Journal of Structural Geology - Géosciences Montpellier · 2012. 5. 21. · Deformation processes and rheology of pyroxenites under lithospheric mantle conditions Erwin Fretsa,b,*,

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Journal of Structural Geology 39 (2012) 138e157

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Journal of Structural Geology

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Deformation processes and rheology of pyroxenites under lithospheric mantleconditions

Erwin Frets a,b,*, Andréa Tommasi b, Carlos J. Garrido a, José Alberto Padrón-Navarta b,c, Isma Amri d,Kamal Targuisti d

a Instituto Andaluz de Ciencias de la Tierra (IACT), CSIC and UGR, Avenida de las Palmeras 4, 18100 Armilla, Granada, SpainbGeosciences Montpellier, CNRS & Université de Montpellier 2, F-34095 Montpellier Cedex 5, FrancecResearch School of Earth Sciences, The Australian National University, Building 61, Mills Road, ACT0200 Canberra, AustraliadDépartement de Géologie, Faculté des Sciences, Université Abdelmalek Essaâdi, Tetouan, Morocco

a r t i c l e i n f o

Article history:Received 17 November 2011Received in revised form27 February 2012Accepted 29 February 2012Available online 15 March 2012

Keywords:MantlePyroxenitePlastic deformationRheologyMicrostructureCrystal preferred orientationsGarnetClinopyroxeneOrthopyroxeneBeni Bousera

* Corresponding author. Instituto Andaluz de Cienand UGR, Avenida de las Palmeras 4, 18100 Armilla, G

E-mail address: [email protected] (E. Fret

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a b s t r a c t

We combined microstructural observations and high-resolution crystallographic preferred orientation(CPO) mapping to unravel the active deformation mechanisms in garnet clinopyroxenites, garnetespinel websterites, and spinel websterites from the Beni Bousera peridotite massif. All pyroxenitesdisplay microstructures recording plastic deformation by dislocation creep. Pyroxene CPOs areconsistent with dominant slip on [001]{110} in clinopyroxene and on [001](100) or [001](010) inorthopyroxene. Garnet clinopyroxenites have however high recrystallized fractions and finer grain sizesthan spinel websterites. Recrystallization mechanisms also differ: subgrain rotation dominates ingarnet clinopyroxenites, whereas in spinel websterites nucleation and growth also contribute. Elon-gated shapes and strong intracrystalline misorientations suggest plastic deformation of garnet, butCPOs are weak. Clinopyroxene porphyroclasts in spinel websterites show deformation twins under-lined by orthopyroxene exsolutions. Thermodynamic calculations indicate that garnet clinopyroxenitesdeformed at 2.0 GPa and 950e1000 �C and spinel pyroxenites at 1.8 GPa and 1100e1150 �C. The lowertemperatures may explain the faster work rates implied by the finer grained microstructures in garnetclinopyroxenites. Greater stresses may have also reduced the competence contrast between garnet andpyroxene in the garnet pyroxenites and, at the outcrop scale, lowered the competence contrastbetween pyroxenites and peridotites, favoring mechanical dispersion of pyroxenites in the coolerlithospheric mantle.

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1. Introduction

Pyroxenite is an important constituent of the upper mantle.Lithological mapping in continental peridotite massifs, like Lherz inthe Pyrenees, Lanzo in the Alps, Beni Bousera and Ronda in theBetic-Rif belt, shows that pyroxenite layers are ubiquitous in thesemassifs (Kornprobst, 1969, 1970; Dickey, 1970; Garrido andBodinier, 1999; Bodinier et al., 2008; Gysi et al., 2011). Mantlepyroxenites have been inferred as source material of ocean islandbasalts in Hawaii (Sobolev et al., 2005) and, to a lesser extent, ofmid-ocean ridge basalts (Hirschmann and Stolper, 1996).

cias de la Tierra (IACT), CSICranada, Spain.s).

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Three origins are classically proposed for mantle pyroxenites:recycling of oceanic crust into the convective mantle due tosubduction (Polvé and Allègre, 1980; Allègre and Turcotte, 1986),partial crystallization of basaltic melts at depth, or melting andmelt-rock reaction products (Loubet and Allègre, 1982; Bodinieret al., 1987, 2008; Suen and Frey, 1987; Pearson et al., 1993;Garrido and Bodinier, 1999). Regardless of their origin, pyroxe-nites usually occur as layers parallel to the peridotite foliation,suggesting that deformation during mantle flow probably controlstheir distribution and, hence, the scale of expression of composi-tional heterogeneity in the upper mantle. However, data onpyroxene deformation mechanisms and rheology are still limited.

Deformation experiments and TEM observations on diopside(Avé Lallemant, 1978; Kollé and Blacic, 1982, 1983; Raterron andJaoul, 1991; Ingrin et al., 1992; Jaoul and Raterron, 1994; Zhanget al., 2006; Amiguet et al., 2009), orthopyroxene (Turner et al.,

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E. Frets et al. / Journal of Structural Geology 39 (2012) 138e157 139

1960; Raleigh, 1965; Green and Radcliffe, 1972; Lally et al., 1972;Kohlstedt and Vandersande, 1973; Ross and Nielsen, 1978;Skrotzki, 1994), and garnet (Ando et al., 1993; Doukhan et al.,1994; Voegelé, 1998; Voegelé et al., 1998a,b; Ji et al., 2003)showed that these minerals may deform by crystal-plasticprocesses and allowed identifying the active slip systems. Fordiopside, these studies also determined the pressure and temper-ature dependence of the critical resolved shear stresses of thevarious slip and twinning systems. In addition, flow laws weredetermined for diopside polycrystals under dry (Bystricky andMackwell, 2001) and hydrous conditions (Chen et al., 2006). Yet,the extrapolation of these results to natural conditions remainsdifficult. Most studies on natural garnet-bearing pyroxene-richlithologies focused on eclogites from subduction-related meta-morphic terranes deformed under high to ultra-high pressure andlow temperature conditions or high-pressure granulitic conditions(e.g., Buatier et al., 1991; Abalos, 1997; Bascou et al., 2001, 2002;Mainprice et al., 2004; Padrón-Navarta et al., 2008). Fewer dataexist for mantle pyroxenites deformed under lithospheric orasthenospheric mantle conditions (e.g., Muramoto et al., 2011).

The Beni Bousera peridotite massif encompasses a large varietyof mantle pyroxenites deformed under a variety of pressure andtemperature conditions (Kornprobst, 1969, 1970; Pearson et al.,1989, 1991, 1992; Targuisti, 1994). Here we present detailed anal-yses of outcrop-scale structures, microstructures, and crystallo-graphic preferred orientation (CPO) of garnet pyroxenites,

Fig. 1. Geological map of the Beni Bousera peridotite massif showing the distribution ofPhotomicrographs illustrate the variation of pyroxenites microstructures at the scale of the

garnetespinel websterites, and spinel websterites from thismassif. We combine these microstructural data with petrologicalmodeling to constrain the deformation mechanisms and rheolog-ical behavior of pyroxenite in the subcontinental lithosphericmantle for a range of pressure and temperature conditions.

2. The Beni Bousera peridotite

The Beni Bousera peridotite massif crops out in the Rif orogenicbelt, which forms the southern limb of the Betic-Rif arcuate Alpineorogen surrounding the Alboran sea in the westernmost Mediter-ranean (Fig. 1). The Beni Bousera massif is mainly composed oflherzolite with minor harzburgite and dunite (Kornprobst, 1969;Reuber et al., 1982). Three main tectono-metamorphic domainscan be distinguished from SW to NE (Fig.1): (i) mylonitic garnet andspinel mylonites (Kornprobst, 1969; Reuber et al., 1982; Saddiqiet al., 1988; Tabit et al., 1997) that overlie (ii) mylonitic to por-phyroclastic peridotites containing garnet pyroxenite layers (Ariè-gite subfacies; Targuisti, 1994), which in turn overlie (iii) coarse-grained porphyroclastic to granular spinel peridotites withdiffering amounts of spinel pyroxenite layers (Seiland subfacies;Targuisti, 1994). Garnet- and/or spinel pyroxenite layers arecommon in the entire massif and may locally comprise up to 50% ofan outcrop section (Fig. 2). Available structural data (Reuber et al.,1982), which are confirmed by our ongoing structural mapping of

tectono-metamorphic domains and the location of the studied pyroxenite samples.massif.

Fig. 2. Field occurrences of Beni Bousera pyroxenites (px). (a) Garnet clinopyroxenite layers (px) parallel to the peridotite foliation (S1) from the garnet-spinel mylonite domain.Note the boudinage of the finer pyroxenite layers (white arrow). (b) Flattened garnets marking the foliation (S1) in a garnet clinopyroxenite. (c) Extreme boudinage of a garnetclinopyroxenite (px) in the Ariègite domain. Note the peridotite foliation warping around the boudin. (d) Elongate garnets (white arrows) marking the lineation (L1) in a garnetclinopyroxenite. (e) Garnetespinel websterites layers (px) intermixed with peridotite from the AriègiteeSeiland transition. Note the absence of boudins. (f) Thick compositepyroxenite from the AriègiteeSeiland transition.

