Contributions to Mineralogy and Petrology

, Volume 165, Issue 3, pp 543–562

Interaction of chemical and physical processes during deformation at fluid-present conditions: a case study from an anorthosite–leucogabbro deformed at amphibolite facies conditions

Authors

    • Department of Geological SciencesStockholm University
  • Sandra Piazolo
    • Department of Geological SciencesStockholm University
    • Department of Earth and Planetary Sciences, GEMOC ARC Key CentreMacquarie University
Original Paper

DOI: 10.1007/s00410-012-0822-9

Cite this article as:
Svahnberg, H. & Piazolo, S. Contrib Mineral Petrol (2013) 165: 543. doi:10.1007/s00410-012-0822-9
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Abstract

We present microstructural and chemical analyses of chemically zoned and recrystallized plagioclase grains in variably strained samples of a naturally deformed anorthosite–leucogabbro, southern West Greenland. The recorded microstructures formed in the presence of fluids at mid-crustal conditions (620–640 °C, 7.4–8.6 kbar). Recrystallized plagioclase grains (average grain size 342 μm) with a random crystallographic orientation are volumetrically dominant in high-strain areas. They are characterized by asymmetric chemical zoning (An80 cores and An64 rims) that are directly associated with areas exhibiting high amphibole content and phase mixing. Analyses of zoning indicate anisotropic behaviour of bytownite plagioclase with a preferred replacement in the \( \left\langle {0 10} \right\rangle \) direction and along the (001) plane. In areas of high finite strain, recrystallization of plagioclase dominantly occurred by bulging recrystallization and is intimately linked to the chemical zoning. The lack of CPO as well as the developed asymmetric zoning can be explained by the activity of grain boundary sliding accommodated by dissolution and precipitation creep (DPC). In low-strain domains, grain size is on average larger and the rim distribution is not related to the inferred stress axes indicating chemically induced grain replacement instead of stress-related DPC. We suggest that during deformation, in high-strain areas, pre-existing phase mixture and stress induced DPC-caused grain rotations that allowed a deformation-enhanced heterogeneous fluid influx. This resulted in local plagioclase replacement through interface-coupled dissolution and precipitation and chemically induced grain boundary migration, accompanied by bulging recrystallization, along with neocrystallization of other phases. This study illustrates a strong interaction and feedback between physical and chemical processes where the amount of stress and fluids dictates the dominant active process. The interaction is a cause of deformation and external fluid infiltration with a result of strain localization and chemical re-equilibration at amphibolite facies conditions.

Keywords

PlagioclaseRecrystallizationFluid–rock interactionChemically induced grain boundary migrationInterface-coupled dissolution and precipitationElectron backscatter diffraction (EBSD)

Introduction

At mid-crustal levels, plastic deformation commonly occurs in the presence of fluids. Such fluids are, at the same time, important in enabling or even inducing chemical reactions and replacement in minerals (e.g. Fyfe et al. 1978). Fluids are commonly localized in deformation zones such as ductile shear zones and brittle faults (e.g. Beach and Fyfe 1973; Carter and Dworkin 1990; Streit and Cox 1998; Cartwright and Barnicoat 2003), while deformation is commonly localized in weak, hydrous metamorphic rocks (e.g. Piazolo et al. 2004). Even though the simultaneous operation of fluid–rock interaction and plastic deformation is common at mid-crustal levels, the role of fluids during deformation and vice versa is still debated (e.g. Fyfe et al. 1978; Brander et al. 2011 and references therein). Intracrystalline plastic processes have been shown to be enhanced by fluid influx (Post and Tullis 1998; Fusseis and Handy 2008, Jamtveit et al. 2008). One of the main processes which operate at fluid-present conditions and under differential stress conditions is dissolution and precipitation creep (DPC). This process of removing impinging parts of grains which allows easier rotation may play a significant role during deformation (Tullis and Yund 1980; den Brok and Spiers 1991; Fitz Gerald and Stünitz 1993; Post and Tullis 1998; Ford et al. 2002). DPC can be purely driven by differential stress (e.g. Durney 1972, Rutter 1976, Wintsch 1985, Wintsch and Yi 2002, Smit et al. 2011) where dissolution occurs at the area of highest stress and precipitation at the area of lowest stress. However, strain rate is not only linearly dependent on the stress, but also inversely dependent on the grain size where the precise relationship depends on whether the transport rate or the dissolution/precipitation rate is rate limiting (Ranalli 1995). The presence of fluid will influence the transport rate and/or the dissolution/precipitation rate. In the presence of fluid material transport are several orders of magnitude faster than diffusion through the crystal lattice (Farver and Yund 1992). If there is a chemical disequilibrium between fluid and mineral present, the composition of the precipitated material may be different to the original. As a consequence, an asymmetric rim of different composition grows on the original grain (Wintsch and Yi 2002). Fluids expand the PT strain and grain size field for DPC mechanisms (Wintsch and Yi 2002) and may result in changes in deformation mechanism by accelerating reaction softening and ease fluid-assisted grain boundary sliding (GBS; e.g. Tullis et al. 1996). However, in a stress-free environment, chemical disequilibrium between fluid phase and solid phase will result in an intimate coupling of dissolution and precipitation. In this case, the chemically unstable phase may be pseudomorphically replaced by a mineral more stable with the reactive fluid. Carmichael (1969) suggested that mineral re-equilibration at the grain scale can be modelled by a sequence of dissolution and precipitation subreactions, each process involving local changes in the composition and redistribution of chemical components. This concept is currently being revisited and is often referred to as interface-coupled dissolution and precipitation (icDP; e.g. Cardew and Davey 1985; Putnis 2002, 2009; Xia et al. 2009; Putnis and Austrheim 2010; Hövelmann et al. 2010). Results suggest that, in reactions where free fluid is present, mineral equilibration occurs by fluid-mediated, interface-coupled dissolution re-precipitation.

During plastic deformation, the physical process of dynamic recrystallization sensu stricto is assumed to produce new grains without compositional change (e.g. Passchier and Trouw 2005) and dominantly occurs in the ductile field. Grains are formed by either grain boundary migration (bulging recrystallization) or by subgrain rotation recrystallization (SGR) and are dominantly driven by elastic strain energy (Urai et al. 1986). In the most general sense, grain boundary migration refers to the migration of boundaries in order to attain a lower energy state (Jessell et al. 2003 and references therein) where relevant driving forces include lowering of strain, chemical and surface energy. (e.g. Urai et al. 1986; Hirth and Tullis 1992; Jessell et al. 2003). Chemically induced grain boundary migration (CIGBM) occurs when there are differences in chemical potentials and coherency strain energies between the neighbouring grains. CIGBM occurs in solid solution series and is well documented in several metal systems (cf. Yoon 1995). In geological materials, it has been observed in calcite experimentally (Hay and Evans 1987) and is suggested to occur in amphiboles (Cumbest et al. 1989), pyroxenes (Piepenbreier and Stöckhert 2001) and plagioclase (e.g. Stünitz 1998). Since, in geological systems, microstructures are easily overprinted and short-lived, this process may have been largely overlooked (Means 1989; Hay and Evans 1987). There is probably more than one driving force active during deformation (e.g. Stünitz 1998; Piepenbreier and Stöckhert 2001; Piazolo et al. 2010). Fluid presence enhances grain boundary migration mobilities and therefore grain boundary migration rates (Urai et al. 1986; Mancktelow and Pennacchioni 2004).

