1. metamorphic Geol., 1995, 13, 553-558

Local disequilibrium of plagioclase in high-temperature shear zones of the lvrea Zone, Italy U. ALTENBERGER lnstitut f u r Geowissenschaften, Johannes Gutenberg Universitat Mainz, Saarstr. 2 1 , D 55099 Mainz, Germany

ABSTRACT Microstructural and chemical analysis of plagioclase in 20 superficially similar amphibolite facies ductile shear zones in metagabbors and amphibolites of the Ivrea Zone in Italy reveals significant differences in An and Ba contents. Plagioclase, which was deformed at P-T conditions lower than those of the wall rocks, occurs in the following four different microstructural situations with different chemical composi- tions: (i) relatively undeformed porphyroclasts, (ii) dynamically recrystallized grains and subgrains rimming the porphyroclasts, (iii) infill of microcracks cross-cutting the porphyroclasts and (iv) fine-grained recrystallized grains in the matrix of the shear zones. The differences in the An and Ba contents are caused by partial chemical equilibration of plagioclase in the shear zones during and partly after deformation. Changes in An and Ba contents were caused by fluid-assisted grain-boundary migration recrystallization, as well as by solid-state diffusion, while fluid activity was high. The relation between the composition and microstructures of the plagioclase in the shear zones indicates that in the different shear zones, fluids ceased to be active during different stages in the late shear zone deformation history.

The interpretation of the variations in composition and microstructures reveals that only grains that developed by grain-boundary migration recrystallization and that are not adjacent to porphyroclasts reflect P-T conditions during the dominant shear-zone deformation.

Key words: disequilibrium; Ivrea Zone; microfabrics; plagioclase; shear zone; thermobarometry.

I N T R O D U C T I O N

The An content of plagioclase can be used as a rough geothennometer if an exchange partner, such as amphibole, is present (e.g. Wenk ef al., 1974; Plyusnina, 1982; Altenberger, 1991a). However, its use in deformed, dynamically recrystallized, plagioclase-bearing rocks is problematic. Several authors have described dynamically recrystallized grains (i.e. grains that developed during deformation) that significantly differ in their chemical composition from their host grains (e.g. Brodie, 1980; Olsen & Kohlstedt, 1985; Brodie & Rutter, 1987; Altenberger, 1991~; Yund & Tullis, 1991). The question in such rocks is: what part of the plagioclase should be used to determine P-T conditions during deformation?

This paper presents results of a detailed microstructural and chemical study of the compositional relationship between plagioclase porphyroclasts and recrystallized grains in shear zones in which dynamic recrystallization of plagioclase was important. Local-scale variations in An and Ba content show that one should be very careful in selecting the right grains for determining P-T conditions during deformation.

ANALYTICAL PROCEDURES

Mineral analyses were performed on a CAMEBAX electron microprobe using ZAF data reduction (Hennoc &

Tong, 1977). The standards used were natural wollastonite (for Si & Ca), natural feldspars (for Al, Na & K), and synthetic oxides (for Ti, Fe, Mg, Ba & F). Chemical data, other than those presented, can be obtained from the author on request.

G E O L O G I C A L SETTING

The studied samples were collected in 20 different ductile shear zones in the Ivrea Zone in the Italian Southern Alps (Fig. 1). This zone basically represents a section through the lower part of the pre-Alpine (Variscan) continental crust, and mainly consists of metamorphosed ultramafic, mafic and pelitic rocks. It was deformed and metamor- phosed under amphibolite to granulite facies conditions during the Caledonian and Variscan orogenic phases. During the subsequent Alpine phase, it was exhumed (Koppel, 1974; Zingg, 1983; Teufel & Scharer, 1989; Burgi & Klotzli, 1990 Zingg ef af., 1990). The investigated (Variscan) shear zones developed under amphibolite facies metamorphic conditions, at temperatures between 550 and 650°C (Altenberger, 1991b, using thermometers of Spear, 1981; Plyusnina, 1982; Graham & Powell, 1984; Colombi, 1988). In the Val d'Ossola region, maximurn temperatures were 675 f 50" C in the north-west, and 530 f 50" C in the south-east (Altenberger, 1991b). The shear zone wall rocks give maximum temperatures of about 750°C in the north-west (Zingg, 1983; Strackenbrock-Gehrke, 1989).

