Lu–Hf Dating, Petrography, and Tectonic Implications of the Youngest Alpine Eclogites (Tauern Window, Austria)

Lithos 170–171 (2013) 179–190

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Lu–Hf dating, petrography, and tectonic implications of the youngest Alpine eclogites (Tauern Window, Austria)

T.J. Nagel a,⁎, D. Herwartz b,1, S. Rexroth c,2, C. Münker d,3, N. Froitzheim c,4, W. Kurz e,5 a Universität Bonn, Steinmann-Institut, Poppelsdorfer Schloss, D-53115 Bonn, Germany b Georg-August-Universität, Geowissenschaftliches Zentrum, Abteilung Isotopengeologie, Goldschmidtstraße 1, D-37073 Göttingen, Germany c Steinmann-Institut, Universität Bonn, Poppelsdorfer Schloss, D-53115 Bonn, Germany d Universitat Köln, Geowissenschaften, Geo-/Kosmochemie, Greinstr. 4-6, Gebäude 902, D-50939 Köln, Germany e Karl-Franzens Universität Graz, Institut für Erdwissenschaften, Heinrichstraße 26, A- 8010 Graz, Austria article info abstract

Article history: Isotopic dating of metamorphic minerals places fundamental constraints on the rates and mechanisms of Received 17 November 2012 burial and exhumation in collisional orogens. The Eclogite Zone in the Tauern Window has been the focus Accepted 17 February 2013 of many studies on subduction-related high-pressure metamorphism. However, the age and duration of Available online 1 March 2013 the high-pressure stage remains the subject of ongoing debate. 32 Ma Sr–Rb ages interpreted to date eclogite-facies metamorphism (Glodny et al., 2005) appear too young in traditional tectonic reconstructions Keywords: of plate collision in the Alps. These ages have either been interpreted to indicate extremely rapid exhumation Eclogite Garnet from more than 60 kilometre depth to mid-crustal levels within 1 Ma years or to date retrogression subse- Lu–Hf dating quent to high-pressure metamorphism. Alps We present element distribution maps and lutetium–hafnium (Lu–Hf) garnet ages of three samples from the Tauern Window Eclogite Zone. All samples display almost unaltered eclogite-facies assemblages and garnets preserve growth zoning. Lu–Hf ages are thus considered as formation ages recording metamorphism towards peak-pressure conditions. In the sample with the smallest grain size, garnet shows regular bell-shaped element distributions with respect to manganese and the iron–magnesium ratio. A six-point isochron of this sample yields 32.8 ± 0.5 Ma (MSWD = 1.06), interpreted as the age of Alpine eclogite-facies metamorphism. In one of the other two, coarser-grained samples' garnet chemistry is identical. The third sample, however, shows complex zoning in large garnet crystals. Cores with a very low iron-magnesium ratio are surrounded by a second garnet generation which is very similar to the Alpine generation in the other two samples. The two coarser-grained samples yield scattered ages between 26.9 ± 9.8 Ma and 62.7 ± 1.8 Ma for individual garnet-whole-rock pairs as the analysed garnet fractions display very different 176Hf/177Hf vs. 176Lu/177Hf ratios. This scatter reflects varying degrees of mixing between Alpine and pre-Alpine garnet fractions as repre- sented by the cores of the third sample. The results confirm the Rb–Sr-whole rock ages of Glodny et al. (2005). Despite the problems this result causes for conventional tectonic reconstructions, the eclogites from the Eclogite Zone in the Tauern Window have to be considered as Lower Oligocene in age and are thus the youngest eclogites of the Alps identified so far. © 2013 Elsevier B.V. All rights reserved.

1. Introduction metamorphic cycle, with well-preserved high-pressure equilibration under eclogite-facies conditions followed by regional Barrovian over- The Tauern Window in the Eastern Alps (Austria) is a classical area print at greenschist- to amphibolite-facies conditions. In the Tauern of orogenic metamorphism. Rocks document the complete Cenozoic Window, eclogite-facies assemblages attributed to the Alpine orogeny occur in different tectonic levels: as relics in the Glockner Nappe and the Rote-Wand-Modereck Nappe (Dachs and Proyer, 2001; Kurz ⁎ Corresponding author. Tel.: +49 228 732760; fax: +49 228 732763. E-mail addresses: [email protected] (T.J. Nagel), [email protected] (D. Herwartz), et al., 2008) and almost pervasively in the Eclogite Zone (Holland, [email protected] (S. Rexroth), [email protected] (C. Münker), 1979; Hoschek, 2001; Miller, 1977)(Fig. 1). In the Glockner Nappe [email protected] (N. Froitzheim), [email protected] (W. Kurz). eclogites occur in Mesozoic host rocks and are thus of unambiguous 1 Tel.: +49 551 395253; fax: +49 551 3910452. Alpine origin, whereas the age of eclogite-facies conditions in the 2 Tel.: +49 228 732761; fax: +49 228 732763. Rote-Wand-Modereck Nappe is only poorly constrained. Based on 3 Tel.: +49 221 4703198; fax: +49 221 4705199. 4 Tel.: +49 228 732463; fax: +49 228 732763. several datasets summarized below, high-pressure metamorphism 5 Tel.: +43 316 3805588; fax.: +43 316 3809870. in the Eclogite Zone is now generally accepted to be Alpine, i.e. Eocene

Cenozoic a 50 km intrusives Upper c Austroalpine Helvetic and Penninic units Lower NNW SSE Southalpine Austroalpine A Gr. Venediger A' 4000 m Win 2000 m x uern dow Graz Ta 0 m

Maribor Bolzano Periadriatic lineament N 10 km d 47°20’N Klammkalk A b Matrei Zone

Glockner Nappe

Fig. 2 Rote Wand - Modereck Nappe 47°00’N A' Wolfendorn Nappe Eclogite Zone Storz Nappe 12°00’E 13°00’E Venediger Nappe

