Eclogite-hosting metapelites from the Pohorje Mountains (Eastern ...

Eur. J. Mineral. 2009, 21, 1191–1212 Published online September 2009

25 years of UHP metamorphism

Eclogite-hosting metapelites from the Pohorje Mountains (Eastern Alps): P-T evolution, zircon geochronology and tectonic implications ´ K1,*, DAVID CORNELL2, NIKOLAUS FROITZHEIM3, JAN C.M. DE HOOG4, IGOR BROSKA1, MARIAN JANA MIRIJAM VRABEC5 and VRATISLAV HURAI1 1

Geological Institute, Slovak Academy of Sciences, Du´bravska´ 9, P.O. Box 106, 840 05 Bratislava 45, Slovak Republic *Corresponding author, e-mail: [email protected] 2 Geovetarcentrum, Go¨teborg University, 405 30 Go¨teborg, Sweden 3 Steinmann-Institut, Poppelsdorfer Schloss, 53115 Bonn, Germany 4 Department of Earth Sciences, University of Oxford, Parks Road, OX1 3PR, Oxford, UK 5 Department of Geology, University of Ljubljana, Asˇkercˇeva 12, 10000 Ljubljana, Slovenia

Abstract: Phase-equilibrium modelling, geothermobarometry, ion-microprobe dating and mineral chemistry of zircon have been used to constrain the P–T–t evolution of metapelitic kyanite-bearing gneisses from the ultrahigh-pressure (UHP) metamorphic terrane of the Pohorje Mountains in the Eastern Alps. These eclogite-hosting rocks are part of the continental basement of the Austroalpine nappes. Based on calculated phase diagrams in the system Na2O-CaO-K2O-FeO-MgO-MnO-Al2O3-SiO2-H2O (NCKFMMnASH) and conventional geothermobarometry, the garnet-phengite-kyanite-quartz assemblages of gneisses record metamorphic conditions of 2.2–2.7 GPa at 700–800 C. These are considered as minima because of the potential for a diffusion-related modification and reequilibration of the garnet and phengite during early stages of decompression. It is therefore most likely that the gneisses experienced the same peak UHP metamorphism at 3 GPa as associated kyanite eclogites. Decompression and cooling to 0.5 GPa and 550 C led to the consumption of garnet and phengite, and the development of matrix consisting of biotite, plagioclase, K-feldspar sillimanite and staurolite. Textures and phase diagrams suggest a low extent of partial melting during decompression. Cathodoluminescence images as well as zircon chemistry reveal cores encompassed by two types of metamorphic zircon rims. Ion probe U-Pb dating of three zircon cores yielded Permian (286 10, 258 7 Ma) and Triassic (238 7 Ma) concordia ages. The zircon rims are Cretaceous with a mean concordia age of 92.0 0.5 Ma and some cores gave a similar age. The Cretaceous zircons all exhibit very low Th/U ratio (,0.02) typical of metamorphic origin. In these zircons, nearly flat HREE patterns, (Lu/Gd)N ¼ 1–4, and only small negative Eu anomalies indicate formation in the presence of garnet and absence of plagioclase, which is corroborated by occurrence of Mg- and Ca-rich garnet inclusions. Therefore, these zircons are interpreted to record the Cretaceous HP/UHP metamorphism. The 92.0 0.5 Ma age obtained in this study agrees with that (93–91 Ma) determined earlier in the Pohorje eclogites from U/Pb zircon, Sm-Nd and Lu-Hf garnet-whole-rock dating. This implies that the eclogites and their country rocks were subducted and exhumed together as a coherent piece of continental crust. There is no evidence for a me´lange-like assemblage of rocks, which followed different P–T–t paths, or several subduction and exhumation cycles as proposed for some other UHP metamorphic terranes. Key-words: UHP metamorphism, continental crust subduction, Eastern Alps, ion probe zircon dating, phase equilibrium modelling.

1. Introduction

et al., 1995), Saxonian Erzgebirge, Germany (Nasdala & Massonne, 2000), Kokchetav Massif, Kazakhstan (Sobolev & Shatsky, 1990), Dabie Shan-Sulu terrane, China (Liu et al., 2002) or the North-East Greenland, Caledonides (McClelland et al., 2006), however, the volume of rocks actually observed to contain coesite or diamond is very limited. In many cases the host rocks do not record peak P–T conditions as high as those in the associated eclogites. There are two possible explanations. The first is that the host rocks also experienced the HP/UHP conditions but either did not equilibrate for kinetic reasons or their peak metamorphic assemblages have been subsequently

Eclogite-hosting gneisses in ultrahigh pressure (UHP) metamorphic terranes may provide evidence whether or not the felsic crustal rocks experienced the same P–T–t history as the mafic and ultramafic ones and thereby constrain the mechanisms of subduction and exhumation (e.g., Carswell et al., 2000; Lang & Gilotti, 2007; Gross et al., 2008). The best evidence for a shared history is the presence of coesite and/or diamond in the host gneiss, for example in the DoraMaira Massif, Western Alps (Chopin, 1984), Western Gneiss Region, Norwegian Caledonides (Dobrzhinetskaya eschweizerbart_xxx

0935-1221/09/0021-1966 $ 9.90 DOI: 10.1127/0935-1221/2009/0021-1966

# 2009 E. Schweizerbart’sche Verlagsbuchhandlung, D-70176 Stuttgart

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M. Jana´k, D. Cornell, N. Froitzheim, J.C.M. De Hoog, I. Broska, M. Vrabec, V. Hurai

Sm-Nd ages of garnet from gneisses (Tho¨ni, 2002) as well as 93–91 Ma Sm-Nd, Lu-Hf, and U-Pb ages of garnet and zircon from eclogites (Miller et al., 2005; Tho¨ni et al., 2008). In this paper we present data on the metapelitic gneisses that host the UHP eclogites in the southeastern part of the Pohorje Mts. Their mineralogical and petrological features are described and are supported by pseudosection modelling and thermobarometry, which constrain their P–T evolution. We present an isotopic study of zircon from these gneisses. Different domains in the zircons have been identified, using transmitted light microscopy, CL images and REE geochemistry, and dated in situ by ion microprobe. Two metamorphic events are distinguished. The Permian– Triassic event, not previously recognized, is proposed to reflect continental rifting. The second event is Cretaceous HP/UHP metamorphism, with almost identical age to that recorded by zircon from the eclogites (Miller et al., 2005). We suggest that the metamorphism of gneisses and associated eclogites was coeval and that these rocks were subducted and exhumed together as a coherent piece of continental crust.

overprinted during retrogression. The second explanation is that eclogites and surrounding rocks did not experience the same P–T conditions and were only juxtaposed at a later stage. As both explanations are geologically feasible, different tectonic interpretations of such terranes are possible. For example, the Western Gneiss Region of Norway and the Adula nappe in the Central Alps have been interpreted as coherent tracts of continental crust that were subducted during a single event and strongly sheared during exhumation (Krogh, 1977; Carswell & Cuthbert, 2003; Walsh et al., 2007; Nagel, 2008). Alternatively, they have been explained as me´langes or accretion channels resulting from a protracted history of subduction and exhumation that contain elements of different origin (e.g., continental and oceanic crust) subducted and exhumed at different times and along different trajectories (e.g., Smith, 1988; Brouwer et al., 2005). In order to distinguish between these hypotheses, it is important to gain information on the P–T paths and timing of subduction and exhumation from different components of the UHP complex. Zircon is the most commonly used accessory mineral for U-Pb geochronology of crustal rocks that experienced HP and UHP metamorphism (e.g., Gebauer, 1996; Gebauer et al., 1997; Rubatto et al., 2003; Rubatto & Hermann, 2007). Metamorphic zircon may form by the recrystallization of older (magmatic or detrital) zircon or by new growth, often encapsulating the older grains. Discrete rims or domains are commonly observed in backscattered electron (BSE) or cathodoluminescence (CL) images, providing an opportunity to obtain reliable ages for the protolith and for metamorphic growth. This can be best achieved with microbeam techniques such as the ion microprobe. However, zircon can form over a wide range of P–T conditions, from prograde to peak metamorphism and exhumation (e.g., Harley et al., 2007). Although in situ dating allows more accurate unravelling of the history of metamorphic rocks, linking zircon ages to specific growth episodes is not always straightforward. Therefore, the ages should be combined with petrological data in order to relate metamorphic evolution to zircon crystallization events (e.g., Liati & Gebauer, 1999; Hermann et al., 2001; Katayama et al., 2001; Mo¨ller et al., 2003; Whitehouse & Platt, 2003; Gilotti et al., 2004). Traceelement patterns of zircon, in particular the rare earth elements (REE), have been increasingly used to link age domains of the zircons with specific metamorphic assemblages (e.g., Schersteń et al., 2000; Rubatto, 2002; Whitehouse & Platt, 2003; Kelly & Harley, 2005; Miller et al., 2005, 2007). The Pohorje Mountains in the Eastern Alps are an important area to study UHP metamorphism and the exhumation of deeply subducted continental crust (Jana´k et al., 2004, 2006). The eclogite-bearing unit in the Pohorje Mts. is characterized by lenses of various rock types (eclogite, peridotite, gneiss, marble, quartzite) within a strongly foliated gneiss host. Previous dating of rocks from the Pohorje Mts. has established that HP/UHP metamorphism is Cretaceous, recorded by 93–87 Ma

