geodynamic implications of ﬂattened tabular equigranular...

Journal of Geodynamics 43 (2007) 484–503

Geodynamic implications of flattened tabular equigranular texturedperidotites from the Bakony-Balaton Highland Volcanic

Field (Western Hungary)�

Károly Hidas a, György Falus a,b, Csaba Szabó a,∗,Péter János Szabó c, István Kovács a,d, Tamás Földes e

a Lithosphere Fluid Research Lab, Department of Petrology and Geochemistry, Eötvös University, H-1117 Budapest, Hungaryb Eötvös Loránd Geophysical Institute of Hungary, H-1145 Budapest, Hungary

c Department of Materials Science and Engineering, Budapest University of Technology and Economics, H-1111 Budapest, Hungaryd Research School of Earth Sciences, The Australian National University, ACT 0200 Canberra, Australia

e Institute of Diagnostic Imaging and Radiation Oncology, University of Kaposvár, H-7400 Kaposvár, Hungary

Received 10 February 2006; received in revised form 8 September 2006; accepted 18 October 2006

Abstract

Peridotite xenoliths showing unusual tabular equigranular textures (addressed as flattened tabular equigranular) were found inNeogene alkali basalts from the Bakony-Balaton Highland Volcanic Field (Western Hungary), Carpathian-Pannonian Region. Theolivines have a characteristic crystallographic preferred orientation (CPO) with [0 1 0]-axes perpendicular to the foliation and the[1 0 0]- and [0 0 1]-axes forming a continuous girdle in the foliation plane. Contrarily, the CPO pattern of orthopyroxene is muchmore scattered, although a single maximum can be observed in [0 0 1] axes subparallel to the plane of foliation. In case of olivine,the activation of (0 1 0)[1 0 0] and also probably (0 1 0)[0 0 1] is suggested. The deformation micro-mechanisms of orthopyroxenesare suggested to be a combination of intracrystalline glide on the (1 0 0)[0 0 1] system and some kind of other mechanism resultingin quite scattered patterns.

We suggest that the unusual orientation patterns of olivines and orthopyroxenes are the result of the complex tectonic evolutionof the region. The flattened tabular equigranular xenoliths represent a structural domain within the subcontinental lithosphericmantle beneath the volcanic field with particular seismic characteristics. The occurrence of flattened domains in the upper mantlemay considerably influence the percolation and residence time of the mantle melts and fluids, which could promote or preventmelt/wall-rock interaction.© 2006 Elsevier Ltd. All rights reserved.

Keywords: Bakony-Balaton Highland Volcanic Field; Carpathian-Pannonian Region; Peridotite xenoliths; Flattened tabular equigranular texture;Crystallographic preferred orientation (CPO); EBSD

� Paper presented at the Peridotite Workshop 2005, held in Lanzo, 27–30 September 2005, organized by Alessandra Montanini and Giovanni B.Piccardo, and sponsored by the Italian Working Group on the Mediterranean Ophiolites.

∗ Corresponding author. Tel.: +36 1 2090555x8338; fax: +36 1 3812108.E-mail address: [email protected] (C. Szabó).

0264-3707/$ – see front matter © 2006 Elsevier Ltd. All rights reserved.doi:10.1016/j.jog.2006.10.007

mailto:[email protected]/10.1016/j.jog.2006.10.007

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K. Hidas et al. / Journal of Geodynamics 43 (2007) 484–503 485

. Introduction

The most accurate way to describe the real rheological state of the upper mantle is studying upper mantle peridotitesn the surface found as xenoliths, Alpine-type peridotites or obducted ophiolites. In the last few years, beside traditionalextural characterization (e.g., grain size and boundary, etc.) and geochemical investigation of the peridotites (e.g., majornd trace element composition, stable and radiogenic isotopic ratios, etc.), their crystallographic preferred orientationCPO) analysis came to the foreground. Whilst the geochemical characteristics provide information mainly on theeodynamic and different geochemical processes of the (upper) mantle, the orientation analysis is used to describe itshysico-mechanical (e.g., deformation) state and evolution. Nevertheless, studies dealing with complex geochemicalnd fabric analysis are rare.

The upper mantle peridotites beneath the Carpathian-Pannonian Region are extensively studied (e.g., Downest al., 1992; Embey-Isztin et al., 2003; Szabó et al., 2004, and references therein). Previous papers described onlyhe basic textural features and focused rather on the geochemical characterization of the upper mantle. However,ecause solely “representative” xenoliths have been studied only slight chemical and/or textural differences haveeen recognized compared to other upper mantle xenolith suits worldwide derived from alkali basalts. Merely veryittle is unraveled from the special phenomenon that are only characteristic for the Carpathian-Pannonian Region andhich could be the products of the local environment or evolutional history. Recently, there have been many efforts

o characterize special, rare xenoliths found in the alkali basalt occurrences of the Carpathian-Pannonian Region (e.g.,ali, 2004; Cvetkovic et al., 2007; Falus et al., 2007). These studies provide invaluable information on the role andehavior of the upper mantle and elucidate the special geodynamic environment during the formation of the Pannonianasin.