E. Frets et al. / Journal of Structural Geology 39 (2012) 138e157140

peridotite foliations and lineations in the massif, indicate consis-tent kinematics across the three metamorphic domains.

3. Field occurrence and macroscopic structures

Garnet pyroxenites crop out within the garnetespinel myloniteand the spinel tectonites of the Ariègite domain (Fig. 1). They occuras centimeter- to meter-scale layers with rather sharp contacts tothe host peridotite and are commonly isoclinally folded or bou-dinaged (Fig. 2aec). Pyroxenite layering is parallel to the peridotitefoliation (Fig. 2a). At the outcrop scale, the foliation in the clino-pyroxenite, which is marked by the shape-preferred orientation ofgarnet, is parallel to that of the host peridotite. In most cases, garnetis pancake-shaped (Fig. 2b), but in some pyroxenites, garnets areelongate and define a stretching lineation on the foliation plane(Fig. 2d). In thicker pyroxenite layers, variation of the garnet modalcontent defines a compositional layering parallel to the foliation.

Garnetespinel websterites characterize the transition betweenthe Ariègite and Seiland subfacies domains (Fig. 1). Similarly to thegarnet clinopyroxenites, these websterites crop out as layersparallel to the peridotite foliation, which are often boudinaged(Fig. 2e). Thick websterite layers in the central part of the massifmay have a composite structure (Fig. 2f), containing spinel in itsexternal part and keliphitized garnet in its central part (Fig. 2f).Finally, the structurally lowest part of the Seiland subfacies domainis characterized by centimeter-thick olivine-bearing websteritelayers with diffuse limits.

4. Sampling and analytical procedures

We collected samples covering a wide range of pyroxenitemicrostructures and rock types (Fig. 1). Pyroxenites were sampledat varying distances to the AriègiteeSeiland transition, from thegarnetespinel mylonite, through the Ariègite domain to theAriègiteeSeiland transition (Targuisti, 1994). Detailed microstructural


analyses were conducted on (Table 1): (i) five orthopyroxene-bearing garnet clinopyroxenites (<10 vol.% of orthopyroxene),(ii) four garnetespinel websterites (BB033W, BB068E, BB075W,and BB076BBE), and (iii) two spinel websterites (BB073CWand BB018W). From group (i), four were collected from thegarnetespinel mylonite domain (BB007, BB025E, BB031 andBB125BW) and one from the Ariègite domain (BB034W) nearthe AriègiteeSeiland transition. Group (ii) pyroxenites weresampled at varying distance to the AriègiteeSeiland transition; thisgroup includes an orthopyroxene-rich websterite (opx/cpx> 3:1;BB033W). The spinel websterites from group (iii) were sampledfrom thick websterite layers (Fig. 2f) that preserve garnetespinelwebsterite in their central part. We did not analyze centimeter-thick websterite layers with diffuse limits that crop out in thelower parts of the Seiland subdomain because they contain largeamounts of olivine (up to 30 vol.%).

Thin sections for EBSD analysis were carefully polished at CSIC-IACT (Armilla, Spain). After two 90 min-long standard polishingwith respectively 6 and 3 mm polycrystalline diamond suspensionthin sections were polished for 1e2 h with 1 mm polycrystallinediamond suspension at 180 rpm. The last step of the procedureconsisted of a chemical polish using colloidal silica for 45 min at110e135 rpm.

Clino- and orthopyroxene, garnet, and spinel CPOs wereobtained by indexing of electron backscattered diffraction (EBSD)patterns using a JEOL JSM 5600 scanning electron microscope atGeosciences Montpellier (Université Montpellier 2, France).Diffraction patterns (Kikuchi bands) are generated by interactionof an electron beam directed at a thin section tilted at 20� to thebeam. We used an acceleration voltage of 17 kV and a workingdistance of 23 mm. A detailed description of acquisition param-eters is provided in the Appendix. CHANNEL5 software fromOxford-HKL Technology was used for indexing and data acqui-sition. Orientation maps of 80e90% of the thin section surfacewere acquired in automatic acquisition mode with a step size of25e50 mm, depending on the sample grain size. For all samples,the step size is at least 5 times smaller than the average grainsize of the recrystallized grains. High-resolution orientationmaps (5 mm step) of a 9 mm2 area were also performed ona typical garnet pyroxenite (BB007) and a garnetespinel

Table 1Sample location, mineralogy, and calculated mineral J-indexes.

Facies Grt-Spl Lherzolite Spinel Lherz

Subfacies Ariègite

Rock type Grt clinopyroxenites

Microstructure Fine-grained porphyroclastic

Sample BB007 BB031 BB125BW BB025E BB034W

Lat (�N) 35�1401000 35�1401800 35�1405700 35�1201000 35�1402600

Long (�W) 4�5202900 4�5200300 4�5304100 4�5004400 4�5105600

Mineralogy (modal vol.%)Grt 65 60 55 50 42e44Cpx 35 40 45 40 55Opx <1 <1 <1 10 1e3Spl e e e e e

Ol e e e e e

DSe-Ar (m) 847 780 1047 1082 5Jcpx (raw) 2.59 3.37 1.77 3.93 4.59Jcpx (1 ppg) 2.24 2.94 1.64 2.97 4.1Jopx (raw) e e e e e

Jopx (1 ppg) e e e e e

*Jgrt (1 ppg) 1.02 1.14 1.01 1.04 1.31

DSe-Ar = distance from the Seiland-Ariègite transition; 1 ppg = 1 point per grain calculati

websterite (BB076BBE) for detailed analyses of the intracrystal-line deformation structures and determination of the recrystal-lization processes. Indexing rates attained 75e80% for thefreshest samples. The smallest indexing rates of 35e40% wereobtained in the most kelyphitized samples due to non-indexation of kelyphite. Post-acquisition data treatmentincreased indexing rates by: (i) filling non-indexed pixels thathave up to 8 identical neighbors with the same orientation and(ii) repeating this operation for respectively 7, 6 and 5 identicalneighbors. As confirmed by petrographic, EPMA analysis, andEDX mapping of thin sections, some small diopside grains weremisindexed as enstatite, whereas the opposite rarely occurred.

To avoid artifacts in the pole figures due to oversampling of thelarger grains, CPOs were plotted as one average orientationmeasurement per grain on equal-area lower-hemisphere stereo-nets. Geographically oriented thin sections were used to check fieldmeasurements of foliation and lineation. CPO data acquired ongeographically referenced thin sections were then rotated into theXZ reference frame for easier comparison among different samples.Additional thin sections cut parallel to the XZ structural referenceframe, i.e., perpendicular to foliation and parallel to mineral line-ation, were used for more detailed analyses of the deformationmicrostructures.

The CPO strengthwas determined using the J-index, which is thevolume-averaged integral of the squared orientation densities(Bunge, 1982). The J-index ranges between 1 for a random distri-bution and infinity for a crystal of single orientation, but inpractice ithas a maximum of around 250 due to truncation at expansion 22 ofthe spherical harmonic series (Ben-Ismaïl andMainprice,1998). TheJ-index for all samples was calculated using SuperJctf and SuperJ7xprograms by D. Mainprice (ftp://www.gm.univ-montp2.fr/mainprice//CareWare_Unicef_Programs/) with a 10� Gaussian half-width, data at 1� bins, and truncation of the orientation distribu-tion function (ODF) at degree 22. We used both one orientationdatum per pixel and one average orientation per grain, analyzing�100 grains per sample to be statistically representative. Modalcompositions were obtained by combining EBSD data and imageanalysis for estimation of the composition of non-indexed areas, inparticular of kelyphite coronas. Modal compositions, microstruc-tures, and J-indexes of all samples are presented in Table 1

olite

AriègiteeSeiland

Grt-Spl websterites Spl websterites

Coarse porphyroclastic

BB068E BB033W BB075W BB076BBE BB018W BB073CW

35�1700300 35�1402600 35�1700000 35�1605500 35�1601800 35�1700000

4�5304100 4�5105600 4�5304700 4�5304600 4�5200000 4�5304800

34e38 15 30 15e17 e e

60 35 55 65 50 401e3 45 15 15 35 351e3 5 1e3 3e5 10e12 12e e e e 3e5 13

102 0 29 0 �195 �299.51 4 7.1 11.73 7.08 5.437.6 3.23 4.76 9.13 5.34 3.36e 3.83 4.13 5.64 8.72 5.33e 2.69 3.76 6.04 4.58 4.8e e e e e e

on; *Jgrt ¼ J-indexes calculated using the MTEX code.

ftp://www.gm.univ-montp2.fr/mainprice//CareWare_Unicef_Programs/

ftp://www.gm.univ-montp2.fr/mainprice//CareWare_Unicef_Programs/

Table 2Representative mineral compositions.