Fig. 1. Tectonic sketches illustrating the location and structural position of the eclogite zone. a Sketch map of the Eastern Alps. b Tectonic map of the Tauern Window. c Cross section through the Tauern Window. d Tectono-stratigraphic column of nappes exposed in the Tauern Window (modified after Kurz et al., 2008). or Early Oligocene in age. Several studies used this zone as a natural 2. Geological setting laboratory to develop models for high-pressure rock exhumation (e.g. Behrmann and Ratschbacher, 1989; Kurz, 2005; Kurz et al., The Tauern Window is defined by large-scale exposure of 1998a; Neufeld et al., 2008). Attempts to reconstruct the scenario of Subpenninic and Penninic nappes framed by the overlying Austroalpine exhumation, however, are limited by the uncertainty about the exact units (Fig. 1). The Subpenninic and Penninic nappes represent the age of eclogite-facies metamorphism, and, in consequence, the rates lower plate of the north-verging, Cenozoic Alpine orogen while the of subduction and exhumation. Austroalpine units are derived from the upper plate. Subpenninic and Based on K/Ar data from amphibole, it was originally thought that Penninic units were imbricated by south-over-north directed thrusting the pressure peak was Cretaceous (Raith et al., 1978), and the Barrovian and later overprinted by a second phase of steep thrusts leading to an overprint Tertiary in age which implied extremely slow exhumation. antiformal stack geometry particularly in the lower basement units. Zimmermann et al. (1994) determined Late Eocene to Oligocene ages The lowermost sheet is the Venediger Nappe (Subpenninic), derived (36 to 32 Ma) using 40Ar/39Ar geochronology and interpreted these to from the European continental margin. It is exposed in antiformal fold date a blueschist-facies stage after the eclogite-facies pressure peak. cores in the centre of the window and comprises Late Variscan granit- Other 40Ar/39Ar studies, however, suggested that exhumation after oids (the Zentralgneis), their older country rocks including probably peak-pressure conditions took place at 42 to 39 Ma (Ratschbacher et pre-Alpine eclogites (Zimmermann and Franz, 1989), and Late Carbon- al., 2004). Glodny et al. (2005) employed multi-mineral Rb–Sr dating iferous to Mesozoic sedimentary cover rocks (Vesela and Lammerer, and determined an Oligocene age for eclogite formation, 31.5 ± 0.7 Ma, 2008). Parts of the continental margin have been sheared off and thrust the youngest age for eclogites in the Alps. In contrast, Kurz et al. (2008) towards north over the Venediger Nappe to form the Storz Nappe, the suggested, based on more 40Ar/39Ar data, that Glodny et al. (2005) Wolfendorn Nappe, and some other, similar units (Fig. 1). The next dated a stage of exhumation, and that the pressure peak was already higher sheet is the Eclogite Zone which is only present in the central reached prior to 38 Ma. Recently, Smye et al. (2011) determined a U/Pb part of the Tauern Window south of the Zentralgneis cores (Figs. 1 and age of 34.2 ± 3.6 Ma on allanite grown before the pressure peak, giving 2). It is commonly attributed to the Penninic nappes. The Eclogite Zone support to the age of Glodny et al. (2005). is a mixture of various strongly sheared and folded rock types: eclogite, Here, we present Lu–Hf garnet-whole-rock ages of three eclogite serpentinite, garnet micaschist, quartzite, marble, and calcschist. Most samples from the Eclogite Zone in order to contribute to solving the authors assume that the Eclogite Zone has been derived from transition- age controversy about eclogite-facies metamorphism in the Tauern Win- al crust located between the European continental margin and the ocean dow. This method is particularly useful because the formation of garnet basin following to the South in Mesozoic times (e.g., Kurz et al., 1998b). can be firmly attributed to a certain segment of the P-T path, that is, in a Above the Eclogite Zone follows another sheet of continental basement clockwise PT-evolution to the prograde branch of the path (e.g. Herwartz and its Mesozoic sedimentary cover, the Rote-Wand-Modereck Nappe et al., 2008; Klemd et al., 2011). Metamorphic temperatures in the (Kurz et al., 1998b). This is in turn overlain by the Glockner Nappe Eclogite Zone are 600–630 °C (Hoschek, 2001) and thus did not exceed which consists mainly of serpentinite, greenschist, amphibolite, and the closure temperature of the Lu–Hf system in garnet (Carlson, 2012; calcschists. As mentioned above, the Rote-Wand-Modereck Nappe and Shu et al., 2012; Skora et al., 2008). Hence, Lu–Hf ages can be assumed the Glockner Nappe both contain scarce relics of eclogites. The Glockner to date the growth of garnet. Our results confirmtheOligoceneageof Nappe is generally assumed to be derived from a Mesozoic ocean basin. eclogite-facies metamorphism. Furthermore, two of the three eclogite The uppermost unit, exposed along the southern border of the Tauern samples contain an older, pre-Alpine, garnet component, and thus repre- Window, is the Matrei Zone, a mixed zone with oceanic rock types sim- sent recycled Variscan crust or Permian mafic intrusions. ilar to the Glockner Nappe and continental slivers derived from the T.J. Nagel et al. / Lithos 170–171 (2013) 179–190 181

Venediger Nappe Complex Dabernitzkogel Eclogite Zone

FRT 2 FRT 5 & FRT 8 Rote Wand - Modereck Nappe Eissee Glockner Nappe Frosnitztal

Matrei Zone Timmeltal

Thrust Felbertal

Prägraten

0 5 km Virgen Matrei

Fig. 2. Geological sketch map of the southern central Tauern Window (after Kurz et al., 2008) with the sample locations (FRT2: 47°04′20.36″N/12°27′01.44″E; FRT5, FRT8: 47°03′ 56.20 N/12°27′17.37″E).

overlying Austroalpine nappes. The unroofing of the window occurred et al., 2004). Hence, 32 Ma old eclogite-facies metamorphism in the mainly in Miocene times in a setting of N–S shortening and E–W exten- Eclogite Zone would require major modifications of existing tectonic sion, though the respective contributions of orogen-parallel extension reconstructions. and orogen-perpendicular compression and associated erosion are a The three samples presented here are from the Eclogite Zone in matter of debate (Ratschbacher et al., 1991; Rosenberg and Garcia, the Frosnitz Valley (Frosnitztal; Fig. 2). In this unit kyanite eclogites 2011; Rosenberg et al., 2004; Selverstone, 1988). The Miocene doming and high-pressure garnet-kyanite-phengite schist are particularly and the associated formation of large-scale antiformal structures in the well-preserved and have been studied for decades (Holland, 1979; Subpenninic nappes clearly postdate the formation of the principal Hoschek, 2001, 2007; Hoschek et al., 2010; Miller, 1977; Selverstone nappe stack presented above (Fig. 1d). et al., 1992; Stöckhert et al., 1997; Zimmermann and Franz, 1989; Eclogite-facies metamorphism in the Eclogite Zone, the Rote-Wand- Zimmermann et al., 1994). These two rock types underwent the Modereck Nappe, and the Glockner Nappe is generally attributed to the same P–T path, culminating at 600–630 °C and 20 –25 kbar which southward subduction of the Mesozoic oceanic domain between suggests that the Eclogite Zone remained a coherent rock slice during Europe and Adria which led to the closure of the ocean and the subduc- the Alpine tectonic history. tion of the European margin in Cenozoic times. As summarized in the Introduction, a Cenozoic age of high-pressure metamorphism in the Eclogite Zone as part of the European margin is now generally accepted. Table 1 However, the Oligocene age proposed by Glodny et al. (2005) and Smye Bulk chemical compositions of the three investigated samples. et al. (2011) has been called into question as it is hard to incorporate in Sample FRT5 FRT8 FRT2 tectonic reconstructions (e.g. Handy et al., 2010; Kurz et al., 2008). The SiO2 (%) 46.46 47.09 50.32 reason for this is that classic schemes picture the Alps in an advanced Al2O3 (%) 18.79 15.41 17.23 state of collision at the Eocene–Oligocene boundary. Several events in Fe2O3 (%) 8.88 10.74 10.34 the Tauern Window and in its surroundings which are viewed as MnO (%) 0.14 0.15 0.14 postdating exhumation and major thrusting, have been dated at the MgO (%) 4.78 8.99 7.18 CaO (%) 12.41 10.30 7.55 same or very similar Oligocene ages. Glodny et al. (2005, 2008) them- Na2O (%) 3.54 2.46 4.10 – selves present identical Rb Sr internal isochron ages between 32 Ma K2O (%) 0.24 0.45 0.87 and 30 Ma for eclogite facies assemblages in the Eclogite Zone and for TiO2 (%) 1.35 1.58 1.50 clearly post-eclogitic amphibolite-facies ductile shear zones in various P2O5 (%) 0.12 0.13 0.20 structural levels throughout the Tauern Window. This coincidence SO3 (%) 0.00 0.22 0.09 LOI (%) 3.18 2.75 0.30 could either indicate extremely rapid exhumation from eclogite- to Sum (%) 99.89 100.27 99.82 amphibolite-facies conditions (Glodny et al., 2008) or simply a common Ba (ppm) 203 232 403 retrogression event being recorded in the Rb–Sr ages which would be Co (ppm) 28 36 33 younger than both eclogite-facies metamorphism and amphibolite- Cr (ppm) 163 377 314 Cu (ppm) 55 50 36 facies shearing (Kurz et al., 2008). The Rieserferner and Rensen Plutons Ga (ppm) 24 22 27 emplaced in the Austroalpine nappe stack immediately south of the Nb (ppm) b20 b20 27 Tauern Window are also of the same age, i.e. 32 Ma (Barth et al., Ni (ppm) 65 307 49 1989; Borsi et al., 1979; Romer and Siegesmund, 2003), and belong to Pb (ppm) b20 b20 b20 b b the 32–30 Ma old suite of intrusions found all along the Periadriatic Rb (ppm) 20 20 34 Sc (ppm) 32 24 27 Line (Rosenberg, 2004). In most tectonic schemes, these intrusions Sr (ppm) 160 296 210 are considered to be syncollisional and to postdate stacking of the V (ppm) 153 174 167 Penninic-Subpenninnic nappe edifice, because the Periadriatic fault ap- Y (ppm) 39 36 39 pears to (1) crosscut the entire nappe pile and to (2) control magma as- Zn (ppm) 69 87 80 Zr (ppm) 128 160 164 cent and emplacement (Pomella et al., 2011; Rosenberg, 2004; Stipp 182 T.J. Nagel et al. / Lithos 170–171 (2013) 179–190