2. Geological setting The Austroalpine nappes in the Eastern Alps were partly metamorphosed during Cretaceous times. This so-called eoAlpine metamorphism reached eclogite-facies conditions in Pohorje, Koralpe, Saualpe (Fig. 1), and some other areas. The highest pressures were reached in the Pohorje Mountains, where Jana´k et al. (2004, 2005) calculated P–T conditions of 3.0–3.1 GPa and 760–825 C for the kyanite eclogites, corresponding to the stability field of coesite. Garnet peridotites yielded UHP conditions of ca. 4.0 GPa and 900 C (Jana´k et al., 2006). In contrast, Sassi et al. (2004), Miller et al. (2005) and Miller & Konzett (2005) reported P–T conditions for the eclogites in the stability field of quartz. The eclogite-facies overprint occurred in these units over a relatively short time span between 93 and 90 Ma, corresponding to the Turonian (Tho¨ni, 2006). It is not clear whether the subduction process, particularly in the case of Pohorje, involved oceanic crust (Sassi et al., 2004), or was entirely intracontinental (Jana´k et al., 2004, 2006). A MORB-like geochemistry of the eclogites points to an oceanic crust subducted during convergence of the European plate to the North and the Apulian (Adriatic) plate, represented by the Southern Alps (Fig. 1), to the South (Sassi et al., 2004). In contrast, Jana´k et al. (2004, 2006) suggest that northwestern parts of the Austroalpine continental crust (Lower Central Austroalpine, Fig. 1) were subducted under southeastern parts (Upper Central Austroalpine), and that no ocean existed between them. They assumed that an intracontinental subduction zone formed within a Permian rift. This inference was drawn by analogy with the Koralpe complex, where Permian gabbros had been described (Tho¨ni & Jagoutz, 1992, eschweizerbart_xxx

Eclogite-hosting metapelites from the Pohorje

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Fig. 1. Tectonic map of the Eastern Alps. Abbreviations: B – Bajuvaric, K – Koralpe, P – Pohorje, S – Saualpe, T – Tirolic, GP– Graz Paleozoic. Modified after Neubauer & Ho¨ck (2000) and Schmid et al. (2004).

Fig. 2. Simplified geological map of Pohorje and adjacent areas (modified from Miocˇ & Zˇnidarcˇicˇ 1997), showing locations of investigated kyanite-garnet gneisses.

1993). In Pohorje, however, no Permian ages have been reported yet. The area of the Pohorje Mts (Fig. 1 and 2) is built up of two major tectonic units or nappes emplaced during the Cretaceous. The lower nappe consists of medium- to highgrade metamorphic rocks and represents the Lower Central Austroalpine. The upper nappe is formed by phyllites and

other low-grade metamorphic rocks and their PermoTriassic sedimentary cover, and represents the Upper Central Austroalpine. This nappe stack is overlain by Early Miocene sedimentary rocks that belong to the synrift basin fill of the Styrian Basin. The structure of the Pohorje Mts. is that of a large antiform with an ESE-WNW-striking axis, the core of which is intruded eschweizerbart_xxx

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by a granodioritic to tonalitic pluton of Miocene age (19–18 Ma; Altherr et al., 1995; Fodor et al., 2008; Trajanova et al., 2008). The lower nappe consists predominantly of mica schist, gneiss, and amphibolite with marble and quartzite lenses. In the south-eastern part, numerous eclogite lenses and a large body of ultramafic rocks (serpentinites and garnet peridotites) occur within this unit. All these rocks are strongly foliated and experienced high- to ultrahigh-pressure metamorphism.

atmosphere, and the ablated material was swept into the ICP after admixing of Ar gas. A laser spot diameter of 30 mm, occasionally 40 mm, was used. The laser repetition rate was 6 Hz with an energy density of 4.5 J/cm2. A typical analysis consisted of a minute of recording without firing the laser to determine the background signal, and another minute with the laser firing. Isotopes monitored were 139 La, 140Ce, 141Pr, 146Nd, 152Sm, 153Eu, 158Gd, 163Dy, 166 Er, 173Yb, 175Lu and 179Hf. Small contributions of 158 Dy and 152Gd were corrected for using the known natural abundances of these isotopes. Isotopes were measured in low resolution in peak-hopping mode. The scan range of the Element 2 was increased to 30 % so that all isotopes could be counted without the need of a magnet jump, allowing an increased scan speed of 303 scans of all isotopes in 60 s. Signal intensities were converted to concentrations using the concentration response factors calculated from repeated analysis of zircon standard 91500 (Wiedenbeck et al., 2004) and using Hf as an internal standard. HfO2 contents of 12 spots from different zircons were measured by SEMEDS at Go¨teborg University; the average of 1.45 0.18 was used for all laser spots. Signal drift was monitored using NIST SRM-610 and SRM-612 standards and was found to be less than 3 % for all elements during the analytical session. Therefore, no drift correction was applied. Fifteen repeats on zircon 91500 indicated a reproducibility of 5–7 % for all REE, except Pr (16 %) and La (62 %) due to the low concentrations of these elements in the 91500 standard. The higher standard deviations on 91500 relative to those on NIST SRM-610 and SRM-612 suggest that this material is slightly inhomogeneous, as reported by Wiedenbeck et al. (2004). All data reduction was performed offline with an inhouse developed spreadsheet.

3. Analytical methods The composition of minerals was determined by WDS analysis using a CAMECA SX-100 electron microprobe at the Diony´z Sˇtu´r Institute of Geology in Bratislava. Analytical conditions were 15 kV accelerating voltage and 20 nA beam current, with peak counting time of 20 s and beam diameter of 2–10 mm. Raw counts were corrected using a PAP routine. Mineral standards (Si, Ca: wollastonite, Na: albite, K: orthoclase, Fe: fayalite, Mn: rhodonite), pure element oxides (TiO2, Al2O3, Cr2O3, MgO) and metals (Ni) were used for calibration. Whole-rock chemistry was obtained by standard XRF methods with a Bruker S4 Pioneer WD spectrometer at Salzburg University. Zircon grains were separated using panning, magnetic separation and hand-picking. The grains were mounted in epoxy resin, together with standard grains, and polished with 1 mm diamond paste. Spots suitable for analysis, i.e. clean, inclusion-free zircon domains, were chosen using backscattered electron and CL images obtained with a Hitachi S-3400N scanning electron microscope with an Oxford EDS system at Go¨teborg University. The inclusions in zircon were analysed and their CL images were obtained with the CAMECA SX-100 electron microprobe at the Diony´z Sˇtu´r Institute of Geology in Bratislava. Ion probe dating was done at the NordSIMS facility at the Swedish Museum of Natural History, Stockholm, using a Cameca IMS1270 ion microprobe with a 20 mm spot size. Zircon analyses closely followed the routines described by Whitehouse et al. (1999) and Whitehouse & Kamber (2005); Pb/U ratios were calibrated using the 1065 Ma Geostandards 91500 zircon (Wiedenbeck et al., 1995). The generally small common lead corrections based on 204Pb were made using Stacey & Kramers (1975) zero age lead isotopic ratios because the common lead is commonly due to surface contamination (Zeck & Whitehouse, 1999). The position of each spot relative to the CL domain was checked by SEM after ion probe dating. Age calculations were made using the Isoplot 3.0 programme of Ludwig (1991, 1998). Uncertainties of age calculations are all given at the 2s level, ignoring decay constant errors. Rare-earth-element concentrations of the dated zircons were obtained by laser ablation ICP-MS at the Department of Earth Sciences at Bristol University, United Kingdom. The apparatus consists of a New Wave 193 nm Excimer laser system attached to a Thermo-Finnigan Element 2 magnetic sector ICP-MS. Ablation occurred in a He

4. Petrography and mineral chemistry of gneisses The investigated metapelitic gneisses occur in the southern part of the Pohorje Mountains near Slovenska Bistrica (Fig. 2) and host several eclogite lenses. No structural discontinuities have been identified between eclogites and their host rocks. The investigated gneisses are well foliated, with a strong shear fabric and minor leucocratic veins and pods (Fig. 3a). The hand specimens (Fig. 3b) are finely laminated (,2–3 mm thick layers) with light quartzo-feldspathic layers alternating with darker layers rich in white mica, biotite, garnet and kyanite. In thin section, the light layers contain patches of plagioclase, Kfeldspar and quartz with minor white mica and biotite. The gneisses have a peraluminous, Na2O and CaO-poor, K2O-rich pelitic composition (e.g., sample PO6MS, Table 1). The major minerals of the gneiss with estimated modal abundances are garnet (7–10 vol%), white mica (30–40 vol%), biotite (20–25 vol%), kyanite (2–5 vol%), plagioclase (5–10 vol%), K-feldspar (3–5 vol%) and quartz (20 vol%). Representative analyses are given in Tables 2–4. eschweizerbart_xxx


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of kyanite and rutile occur together with zircon in the Mg-rich garnet cores (Fig. 4d). Polyphase aggregates of staurolite, biotite and quartz (Fig. 4e) are mostly connected with matrix by fractures penetrating the garnet interiors. Along these fractures, garnet shows bright haloes (Fig. 4e) due to increased Fe and Mn contents. White mica occurs in the matrix as fine grains associated with K-feldspar, plagioclase, biotite and quartz (Fig. 4f). This white mica is phengite with up to 3.34 Si atoms per formula unit and 0.77 wt% TiO2 (Table 3). Large white mica porphyroblasts are muscovite; they are almost homogeneous with 3.13–3.08 Si atoms per formula unit. Biotite forms minor inclusions in garnet and subhedral, lath-shaped porphyroblasts in matrix. Composition of biotite inclusions in garnet is Mg-rich (XMg 0.76–0.79), corresponding to phlogopite. Biotite in the matrix, along the retrograded rims of garnet, or in fractures penetrating the garnet, is less magnesian (XMg ¼ 0.42–0.69) siderophyllite. Kyanite occurs as inclusions in garnet and as prismatic, subhedral grains of fish-like shape in the matrix. Some of the matrix kyanite grains are partly replaced by fine sillimanite needles, but most are unaltered. Plagioclase and K-feldspar vary in grain size and may occur as separate grains in the felsic layers or intimately intergrown, with minor K-feldspar in plagioclase (Fig. 4f). Plagioclase composition ranges from XAn 0.17 to 0.26. Accessory minerals in the matrix include zircon, monazite, xenotime and apatite. Some of the apatite crystals contain oriented needle-like monazite (Krenn et al., 2009).

