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LITHOS-04060; No of Pages 15 Lithos xxx (2016) xxx–xxx

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Thermal and metasomatic rejuvenation and dunitization in lithospheric mantle beneath Central Europe – The Grodziec (SW Poland) case study Magdalena Matusiak-Małek a,⁎, Mateusz Ćwiek a, Jacek Puziewicz a, Theodoros Ntaflos b a b

Institute of Geological Sciences, University of Wrocław, Pl. M. Borna 9, 50-204 Wrocław, Poland Department of Lithospheric Research, University of Vienna, Althanstrasse 14, 1090 Vienna

a r t i c l e

i n f o

Article history: Received 31 March 2016 Accepted 30 August 2016 Available online xxxx Keywords: Lithospheric mantle Xenoliths Stealth metasomatism Dunitization Bohemian Massif

a b s t r a c t The 32 Ma Grodziec nephelinite (Lower Silesia, SW Poland) contains xenolith of peridotite (mostly lherzolite) and clinopyroxenite/olivine clinopyroxenite composition. The forsterite content in olivine classifies these rocks into three groups: groups A and B consist of peridotites, while group C xenoliths are pyroxenitic cumulates. Group A xenoliths contain olivine Fo 87.90–91.8% and pyroxenes with high Mg# (~0.91–0.92); clinopyroxene is strongly LREE-enriched (LaN/LuN = 2.19–17.74) and strongly impoverished in Zr, Hf and Ti relative to primitive mantle. The group B xenoliths (dunites and wehrlite) are orthopyroxene-free, olivine and clinopyroxene are less magnesian than those in the A group (Fo = 85.2–87.2%, Mg# = 0.86–0.88), clinopyroxene is less LREE-enriched (LaN/LuN = 4.07–4.15) and only slightly impoverished in Zr, Hf and Ti. Group C xenoliths contain olivine with forsterite content from 78.6 to 86.6% and clinopyroxene of Mg# from 0.84 to 0.85, with LREE/trace element characteristics similar to those of B group (LaN/LuN = 1.96–3.10). Group A xenoliths from Grodziec record migration of mixed carbonatite-alkaline silicate melts through the subcontinental lithospheric mantle beneath Lower Silesia, which preceded the migration of melts similar to the Grodziec nephelinite. The peridotitic protoliths were dunitized at the direct contacts with the migrating nephelinite melt and are now represented by group B. Group C pyroxenites originated in mantle conditions by crystal settling in places of transient nephelinite melt stagnation. The mantle section beneath Grodziec was reheated to ca 1000–1100 °C. The Grodziec scenario is similar to that of Księginki (northern extension of Eger Rift, SW Poland), which shares a similar age of xenolith entrainment. Both sites show that the processes of mantle metasomatism and thermal rejuvenation of subcontinental lithospheric mantle were more intense during the Lower Oligocene volcanic climax compared to those recorded in younger (ca 20 Ma) xenolith suites from Lower Silesia. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Mafic lavas often carry pieces of the lithospheric mantle which allow direct examination of its composition and structure. Such xenoliths from Cenozoic volcanic rocks from Czech Republic and Poland (central Europe) have been the focus of numerous recent studies (Ackerman et al., 2007, 2012, 2013, 2015; Kochergina et al., 2016; Kukuła et al., 2015; Matusiak-Małek et al., 2010, 2014; Puziewicz et al., 2011, 2015). These show that subcontinental lithospheric mantle (SCLM) beneath SW Poland (Lower Silesia) is dominated by depleted harzburgites which record significant (up to 35%) melt extraction. These harzburgites were overprinted by metasomatic events, mostly cryptic/stealth in style. Despite general similarity within the xenolith occurrences of Lower Silesia, details of the evolution of mantle rocks from each locality

⁎ Corresponding author. E-mail address: [email protected] (M. Matusiak-Małek).

differ ,thus giving additional information about variations in the regional SCLM. In this study we present data from peridotite and clinopyroxenite xenolith suites from the Grodziec nephelinite (SW Poland). The aim of this study is to decipher the evolution of lithospheric mantle sampled by the Grodziec nephelinite. Furthermore, we compare Grodziec xenoliths to other xenolith localities in the region and show differences in the intensity and nature of processes affecting the SCLM beneath the northern margin of the Bohemian Massif in Oligocene and Miocene times. Moreover, we document that the process of dunitization of peridotites, well known from ophiolites and peridotite massifs, is also widespread in the SCLM. 2. Geological settings The latest volcanic activity in Central Europe took place in Cenozoic times and was related to uplift and rifting of Variscan basement in the foreland of the growing Alpine orogen (Wilson and Downes, 1991,

http://dx.doi.org/10.1016/j.lithos.2016.08.041 0024-4937/© 2016 Elsevier B.V. All rights reserved.

Please cite this article as: Matusiak-Małek, M., et al., Thermal and metasomatic rejuvenation and dunitization in lithospheric mantle beneath Central Europe – The Grodziec (SW Poland) case study, Lithos (2016), http://dx.doi.org/10.1016/j.lithos.2016.08.041

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2006). This volcanic activity produced a vast number of mostly mafic lavas with outcrops which spread from France, through Germany, Austria to Czech Republic and Poland and are collectively termed the Central European Volcanic Province (CEVP, Wimmenauer, 1974). Volcanism in eastern part of CEVP is concentrated in the Eger (Ohře) Rift in the Bohemian Massif. Volcanic rocks occur also in the Lower Silesia in SW Poland, at the NE extension of the Eger Rift and along the Labe-Odra fault system, located to NE of the Rift (Ulrych et al., 2011 and references therein, Fig. 1). Cenozoic volcanic rocks in SW Poland (Lower and Opole Silesia region, Sudetes Mountains) form five “volcanic complexes” (from west

to east): Lubań-Frydlant, Złotoryja-Jawor, Niemcza-Strzelin, Niemodlin and Lądek Zdrój (Fig. 1). Volcanic activity in this region occurred in two main episodes: (1) 30–26 Ma (Oligocene) and (2) 22–18 Ma (Late Oligocene- Early Miocene). The lava occurrences close to Lądek Zdrój originated at 5.5–3.8 Ma (Pécskay and Birenmajer, 2013). Around 3% of the outcrops of volcanic rocks contain xenoliths of mafic or ultramafic rocks and clinopyroxene megacrysts. Grodziec hill, located in the Złotoryja-Jawor volcanic complex (Fig. 1), is a remnant of a volcanic plug formed at ca. 32.16 ± 1.37 Ma (K-Ar ages, Badura et al., 2006). The plug (384 m. a.s.l) is crowned by a medieval castle, constructed from material excavated from its conical

Fig. 1. (a) Outcrops of Cenozoic volcanic rocks in SW Poland (based on Sawicki, 1995) and location of Variscan Massifs and associated Cenozoic Rifts (inset, based on Ziegler and Dézes, 2005). Red square in inset marks the Polish part of CEVP. Green squares show approximate extent of “volcanic complexes”, gray dashed lines show major faults in the region; (b) Geological settings (after Sawicki, 1995) of Jawor-Złotoryja volcanic complex with location of Grodziec. Names in italic shows localities of mantle xenoliths mentioned in the text.