Furthermore, the presence of a trace amount of water in the crystals will have profound effects on the behaviour during deformation through hydrolytical weakening, which lowers the temperature for onset of effective dislocation creep (Tullis and Yund 1980). In summary, presence and/or influx of fluids may lower the strength of the rock and localize strain.

Of importance to the rock rheology during deformation is also the distribution, and pathways of the fluid media which may be heterogeneously distributed over short distances (micro-scale) and cause various deformation mechanisms to be activated during the same event, even simultaneously (e.g. Reinecke et al. 2000; Fusseis et al. 2006; Brander et al. 2011). The distribution of fluids is shown to be affected by deformation leading to wetted and open grain boundaries (Tullis et al. 1996; Reinecke et al. 2000). Transport of fluids may occur via fractures, along grain boundaries and triple-junction channels, but also through interconnected pore systems in reaction products (e.g. Putnis 2009).

This paper presents detailed microstructural and chemical analysis of a recrystallized plagioclase-dominated rock deformed at wet amphibolite facies conditions. Observations suggest a close link between physical and chemical processes where fluid played a major role. Furthermore, analyses shows possible anisotropic diffusion/dissolution rates in different crystallographic directions of plagioclase.

Geological background

The investigated samples stem from a variably strained amphibolite facies grade anorthosite–leucogabbro unit on Storø, an island located in the Nuuk region, southern West Greenland (Fig. 1). The region is dominated by rocks of Archaean age (>2.5 Ga) which according to current models consist of several (amalgamated) terranes displaying a complex geological history (e.g. Nutman et al. 2007 and references therein). In the region, anorthosite complexes are commonly associated with greenstone belts and ultramafic units (e.g. Windley and Garde 2009 and references therein).
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Fig. 1

Geological map over central Storø Island and sample location. Inset shows position of Storø (black dot) in southern West Greenland. Modified and simplified from Grims (in Hollis et al. 2004)

At present day levels, the studied anorthosite–leucogabbro is structurally on top of the Storø greenstone belt (SGB; van Gool et al. 2007). It is positioned within an overturned fold limb of an antiform, in the hanging wall of the Storø shear zone (SSZ; Fig. 1) which according to Nutman et al. (2007) represent a terrane boundary between two plates. This terrane boundary suture formed at 2.65 Ga (D1) causing high-pressure assemblages in the region. Initial deformation was followed by a protracted reworking (D2) and exhumation of the boundary only shortly after D1 including development of the SSZ at 2.63 Ga. Plagioclase coronas surrounding garnets in some of the garnet-bearing amphibolites in the SGB (van Gool et al. 2007) may indicate a decompression related to the inferred exhumation of the SGB. The SGB is host to a gold mineralization. Early mineralization occurred at ca. 2.71 Ga (arsenopyrite Re–Os isochron; Scherstén et al. 2012) and occurred coeval with hydrothermal alteration shortly after sediment deposition. At 2.635 Ga, the gold was remobilized during amphibolite facies retrogression (Juul-Pedersen et al. 2007; Knudsen et al. 2007; Nutman et al. 2007; Scherstén et al. 2012). This resulted in further gold concentration where fluid transport may have occurred over large distances (100 m to km distances) from SSZ into the hangingwall antiform–synform pair (van Gool et al. 2007).

We suggest that the main deformation events documented in the studied anorthosite–leucogabbro are the regionally identified D1–D2 events. According to thermobarometric investigations by Hollis and Persson (in Hollis 2005), which were carried out on nearby garnet (Grt) + sillimanite (Sil) + biotite (Bt) gneisses and Grt-amphibolites, temperature and pressure reached a range of 550–620 °C and 4.5–5.1 kbar, representing the conditions in the central and upper supracrustal package during exhumation (D2). These PT conditions are consistent with the observed mineral assemblage in studied rocks which are dominated by plagioclase and amphibole (Amp), without any metamorphic high-pressure and temperature minerals. Mineral abbreviations used follow Whitney and Evans (2010).

The studied anorthosite–leucogabbro is ~400 m wide and folded on a regional scale (Fig. 1). Within the unit, slivers of interleaved tonalitic orthogneisses and amphibolites are present. The anorthosite–leucogabbro is characterized by significant strain partitioning ranging from coarse-grained anorthosite with preserved igneous textures, to highly strained anorthositic gneiss, characterized by completely or near complete recrystallization and general fine grain size and strong mylonitic foliation (Fig. 2). The gneissic fabric exhibits no obvious lineation; however, D2 is characterized by a weak mineral lineation produced by aggregates of feldspar grains and Amp-preferred orientation.
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Fig. 2

a Outcrop photo of anorthosite–leucogabbro showing centimetre-scale-fingering shear zones. b Gradation from protomylonitic to mylonitic shear zone in anorthosite–leucogabbro

Methods

Mineral chemistry and geothermobarometry

Mineral chemical compositions were analysed using a JEOL Superprobe JXL 8200 (University of Copenhagen, Denmark) and a Cameca SX50 (Uppsala University, Sweden). For the JEOL superprobe standard online ZAF correction was performed during acquisition and the analytical conditions used were a 15 kV acceleration voltage, 10 nA sample current and 10 s counting time for peak and background using a 5 μm beam spot size. Analytical settings for the Cameca were a 20 kV acceleration voltage and 15 nA sample current with a 3 μm beam spot size. Here, analyses were corrected for by a PAP-modified ZAF correction method. The presence, position and symmetry of chemical zoning were additionally documented by backscattered electron imaging (BSE). Pressure and temperature conditions during deformation were determined from Ti-in-amphibole (Amp) thermometry (Spears 1981) and Al-in-Amp barometer (Anderson and Smith 1995).

Acquisition and analysis of crystallographic orientation

Crystallographic data were collected using the electron backscatter diffraction (EBSD) technique (Adams et al. 1993; Prior et al. 1999). Analyses were performed on thin sections at Stockholm University using a Phillips XL-30 FEG-ESEM equipped with a Nordlys detector and Channel 5 analysis suite from HKL Technology (Oxford instruments). Thin sections were chemically polished using colloidal silica and left uncoated during analyses. EBSD settings used were a working distance of between 19 and 21 mm and an accelerating voltage of 20 keV (~0.8 nA). Triclinic bytownite I-1 (Facchinelli et al. 1979) was used as the theoretical Pl match unit. To analyse the crystal orientation differences between a core and rim, the rim structure was transformed and plotted as triclinic labradorite (C-1) using the lattice parameters of Wenk et al. (1980). A step size of between 2 and 10 μm, depending on the crystallographic detail needed, was used during automated data collections in a rectangular grid using a beam scan. Processing of automatically acquired orientation data was performed to enhance the continuity of data over the microstructures following procedures suggested by Prior et al. (2002) and Bestmann and Prior (2003). Following Piazolo et al. (2006), a grain is denoted “strain free” when the average internal misorientation (along a profile) is <1°.