Table 2 Selected microprobe analyses from minerals in the three investigated samples.

FRT5 FRT2 FRT8

phe czo am omp omp pa dol phe zo/ czo am omp ab pl mrg bt phe zo/ czo am omp tlc czo czo

SiO2 52.32 38.61 50.84 56.75 55.46 48.7 0.21 53.52 39.30 38.28 51.09 56.54 66.71 62.10 32.32 35.9 52.03 39.10 38.61 51.39 56.73 62.26 TiO2 0.29 0.24 0.11 b.d. 0.03 0.13 b.d. 0.26 0.08 0.12 0.11 0.05 0.01 0.03 0.03 0.44 0.27 0.11 0.24 0.13 0.00 0.08 Al2O3 27.95 30.28 11.36 13.24 9.99 39.9 0.00 27.96 32.70 28.62 8.70 14.28 20.96 23.74 49.27 20.2 28.57 32.58 30.28 10.49 12.61 0.65 FeO* 1.15 4.96 10.92 3.51 8.95 0.42 8.35 0.93 1.04 5.16 6.24 2.30 0.11 0.06 0.30 14.2 0.94 1.22 4.96 8.54 2.45 3.18 MnO 0.03 0.17 0.06 b.d. b.d. b.d. 0.06 b.d. 0.04 b.d. 0.05 0.03 0.02 b.d. 0.04 0.2 0.01 b.d. 0.17 b.d. 0.03 0.04 MgO 3.75 0.19 12.33 7.24 6.33 0.19 16.75 3.92 0.04 0.14 16.98 7.14 0.08 b.d. 0.48 14.1 3.68 0.19 0.19 14.79 8.60 28.65 CaO 0.00 23.58 6.27 11.26 10.39 0.3 28.36 0.05 24.66 23.63 10.56 10.71 1.77 5.06 10.22 b.d. b.d. 24.46 23.58 8.53 12.83 0.01 Na2O 0.55 0.03 5.14 8.56 8.58 6.65 b.d. 0.74 0.04 b.d. 2.89 8.71 11.06 8.95 2.10 0.39 0.61 0.03 0.03 3.46 7.56 0.01 K2O 9.62 b.d. 0.18 0.01 0.01 0.72 b.d. 9.61 0.02 0.05 0.29 b.d. 0.09 0.11 0.17 9.82 9.07 0.01 0.00 0.20 0.02 b.d. Cr2O3 0.04 0.09 0.01 0.04 b.d. 0.06 b.d. 0.02 0.01 b.d. 0.01 0.03 b.d. 0.01 0.00 0.02 0.06 0.07 0.09 b.d. 0.04 0.01 Sum 95.71 98.13 97.21 100.61 99.73 97.06 53.72 97.02 97.92 96.00 96.92 99.77 100.80 100.07 94.94 95.28 95.24 97.78 98.13 97.51 100.87 94.87 Si 6.86 3.00 7.25 1.99 2.02 6.08 0.01 6.91 3.00 3.04 7.23 1.98 2.91 2.75 4.29 5.34 6.83 2.99 3.00 7.24 1.98 8.00 Ti 0.03 0.01 0.01 0.00 0.00 0.01 0.00 0.03 0.00 0.01 0.01 0.00 0.00 0.00 0.00 0.05 0.03 0.01 0.01 0.01 0.00 0.01 Al 4.32 2.77 1.91 0.55 0.43 5.88 0.00 4.25 2.94 2.68 1.45 0.59 1.08 1.24 7.70 3.54 4.42 2.93 2.77 1.74 0.52 0.10 Fe 0.13 0.32 1.30 0.10 0.27 0.04 0.22 0.10 0.07 0.34 0.74 0.07 0.00 0.00 0.03 1.76 0.10 0.08 0.32 1.01 0.07 0.34 Mn 0.00 0.01 0.01 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.01 0.00 0.00 0.00 0.00 0.03 0.00 0.00 0.01 0.00 0.00 0.00 Mg 0.73 0.02 2.62 0.38 0.34 0.04 0.80 0.75 0.00 0.02 3.58 0.37 0.01 0.00 0.10 3.13 0.72 0.02 0.02 3.10 0.45 5.49 Ca 0.00 1.96 0.96 0.42 0.40 0.04 0.97 0.01 2.01 2.01 1.60 0.40 0.08 0.24 1.45 0.00 0.00 2.00 1.96 1.29 0.48 0.00 Na 0.14 0.00 1.42 0.58 0.60 1.61 0.00 0.19 0.01 0.00 0.79 0.59 0.94 0.77 0.54 0.11 0.15 0.00 0.00 0.94 0.51 0.00 K 1.61 0.00 0.03 0.00 0.00 0.12 0.00 1.58 0.00 0.01 0.05 0.00 0.00 0.01 0.03 1.86 1.52 0.00 0.00 0.04 0.00 0.00 Cr 0.00 0.01 0.00 0.00 0.00 0.01 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.01 0.00 0.01 0.00 0.00 0.00 Sum 13.82 8.10 15.51 4.03 4.07 13.82 1.99 13.82 8.03 8.11 15.46 4.01 5.02 5.01 14.14 15.82 13.77 8.04 8.10 15.37 4.01 13.94 O 22.00 12.50 23.00 6.00 6.00 22.00 4.00 22.00 12.50 12.50 23.00 6.00 8.00 8.00 22.00 22.00 22.00 12.50 12.50 23.00 6.00 22.00

