been produced syndeformationally and may be used as a tool to distinguish between a dominance of recovery and recrystallization processes. The described e€ects appear to be very common during deformation along a typical P,T-path during orogenesis, and therefore, compositional driving potentials are expected to occur in most solid solution minerals of most rock types during syndeformational recrystallization. The term ``syndeformational'', ``syntectonic'', or ``synkinematic'' recrystallization is preferred in comparison to ``dynamic recrystallization'' because it does not dispose a recrystallization mechanism. The described combination of SIGM and CIGM is probably the most ecient way to produce ®ne grained recrystallization products that are the prerequisite for grainsize-sensitive deformation mechanisms. Therefore, it may be the favoured way, in which a change in deformation mechanism from crystal plasticity to grainsize-sensitive mechanisms can be achieved during deformation. Acknowledgements This work has bene®tted greatly from comments by and discussions with Martyn Drury, John FitzGerald, ReneÂe Heilbronner, Robert Kruse, Mervyn Paterson, Stefan Schmid, John Schumacher, and, especially, Christian de Capitani and James Connolly. Dani Mathys from the central SEM facilities of Basel University helped with the SEM work. Susanne Schmidt helped with the microprobe analyses, Christian de Capitani with the thermodynamic calculations, and ReneÂe Heilbronner with computer graphics. Stefan Schmid and Robert Kruse provided ®eld photographs. I would like to thank all of them very much. The manuscript has been substantially improved by the careful reviews of Neil Mancktelow, Janos Urai, and an anonymous reviewer. Special thanks to Andre Zingg, who provided the sample for this study. This research has been supported by the Swiss Nationalfonds Grant No. 20-42 134. 94.

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