5. P–T evolution

Fig. 3. (a) Photograph of metapelitic gneiss outcrop with segregations of leucocratic veins and pods, indicating partial melting. (b) Hand specimen of gneiss with kyanite and garnet porphyroblasts. Light quartzo-feldspathic layers alternate with darker layers rich in white mica and biotite.

5.1. Phase equilibrium modelling and geothermobarometry The P–T evolution of the investigated gneisses was reconstructed from textural relationships, phase-equilibrium modelling, and conventional thermobarometry. The P–T sections were calculated using Perple_X´ 07 thermodynamic software (Connolly, 2005: version 07) with the internally consistent thermodynamic dataset of Holland & Powell (1998). Stable assemblages in the system Na2O-CaO-K2O-FeO-MgO-MnO-Al2O3-SiO2-H2O (NC KFMMnASH) were constrained by whole-rock composition (Table 1), mineral chemistry, and modal proportions of the observed phases. From minor components (such as Mn and Ti), manganese is preferred in our modelling due to its striking effect on the stability of garnet (Tinkham et al., 2001). The influence of titanium is considered to play a significant role in the stability of biotite

Garnet forms porphyroblasts, up to 0.5 cm in diameter, which are mostly subhedral, and elongated parallel to the foliation (Fig. 4a). Some grains have experienced substantial reduction in size by resorption and fracturing. Microprobe traverses (Fig. 4c and 5) show that the garnets have homogeneous cores with higher pyrope (25–27 mol%) and grossular (9–10 mol%) contents than the rims (12–19 mol% prp; 6–9 mol% grs). A decrease in Mg and Ca is balanced by an increase in Fe and Mn. Inclusions of biotite, kyanite, quartz, staurolite, rutile, zircon, apatite and graphite (identified by Raman spectroscopy) occur in the garnet. Rutile appears in two varieties: (a) thin, oriented blue lamellae and (b) coarser, randomly oriented red-brown grains (Fig. 4b). Inclusions Table 1. Bulk rock composition of sample PO6MS (wt% oxides). SiO2

TiO2

Al2O3

Fe2O3 tot

MnO

MgO

CaO

Na2O

K2O

P2O5

SO3

F

LOI

Total

56.81

1.02

21.52

7.97

0.11

2.18

0.7

0.65

4.23

0.2

0.03

0.04

3.64

99.1

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Table 2. Representative microprobe analyses of garnet from the matrix and inclusions in zircon.

Sample

PO6MS

PO6MS

JV2/ 04

JV2/ 04

PO6OR

PO6OR

PO6MS

PO6MS

JV2/ 04

JV2/ 04

PO6OR

PO6OR

Mineral Position SiO2 TiO2 Al2O3 Cr2O3 FeOt MnO MgO CaO Totals Oxygens Si Ti Al Cr Fe Mn Mg Ca Sum Xalm Xsps Xprp Xgrs XMg

Grt Core 38.28 0 21.53 0.04 30.19 0.61 6.21 3.24 100.1 12 3.00 0.00 1.99 0.00 1.98 0.04 0.73 0.27 8.00 0.66 0.01 0.24 0.09 0.27

Grt Rim 37.44 0.13 20.97 0.02 34.86 2.52 3.17 2.18 101.29 12 2.99 0.01 1.97 0.00 2.32 0.17 0.38 0.19 8.02 0.76 0.06 0.12 0.06 0.14

Grt Core 38.14 0.22 21.76 0.1 27.06 1.68 6.57 4.28 99.81 12 2.98 0.01 2.00 0.01 1.77 0.11 0.76 0.36 8.00 0.59 0.04 0.25 0.12 0.30

Grt Rim 37.82 0 21.4 0.03 31.26 5.86 3.51 1.67 101.55 12 2.99 0.00 2.00 0.00 2.07 0.39 0.41 0.14 8.01 0.69 0.13 0.14 0.05 0.17

Grt Core 38.34 0 21.71 0.01 30.36 0.58 6.38 3.17 100.55 12 2.99 0.00 2.00 0.00 1.98 0.04 0.74 0.27 8.01 0.65 0.01 0.24 0.09 0.27

Grt Rim 37.29 0 21.11 0.03 33.92 5.15 2.4 1.65 101.55 12 2.98 0.00 1.99 0.00 2.27 0.35 0.29 0.14 8.02 0.74 0.11 0.09 0.05 0.11

Grt In Zir 38.53 0.02 21.99 0.06 30.14 0.66 5.85 3.55 100.8 12 3.00 0.00 2.02 0.00 1.96 0.04 0.68 0.30 8.00 0.66 0.01 0.23 0.10 0.26

Grt In Zir 38.42 0.04 21.9 0.05 30.34 1.6 5.44 3.35 101.14 12 2.99 0.00 2.01 0.00 1.98 0.11 0.63 0.28 8.00 0.66 0.04 0.21 0.09 0.24

Grt In Zir 39.05 0.02 21.99 0.04 26.16 1.64 5.77 6.15 100.82 12 3.01 0.00 2.00 0.00 1.69 0.11 0.66 0.51 7.98 0.57 0.04 0.22 0.17 0.28

Grt In Zir 39.27 0 21.96 0.03 27.54 0.83 5.9 5.7 101.23 12 3.02 0.00 1.99 0.00 1.77 0.05 0.68 0.47 7.98 0.60 0.02 0.23 0.16 0.28

Grt In zir 38.64 0.01 21.61 0 28.11 1.01 6.11 4.67 100.16 12 3.01 0.00 1.99 0.00 1.83 0.07 0.71 0.39 7.99 0.61 0.02 0.24 0.13 0.28

Grt In Zir 38.37 0.01 21.07 0 29.82 0.99 5.86 4.23 100.35 12 3.01 0.00 1.95 0.00 1.95 0.07 0.68 0.36 8.02 0.64 0.02 0.22 0.12 0.26

Table 3. Representative microprobe analyses of white mica and biotite.

Sample

PO6MS PO6MS

Mineral Phn Position Matrix SiO2 50.74 0.23 TiO2 30.48 Al2O3 0.01 Cr2O3 FeOt 3.29 MnO 0.00 MgO 1.67 CaO 0.25 0.10 Na2O 8.93 K2O Totals 95.7 Oxygens 11 Si 3.34 Ti 0.01 Al 2.37 Cr 0.00 0.18 Fe2þ Mn 0.00 Mg 0.16 Ca 0.02 Na 0.01 K 0.75 Sum 6.85 XMg 0.48

Ms Matrix 46.98 0.65 34.19 0.05 1.2 0 1.02 0 0.84 9.73 94.66 11 3.13 0.03 2.69 0.00 0.07 0.00 0.10 0.00 0.11 0.83 6.96 0.60

JV2/ 04

JV2/ 04

Phn Matrix 49.63 0.77 32.5 0.05 1.57 0.05 1.79 0 0.59 9.39 96.34 11 3.24 0.04 2.50 0.00 0.09 0.00 0.17 0.00 0.08 0.78 6.90 0.67

Phn Matrix 49.74 0.75 32.35 0.04 1.48 0.01 1.96 0.02 0.54 9.3 96.19 11 3.25 0.04 2.50 0.00 0.08 0.00 0.19 0.00 0.07 0.78 6.91 0.70

JV2/ 04

PO6OR PO6OR PO6MS PO6MS

PO6MS

Phn Matrix 50.39 0.36 32.2 0 2.7 0 2.16 0.09 0.02 8.52 96.44 11 3.27 0.02 2.47 0.00 0.15 0.00 0.21 0.01 0.00 0.71 6.84 0.58

Bi Bi Grt fract. In Grt 35.86 39.69 2.61 2.08 19.39 18.07 0.08 0.1 20.48 9.66 0.2 0.04 8.16 18.07 0.02 0.02 0.28 0.54 8.79 8.05 95.87 96.32 11 11 2.71 2.81 0.15 0.11 1.73 1.51 0.01 0.01 1.30 0.57 0.01 0.00 0.92 1.91 0.00 0.00 0.04 0.07 0.85 0.73 7.72 7.72 0.42 0.77

Phn Matrix 49.5 0.62 30.9 0.05 3.23 0.01 1.94 0.1 0.17 9.11 95.63 11 3.27 0.03 2.41 0.00 0.18 0.00 0.19 0.01 0.02 0.77 6.88 0.51

Bi In Grt 38.51 0.67 20.11 0 10.06 0.01 17.36 0.03 0.45 9.4 96.6 11 2.74 0.04 1.69 0.00 0.60 0.00 1.84 0.00 0.06 0.86 7.83 0.75 eschweizerbart_xxx