Relative finite strain analysis and representative thin section selection

To select thin section representing different finite strain, we used the porphyroclast/recrystallized grain ratio Rpr as a proxy for the relative finite strain experienced by the sample analysed. To do this, we counted the number of plagioclase porphyroclasts and recrystallized grains at 336 intersections on a grid with a line spacing of 1,150 μm.

Analysis of chemical zoning related to crystal structure in plagioclase

To determine the position of the chemical zoning related to the crystal structure and to the foliation and lineation, we used information from the collected EBSD data. The Channel 5 software provides a 3-D feature that visualizes a box-mineral with the crystallographic a-/b- and c-axis for each analysed point. We used this 3-D feature in combination with pole figures, backscattered electron images and photomicrographs to deduce the position of the rims on the most likely plane and direction in the bytownite atomic structure (I-1).

Porosity measurements via surface topography analysis

In order to evaluate the porosity of the investigated feldspar, surface topography measurements across rim–core boundaries of zoned Pl grains were performed using a SENSOFAR Plμ 2300 optical imaging profiler with accompanied SensoScan 2.45 software at Stockholm University. Confocal profilometry data are collected by vertical scan of the surface in multiple 100-nm steps. The high numerical aperture (0.95) and a 150× magnification lens allow a lateral and vertical resolution of 111 and 1 nm, respectively. In addition surface slopes of up to 71° can be detected. Since the reflectivity of the surface is dependent on its chemical composition, the intensity of light detected makes possible to distinguish different solid phases. Before analyses, the surface of the sample was chemically polished using colloidal silica (as for the EBSD analyses, see above) to remove irregularities produced during mechanical polishing. Etching of control samples without porosity showed that no porosity was induced during the etching process.

Results

Thin section description

For in depth analysis, three thin sections were selected out of a suite of 10 thin sections using the described relative finite strain analysis. Samples hso486009, hso486006c and hso486006b represent low (Rpr = 4.6), medium (Rpr = 0.22) and high strain (Rpr = 0.05), respectively (Fig. 3).
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Fig. 3

a Sketch illustrating position of analysed thin section from representative low, transitional and high-strain areas. b Thin section 486009 from low-strain area. Area in box labelled 1 is used for phase and Pl rim distributions in Fig. 4. c Thin section 486006c from transition between low- and high-strain area. Rotated 90° anticlockwise from a. d Thin section 486006b from high-strain area. Stippled lines encircle former, now nearly completely recrystallized porphyroclasts (Cl). Areas in boxes labelled 2 and 3 are used for phase and Pl rim distributions in Fig. 4. Foliation (S) is shown. X are holes (epoxy) in thin section. Numbers I and II refer to recrystallized grain size groups, see text for definition

The low-strain sample contains large (up to several centimetres) Pl crystals, that is, porphyroclasts, while smaller and recrystallized Pl grains are found as clusters between the large grains. These are more seriticised than their equivalents in higher strained counterparts (Fig. 3b, d). Fractures are found inside and across Pl porphyroclasts and small Pl grains. Asymmetric chemical zoning occurs as rims around porphyroclasts, recrystallized grains and along some fractures. Transgranular fractures without chemical zoning contain hydrous minerals phases such as micas. Amphiboles are partially replaced by randomly oriented Bt (Fig. 3b). The medium-strain sample is finer grained than its low-strain equivalent and exhibits mineral segregation into Pl- and Amp-dominated foliation parallel bands (Fig. 3c). Biotite, clinozoisite (Czo), opaque minerals and muscovite (Ms) are mostly found in association with Amp in these Amp-rich foliation parallel horizons. In addition, the number of recrystallized Pl grains that exhibit chemical zoning increases in the vicinity of Amp-rich horizons and hydrous phases (Fig. 4). Plagioclase porphyroclasts are smaller and more deformed than those seen in its low-strain equivalent.
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Fig. 4

ac Photomicrographs and a*–c* respective line drawings illustrating the correlation between the amount of chemical zonation (rims) on Pl grains (Group I) and the amount of other phases in representative areas in a the low-strain zone and b, c in the high-strain zone. See Fig. 2b, d for areas 1–3 in thin section. The “other phases” are dominantly amphibole with minor amounts of quartz, biotite and clinozoisite. a Near monomineralic area in the low-strain sample where recrystallized Pl grains display asymmetric rims and near absence of lobate grain boundaries and bulges; b near monomineralic area in the high-strain sample where recrystallized Pl grains display few rims and rarely lobate grain boundaries and bulges; c area in high-strain sample with an increased phase mixture of Pl–Amp–Qz and increase of rims, bulges and lobate grain boundaries in Pl. Legend at bottom is for line drawings

The high-strain sample shows similar features to the medium-strain sample. However, it is finer grained. Only a few porphyroclast remnants exist. These are less than 2 mm across and show significant undulose extinction.

Detailed microstructural grain analysis: shape, size, boundary characteristics, crystal orientations and chemical composition

Plagioclase porphyroclasts

Pl porphyroclast sizes range from millimetres up to several centimetres in diameter, where the larger grains are found in low-strain areas and the smaller in high-strain areas (Fig. 3). In low-strain areas, they display growth twins and show a low internal lattice distortion. Fractures are common and found as irregular, transgranular fractures and as crystal lattice bound fractures (Fig. 5a), following the perfect cleavage planes known in feldspar {001} and {010}. Micas decorate parts of the transgranular fractures. The average anorthite (An) content of porphyroclasts in all areas is An80 ± An2 (Table 1). Chemical zonation (rims of An64) is frequent along lattice bound fractures and is also found at grain boundaries. Hüttenlocher intergrowths (Jäger and Huttenloch 1955; Smith and Brown 1988) with a herring bone–type structure are found in Pl porphyroclasts (inset Fig. 5a).
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Fig. 5

Representative Pl pophyroclasts from low, transitional and high-strain zone. Note the increased abundance of microstructures indicative for crystal plastic deformation from low to high strain. a Low-strain zone (sample 486009) where arrow A and B point to irregular fractures and crystal plane bound fractures, respectively. Note chemical change associated with fractures and grain boundaries (dark grey). Inset shows BSE image of herring bone–like Hüttenlocher intergrowths from top bright porphyroclast, scale bar is 25 μm. b Transition zone with twin boundary migration in porphyroclast (arrow; sample 486006c). c Amoeboid-like remnant of porphyroclast and Group I recrystallized grains (sample 486006b). Arrows point to truncation of porphyroclast by recrystallized grains

Table 1

Chemical analyses of plagioclase and amphibole

Wt%

Porphyroclast

1st group recryst. core

1st group recryst. rim

Amphibole for PT calc

SiO2

47.86

47.96

52.02

42.19–42.65

TiO2

<0.01

<0.01

<0.01

0.58–0.63

AI2O3

33.58

33.37

30.72

12.10–12.92

 

0.00

0.00

0.00

0.00–0.05

FeO

0.04

0.14

0.04

18.70–19.43

MnO

<0.01

<0.01

<0.01

0.22–0.28

MgO

<0.01

<0.01

<0.01

10.01–10.05

CaO

16.57

16.55

13.38

11.61–11.95

Na2O

2.24

2.26

4.10

1.27–1.38

K2O

0.01

0.01

0.02

0.89–0.93

CI

0.00

0.00

0.00

0.04–0.06

Total

100.30

100.30

100.28

98.47–99.21

Mol %

    