3. Sample petrology corresponding higher aegirine content (Table 2) which results in a patchy zoning (Figs. 3 and 4; Table 2).Thesampledisplaysa Microprobe and X-ray flourescence and microprobe data (Tables 1 well-developed foliation defined by elongated omphacite, aligned zois- and 2) were obtained at the Steinmann-Institut Bonn using a ite/clinozoisite, phengite, and amphibole as well as a certain degree of PANalytical-Axios and a JEOL-JXA-8900, respectively. All three eclogite compositional layering of relatively dolomite-rich and relatively samples FRT2, FRT5, and FRT8 (Fig. 3) are fresh, i.e. they show only garnet-rich domains. Garnets with slightly elongate grain shapes also minor retrogression under post-eclogite-facies conditions. Their miner- show a preferred orientation with their long axes parallel to the folia- alogical appearance is very similar to assemblages described in other tion (Fig. 3; the foliation has a vertical orientation in this figure). The studies on the Eclogite Zone referenced above. In hand specimen, all sample contains about 5% of iron-rich dolomite and 10% of zoisite/ three samples display abundant euhedral garnet in a mint-green matrix clinozoisite which is more than in the other samples. Abundant of omphacite, varying amounts of white mica, bright clinozoisite, and paragonite, partly containing relics of kyanite, postdates the foliation some dark blue amphiboles. They differ, however, in grain size with while silicon-rich phengite is aligned parallel to it. Rare amphiboles FRT2 having the largest and FRT5 the smallest grain size. Bulk chemical are rich in pargasite and glaucophane component and euhedral, i.e. compositions of major and trace elements are generally in agreement they are not associated with symplectitic breakdown of omphacite. with basaltic protoliths as for instance indicated by their magnesium The sample contains some quartz typically in the pressure shadows of numbers (56–66) and chromium and nickel contents (163 ppm– garnet but also dispersed. Formation of the foliation clearly occurred 377 ppm, 49 ppm to 307 ppm, respectively) (Table 1). Slightly elevated under high-pressure conditions, as the sample contains abundant contents of yttrium (36–39 ppm) and zirconium (128 ppm–164 ppm) synkinematic omphacite, silicon-rich phengite, sodium-rich amphibole, together with low niobium contents, suggest affinities towards Phanero- and no feldspar. Garnets do not show any sign of resorption. Dolomite zoic MORBs, but altogether the data does not allow to clearly distinguish and zoisite/clinozoisite display straight grain bondaries against garnet between different tectonic settings. The mineralogical and structural ap- and omphacite. They were probably both stable under eclogite-facies pearance described below also points towards a certain degree of chem- conditions and are part of the high-pressure assemblage. Only ical alteration during high-pressure metamorphism that may have paragonite and some of the clinozoisite minerals appear to be retrogres- changed primary compositions. For example, the low SiO2 contents of sive, a view also supported by observations in the other samples. samples FRT5 and FRT8 may be explained by mobilization of SiO2,assug- Selverstone et al. (1992) describe very similar samples from the gested by abundant quartz-kyanite segregations. The high loss on igni- Frosnitztal in detail and also conclude that these rocks underwent al- tion in the same samples corresponds to a few percent of dolomite in most no overprint after peak-pressure conditions of 625 °C and 20 kbar. the high-pressure assemblage. Sample FRT2 is much coarser grained than FRT5 with garnets up to a few millimetres and omphacite up to 1 cm in diameter. These two 3.1. Mineralogy minerals together with paragonite and phengite make up the bulk of the rock with additional minor amounts of zoisite/clinozoisite, plagio- Sample FRT5 (Fig. 3 a and b) corresponds very much to the banded clase, albite, amphibole, quartz, rutile, margarite, biotite, and apatite. maficeclogitesdescribedinSelverstone et al. (1992).Itcontainsas Randomly oriented paragonite locally contains relics of corroded kya- rock-forming minerals garnet, omphacite, zoisite/clinozoisite, paragonite, nite and a thin rim of clinozoisite. Most of the other minerals define a phengite, quartz, and dolomite and small amounts of kyanite, amphi- weak foliation. Large, slightly elongated omphacite phenocrysts are bole and rutile. It is the most fine-grained of the studied samples with surrounded by a network of fine-grained omphacite and the other perfectly euhedral garnets up to 500 μm in diameter. The sample con- minerals (Fig. 3c and d). Omphacite phenocrysts display clear and tains abundant jadeite-rich omphacite which appears dynamically shaded domains. The shaded domains are related to microinclusions, recrystallized. It shows older domains with a jadeite component up to probably rutiles exsolved from magmatic clinopyroxe on its transfor- 0.55 crosscut by a network of omphacite with a lower jadeite and mation into omphacite. Plagioclase and albite are clearly retrogressive T.J. Nagel et al. / Lithos 170–171 (2013) 179–190 183

aabb

ccdd

eeff

Fig. 3. Photo micrographs of the three investigated samples FRT5 (a and b), FRT2 (c and d), and FRT8 (e and f). Photos a, c, and e using regular, polarized light, photos b, d, and f show the same locations, respectively, under cross-polarized light. (cpx, clinopyroxene; czo, clinozoisite; grt, garnet; ky; kyanite; omp, omphacite; pa, paragonite; phe, phengite; qtz, quartz; sym, symplectite). and exclusively occur together with amphibole or clinopyroxene 3.2. Garnet chemistry and garnet inclusions in fine-grained symplectites which locally replace omphacite. Also small flakes of biotite and margarite are probably post-eclogitic. The Chemical zoning and the identity and distribution of inclusions microstructures of this sample denote a coarse-grained, gabbroic in garnet were studied in detail for each of the samples (Figs. 4 host rock. Of the three investigated samples, FRT2 seems the chemi- to 7). Elemental x-ray maps (Figs. 4, 6, and 7) were obtained with cally least altered as it does not contain dolomite as a matrix phase. the microprobe using a sample current of 5 ∗ 10−8 Ampere, an accel- The chemical and mineralogical composition of the last sample, eration voltage of 15 kV and a dwell time of 100 ms per spot. FRT8, is intermediate between the two other ones. It contains fairly Garnets in sample FRT5 (Fig. 4) are euhedral and contain few in- large omphacite and euhedral garnet up to 1 mm and 500 μm, respec- clusions, mostly quartz, zoisite/clinozoisite, and rutile. They display tively, in size (Fig. 3e and f). Abundant paragonite locally contains ky- typical prograde zoning with decreasing manganese content and de- anite relics but also large euhedral kyanite has been preserved. Zoisite/ creasing iron–magnesium ratio from core to rim (Fig. 5). The outer- clinozoisite, sodium-rich amphibole, dolomite, phengite, and rutile most rim shows a marked decrease in iron, a corresponding occur as minor phases and feldspar is very rare. Different from the pre- increase in magnesium and slightly oscillatory manganese zoning. vious samples, FRT8 contains a few percent of talc, mostly present as Calcium displays relatively little variation with only a minor increase domains of small, randomly oriented crystals but also dispersed in at an intermediate growth stage. A slight reequilibration at a later the matrix and intergrown with phengite. The rock displays a weak fo- metamorphic stage is indicated by higher magnesium- and liation defined by elongate omphacite, and aligned phengite, zoisite/ manganese- and lower iron-contents along probable microcracks. clinozoisite and kyanite. Again, growth of paragonite seems to post- By and large, however, garnet in this sample preserves its prograde date the foliation. growth history. The garnet crystal in Figs. 3 and 4 is in direct contact 184 T.J. Nagel et al. / Lithos 170–171 (2013) 179–190