Please cite this article as: Matusiak-Małek, M., et al., Thermal and metasomatic rejuvenation and dunitization in lithospheric mantle beneath Central Europe – The Grodziec (SW Poland) case study, Lithos (2016), http://dx.doi.org/10.1016/j.lithos.2016.08.041

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peak (Olczak, 2008). Large (up to 14 cm in diameter) xenoliths occur in the nephelinite pieces forming the castle walls but are unavailable for scientific studies. The volcanic rocks are underlain by Triassic sandstones and Permian limestones and dolomites of the North Sudetic Basin (Fig. 1). Xenoliths from Grodziec were briefly reported by Chodyniecka and Kapuściński (1969), who described olivine-rich rocks of composition similar to harzburgite and noticed 2 vol.% of amphibole.

olivine formula unit. Cr-number (Cr#) denotes the atomic ratio of Cr/(Cr + Al), Mg-number (Mg#) stands for atomic Mg/(Mg + Fe2++ Fe3+). Contents of trace elements were normalized to primitive mantle values after McDonough and Sun (1995). The mineral abbreviations are as follows: Ol – olivine, Opx- orthopyroxene, Cpx – clinopyroxene, Spl – spinel, Alk. Fsp – alkali feldspar, Pl - plagioclase.

3. Analytical methods

Chodyniecka and Kapuściński (1969) described the lava from Grodziec as nephelinite consisting of augite, nepheline, olivine and magnetite. Plagioclase was detected in very low amounts (0.5 vol.%), the measured SiO2 content was 39.28 wt.%, K2O + Na2O was 4.31 wt.%. Badura et al. (2006), Wolska et al. (2007), and Ladenberger (2006) classified the rock on the Total Alkali Silica diagram as basanite due to its SiO2 concentration reaching 42–43 wt.% and sum of alkalis from 3.8 to 4.1 wt.%. In this paper we use the term “nephelinite”, because we did not detect (optical microscope and Back Scattered Electron (BSE) images) the occurrence of plagioclase, while nepheline is an abundant phase. Locally, 200 μm – long crystals of rhönite poikilitically enclosing clinopyroxene and olivine have been found in the nephelinite.

The present study is based on 30 xenoliths collected from nephelinite exposures on the slopes of the Grodziec hill and from destroyed parts of the wall surrounding the castle (with permission of the castle manager). The xenoliths were cut and sliced into 150 μm thick sections. Pieces for bulk rock analyses were cut from the biggest samples. Modal composition of the xenoliths was estimated by pointcounting method (minimum 300 points per section) using highresolution images and JMicroVison software (Roduit, 2007). Observations of selected thick sections were conducted under scanning electron microscope (SEM) QEMSCAN 650 FEG produced by FEI at the Wrocław Research Centre EIT + (Poland). The mineral maps were constructed by imposition of the BSE images and detected and automatically interpreted X-ray spectra from the sample. The automatic interpretation of X-ray spectra was based on a self-build mineral database. One pixel in the image corresponds to 15 × 15 μm square in the sample. Major element compositions of minerals have been analysed by Cameca SX-100 electron microprobes. Most analyses were undertaken at the Department of Lithospheric Research at the University of Vienna (Austria), but some data were collected at the Faculty of Geology, at the University of Warsaw (Poland). In both laboratories the machines worked under standard conditions of acceleration voltage 15 kV, sample current 15 nA, counting times 10 or 20 s, using natural silicates and synthetic oxides as standards. PAP correction procedure was applied accordingly. Counting times were increased up to 40 s to improve detection limits for Ni (500 ppm) and Ca (280 ppm) in olivine in the Vienna laboratory. The in situ trace-element compositions of ortho- and clinopyroxene have been measured at the Institute of Geology, Academy of Sciences Czech Republic, Prague using an Element 2 ICP-MS coupled with an UP-213,213-nm NdYAG laser ablation system (New Wave Research). Two or three grains of each phase per sample were analysed. The repetition rate of 20 Hz and the output energy of 12 J/cm2 were applied. The beam spot size varied from 55 to 100 μm, depending on the phase and its size. For calibration of relative sensitivity of the equipment, we used glasses NIST612 and BCR-2G as external standards (Jochum et al., 2011). Internal standardization was performed using Ca content measured by electron microprobe. The time-resolved signals were processed using Glitter software (Van Achterberg et al., 2001). Internal precision was typically ±5%. Titanium content was recalculated from microprobe analyses. We measured composition of both massive and spongy clinopyroxene (where present). In the former, the analyses were typically similar in single xenoliths and could be averaged. The spongy clinopyroxene has strongly variable trace element compositions, even on the scale of individual grains. This suggests that they are not equilibrated and therefore we did not include them in the following discussion. Only three xenoliths were sufficiently large to allow determination of bulk rock composition. The pieces were sent to Bureau Veritas Commodities laboratories (Vancouver, Canada) where they were pulverized, and analysed by Inductively Coupled Plasma Mass Spectrometry (procedure code LF202). Pyroxene classification after Morimoto (1989) was used. Spinel compositions were recalculated on the basis of 3 cations, with Fe3 + and Fe2 + calculated by charge balance (Deer et al., 1993). Forsterite (Fo) content of olivine is calculated as atomic Mg/(Mg + Fe)*100 per

4. Composition of the host rock

5. Petrography of xenoliths Xenoliths in the Grodziec nephelinite are oval to rectangular, usually from 2 to 7 cm long, but some reach up to 12 cm in diameter. Xenolithnephelinite contacts are sharp. Most xenoliths are strongly altered and yellow-red, whereas fresh ones are either emerald green or blackish. The altered xenoliths exhibit linear arrangements of greenish clinopyroxene and blackish orthopyroxene. Only the fresh rocks were chosen for study. The xenoliths can be grouped into two suites: peridotitic (greenish xenoliths) and pyroxenitic (blackish xenoliths). Three textural classes of minerals occur in both suites: 1) rock-forming phases (olivine, orthopyroxene, clinopyroxene, spinel), denoted as “I″; 2) phases forming lamellae in group I (orthopyroxene, clinopyroxene, spinel), denoted as “II”, and 3) phases occurring in intergranular, fine-grained aggregates (olivine, clinopyroxene, spinel, feldspar, amphibole, rhönite, mica, denoted as “III”; for details of the classification, see MatusiakMałek et al., 2014). Detailed description and interpretation of origin of intergranular aggregates will be presented elsewhere, this paper describes only the host peridotites and pyroxenites. Oval vugs (200 μm in diameter) filled with hydrothermal phases (feldspar, rhodochrosite, allophane, scarce niobates, and ilmenite) and occasionally rhönite occur rarely. 5.1. Peridotites Most of the peridotites are spinel lherzolites (spongy clinopyroxene is treated as clinopyroxene I in modal analyses), but other peridotitic compositions are present as well (Fig. 2, Table 1). The xenoliths are anhydrous and typically show protogranular to porphyroclastic texture (Fig. 3a; Mercier and Nicolas, 1975), but in dunites Gro2D and Gro6A the texture resembles an adcumulate (Fig. 3b). In most xenoliths, minerals are evenly distributed, but in some samples, orthopyroxene and clinopyroxene (± spinel) are concentrated in separate areas, sometimes forming elongated, discontinuous trails (Fig. 3b, c). Clinopyroxene I and spinel I (± olivine I) often form oval to elongated clusters from 0.5 to 10 mm long, in which 30 μm-1 mm long, oval crystals of spinel are enveloped by usually spongy, anhedral clinopyroxene (Fig. 3d). Olivine I forms subhedral, tabular crystals with the longer axis from 0.4 to 2 mm or from 4 to 8 mm. Crystals of olivine I show kink banding and numerous fractures filled with iron oxides and serpentine group minerals. Orthopyroxene I forms subhedral, greyish to yellowish crystals with longer axis from 0.5 to 8 mm. Crystals of orthopyroxene I are