Bi Matrix 36.21 2.45 19.31 0.06 20.14 0.16 8.04 0.07 0.26 8.58 95.28 11 2.75 0.14 1.73 0.00 1.28 0.01 0.91 0.01 0.04 0.83 7.69 0.42

JV2/ 04 Bi Matrix 35.26 1.6 19.6 0.03 19 0.51 9.6 0.08 0.15 9.15 94.98 11 2.69 0.09 1.76 0.00 1.21 0.03 1.09 0.01 0.02 0.89 7.80 0.47

PO6OR PO6OR Bi In Grt 37.4 0.44 19.59 0.04 13.4 0.02 14.84 0.07 0.45 8.49 94.74 11 2.75 0.02 1.70 0.00 0.83 0.00 1.63 0.01 0.06 0.80 7.81 0.66

Bi Matrix 35.82 1.59 20.44 0.11 20.84 0.2 8.41 0.05 0.32 8.98 96.76 11 2.69 0.09 1.81 0.01 1.31 0.01 0.94 0.00 0.05 0.86 7.77 0.42


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Table 4. Representative microprobe analyses of feldspars. Sample

PO6MS

PO6MS

JV2/04

JV2/04

PO6OR

PO6OR

PO6MS

JV2/04

Mineral Position SiO2 Al2O3 CaO Na2O K2O Totals Oxygens Si Al Ca Na K Sum XAn XAb XOr

Pl core Matrix 65.38 22.6 3.52 9.62 0.2 101.32 8 2.85 1.16 0.16 0.81 0.01 5.00 0.17 0.82 0.01

Pl rim Matrix 62.73 23.93 5.19 8.53 0.19 100.57 8 2.77 1.24 0.25 0.73 0.01 5.00 0.25 0.74 0.01

Pl Matrix 63.37 24.15 4.92 9.08 0.18 101.7 8 2.76 1.24 0.23 0.77 0.01 5.01 0.23 0.76 0.01

Pl Matrix 62.51 23.34 4.72 9.03 0.17 99.77 8 2.77 1.22 0.23 0.78 0.01 5.00 0.22 0.77 0.01

Pl Matrix 62.52 24.45 5.46 8.67 0.14 101.24 8 2.74 1.26 0.26 0.74 0.01 5.01 0.26 0.73 0.01

Pl Matrix 62.45 24.83 5.25 8.59 0.13 101.25 8 2.74 1.28 0.25 0.73 0.01 5.01 0.25 0.74 0.01

Kfs Matrix 64.5 18.46 0 0.29 16.03 99.28 8 3.01 1.01 0.00 0.03 0.95 4.99 0.00 0.03 0.97

Kfs Matrix 63.9 18.87 0.04 0.39 16.23 99.43 8 2.97 1.03 0.00 0.04 0.96 5.00 0.00 0.04 0.96

(White et al., 2007). Regarding the inconsistency of Mn and Ti end-members in the Ti-biotite solution model (White et al., 2007), the Ti component was ignored in the present modelling. Solution models for garnet (Holland & Powell, 1998), plagioclase (Newton et al., 1980), phengite, biotite, staurolite (Powell & Holland, 1999, 2001) and haplogranitic melt (Holland & Powell, 2001; White et al., 2001) were used, as available from the Perple_X data file (solut_07.dat). We performed the calculations with bulk water content of 2 wt%, as constrained by phase equilibria, composition of retrograde mineral assemblages, and fluid inclusions in retrograde garnet rims (Hurai et al., 2008). Mineral compositions and modal proportions were computed for a fixed bulk composition, which is assumed to represent the near-peak and retrograde evolution. In contrast, the prograde evolution is obscured, as indicated by the chemically homogeneous garnet cores and the absence of prograde growth zoning in garnet. We assume that bulk composition is essentially homogeneous on the scale of measured sample. Textural and chemical heterogeneities (e.g., garnet resorption) imply that effective bulk composition may diverge from the average one, but the difference is not significant since the calculated topologies are consistent with both textural observations and measured mineral compositions. The calculated P–T sections are shown in Fig. 6a, b. To determine the peak metamorphic conditions from the observed garnet þ phengite þ kyanite þ quartz assemblage, the following net transfer reaction was used: 3 celadonite þ 4 kyanite ¼ 3 muscovite þ 4 SiO2 þ pyrope

Holland, 2002; Hermann, 2003; Krogh Ravna & Terry, 2004). We used a combination of the ideal activity model for the phengite solid solution (Holland & Powell, 1998) and the garnet activity model of Ganguly et al. (1996), as recommended by Krogh Ravna & Terry (2004). For temperature evaluation, we used the garnet-phengite exchange geothermometer calibrated by Green & Hellman (1982) for pelites with low-Ca and MgO/(MgO þ FeO) of about 0.2–0.3. As with their experimental results, we made calculations on the assumption of all Fe as Fe2þ both in garnet and phengite. In P–T calculations we used the compositions of phengite with maximum Si coupled with garnet cores (Tables 2 and 3). P–T conditions for the retrograde stage have been calculated from a garnet-biotite geothermometer (Ferry & Spear, 1978) with the solution model of Berman (1990), garnet-plagioclase-Al2SiO5 (GASP) barometry (Koziol & Newton, 1988) and garnet-plagioclase-rutile-ilmenite (GRIPS) barometry (Bohlen & Liotta, 1986) using compositions of the garnet rim, adjacent low XMg biotite and high XAn plagioclase (Tables 2–4). The results of geothermobarometry are summarized in Table 5. 5.2. Derivation of the P–T path We infer that metamorphic assemblages stable at peak HP/ UHP conditions included garnet, kyanite, phengite, rutile, quartz or coesite and jadeite-rich clinopyroxene (Fig. 6a). According to our modelling, the modal proportions would correspond to 17–18 vol% garnet, 7–8 vol% kyanite, 42 vol% phengite, 27–28 vol% quartz/coesite, and 4 vol% omphacite, with minor amount (0.1 vol%) of aqueous fluid. The wet metapelite solidus is calculated to lie between 750–800 C at 2.5 GPa (Fig. 6a), which is in good agreement with experimental metapelite (Hermann & Spandler, 2008) and metagreywacke systems (Auzanneau et al., 2006).

(1)

calibrated as a geobarometer by Krogh Ravna & Terry (2004). This geobarometer is suggested to be useful for clinopyroxene-free UHP/HP metapelites (e.g., Coggon & eschweizerbart_xxx

1198


Fig. 4. Photomicrographs of kyanite-garnet gneisses. (a) Garnet (Grt) porphyroblast with rutile (Rt) inclusion. (b) Enlarged view of (a) shows that rutile also occurs as thin, oriented needles indicating exsolutions from garnet. (c) BSE image of garnet porphyroblast with line marking the location of the analysed profile shown in Fig. 5. Bright rims due to increase in Fe close to the garnet edge indicate diffusion. (d) BSE image of garnet with inclusions of kyanite (Ky), rutile (Rt) and zircon (Zir) in the Mg-rich core. (e) BSE image of garnet with composite inclusions of biotite (Bi) and staurolite (St). Note bright haloes due to increase in Fe around the inclusions and fractures connecting these inclusions with matrix. (f) Breakdown of phengite (Ph) to biotite (Bi), plagioclase (Pl), K-feldspar (Kfs) and quartz (Qtz).

garnet cores at 2.4–2.7 GPa and 700–770 C. The 0.08 XGrs contour of the garnet cores comprises a broader range of P–T conditions, but mostly in excess of 2 GPa. Geobarometry based on the net-transfer reaction (1) yields a pressure of 1.9–2.6 GPa at a nominal temperature of 700–850 C (Table 5). Application of the garnet-phengite geothermometer (Green & Hellman, 1982) yields a temperature of 760–980 C at a pressure of 2–3 GPa. We consider the temperatures derived on the basis of the

The silica content of the phengite is pressure sensitive, with a positive slope of isopleths as shown in Fig. 6a. This is consistent with available experimental data on metapelitic systems (Massonne & Szpurka, 1997; Hermann, 2002, 2003). In garnet, the XMg ¼ Mg/(Mg þ Fe) value is temperature sensitive, whereas the grossular content (XGrs) is more pressure sensitive, but the isopleths are widely spaced (Fig. 6a). The 3.34 Si isopleth of phengite intersects the 0.27–0.28 XMg isopleths relevant to the eschweizerbart_xxx


1199

Fig. 5. Compositional profile across garnet shown in Fig. 4c with mole fractions of pyrope, grossular, spessartine and almandine endmembers.

garnet-phengite thermometry to be maximum values, subject to large uncertainties due to a possible Fe3þ content in white mica (e.g., Carswell et al., 2000). From textural observations, phase equilibrium modelling and conventional geothermobarometry, we deduce that the decompression from the peak metamorphic conditions to 0.5–1 GPa and 700–750 C (Fig. 6b) led to the consumption of phengite and garnet, growth of biotite, plagioclase and K-feldspar, while kyanite was partly transformed to sillimanite. Jadeite was most probably totally decomposed to plagioclase and quartz. Phase equilibrium calculations and textural observations (formation of quartzofeldspathic layers) predict a low-degree of partial melting during the decompression (Fig. 6b). Decrease in garnet volumes and grossular contents suggest no re-growth of garnet related to a decompression (e.g., Auzanneau et al., 2006). The subsequent cooling led to crystallization of melt and liberation of aqueous fluid near solidus at 650 C in the sillimanite stability field. Further decrease in temperature to 550 C accompanied by hydration led to formation of staurolite. An inferred P–T path is shown in Fig. 6b.