In this paper we present a basic (major and trace element) geochemical, detailed fabric (polarized light microscope,nd CT) and EBSD analysis of CPO of both olivine and orthopyroxene in three peridotite xenoliths from the Bakony-alaton Highland Volcanic Field (BBHVF), marked by a characteristic and rare tabular equigranular texture. Theetrographic features and uniqueness of the observed texture inspired us to address it flattened tabular equigranularexture. Furthermore, the studied upper mantle xenoliths may provide insight into anisotropic nature of the lithospheric

antle beneath the central part of the Carpathian-Pannonian Region. We propose that anisotropic nature is very likelyssociated with the tectonic evolution of the Carpathian-Pannonian Region. In fact, resulting anisotropy in the upperantle of the volcanic field has severe effect on melt migration and may also be traceable by geophysical soundingethods.

. Geological setting

The Carpathian-Pannonian Region is situated in Eastern Central Europe and includes the Pannonian Basin sur-ounded by the Carpathian Mountains (Fig. 1a). The Pannonian Basin is considered to represent a back-arc basin ofhe Carpathian Arc that was formed at late stages of the Alpine orogenesis, due to the convergence of Apulia andurope (e.g., Stegena et al., 1975; Horváth, 1993; Csontos, 1995; Fodor et al., 1999). The major driving forces of basin

ormation are controlled by continuous subduction of the European plate on the northern and eastern contact of theasin along the outer Carpathians (Csontos et al., 1992; Horváth, 1993), and synchronous eastward extrusion of theasement units from the Alpine compressional belt. In the Early to Late Miocene the subduction related rollback effects thought to be responsible for equal thinning in the crust and the mantle (Lankreijer et al., 1995). After a short periodf inversion in the Late Miocene to Pliocene, the Pannonian Basin was affected by an active rift phase. Consequently,he Pannonian Basin is characterized by anomalously thin lithosphere (crust and upper mantle), ∼60 km (Horváth,993) (Fig. 1a).

The Carpathian-Pannonian Region became “locked” during the Late Pliocene to Quaternary. Post-extensional alkaliasaltic activity took place sporadically in the basin (e.g., Szabó et al., 1992; Embey-Isztin et al., 1993) at that time.pper mantle ultramafic xenoliths occur in these mafic volcanic rocks and can be found in the Carpathian-Pannonianegion from west to east in the Styrian Basin, Little Hungarian Plain, Bakony-Balaton Highland, Nógrád-Gömör and

astern Transylvanian Basin (e.g., Szabó et al., 2004 and references therein) (Fig. 1a).

Among these localities, the Bakony-Balaton Highland Volcanic Field (BBHVF) (Fig. 1b) is of extreme importanceecause this location contains wide spectrum of xenolith lithologies and different textural types. Besides the peridotites,ranulite, (e.g., Embey-Isztin et al., 2003; Török et al., 2005), pyroxenite (Dobosi et al., 2003; Bali et al., 2006) xenoliths

486 K. Hidas et al. / Journal of Geodynamics 43 (2007) 484–503

are also abundant. A full variety of mantle xenolith textures from protogranular to recrystallized (Embey-Isztin et al.,1989; Downes et al., 1992; Szabó et al., 1995) types and some cataclastic (Falus et al., 2004) mantle fragments alsooccur.

3. Sampling and applied techniques

Three representative peridotite xenoliths, derived from the BBHVF, were studied in detail. Two of them werecollected near Szentbékkálla village (Szba-1 and Szbd-15), whereas one was found around Szigliget village (Szgk-0301)(Fig. 1b).

The thin sections of the studied xenoliths had been cut oriented in xz- (i.e. perpendicular to the foliation and parallelto the lineation) and yz-plane (i.e. perpendicular to the foliation and the lineation). Polarized light microscope (NikonEclipse E600 POL, Lithosphere Fluid Research Lab, Eötvös University Budapest, Hungary) and Computer Tomography(CT) were used for detailed petrographic description. CT analysis was carried out using a Siemens Somatom Plus 4 CT

(Institute of Diagnostic Imaging and Radiation Oncology, University of Kaposvár, Hungary). The peak acceleratingvoltage was 140 kV, slice thickness 1 mm, X-ray tube current 189 mA. Osiris Medical Imaging Software v4.19 andMVE, Medical Volume Explorer v0.8.17.0 software were used for data processing. Major element mineral chemistryof rock forming minerals was measured by JEOL Superprobe JXA-8600 microprobe equipped with WDS detectors

Fig. 1. (a) Map of lithospheric thickness (units in km) beneath the Carpathian-Pannonian Region. The Bakony-Balaton Highland Volcanic Field(BBHVF) is situated in the center of the Pannonian Basin with anomalously thin lithosphere after Lenkey (1999), according to Horváth (1993).SBVF: Styrian Basin Volcanic Field; BBHVF: Bakony-Balaton Highland Volcanic Field; LHPVF: Little Hungarian Plain Volcanic Field; NGVF:Nógrád-Gömör Volcanic Field; ETBVF: Eastern Transylvanian Basin Volcanic Field. (b) Simplified geological map of the BBHVF with the uppermantle and lower crustal xenolith bearing locations (after Jugovics, 1968; Harangi, 2001).

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Department of Earth Sciences, University of Florence, Italy). Trace element mineral chemistry was determined byA-ICPMS (Institute of Isotope Geochemistry and Mineral Resources, ETH-Zürich, Switzerland). The quantitativeater determination in silicate minerals was carried out by a Bruker IFS 28 type infrared microscope (Research Schoolf Earth Sciences, Australian National University, Canberra, Australia) using unpolarized light, 105 �m sized aperturend the calibration of Bell et al. (2003) and Kovács et al. (2006) for quantification.