Please cite this article as: Matusiak-Małek, M., et al., Thermal and metasomatic rejuvenation and dunitization in lithospheric mantle beneath Central Europe – The Grodziec (SW Poland) case study, Lithos (2016), http://dx.doi.org/10.1016/j.lithos.2016.08.041

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Fig. 2. Modal composition of the Grodziec xenoliths in the clinopyroxene-olivine-orthopyroxene diagram.

homogeneous, only in dunite Gro2D their core parts contain up to 20 μm long, parallel lamellae of spinel II. Orthopyroxene II forms extremely rare inclusions in olivine I in lherzolite Gro2A.

Table 1 Modal composition of Grodziec xenoliths. Classification into groups according to Fig.5. Sample

Lithotype

Ol [%]

Opx

Cpx

Spl/Opq

Group A Gro2A Gro12 Gro13A Gro15 Gro20C Gro23D Gro23E Gro28 Gro31 Gro32 Gro34A Gro34B Gro35 Gro38B Gro40A Gro40C

lherzolite lherzolite lherzolite lherzolite lherzolite lherzolite lherzolite harzburgite lherzolite lherzolite harzburgite lherzolite lherzolite wehrlite lherzolite lherzolite

83.8 68.0 68.8 77.8 67.3 58.1 45.8 76.2 61.7 69.0 74.7 56.6 74.5 85.6 76.1 70.1

8.5 18.8 15.7 6.5 19.6 26.1 35.2 22.6 27.3 15.8 18.3 29.7 13.8 4.4 11.2 20.1

7.7 5.6 9.9 15.7 8.40 10.9 7.4 0.7 7.7 9.1 1.3 6.3 8.4 10.0 7.2 8.2

traces 7.7 5.5 traces 4.7 4.7 11.6 0.5 3.2 6.1 5.6 7.3 3.3 0.0 5.4 1.7

Group B Gro2D Gro6A Gro35A

dunite dunite wehrlite

92.7 93.7 81.4

traces traces 0.0

6.7 2.4 15.7

0.6 3.9 2.9

Group C GroZYA Gro18B Gro19 Gro23B Gro36A Gro36B Gro37B Gro39

dunite Ol clinopyroxenite Ol clinopyroxenite Ol clinopyroxenite Ol clinopyroxenite dunite harzburgite websterite

100.0 28.5 38.3 32.0 20.4 100.0 42.4 1.2

0.0 0.0 traces 0.0 0.0 0.0 50.0 20.0

0.0 65.9 61.0 67.4 79.6 0.0 0.0 78.8

0.0 5.6 0.7 0.6 0.0 0.0 7.6 traces

Clinopyroxene forms anhedral, green grains with longer axis from 0.5 to 6 mm. The crystals are either spongy (Fig. 3e) or massive but surrounded by a thick (100–200 μm) spongy rim. Vugs in spongy clinopyroxene are filled with alkali felspar/plagioclase and minute square crystals of ulvöspinel. Ameboidal crystals of olivine III, up to 25 μm long, occur locally in the spongy clinopyroxene (Fig. 3d, e). Clinopyroxene I encloses rare lamellae of orthopyroxene II in wehrlite Gro38B. Spinel typically occurs in clusters with clinopyroxene, only rarely it forms 10–25 μm in diameter inclusions in other silicates. If spinel I in a cluster is in contact with feldspar, it is surrounded by a thin spongy rim (Fig. 3e). 5.2. Pyroxenites Olivine clinopyroxenites and clinopyroxenite (Fig. 2, Table 1) form oval blackish xenoliths from 3 to 12 cm in diameter. Their contact with the host rock is sharp, but is locally underlined by concentrations of opaques. Both types show adcumulate textures (Fig. 4a). Grains in olivine clinopyroxenites are usually 1–5 mm long, but some may exceed 1 cm; clinopyroxene crystals are usually slightly larger than those of olivine. Only clinopyroxenite Gro39 is finergrained – crystals hardly reach 1 mm in length. Clinopyroxene may enclose large (up to 2 mm) sub- to euhedral crystals of spinel, while olivine encloses rare, oval inclusions of sulfides which are 20–25 μm in diameter. Olivine I is not kink banded, but in some xenoliths it is strongly fractured. Clinopyroxene I forms anhedral crystals which usually comprise massive cores and spongy rims. Proportions between the massive and spongy parts vary between crystals, in some crystals the massive part is not present. Locally, at the contact between two clinopyroxene I crystals, 50–200 μm wide spongy rims composed of clinopyroxene III, plagioclase, spinel III and rhönite are formed (Fig. 4b). In clinopyroxenite Gro39 clinopyroxene I encloses parallel lamellae or elongated intergrowths of clinopyroxene II or vugs of similar size (Fig. 4c). The lamellae are accompanied by small (b 30 μm long) pools

Please cite this article as: Matusiak-Małek, M., et al., Thermal and metasomatic rejuvenation and dunitization in lithospheric mantle beneath Central Europe – The Grodziec (SW Poland) case study, Lithos (2016), http://dx.doi.org/10.1016/j.lithos.2016.08.041

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Fig. 3. Textural features of Grodziec peridotites. (a) Protogranular texture. Clinopyroxene-spinel clusters in lower right and upper left corners (lherzolite Gro12, optical image); (b) Adcumulative texture in dunite Gro2D (optical image). Note the pseudoparallel trails formed of clinopyroxene and product of its disintegration. Int. aggr. – intergranular aggregates formed due to clinopyroxene disintegration; (c) Trails of clinopyroxene and orthopyroxene (xenolith Gro15, mineral map, QUEMSCAN); (d) Spinel-clinopyroxene cluster (xenolith Gro28, BSE image); (e) details of texture of spongy clinopyroxene (xenolith Gro35A, BSE image).