An

80.27

80.20

64.28

 

Ab

19.68

19.80

35.60

 

Or

0.05

0.00

0.12

 

No. of analyses

11

61

30

3

An = Ca × 100/(Ca + Na + K); Ab = Na × 100/(Ca + Na + K); Or = K × 100/(Ca + Na + K)

At the transition from low to high strain, the porphyroclasts display the same features as in low strain, but in addition exhibit migration of twin boundaries (Fig. 5b), substructure and subgrain development and presence of small recrystallized grains with similar sizes as the subgrains. These substructures are to a large extent confined to twins, as are chemical changes. Still, recrystallized grains dominantly occur at grain boundaries. Porphyroclasts in high-strain areas only display a few fractures. These are mostly late transgranular, that is, they cut across grain boundaries. These porphyroclasts display an intense development of substructures and recrystallization (Fig. 5c). The shape of the porphyroclasts is irregular and amoeboidal, and large recrystallized grains grow on the expense of the clast as evidenced by truncation of twins and fractures (Figs. 3d, 5c).

Recrystallized plagioclase

In general, recrystallized Pl grains exhibit growth twins and low internal deformation. They are cut by thin, late, transgranular fractures that are decorated with hydrous phases. For the sake of clarity, we divide recrystallized plagioclase grains into two groups on the basis of their size, occurrence and chemical characteristics.

Grains of Group I are characterized by chemical zonations where the core chemical composition is An80, and for rims (chemical zoning), An64 (Table 1). The chemical zoning is characteristically asymmetric (Fig. 4). Group I grains are either seen as relatively large (average 720 μm) grains found around and inside highly strained remnants of porphyroclasts in the high-strain sample (Fig. 3d) or as smaller grains with an average grain size of 314 μm in the low-strain sample and 240 μm in the high-strain sample (Fig. 3b, d). Larger grains show grain orientations that scatter around the adjacent host porphyroclast orientation (Fig. 6a). In contrast, small Group I grains show random crystallographic orientations in both low- and high-strain areas (Fig. 6b, c). The latter Group I grains dominate the bulk of the recrystallized grains and occur in all samples. Their grain shape–preferred orientation (SPO) is more pronounced in the high-strain areas where long axis of grains is subparallel to the foliation (Fig. 7). Grain boundaries are commonly smooth and straight. In high-strain areas, lobate boundaries with increased boundary curvatures are present which are always associated with asymmetric chemical zoning (e.g. Fig. 8b). Grain boundary bulging is observed in both low- and high-strain samples and is closely associated with chemical zonation (Figs. 4, 8, 9, 10 and 11). The bulges have variable sizes (~50–150 μm) corresponding to the grain size range of Group II grains (Fig. 8). Some of the larger grains display subgrain structures and deformation twins. Hüttenlocher intergrowths are found in the larger recrystallized grains in the lower-strain area where they are truncated by rims (chemical zonation) with lower anorthite content (cf. Fig. 9).
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Fig. 6

Pole figures showing poles to planes for recrystallized Pl (ad) and amphibole grains (e) represented by: a high-strain area (in a strain shadow and within a porphyroclast); Group I; b transition to and from high-strain area; Group I; c low-strain area; Group I; d high-strain area; Group II, that is, grains <150 μm grain size; e amphibole from high-strain area. Pole figures are presented as one point per grain, equal area lower hemispheres. Horizontal line is foliation, and lineation is at X

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Fig. 7

Rose diagrams showing 2-D orientation of Group I recrystallized grains as seen in thin section; a grain long axis to foliation (horizontal plane) and b thickest rim to foliation. Data are presented from low- and high-strain areas (data from area shown as a box in Fig. 3b and area next to box (3) in Fig. 3d, respectively). Sense of shear is indicated

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Fig. 8

Photomicrographs showing Pl rim and grain boundary structures from a to c a high-strain area and d a low-strain area. a Asymmetric rim and bulging. Note angle between twins in core and rim. Solid arrow point at position where an old grain will be separated into two smaller grains, retaining the old grain chemistry. b Lobate and facetted grain boundaries. Note grain size differences. c Faceted grain boundaries (solid arrow) related to rim growth direction (stippled arrow). d Zig-zag-shaped boundary between Pl core and rim in low-strain area. Inset shows orientation of selected crystallographic axes from core (An80) and rim (An64). Note limited deviations in major axis directions. Pole figure is presented as equal area and upper hemisphere. 3-D representation of mineral orientation is shown for some selected grains. Dotted lines are boundaries between core and rims. Stippled arrows infer growth/dissolution directions

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Fig. 9

Photomicrograph of characteristic Pl chemical zonation rims in low-strain area. a Fluid inclusions along core–rim boundary. b Asymmetric rims in Pl; positions for analytical points (EPMA) are shown. White arrow point at an irregular interface boundary (inferred replacement boundary with inward dissolution and precipitation). Box outlines area for confocal profilometry analysis viewed in d across a concave interface boundary. c Backscattered electron image zooming into the central grain boundaries in b. Solid arrow points at weakly developed Hüttenlocher intergrowths in the core of the grain. Stippled arrow point at core-and-rim boundary. d Confocal profilometric surface topography across core-and-rim boundary (stipple line) illustrating an abundance of pits, that is, porosity in the rim (arrow). Weak flame structures in the core (discordantly to the rim boundary) are Hüttenlocher intergrowths

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Fig. 10

a Photomicrograph of Pl grains with asymmetric chemical zoning. 3-D representation of mineral orientation is shown for selected grains. Solid lineab across core–rim boundary is the line for the chemical profile in b. Stippled arrow mark inferred growth/dissolution directions. b Step function (sharp chemical interface) between Ca and Na content in chemical profile ab. c Backscatter electron image over the chemical profile area in a

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Fig. 11

a Pinning of Pl bulge between Amp grains in high-strain area. b Misorientation profile, ab, showing a low-angle grain boundary (6°) across the pinned bulge in a. c Photomicrograph from high-strain area showing lobate grain boundaries. Photo is XY. d 3-D representation of mineral orientation is shown for selected grains in c. Dotted lines are boundaries between core and rims

Group II grains are defined as grains with an anorthite content of An64 lacking to a large extent a core of An80. Such grains are less than 150 μm with an average grain size of ~100 μm, where grains are larger in low-strain areas and smaller in high-strain areas (Table 1). Group II grains are dominantly found in the high-strain area (Figs. 4c, 8 and 11). The internal strain is low as indicated by misorientation profiles (i.e. <1° on average) and grains (n = 256) from the high-strain sample indicate random grain orientations (Fig. 6d).