Olivine and orthopyroxene crystallographic preferred orientation (CPO) was measured in syton-polished xz- and yz-hin sections using a Philips XL30 SEM system equipped with TSL-manufactured electron back-scattered diffractionEBSD) (2 points/grain; at least 100 grains/thin section; the fabric uniformity was tested previously, see Fig. 3a and bor details) (Department of Materials Science and Engineering, Budapest University of Technology and Economics,ungary). Accelerating voltage was set to 30 kV with 7 spot size and 13.0 mm working distance. The confidence index

CI) was 0.19 (±0.08) for olivine and 0.30 (±0.06) for orthopyroxene, which means that the patterns are correctlyndexed at least 95% of the time (Wright, 2000). The EBSD data were processed by Orientation Imaging Microscopy4.51 software package for TSL EBSD. Pole figures were generated by the PF2k careware software of Mainprice2003).

. Petrography

The size of the mantle xenoliths selected for detailed study is smaller (maximum 3 cm × 4 cm in diameter) thanhe usual mantle xenoliths from the BBHVF (often 10 cm × 15 cm in diameter). The shape of the xenoliths is roundedFig. 2a). Macroscopic foliation in the rocks and mineral lineation are visible in hand specimens being defined byattening and stretching of all mineral phases (Fig. 2a). On the CT image foliation is also shown in 3D by the olivinesFig. 2b).

The modal compositions of the samples in xz- and yz-sections were analyzed with an optical microscope using theraditional grid (area counting) method and more than 2000 points were analyzed in each sample (Table 1). Two of thehree samples are spinel lherzolites (Szba-1 and Szbd-15), whereas one is spinel harzburgite (Szgk-0301) (Table 1).he grain size and aspect ratio distribution was determined manually by digital image analysis, using the Scion Image4.0.3.2 software.

The grain size distribution is uniform, and the average grain size of both olivine and pyroxene in the studiederidotites is ranging from 200 �m × 300 �m to 0.5 mm × 1.5 mm (Fig. 3a and b). The majority (80%) of the olivinesas 200–400 �m size (Fig. 3a), whereas the orthopyroxenes are somewhat smaller with 150–200 �m size (Fig. 3b).he rock forming minerals of the peridotites have very strong shape preferred orientation (SPO); no banding or veiningas observed (Fig. 4a). The CT analysis also support that the position of the minerals in the fabric of the peridotites isomogeneously dispersed (shown by olivines in Fig. 2b). The rock-forming minerals are elongated (in the 2D sections)ith a typical aspect ratio of 1:∼3 for olivine and 1:∼2 for orthopyroxene (Figs. 3a and b and 4a). The minerals in

ll three samples are anhedral. The grain boundaries in the studied xenoliths are straight, and internal strain features

re absent, or only rarely occur in olivines as widely spaced (100–150 �m) dislocation walls. In Szgk-0301 xenolithhe grain boundaries are slightly curved, but the mineral elongation is still well defined. Foliations and lineationsithin the peridotites are defined by the alignment of spinel grains, however the macroscopic long axis of the otherantle silicates (olivine, orthopyroxene, clinopyroxene) is subparallel to this lineation, whereas orientation of some

able 1odal composition (in xz and yz sections), equilibrium temperature and oxygen fugacity of the studied samples

ample Olivine(modal%)

Orthopyroxene(modal%)

Clinopyroxene(modal%)

Spinel (modal%) Rock name Temperaturea

(◦C)�log fO2b

(at 10 kbar)

xz yz xz yz xz yz xz yz

zba-1 78 79 16 16 5 4 1 1 Spinel lherzolite 1020 −0.4922zbd-15 86 80 10 13 3 4 1 1 Spinel lherzolitec 980 −0.3755zgk-0301 82 78 13 16 4 4 1 2 Spinel harzburgite 1010 0.0898

a The estimation of temperature was made according to the method of Brey and Köhler (1990).b The oxygen fugacity was calculated at 10 kbar using the equation of Ballhaus et al. (1991).c The actual modal composition of the sample is harzburgitic. However, melt pockets (3 vol.%) are suggested to represent former clinopyroxenes

Fig. 3c).


Fig. 2. (a) Photomicrograph of Szgk-0301 spinel harzburgite showing the macroscopic foliation (black solid lines) and the orientation of xz- andyz-thin sections. White dashed lines outline the approximate position of the (b) CT image. (b) Three-dimensional (3D) CT image of the Szgk-0301spinel harzburgite. The isodensity contours show the heterogeneously dispersed olivine distribution within the xenolith. The flattening of olivine

shows the plane of the foliation too, especially in the upper left side of the picture. The image was taken at the level of 3980 HU (Housefield Unit)with the window width of 230. These values refer to the average surface density of olivine (3.22 + 1.32Fe# [g cm−3], after James et al., 2004). Theorientation, scale and the presented volume of the peridotite are shown in (a).

orthopyroxenes is perpendicular to it (Fig. 4a). Spinels are mostly interstitial, however in xenolith Szbd-15 they formrounded inclusions within orthopyroxenes 0.1 mm × 0.3 mm in size (Fig. 4b). In xenolith Szbd-15 melt pockets (i.e.pocket-shaped spaces, formed by in situ melting process, which mainly consist of glass, secondary clinopyroxene,spinel and olivine) appear, most probably associated with the melting of clinopyroxenes (Fig. 4c). Melt pockets showSPO, subparallel to the mineral lineation. Moreover, they have straight, gently curved boundaries toward the adjacentminerals and their dimension is similar to the clinopyroxene.