80 AA’ XFe/(XFe+XMg) A’ 60 XFe 40 FRT 5 XCa 20 Mg XMn X 0 A 0.35 mm AA’ 0.2 mm Fe Mg 80

40 FRT 2

0 2.1 mm

80 AA’

Ca Mn 60

40 Fig. 4. Element map of major bivalent cations in garnet from a typical euhedral garnet in FRT 8 sample FRT5, also displayed in Fig. 3a and b. Warm colours correspond to high contents, cold colours to low contents. The scale was adjusted to optimize visibility. Garnet preserves 20 typical growth zoning with decreasing manganese content and decreasing iron–magnesium ratio from core to rim. Only very slight retrogression occurs along microcracks. Line – ′ fi 0 A A indicates pro le of quantitative microprobe measurements displayed in Fig. 5. 0.63 mm with patchy omphacite in the upper right and lower left of the figure Fig. 5. Profiles of quantitative analyses through garnets displayed in element maps (Figs. 4, and we therefore interpret it to be part of the peak-pressure 6, and 7). Y-axis shows fractions of major bivalent cations and the Fe/(Fe + Mg) ratio. Gaps assemblage. correspond to inclusions as can be seen on the respective element map. Garnets of samples FRT5 and FRT8 show regular bell-shaped element distributions pointing to a monocyclic Garnets in sample FRT2 preserve a complex multi-stage growth his- PT-evolution. Sample FRT2 contains erratic cores with a very low Fe/(Fe + Mg) ratio, tory (Fig. 6). Large euhedral crystals in some cases enclose composite interpreted to represent relics from an earlier metamorphic cycle. cores of an earlier, chemically distinct garnet generation 1 together with quartz, apatite and paragonite. Garnets in these cores are locally rectangular as if pseudomorphic but locally also irregularly-shaped garnet generation 2 started growing once this phase was completely and partially resorbed (Fig. 6). They are unzoned and their composition consumed. Two-stage garnet growth along a simple prograde P-T path is particularly poor in iron, rich in magnesium and relatively rich in is common in garnets from eclogites and also predicted by thermody- manganese. Overgrowths over these composite cores are formed by a namic modelling (e.g. Konrad-Schmolke et al., 2008). However, the second garnet generation 2. This is characterized by a typical mono- first prograde garnet in mafic compositions should display the highest cyclic zoning similar to the pattern observed in sample FRT5. Garnet iron-magnesium ratio and would typically be zoned (e.g. Konrad- generation 2 starts with high and outward decreasing manganese Schmolke et al., 2005). Also the spike in manganese observed in the ear- and calcium contents and displays regular outward decreasing iron- liest portion of generation 2 in sample FRT2 is hard to explain in terms magnesium ratios (Fig. 5). At the boundary between garnet generations of a simple prograde evolution. Hence, we prefer the interpretation of 1 and 2, a sharp rise of the iron, manganese and calcium contents and a garnet generation 1 as a relic. sharp drop of the magnesium content are observed (Fig. 5). Inclusions Garnet generation 2 probably grew during the Alpine history to- in garnet generation 2 are quartz, zoisite/clinozoiste, paragonite, wards peak-pressure conditions and this metamorphic cycle did not phengite, sodium-rich amphibole, omphacite, rutile, and apatite. reach temperatures at which garnet would have lost its growth Zoisite/clinozoisite and paragonite occur only in the earlier portion of zoning through diffusive reequilibration. This interpretation is also sup- garnet generation 2. We interpret garnet generation 1 to represent a ported by the isotopic data presented below. For comparison, eclogites relic from an earlier metamorphic event and attribute the absence of from the Adula Nappe in eastern Switzerland which is in a tectonic po- any growth zoning to diffusion after garnet growth. The diffusive oblit- sition identical to the one of the Eclogite Zone, display a similar eration of zoning must have happened during the first metamorphic two-stage garnet growth history with pre-Alpine relics preserved in cycle still, as garnets of the second generation show almost no signs of overall Alpine eclogites (Herwartz et al., 2011). We emphasize that diffusive reequilibration. Therefore, the first metamorphic event was most garnet in sample FRT2 belongs to the second, Alpine garnet gener- probably associated with rather high temperatures. An alternative ex- ation and only the large grains show the relic cores described above. planation is that garnet generation 1 was associated with the break- In sample FRT8, mapped garnets display inclusions and a zoning pat- down of a particular phase during the early Alpine evolution, and that tern very similar to the ones in sample FRT5. They show the same hump T.J. Nagel et al. / Lithos 170–171 (2013) 179–190 185

A’ 2 mm Fe Mg

Ca Mn

Fe Mg

Ca Mn

Fig. 6. Element maps of major bivalent cations in garnet from sample FRT2. Four top panels show a relatively large garnet with a composite core of a garnet generation1, quartz, paragonite, and apatite (white in calcium map). As an overgrowth over this core a garnet generation 2 is seen. This shows a chemical zoning similar to garnets in sample FRT5 (Fig. 4). Four lower panels are a higher resolution map of the composite core. Note that garnet generation 2 starts with a sharp rise in iron, calcium and manganese and a sharp drop in magnesium. 186 T.J. Nagel et al. / Lithos 170–171 (2013) 179–190

A’ 0.5 mm Fe Mg

Ca Mn

Fig. 7. Element maps of major bivalent cations in garnet from sample FRT8. Compositional zoning in garnet is very similar to the one in sample FRT5 but less affected by retrogression as the crystal is larger. of manganese in the core, the outward decreasing iron-magnesium Garnet separates and whole rock powders were spiked with a mixed ratio and the slight rise of calcium at an intermediate growth stage 176Lu–180Hf tracer, prior to sample digestion. Two different procedures (Figs. 5, 7). Inclusions are quartz, zoisite/clinozoisite, rutile and some were applied: (1) The garnet and one set of whole rock powders were phengite. As in sample FRT5, garnets are affected by subsequent diffu- digested using a selective tabletop procedure which efficiently dissolves sive reequilibration along microcracks. However, as the grain shown the target phase (e.g. garnet), but leaves behind microscopic refractory in Fig. 7 is larger than the ones in FRT5 (Fig. 4), the relative effect is Hf-bearing phases such as zircon (Herwartz et al., 2008, 2011; Lagos et smaller. We interpret the garnet displayed in Fig. 7 to belong to the Al- al., 2007). The samples were attacked by a mixture of HF:HNO3 (2:1) in pine metamorphic cycle. We did not find relic garnet cores as in sample closed Teflon vials on 120 °C hotplates overnight. Subsequently, be-