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MICROSTRUCTURES

The shear zones cut through medium- to coarse-grained rocks consisting of granoblastic metagabbros and strongly foliated amphibolites. They are fine-grained mylonites and ultramylonites, following the classification of Sibson (1977). The dominant minerals in the shear zones are plagioclase, amphibole and clinopyroxene that occur as host-rock relics (i.e. porphyroclasts). These are embedded in a recrystallized matrix that also consists of plagioclase, amphibole and clinopyroxene, although the latter occurs only in shear zones that have been formed under uppermost amphibolite facies conditions. Garnet and orthopyroxene also occur as prophyroclasts and as matrix grains, but in much smaller quantities.

The plagioclase occurs in the following four different microstructural forms; (i) relatively undeformed porphyra- clasts in the shear zones, (ii) dynamically recrystallized grains and subgrains rimming the porphyroclasts, (iii) infill in microcracks cross-cutting the porphyroclasts and (iv) fine-grained recrystallized grains in the matrix of the shear zones. The plagioclase porphyroclasts show a typical core-and-mantle structure, subgrains and/or recrystallized grains having similar sizes (Fig. 2a).

fig. 1. Geological sketch map of the Ivrea Zone, redrawn after Z i n g (1983).

INTERPRETATION OF MICROSTRUCTURES

The grain-size reduction of plagioclase and amphibole associated with the formation of the (ductile). shear zones is interpreted as due to deformation by a recrystallization- accommodated dislocation creep mechanism. Other possible grain-size reducing deformation mechanisms, such as rotation-recrystallization during dislocation creep, only occur at much lower temperatures than the 550-650°C characteristic for the shear zones studied (e.g. FitzGerald el al., 1983; Pokier, 1985; Tullis & Yund 1985; Urai ef al., 1986). Simpson (1985) also described deformation of plagioclase by grain-boundary migration recrystallization at 450-650" C. No evidence for grain-size reduction by cataclasis was observed. However, the plagioclase porphyroclasts show core-and-mantle structures, which indicate that some rotation-recrystallization occurred. I argue here that this must have occurred at a lower temperature after the main recrystallization- accommodated dislocation creep event that produced the fine-grained (recrystallized) matrix.

To summarize, the deformation in the shear zones is inferred to have started with recrystallization- accommodated dislocation creep (producing a fine-grained

DISEQUILIBRIUM IN SHEAR ZONES 555

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Fig. 2. (a) Amphibolite facies shear zone from Val d’Ossola, showing plagioclase porphyroclasts embedded in a fine-grained plagioclase matrix. (Crossed polars; long side of picture 3.35 mm.) (b) Chemical profile across shear zone shown in (a). (An content: black columns; Ba content: squares.) Variations in An and Ba content are small, but reproducible. (c) Correlation between grain size of recrystallized plagioclase and An content.

matrix), and ended with climb-accommodated dislocation creep (producing the core and mantle structures in the porphyroclasts).

CHEMICAL COMPOSITION OF PLACIOCLASE AND A M P H I B O L E

Chemical composition of plagioclase

The plagioclase grains show large variations in An and Ba contents (typically 2011101%). The An and Ba contents

were determined in porphyroclasts, recrystallized grains and subgrains at the rim of the porphyroclasts, recrystallized grains within microcracks cross-cutting the porphyroclasts and recrystallized plagioclase of the matrix. For comparison, the An and Ba contents were also measured in relatively undeformed plagioclase grains in the shear-zone wall rocks. On the basis of variations in An and Ba contents, the following three different types of shear zones were distinguished. 1 In type 1 shear zones the An and Ba contents of the recrystallized plagioclase that rims the porphyroclasts, plagioclase filling microcracks and plagioclase in the cores of the porphyroclasts are similar (Figs 2a,b & 3a,b). The An and Ba contents of recrystallized plagioclase grains in the matrix are lower. A correlation between An content and grain size of the recrystallized plagioclase in the matrix can also be seen: the larger grains have a higher An content (Fig. 2c). 2 In type 2 shear zones, the An and Ba contents of the fine-grained plagioclase in the matrix are similar to those of the recrystallized rim of the porphyroclasts, but differ from those of the cores of the porphyroclasts (Fig. 4). Diffusion rims occur in the plagioclase grains in the