Regarding the textural type, the studied xenoliths are not common in the BBHVF and were reported very rarely among

the worldwide-studied upper mantle peridotites. Mercier and Nicolas (1975) called similar fabrics tabular equigranular;however they have not suggested a clear implication for those textures. Pike and Schwarzman (1977) interpreted similartextural types as metamorphic foliated one produced due to simple shear during a mantle deformation. However, in


Fig. 3. (a) Grain size- and aspect ratio distribution of olivine in peridotites studied. Aspect ratio is dimensionless and expressed as longest axis/shortestaxis on 2D pictures. n: number of analyzed grains. (b) Grain size- and aspect ratio distribution of orthopyroxene in peridotites studied. Aspect ratiois dimensionless and expressed as longest axis/shortest axis on 2D pictures. n: number of analyzed grains.


Fig. 4. Photomicrographs of textural features in the studied xenoliths. ol: olivine; opx: orthopyroxene; cpx: clinopyroxene; sp: spinel; gl: glass; ol2:secondary olivine; cpx2: secondary clinopyroxene; sp2: secondary spinel. (a) Flattened equigranular texture of the studied peridotites, represented bySzba-1 spinel lherzolite. Solid gray line denotes the direction of the lineation in the oriented thin section. Photo was taken under stereomicroscope.(b) Interstitial spinel and spinel inclusions in orthopyroxenes in Szbd-15 spinel lherzolite (further information in Table 1, see text for details).Plane-polarized light image. (c) Melt pocket in Szbd-15 spinel lherzolite (further information in Table 1, see text for details). Plane-polarized lightimage.


Pike and Schwarzman’s xenoliths orthopyroxenes and clinopyroxenes often have undulatory extinction, which featureis not observed in our samples. Based on the macroscopic characteristics of the constituent phases (and the later shownEBSD orientation analysis), we called the studied samples ‘flattened tabular equigranular’ textured peridotites.

5. Major element mineral chemistry

Major element analysis of mineral phases in the studied xenoliths was carried out in order to define the principalgeochemical characteristics of segment of the mantle, represented by the studied xenoliths. Based on point analysiscarried out on mineral cores and rims and the careful analysis of digital SEM images, it is clear that no chemical zoningoccurs (Table 2).

Generally, the mg# [=100 × Mg/(Mg + Fe2+)] of olivines, orthopyroxenes and clinopyroxenes in the studied peri-dotites is almost constant (ol: 90.8–91.6; opx: 91.0–92.1; cpx: 92.2–93.3), whereas the cr# [=100 × Cr/(Cr + Al)] inspinel is strongly variable (16.0–41.6; Table 2 and Fig. 5a and b). Clinopyroxenes are TiO2-poor (up to 0.18 wt.%)and rich in Cr2O3 (up to 1.05 wt.%) (Table 2). The Al2O3 content of the clinopyroxenes is variable ranging fromthe highest (most fertile) composition of Szba-1 (4.54 wt.%) to the lowest (most depleted) values of Szgk-0301(2.89 wt.%). A positive correlation is seen between Al and Na in clinopyroxene (Fig. 5a) and a negative correla-tion was found between Al and Cr in the spinels of the studied peridotite. The highest Cr2O3 content (35.0 wt.%)is characteristic for spinels in Szgk-0301 xenolith and lowest one (15.0 wt.%) for Szbd-15. All of these are com-mon features of peridotite xenoliths derived from the subcontinental lithospheric mantle. Szbd-15 spinel lherzolitepossesses the lowest mg# and cr# in all mineral phases, whereas Szgk-0301 displays the most depleted overallmajor element compositions (Table 2 and Fig. 5a and b) compared to data of Downes et al. (1992) and Szabó etal. (1995).

The water content of the major silicates in two of the three studied peridotites was analyzed by IR spectroscopy(Table 2). Olivines possess the lowest water content (∼1 ppm), orthopyroxenes have a water concentration of90–60 ppm, whereas clinopyroxenes show values ranging between 250 and 480 ppm. This means that the studiedperidotites represent relatively dry fragments of the upper mantle (i.e., contains less water than the normal MORBmantle).

Table 2Major element composition (in wt.%) and semi-quantitative water content (in ppm) of the rock forming minerals in the studied peridotite xenoliths

Sample Olivine Orthopyroxene Clinopyroxene Spinel

Szba-1 Szbd-15 Szgk-0301 Szba-1 Szbd-15 Szgk-0301 Szba-1 Szbd-15 Szgk-0301 Szba-1 Szbd-15 Szgk-0301

SiO2 40.4 41.5 41.3 55.0 56.0 57.1 51.8 53.2 53.3 0.05 n.d. 0.05TiO2 n.a. n.d. n.d. n.d. 0.05 n.d. 0.10 0.18 0.04 0.07 0.09 0.05Al2O3 n.a. n.d. n.d. 3.84 3.75 2.66 4.54 3.99 2.89 44.2 52.8 32.9Cr2O3 n.a. n.d. n.d. 0.64 0.38 0.59 1.05 0.49 0.92 22.6 15.0 35.0FeOta 8.35 9.05 8.25 5.33 5.86 5.21 2.53 2.52 2.24 11.1 12.0 14.0MnO 0.14 0.17 0.12 0.12 0.14 0.13 0.12 0.09 0.10 0.12 0.13 0.15NiO 0.36 0.37 0.36 n.d. n.a. n.a. 0.09 n.d. n.a. 0.32 0.31 0.20MgO 50.1 50.0 50.5 33.6 33.4 34.3 17.0 16.8 17.5 19.3 19.8 17.1CaO 0.08 0.05 0.09 0.92 0.63 0.85 20.8 22.3 21.8 n.a. n.d. n.d.Na2O n.a. n.a. n.a. 0.07 n.d. 0.05 0.93 0.43 0.51 n.a. n.a. n.a.