6. Zircon geochronology 6.1. Characterization of zircon Zircon was investigated in three samples (JV2/04, PO6MS and PO6OR) of metapelitic gneisses in transmitted light, backscattered electron, and CL images. The zircons show three distinct domains in CL images (Fig. 7). Cores generally exhibit truncated growth zonation overgrown by dark (rim 1) and bright (rim 2) rims (Fig. 7a), reflecting probably two stages of the metamorphic growth or recrystallization. Rim domains of both types often show features such as relict zonation (e.g., grain 2 and 16 in Fig. 7a or 27 in Fig. 7b) and texturally younger domains cutting across

Fig. 6. P–T sections calculated for garnet-kyanite gneiss in the MnNCKFMASH system using the Perple_X thermodynamic software (Connolly, 2005: version 07). (a) Stable mineral assemblages constrained from the measured bulk-rock composition of sample PO6MS with 2 wt% of H2O. The shaded areas correspond to the selected assemblages observed and discussed in the text. The isopleths of XMg and XGrs in garnet, and Si a.p.f.u in phengite are shown with the ellipsoid encompassing the peak P–T conditions. (b) The same P–T section with calculated proportions (vol%) of garnet and melt. The retrograde P–T path of the gneiss is indicated by arrow. See text for further explanations. eschweizerbart_xxx

1200


Table 5. Summary of P(GPa) – T ( C) estimates based on thermobarometric calculations. Sample

Assemblage

Peak metamorphic conditions PO6MS Grt(core)-Phn-Ky-Qtz JV2/04

Grt(core)-Phn-Ky-Qtz

PO6OR

Grt(core)-Phn-Ky-Qtz

Retrograde evolution PO6MS Grt(rim)-Bi-Pl-Ky JV2/04

Grt(rim)-Bi-Pl-Ky

PO6OR

Grt(rim)-Bi-Pl-Sil

PO6MS Grt(rim)-Pl-Ilm JV2/04

Grt(rim)-Pl-Ilm

PO6OR

Grt(rim)-Pl-Ilm

T ref

Pa

P ref

Tb

700 850 700 850 700 850 T ref 600 700 600 700 600 700 T ref 600 700 600 700 600 700

2.08 2.58 1.85 2.3 2.13 2.61 Pc 0.7 0.91 0.68 0.89 0.58 0.77 Pe 0.6 0.73 0.53 0.65 0.48 0.6

2 3 2 3 2 3 P ref 0.5 1 0.5 1 0.5 1

915 982 764 822 827 889 Td 696 720 665 687 589 610

a

Krogh Ravna & Terry (2004), bGreen & Hellman (1982), cKoziol & Newton (1988), dFerry & Spear (1978) and Berman (1990), e Bohlen & Liotta (1986).

older rims or cores (grain 22 in Fig. 7b or 10, 23 and 25 in Fig. 7c), indicating that rim development proceeded by recrystallization of the pre-existing zircon. However, newly grown rim 1 on a resorbed core of zircon is also clearly shown in Fig. 7a (grain 7). Uranium concentrations of the two rim types overlap; whereas in some examples in the literature dark rims have higher U levels (e.g., Schersteń et al., 2000). Inclusions of garnet (Fig. 8), white mica and rutile are present mostly in the rims of zircon. The composition of garnet inclusions in zircon (Table 2) corresponds to that of matrix garnet cores. 6.2. Age determinations Positions of spots in the zircons dated by the ion probe are illustrated in Fig. 7, the U-Pb data are given in Table 6 with concordia and weighted-mean calculations shown in Table 7, and concordia diagrams plotted in Fig. 9.

Fig. 7. Cathodoluminescence images of dated zircons, showing cores, dark rim 1 and bright rim 2 domains, and the location of ion probe spots.

6.2.1. Sample JV2/04 Two of the three cores analysed gave concordant Permian (252 Ma) and Triassic (238 Ma) ages (spots 2a and 16a). The third core spot (7a) has the same age (92 Ma) as the main group of data from rims. The rims are all concordant but show some scatter along concordia (Fig. 9a) and the precision is not good enough to say whether the two rim generations have significantly different ages. The main group of six rims and one core yields a concordia age of 91.3 1.0 Ma (Fig. 9a). Spot 16b shows relict zonation

(Fig. 7a) and its slightly higher age was probably influenced by the 238 Ma core. Spot 1a with 77 Ma age has high common Pb and is therefore ignored, but spots 9a (rim 2) and 10b (rim 1) group at 86.2 1.6 Ma. 6.2.2. Sample PO6 MS One core (spot 22a) was dated at 286 10 Ma. The dark rim 1 spots (Fig. 7b) gave a slightly older concordia age of eschweizerbart_xxx


1201

negative Eu anomalies (Eu/Eu* ¼ 0.2–0.6). One core (JV2/04, grain 16) has much higher Th/U ratio (0.24) and a more pronounced Ce anomaly (Ce/Ce* ¼ 17.4) than the two other cores (Th/U ¼ 0.008–0.016 and Ce/Ce* ¼ 3–4.8). We also analysed undated deeper parts of the zircons by laser ablation, penetrating the crystal rims. These domains show similar compositions as the old cores (Fig. 10d and 11). They are characterized by high HREE contents (180–660 ppm Yb), very steep HREE patterns ((Lu/Gd)N ¼ 33–200), variably negative Eu anomalies (Eu/Eu* ¼ 0.1–0.7) and variably positive Ce anomalies (Ce/Ce* ¼ 6–62). Hf contents were assumed to be similar to other zircon domains.

Fig. 8. Cathodoluminescence image of zircon with garnet inclusions. Note position of inclusions in the rims.

7. Discussion 7.1. Interpretation of zircon ages The three dated samples show a coherent pattern of ages that allowed weighted-mean concordia ages to be calculated (Table 7). Three of the dated cores yielded Permian to Triassic ages, which suggest that the protolith of the gneiss experienced a Permian to Triassic orogenic event. Zircon cores in two samples (JV2/04 and PO6OR) gave Cretaceous ages within error of that of the metamorphic rims. The identification of spot 7a in JV2/04 in a zoned truncated core (Fig. 7a) seems certain, whereas the cores in PO6OR (Fig. 7c) are less well defined and might have been recrystallized during metamorphism as seen in grain 23. It is probable that all the cores were originally significantly older than the rims, but only some of them have been reset by recrystallization or possibly by total lead loss during the Cretaceous UHP metamorphism. The very low Th/U ratios (,0.02) of all zircon spots (Table 6) with exception of one Triassic core (0.24), point to their formation by metamorphic crystallization. Metamorphic zircon typically has low Th/U ratios (,0.1, e.g., Schaltegger et al., 1999; Rubatto, 2002), however, values greater than 0.1, similar to magmatic ones, have also been observed (Schersteń et al., 2000; Raith et al., 2003; Hoskin & Schaltegger, 2003; Mo¨ller et al., 2003; Harley et al., 2007). The first generation of dark rims recognized in all three samples has a weighted mean age of 92.6 1.0 Ma compared with 91.0 1.0 Ma (calculated for two samples, 2s confidence levels) for the second generation of bright rims, which clearly overgrow the dark rims. These ages are different only at the 1s confidence level, but together with the textural evidence indicate that the two stages of metamorphic zircon formation might have occurred 1–2 million years apart. A more detailed study might be able to statistically resolve this difference. The combined age of 92.0 0.5 Ma for the rims is a statistically valid general age for the metamorphic zircon rim formation. A group of four rim spots, two in each of samples JV2/04 and PO6OR, has a clearly younger age of 86.4 1.1 Ma

92.8 1.5 Ma (Fig. 9b) than the bright rim 2 spots (90.7 1.5 Ma), but although the bright rims are clearly younger in a textural sense, the age difference is only significant at the 1s confidence level. Taken together, ten rim spots yield a concordia age of 91.7 1.1 Ma (Table 7). 6.2.3. Sample PO6 OR Zircons in this sample also exhibit three different domains, although the CL contrast between them is lower (Fig. 7c). All three cores gave the same age as the main group of seven rims and their combined concordia age corresponds to 92.4 0.7 Ma (Fig. 9c). Two younger concordant rims gave a concordia age of 86.4 1.5 Ma, similar to that found in sample JV2/04. 6.3. Rare earth element concentrations of zircon Rare earth element concentrations are presented in Table 8 and chondrite-normalized patterns are shown in Fig. 10. The zircon rims (92–91 Ma) are characterized by low HREE contents (4–22 ppm Yb), nearly flat HREE patterns ((Lu/Gd)N ¼ 1.3–4.5), with the exception of 11.7 (Fig. 11), small negative Eu anomalies (Eu/Eu* ¼ 0.6–0.85) and positive Ce anomalies (Ce/Ce* ¼ 4–14, with the exception of 21). The REE patterns of zircon rims are very similar for all three samples. Zircons from sample JV2/04 have slightly lower and those from sample PO6MS slightly higher HREE abundances than those from sample PO6OR. Zircons from sample JV2/04 also have the lowest (Lu/ Gd)N ratios. Young core (92–91 Ma) domains of zircon grains show trace-element patterns indistinguishable from the rims, except for one grain from sample JV2/04 (zircon 10), which is considerably enriched in MREE compared to the rims. The three old cores (290–240 Ma) have REE patterns that are easily distinguishable from the rims by higher HREE contents (140–830 ppm Yb), much steeper HREE patterns ((Lu/Gd)N ¼ 22–46) and more eschweizerbart_xxx

eschweizerbart_xxx

% 6.8 4.0 2.0 2.4 2.3 4.0 2.7 3.1 2.8 3.0 2.7

Pb

206 Pb 0.0409 0.0486 0.0487 0.0488 0.0502 0.0460 0.0460 0.0469 0.0479 0.0459 0.0467

14c1 14c2 15b 16b 22a 22b1 22b2 22b3 27b 27c 31c

Rim 2 Rim 2 Rim 1 Rim 1 Core Rim 1 Rim 1 Rim 2 Rim 1 Rim 2 Rim 2

s

207

PO6MS

10.1 0.9 1.5 2.3 2.3 2.0 1.6 1.6 1.8 1.0 2.0 1.3 2.8

0.0430 0.0519 0.0491 0.0489 0.0496 0.0469 0.0479 0.0483 0.0486 0.0474 0.0468 0.0516 0.0480