_- I

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(b) Fig. 3. (a) Microcrack through a plagioclase porphyroclast in type 1 shear zone. The crack is outlined by a ribbon of recrystallized grains. Note that mechanical twinning post-dates the recrystallization. (Crossed polars; long side of picture 1.5 mm.) (b) Chemical profile across microcrack shown in (a). Distances are given in tens of micrometres.

556 U . A L T E N B E R C E R

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distance (cm) Fig. 4. Chemical profile across plagioclase and amphiboles in wallrock and shear zone of type 2.

relatively undeformed wallrocks. The An content of these diffusion rims is similar to that of both the recrystallized grains in the matrix and the recrystallized grains rimming the porphyroclasts. The difference in An content between the latter two types of recrystallized grains is <3.5 mol%, whereas the difference between recrystallized grains and porphyroclasts is up to 25mol%. Some samples of wall rocks adjacent to type 2 shear zones show subgrain boundaries in the d ihs ion rims of the plagioclase grains. The diameter of these subgrains is smaller than the diameter of the diffusion rims. 3 In type 3 shear zones, the An and Ba contents of the plagioclase in the matrix, in the recrystallized rim and in the core of the porphyroclasts are similar, but different from the plagioclase in the wall rocks (Fig. 5; see also Altenberger, 1991~). In the weakly deformed wall rocks (<lo vol.% recrystallized grains), a large contrast in An

1

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distance (cm) Fig. 5. Chemical profile across plagioclase and amphibole in wallrock and shear zone of type 3.

content between old grains, rims and the few recrystallized grains occurs (Fig. 5). This contrast decreases towards the centre of the shear zone, where compositional differences between porphyroclasts and recrystallized grains d o not exceed 3-5 mol%. The smallest chemical variation exists between the recrystallized grains in the rim of the porphyroclasts and in the matrix.

Chemical composition of amphibole

In some samples, . amphibole compositions were also determined, but less systematically. A chemical profile across a type 2 shear zone shows that the Ti content of hornblende vanes sympathetically with the An content of the plagioclase (Fig. 4). In type 3 shear zones, amphibole shows a similar behaviour to that of the plagioclase, with decreasing Ti content towards the shear zone (Fig. S), as expected from the decreasing temperature towards the shear zone.

INTERPRETATION OF CHEMICAL DIFFERENCES OF PLACIOCLASE

Yund & Tullis (1991) have shown experimentally that during recrystallization-accommodated dislocation creep in plagioclase, the ion-exchange rate was high only when water was present on migrating grain boundaries. That the matrix plagioclase was entirely recrystallized via grain- boundary migration recrystallization and that the composi- tion was completely altered towards low-T compositions indicate that the activity of water was high during this (initial) deformation phase of the shear zone. This is consistent with evidence for fluid-enhanced material transport in these shear zones, as indicated by bulk compositional changes (Altenberger, 1991c, 1992).

During rotation-recrystallization, grain boundaries do not necessarily migrate. They are assumed to be essentially tight, i.e. to consist of dislocation networks, and hence should not allow rapid mass transport. So, during rotation-recrystallization, equilibration of plagioclase does not occur by migration of boundaries. Instead, equilibration of the plagioclase must occur more slowly by solid-state diffusion.

In type 1 shear zones, the porphyroclasts plus rims show ‘old’ (high) An and Ba contents. Hence, during the later deformational stage, during which rotation- recrystallization occurred, no solid-state equilibration of plagioclase occurred. This means that fluid activity during development of the core and mantle structure, i.e. during the later stages of the deformation, must have remained low.