Amphibole

Amphiboles (hornblende) is the second most abundant mineral in the investigated samples and occurs between large Pl porphyroclast in the low-strain area (Fig. 3b) and dominantly as elongated grains (up to 1 mm long) aligned with foliation in higher-strain areas. Amphibole grains from the high-strain area (Fig. 3d) display a CPO with the (100) plane aligned with foliation and the \( \left\langle {00 1} \right\rangle \) axis with lineation (Fig. 6e). Amphibole is also found in layers with a phase mixture of Pl + Amp + Qz (Fig. 4c) and with an average grain size of 190 μm and aspect ratio of 2.27. Some grains are strained with an apparent bending of the crystallographic lattice from edge to edge in elongation. Subgrain walls (up to 8°) are seen perpendicular to the grains long axis. Some are also observed along long-axis parallel grain boundaries. Hydrous (e.g. Ms and Bt) and opaque phases are commonly seen in Amp-rich bands.

Quartz

Quartz is less abundant than Amp and Pl and mostly occurs in layers with a mixture of Pl and Amp (Fig. 4c). Grains are up to 800 μm large but more frequently less than 200 μm, with an average of 130 μm. They commonly have smooth grain boundaries and an average aspect ratio of 1.97 in high-strain areas, with an alignment of the long axis inclined to foliation (average 13°). Grains commonly show undulose extinction and some subgrain boundaries. Grains (n = 36) from the high-strain sample (Fig. 3d) indicate random grain orientations (not shown).

Characteristics of plagioclase chemical zoning

Asymmetric chemical zonation is seen in feldspar grains as distinct rims of lower Ca content than the grains’ interior (An64 and An80, respectively; for example, Figs. 9, 10; Table 1). Such rims are most abundant in the smaller fraction of Group I grains while such zonation is rare in large Group I grains and porphyroclasts.

Several characteristic features are associated with these rims:
  1. (a)

    sharp compositional change from rim to core (e.g. Fig. 10),

     
  2. (b)

    smooth and sharp boundaries towards the adjacent grain and grain cores, respectively (Figs. 8, 9).

     
  3. (c)

    common development on more than one side of a grain, along boundaries as well as at triple junctions (cf. Fig. 4).

     
  4. (d)

    visible porosity in some cases (Fig. 9d).

     
  5. (e)

    bulging structures and associated pinning features with or without a bridging subgrain boundary (e.g. Figs. 8a, 11, respectively), where the bulge size is comparable to the Group II smaller grains (e.g. Fig. 11).

     
  6. (f)

    higher internal misorientations in rims than in cores (i.e. more deformed); this is most pronounced in high-strain areas (Fig. 11c, d).

     
  7. (g)

    facetted grain edges where a rim with straight grain boundaries cuts growth twins of the neighbouring grain (e.g. in Fig. 8).

     
  8. (h)

    lobate grain boundaries with increased curvature in the high-strain areas (Fig. 8b).

     
  9. (i)

    in low-strain areas, zig-zag-shaped core–rim boundaries where at the 1 μm scale no micro-fractures could be identified (Fig. 8d). The zig-zag step coincides with twin boundaries.

     
  10. (j)

    crystallographic orientations consistent across core–rim boundary (Fig. 8d).

     
  11. (k)

    fluid inclusions are observed within the rims and along core-and-rim boundary (Fig. 9a).

     
In low-strain areas, the position of rims is not related to the orientation of foliation, whereas in high-strain areas, thickest rims dominantly occur in the elongation direction, that is, subparallel to foliation (Table 2; Fig. 7b). Thus, in high-strain zones, grain boundaries which are at low angles to the foliation only rarely exhibit chemical zonation. In low-strain areas, rims are seen in 85 % of the recrystallized grains (Group I) while in high-strain areas, 75 % of grains show rims. There is a positive correlation between the amount of rims and the presence of hydrous phases (cf. Fig. 4b, c).
Table 2

Measurement of plagioclase rims

 

High-strain area

Low-strain area

 

Tot. no. of measured grains

104

97

https://static-content.springer.com/image/art%3A10.1007%2Fs00410-012-0822-9/MediaObjects/410_2012_822_Figa_HTML.gif

Average grain aspect ratio

2.2

1.8

% has no rim

25

15

% thickest rim in grain elongation

90

49

Rim thicknesses, t (μm)

16–240

16–360

Average rim thickness, t (μm)

85

64

Rim lengths, l (μm)

56–576

40–744

Average rim length, l (μm)

209

238

Length/thickness (average), l/t

3.2

3.7

Chemical zoning related to crystallography

In order to evaluate the possible link between crystal orientation and presence and extent of chemical zoning, we performed detailed analysis using the crystallographic data collected with the EBSD system (Table 3). We considered the three principal crystallographic axes (a-/b- and c-axis). We did not take into consideration the crystal orientation of the adjacent grain, that is, our measurements allow evaluation of the surface and crystal direction most prone to the process responsible for the chemical zoning. Results show that ~60 % of the thickest rims occur along the (001) plane and ~61 % in the \( \left\langle {0 10} \right\rangle \)b” direction (i.e. on the (010) crystal face, however, not always along the (001) plane) (Figs. 8c, 10a and 11a). We note that when grains are randomly oriented, it is difficult to separate between, for example, the (100) and (001) plane, when these planes are at a large angle to the foliation. The results therefore only indicate the most likely plane and direction and do not present a full set of combined planes and directions.
Table 3

Position of thickest rim related to crystallography

Parallel to

High strain

Low strain

Tot.

No.

%

No.

%

No.

%

(001)

30

59

32

62

62

60

(010)

7

14

7

13

14

14

(100)

14

27

13

25

27

26

Tot.

51

100

52

100

103

100

\( \left\langle {00 1} \right\rangle \)

5

10

6

12

11

11

\( \left\langle {0 10} \right\rangle \)

33

65

30

58

63

61

\( \left\langle { 100} \right\rangle \)

13

25

16

30

29

28

Tot.

51

100

52

100

103

100

The replacement product, that is, the rim, shows only a minor deviation in orientation from the core as illustrated by pole figures with data from a core (bytownite, I-1) and a rim (labradorite, C-1) from a grain with a zig-zag-shaped interface boundary (Fig. 8d). A minor deviation in orientation is expected even with strict epitaxial growth, considering the change in lattice parameters.

Interpretation and discussion

Conditions of deformation

Geothermobarometry on the nearby Grt–Sil–Bt gneiss estimates 550–620 °C and 4.5–5.1 kbar (Hollis and Persson in Hollis 2005) for the D2 event. Geothermometry (Ti-in-Amp; Spears 1981) together with geobarometry (Al-in-Amp; Anderson and Smith 1995) on co-existing Amp–Pl pairs from the medium-strained sample yield 620–640 °C and 7.4–8.6 kbar and is interpreted as the D2 conditions. This interpretation is based on the fact that amphibole compositions are homogeneous throughout grains and therefore fully equilibrated at the last major tectonometamorphic event. The obliquity of the grain elongation (Fig. 7) indicates that a transpressional general shear is present and that the simple shear component becomes more dominant in high-strain zones.

It should be noted that the replacement of amphibole by randomly oriented biotite is interpreted as a late static event associated with late fractures seen in feldspars devoid of chemical changes.