Total 99.4 101.1 100.6 99.5 100.2 100.9 98.7 100.0 99.3 97.8 100.1 99.5mg#b 91.4 90.8 91.6 91.8 91.0 92.1 92.3 92.2 93.3cr#c 25.5 16.0 41.6H2O (ppm)d 1.0 n.a. 2.3 90 n.a. 60 480 n.a. 250 n.a. n.a. n.a.

n.a.: not analyzed; n.d.: not detected.a All Fe given as FeO.b mg# = 100 × [Mg/(Mg + Fe2+)] in atomic ratio.c cr# = 100 × [Cr/(Cr + Al)] in atomic ratio.d Water content is semi-quantitative (details in text).


Fig. 5. (a) Variation in cation numbers of Na vs. Al in clinopyroxenes of peridotite xenoliths from the Bakony-Balaton Highland Volcanic Field(Downes et al., 1992; Embey-Isztin et al., 2003; Falus, 2004) and in the xenoliths studied. (b) Relationship between the Fo (forsterite) content of

olivines and cr# [=100 × Cr/(Cr + Al) in atomic ratio] in spinels of peridotite xenoliths from the Bakony-Balaton Highland Volcanic Field (Downeset al., 1992; Embey-Isztin et al., 2003; Falus, 2004), and in the xenoliths studied. Also, calculated compositions of 20% and 30% partial melting(Arai, 1994) are indicated.

6. Trace element composition of clinopyroxenes

Trace element composition of the clinopyroxenes in the studied peridotites is shown in Table 3. Incompatible traceelements in clinopyroxenes of the studied xenoliths show Pb, Th and U values up to 1.91, 5.80, 1.42 ppm, respectively.Concentration of the high field strength elements: Zr, Nb, Hf, Ta is up to 21.5, 0.58, 0.64, 0.04 ppm, respectively.Szbd-15 spinel lherzolite, however, shows an overall elevated trace element concentration, compared to other twoxenoliths studied.

Clinopyroxenes in the studied peridotites are enriched (cf. chondrites; Nakamura, 1974) in light rare earth elementscompared to heavy rare earth elements (Fig. 6), which is indicated by their high LaN/LuN (0.79–11.80) ratios (Table 3).

Szbd-15 spinel lherzolite is the most enriched in light rare earth elements, whereas Szgk-0301 spinel harzburgite is themost depleted in these elements among the peridotites studied (Fig. 6). However, the latter still remains enriched in rareearth elements (1


Table 3Trace element composition (in ppm) of clinopyroxenes in the studied peridotites

Sample Clinopyroxene, Szba-1 Clinopyroxene, Szbd-15 Clinopyroxene, Szgk-0301

Average (n = 4) Standard deviation Average (n = 3) Standard deviation Average (n = 4) Standard deviation

Sc 54.3 3.1 53.4 0.2 55.6 1.0Cr 6410 504 3346 81 6305 205Ni 405 9 365 2 424 12Sr 23.7 0.5 81.6 0.5 25.9 0.7Y 6.7


Fig. 7. Crystallographic preferred orientation (CPO) patterns and J-factor of olivine in peridotites studied. Pole figures are lower hemisphere, equalarea projections, using PF2k (Mainprice, 2003). Horizontal lines denote the foliation, lineation at 90◦/0◦. Sectioning inaccuracies were corrected,by rotating the data.


Fig. 8. Crystallographic preferred orientation (CPO) patterns of orthopyroxene in the studied peridotites. Pole figures are lower hemisphere, equalarea projections, using PF2k (Mainprice, 2003). Horizontal lines denote the foliation, lineation at 90◦/0◦. Sectioning inaccuracies were corrected,by rotating the data.


Usually, mantle peridotites have olivine J-factor (JOl) in the range of 2–25 and only a few have JOl > 20 (Ben Ismail andMainprice, 1998). The calculated J-factor of the olivines in the studied peridotites is ranging between 18.8 and 23.58(Fig. 7). The highest value is obtained in Szgk-0301 spinel harzburgite, whereas the smallest value is from Szbd-15spinel lherzolite.

The CPO of orthopyroxene is more scattered than that of the olivines (Fig. 8). The [0 0 1]-axes roughly follow thelineation and show a single maximum, the [0 1 0]-axes exhibit a single maximum perpendicular to the foliation, whereasthe orientation distribution of [1 0 0]-axes are scattered. Only few orthopyroxenes show the expected (1 0 0)[0 0 1]“normal” CPO pattern, which is the dominant slip system under upper mantle conditions (e.g., Coe and Kirby, 1975;Dornbush et al., 1994).