In type 2 shear zones, the porphyroclast rims have the same ‘new’ (low) An and Ba contents as those of the matrix plagioclase. Only the cores of the porphyroclasts show ‘old’ (high) An and Ba contents. This means that not only before, but also during the later deformational stage during which rotation-recrystallization took place, the fluid activity must have remained high.

In type 3 shear zones, the entire porphyroclasts show

DISEQUILIBRIUM IN S H E A R ZONES 557

‘new’ (low) An and Ba contents. This means that re-equilibration continued after the last deformational stage, so that fluid activity must have remained high until after the last deformational stage.

Fluids must have been more active in the shear zones than in the wall rocks, since the plagioclase cores in the wall rocks have an ‘old’ chemical composition, i.e. were not equilibrated. This means that fluid permeability in the wall rocks must have been lower, thus inhibiting ion exchange by fluid-phase grain-boundary diffusion.

DISCUSSION AND CONCLUSION

The differences in plagioclase composition in all of the amphibolite facies ductile shear zones discussed indicate that they all must once have represented zones of high fluid permeability and activity during mid to lower crustal shearing. However, in different shear zones, fluid ceased to be active during different stages during their late deformation history.

Solid diffusion through the crystals must have been of minor importance in the shear-zone environment, because deformation-enhanced reactions are short-term processes (e.g. Newton, 1990). The results of the present study also show that one should be very careful in selecting the right grains for thermobarometry in deformed rocks. Not all the grains yield P - T conditions that prevailed during the main deformational event. For example, in shear zones of types 1 and 2, the porphyroclasts and their recrystallized rims indicate P-T conditions that prevailed during pre-shear- zone metamorphism and deformation of the wallrocks. Only grains developed by grain-boundary migration- recrystallization that are not adjacent to porphyroclasts reflect P-T conditions during the dominant shear-zone deformation. In large shear zones, in which no unaffected precursor rock survives, the composition of the porphyroclasts could be the key for the pre-shearing history. However, the present study shows that porphyro- clasts should only be used if there is a marked difference in composition between recrystallized grains and the porphyroclasts. When both have the same composition, the interpretation remains ambiguous.

ACKNOWLEDGEMENTS

I am grateful to B. Schulz-Dobrick for his help during the microprobe sessions, to B. Lafrance, J. H. Kruhl and B. den Brook for their critical reviews, and to A. Priewisch and B. Shannon for correcting the English. Funding for field work and mineral analyses was provided by the Deutsche Forschungsgemeinschaft (DFG).

REFERENCES Altenberger, U., 1991a. Geochemie und Metamorphose der

Amphibole der Sesia-Lanzo-Zone nord6stlich des Val d’Ossola, Prov.di Novara/Norditalien. Mainzer geowissemchaftliche Mitteilungen, 20, 91-118.

Altenberger, U., 1991b. The ductile deformation of the lvrea Zone - a study from micro to mesoscale. Mitteilungen Geologisches Instirut. ETH Zurich, pp. 91-92.

Altenberger, U., 1991~. Hochtemperierte Scherzonen in der Ivrea-Zone/N. Italien. Ein Beitrag zu deren Mikrogefiige, Metamorphose und Geochemie. Zentralblatt Geologie Palatontoligie, Teil 1, 3-20.

Altenberger, U., 1992. Fluid enhanced element mobility and mass transport in a Variscan peridotite shear zone in the Ivrea Zone/Northern Italy. 8. Rundgesprach: Geodynamik des europaischen Variszikums, CheblCSFR, Abstracts, p . 2 .

Brodie, K. H., 1980. Variations in mineral chemistry across a phlogopite peridotite shear zone. Journal of Structural Geology,

Brodie, K. H. & Rutter, E. H., 1987. Deep crustal extensional faulting in the Ivrea Zone of Northern Italy. Tecfonophysics,

Biirgi, A. & Klotzli, U., 1990. New data on the evolutionary history of the Ivrea-Verbano ZonelNorthern Italy). Bulletin Swiss Association of Petroleum Geologists and Engineers, 56, 49-70.