Rim 2 Core Rim 1 Rim 2 Rim 2 Core Rim 1 Rim 2 Rim 2 Rim 1 Rim 2 Core Rim 1

1a 2a 2b 5a 5b 7a 7b 9a 9b 10a 10b 16a 16b

%

s

Pb

206

Pb

207

Spot

JV2/04

Sample

U

Pb

Pb

U 0.0819 0.0918 0.1003 0.0925 0.3149 0.0915 0.0930 0.0926 0.0981 0.0895 0.0920

235

207

0.0710 0.2845 0.0984 0.0954 0.0976 0.0926 0.0925 0.0901 0.0965 0.0925 0.0864 0.2671 0.0999

235

207

% 7.1 4.4 2.7 3.0 3.0 4.5 3.3 3.7 3.4 3.6 3.3

s

10.2 1.7 2.1 2.7 2.7 2.5 2.2 2.1 2.2 1.8 2.4 1.9 3.0

%

s U

Pb

Pb

U 0.0145 0.0137 0.0150 0.0137 0.0455 0.0144 0.0147 0.0143 0.0149 0.0142 0.0143

238

206

0.0120 0.0398 0.0145 0.0142 0.0143 0.0143 0.0140 0.0135 0.0144 0.0142 0.0134 0.0375 0.0151

238

206

% 1.95 1.86 1.88 1.86 1.86 1.89 1.87 1.91 1.86 1.86 1.86

s

1.30 1.42 1.46 1.37 1.32 1.37 1.40 1.36 1.38 1.56 1.37 1.39 1.30

%

correl. 0.276 0.423 0.687 0.618 0.625 0.424 0.565 0.520 0.548 0.523 0.562

error

0.128 0.840 0.701 0.503 0.495 0.558 0.648 0.656 0.619 0.854 0.564 0.736 0.427

Error correl.

Coordinates: s

Table 6. U-Pb ion probe data for spots in zircons.

U no data 0.0008 0.0027 0.0052 0.0126 no data no data 0.0024 no data 0.0018 0.0006

232

Pb

208

no data 0.0139 0.0063 0.0050 0.0053 0.0061 0.0045 0.0053 0.0059 0.0032 0.0009 0.0139 0.0108

U

232

Pb

208

9.3 1.1 2.4 2.1 3.5 1.9 0.3 1.1 1.4 0.9 1.8 1.2 0.1

% no data 270.5 38.6 13.2 16.0 no data no data 99.0 no data 110.2 312.6

s

%

Discordance

% 14.1 1.6 1.4 2.1 3.1 3.8 3.8 1.8 0.0 3.9 2.2

Discordance

46 25.12Ń 15 31.23E

no data 7.6 10.8 10.9 10.6 10.9 9.8 10.1 13.2 13.3 245.0 6.8 19.0

%

s

46 24.510 N 15 31.230 E

Pb% 0.65 *0.33 *0.08 *0.00 0.02 0.42 0.24 *0.20 0.22 0.16 0.27

206

Common

2.30 {0.02} {0.07} {0.17} {0.08} {0.19} {0.07} {0.02} {0.03} 0.06 0.20 0.11 {0.13}

Pb%

206

Common Pb ## 21 34 54 53 48 38 36 41 22 47 29 64

s

Pb 206 Pb 297 127 131 140 204 3 2 45 94 9 35

207

165 91 46 55 53 95 64 73 66 72 64

s

Ages Ma

168 280 151 142 175 44 96 113 127 67 40 269 100

206

Pb

Ages Ma 207

U

Pb

Pb U 80 89 97 90 278 89 90 90 95 87 89

235

207

70 254 95 93 95 90 90 88 94 90 84 240 97

235

207

5 4 3 3 7 4 3 3 3 3 3

s

7 4 2 2 2 2 2 2 2 2 2 4 3

s U

Pb

Pb U 93 88 96 88 287 92 94 92 95 91 91

238

206

77 251 93 91 91 92 90 87 92 91 86 237 97

238

206

2 2 2 2 5 2 2 2 2 2 2

s

1 3 1 1 1 1 1 1 1 1 1 3 1

s

ppm 336 397 1289 772 499 503 476 368 400 419 422

[U]

462 620 680 376 386 380 566 622 696 1956 668 408 172

ppm

[U]

ppm 5.6 7.2 18.4 10.3 7.9 5.3 4.2 4.4 2.9 5.8 6.0

[Th]

5.4 4.8 3.9 7.4 7.5 4.5 6.8 6.1 5.6 28.9 6.9 99.4 0.8

ppm

[Th]

ppm 5.0 5.7 20.3 11.2 23.9 7.0 7.0 5.5 6.0 6.2 6.3

[Pb]

5.0 26.0 10.4 5.6 5.8 5.7 8.3 8.9 10.5 29.1 9.4 17.5 2.7

ppm

[Pb]

0.017 0.018 0.014 0.013 0.016 0.011 0.009 0.012 0.007 0.014 0.014

Th/U

0.012 0.008 0.006 0.020 0.020 0.012 0.012 0.010 0.008 0.015 0.010 0.244 0.005

Th/U

1202 M. Jana´k, D. Cornell, N. Froitzheim, J.C.M. De Hoog, I. Broska, M. Vrabec, V. Hurai

eschweizerbart_xxx

Rim 2 Core Rim 1 Rim 2 Rim 1 Rim 2 Core Rim 2 Rim 1 Rim 2 Core Rim 2

Pb 0.0495 0.0438 0.0468 0.0511 0.0461 0.0462 0.0476 0.0491 0.0464 0.0475 0.0464 0.0465

206

Pb

207

% 2.0 3.6 2.2 2.2 2.0 3.3 2.4 2.2 1.7 3.0 2.0 3.0

s

U 0.0984 0.0884 0.0879 0.1004 0.0927 0.0930 0.0946 0.0970 0.0921 0.0930 0.0931 0.0859

235

Pb

207

% 2.4 3.8 2.5 2.6 2.3 3.5 2.7 2.5 2.2 3.2 2.3 3.2

s Pb

U 0.0144 0.0146 0.0136 0.0143 0.0146 0.0146 0.0144 0.0143 0.0144 0.0142 0.0145 0.0134

238

206

% 1.22 1.20 1.29 1.30 1.23 1.20 1.21 1.21 1.39 1.20 1.20 1.20

s

0.516 0.316 0.507 0.502 0.524 0.344 0.443 0.488 0.640 0.373 0.515 0.370

Error correl. U 0.0068 no data no data 0.0048 0.0018 0.0001 no data 0.0051 0.0059 0.0069 0.0073 0.0001

232

Pb

208

% 11.3 no data no data 11.9 98.5 4426.0 no data 9.3 8.5 12.7 10.8 1807.4

s % 3.3 8.1 2.0 6.5 3.6 3.4 0.5 2.5 3.0 0.6 2.9 2.4

Discordance

46 24.56Ń 15 31.52E

Pb% {0.27} 0.36 0.31 {0.17} 0.18 0.35 0.26 {0.00} {0.02} {0.08} {0.12} 0.26

206

Common Pb 172 121 37 245 0 0 79 152 0 77 20 25

206

Pb

46 87 52 51 49 85 57 50 57 70 47 71

s

Ages Ma 207

Explanations. Rim 1: CL-dark, rim 2: CL-bright. For common Pb corrections, {}: not corrected, *: common Pb not significant. Discordance is the difference between the U-Pb ages. Analytical and age errors are all given at the 1s level in this table, but age calculation errors in Table 7, the text and figures are given at the 2s level.

3a 3b 3c 10a 10b 14a 14b 15a 15b 23a 25a 25b

PO6OR Pb

U 95 86 86 97 90 90 92 94 89 90 90 84

235

207

2 3 2 2 2 3 2 2 2 3 2 3

s Pb

U 92 94 87 91 93 93 92 92 92 91 93 86

238

206

1 1 1 1 1 1 1 1 1 1 1 1

s ppm 382 252 602 348 572 329 443 337 510 299 451 397

[U] ppm 3.6 1.8 4.5 5.2 6.4 6.3 2.9 6.2 7.6 4.2 3.6 5.9

[Th]

ppm 5.8 3.0 8.0 5.2 8.7 5.0 6.0 5.1 7.7 4.5 6.9 5.6

[Pb]

0.009 0.007 0.008 0.015 0.011 0.019 0.007 0.018 0.015 0.014 0.008 0.015

Th/U

Eclogite-hosting metapelites from the Pohorje 1203


1204

Table 7. Concordia and weighted mean calculations of ion probe zircon dates. 2s Calc. no.