Garnet clinopyroxenite Spinel-garnet websterite Spinel websterite

Sample BB034W BB025E BB076BBE BB033W BB076BBE BB073CW

Texture porphyroclasts neoblasts porphyroclasts neoblasts porphyroclasts neoblasts interstitial

Core/Rim c r c c c c r c r c c c c r c r c c c

Mineral Grt Grt Grt Cpx Opx Cpx Cpx Opx Opx Cpx Opx Spl Cpx Cpx Opx Opx Cpx Opx Ol Spl

SiO2 (wt.%) 41.26 41.36 41.13 52.28 53.09 50.68 50.99 54.53 54.98 51.13 53.90 0.02 51.00 51.40 53.12 54.04 51.53 53.82 40.05 0.01TiO2 0.14 0.17 0.11 0.41 0.16 0.48 0.43 0.18 0.11 0.46 0.04 0.08 0.68 0.72 0.13 0.13 0.70 0.16 0.00 0.05Al2O3 22.90 22.86 22.88 7.01 3.46 9.05 8.86 5.77 5.37 8.86 5.34 60.69 9.19 7.74 6.76 5.54 8.22 5.61 0.00 63.09Cr2O3 0.10 0.12 0.10 0.15 0.08 0.22 0.22 0.31 0.35 0.13 0.08 3.13 0.45 0.33 0.25 0.21 0.40 0.18 0.01 3.77FeOa 15.36 15.43 15.86 6.46 17.07 4.01 4.84 6.54 6.63 4.28 10.29 18.42 3.44 3.84 9.15 9.50 3.73 9.19 15.19 13.19MnO 0.35 0.36 0.37 0.16 0.25 0.13 0.15 0.15 0.19 0.13 0.21 0.26 0.09 0.12 0.17 0.22 0.07 0.15 0.17 0.17NiO 0.00 0.00 0.01 0.06 0.17 0.00 0.04 0.10 0.10 0.03 0.08 0.37 0.05 0.01 0.05 0.09 0.04 0.10 0.34 0.51MgO 16.08 15.90 15.77 12.95 25.93 13.00 15.10 32.10 32.58 13.65 30.06 16.94 12.84 13.85 29.98 30.26 13.70 30.02 45.03 19.28CaO 4.65 4.71 4.70 18.54 0.58 20.21 17.22 0.58 0.37 19.30 0.47 0.00 19.72 19.98 0.42 0.48 19.48 0.80 0.02 0.00Na2O 0.02 0.03 0.01 2.11 0.03 1.82 1.68 0.02 0.01 1.71 0.01 0.00 2.14 1.69 0.02 0.02 1.87 0.04 0.00 0.01K2O 0.01 0.01 0.01 0.02 0.02 0.03 0.02 0.03 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.01 0.02 0.01 0.01 0.01

Total 100.95 101.05 101.05 100.17 100.83 99.64 99.56 100.33 100.69 99.69 100.50 99.92 99.61 99.69 100.09 100.50 99.77 100.10 100.82 100.09

Ob 12 12 12 6 6 6 6 6 6 6 6 4 6 6 6 6 6 6 4 4

Si 2.986 2.995 2.981 1.906 1.913 1.847 1.8501 1.879 1.887 1.857 1.885 0.001 1.853 1.869 1.856 1.883 1.869 1.882 0.999 0.000Ti 0.006 0.009 0.006 0.011 0.004 0.013 0.0117 0.005 0.003 0.013 0.001 0.002 0.018 0.020 0.004 0.003 0.019 0.004 0.000 0.001Altot 1.953 1.951 1.954 0.301 0.147 0.389 0.3788 0.234 0.217 0.379 0.220 1.878 0.394 0.332 0.278 0.228 0.351 0.231 0.000 1.901IVAl 0.094 0.087 0.153 0.1499 0.121 0.113 0.143 0.115 0.147 0.131 0.144 0.117 0.131 0.118VIAl 0.207 0.059 0.236 0.2289 0.114 0.104 0.237 0.105 0.247 0.201 0.134 0.111 0.220 0.113Cr 0.006 0.007 0.006 0.004 0.002 0.006 0.0063 0.009 0.009 0.004 0.002 0.065 0.013 0.009 0.007 0.006 0.012 0.005 0.000 0.076Fetota 0.930 0.935 0.962 0.197 0.514 0.122 0.1467 0.189 0.190 0.130 0.301 0.404 0.104 0.117 0.267 0.277 0.113 0.269 0.317 0.282Fe2þc 0.877 0.900 0.895 0.182 0.100 0.131 0.130 0.100 0.115 0.113Fe3þc 0.053 0.035 0.067 0.015 0.022 0.016 0.000 0.004 0.002 0.000Mn 0.022 0.022 0.023 0.005 0.008 0.004 0.0047 0.005 0.005 0.004 0.006 0.006 0.003 0.004 0.005 0.007 0.002 0.005 0.004 0.004Mg 1.735 1.716 1.704 0.704 1.392 0.707 0.8168 1.649 1.667 0.739 1.567 0.663 0.696 0.751 1.562 1.572 0.741 1.565 1.674 0.735Ca 0.360 0.365 0.365 0.724 0.022 0.789 0.6694 0.022 0.014 0.751 0.018 0.000 0.767 0.778 0.016 0.018 0.757 0.030 0.000 0.000Na 0.149 0.002 0.129 0.1183 0.001 0.000 0.121 0.001 0.000 0.151 0.119 0.002 0.001 0.131 0.003 0.000 0.001K 0.001 0.000 0.001 0 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.000 0.000 0.000Ni 0.002 0.005 0.000 0.0012 0.003 0.003 0.001 0.002 0.008 0.001 0.000 0.001 0.002 0.001 0.003 0.007 0.010

Total 8.000 8.000 8.000 4.005 4.010 4.007 4.005 3.996 3.997 3.999 4.004 3.027 4.001 4.001 3.999 3.998 3.997 3.997 3.001 3.011

XMgd 0.664 0.656 0.656 0.782 0.730 0.876 0.862 0.897 0.898 0.850 0.839 0.621 0.874 0.867 0.854 0.850 0.868 0.853 0.841 0.723

Cr#d 0.033 0.039

XPrp 0.579 0.571 0.570XAlm 0.300 0.299 0.299XGrs 0.088 0.098 0.084XSps 0.007 0.007 0.008XAdr 0.028 0.019 0.035

CaTs 0.101 0.133 0.143 0.144 0.094 0.107 0.117Ac 0.036 0.016 0.015 0.017 0.002 0 0.003Jd 0.112 0.103 0.063 0.100 0.119 0.128 0.116AcJd 0.148 0.119 0.078 0.117 0.121 0.128 0.119

Ac¼ Fe3þ, CaTs¼ CrCaTsþAlCaTS, CrCaTs¼ Cr, AlCaTs¼ VIAl-Jd, Jd¼Na-Ac (if Na-Ac< VIAl), Jd¼ VIAl (if Na-Ac� VIAl).a FeO and Fetot calculated as total iron.b Number of oxygens for which cations are calculated.c Fe2þ and Fe3þ based on garnet and clinopyroxene stoichiometry.d XMg¼Mg/(Mgþ Fe2þ). Fetot was used for orthopyroxene, olivine and spinel. Cr#¼ Cr/(CrþAl).

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Mineral composition analyses were performed using a CamecaSX100 electron microprobe at CIC (Universidad de Granada, Spain)with operating conditions of 15 kV accelerating voltage, 20 nAbeam current and a range of 20e50 s peak counting timedepending on the analyzed element. At least three analyses of thecore and rim of each mineral were performed to obtain represen-tative average compositions and detect possible compositionalzoning. Representative electron microprobe analyses of mineralsare given in Table 2. Bulk-rock major elements were analyzed usingstandard X-ray fluorescence procedures at the analytical facilities ofIACT (Armilla, Granada).

5. Microstructures

5.1. Garnet pyroxenites

Both garnet clinopyroxenite and opx-bearing garnet clinopyr-oxenite have porphyroclastic microstructures characterized bycoarse garnet porphyroclasts (up to 10 mm long) enclosed in a finergrained (250e300 mm) matrix of recrystallized clinopyroxene andgarnet neoblasts (Fig. 3a). Garnet porphyroclasts occur either asisolated grains or as aggregates forming a compositional layering

Fig. 3. Photomicrographs of typical microstructures of garnet pyroxenites (cross-polarized ligarnet clinopyroxenite. Flattening of garnet porphyroclasts (grt-p) marks the foliation (S1)oxene porphyroclasts (cpx-p) and the embayments in garnet porphyroclasts filled with clincontent and by the elongation of garnet porphyroclasts (grt-p, garnet porphyroclasts; grt-n(garnet marked by white arrow). (d) Garnet porphyroclasts (grt-p) with rutile exsolutions pand subgrains in clinopyroxene; grain boundaries are coated by a fine-grained matrix (cpx-nporphyroclasts and -n, neoblasts.