Mechanisms of recrystallization

The two different groups of recrystallized Pl grains can be directly linked to two successive events (D1 and D2) of recrystallization (i.e. grain size reduction) where Group I grains were formed in an early event and those of Group II in a subsequent event. Considering that recrystallized grains of Group I still show in some cases an orientation relationship with their inferred host (Fig. 6a) we suggest that these were formed by a host controlled recrystallization process such as SGR or bulging recrystallization (e.g. Kruse et al. 2001). However, we also observe that grains seem to have nucleated in these strained host porphyroclasts, and subsequently replaced parts of the host grain (Fig. 5c), suggesting heterogeneous nucleation (e.g. Jeřábek et al. 2007; Brander et al. 2011). The smaller grains of Group I show no CPO and occur both in the low- and high-strain areas. Substructures such as low-angle boundary walls and subgrains are near absent, and chemical zonation is common. We suggest that grains in Group I were formed by SGR or bulging recrystallization since they have the same composition as the clasts and that the crystallographic relationship to host clast were obliterated in the subsequent deformation event. In contrast to low-strain areas dominated by Group I, lobes and bulges are common in the high-strain area and are of similar size as the Group II grains (<150 μm).

For the formation of Group II grains, we propose the activity of three simultaneously operating processes:
  1. 1.

    Bulging recrystallization.

     
  2. 2.

    Left-over-grains by incomplete consumption during grain boundary migration.

     
  3. 3.

    To a minor extent: Development of subgrains and subgrain rotation inside rims.

     

The first process, bulging recrystallization, is evident in, for example, Figs. 8 and 11, where the bulges may or may not show a bridging subgrain boundary (cf. Halfpenny et al. 2006). Boundary migration resulting in bulging recrystallization is a low-temperature strain-induced process (Hirth and Tullis 1992; Tullis 2002; Stipp et al. 2002). Driving forces for strain-enhanced bulging may, however, be enhanced if a chemical driving potential is added to the process (Stünitz 1998; Baxter and DePaolo 2004). Such an effect is well documented for metal systems (diffusion-induced recrystallization (DIR); e.g. Yoon 1995). A bridging subgrain boundary seen in Fig. 11a, b may be explained by dislocations built up at the bulge neck due to a rotation of the host grain and pinning of the bulge between other grains during deformation. This physical detachment of a bulge with a lower anorthite content also explains why some of the smaller grains (<150 μm) have the same composition as the rims. In the second process, left-over-grains are developed when a grain with the higher anorthite content is partly consumed by a growing grain with a lower An content. This explains why some, yet only a few, of the grains in Group II show a composition of An80. The third process, where a subgrain is developed and presumably rotated (as seen by low-angle boundaries and “off-setting” twins), is only rarely seen and only observed within rims (Fig. 11c, d). The restriction to rims may suggest that only there the dislocations were able to build up sufficiently to cause subgrain rotation. The activation energy for dislocation creep may have been lowered by the introduction of water into the lattice during the process of chemical change (see below for process discussion; Tullis and Yund 1980). Consequently, subgrain rotation may have been operated. Group II grains adjacent to chemical zoning dominantly show a low internal distortion and that is not consistent with a solely strain-induced GBM (e.g. Urai et al. 1986). However, the fact that lobate and bulging structures are more frequent in the high-strain areas, even though rims are thicker in the low-strain area, indicates that both driving forces (i.e. strain energy and chemical energy) were significant. The random grain orientations of Group II grains (Fig. 6d) can be explained by inheriting the orientation of the random porphyroclasts but also by subsequent rotation by fluid-induced grain boundary sliding.

In summary, we suggest that the grain size–reducing processes dominantly resulted in a near random orientation of grains as no CPO is seen in both low- and high-strain areas and in all grains apart from some shielded Group I grains (strain shadow in the tail of a porphyroclast). During subsequent deformation, none of the recrystallized grain groups developed a significant CPO (Fig. 6). A lacking CPO, as is seen among the recrystallized grains, would not be changed to a preferred crystallographic orientation by annealing alone (Piazolo et al. 2010).

Origin of chemical zoning and associated grain characteristics

In principle, possible external factors that may influence the thickness and location of these rims are finite strain, orientation of the stress axes and grain elongation, location of grain with respect to other phases, crystallographic orientation of boundaries between adjacent grains and the presence of fluids.

Combined microstructural and chemical analyses indicate that the character and extent of chemical modifications are directly but not exclusively related to finite strain and the kinematic axes orientation. An increase in SPO is directly linked to a higher abundance of asymmetric chemical rims in the direction of elongation (Fig. 7). In the high-strain areas, rims become more asymmetric resulting in a high percentage of grain boundaries without rims that are orientated subparallel to foliation (e.g. Figs. 4b, c, 7; Table 2). At the same time, grain boundaries become increasingly irregular and show bulging (e.g. compare Fig. 4b with c).

Processes potentially responsible for the chemical zoning are as follows: a) lattice diffusion; b) DPC occurring under differential stress conditions; c) CIGBM; and d) grain-inward replacement by the process of icDP (e.g. O’Neil and Taylor 1967; Putnis 2002; Putnis and Putnis 2007; Engvik et al. 2008; Putnis 2009; Hövelmann et al. 2010; Putnis and Austrheim 2010),

In principle, lattice diffusion can result in chemical zoning, however, this process would result in a smooth chemical change across the chemical zoning and not the observed distinct chemical step between rim and core compositions (Fig. 10). Furthermore, volume diffusion is too slow to account for the large rims in the studied samples (Yund 1986; Yund and Snow 1989; Yund and Tullis 1991).

Dissolution and precipitation creep is strongly dependent on the orientation of the stress axes, where material at boundaries perpendicular to σ1 dissolves while material is added on boundaries perpendicular to σ3 (e.g. Wintsch and Yi 2002; Menegon et al. 2008). Statistically, rim distributions in the studied high-strain zones show this relationship. This process would further result in an increase in SPO, as is also observed (Fig. 7). Due to geometric constraints if DPC is active, it is inevitable that some grain boundary sliding occurs (Ford et al. 2002). The fact that several elongated grains do not contain their rims in the extensional position but instead show zonation towards the shortening direction further support that some grain boundary sliding must have occurred (Figs. 4, 9, 10 and 11).

We suggest that grain boundary migration in our samples is to a large part driven by chemical differences, also termed CIGBM (e.g. Hillert and Purdy 1978; Balluffi and Cahn 1981; Hay and Evans 1987; Stünitz 1998). Chemically induced GBM may be directly associated with DPC (Baxter and DePaolo 2004) if fluids are present. Observed characteristic features for CIGBM include zig-zag-shaped chemical boundaries, lobate and bulging boundaries and facetted grains (Figs. 4, 8) (Hay and Evans 1987). Since some of the grains in high-strain areas do show evidence for plastic behaviour such as slight bending of twins (Fig. 8c), some strain energy difference was a contributing driving force. However, no boundaries are found with lobate or bulging structures without the chemical zonation, indicating that the chemical driving force for GBM played a dominant role. Microstructural observations such as the rim (An64) of Pl grains growing into other An80 grains and cutting through twins (Figs. 8a, c and 11), as well as the microstructures supporting bulging recrystallization for Group II grains, support the activity of CIGBM. The mechanism by which grain boundaries migrate is likely to be the so-called icDP mechanism suggested to be a common mineral replacement mechanism in the presence of reactive fluids (e.g. Putnis 2002). Characteristics of icDP (e.g. Putnis 2009; Engvik et al. 2008; Niedermaier et al. 2009; Hövelmann et al. 2010) are as follows: (1) chemical profiles with an apparent step function in Ca and Na content across core–rim boundary (Fig. 10), (2) higher amount of voids in the rim (product phase) than in the core which may represent the relict porosity that is expected as the product phase has a lower volume (~1 vol%) than the original (Fig. 9d), (3) preservation of crystallographic orientation and therefore optical continuity (Fig. 8d; observed minor crystallographic changes are consistent with a change in lattice parameters in the Pl solid solution series).