filled with feldspar and minute crystals of amphibole and spinel (Fig. 4c). Larger (up to 100 μm in diameter) inclusions of orthopyroxene occur in some grains of clinopyroxene I (Fig. 4d). 6. Chemical composition of minerals Chemical composition of olivine offers a good discrimination tool for mantle xenoliths from Lower Silesia (e.g. Puziewicz et al., 2015). The relationships between Fo and NiO in olivine I classify the Grodziec xenoliths into three groups: group A (Fo = 87.9–91.8), group B (Fo = 85.2–87.2) and group C (Fo = 78.8–86.6; Fig. 5a). Groups A and B correspond to the peridotitic suite, while group C refers to pyroxenitic suite. Compositions of olivine and spinel from group A xenoliths plot in the Olivine-Spinel Mantle Array (OSMA; Arai, 1994), whereas those from Groups B and C fall away from the trend (Fig. 5b). 6.1. Mineral chemical composition Olivine I composition is typically homogeneous within a sample. Forsterite and NiO contents in olivine I varies between 87.90–91.80% and 0.29–0.46 wt.%, respectively, in group A, between 85.17–86.14% and 0.22–0.35 wt.%, respectively, in group B and between 78.60–

86.60% and 0.08–0.30 wt.%, respectively in group C (Fig. 5a, Table S1). In all the groups, olivine is usually poor in Ca (below 900 ppm), but locally in group C xenoliths Ca content may reach 2500 ppm. Olivine I from group A lherzolite Gro40A is chemically heterogeneous. It has a strongly variable forsterite content (87.90–89.78%), while NiO and Ca contents are significantly elevated with the highest values in crystal rims (0.27–0.47 wt.% and 700–1860 ppm, respectively; Fig. 5, Table S1). Orthopyroxene I in group A is Al, Cr enstatite, in group C it is Al enstatite. The Mg# is 0.90–0.92, and 0.84–0.85 in groups A and C, respectively. The Al content in group A is 0.05 to 0.16 a. pfu (corresponding to 0.70 to 3.71 wt.% of Al2O3), significantly lower than in group C (0.13 to 0.16 Al pfu; 3.19–3.77 wt.% Al2O3; Table S2; Fig. 6). Orthopyroxene I is usually depleted in Light Rare Earth Elements (LREE). In most xenoliths the LaN/LuN varies from 0.04 to 0.23 (Table S3; Fig. 6). However, in orthopyroxene from group A lherzolites Gro12 and Gro23D Nd and Sm content are elevated (“spoon-shaped” pattern; Fig. 6). In lherzolites Gro20C, Gro31 and Gro40C the REE patterns are almost flat (LaN/LuN = 0.23–0.61; Fig. 6), but trace element concentrations are different. Orthopyroxene from group A lherzolite Gro40A is LREE-enriched (LaN/LuN = 3.09). Orthopyroxene I from most xenoliths has a negative Sr anomaly, its Ti anomaly varies from negative to positive (Fig. 6).

Please cite this article as: Matusiak-Małek, M., et al., Thermal and metasomatic rejuvenation and dunitization in lithospheric mantle beneath Central Europe – The Grodziec (SW Poland) case study, Lithos (2016), http://dx.doi.org/10.1016/j.lithos.2016.08.041

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Fig. 4. Textural features of Grodziec pyroxenites. (a) cumulative texture in olivine clinopyroxenite Gro19I, (optical image); (b) spongy contact between two clinopyroxene I grains (olivine clinopyroxenite Gro19I, BSE image); (c) lamellae and intergrowths of orthopyroxene II in clinopyroxene I. Square shows area enlarged in inset (clinopyroxenite Gro39, BSE image); (d) inclusions of orthopyroxene II in clinopyroxene I (olivine clinopyroxenite Gro19I, BSE image).

Fig. 5. Chemical classification of Grodziec xenoliths. (a) Fo-NiO relationship in olivine I; (b) Fo in olivine- Cr# in spinel relationship. Olivine-Spinel Mantle Array (OSMA) field after Arai (1994). Letters in the diagrams indicate group into which the xenoliths have been classified.

Orthopyroxene II of enstatite composition forms lamellae and inclusions in group B dunites Gro2D and Gro6A. Its Mg# is 0.87 and Al content is 0.08 a. pfu (1.89–1.92 wt.% Al2O3; Table S2, Fig. 6).

Clinopyroxene I in group A xenoliths is typically Al, Cr, Na augite, only in lherzolite Gro20C it is Cr ± Al augite (Table S4; Fig. 7) and grades toward Al, Cr diopside at contact with spongy clinopyroxene. The Mg#

Fig. 6. Chemical composition of orthopyroxene I. (a) Mg#-Al relationships; (b, c) primitive mantle normalized REE and multi-trace-element patterns, respectively, in LREE-depleted orthopyroxene from groups A and C; (d,e) primitive mantle normalized REE and multi-trace-element patterns, respectively, in orthopyroxene with “flat” REE pattern from group A; (f, g) primitive mantle normalized REE and multi-trace-element patterns, respectively, in orthopyroxene with “spoon-shaped” and LREE-enriched REE pattern from group A.

Please cite this article as: Matusiak-Małek, M., et al., Thermal and metasomatic rejuvenation and dunitization in lithospheric mantle beneath Central Europe – The Grodziec (SW Poland) case study, Lithos (2016), http://dx.doi.org/10.1016/j.lithos.2016.08.041

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Please cite this article as: Matusiak-Małek, M., et al., Thermal and metasomatic rejuvenation and dunitization in lithospheric mantle beneath Central Europe – The Grodziec (SW Poland) case study, Lithos (2016), http://dx.doi.org/10.1016/j.lithos.2016.08.041