parallel to the foliation (Fig. 3b). They are usually pancake-shapedwith aspect ratios that vary between 1.5:1 and 4:1. Their elonga-tionmarks the lineation and their flattening the foliation. The latteris also defined by the shape-preferred orientation of garnet andclinopyroxene in thematrix (Fig. 3c). All garnet porphyroclasts haveirregularly shaped grain boundaries with embayments filled byclinopyroxene (arrows in Fig. 3d) and may contain clinopyroxeneinclusions (Fig. 3a,d). The largest garnet porphyroclasts also containoriented rutile inclusions (Fig. 3d). Garnet in the matrix is also lens-shaped, but less elongated than the porphyroclasts, which are up to500 mm long with average aspect ratios of 2:1 (Fig. 3c). Matrix cli-nopyroxenes have more irregular shapes and tend to be less elon-gated (300e400-mm long on average) than matrix garnets; mm-size porphyroclasts are rare (Fig. 3a). They usually display undu-lose extinction with local occurrence of subgrains (Fig. 3e). Clino-pyroxene grain boundaries are serrated, with very short length-scale embayments, which grade into a very fine-grained matrix(5e10 mm, Fig. 3e,f) that coats most grain boundaries. Most clino-pyroxenes show fine, closely-spaced orthopyroxene exsolutions inthe core of crystals (Fig. 3e,f). Orthopyroxene occurs as isolatedrecrystallized grains within the clinopyroxenematrix and has a sizesimilar to recrystallized clinopyroxene neoblasts.

ght, except b, plane polarized light). (a) Fine-grained porphyroclastic microstructure ofand clinopyroxene is almost entirely recrystallized. Note the few remaining clinopyr-opyroxene (white arrows). (b) Foliation (S1) defined by variations in the garnet modal, garnet neoblasts). (c) Detail of the recrystallized matrix of a garnet clinopyroxeniteresenting sinuous grain boundaries filled with clinopyroxene. (e) Undulose extinction, clinopyroxene neoblasts). (f) Detail of recrystallized clinopyroxene crystals -p indicates


5.2. Garnetespinel websterites

Garnetespinel websterites differ from the garnet pyroxenites bytheir coarser clinopyroxene grain sizes (Fig. 4), their higher ortho-pyroxenemodal contents, and the presence of spinel. Garnet in theserocks is completely transformed to kelyphite aggregates up to 5 mmwide.Kelyphite aggregateshave irregular shapes, but tend tobemorerounded than garnet porphyroclasts in garnet clinopyroxenites.However, the compositional layering parallel to the foliation ispreserved in some samples (Fig. 4b). Spinel occurs as a minor phasewithin kelyphite or along pyroxene grain boundaries. Clinopyroxeneoccurs as large porphyroclasts up to 2 cm long enclosed in a rather

Fig. 4. Photomicrographs of coarse porphyroclastic garnetespinel websterites (cross-polariz(S1, dashed line). Garnet is completely transformed into greenish post-kinematic kelyphite aDetail of a displaying a large elongated orthopyroxene crystal and a large clinopyroxene pa showing a bent and twinned clinopyroxene crystal with orthopyroxene exsolutions alongextinction in clinopyroxene, elongated orthopyroxene crystals, and the intergranular fine-oblique to the foliation. (g) Detail showing the shape of recrystallized clinopyroxene grains. (and exsolution planes. -p indicates porphyroclasts, -n, neoblasts, kel means kelyphitized ga

coarse grained (w1 mm on average) matrix of ortho- and clinopyr-oxene (Fig. 4aed). Clinopyroxene porphyroclasts are stronglydeformed and often bent. They show thick orthopyroxene exsolu-tions up to 15 mmwide parallel to (100), strong undulose extinction,and kinks (Fig. 4aed). They also have very irregular shapes, beingpartially replaced by the recrystallized matrix that occurs preferen-tially along exsolution planes (Fig. 4a,d). Clinopyroxene porphyr-oclasts may also enclose orthopyroxene crystals (Fig. 4c).Orthopyroxene porphyroclasts are rarer and smaller than clinopyr-oxene ones, but extremely deformed, reaching aspect ratios of 10:1(Fig. 4c). Larger, but less deformed orthopyroxene crystals charac-terize the orthopyroxene-rich websterite BB033W.

ed light). (a and b) Elongation of large clinopyroxene porphyroclasts marks the foliationggregates, which are more rounded than garnet crystals in garnet clinopyroxenites. (c)orphyroclast (cpx-p) with thick orthopyroxene lamellae parallel to (100). (d) Detail ofthe (100) twin lamellae. (e) Detail of the recrystallized matrix in b showing undulosegrained matrix. (f) Detail of b; note the elongation of orthopyroxene crystals slightlyh) Fine-grained matrix partially replacing clinopyroxene crystals along grain boundariesrnet.


Recrystallized grains have irregular, interlocking shapes(Fig. 4cef). They have no clear shape-preferred orientation, buttabular clinopyroxene crystals rimmed by orthopyroxene occurdispersed in the matrix (arrow in Fig. 4f). These tabular crystals areinterpreted to represent fragments of ancient porphyroclastslimited by orthopyroxene exsolution lamellae. Recrystallized cli-nopyroxenes display undulose extinction and subgrains (Fig. 4e,f),but are not twinned. They have serrated grain boundaries (Fig. 4g)that are often rimmed by a very fine-grained matrix composed ofortho- and clinopyroxene. This matrix differs in abundance through

Fig. 5. Photomicrographs of coarse porphyroclastic spinel websterites (cross-polarized light(cpx-p) and orthopyroxene (opx-p) porphyroclasts. (c) Detail of a showing clinopyroxene crRecrystallized clinopyroxene shows undulose extinction and straight grain boundeneeorthopyroxeneespinel symplectite. (e) Detail of a displaying a partially recrystallized ofine-grained matrix along boundaries and exsolution planes (white arrows). (f) Detail of a selongated parallel to the foliation (S1). (g) Olivine crystal (ol) showing well developed sub(cpx-n) grain boundaries. -p indicates porphyroclasts, -n, neoblasts, and -i, interstitial cryst

the samples andwhenmost abundant, it completely coats all grains(Fig. 4eeg) and partially replaces them (Fig. 4h).

5.3. Spinel websterites

Compared to the garnetespinel websterites, kelyphite is absentand spinel is a major phase (5e10 vol.%) of spinel websterites.Spinel occurs either in intergrowth with ortho- and clinopyroxene(Fig. 5b,d) or forming interstitial aggregates along pyroxene grainboundaries aligned parallel to the foliation (Fig. 5a,c,e).

). (a and b) Foliation (S1) defined by the shape-preferred orientation of clinopyroxeneystals with coarse exsolutions and subgrains and spinel (spl) with interstitial habitus.aries (white arrows). (d) Detail of b showing an undeformed clinopyrox-rthopyroxene (opx) and an elongated clinopyroxene porphyroclast (cpx-p) replaced byhowing a “pocket” of fine-grained plagioclase-bearing aggregates (fine grained matrix)grains and undulose extinction. (h) Fine-grained matrix along clinopyroxene neoblastals.


Automorphic spinel is rare. Close to the contacts with the perido-tite, olivine occurs as coarse (w1 cm) crystals elongated parallel tothe foliation. Farther within the pyroxenite, olivine occurs as raresmall interstitial crystals (w1 mm or less). Both show unduloseextinction and subgrains (Fig. 5g).

Clinopyroxene occurs as large porphyroclasts up to 2 cm longelongated parallel to the foliation (Fig. 5a,b). These porphyroclastsare strongly deformed, often bent, and show mechanical twinsparallel to (100), strong undulose extinction, and kinks. They havevery irregular shapes, being partially replaced by recrystallizedgrains. This recrystallization occurred preferentially along twinningplanes that are usually marked by orthopyroxene exsolution(Fig. 5c,e). Orthopyroxene porphyroclasts are less elongated andshow strong undulose extinction and kinks. They display thinnerand more closely spaced clinopyroxene exsolution (Fig. 5a). Theirboundaries are irregular with embayments filled by ortho- andclinopyroxene. Large, undeformed ortho- and clinopyroxene grainsmostly occur in rounded aggregates with spinel (Fig. 5d). Pyroxeneporphyroclasts are surrounded by medium sized (0.5e1 mm)anhedral pyroxene crystals with sutured grain boundaries. Somepyroxenes show intracrystalline structures such as unduloseextinction. Porphyroclasts and matrix neoblasts are commonlycoated by a very fine-grained matrix composed of ortho- and cli-nopyroxene grains (Fig. 5cef, h). This matrix occurs in variable

Fig. 6. Clinopyroxene, orthopyroxene, and garnet CPO. PF¼ Pole Figure, IPF¼ Inverse Pola uniform distribution (m.u.d.) for clinopyroxene and 0.25 m.u.d. for garnet. In garnet IPFs,where contours are at 0.02 m.u.d.).

proportions through the samples. In spinel pyroxenite BB018W, itforms lens-shaped pockets, slightly flattened in the foliation planethat contain fibrous orthopyroxene, clinopyroxene, olivine, andplagioclase (Fig. 5f).