The consistency of observed features with those resulting from CIGBM through the fluid-induced grain replacement mechanism icDP strongly suggests the activity of this mechanism in both low- and high-strain samples.

Chemical replacement and grain boundary migration is envisaged to have occurred as follows (Fig. 12):
https://static-content.springer.com/image/art%3A10.1007%2Fs00410-012-0822-9/MediaObjects/410_2012_822_Fig12_HTML.gif
Fig. 12

Cartoon illustrating the inferred replacement mechanism for grain-inward replacement together with chemically induced grain boundary migration (CIGBM). a Fluid influx along grain boundary between grain no. 1 and no. 2. b Mineral–fluid reaction start by dissolution of grain no.1 and proceeds into the grain. c Start of CIGBM of grain no. 1 into grain no. 2. The reaction front is ahead of the advancing grain no. 1. CIGBM facilitated by icDP opens up fluid pathways through porosity generation allowing replacement at both reaction fronts

  1. (a)

    Deformation-enhanced fluid distribution promotes fluid influx along grain boundaries and triple junctions,

     
  2. (b)

    Dissolution starts on the grain with the most soluble surface (i.e. crystallographically and chemically controlled). When the thin fluid film is supersaturated, precipitation occurs and coupled with the dissolution follows the reaction front into the grain (Grain 1). The icDP has now produced a product (An64) more stable with the incoming fluid. Chemical changes as rims of higher Ca are not seen by BSE on Amp as we interpret the system to be open on a thin section scale and that the excess Ca has been removed.

     
  3. (c)

    As the product is less soluble than the adjacent Pl of An80 (Grain 2), dissolution may start on this grain too if grain orientation permits (i.e. coherency strains). However, the solution will precipitate on the product of Grain 1, which will advance (grow) into the Grain 2 in the wake of the reaction front by CIGBM. However, since porosity is developed in the product from the initial starting point (line) during icDP, fluids may continue to reach the first reaction front (inside Grain 1). Thus, a consequence of the simultaneous reaction fronts may be an enhanced growth rate limited by the availability of porosity in the products and fluids and its local chemical composition.

     

Migration and replacement directions: crystallographically controlled anisotropy of plagioclase

Any consistency in the position of the thickest rim to specific crystal planes and directions would imply anisotropic behaviour due to differences in elastic compliances. These differences will result in differences in the rate to adjust or dissolve and replace the crystal lattice in a given crystal direction (e.g. Lee et al. 1993). There is a twofold influence of anisotropy related to crystallography in our studied samples involving the rate of dissolution important to the rate of icDP and therefore the rate of grain boundary migration. The direction in which grain boundaries will migrate or dissolve is dependent on external and internal factors:

Externally, a strong stress field will support growth in the extension direction and inhibit growth and/or replacement normal to the principal stress axis (e.g. Wintsch and Yi 2002). The presence of different rim thickness around a grain will also be a result of a heterogeneous fluid–solid ratio in a sample, that is, the thickness of the fluid film will most certainly be variable around the grain (Huang et al. 1986; Reinecke et al. 2000; Hövelmann et al. 2010).

Internally, the atomic structure will have an effect of replacement (dissolution/migration) direction. Dissolution studies of feldspars have shown that it is easier to break Al–O bonds than Si–O bonds (Xiao and Lasaga 1994; Oelkers and Schott 1995); hence, the crystal lattice configuration should show an anisotropy in dissolution rates as the structure is weaker at Al–O bonding dominating directions (Arvidson et al. 2004). Dissolution experiments on albite have shown that dissolution rates were faster normal to the (010) surface than to the (001) surface and even faster on anorthite crystals (on the (010) surface (Arvidson et al. 2004). The atomic structure, apart from the ease of breaking certain bonds, displays different elastic constants in different directions. Experiments on albite by Brown et al. (2006) showed that the (001) and the (010) surfaces have the highest constants (elastic modulus; 179.5 and 183.5, respectively) that can be compared to, for example, (100) with a constant of 69.9. Coherency strain differences along different crystallographic directions are shown to promote growth and/or diffusion in different crystal directions. The initiation of migration and the direction of migration occur towards the grain with the surface of the highest coherency strain energy, for example, Lee et al. (1993). Twinning in plagioclase might also be an extra internal factor influencing the dissolution rates. Albite twins may act as obstacles leading to slower dissolution of the (001) surface than on the (010) surface as indicated in experiments by Arvidson et al. (2004). Our measurements of position of thickest rim to the three principal axis show that the rims are dominantly positioned on the (010) surface (Table 3), preferably on grains with the (001) surface subparallel to foliation. Rims are not always observed on neighbouring grains, nor are bulges into neighbouring grain indicating that in these cases, either fluid flow was minimized or that the coherency strain in the neighbour grain prevented growth. Our results thus indicate a preferred dissolution direction in Pl and that replacement starts inwards (icDP) since that is the easiest direction, that is, towards the highest elastic constant (e.g. Lee et al. 1993).

Our observations confirm previous studies and suggestions (e.g. Heidelbach et al. 2000; Arvidson et al. 2004; Niedermeier et al. 2009; Hövelmann et al. 2010) that there is a crystallographic control on growth and dissolution directions in Pl feldspars, namely in the \( \left\langle {0 10} \right\rangle \) direction and along the (001) plane. There is therefore a resistance for chemically induced boundary migration in certain directions depending on the orientation of the neighbouring grains. We have evaluated rim positions only in relation to the three principal crystallographic axes and their respective planes. Other planes such as preferred fracture planes may also be of significance (cf. e.g. Marshall and McLaren 1977; Stünitz et al. 2003). We do note that the (010) is a perfect cleavage plane in feldspar, and since microcracks are common in feldspars (and reported up to granulite facies conditions), the preferred growth of rims on the (010) plane may (at least partly) be explained with an enhanced fluid–rock interaction along microcracks, and not exclusively with the anisotropy in elastic properties of feldspar during deformation.

Interaction of physical and chemical processes in deforming mid-crustal plag-rich rocks: the role of fluids

In our samples there is a direct relationship between phase distribution and chemical changes in Pl grains (cf. Fig. 4a, b). High-strain zones with increased number of small grains are predominately polymineralic and show an abundance of asymmetric chemical zoning in Pl and related grain boundary migration and recrystallization (Figs. 4c, 8). Computer modelling by Ford et al. (2002) and Wheeler (2009) showed that if grain boundary diffusion creep (DPC) is operative, aggregates with marked differences in grain size and irregular grain shapes and/or different mineral species exhibit significant grain rotations during compression. We suggest that strain localization started in polymineralic areas. Monomineralic areas with a more regular grain structure could not accommodate strain sufficiently and little or no grain rotation occurred; hence, no fluid pathways were produced. In the polymineralic areas, grain boundary diffusion and dissolution was enhanced at boundaries of unlike phases (Renard et al. 2001). In addition, the lack of coherence at these boundaries allowed significant grain boundary sliding to occur. Both processes resulted in physical grain rotations which assisted fluid influx along the grain boundaries and triple junctions, chemical reactions as well as destruction of any pre-existing crystallographic preferred orientation (e.g. Wightman et al. 2006).