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varies from 0.89 to 0.92, in diopside it is up to 0.93. The Al content varies from 0.09 to 0.21 a. pfu (2.00–4.91 wt.% Al2O3), Na content varies from 0.10 to 0.14 a. pfu (0.75–1.92 wt.% Na2O) in most xenoliths, and from 0.04 to 0.09 a. pfu (0.53–1.27 wt.% Na2O) in lherzolite Gro20C. Clinopyroxene I is LREE-enriched (LaN/LuN = 2.19–17.74) with convex downward REE pattern peaking at Nd. Clinopyroxene in lherzolite Gro20C is strongly and continuously enriched in LREE (LaN/LuN = 36.16). All the types show strong negative Sr, Zr-Hf and Ti anomalies (Fig. 7). Clinopyroxene I in group B xenoliths is Al ± Cr augite (Table S4). Its Mg# varies from 0.86 to 0.88, while Al and Na content are from 0.10 to 0.17 a. pfu (2.23–4.00 wt.% Al2O3) and from 0.09 to 0.11 a. pfu (0.74–1.51 wt.% Na2O), respectively (Fig. 7). It is LREE-enriched (LaN/LuN = 4.07–4.15), but its REE pattern is only slightly convex downward (Table S5; Fig. 7). Small negative Ti and Nb-Ta anomalies are also present. Clinopyroxene I in group C xenoliths is Al, Cr augite/diopside with Mg# from 0.80 to 0.86 (Table S4). It is richer in Al than the previous groups (0.19 to 0.41 a. pfu of Al; 4.44–9.34 wt.% Al2O3) and poorer in Na (0.04 to 0.09 a. pfu of Na; 0.62–1.20 wt.% Na2O; Fig. 7). It is slightly LREE-enriched (LaN/LuN = 1.96–3.10), and has a convex-downward REE pattern with its peak at Nd-Sm (Table S5; Fig. 7) and weak negative Sr and Zr anomalies. Spongy clinopyroxene from all the groups is chemically heterogeneous even at the scale of a single grain (Fig. 7a, b). It is typically Cr, Al, Ti diopside with rare transitions to Cr ± Al augite (Table S4). Its Mg# is higher than in massive clinopyroxene I in the same sample, the Al and Na contents are similar or lower (Table S4, Fig. 7). The Cr# of spinel I in group A varies from 0.45–0.68, the Mg# is from 0.49 to 0.70 (Table S6; Fig. 8). The Mg# is negatively correlated with TiO2 content, which varies from 0.06 to 5.58 wt.% (Fig. 8b); the highest contents of TiO2 in spinel always occur in crystals surrounded by spongy clinopyroxene. Composition of spinel I in group B xenoliths is diverse: in dunite Gro2D the Cr# is 0.60 to 0.67, the Mg# varies from 0.35–0.36, and the TiO2 varies from 2.58–3.84 wt.%; in wehrlite Gro35A spinel is poorer in TiO2 (0.77–0.87 wt.%) and has lower Cr# (0.13), but its Mg# is higher (0.63; Table S6; Fig. 8). Spinel in group C xenoliths is Al-rich and Cr-poor (Cr# b 0.05), while Mg# is relatively high (0.58–0.67; Table S6; Fig. 8). TiO2 content varies from 0.42 to 0.88 wt.%. Vugs in spongy clinopyroxene in group A xenoliths are filled with alkali feldspar of composition An0.85–7.25Or20.99–50.88 (Table S7. Fig. 9). In groups B and C the vugs are filled with plagioclase (An33.58–55.73 Or2.36–6.77; Table S7; Fig. 9). Sulfides in group C olivine clinopyroxenite Gro19 contain 39–40% S, 56–58% Fe, and 1.6–2.6% Ni, indicating a composition of Ni-poor monosulfide solid solution (MSS; Guo et al., 1999). Amphibole III in clinopyroxene I in websterite Gro39 has composition similar to kaersutite with Mg# ~ 0.76. 6.2. Bulk rock chemical composition Bulk rock chemical analysis was possible for samples Gro20C (group A), Gro19 and Gro39 (group C). Modal compositions calculated from the whole-rock chemical composition of xenoliths fit quite well to those established by point counting (Table 1 vs. S8) and classify the rocks as spinel lherzolite (Gro20C), olivine clinopyroxenite (Gro19) and olivine websterite (Gro39). The Mg# is 0.69 for peridotite Gro20C, 0.76 for olivine clinopyroxenite and 0.67 for olivine websterite. The primitive-mantle normalized REE patterns show enrichment in LREE in all the samples. Concentrations of trace elements in pyroxenites are higher than in peridotite, the (La/Lu)N = 43.23 for the latter, for olivine clinopyroxenite it is 6.46, and 9.97 for olivine websterite (Fig. S1). All the xenoliths show slightly negative Zr anomaly, the peridotite shows elevated concentrations of Nb-Ta. Low Mg# values for the xenoliths probably result from the abundant fine-grained aggregates (up to 35 vol.%). These aggregates “contaminate”

the peridotite analyses and therefore the bulk rock analyses will not be used for further discussion. 7. Temperatures and pressures of formation Calculations of temperature of peridotite equilibration of Grodziec peridotites are based on two pyroxene geothermometers (Brey and Köhler, 1990; Liang et al., 2013); such pairs occur only in group A xenoliths (Table 1). We did not calculate temperatures for samples where only spongy clinopyroxene occurs. For those xenoliths we have tried to use the Al-in-orthopyroxene geothermometer by Witt-Eickschen and Seck (1991), but the composition of the Grodziec orthopyroxene does not meet the basic conditions of the algorithm (i.e. Al content on M1 position is too high to keep the linear correlation between Al and Cr contents). Clinopyroxene has spongy rims in practically all samples, so for temperature calculations with the Brey and Köhler (1990) method we had to use the composition of the massive cores, often of grains which are not in direct contact. A pressure of 1.5 GPa was assumed. The calculated temperatures range from 1006 to 1112 °C (Fig. 10), in a single sample the temperatures varies up to 50 °C. Temperatures based on trace element composition of pyroxenes (Liang et al., 2013) are either 20 to 70 °C higher or comparable with that calculated with major elements (Fig. 10). But in xenoliths where orthopyroxene has a flat or “spoon shaped” REE pattern, the temperatures are significantly lower (Gro12, Gro 20C, Gro23D, Gro40C), while in xenolith Gro40A where orthopyroxene is LREE-enriched (Fig. 6) the calculated temperatures are unrealistically high. Texture of group C xenoliths is cumulative pointing to magmatic origin of those rocks. It is therefore possible to estimate the pressure of formations using the Nimis (2000) algorithm based on relations between unit-cell volume, M1-polyhedron volume and Mg# in clinopyroxene. The calculated pressures for olivine clinopyroxenites are from 0.86 to 1.11 GPa, corresponding to depths of 33–42 km. 8. Discussion 8.1. Origin of group A peridotites Group A xenoliths are peridotites formed of Mg-rich minerals, whose composition allows them to be classified as mantle-derived rocks (Fig. 5b). They are all clinopyroxene-bearing, the clinopyroxene content (0.4–15.7 vol.%) is high in comparison to other xenolith suites in Lower Silesia, where it seldom reaches 5 vol.% (Puziewicz et al., 2015). Nevertheless, the content of clinopyroxene in Grodziec xenoliths is lower than that in primitive mantle (20% assumed by Norman, 1998), so some melt must have been extracted from the peridotites if the mineral is restitic. Depletion in basaltic components is, however, not reflected by clinopyroxene trace element composition, which is strongly LREE-enriched (Fig. 7). Moreover, the major element composition of clinopyroxene does not follow the trends which would be expected to result from partial melting, for example a negative Al-Na correlation does not occur (Fig. 7b). We therefore suggest that clinopyroxene is either a secondary phase and was introduced into the peridotites during a metasomatic (modal/“stealth”; O'Reilly and Griffin, 2013) event or was completely chemically reequilibrated during cryptic metasomatism and preserved no record of partial melting. The Mg# and Cr# in in group A spinel I are negatively correlated, suggesting a restitic origin (Fig. 7a). Some group A spinels are, however, characterized by TiO2 content N 0.2 wt.% (0.5 wt.% in Pearson et al., 2003), which is believed to be the maximum TiO2 concentration in spinel formed during partial melting (Hellebrand et al., 2002; Fig. 7b). Therefore, spinel in Grodziec group A xenoliths is not a reliable indicator of partial melting degree. As clinopyroxene and spinel cannot be used as indicators of partial melting, the only phase which records this process in group A peridotites