6. Crystallographic preferred orientation (CPO) data

6.1. Clinopyroxene, orthopyroxene, and garnet CPO

All pyroxenite samples display a clear clinopyroxene CPO (Figs. 6and 7) characterized by a concentration of [001] axes subparallel tothe lineation and, in most cases, [010] axes at high angle to thefoliation. In most samples, the clinopyroxene CPO has a clearasymmetry to the foliation that indicates simple shear deformation.However, detailed analyses highlight second-order variations inclinopyroxene CPO that do not correlate to provenance norlithology. In all samples, [001] axes show some dispersion in the XYplane (foliation) or in the XZ plane. [001] forms aweak girdle in thefoliation plane in garnet pyroxenites BB025E, BB034W, BB125W, ingarnetespinel websterites BB076BBE and BB068E, and in spinelwebsterites BB073CW and BB018W. It is dispersed in the XZ planein garnet pyroxenites BB007 and BB0125BW and in garnetespinelwebsterites BB033W and BB075W. Dispersion of [001] in the foli-ation plane is accompanied by a [010] maximum normal to the

e Figure. Pole figures are lower-hemisphere stereoplots; contours at 0.5 multiples ofcontours at 0.05 m.u.d. (except for the normal to the foliation IPF of sample BB125BW

Fig. 7. Clinopyroxene and orthopyroxene CPO in garnetespinel and spinel websterites. Pole figures are lower-hemisphere stereoplots; contours at 0.5 multiples of a uniformdistribution (m.u.d.).



foliation and preferred orientation of [100] axes in the foliationplane with weak maxima normal to the lineation. In contrast,dispersion of [001] in the XZ plane is associated with dispersion of[010] in a plane normal to the lineation. Among the first group,spinel websterite BB073CW and, to a lesser extent, garnetespinelwebsterite BB068E exhibit clinopyroxene CPOs characterized bybimodal [001] and [100] maxima in the foliation plane with thelineation as the bissectrix of the angle formed by the [001] maxima.

Although the patterns are similar, the intensity of the clino-pyroxene CPOs varies as a function of the lithology and provenance(Fig. 8). Garnet clinopyroxenites from the mylonitic domaingenerally display weak clinopyroxene CPO, characterized by J-indexes <4, while garnetespinel websterites from theAriègiteeSeiland transition have strong clinopyroxene CPO with J-indexes as high as 12 (average of 8.1). Spinel websterites displayintermediate J-indexes. In addition, the abundance of very coarseclinopyroxene porphyroclasts in the spinel websterites results insignificantly higher J-indexes when the latter is calculated usingone measurement point per pixel (raw EBSD data, full symbols inFig. 8).

Orthopyroxene displays a clear CPO, which is well-correlatedwith the clinopyroxene CPO when orthopyroxene is present asa major phase (Fig. 7). Orthopyroxene [001] axes cluster at lowangle to the lineation and either [100] or [010] form a slightlyweaker maximum at high angle to the foliation. It is striking to notethat the bimodal distribution of clinopyroxene [001] axes in sampleBB073CW is accompanied by a bimodal distribution of orthopyr-oxene [001] axes. J-indexes for orthopyroxene (Table 1) varybetween 2.7 and 6. Since most orthopyroxene-bearing sampleswere collected close to the AriègiteeSeiland transition, the relationbetween CPO strength and provenance could not be tested.

Garnet CPO could only be measured in the garnet clinopyrox-enites. It is always veryweak with J-indexes close to 1 (Table 1). Themore pronounced CPO of sample BB034W might be due to thesmaller number of grainsmeasured, which were at least an order ofmagnitude less than in the other samples. Garnet CPO patterns varyfrom sample to sample (Fig. 6). Samples BB125BW and BB007 arecharacterized by weak h110i clusters subparallel to the lineation.Sample BB125BW shows weak h111i clusters normal to the folia-tion. The remaining samples have low Miller indices directionsh100i, h110i, or h111imaxima neither parallel to the lineation, nor tothe foliation.

Fig. 8. J-index as function of the distance to the AriègiteeSeiland transition. Fullsymbols are calculated using one data point per pixel of the orientation map; emptysymbols are calculated using one point per grain.

6.2. Crystallographic orientation maps

Analysis of the crystallographic orientation maps shows thatgarnet porphyroclasts in garnet clinopyroxenites are strongly bentand display continuous crystallographic orientation gradients andeven subgrains (Fig. 9a). Within a single porphyroclast, the rotationof the crystalline lattice may be as large as 30� over a distance of5 mm (Fig. 9b,d). When the magnitude of the gradient is greaterthan 10�/mm, subgrains are present; they are more common at theedges of the garnet porphyroclasts (white arrows in Fig. 9a).Misorientation profiles parallel to the long axis of a garnet por-phyroclast highlight rotations dominantly around h111i, whereasprofiles perpendicular to the crystal elongation show rotationsaround both h111i and h110i (Fig. 9c). The strongest orientationgradients are observed in themost elongated crystals, such as thosein garnet clinopyroxenites BB031 and BB125BW that transition intorecrystallized aggregates with average grain sizes of 500 mm(Fig. 9e). Misorientation between neighboring grains within theseaggregates may be greater than 45� (Fig. 9e). In more equidimen-sional garnets from websterites BB007 and BB125BW, orientationgradients are limited to the crystal rims.

Fig. 10 illustrates the differences in microstructure betweengarnet clinopyroxenites and spinel websterites. First, clinopyroxenegrain sizes in garnet pyroxenites are noticeably smaller than in spinelwebsterites and show a rather continuous grain-size distribution(20 mm to 1 mm), which renders the discrimination between por-phyroclasts and recrystallized grains difficult. In contrast, spinelwebsterites show a strongly bimodal grain-size distribution charac-terized by recrystallized grains 80e100 mmwide or even larger thatsurround and partially replace large strained porphyroclasts thatmay attain several mm. Analysis of misorientations within theseporphyroclasts reveals 180� rotations around [001] that representdeformation twins (Fig. 10a and c). These twins, which are alsohighlighted in the correlated (between neighboring points in amap)misorientation distribution (Fig. 10f), are almost always outlined bycoarse orthopyroxene exsolution lamellae (Fig. 10a). The highproportion of low-angle grain boundaries in the correlated misori-entation distribution of both garnet clinopyroxenite and spinelpyroxenite (Fig. 10f) highlights the abundance of subgrains in bothrocks (in red in Fig. 10a,b). On the other hand, uncorrelated misori-entationdistributionsdiffersmore fromthe randomone in the spinelpyroxenite than in the garnet clinopyroxenite (Fig.10f), in agreementwith the stronger clinopyroxene CPO of the former (Table 1).

The intracrystalline structures also differ between garnetpyroxenites and spinel websterites. Most clinopyroxene crystals ingarnet pyroxenites are segmented in subgrains (in red in Fig. 10d,e)with sizes similar to the recrystallized grains in thematrix. In spinelwebsterites, subgrains are less common (cf. lower proportion oflow-angle grain boundaries in Fig. 10f), occurring mainly parallel tothe (100) exsolution planes of the large clinopyroxene porphyr-oclasts (Fig. 10a,b). This occurrence suggests that shearing alongtwinning lamellae produced an additional rotation of a few degreesthat transformed the lamellae into subgrains. In the spinel web-sterites, clinopyroxene and orthopyroxene neoblasts located up to2 mm away from the porphyroclast and its exsolutions, respec-tively, show similar crystallographic orientations (Fig. 10b). Thisgeometry illustrates how dynamic recrystallization created signif-icant grain-size reduction in these initially very coarse-grainedrocks. Presence of neoblasts with strong misorientations relativeto the porphyroclast within domains where the continuity ofexsolutions indicates that they were initially part of the porphyr-oclast (upper left corner of Fig. 10a), implying that nucleation andgrowth are also involved in the recrystallization process.

Finally, EBSD analyses on undeformed clinopyroxeneeorthopyroxeneespinel aggregates in spinel websterite BB073CW

Fig. 9. (a) Crystal orientation map of a garnet pyroxenite; garnet crystals are colored as a function of the X0 Inverse Pole Figure (insert on the left), pyroxene crystals are black.White arrow indicates recrystallized crystals at the rim of large porphyroclast. (b) Misorientation profile AeA0 parallel to the long axis of a porphyroclast; note the continuousvariation in orientation at the core and the development of subgrains toward the rim. (c) Stereoplots showing the internal misorientation in the same crystal; coloring as in (a);black arrows indicate rotation axes. (d) Misorientation profile CeC0 . (e) Misorientation profile DeD0 along a polycrystalline aggregate; weak misorientations on the left part of theprofile are consistent with an origin by recrystallization.


(Fig. 11a) highlight topotaxic crystallographic relations between thethreephases. Orthopyroxene and clinopyroxenehave almost identicalCPOs and their [010] axes are parallel to the spinel [110] axes. Inaddition, orthopyroxene [100] axes are parallel to spinel h110i axes. IngarnetespinelwebsteriteBB075W,orthopyroxenecoronasdevelopedaround kelyphitized garnets also show almost single-crystal CPOs(Fig. 11b). Similar topotaxial relationships among spinel and pyrox-eneswereobserved inkelyphite fromretrogradegarnet peridotitesbyOdashima et al. (2008) and Obata and Ozawa (2011). Padrón-Navartaet al. (2008) also identified topotaxial and homoepitaxial relationsduring the formation of garnet after pyroxenes from prograde maficgarnet granulites.