FRT2. The average crystal size in this sample, however, is much larger tween 0.5 and 1 ml of HClO4 was added to the samples which were than in sample FRT5 and the existence of such cores appears more likely then dried down and re-dissolved in 6 N HCl. In this study the digestion than in the consistently ﬁne-grained sample FRT5. Though these cores procedure was repeated at least one more time for all samples, to en- were not found in sample FRT8 by microprobe analyses, the isotopic sure full digestion of the target phase. Column chemistry was carried data presented below point to the presence of pre-Alpine relics also in out once sample solutions were clear and assumed to be fully equilibrat- this sample. ed with the 176Lu–180Hf tracer. (2) One whole rock powder split of each sample, as well as the solid leftovers from the tabletop procedure (‘Res- 4. Lu–Hf geochronology idues’) were digested by a second protocol which efﬁciently dissolves theentiresample(Herwartz et al., 2008, 2011; Lagos et al., 2007). Fol- 4.1. Sample preparation lowing a table-top technique as described above, the vials were placed in steel-jacketed PARR bombs at 180 °C for 72 h, again using a mixture

Sample preparation and the analytical protocol for the isotopic in- of HF:HNO3 (2:1). Once more, HClO4 was added to the samples, after vestigation are similar to previous publications (e.g. Herwartz et al., which the samples were dried down and re-dissolved in 6 N HCl. Sam- 2008, 2011; Kirchenbaur et al., 2012; Lagos et al., 2007). The samples ple solutions were always clear and no residual phases were identified. were crushed in a steel mortar and divided into two splits. One split In preparation for the column chemistry (after Münker et al., was powdered in an agate mill and used for whole rock analyses. 2001), samples were dissolved in 4 ml of 3 M HCl on the hotplate at The second split was washed to remove the clay-size fraction and 120 °C. After the solution cooled down to room temperature, 1 ml sieved into several grain size fractions. Garnet concentrations were sig- of ascorbic acid was added to the samples to reduce Fe3+ to Fe2+. nificantly enriched in the sieve fractions using a Franz LB-1 magnetic All samples were centrifuged aiming to remove any solid residues separator, before final garnet separation by hand picking under a binoc- and subsequently loaded on to the Ln-Spec cation exchange columns ular microscope, from the 128–180 μmto180–250 μm size fractions. (see Münker et al., 2001 for details on the column chemistry). All Hf Garnet was cleaned in dilute 0.25 N HCl, in an ultrasonic bath, and re- cuts were processed twice, to ensure the best possible removal of Lu peatedly washed with deionized water. and Yb, thus minimizing interferences on 176Hf. T.J. Nagel et al. / Lithos 170–171 (2013) 179–190 187

The analyses of Lu and Hf were carried out in static mode using the Lagos et al., 2007; Vervoort et al., 2004). The reported errors for Thermo Finnigan Neptune multi-collector inductively coupled plasma 176Lu/177Hf include an error-propagation for non-ideal sample mass spectrometer (MC-ICPMS) based at the Steinmann Institute in spike ratios that slightly increases errors to 0.22–0.27%. Procedual Bonn. Measured Hf isotope compositions were corrected for (1) mass blanks were <25 pg for Hf and <10 pg for Lu and negligible. Isochron bias using 179Hf/177Hf of 0.7325 (Patchett and Tatsumoto, 1980)and regressions were calculated using ISOPLOT v. 2.49 (Ludwig, 2001)and the exponential law, and (2) interferences on 176Hf and 180Hf by moni- the decay constant of λ176Lu (β-) = 1.867 × 10−11 yr−1 (Scherer toring 173Yb, 175Lu, 181Ta, 182 W signals. Because the true isotopic et al., 2001; Söderlund et al., 2004). composition of Lu in the Hf cuts is essentially unknown we calculated 176Hf/177Hf using both the spiked and natural 175Lu/176Lu composition 4.2. Results for interference correction and used the mean 176Hf/177Hf. The respective uncertainty was added to the 2σ external error in 176Hf/177Hf that Fig. 8 illustrates the isotopic composition of samples FRT2, FRT5 and was estimated using an empirical relationship (Bizzarro et al., 2003) FRT8 in 176Lu/177Hf vs. 176Hf/177Hf space. Garnet, both whole-rocks and and used for isochron regressions. All 176Hf/177Hf ratios are reported a ‘residue’ from sample FRT5 define an isochron of 32.76 ± 0.5 Ma relative to 176Hf/177Hf = 0.282160 for the Münster AMES Hf standard (MSWD = 1.06; n = 6). The other two samples yield a significant which is isotopically identical to the JMC-475 standard. For the Lu mea- scatter in ages calculated from garnet + whole-rock pairs. The two surements, naturally occurring Yb in the Lu cuts was used to correct for youngest individual two-point ages using one garnet fraction and the mass bias. For this purpose, the typical 176Yb/173Yb value was measured whole rock are near the Early Oligocene age obtained in sample FRT5, from pure Yb standards at the beginning of each run session. Interfer- 26.9 ± 9.8 Ma in FRT 2 and 30.4 ± 8.9 Ma in FRT8, respectively. All ences of 176Hf on 176Lu were corrected by monitoring 177Hf. This proce- other two-point ages are significantly older up to 62.7 ± 1.8 Ma in dure typically results in external reproducibilities of ± 0.2% (2σ)for sample FRT2. the 176Lu/177Hf of ideally spiked samples (Blichert-Toft et al., 2002; 5. Discussion

0.288 We interpret the age of 32.76 ± 0.5 Ma obtained in sample FRT5 to a) FRT 5 indicate garnet growth associated with increasing pressures and 0.287 temperatures in a subduction channel shortly before peak-pressure conditions were reached. For many rock compositions garnet starts to 0.286 grow between 400 and 500 °C, hence, in a subduction channel at Hf

177 blueschist-facies conditions and is most abundant at eclogite-facies 0.285

Hf/ conditions (e.g. Konrad-Schmolke et al., 2005; Nagel et al., 2002a).