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Fig. 7. Chemical composition of clinopyroxene. (a) Mg#-Al relationship and (b) Al-Na relationship in clinopyroxene I and spongy clinopyroxene; (c) primitive mantle normalized REE patterns in group A clinopyroxene; insets show REE patterns in lherzolite Gro20C; (d) primitive mantle normalized multi-trace-element pattern in group A clinopyroxene; (e) primitive mantle normalized REE patterns in groups B and C clinopyroxene; (f) primitive mantle normalized multi-trace-element pattern in groups B and C clinopyroxene.

is orthopyroxene. The REE patterns of most group A orthopyroxene is LREE-depleted pointing to its restitic origin (e.g. Frei et al., 2009), but in a few samples orthopyroxene is not LREE-depleted (Fig. 6). The flat and “spoon-shaped” patterns cannot be attributed to mixing with clinopyroxene lamellae during analysis, as the lamellae do not occur. Therefore these REE patterns in orthopyroxene are considered to result from metasomatism or from intergrowths finer than the BSE image

resolution. Consequently, the xenoliths with LREE-enriched patterns, as well as those where the REE composition in orthopyroxene was not analysed, were not plotted in the MgO-Al2O3 melting line proposed by Upton et al. (2011; Fig. 11). Xenoliths containing LREE-depleted orthopyroxene experienced ~17 to ~20% of melt extraction (Fig. 11). Orthopyroxene from lherzolites Gro 2 A, Gro15, Gro32, Gro 40 A, harzburgites Gro18A and wehrlite Gro38B depart from the melting

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Fig. 8. Chemical composition of spinel I. (a) Mg#-Cr# relationships; (b) Mg#-TiO2 relationships.

trend towards lower Al contents (0.05–0.09 a. pfu corresponding to 1.29–2.27 wt.% of Al2O3). Such a composition would require a high degree of melting of a harzburgitic source, as proposed for the A0 group xenoliths from Krzeniów (Matusiak-Małek et al., 2014). After the depletion event, the restitic peridotite was further affected by metasomatism which led to the introduction of clinopyroxene and possibly spinel. Group A clinopyroxene are LREE-enriched, but their convex downward shapes suggest a reaction with an alkaline silicate melt. On the other hand, low Ti/Eu ratios and negative Zr-Hf anomalies indicate the effect of a carbonatitic component. To assess the relative silicate and carbonatitic effects, we have calculated composition of melts in equilibrium with clinopyroxene (Fig. 12). A hypothetical alkali

Fig. 9. An-Or-Ab contents in feldspar filling vugs in spongy clinopyroxene.

basalt (partition coefficients after Hart and Dunn, 1993) in equilibrium with group A clinopyroxene (Fig. 12a) is strongly LREE-enriched, and trace element concentrations are higher than in natural lavas occurring in the region. Moreover, the hypothetical melt has negative Zr, Hf, Sr and Ti anomalies, which do not occur in natural melts. A calculated carbonatitic melt in equilibrium with group A clinopyroxene (partition coefficients after Walker et al., 1992; Klemme et al., 1995, and Blundy and Dalton, 2000) is characterized by high Th, U, REE and Sr content, and negative Zr and Ti anomalies (Fig. 12b). This composition fits the general features of natural carbonatites (e.g. Coltorti et al., 1999; Woolley and Kempe, 1989; Fig. 12c), but again it is far from those of the host lava and other Cenozoic lavas in the region. Therefore, we suggest that the metasomatic medium reacting with Grodziec group A xenoliths had mixed carbonatitic–silicate composition but did not directly correspond to the host lava. Small variations in trace element composition within the group A clinopyroxenes may be explained by chromatographic fractionation of the metasomatic melt during percolation (Ionov et al., 2002). Such fractionation would result in variable proportions of carbonatite and silicate components of the melt.

Fig. 10. Temperatures calculated for the Grodziec xenoliths. Points show temperatures calculated using Brey and Köhler (1990) algorithm. One point represents one orthoclinopyroxene pair. Crosses with error bars represent temperatures calculated using Liang et al. (2013) algorithm.

Please cite this article as: Matusiak-Małek, M., et al., Thermal and metasomatic rejuvenation and dunitization in lithospheric mantle beneath Central Europe – The Grodziec (SW Poland) case study, Lithos (2016), http://dx.doi.org/10.1016/j.lithos.2016.08.041

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Fig. 11. Relationship between Al2O3 and MgO contents in orthopyroxene as an indicator of partial melting; melting trend after Upton et al. (2011).

8.2. Origin of groups B and C xenoliths Group B xenoliths have been described in Lower Silesian xenolith suites from Krzeniów (Matusiak-Małek et al., 2014), Wilcza Góra (unpublished) and Steinberg (Kukuła et al., 2015). Group B xenoliths occur also in Księginki, although the classification into groups A and B

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was not used by Puziewicz et al. (2011) in their study. These peridotites are characterized by Fo content in olivine lower than that in group A ones (typically 89 to 84%) and by harzburgitic, dunitic or rarely wehrlitic composition; dunitic group B xenoliths are usually orthopyroxenebearing. Despite significant differences in chemical composition of minerals between groups A and B xenoliths, their textures are identical, protogranular to porphyroclastic. Group B xenoliths have been interpreted as product of interaction between primary peridotite and silicate melt (Puziewicz et al., 2015 and references therein; Kukuła et al., 2015). Group B xenoliths from Grodziec differ from typical group B xenoliths from Lower Silesia by having adcumulative, layered textures and consisting only of olivine and clinopyroxene; two of them are dunites and one is wehrlite (Fig. 2). Dunite formation in the mantle rarely results from depletion of primary peridotite, as the degree of melting would have to be N 40% (Pearson et al., 2003). More often it is a result of reaction of harzburgite with silicate melt leading to orthopyroxene dissolution and crystallization of olivine (e.g. Kelemen, 1990; Tursack and Liang, 2012 and references therein). Reactive dunites, which result from reaction between peridotite and tholeiitic melt, are common in ophiolitic complexes and may be characterized by Fo and NiO contents in olivine lower, similar or higher than those in the surrounding mantle peridotites (e. g. Morgan et al., 2008; Suhr et al., 2003). The Fo content in reactive olivine is usually not b 88% (e.g. Kelemen et al., 1995; Kubo, 2002; Mazzucchelli et al., 2009; Morgan et al., 2008; Santos et al., 2002; Suhr et al., 2003). Experimental studies show, however, that dunite formed by reaction of peridotite with mafic melts can contain olivine in which Fo content

Fig. 12. Primitive-mantle normalized (McDonough and Sun, 1995) composition of mafic silicate melts (a, d) and carbonatites (b) in equilibrium with Grodziec group A (a, b) and B and C (d) clinopyroxene; data from Grodziec nephelinite after Ladenberger (2006); c) (Ti/Eu)/((La/Yb)N) ratio in clinopyroxene as an indicator of nature of metasomatic agent (Coltorti et al., 1999). See text for details.