7. Mineral chemistry

Table 2 lists representative analyses of pyroxenite constituentminerals. Garnet is always pyrope-rich: Prp35e70 , Alm17e45,Grs3-26,

with minor spessartine (Sps) and andradite (Adr) components(Sps0e2 Adr0e4). Garnet porphyroclasts show no significant zoning,except for garnets in samples BB007 and BB031 that show a rim-ward decrease of the Prp content accompanied by an increase in theGrs content.

In all studied pyroxenites, clinopyroxene is rich in Al (0.37� 0.07atoms per formula unit [a.p.f.u.], calculated on the basis of 6oxygens), and Na (0.13� 0.03). It also has large amounts of non-quadrilateral Tschermak (0.12� 0.04) and acmite-jadeite(0.13� 0.03) components. These non-quadrilateral componentsshow a slight rimward increase in the few preserved porphyroclastsfrom garnet clinopyroxenites, but a rimward decrease in the largeporphyroclasts of the spinel websterites. No significant zoning wasobserved in clinopyroxene neoblasts.

Orthopyroxene in websterites is Ca-rich (0.02� 0.01 a.p.f.u.based on 6 oxygens) and in Al (0.26� 0.04). Porphyroclasts may beslightly zoned with a rimward decrease in Al coeval with a slight

Fig. 10. Detailed (5 mm step size) crystal orientation maps of a spinel websterite (a,d) and a garnet pyroxenite (d,e). (a,d) Phase distribution and grain boundary structure: cli-nopyroxene in white, orthopyroxene and garnet in gray, respectively. Red, black, and blue lines indicate grain boundaries with misorientations between 2�e15� , 15�e179� , and>179� (twins), respectively. Thick black line AeA0 in a indicates the trace of the misorientation profile in c. (b,e) Crystallographic misorientation maps relative to an “ideal”orientation (black arrows indicate the grains used for defining this orientation); clinopyroxene are shown in yellow and garnet or orthopyroxene in blue. Grain boundaries as in(a,d). (c) Misorientation profile (AA0) in a clinopyroxene porphyroclast: 180� misorientations with [001] rotation axes correspond to twinning. Inset show that rotation axes incrystal coordinates for misorientations >179� in the entire map strong cluster around [001]. (f) Clinopyroxene misorientation histogram for correlated and uncorrelated distri-butions in the spinel websterite (top) and in the garnet pyroxenite (bottom). Both show a predominance of low-angle misorientations, that is of subgrain boundaries. Twinning,recorded as 180� misorientations, is only observed in the spinel websterite. (For interpretation of the references to color in this figure legend, the reader is referred to the webversion of this article.)


increase in Ca. Small orthopyroxene neoblasts in garnet clinopyr-oxenites (sample BB025E) contain much less Al (0.12� 0.04).

Spinels are Mg-rich (average Mg #¼ 0.73� 0.04) in bothgarnetespinel websterites and spinel websterites and yield averageCr # of 0.03� 0.01, except for sample BB033W that yields a higherCr # of 0.07. Finally, olivine in the olivine-spinel websteriteBB073CW is rather Mg-poor (Mg#¼ 0.85� 0.01).

8. Geothermobarometry

Phase equilibria were computed by free energy minimization inthe system Cr2O3eNa2OeCaOeFeOeMgOeAl2O3eSiO2

(CrNCFMAS) using Perple_X (Connolly, 2009) and the internallyconsistent thermodynamic data of Holland and Powell (1998) withadditional data for Cr-spinels and Cr-garnets from Klemme et al.(2009). Other solid solutions used were orthopyroxene and clino-pyroxene (Holland and Powell, 1996; both modified for ideal Cr, seePerple_X documentation for more details), olivine (Holland andPowell, 1998), and plagioclase (Newton et al., 1980).

Bulk-rock compositions of five representative garnet clinopyr-oxenite, garnet and spinel websterite, and spinel websteritesamples projected into the SiO2eAl2O3eMgO compatibilitydiagram from the NCFMAS system are presented in Fig. 12. Irre-spective of the mineral assemblage, all samples plot in the same

Fig. 12. Bulk-rock composition of selected samples projected from the NCFMASsystem. Projection through the exchange vector MgFe�1 using an average clinopyr-oxene composition (cf. Table 2) computed with CSpace (Torres-Roldán et al., 2000).

Fig. 11. (a) Phase distribution map showing an undeformed clinopyroxene (green),orthopyroxene (blue), and spinel (yellow) symplectite sharing topotactic relationsrelated to the breakdown of former garnet. Black, gray and white arrows indicateshared crystallographic orientations between the different minerals in stereoplots. (b)


region supporting the application of equilibrium thermodynamiccalculation assuming the same bulk-rock composition so as toconstrain their pressure and temperature evolution. Fig. 13adisplays the pseudosection computed for the bulk-rock composi-tion of garnet clinopyroxenite BB034W, which has a homogeneoustexture, in the NCFMAS system between 700 and 1300 �C anda pressure range of 1.0e3.0 GPa. A pseudosection for the same bulkcompositionwas also calculated in the CMAS system (not shown) totest the effect of the Cr2O3 component. The phase relation topologyremains the same except for the reaction,

grtþ ol ¼ opxþ splþ cpx (1)

that is invariant in the CMAS simplified system, which is displacedto lower pressure (ca. 0.4 GPa). This supports the importance of theCr2O3 component in modeling the garnet to spinel transition aspreviously stressed by Klemme (2004) and Klemme et al. (2009).

The pseudosection in Fig. 13a shows that for the computedtemperature range, the garnet clinopyroxenite field is constrainedto pressures above 1.2 GPa. The olivine-bearing garnet spinelwebsterites that characterize the AriègiteeSeiland transition inBeni Bousera are limited to a narrow 5-phase field (orthopyroxene,olivine, garnet, clinopyroxene, and spinel), constraining equilibra-tion pressure to 1.1e1.2 GPa at 700 �C and 1.8 GPa at 1200 �C. Thegarnet-out curve and the pyroxenite solidus for similar bulkcompositions from Lambart et al. (2009) constrain the spinelwebsterite to have equilibrated at pressures less than 1.8 GPa(Fig. 13a). As mentioned above, at the sample/outcrop scales, theserocks still exhibit kelyphitized garnet and could therefore havebeen equilibrated close to the AriègiteeSeiland transition. Thepresence of fine-grained plagioclase-bearing aggregates in thematrix of some spinel websterites indicates late to post-kinematicpartial re-equilibration in the plagioclase stability field(P� 1.2e1.3 GPa).

Synkinematic PT conditions for garnet clinopyroxenites wereestimated from the composition of the recrystallized exsolution-free pyroxenes in the matrix using several well-calibratedconventional barometers and thermometers. Pressure was esti-mated from the Al content in orthopyroxene of Nickel and Green(1985) and Taylor (1998) using the orthopyroxene compositionsin sample BB025E. Temperatures were estimated using the FeeMg

Orthopyroxene rims around kelyphite with quasi-single-crystal orientations suggest-ing topotactic growth at the expense of garnet. (For interpretation of the references tocolor in this figure legend, the reader is referred to the web version of this article.)

Fig. 13. (a) PeT-phase diagram for the bulk-rock composition of garnet clinopyroxenite BB034W. Experimental solidus of Beni Bousera garnet clinopyroxenite from Lambart et al.(2009). (b) Variation of garnet and orthopyroxene modal volumes during the proposed PT path. Mineral abbreviations after Whitney and Evans (2010).


exchange in clinopyroxeneeorthopyroxene pairs (Taylor, 1998), theCa content in orthopyroxene (Brey and Köhler, 1990, modified byNimis and Grütter, 2010), and the enstatite activity in clinopyrox-ene (Nimis and Taylor, 2000). These three methods give similarresults. Synkinematic recrystallized assemblages in garnet clino-pyroxenites yield pressures of 1.9e2.2 GPa and temperatures of950e1000 �C. The modeled phase assemblage in the pseudosectionunder these PT conditions (orthopyroxene, olivine, garnet and cli-nopyroxene, Fig. 13a) is in good agreement with observed modalcomposition of garnet clinopyroxenites, where garnet and clino-pyroxene represent more than 90 vol.% and orthopyroxene andolivine are minor phases.

The transition between garnet clinopyroxenite and spinelpyroxenites could be reached either by a decrease in pressureresulting in the assemblage orthopyroxeneeclinopyroxeneespinelor by an increase in temperature during decompression. Thelatter path should however result in small amounts of olivine (ca.2 vol.%) in the spinel pyroxenite assemblage (Fig. 12a). The obser-vation of olivine (Table 1) together with the microstructural data inthe spinel pyroxenites, in particular the larger recrystallized grainsizes and the evidence for grain boundary migration supportshigher temperatures (T> 1040e1050 �C) during the deformation ofthe spinel websterites. Conventional geothermometers that satis-fied T> 1040e1050 �C yielded 1113� 31 �C for the Taylor (1998)


thermometer, 1151�16 �C for the Brey and Köhler (1990) two-pyroxene thermometer, and 1157 �C for the Wood and Banno(1973) one.