The presence of aqueous fluids is evidenced by hydrous phases such as Czo and Ms and by the observed fluid inclusions inside and along the rims. These hydrous phases are mostly found associated with amphibole-rich horizons in polymineralic areas. Here, amphiboles show some subgrain boundaries and a well-developed CPO (Fig. 6e). In contrast, plagioclase and to a much lesser extent quartz display no CPOs. This feature can be explained by rigid body rotation of elongate amphiboles which experienced some limited crystal plastic deformation in a rheologically softer deforming matrix (e.g. Baratoux et al. 2005: Díaz Aspiroz et al. 2007). Rigid rotation produced voids and/or open grain boundaries creating fluid channels and high-diffusivity pathways. The fluid presence, dynamic void and fluid channel generation caused complete re-equilibration of amphiboles while in feldspars only partial equilibrium was reached. These points to the influx of an externally derived fluid in a chemically open system.

Besides facilitating for replacement of Pl in low-strain areas along grain boundaries and in high-strain areas dominantly in low-stress boundaries, rates of DPC increased. This resulted in an increase in the aspect ratios of grains and asymmetric chemical zonation rims and effective accommodation of grain boundary sliding in higher-strain areas. The fluids decreased the coherence, that is, friction along grain boundaries and thus enabled easy grain rotations allowing grain boundary sliding to be effective. Furthermore, fluids increased the mobility of grain boundaries in the high-strain area enabling significant GBM and bulging recrystallization. In areas of replacement, a higher amount of water in the crystal lattice is indicated by the presence of fluid inclusions resulted in hydrolytical weakening that allowed some crystal plastic deformation to be active.

The link between physical processes such as grain rotations, grain boundary sliding and crystal plastic deformation and chemical processes such as replacement reactions and dissolution and precipitation is highly dependent on the influx of fluid, its composition and its continuous or limited availability. The fact that some Pl-rich horizons contain an abundance of Pl with zonation mixed with hydrous phases while some do not, for example, close to relict porphyroclast pressure shadows and horizons with few amphiboles (e.g. box (1) in Fig. 3d) indicate that the fluid distribution is heterogeneous and enhanced by deformation. This interpretation is in line with previous work, which showed that a deformation-enhanced fluid distribution is important in promoting a change in deformation mechanisms, strength and bulk transport in Pl aggregates (Tullis et al. 1996). Furthermore, Reinecke et al. (2000) suggested that a heterogeneous distribution of CIGBM in calcite aggregates may be linked to a heterogeneous grain-scale fluid distribution. In addition, externally derived fluids have been shown to be common at mid-crustal levels and are likely to play a significant role in the activity and rates of chemical and physical processes (e.g. Wintsch and Yi 2002; Menegon et al. 2008; Putnis 2009).

Regional context of samples and their deformation and chemical reaction history

According to regional metamorphic studies, two main deformation events D1 and D2 can be distinguished in the area. The first major event D1, in our sample, was initiated during an inferred terrane amalgamation at relatively high T and little fluid influx at mid- to lower-crustal levels (Juul-Pedersen et al. 2007; van Gool et al. 2007) and can be directly linked to the main recrystallization event forming Pl Group I grains. The observed Hüttenlocher intergrowths in Group I recrystallized grains may be related to the exhumation process proposed by Nutman et al. (2007) to have occurred after D1. The regional, large-scale folding event (D2) is in our samples seen as the second recrystallization event with significant fluid influx. Both deformation conditions and fluid influx is in line with the regionally observed increased fluid flow, large-scale shearing and ore genesis (Hollis 2005; van Gool et al. 2007). During this event, externally derived reactive fluids together with differential stress induced DPC, GBS and strain- and chemically induced GBM through icDP. Grain size reduction due to bulging recrystallization of fluid-enhanced fast-migrating boundaries generated Pl Group II grains. Intracrystalline deformation of Amp during D2 is consistent with the regionally thermobarometrically derived amphibolite facies conditions for D2 as well (e.g. Biermann and van Roermund 1983).

Conclusions

This study shows that the deformation of plagioclase in an anorthosite–leucogabbro deformed during exhumation at wet amphibolite facies conditions (620–640 °C, P 7.4–8.6 kbar) is strongly governed by the feedback between physical and chemical processes.

During deformation, fluid presence and distribution are deformation enhanced. Influx of external fluid which is in chemical disequilibrium with the original rock is promoted by grain rotations caused by heterogeneities in grain size, mineral phases and grain shape during incipient DPC. Several physical and chemical processes operate simultaneously. Disequilibrium of the incoming, externally derived fluid with the bytownite plagioclase (An80) causes icDP to occur, which transforms the bytownite to labradorite composition (An64). Grain boundary migration is present and both strain and chemically induced while rates are increased where fluid is present.

In high-strain areas, grain boundary sliding and DPC are dominant mechanisms combined with some strain- and chemically induced GBM. The latter resulted in formation of new recrystallized grains through bulging recrystallization. Replacement by icDP is in high-strain areas only active to a minor extent.

In low-strain areas, CIGBM facilitated by icDP is dominant.

Observed geometric relationships of chemical changes and crystallographic orientation of Pl (An80) grains indicate a marked anisotropy of the reaction rate (dissolution and coupled precipitation). Reaction is the fastest on the (010) cleavage plane (along the \( \left\langle {0 10} \right\rangle \) axis) and preferentially occurs on grains that have the (001) plane subparallel to foliation. At low-strain Pl orientations govern the rate of reaction rather than the orientation of the main stress axes.

Our study highlights the multiple feedbacks between physical and chemical processes in rocks undergoing simultaneous deformation and external fluid infiltration and shows that these may result in enhanced strain localization. Such a scenario is commonly observed in the continental crust during amphibolite facies conditions.

Acknowledgments

The Geological Survey of Denmark and Greenland (GEUS) approved publication of this paper and is further acknowledged for financing of fieldwork and logistics. Knut and Alice Wallenberg foundation is acknowledge for funding the EBSD facility at the Department of Geological Sciences, Stockholm University. We thank Timothy Grove for editorial handling, an anonymous reviewer and especially reviewer Luca Menegon for helpful and constructive comments and suggestions that significantly improved the manuscript. Marianne Ahlbom (Stockholm University) is acknowledged for helpful assistance with the SEM and EBSD analyses and José Godinho for introduction to and guidance during confocal profilometry measurements. We are grateful to the Nordic Mineralogical Network (Minnet) for partly financing the electron microprobe analyses (EPMA) and to Alfons Berger (University of Copenhagen) and Hans Harryson (Uppsala University) for assistance with the EPMA analyses.

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© Springer-Verlag Berlin Heidelberg 2012