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decreases to 80% (Morgan and Liang, 2005; Tursack and Liang, 2012; Van Den Bleeken et al., 2010). The NiO content in olivine in reactive dunites, natural as well as experimental, is lower than 0.25 wt.%. Spinel in replacive dunites has high Cr# and contains elevated amounts of TiO2 (Batanova and Savalieva, 2009; Morgan et al., 2008). Dunites can originate also by crystal settling from melts of basaltic composition in various geological settings. Olivine occurring in ultramafic cumulates has Fo content down to 70%, NiO content may decrease below detection limits (Elthon and Steward, 1992), the Ca content may be from below 1000 ppm (Li et al., 2012) to few tenths of percent (Elthon and Steward, 1992). As the composition of olivine from cumulative and reactive mantle dunites can overlap, their texture seems to be the reasonable tool for assessing their origin. However, Mazzucchelli et al. (2009) showed that reactive dunite may have granoblastic texture, which in fact, is identical to adcumulative one. Below we discuss possible cumulative and replacive origin of group B xenoliths. Group B xenoliths from Grodziec are formed of olivine with Fo content 85–86%, NiO content from 0.22 to 0.35 wt.% (Fig. 5) and low Ca content; the xenoliths show adcumulative textures. As discussed above, those features does not allow to clearly define origin of the xenoliths. Composition of olivine and clinopyroxene from group B xenoliths plot close to that of group C xenoliths (Figs. 5, 7), and we therefore suggest that the two groups may have a common origin. We have calculated composition of melt in equilibrium with clinopyroxene in group B and C xenoliths. The calculated melts are LREE-enriched and show a positive Nd anomaly (Fig. 12d). Despite enrichment in Th and U in olivine clinopyroxenites Gro36A and Gro 39 and slight variations in normalized concentrations, the composition of the calculated melt resembles that of the host nephelinite. This result, together with cumulative texture in group C, indicates that the pyroxenites are precipitates from an alkaline melt. If crystallization of alkaline melt produces pyroxenites, group B dunites/wehrlite cannot be formed by the same process, but may be associated with it. Small amounts of clinopyroxene was reported in replacive dunites worldwide (Kelemen et al., 1995; Morgan et al., 2008; Suhr et al., 2003) and have been interpreted as precipitates from percolating, cooling melt after its saturation in both clinopyroxene and olivine (Batanova and Savalieva, 2009). Such an interpretation is supported by experimental results (Tursack and Liang, 2012). This suggests that the Grodziec group B dunites and wehrlite are replacive clinopyroxene- and spinel-bearing rocks formed by reaction between (depleted?) peridotite and percolating alkaline melt. Locally, the clinopyroxene might have been concentrated to make the rock composition wehrlitic (xenolith Gro35A). Furthermore, the reaction caused Fe-enrichment of the dunite-forming phases (“Fe-metasomatism”), as well as increased Cr# and TiO2 content in spinel (Fig. 8). These arguments suggest that the group C xenoliths are upper-mantle cumulates from a melt which also caused local dunitization and Fe-metasomatism of upper mantle. Not much attention have been paid so far to dunitic xenoliths from Lower Silesian occurrences. Dunites have been found in Krzeniów, Steinberg, Księginki and Wilcza Góra (Puziewicz et al., 2015); in the great majority of those xenoliths, olivine is enriched in Fe (down to 84.5% in Księginki), orthopyroxene is not present, and clinopyroxene (if present) shows LREE-enriched composition resembling that of group B clinopyroxene from Grodziec. We therefore suspect that dunitization may be more widespread in uppermost regional SCLM. Dunitization takes place in channels of limited width (from single centimetes to meters) and therefore replacive dunites are subordinate components of oceanic mantle, e.g. dunites form only 5 to 15% of the exposed mantle section in the Oman ophiolite (Kelemen et al., 1995). In the Grodziec xenolith suite they constitute 13.6% of the collected mantle-derived samples. In other Lower Silesian localities the amount of Fe-rich, orthopyroxene-free dunites varies from 3 to 9%. Therefore we suggest that proportions between “non-dunitized” and “dunitized” peridotites in the SCLM may resemble those in oceanic lithosphere.

8.3. Regional geological considerations Mantle xenolith suites from Lower Silesia can be grouped into the “older” ones, erupted at ca. 30–32 Ma, and the “younger” ones, erupted at ca 20 Ma. The exception is Lutynia xenolith suite (Lądek Zdrój volcanic complex) which was brought to the surface ca. 3.8–5.7 Ma. The older suites comprise the xenoliths from Grodziec nephelinite, from Księginki nephelinite (Puziewicz et al., 2011) and from Steinberg basanite (Kukuła et al., 2015). Księginki and Steinberg are located in the NE extension of the Eger Rift (Fig. 1), whereas Grodziec (Złotoryja-Jawor volcanic complex) is in an off-rift location. The “younger” xenolith suites occur in Krzeniów (Matusiak-Małek et al., 2014), Wilcza Góra (unpublished), Winna Góra (not published; all in the Złotoryja-Jawor volcanic complex), Pilchowice (Puziewicz et al., 2015; located outside the major volcanic complexes), Targowica (not published; NiemczaStrzelin volcanic complex) and Lutynia (Matusiak-Małek et al., 2010). The xenolith suites from Grodziec and Księginki are in fact the only ones in Lower Silesia showing consistent and narrow-range equilibration temperatures (Grodziec: 1010–1120 °C Księginki: 1060–1120 °C). Clinopyroxenites and olivine clinopyroxenites which originated by crystal settling from a magma chemically similar to the host nephelinite are common in both sites (termed “pyroxenite suite” by Puziewicz et al., 2011, and group “C″ in this study). In both sites these rocks were shown to precipitate from a melt which reactively percolated through the surrounding mantle peridotites leading to their “Fe-metasomatism”. Clinopyroxene, which was added to the peridotites during this process, is chemically similar in both xenolith suites (Fig. 13a, b); the amount of clinopyroxene incorporation was greater in Grodziec. The orthopyroxene differs (Fig. 13d), suggesting that peridotitic protoliths in both sites were different. Our studies of “younger’ xenolith suites from Krzeniów (MatusiakMałek et al., 2014), Wilcza Góra (not published) and Lutynia (Matusiak-Małek et al., 2010) show that they are dominated by harzburgites belonging to groups A and B. Cumulative rocks of group C are not abundant. These xenolith suites yield no consistent equilibration temperatures. This suggests that the “younger” xenolith suites represent SCLM regions which were not strongly thermally and chemically rejuvenated by migration of the melt which eventually brought them to the surface. The “older” xenolith suite from Steinberg (Kukuła et al., 2015) shares these characteristics. The similar characteristics of the A group xenoliths from Steinberg and those from the “younger” suites show that these xenoliths represent SCLM which escaped extensive Cenozoic syn-volcanic rejuvenation. 8.4. Origin of spongy clinopyroxene Clinopyroxene in most Grodziec xenoliths has spongy (sieve) texture. Similar textures were described in many mantle xenoliths worldwide (e.g. Carpenter et al., 2002; Ma et al., 2015; Shaw and Dingwell, 2008; Shaw et al., 2006; Su et al., 2011). Spongy clinopyroxene from Grodziec xenoliths is characterized by lower Al, Na (Fig. 7) and higher Ca than the coexisting massive one. Vugs are filled with feldspar of highly variable composition (Fig. 9). Alkali feldspar occurs in spongy clinopyroxene from orthopyroxene-bearing xenoliths (group A), whereas andesine/labradorite plagioclase appears in that from orthopyroxenefree xenoliths (groups B and C). The described relationships are a record of partial melting of the clinopyroxene which was frozen at a transient stage, before any textural maturation happened. Partial melting removes fusible elements (such as Al and Na) from clinopyroxene and concentrates Ca and Cr (Hirose and Kawamoto, 1995), which is visible also in the Grodziec xenoliths, albeit the Cr increase is very slight. This process is short-lived and abrupt cooling at its end preserves the transient textures. Melting of clinopyroxene may be triggered by variable mechanisms: decompression (Su et al., 2011), reaction with host melt (Shaw and Dingwell, 2008), or reaction with infiltrating metasomatic melt