8. Discussion

8.1. Geochemical characteristics of the studied peridotites and the upper mantle beneath BBHVF

Based on the previous and recent studies on peridotite xenoliths from the Carpathian-Pannonian Region, a generalgeochemical feature of the subcontinental lithospheric mantle beneath the central part of the region, in the Bakony-Balaton Highland Volcanic Field (BBHVF) can be outlined. Clinopyroxene compositions, being the most sensitivelithospheric mantle rock forming mineral, show a continuous trend from the undepleted high Al (0.38) and Na (0.13)contents, related to protogranular and porphyroclastic xenoliths, to highly depleted Al (0.07) and Na (0.02) com-positions. The major element chemical compositions of clinopyroxenes from the studied peridotite xenoliths fit thistrend well and can be seen close to the most depleted equigranular and secondary recrystallized peridotite xenoliths(Fig. 5a). Plot of forsterite content of olivine versus cr# of coexisting spinel also shows a wide geochemical varianceof the BBHVF mantle xenoliths from “fertile” field to highly “depleted” area, data falling below or close to the 20%partial melting line (Fig. 5b). In this widely accepted diagram the studied peridotites suggest that they went through20–25 vol.% of partial melting (Fig. 5b). This is suggested to be a common ancient mantle process, active well beforethe formation of the Pannonian Basin (e.g., Downes et al., 1992; Szabó et al., 1995).

Spinel lherzolite of Szbd-15 possesses the lowest mg# in all silicate minerals and spinel, respectively. In contrast,Szgk-0301 spinel harzburgite displays the most depleted overall major element compositions, which corresponds tothe modal composition of the peridotites: Szgk-0301 xenolith is the richest in olivine (Table 1), even if Szbd-15 samplehas lower pyroxene content. Furthermore, the studied peridotites have 3–4 times more orthopyroxene (10–16 vol.%)than clinopyroxene (3–4 vol.%) in their modal composition (Table 1). This feature suggests significant depletionin clinopyroxene content (due to partial melting) compared not merely to the BBHVF but also to the Phanerozoicsubcontinental lithospheric mantle peridotites of the world, which show orthopyroxene/clinopyroxene ratio around 2(Griffin et al., 1999; Downes, 2001).

Among the trace elements, distribution of the rare earth elements (being the most sensitive element group) aregenerally used to depict such major geochemical processes as partial melting and mantle metasomatism, whichcause characteristic depletion or enrichment in these elements, respectively. Clinopyroxenes in peridotite xenolithswith equigranular and secondary recrystallized texture from the BBHVF show relative enrichment in light rare earthelements (Fig. 6), which is explained by fluid activity in the mantle beneath the Pannonian Basin (e.g., Downeset al., 1992). In contrast, clinopyroxene from protogranular and porphyroclastic xenoliths have flat rare earth ele-ment patterns, which refer to only slight enrichment in the light rare earth element content (Fig. 6). This minorgeochemical modification could also be the result of high fluid activity (Downes et al., 1992). The trace elementcomposition of the studied peridotites suggests a complex geochemical evolution, in which early depletion in rareearth elements was followed by enrichment in light rare earth elements (Fig. 6). The present rare earth element con-tent of the studied peridotites is not the same, however, it is likely that the same geochemical evolution affectedthem in different degrees. The occurrence of melt pockets in Szbd-15 xenolith implies intense melt/wall rock inter-action (e.g., Bali et al., 2002 and references therein). However, the formation of these melt pockets is suggestedto be a late, mostly in situ process (Bali et al., 2002; Bali, 2004), with little or no influence to observed preferred

orientations.

As a summary, based on their major element composition, the studied peridotites went through high degree partialmelting (20–25%), which is higher than the usual observed in common upper mantle peridotites of the BBHVF (∼20%)(Fig. 5b). Furthermore, the mg#s of the common rock types of the BBHVF are usually lower (89–90) than those of

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he studied xenoliths (91–92), which correspond to the above mentioned higher degree of partial melting observed inhe studied peridotites (Fig. 5a). Therefore, this depletion is also confirmed by the high orthopyroxene/clinopyroxeneatio (3–4) in the studied xenoliths, which means significant clinopyroxene loss during partial melting (Table 1). Therace element composition, despite the differences, suggests similar geochemical evolution (early depletion followedy different degree enrichment in light rare earth elements) (Fig. 6).

.2. Origin of crystallographic preferred orientations

The development of crystallographic preferred orientation (CPO) of minerals in mantle rocks is attributed to defor-ation related to major tectonic processes within the lithospheric mantle. The regularities in olivine CPOs of the

arge number of mantle derived rocks combined with theoretical modeling enable kinematical interpretation of severalPO-types (e.g., Ben Ismail and Mainprice, 1998).

The studied xenoliths from the BBHVF have flattened tabular equigranular textures with straight, gently curved grainoundaries typical of dynamic (± static) recrystallization at high temperatures (Fig. 4a–c). Major and trace elementomposition (Tables 2 and 3 and Figs. 5a and b and 6), equilibrium temperatures and oxygen fugacity (Table 1) of thetudied peridotites are unambiguously similar to the previously studied peridotite xenoliths with equigranular texturerom the Bakony-Balaton Highland Volcanic Field (e.g., Embey-Isztin et al., 1989, 2003; Downes et al., 1992; Szabó etl., 2004). These features indicate that annealing could have also been a characteristic process in the lithospheric upperantle portion represented by the studied xenoliths. In fact, early xenolith studies from the BBHVF (e.g., Embey-Isztin