176 Whatever the protolith history of sample FRT5 sample was, it seems 0.284 Age: 32.76 ± 0.5 Ma that the Lu–Hf system was completely reset during formation of the 176 177 Initial Hf/ Hf = 0.283150 (10) eclogite-facies assemblage. Neither the preserved prograde chemical 0.283 MSWD =1.06; n = 6 zoning in garnet nor generally accepted metamorphic conditions dur- 0.282 ing the Alpine cycle (maximum temperature of 600 to 630 °C) allow 0.288 the interpretation of this age as a cooling age. Diffusive reequilibration b) FRT 2 in garnet is much faster for bivalent cations as compared to trivalent 0.287 Lu and tetravalent Hf (Carlson, 2012). Hence, a growth zoning with respect to major bivalent cations as seen in Fig. 4 strongly points to pre- 0.286 Age: 52.1Age: ± 4.5 39.5 Ma ± 4.5 Ma

Hf Age: 62.7 ± 1.8 Ma served Lu and Hf compositions and thus garnet formation ages. Age: 29.9 ± 9.8 Ma – 177 Published estimates of the closing temperature of the Lu Hf system 0.285

Hf/ are >630 °C (Skora et al., 2008), >800 °C (Schmidtetal.,2011), and

176 around 1000 °C (Shu et al., 2012), hence, well above Alpine peak tem- 0.284 peratures in the Eclogite Zone. An interpretation of garnet as grown or 0.283 recrystallized during subsequent amphibolite-facies conditions seems equally far-fetched. Garnet in FRT5 clearly coexists with omphacite 0.282 (Figs. 3, 4) and belongs to the eclogite-facies assemblage. There is no 0.288 signiﬁcant degree of subsequent amphibolite-facies overprint in the c) FRT 8 sample. Generally, garnet in maﬁc compositions is most abundant at 0.287 eclogite-facies conditions, hence, a transition from eclogite- to amphibolite-facies assemblages is associated with garnet breakdown 0.286

Hf Age: 30.4 ± 8.9 Ma rather than garnet growth (e.g. Konrad-Schmolke et al., 2008; O'Brien

177 et al., 1992). Recrystallization during deformation can be ruled out as 0.285

Hf/ a possibility, since this also should annihilate the prograde growth

176 0.284 Age: 43.9 ± 9 Ma zoning. whole-rock 1 (full digestion) The 32.76 ± 0.5 Ma-age accords with Rb–Sr multi-mineral iso- whole-rock 2 (selective digestion) chrons of Glodny et al. (2005) which are derived from eclogite- 0.283 garnet solid residues of whole-rock2 facies assemblages and interpreted to capture prograde metamorphic 0.282 reactions towards peak-pressure conditions as well as deformation at 012345678 peak-pressure conditions. Glodny et al. (2005) originally proposed a 176Lu/177Hf slightly younger age of 31.5 ± 0.7 Ma. However, correcting their age for the recently improved decay constant of the Rb–Sr system Fig. 8. Lu–Hf isochron plots for samples A) FRT5, B) FRT2 and C) FRT8. If not shown, 2σ (Nebel et al., 2011) leads to an age of 32.1 ± 0.7 Ma, in line with uncertainties used in regressions are smaller than symbol sizes. The Program ‘ISOPLOT our garnet age. v. 2.49′ (Ludwig, 2001) was used to calculate isochron regressions using the errors reported in Table 3. Ages are based on λ176Lu = 1.867 × 10–11 yr–1 (Scherer et al., We propose that the scattered and inconsistent garnet-whole- 2001; Söderlund et al., 2004). rock ages obtained in samples FRT2 and FRT8 result from variable 188 T.J. Nagel et al. / Lithos 170–171 (2013) 179–190 mixing of Alpine and relic pre-Alpine garnet. This interpretation is the the fact that the adjacent gneisses experienced the same metamor- fully in accord with the observation of an earlier generation of cores phic peak conditions (e.g. Miller et al., 2007; Hoschek et al., 2010). in large garnets of sample FRT2 which very likely represent a pre- As stated already in the Introduction, eclogite-facies conditions in Alpine garnet generation. As a consequence, the Alpine garnet is not Lower Oligocene times do not blend very well into existing tectonic equilibrated with the entire whole-rock. Because the radiogenic models of the nappe stack exposed in the Tauern Window. Most Hf from pre-Alpine garnet was not available at the time of Alpine schemes propose nappe stacking of Penninic and Subpenninic units be- garnet growth, the initial 176Hf/177Hf composition of the whole-rock fore the intrusion of the Rieserferner Pluton and the Rensen Pluton is systematically overestimated with respect to Alpine garnet (c.f. around 32 Ma. On the other hand, nappe stacking has to postdate Herwartz et al., 2011). Hence, disequilibrium can result in both eclogite-facies conditions in the Eclogite Zone because the nappes older and younger apparent ages. Although the youngest garnet- below and above were affected by peak pressures far beneath the whole-rock ages in sample FRT2 and FRT8 are, within the large errors, peak pressures observed within the Eclogite Zone and are characterized identical to the inferred Alpine high-pressure age from sample FRT5, by a different retrograde P–Tpath(Kurz et al., 2008). Therefore, most underestimated ages may also be expected from such samples. authors favour an Eocene age for this high-pressure event, i.e. an age In the fully equilibrated case of sample FRT5, the initial is well de- between 42 and 35 Ma (Kurz et al., 2008; Ratschbacher et al., 2004; fined by two fully dissolved whole rocks and the solid residue of one Zimmermann et al., 1994). The tectonic scheme for the early collision separate garnet. This age could only be too old, if the fully digested history in the Tauern area is heavily influenced by observations from whole rock is affected by old zircon cores (Scherer et al., 2000)orif the Lepontine Dome 300 km further West in eastern Switzerland and the garnet separates comprise any pre-Alpine garnet. However, the the adjacent Italy. There, the magma ascent of the 32–30 Ma-old Bergell good fit of the isochron gives confidence in the obtained age and intrusion (von Blanckenburg, 1992) is controlled by the Insubric the presence of pre-Alpine relics appears unlikely in sample FRT5. mylonite belt that clearly crosscuts the Sub-Penninic (Lepontine) The age of the pre-Alpine cores in samples FRT2 and FRT8 could not be nappe stack (Berger et al., 1996; Nagel, 2008; Nagel et al., 2002b). This confined in the present study. They may be Variscan as in the Adula nappe stack again postdates 37–35 Ma-old eclogite-facies conditions Nappe. A likely alternative is that they represent relics from the Permian in the Adula Nappe (Gebauer, 1996; Herwartz et al., 2011). Along the high-temperature event widely recorded in the Alps (e.g. Schuster and Lepontine cross section the Alps became a high mountain range, i.e. a Stüwe, 2008). The limited whole rock data in Table 1 are inconclusive source area for abundant synorogenic detritus, at the Lower-to-Upper with respect to the origin of the protoliths of eclogites as are εHf initials Oligocene boundary (Garzanti and Malusa, 2008). Models for the for- calculated from the bombed whole rocks in Table 3 (Blichert-Toft et al., mation of the nappe stack in the Tauern Window have been constructed 1999). Assuming a magmatic age between 550 Ma and 250 Ma, εHf conforming to this Lepontine situation. However, the Rieserferner and initials are 15.1–17.3 (FRT5), 3.8–7.8 (FRT2), and 8.7–12.0 (FRT8), respec- Rensen Pluton which are coeval to the Bergell intrusion, are emplaced tively, and thus not consistent if used as a proxy for a certain formation in the Austroalpine units south of the window and not in the environment. In any case, the pre-Alpine garnet relics indicate that Penninic–Subpenninic units. There is no overprinting relationship these rocks were incorporated in the continental basement of the Euro- such as an intrusive contact which would indicate that the Periadriatic pean margin at the start of the alpine orogeny and are not derived from magmatic rocks had to be younger than the Penninic–Subpenninic Mesozoic ocean crust although they have certain MORB characteristics nappe stack in the Tauern Window. (Raith et al., 1977). Mafic bodies in the European basement immediately Eclogite-facies conditions at 32 Ma in the Eclogite Zone are not in north of and structurally below the Eclogite Zone have protolith ages conflict with structural observations nor with absolute ages obtained around 490 Ma (von Quadt et al., 1997). The interpretation of the eclogite in the vicinity of the Tauern Window exept for Ar–Ar ages around zone as a coherent piece of continental crust is additionally supported by 42 Ma in amphibole and white mica in eclogites from the Eclogite