Data type

No. Spots included

No. spots excluded

Age Ma

Err. Ma

MSWD Probability

252 238

7 7

1.8 1.1

0.18 0.28

2.5 1.3 1.4 1 1.6

1 0.002 3.3 0.35 0.003

0.32 0.97 0.07 0.55 0.96

2.4

0.12

Interpretation

JV2/04 Concordia calculations 1 Core 2a 2 Core 16a

1 1

3 4 5 6 7

1 4 3 7 2

0 1a, 9a, 10b 4 rims 4

1

0

6 4 10

0 0 0

92.8 90.7 91.7

1.5 1.5 1.1

0.08 3.5 1.6

0.78 0.06 0.21

3 7

0 2

92.9 92.1

1.3 0.8

5.2 0.001

0.02 0.97

10 2

2

92.4 86.4

0.7 1.5

1.4 1.6

0.24 0.2

Reset by metamorphism Growth of rims during metamorphism General age of metamorphism Possible lead loss event

10 7

92.6 91

1 1.1

0.01 0.67

0.92 0.41

Early metamorphic zircon growth Later metamorphic zircon growth

27 4

92 86.4

0.5 1.5

1.8 0.01

0.17 0.93

General age of metamorphism Late-metamorphic Pb-loss

Core 7a All CL-dk rims 3 CL-brt rims 6 rims (& 1 core) Younger CL-brt rims PO6MS Concordia calculations 8 Core 22a 9 10 11 PO6OR 12 13

CL-dark rim 1 CL-bright rim 2 All rims Concordia calculations Cores Rims

14 All spots 15 Younger rims Weighted means of above calculations 16 CL-dark rims 4 & 9 17 CL-bright rims 5 & 10 18 All rims 6,11 & 14 19 Young rims 7 & 15

91.7 92.5 91.4 91.3 86.2

286

than the main rim groups, although these spots belong to both types. This suggests that zircon may have experienced some event at that time, possibly another growth event, or lead-loss related to fluid activity during retrogression. Such influx of aqueous fluid may have been harmful for preservation of coesite inclusions due to enhancement of the kinetics of the coesite-quartz transition (Mosenfelder et al., 2005). Rare-earth-element content of zircon was used to relate the age domains to their metamorphic evolution. It has been widely documented that REE patterns of zircons of all origins are characterized by strong enrichment of HREE over MREE and low LREE contents, while positive Ce anomalies and negative Eu anomalies are nearly ubiquitous in crustal magmatic zircon (Schersteń et al., 2000; Hoskin & Schaltegger, 2003). Metamorphic recrystallization or growth may alter these patterns, depending on the mineral assemblage present in the rock (e.g., Rubatto, 2002; Whitehouse & Platt, 2003).

10

Metamorphic protolith (low Th/U) Metamorphic? protolith (higher Th/U) Reset by metamorphism Early metamorphic zircon growth Later metamorphic zircon growth General age of metamorphism Possible lead loss event

Permian metamorphism (low Th/U) Early metamorphic zircon growth Later metamorphic zircon growth General age of metamorphism

There are clear distinctions between REE patterns of old and young domains in the investigated zircons (Fig. 10). Old cores are characterized by strongly negative Eu anomalies, which indicate the presence of plagioclase during the zircon growth (Rubatto, 2002; Hoskin & Schaltegger, 2003). Moreover, relatively steep HREE patterns suggest absence of garnet and low pressure conditions during the formation of these old zircon domains. In contrast, the young zircon domains show the nearly flat HREE patterns and low (Lu/Gd)N ratios, which indicate that these are of metamorphic origin and formed in the presence of a HREE-bearing phase, most likely garnet. The absence of a strong negative Eu anomaly suggests growth of zircons in the absence of plagioclase. This is corroborated by presence of garnet and absence of plagioclase inclusions in the rims of the investigated zircons. The composition of garnet inclusions with high Mg and Ca contents (Table 2) corresponds to that of matrix garnet cores stable at peak metamorphic conditions (Fig. 6a). Therefore, formation of zircon domains adjacent to such eschweizerbart_xxx


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Fig. 9. Concordia diagrams for ion-probe data for samples (a) JV2/04, (b) PO6MS and (c) PO6OR.

garnets may have occurred at the same P–T conditions (2.2–2.7 GPa; 700–800 C). The spread in (Lu/Gd)N ratios of the zircon rims in each sample (Fig. 11) may be the result of continued garnet growth during formation of the zircon rims, as garnet would take up an increasingly large part of

the HREE budget of the rocks (Rubatto, 2002; Whitehouse & Platt, 2003). However, cores and rims with indistinguishable U-Pb ages do not show systematically different (Lu/Gd)N ratios and therefore the variations more likely represent the sample heterogeneity. eschweizerbart_xxx


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Table 8. Rare earth element data for selected zircons by LA-ICP-MS (concentrations in ppm). Sample

Spot

Domain

Young rims and cores PO6MS 27b Rim 1 PO6MS 14c2 Rim 2 JV2/04 7a Core JV2/04 7b Rim 1 JV2/04 16b Rim 1 PO6OR 10b Rim 1 PO6OR 15a Rim 2 Old cores PO6MS 22a Core JV2/04 2a Core JV2/04 16a Core Undated cores PO6MS 22a* b.s. PO6MS 22b3* b.s. JV2/04 2a* b.s. PO6OR 3b* b.s.

La

Ce

Pr

Nd

Sm

Eu

Gd

Dy

Er

Yb

Lu

0.085 0.056 0.022 0.061 0.009 0.021 0.036

0.47 0.84 0.46 0.68 0.32 0.43 0.72

0.03 0.01 0.002 0.011 0.001 0.007 0.009

0.29 0.2 0.14 0.17 0.12 0.14 0.21

1.21 0.84 0.67 0.71 0.33 0.51 0.79

0.57 0.46 0.48 0.44 0.26 0.3 0.45

7.3 4.2 4.4 4 2.5 3.1 4.5

19.6 11.3 9.8 8.2 10.9 6.8 12

17 10.7 6.9 6.1 14.8 5.3 12.1

21.1 12.3 7.6 7.6 22 6.2 15.1

3.14 1.84 1.08 0.96 3.68 0.96 2.32

(Lu/Gd)N Ce/Ce* Eu/Eu* 3.5 3.5 2 2 11.7 2.5 4.2

2.2 8.4 16 6.3 23 8.3 9.6

0.58 0.74 0.85 0.79 0.85 0.73 0.72

0.048 0.3 0.011 0.17 0.92 0.19 7.8 67 0.021 0.26 0.007 0.21 0.79 0.45 6.5 35 0.126 21.6 0.63 6.2 10.8 2.3 46 207

102 81 437

142 202 838

21 36 154

22 46 27

3.1 5.1 18

0.22 0.6 0.32

0.02 11.6 0.013 0.24 0.073 7.7 0.054 6.4

139 114 206 289

259 181 662 533

49 32 136 95

33 48 199 33

65 6.3 24 22

0.24 0.21 0.72 0.07

0.095 0.007 0.085 0.093

0.92 0.1 0.67 1.19

2.4 0.47 1.27 3.9

0.43 12.2 62 0.11 5.4 52 0.63 5.6 35 0.22 24 137

Elements denoted with subscript N, such as LuN, are normalized to C1 chondrite (values from Anders pffi & Grevesse, 1989). pffi Ce* and Eu* are Ce and Eu values expected for a smooth normalized REE pattern, calculated as Ce* ¼ (LaN PrN) and Eu* ¼ (SmN GdN). *Undated cores represent parts of zircon below the surface (b.s.) that were ablated after penetration of the rim.

Fig. 10. Rare-earth-element patterns of zircons from young domains from samples (A) PO6MS, (B) JV2/04, (C) PO6OR, and (D) old domains of all samples normalized to chondrite (Anders & Grevesse, 1989). The x-distance between rare-earth elements is based on ionic radii of trivalent cations. Composition of zircon 91500 standard (thick grey line) is illustrated for comparison (Wiedenbeck et al., 2004). Grey fields indicate composition of zircon rims from Pohorje eclogites (Miller et al., 2007). Dotted lines in A, B and C represent core domains of metamorphic zircon with U-Pb ages around 91 Ma. Thick solid lines in D indicate old cores (237–287 Ma), whereas dashed lines represent core domains for which no age information is available. eschweizerbart_xxx


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Fig. 11. Plot of (Gd/Lu)N versus Eu/Eu* showing the differences between young and old domains. Low Eu/Eu* and high (Gd/Lu)N values are indicative of zircon growth in the presence of plagioclase and the absence of garnet, i.e. at low pressure conditions. High Eu/ Eu* values and low (Gd/Lu)N indicate that the garnet was a stable phase during zircon growth but plagioclase was not, consistent with high pressure conditions.