t al., 1989; Downes et al., 1992; Szabó et al., 1995) demonstrated that equigranular xenoliths with statically annealedextures are quite abundant among the mantle xenoliths found in the central part of the Carpathian-Pannonian Region.his suggests that annealing and static recrystallization affected considerable proportion of the lithospheric mantleeneath the studied region, which may have been related to thermal effects of asthenosphere doming (e.g., Stegenat al., 1975; Huismans et al., 2001) and also probably to the migration of metasomatic melts/fluids indicated by tracelement compositions of equigranular mantle xenoliths (e.g., Downes et al., 1992; shown in Fig. 6). It is a relevantuestion how CPO of olivines and orthopyroxenes, generated during deformation prior to annealing, survived thistatic recrystallization. Detailed fabric analysis on peridotite massifs (Van der Wal and Bodinier, 1996; Vauchez andommasi, 2003), ophiolites (Dijkstra, 2001) and mantle xenoliths (Vauchez and Garrido, 2001; Xu et al., 2003) showffect of strong static recrystallization due to intense interaction with (hot) asthenospheric melt observed both in theirextural and geochemical characteristics. These studies clearly demonstrate that the original CPOs are well preservedfter static recrystallization.

Peridotite xenoliths of this study display extremely similar and strong olivine CPO, with J-factors ranging between8.8 and 23.58, which indicate very strong fabric (Fig. 7). Strong CPO is unlikely to have formed as a consequence oftatic recrystallization because static recrystallization generally results in scattered crystal orientations (Vauchez andommasi, 2003). Thus, it can be stated that the observed CPOs (both for olivines and orthopyroxenes) represent fabric

hat developed due to mantle deformation. As it is shown in Fig. 7 olivines have a characteristic CPO marked by the0 1 0]-axes oriented parallel to the poles of foliation, whilst [1 0 0]- and [0 0 1]-axes show distinctive maximums inhe foliation plane. This CPO pattern implies, that the active slip system was (0 1 0)[1 0 0], but the activation of otherlip systems, e.g., the (0 1 0)[0 0 1], based on similarities with the olivine textures described by Vauchez et al. (2005),s also suggested.

Olivine CPOs similar to that observed in the studied xenoliths were found rarely in other natural samples fromphiolites (e.g., Dijkstra, 2001; Dijkstra et al., 2002; Michibayashi and Mainprice, 2004) and xenoliths from subcratonicithosphere (e.g., Tommasi et al., 2000; Saruwatari et al., 2001; Vauchez et al., 2005). Saruwatari et al. (2001) regardimilar orientation patterns being deformed by dislocation creep at high temperatures (>1000 ◦C) with (0 1 0)[1 0 0]s the dominant slip system, based on the results of Carter and Avé Lallemant (1970). According to Dijkstra (2001),he development of such orientation patterns is indicative of flattening. This could be a suitable explanation for theevelopment of the special CPO in the studied olivines, and explains well the coaxial deformation observed in thetudied samples. However, based on the structural geological studies carried out in the Pannonian Basin, flattening has

ot occurred during basin formation (e.g., Fodor et al., 1999). Nevertheless, we cannot rule out the possibility that broadifting and well-defined extension direction on the surface has manifested in flattening of the upper mantle. Vauchez etl. (2005) argues that similar olivine orientations are formed when beside the (0 1 0)[1 0 0] slip system (Carter and Avéallemant, 1970) the (0 1 0)[0 0 1] slip system also contributes to the deformation. This is suggested to occur at high


pressure corresponding to the lowermost upper mantle (Couvy et al., 2004), which is irrelevant in our samples thatwere derived from the shallow lithospheric mantle (∼30–35 km estimated depth, according to the geotherm-plottedequilibrium temperatures of Table 1). The presence of water could also activate the (0 1 0)[0 0 1] slip system (Jungand Karato, 2001). However, based on the quantitative analysis of the rock forming minerals in the studied peridotitexenoliths, the H2O content is extremely low in the olivines (


Fig. 9. Perspective view of the BBHVF with its post-extensional alkali basalts (Jugovics, 1968; Harangi, 2001) and the supposed isochemical mantledST

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omain beneath the Bakony-Balaton Highland Volcanic Field (spatial body in the center of the figure), from where the studied xenoliths (Szba-1:zentbékkálla village; Szbd-15: Szentbékkálla village; Szgk-0301: Szigliget village) originate. Because of the perspective, the map is not in scale.he distance between Szigliget and Szentbékkálla villages is approximately 15 km (further information in the text).

henomenon worldwide (e.g., Anderson, 1961; Nicolas and Christensen, 1987; Karato, 1992; Ribe, 1992; Artemievat al., 2002), however seismic anisotropy measurements within the Pannonian Basin have not been carried out yet.evertheless, there are strong indications for the existence of seismic anisotropy in the shallow lithospheric mantle

Malinowski, personal communication, 2003). Based on Mainprice (2000), we have calculated seismic anisotropy ofhe mantle domain with the specific orientation pattern using Voigt-Reuss-Hill averaging method (Fig. 10 shown by thexample of Szgk-0301 spinel harzburgite). We have only used olivine orientations because the orthopyroxene orienta-ions were quite scattered. This implies that resulting velocities (Vp = 8.8–8.9 km/s) and anisotropies (AVs = 7–10.4%)re maximum estimates. Nevertheless, they are extremes compared to other data worldwide (e.g., Vauchez et al., 2005here Vs polarization anisotropy = 2.8–8.3%), which is thought to be result of the very strong fabric of the studiederidotites (J-factors: 18.8–23.58; Fig. 7).