Colombi, A., 1988. Metamorphism et geochimique des roches mafiques des Alpes ouest-centrales. These de doctorat, Lausanne, Switzerland.

FitzGerald, J. D., Etheridge, M. A. & Vernon, R. H., 1983. Dynamic recrystallization in a naturally deformed albite. Textures and Microstructures, 5,219-237.

Graham, C. M. & Powell, R., 1984. A garnet-hornblende geothermometer: testing and application to the Pelona Schist, southern California. Journal of Metamorphic Geology, 2, 13-21.

Hennoc, J. & Tong, M., 1977. MBXCOR, a new package for the automatic CAMEBAX microprobe. Conference on X-ray optics and microanalysis, Boston.

Koppel, V., 1974. Isotopic U-Pb ages of monazites and zircons from the crust-mantle transition and adjacent units of the lvrea and Strona-Ceneri Zones Southern Alps. Contributiom to Mineralogy and Petrology, 43, 55-70.

Newton, R. C., 1990. Fluids and shear zones deep in the crust. Tectonophysics. 18521 -37.

Olsen, T. S. & Kohlstedt, D. L., 1985. Natural deformation and recrystallization of some intermediate plagioclase feldspars. Tectonophysics, 111, 107- 131.

Plyusnina, L., 1982. Geothermometry and geobarometry of plagioclase-hornblende bearing assemblages. Contributions to Mineralogy and Petrology, 80, 140-146.

Poirier, J. P. 1985. Creep of Crystah. Cambridge Earth Sciences Series, Cambridge.

Sibson, R. H., 1977. Fault rocks and fault mechanism. Journal of the Geologicul Society of London, U3, 191-213.

Simpson, C. (1985) Deformation of granitic rocks across the brittle-ductile transition. Journal of Structural Geology, 7 , 503-5 11.

Spear, F. S., 1981. An experimental study of hornblende stability and compositional variability in amphibolite. American Journal of Sciences, ’281,697-734.

Strackenbrock-Gehrke, I. 1989. Thermomctrie und Graphitgenese in Metakarbonaten der Ivrea-Zone (Norditalien). Unpubl. PhD Thesis, Glittingen.

Teufcls, S. & Scharer, U., 1989. Unravelling the age of high grade metamorphism of the Ivrea Zone: a monazite single grain and small fraction study. Terra Abstracts, 1, 350.

Tullis, J. & Yund, R. A., 1985. Dynamic recrystallization of feldspar: a mechanism of ductile shear zone formation. Geology, 13,238-241.

Urai, J . L., Means, W. D. & Lister, G. S., 1986. Dynamic recrystallization of minerals. In: Mineral and Rock Deformation: Laboratory Studies. The Paterson Volume. Geophysical Monographs of the American Geophysical Union,

Wenk, E.. Schwander, H. & Stem, W., 1974. On calcic amphiboles and amphibolites from the Lepontine Alps.

2,265-277.

140, 193-212.

36,161-199.

558 U. A L T E N B E R G E R

Schweizer Mineralogische Pefrographische Mineilungen, 54. Perrographirche Mirteilungen, 63,361-392. 97-149. Zingg, A., Handy, M. R., Hunziker, J. C. & Schmid, S. M., 1990.

Yund, R. A. & Tullis, J., 1991. Compositional changes of minerals Tectonometamorphic history of the Ivrea Zone and its associated with dynamic recrystallization. Contributions to relationship to the crustal evolution of the Southern Alps. Mineralogy and Petrology, 108, 346-355.

Zingg, A., 1983. The lvrea and Strona Ceneri Zones (Southern Alps, Ticino and N. Italy) - a review. Schweizer Mineralogische

Tectonophysics, 182, 169- 192.

Received 27 January 1993; revision accepted 14 February 1995.