7.2. Constraints on the Permian–Triassic protoliths Permian (286 10, 258 7 Ma) to Triassic (238 7 Ma) ages and the composition of three zircon cores may indicate that the gneisses hosting the Pohorje Mts eclogites experienced a metamorphic event at that time. The neighbouring eclogite-bearing units of the Koralpe and Saualpe were affected by a low-P, high-T Permian to Early Triassic metamorphism in the stability field of andalusite and sillimanite (Habler & Tho¨ni, 2001), coeval with intrusions of gabbros (Tho¨ni & Jagoutz, 1992; 1993; Miller & Tho¨ni, 1997) and pegmatites (Tho¨ni & Miller, 1996; Habler & Tho¨ni, 2001) at 275 18 to 255 9 Ma (Fig. 12). This metamorphism and magmatism resulted from rifting of the continental crust (Schuster et al., 2001). A similar low-P, high-T event may have affected the Pohorje area, as indicated by the REE patterns of the inherited zircons, although the investigated gneisses show intense recrystallization during the Cretaceous HP/UHP metamorphism. We propose that the eclogitebearing complexes of Pohorje, together with those in Koralpe and Saualpe, represent the basement of a Permian-age rift within the Austroalpine continent (Fig. 13).

Fig. 12. Diagram of Sm-Nd and U-Pb ages of eclogites, schists and gneisses from Pohorje, and other Austroalpine terranes (A) Pre-Alpine Sm-Nd garnet-whole-rock ages of the Austroalpine basement (Tho¨ni, 2002, and references therein) and Eo-Alpine ages. Error bars indicate 2s uncertainties. (B) Detail of A showing similar Eo-Alpine Cretaceous ages of mafic and continental units. Sm-Nd ages are generally garnet-whole-rock isochrones, whereas U-Pb ages are zircon concordia ages. Data sources: this study; Tho¨ni (2002, and references therein); Miller et al. (2005).

from the proximity of pelitic gneisses (Jana´k et al., 2004; Vrabec, 2004, 2007). We consider these peak P–T estimates for eclogite-hosting metapelites as minima because of the potential for a diffusion-related modification of the garnet composition at high temperatures, and re-equilibration of phengite during early stages of decompression. We infer that pelitic gneisses have experienced the same UHP conditions as neighbouring eclogites. The age of the HP/UHP metamorphism in the studied gneisses, 92–91 Ma, is identical with that of the peakpressure metamorphism in the Pohorje eclogites obtained from zircon (90.7 1.0 Ma, Miller et al., 2005), Sm-Nd garnet-whole rock (90.7 3.9 and 90.1 2.0 M, Miller et al., 2005) and Lu-Hf garnet-whole-rock dating (93.3 2.8 Ma, Tho¨ni et al., 2008). Similar ages are

7.3. Cretaceous UHP metamorphism and exhumation Based on the results obtained from the pseudosection modelling and conventional geothermobarometry we estimate that the investigated metapelites record peak P–T conditions of 2.2–2.7 GPa at 700–800 C. The calculated pressures are lower than those recorded in the kyanite eclogites (3.0–3.1 GPa at 760–825 C) eschweizerbart_xxx

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Fig. 13. Hypothetic cross-sections of the Austroalpine orogen. Patterns are the same as in Fig. 1. (a) Upper profile (beginning of Cretaceous) shows the situation after the arc-continent collision which terminated the subduction of the Meliata Ocean. (b) Middle profile (Late Cretaceous, 100–90 Ma) shows Pohorje garnet peridotites, kyanite eclogites and associated kyanite-garnet gneisses deeply buried in an intra-Austroalpine subduction zone. This new subduction zone formed by lithospheric thrusting within a former Permian rift, possibly reactivating an extensional fault. Exhumation was accommodated by later extraction of the Upper Central Austroalpine lower crustal and mantle wedge. The dashed line delineates the extracted wedge and comprises lithospheric mantle (white), lower crust (grey) of the Upper Central Austroalpine, as well as oceanic crust of the Meliata Ocean (black). (c) Lower profile illustrates the situation after slab extraction. Pohorje rocks have been exhumed to a lower crustal level by the extraction of the overlying mantle wedge combined with minor northwest-directed channel flow of the crust. Subduction of the Penninic Ocean under the Austroalpine continental crust has started. The island arc/backarc basin complex southeast of the Austroalpine crust still exists (Campanian intra-oceanic magmatism, Ustaszewski et al., 2008) and will only be eliminated by continental collision during Maastrichtian to Earliest Palaeocene. Beginning at about the stage shown in the lower profile, E–W stretching of the Austroalpine crust further modified the situation by thinning and disrupting the Austroalpine nappe pile, possibly in response to rollback of the Penninic subduction zone (Froitzheim et al., 1997).

known from the Austroalpine eclogite-bearing units (Koralpe-Saualpe) further to northwest (Fig. 12). The eclogite zircons studied by Miller et al. (2005) were completely digested for ID-TIMS analysis; hence it is possible that some older core material was included. Nevertheless, their ages show little scatter, averaging 90.7 1.0 Ma, which suggests that any older age domains are only marginally older than the rims within the quoted uncertainty of 1 Ma. Further ion-probe dating

of eclogite zircons will be needed to confirm these ideas. The Pohorje eclogite dated by Miller et al. (2005) and Tho¨ni et al. (2008) from a locality near Kebelj, ca. 9 km west of our study area, records maximum P–T conditions of ca. 700 C and 2.5 GPa (Miller et al., 2005). This is within the pressure range determined for metapelitic gneisses in this study. Based on these arguments we infer that the Pohorje eclogites and their country rocks were subducted, reached eschweizerbart_xxx


peak pressure, and were exhumed together as one piece of continental crust. This crust was probably already internally imbricated during the subduction and was intensely sheared during the exhumation, resulting in the presently observed mixture of different lithologies. However, there is no evidence for a me´lange-like assemblage of rocks that followed different metamorphic paths, or even several subduction and exhumation cycles as proposed for subduction zones at active continental margins (Gerya & Sto¨ckhert, 2006). Such complex circulation in a subduction zone may take place during a long-lasting oceanic subduction at active continental margins (as in the numerical models) but not during a short-lived and intra-continental subduction, as in the Austroalpine nappes. The model of complex flow in a subduction channel or ‘‘tectonic accretion channel’’, as proposed for other UHP terrains (e.g., Burov et al., 2001; Gerya et al., 2002), does not apply in the case of the Austroalpine HP/UHP province. We assume that the Pohorje rocks were exhumed from mantle depth to lower/middle crustal depth during the Cretaceous by downward extraction of the overlying wedge consisting of mantle, lower crust, and oceanic crust (Fig. 13b), similar to model of slab extraction (Froitzheim et al., 2003; see also Kurz & Fritz, 2003). Isostasy caused the uplift of the subducted slab, probably in combination with northward channel flow of the hot, low-viscosity crustal rocks (Fig. 13c) as suggested by Tenczer & Stu¨we (2003). Further exhumation was accommodated by the immediately ensuing Late Cretaceous extensional tectonics (e.g., Ratschbacher et al., 1989), leading to rapid, uniform cooling between 86 and 78 Ma in the Saualpe area, northwest of Pohorje (Wiesinger et al., 2006). This deformation likely affected the Pohorje area as well. The final step of exhumation of the Pohorje rocks is related to Miocene rifting of the Styrian Basin and is documented by 15.6–16.9 Ma zircon fission-track data from the Pohorje pluton (Fodor et al., 2008). Our data augment the already existing evidence that the Austroalpine eclogite-bearing units were subducted as a coherent piece of crust (Tho¨ni, 2006; Miller et al., 2007), with the possible exception of the ultramafic bodies with garnet peridotites in the southeastern part of Pohorje Mts., which may have been introduced from the mantle wedge above the subduction zone (Jana´k et al., 2006; De Hoog et al., 2009). The size of the coherent HP to UHP body is about 80 km in SE–NW direction. The increase in peak pressure from NW to SE, reaching the highest values in Pohorje Mts. (Jana´k et al., 2004, 2005, 2006), suggests that the subduction zone dipped in a direction between east and south (Fig. 13). This situation is very similar to that in the Norwegian Caledonides, where the entire Western Gneiss Region was subducted to UHP conditions as a coherent crustal piece (Walsh et al., 2007) and retained the pressure gradient acquired in the subduction zone even after exhumation (Young et al., 2007), although the latter was accompanied by strong internal deformation.

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8. Conclusions Eclogite-hosting metapelites of Pohorje Mts. provide further evidence on deep subduction and exhumation of the continental crust in the Eastern Alps during the Cretaceous orogeny. Metapelitic gneisses record pressures of 2.2–2.7 GPa, but they most likely have experienced the same UHP conditions as neighbouring eclogites. Ion microprobe U-Pb dating of zircon records Cretaceous (92–91 Ma) HP/UHP metamorphism. Permian to Triassic zircon cores suggest that gneisses represent Pre-Mesozoic continental basement. This interpretation is corroborated by trends in the REE composition of the zircons and the composition of garnet inclusions in the zircons. The Pohorje Mts represent the most deeply subducted part of a coherent, ca. 80 km large HP/UHP metamorphic terrane of the Eastern Alps, exhumed by extraction of the overlying lower crust and upper mantle wedge. Acknowledgements: We thank S. Elvevold, J.A. Gilotti and R.P. Wintsch for their constructive comments, C. Storey for assistance with laser ablation analysis, F. Finger for whole-rock analysis and M. Whitehouse and the NordSIM staff for their support. This work was financially supported by the Slovak Research and Development Agency (project APVV-51-046105), and the VEGA Scientific Grant Agency (grant No. 2/0031/09). The NordSIM facility is supported by the research councils in Denmark, Norway and Sweden and the Geological Survey of Finland, together with the Swedish Museum of Natural History. This is NordSIM contribution no. 237.

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Received 15 January 2009 Modified version received 15 April 2009 Accepted 15 June 2009 eschweizerbart_xxx