The PT evolution derived from both conventional geo-thermometry and microstructural observations is corroborated bythe observed increase in modal volume of orthopyroxene (from 0 to35%, and up to 45% for orthopyroxene-rich sample BB033W,Table 1) and decrease in garnet modal volume (from 45 to 17% anddown to 0% for the garnet free samples BB073CW and BB018W)from the garnet clinopyroxenites to the spinel websterites. Thecrystallization of orthopyroxene at expenses of olivine can bemodeled in the CMAS system. It results from two different reac-tions: the invariant reaction (1), which is limited by the smallamount of olivine in the garnet clinopyroxenite protolith, and theexchange reaction involving a Tschermaks-type substitution (thesubscript “ss” indicates solid solution):

grtss þ cpxss ¼ opxss þ cpxss; (2)

that occurs in divariant fields containing grt and cpx. Reaction (2)has a positive slope in the PT diagram and therefore the garnetbreakdownmay result either from a decrease in pressure and/or anincrease in temperature (see isopleths of modal proportion inFig. 13b). Crystallization of orthopyroxene at the expense of garnetis also consistent with microstructural observations that showorthopyroxene coronas with single-crystal like orientations aroundformer garnets (Fig. 11b).

The PT conditions calculated for equilibration of thegarnetespinel websterites and spinel websterite are close to thesolidus of Beni Bousera garnet clinopyroxenite (Lambart et al., 2009).This prediction is consistent with previous work on Ronda pyroxe-nites that concluded that olivine-bearing spinel websterite at theRonda recrystallization front formed by melting of garnet clinopyr-oxenites upstream of the peridotites melting front (Garrido andBodinier, 1999; Soustelle et al., 2009).

9. Discussion

9.1. Deformation mechanisms

9.1.1. GarnetClear evidence for plastic deformation of garnet in mantle rocks

is rather rare (Carstens, 1969, 1971). However, the present obser-vations in pyrope-rich garnets (Table 2) are similar to microstruc-tures interpreted from dislocation creep in almandine-rich garnetsin granulites (Dalziel and Bailey,1968; Ross,1973; Ji andMartignole,1994; Kleinschrodt and McGrew, 2000; Kleinschrodt and Duyster,2002; Prior et al., 2000) and in more ferro-magnesian garnets ineclogites from HP/UHP metamorphic terranes (Chen et al., 1996;Bascou et al., 2001; Mainprice et al., 2004). These observationsinclude: the lens shape of garnet porphyroclasts and their align-mentmarking the lineation and foliation in garnet clinopyroxenites(Figs. 2b,c and 3) as well as the strong intracrystalline misorienta-tions accommodated by rotations around low-order crystallo-graphic axes, dominantly h111i, and the presence of subgrainswithin elongated garnet porphyroclasts (Figs. 9 and 10). In themostdeformed samples, garnet porphyroclasts are almost fully recrys-tallized and the pancake-like garnets observed in hand samples areactually polycrystalline aggregates (Fig. 9e).

TEM observations of experimentally (Voegelé, 1998; Voegeléet al., 1998a,b) and naturally deformed garnets (Ando et al., 1993;Doukhan et al., 1994; Voegelé, 1998; Ji et al., 2003) reveal both½h100i and ½h111i dislocations. Viscoplastic self-consistentmodeling of the evolution of garnet CPO in simple and pure shearshows that activation of these slip systems, with h111i slip easier

than h100i, results in alignment of h110i axes parallel to the linea-tion and of {100} and {111} planes parallel to the foliation(Mainprice et al., 2004). CPO is nevertheless always weak, due tothe cubic structure of garnet, which results in a large number ofequivalent slip systems. For instance, the h111i{110} mode thataccommodates most deformation in the models is composed of 12slip planes/directions.

Garnet CPOs in the Beni Bousera garnet clinopyroxenites arealways weaker than themodel predictions (J� 1.3, Table 1), but thisis also the case for the naturally deformed eclogites analyzed byMainprice et al. (2004). Among our samples, only BB007 andBB125BW show concentrations of h110i axes parallel to the linea-tion (Fig. 6) as predicted by the numerical models of garnetdeformation. Garnets in the remaining samples do not showconcentration of any of the main crystallographic axes parallel thelineation or foliation. It is important to note, however, that the VPSCmodels in Mainprice et al. (2004) were calculated for pure garnetaggregates, whereas in the Beni Bousera pyroxenites most defor-mation has probably been accommodated by the pyroxenes, whichare almost fully recrystallized. Rotation of garnet crystals in theless-viscous pyroxene matrix may have contributed to the disper-sion of the garnet CPO.

Strong keliphitization hinders the analysis of garnet deforma-tion in the garnetespinel websterites, but themore rounded shapesof the relics of coarse garnet crystals in these samples suggest thatthey were less deformed. Finally, the embayments filled bypyroxenes along garnet grain boundaries in garnet clinopyrox-enites (Fig. 3d) cannot have formed by plastic deformation. Theyrather suggest syn- to late-kinematic reactions that crystallizepyroxenes at the expense of garnet.

9.1.2. ClinopyroxeneMicrostructures and CPO of clinopyroxenes of the Beni Bousera

pyroxenites record a variety of deformation processes: frommechanical twinning on [001](100) (Fig. 10a) to dislocation glideassisted by dynamic recrystallization by subgrain rotation (Fig. 10)and, to a lesser extent, grain boundary migration (Fig. 3f). Theabundance of subgrains with grain sizes similar to the recrystal-lized matrix (Fig. 10a), together with the large proportion of low-angle grain boundaries with misorientations <15� (Fig. 10f) andtheir dominant rotations around [001] (Fig. 10c), indicates thatsubgrain rotationwas the dominant recrystallizationmechanism ingarnet pyroxenites. Fig. 3f shows that some bulging recrystalliza-tion did occur. However, the grain size of these bulges and of thepyroxene crystals formed by this process (5e10 mm) are an order ofmagnitude smaller than recrystallized crystals that compose mostof the matrix. In the spinel websterites-equilibrated at highertemperatures-nucleation and growth probably contributed more torecrystallization, as indicated by the presence of strongly misor-iented crystals in domains where the continuity of the exsolutionsindicates that they were part of former single porphyroclast(Fig. 10a). Stress concentration at the tips of the exsolution lamellaemight have favored nucleation.

Mechanical twinning of diopside on [001](100) was first recor-ded in the pioneering experiments of Avé Lallemant (1978), whodeformed single crystals at temperatures below 850 �C and strainrates of 10�3 s�1 in a Griggs apparatus. Twinning on [100](001) wasobserved at higher temperatures (1000 �C) and lesser strain rates(10�6 s�1). Similar results were reported by Kollé and Blacic (1982,1983) for Cr-diopside and hedenbergite. However, to our knowl-edge, the present observations are the first that clearly evidencedmechanical twinning along [001](100) in clinopyroxenes deformedat high temperature (1100e1150 �C) and natural strain rates.

TEM observations on experimentally deformed diopside crystalsenabled the identification of dislocation glide systems active as


a function of the temperature for laboratory strain rates: between800 and 900 �C, [001](100) is the easiest slip system, but[100](010), ½h110i{1�10} and [001]{110} are also activated (Ingrinet al., 1992). Above 1000 �C, the most favorable slip system is½h110i{1�10}, but [001]{110} and [001](100) are also active(Raterron and Jaoul, 1991; Jaoul and Raterron, 1994).

Experimentally deformed diopside aggregates (Boland andTullis, 1986; Lavie, 1998; Mauler et al., 2000; Bystricky andMackwell, 2001), naturally deformed omphacite-bearing eclogites(Van Roermund and Boland, 1981; Van Roermund, 1983; Buatieret al., 1991; Godard and Van Roermund, 1995) and lower crustalgabbros (Barruol and Mainprice, 1993) usually have clear clino-pyroxene CPO characterized by [001] and [010] axes alignedsubparallel to the lineation and normal to the foliation, respectively.Al- and Na-rich clinopyroxenes (Table 2) from Beni Bouserapyroxenites show similar CPO patterns, except for garnet pyroxe-nite BB031 (Fig. 6). These patterns are consistently reproduced byviscoplastic self-consistent models in which most of the deforma-tion is accommodated by glide on [001]{110} (Bascou et al., 2002).Based on these results and on the petrological data, we concludethat clinopyroxene in the Beni Bousera pyroxenites deformed bydislocation creep with dominant activation of the [001]{110}system over a temperature range of 950e1150 �C. The transition todominant glide on ½h110i{1�10} observed at 1000 �C in experi-ments occurs therefore at higher temperatures in nature. Thisdifference suggests that the transition might be strain-ratedependent, being favored under high strain rates.

CPO evolution models also showed that the strain regimecontrols the CPO symmetry (Bascou et al., 2002). Transpression, forinstance, results in a dispersion of [001] in the foliation planesimilar to that observed in many Beni Bousera pyroxenites. Theanastomosed pattern of the foliation around the more competentgarnet crystals may account, on the other hand, for the dispersionof [001] in the XZ plane observed in the remaining samples. Thisgeometry may also explain the obliquity between the clinopyrox-ene CPO and the foliation observed in some samples. It cannot,however, explain the bimodal distribution of [001] relative to thelineation displayed by spinel websterites BB076BBE, BB068E, andBB073CW. Finally, BB031 has a clinopyroxene CPO that is notconsistent with glide on any known clinopyroxene slip system.