Table 3 Lu–Hf concentrations and isotopic compositions of samples FRT2, FRT5, FRT8. Uncertainties on the last decimal places (in parentheses) are the estimated2σ external reproducibilities. Isochrons were calculated using the 176Hf/177Hf errors presented in the last column which account for uncertainties resulting from the unknown Lu composition in the measured Hf cuts. ‘Residues’ refer to undigested solids left after the selective dissolution of garnet separates by the tabletop procedure.

ppm Lu ppm Hf 176Lu/177Hf 176Hf/177Hf ±2 s.e.(external) incl. Lu uncertainty

Sample: FRT5 SR2 WR-1a ‘bombed’ 0.384 2.41 0.02265 (6) 0.283162 (16) 0.000016 DH182 WR-1b ‘bombed’ 0.383 2.46 0.02211 (6) 0.283166 (13) 0.000015 SR9 grt-1 1.85 0.0385 6.833 (17) 0.287371 (48) 0.00011 SR14 grt-2 2.12 0.0657 4.591 (11) 0.286026 (86) 0.000102 SR15 grt-3 2.03 0.093 3.094 (8) 0.285017 (27) 0.000038 SR22 residue of grt-1 0.0008686 (22) 0.283154 (29) 0.000037

Sample: FRT2 SR4 WR-1 ‘bombed’ 0.292 3.03 0.01371 (3) 0.282799 (20) 0.000023 SR18 WR-2 ‘table top’ 0.257 0.227 0.1607 (4) 0.282820 (16) 0.00002 SR11 grt-1 0.429 0.0403 1.510 (4) 0.283816 (84) 0.000114 SR12 grt-2 0.503 0.0485 1.473 (4) 0.284098 (58) 0.000111 SR13 grt-3 0.488 0.0387 1.788 (4) 0.283637 (174) 0.000302 DH196 grt-4 1.07 0.0786 1.934 (5) 0.284899 (58) 0.000058

Sample: FRT8 SR3 WR bombed 0.302 2.54 0.01686 (4) 0.282952 (19) 0.000024 SR20 WR tap top 0.292 0.236 0.1758 (4) 0.282993 (424) 0.000426 SR10 grt-1 0.935 0.033 4.034 (10) 0.286111 (78) 0.000508 SR16 grt-2 0.832 0.044 2.702 (7) 0.284401 (74) 0.000106 SR17 grt-3 0.82 0.043 2.733 (7) 0.284468 (75) 0.000114 SR23 residue of grt-1 0.0005015 (13) 0.283001 (38) 0.000184 T.J. Nagel et al. / Lithos 170–171 (2013) 179–190 189

Zone determined by Ratschbacher et al. (2004). Other published Ar–Ar Acknowledgments ages from the Ecogite Zone for white mica are between 36 Ma and 32 Ma (Zimmermann et al., 1994) and around 32 Ma (Kurz et al., We thank Jane Selverstone and Jan Kramers for detailed and spe- 2008). These ages were interpreted to capture cooling during exhuma- cific comments and Marco Scambelluri for speedy editorial handling. tion. We see three possibilities to explain the apparent conflict between Laboratory work was supported by DFG project NA 411/2-1. the 33 and 32 Ma high-pressure ages (Glodny et al., 2005; Smye et al., 2011; this study) and the published 36 to 32 Ma Ar–Ar data of References Zimmermann et al. (1994) and Kurz et al. (2008). Firstly, exhumation and cooling may have occurred extremely fast within one million Barth, S., Oberli, F., Meier, M., 1989. U–Th–Pb systematics of morphologically character- years, i.e. within the error bars of the high-pressure ages and the ized zircon and allanite; a high resolution isotopic study of the Alpine Rensen Pluton (northern Italy). Earth and Planetary Science Letters 95, 235–254. Ar–Ar cooling ages. This explanation was favoured by Glodny et al. Behrmann, J.H., Ratschbacher, L., 1989. Archimedes revisited: a structural test of (2008) who obtained identical Rb–Sr ages from assemblages they eclogite emplacement models in the Austrian Alps. Terra Nova 1, 242–252. viewed as eclogite-facies and amphibolite-facies, respectively. Secondly, Berger, A., Rosenberg, C., Schmid, S.M., 1996. Ascent, emplacement and exhumation of – – the Bergell pluton within the Southern Steep Belt of the Central Alps. Schweizerische the 36 32 Ma Ar Ar ages may actually date high-pressure conditions Mineralogische und Petrographische Mitteilungen 76, 357–382. and not cooling (see Chopin and Maluski, 1980). Thirdly, the Ar–Ar Bizzarro, M., Baker, J.A., Haack, H., Ulfbeck, D., Rosing, M., 2003. Early history of Earth's crust– ages may suffer from excess or inherited Argon and thus be anomalously mantle system inferred from hafnium isotopes in chondrites. Nature 421, 931–933. Blichert-Toft, J., Albarède, F., Rosing, M., Frei, R., Bridgwater, D., 1999. The Nd and Hf old, a phenomenon particularly common in high-pressure phengites isotopic evolution of the mantle through the Archean. Results from the Isua (e.g. Ruffet et al., 1995, 1997; Scaillet, 1996). At least the Ar–Ar ages of supracrustals, West Greenland, and from the Birimian terranes of West Africa. 42 Ma (Ratschbacher et al., 2004) appear to be affected in this way Geochimica et Cosmochimica Acta 63, 3901–3914. Blichert-Toft, J., Boyet, M., Télouk, P., Albarède, F., 2002. 147Sm–143Nd. and 176Lu–176Hf if our Oligocene age indicates high-pressure conditions. In the Alps, in eucrites and the differentiation of the HED parent body. Earth and Planetary Ar–Ar ages from high-pressure rocks have repeatedly turned out to be Science Letters 204, 167–181. too old (Scaillet, 1996). Borsi, S., Del Moro, A., Sassi, F.R., Zirpoli, G., 1979. On the age of the Vedrette de Ries – (Rieserferner) massif and ist geodynamic significance. International Journal of Regional cooling in the Tauern window started at the Oligocene Earth Sciences (Geologische Rundschau) 68, 41–60. Miocene boundary (Luth and Willingshofer, 2008)coevalwithE–Wex- Carlson, W.D., 2012. Rates and mechanism of Y, REE, and Cr diffusion in garnet. American tension and N–S shortening. The intense Early Miocene tectonics and Mineralogist 97, 1598–1618. – metamorphism make it impossible to define a temporal relationship Chopin, C., Maluski, H., 1980. 40Ar 39Ar dating of high pressure metamorphic micas from the Gran Paradiso Area (Western Alps): evidence against the blocking between the Periadriatic intrusions and the formation of the nappe temperature concept. 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