Furthermore, the geodynamic significance of the flattened tabular equigranular texture type resides in the fact that thehape-characteristics and strong crystallographic preferred orientation of the minerals could strongly affect the perco-ation direction of migrating melts. Basaltic melts prefer to wet (0 1 0) crystal faces of olivine due to achieved minimalnterfacial energies (e.g., Waff and Faul, 1992). In mantle segments that exhibit strong crystallographic preferred ori-ntation this results in favorable percolation directions of the infiltrating melts parallel to olivine (0 1 0). Assuming thathe flattening of the mantle domain beneath the central part of the Carpathian-Pannonian Region is subparallel to theurface, and therefore the (0 1 0) faces are also subhorizontal, we suggest that migration of melts/fluids was dominantlyubhorizontal. This is superimposed also by the shape of the flattened grains resulting in surfaces three times largerubparallel to (0 1 0), than that perpendicular to it. Vertical motion is only achieved where (0 1 0) faces are subverticalr the migrating fluids can segregate from the peridotitic matrix (e.g., cracks or veins). Where this segregation can-

ot take place, melts/fluids are retained in the mantle exerting intensive geochemical modification/reaction and staticecrystallization on the deformed peridotite, producing equigranular, secondary recrystallized and at very intensiveelt/solid interaction, poikilitic textured rocks. At the time (and probably somewhat later) of the basin-forming defor-ation the supposed mantle domain(s)/region(s) could have strongly affected the percolation of melts, and therefore


−1
Fig. 10. Calculated compressional velocity (Vp [km s ]) distribution, shear-wave anisotropy (AVs [%]) and the vibration directions of the fastsplit shear waves for the Szgk-0301 spinel harzburgite, indicating the maximum (filled square) and minimum (open circle) values and directions.Contour intervals are 0.1 km s−1 for Vp and 1% for shear-wave anisotropy. Minimum contours are shown as dashed lines. Sectioning inaccuracieswere corrected, by rotating the data. The result is representative for all xenoliths studied here.
could have had a severe effect on the Neogene volcanic activity of the central Carpathian-Pannonian Region (Falus,2004).

9. Conclusions

1. Some rare clinopyroxene poor spinel peridotite xenoliths from the Bakony-Balaton Highland Volcanic Field (Hun-gary) have flattened tabular equigranular texture;

2. The crystallographic preferred orientation pattern of olivine crystal axes suggests the activation of multiple slipsystems: (0 1 0)[1 0 0] and most probably (0 1 0)[0 0 1]. The orientation distribution of orthopyroxene crystals impliesboth intracrystalline slip on the (100)[001] and non-intracrystalline deformation;

3. The crystal axis orientation distribution pattern of both olivine and orthopyroxene is interpreted as the result ofpure shear (coaxial) deformation. Therefore, the fabrics observed in the mantle xenoliths are presumably the conse-quence of tectonic processes of the Carpathian-Pannonian Region, although the preservation of earlier deformationsignatures especially in the shallower mantle portion cannot be excluded;

4. The strong orientation pattern and flattened tabular equigranular texture of the studied peridotites could stronglyaffect the migration of basaltic melts/fluids beneath the Bakony-Balaton Highland Volcanic Field. The mantledomain represented by these peridotites could act either as a barrier, or as a melt guiding plane during the Neogenevolcanic history of the central part of the Carpathian-Pannonian Region;

5. Anisotropic, horizontally persistent upper mantle domains are also suggested to have formed as a result of thedeformation. This anisotropy, having characteristic seismic signals, is suggested to be detectable using geophysicalsounding methods.

Acknowledgements

We are grateful for the kind help of Orlando Vaselli and †Filippo Olmi (EMPA analysis, University of Florence),Zoltán Zajacz (LA-ICPMS analysis, ETH-Zürich), Zoltán Gaál (EBSD analysis, Budapest University of Technologyand Economics). We thank Kálmán Török (Lithosphere Fluid Research Lab and Eötvös Loránd Geophysical Instituteof Hungary) for sample Szba-1 and the fruitful discussions about the evolution of the Pannonian Basin system. Special

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hanks for Enikő Bali (Lithosphere Fluid Research Lab) for her help in interpreting the trace element diagrams. Theuthors owe thank to Keith Benn and an anonymous reviewer for their helpful comments and constructive criticism.e also acknowledge the invitation of Giorgio Ranalli and his efforts in editing this volume. Partial funding for thisork was provided by the Hungarian National Scientific Foundation (OTKA) Grant T043686 to C. Szabó. We are

lso grateful for the support of the Pro Renovanda Cultura Hungariae Foundation and the help of the fellows of theithosphere Fluid Research Lab (especially Tibor Guzmics, János Kodolányi, Attila Tóth and Eszter Badenszki).

This is the 19th publication of the Lithosphere Fluid Research Lab.

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Geodynamic implications of flattened tabular equigranular textured peridotites from the Bakony-Balaton Highland Volcanic Field (Western Hungary)IntroductionGeological settingSampling and applied techniquesPetrographyMajor element mineral chemistryTrace element composition of clinopyroxenesEBSD- measurement and fabric analysisDiscussionGeochemical characteristics of the studied peridotites and the upper mantle beneath BBHVFOrigin of crystallographic preferred orientationsGeodynamic implications of flattened tabular equigranular xenoliths

ConclusionsAcknowledgementsReferences

geodynamic implications of ﬂattened tabular equigranular...

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