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Fig. 13. Comparison of composition of group A phases from Lower Silesian mantle xenoliths localities: (a) Mg#- Al relationship in clinopyroxene; (b) Mg#-Al relationship in clinopyroxene; (c) compared normalized trace element composition of clinopyroxene from Grodziec and from Księginki xenoliths; (d) Mg#-Al relationships in orthopyroxene. Data for Księginki from Puziewicz et al. (2011), for Lutynia from Matusiak-Małek et al. (2010), for Krzeniów from Matusiak-Małek et al. (2014), for Steinberg from Kukuła et al. (2015) and from Wilcza Góra (unpublished).

(Bonadiman et al., 2005; Carpenter et al., 2002). The spongy clinopyroxene in Grodziec xenoliths has a lower AlIV/AlVI ratio than the massive ones, suggesting decompression melting (Fig. 14). However, feldspar enclosed in spongy clinopyroxene may provide additional information on the causes and style of melting. Vugs in spongy clinopyroxene described in literature are usually filled with glass, either devitrified or fresh (Op. Cit), plagioclase is not

Fig. 14. AlIV-AlVI relationships in massive and spongy clinopyroxene as an indicator of decrease in pressure (Su et al., 2011). Fields for eclogites, xenoliths and volcanic rocks after Aoki and Shiba (1973).

reported. Chemical composition of phases filling vugs in Grodziec xenoliths clearly shows it is feldspar (Table S7). On the other hand, size of the vugs does not allow microscopic observations and therefore clear statement if a phase is a mineral or glass cannot be made. So we suggest, that feldspar occurring in Grodziec spongy clinopyroxene may be in fact drops of frozen, recrystallized glass. Alkali feldspar (corresponding to K- and Na-rich glasses) is present in spongy clinopyroxene in group A xenoliths. Shaw and Dingwell (2008) showed experimentally that K-rich melts may be produced under low pressure conditions by reaction of peridotitic orthopyroxene with infiltrating mafic melt. But this explanation cannot be true for Grodziec xenoliths as spongy alkali feldspar in spongy clinopyroxene occur randomly in whole xenoliths (e.g. Fig. 3d) while in experiments this type of glass is present only in proximity to dissolving orthopyroxene (op. Cit.). Clinopyroxene equilibrated in spinel facies is poor in K (Table S4), and its melting cannot produce K-rich melts/ feldspar. Therefore, we suggest that spongy clinopyroxene in group A xenoliths was formed not only by decompression melting, but it records also infiltration of an Na, K- rich silicate melt (Coltorti et al., 2000). Studies of the fine-grained intergranular aggregates may give more information on the origin of the melt and its reaction with the host peridotites. Vugs in group B and C spongy clinopyroxene are filled with plagioclase (corresponding to Ca-rich glasses) containing only minor amounts of K. Doukhan et al. (1993) showed experimentally that melts enriched in Si, Ca, Al, and Na results from small-degree partial melting of clinopyroxene and therefore formation of spongy clinopyroxene in groups B and C was triggered by pure decompression melting.

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9. Conclusions The suite of peridotitic and pyroxenitic xenoliths occurs in the 32 Ma nephelinite plug forming Grodziec hill in Lower Silesia (SW Poland). The peridotite suite comprises harzburgites and lherzolites. Some peridotites (group A) contain clinopyroxene which was introduced during a metasomatic event which happened during migration of mixed carbonatite-silicate melt different from the host nephelinite. The rest of the peridotites (group B) contain clinopyroxene which was introduced during migration of a melt similar in composition to nephelinite, probably immediately before the entrainment of mantle xenoliths into the erupting lava. The same melt induced local, but possibly widespread dunitization. In places where channelized flow was temporarily stopped, crystal settling produced clinopyroxenites and olivine clinopyroxenites. The Grodziec xenoliths are similar to the Księginki peridotite and pyroxenite xenolith suites, also brought to the surface by nephelinite lava at ca 30–32 Ma. The xenoliths document thermal rejuvenation of the lithospheric mantle section beneath both sites at temperatures ca 1000–1100 °C. The xenolith suites from Grodziec and Księginiki are significantly different from the “younger” xenolith suites in Lower Silesia, which occur mostly in ca 20 Ma basanites. The latter are much less affected – both chemically and thermally – by pre- to synvolcanic melt migration through the SCLM. The Grodziec xenolith suite, together with the Księginki one, show that the dynamics of processes affecting the lithospheric differed between the older 30–32 Ma stage and younger 20 Ma stage in Lower Silesia. Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.lithos.2016.08.041.

Acknowledgements We would like to thank the management of the Grodziec castle for support during field work. We thank dr. Jana Ďurišová (Institute of Geology, Czech Academy of Science) for conducting the LA-ICPMS analyses, and our MSc student Michał Dajek for preparation of maps. We are grateful to Hilary Downes, Ioan Seghedi and an anonymous reviewer whose critical comments helped to significantly improve the manuscript. This study was supported by National Centre for Scientific Research (grant number DEC-UMO-2014/15/B/ST10/00095). The analytical work was supported by Austrian-Polish scientific and cultural co-operation agreement (Institute of Geological Sciences University of Wroclaw and Department of Lithospheric Sciences, University of Vienna) 2013-2015 project.

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Please cite this article as: Matusiak-Małek, M., et al., Thermal and metasomatic rejuvenation and dunitization in lithospheric mantle beneath Central Europe – The Grodziec (SW Poland) case study, Lithos (2016), http://dx.doi.org/10.1016/j.lithos.2016.08.041