9.1.3. OrthopyroxeneAs in previous microstructural studies of naturally deformed

orthopyroxenites (Etheridge, 1975), lattice bending and kinking arecommon in our samples. In addition, like clinopyroxene, ortho-pyroxene also shows evidence for dynamic recrystallization. Thestrong misorientation of recrystallized grains (Fig. 10a) suggeststhat nucleation and growth contributed to this process. Theseprocesses are likely favored by the high temperatures recorded inthe spinel websterites.

The most commonly observed slip system in experimentallydeformed enstatite is [001](100) (Turner et al., 1960; Raleigh, 1965;Green and Radcliffe, 1972). Ross and Nielsen (1978) also observedslip on [001](010) in wet polycrystalline enstatite deformed at hightemperature. [010] dislocations (Lally et al., 1972; Kohlstedt andVandersande, 1973) and even longer Burger vectors ones(Skrotzki, 1994) were also reported in naturally deformed rocks, butare rare.

CPOs of Al- and Ca-rich orthopyroxene crystals (Table 2) fromBeni Bousera pyroxenites are characterized by clustering of [001]subparallel to the lineation and [100] axes near the normal to thefoliation. This is consistent with dominant slip on [001](100). Twogarnetespinel pyroxenites, BB076BBE and BB075W, show [010]axes clustered perpendicular to the foliation plane and are moreconsistent with dominant slip on [001](010). There is, however, no

independent indication that these two pyroxenites were deformedunder different hydration conditions. Similar orthopyroxene CPOsare also observed in peridotites deformed under lithospheric andasthenospheric conditions (e.g., Tommasi et al., 2006, 2008;Soustelle et al., 2009), suggesting that both planes may be acti-vated in nature. Finally, the topotaxic relations between orthopyr-oxene and clinopyroxene observed in the symplectite mightexplain the strong correlation with clinopyroxene CPO.

9.2. Changes in deformation processes and rheological contrasts asa function of synkinematic PeT conditions

Based on the microstructures and CPO data analyzed above, weconclude that the deformation of Beni Bousera pyroxenites wasessentially accommodated by dislocation creep of the volumetricallydominant clinopyroxene. Consistent orientations of the pyroxenitelayering and of the foliation of the host peridotite at the massif scale(Reuber et al., 1982) suggest similar kinematics and hence a contin-uous deformation across the different metamorphic domains. Ther-mobarometry indicates however that deformation of garnetclinopyroxenites took place under lower temperatures and higherpressures than the deformation of the spinel websterites, respec-tively 950 �Ce2.0 GPa and 1150 �Ce1.8 GPa (Fig. 13).

The finer recrystallized grain sizes in the garnet clinopyrox-enites relative to the spinel websterites suggests greater work ratesin the former (Austin and Evans, 2007). Assuming a non-linearviscous rheology typical of crystal-plastic deformation by disloca-tion creep, we estimated the stresses during deformation of bothgarnet clinopyroxenites and spinel websterites using experimen-tally derived dislocation creep flow laws for dry (Bystricky andMackwell, 2001) and wet clinopyroxenite (Chen et al., 2006). Asgarnet clinopyroxenites are associated with mylonitic peridotites,we assumed that they deformed under faster strain rates than thespinel websterites. We assumed therefore that the strain rate was10�12 s�1 for garnet clinopyroxenite versus 10�14 s�1 for spinelwebsterites. Calculations for clinopyroxene creep under dryconditions yielded ca. 185 MPa for the garnet clinopyroxenites andca. 7 MPa for the spinel websterites. Stresses calculated for wetconditions are substantially smaller, down to ca. 10 MPa and7�10�2 MPa, respectively. In both cases, however, stresses for thegarnet clinopyroxenites are greater by more than one order ofmagnitude than those predicted for the spinel websterites. If strainrates are supposed similar in both domains, stresses in the garnetpyroxenites are still one order of magnitude larger.

The extremely small stresses obtained using wet clinoyroxenerheologies are consistent with the conclusion by Chen et al. (2006)that for hydrated conditions pyroxenites are weaker than perido-tites. An inversion in the rheology contrast between pyroxenitesand peridotites is however not corroborated by our field observa-tions. The systematic boudinage of the pyroxenite layers (Fig. 2)indicates that pyroxenites are more competent than peridotites inall domains. Thus, although the deformation of garnet in the garnetclinopyroxenites indicates a small rheological contrast relative tothe clinopyroxene, this reduction in competence contrast cannot beexplained by hydration of the garnets, as proposed by Muramotoet al. (2011) to account for plastic deformation of garnet in pyrox-enites in supra-subduction environments. The more roundedshapes of garnets in garnetespinel websterites close to theAriègiteeSeiland transition and the coarser clinopyroxene grainsize suggest a greater competence contrast between garnet andclinopyroxene during deformation of this domain, which occurredat higher temperature and lower pressure conditions. Based onthese observations, we propose that the reduction in the compe-tence contrast between garnet and clinopyroxene is due to the


higher stresses implied in the deformation of the colder garnetpyroxenites.

Although the analysis of field structures does not corroborate aninversion of the competence contrast between the garnet clino-pyroxenites and the host peridotite, it points to a reduced compe-tence contrast between the two rocks. Garnet clinopyroxenites ingarnetespinel mylonites often outcrop as extremely stretched,partially disrupted layers (Fig. 2a,c). Even in thicker garnet pyrox-enites, macroscopic boudins in the Beni Bousera massif aresymmetric boudins that could be classified as “drawn” boudins-inthe sense of Goscombe et al. (2004) -and require small competencecontrasts (Fig. 2c). Deformation under high stress conditions isprobably an effective mechanism for compositional homogeneiza-tion of the lithospheric mantle through mechanical mixing.Stretching of pyroxenite layers also increases the effective contactsurface with the host peridotites and decreases grain sizes,enhancing the kinetics of pyroxenite-peridotite reactions duringheating/exhumation.

10. Conclusions

Analysis of the microstructure, CPO, and thermobarometric dataof pyroxenites from the Beni Bousera massif evidences that underlithospheric and near asthenospheric mantle conditions pyroxe-nites deform by dislocation creep. Strain was essentially accom-modated by the pyroxenes, which deformed by dislocation creepinvolving glide on [001]{110} in clinopyroxene and on [001](100)and [001](010) in orthopyroxene, recovery, and dynamic recrys-tallization by subgrain rotation. Grain boundary migration alsocontributed to recrystallization during the deformation of thespinel websterites that took place under higher synkinematictemperature conditions. In the latter, mechanical twinning alsoplayed an important role in the deformation and recrystallization oflarge clinopyroxene crystals.

Garnet in garnet clinopyroxenites also deformed plastically.Crystal orientation maps show strong intracrystalline misorienta-tions accommodated by rotations around h111i and h110i andsubgrains, suggesting deformation by dislocation creep. However,garnet CPOs are weak and only two out of five analyzed samplesshow CPOs consistent with those predicted by viscoplastic self-consistent modeling for dominant slip h111i{110}.

Garnet pyroxenites and spinel websterites display contrastingmicrostructures characterized by distinct grain-size distributions,as well as different volumes of the recrystallized grain fractions.Plastic deformation of garnet and almost complete recrystallizationof clinopyroxene to fine grain sizes in the garnet clinopyroxenitesindicate deformation for large work rates (and large stresses andstrain rates), consistent with the higher pressure and lowertemperatures conditions inferred from petrological data in thesesamples (2.0 GPa and 950e1000 �C). These strong stress conditionsmight account for the reduction in the competence contrastbetween garnet and clinopyroxene in these rocks.

Acknowledgments

We thank Hassana Malainin for help during field work and D.Mainprice and A. Vauchez for helpful discussions on deformationprocesses in garnet and pyroxenes. R. Reyes-González, C. Nevadoand D. Delmas are thanked for providing high-quality polishing ofsections for EBSD measurements. Electron microprobe analyseswere carried out with the help of C. Merlet at the Service Micro-sonde Sud, Université Montpellier 2. Comments by W. Sullivangreatly improved the readability of the manuscript. We also thankK. Ozawa for constructive comments. The research leading to theseresults has been funded by Crystal2Plate, a EU-FP7 Marie Curie

Action under grant agreement PITN-GA-2008-215353. We alsoacknowledge funding from the Spanish “Ministerio de Ciencia eInnovación” Grant CGL2010-14848, Junta de Andalucía researchgrants RNM-131 and 2009RNM4495, and the International Litho-sphere Program (CC4-MEDYNA). The EBSD-SEM national facility inMontpellier is supported by the Institut National de Sciences del’Univers (INSU) du Centre National de la Recherche Scientifique(CNRS), France and by the Conseil Régional Languedoc-Roussillon,France.

Appendix A. Supplementary data

Supplementary data related to this article can be found online atdoi:10.1016/j.jsg.2012.02.019.

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Journal of Structural Geology - Géosciences Montpellier · 2012. 5. 21. · Deformation processes and rheology of pyroxenites under lithospheric mantle conditions Erwin Fretsa,b,*,