Rapid Formation and Exhumation of the Youngest Alpine Eclogites: a Thermal Conundrum to Barrovian Metamorphism

Home , Central Eastern Alps, Western Tauern Alps

Earth and Planetary Science Letters 306 (2011) 193–204

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Earth and Planetary Science Letters

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Rapid formation and exhumation of the youngest Alpine eclogites: A thermal conundrum to Barrovian metamorphism

Andrew J. Smye a,⁎, Mike J. Bickle a, Tim J.B. Holland a, Randall R. Parrish b, Dan J. Condon b a Department of Earth Sciences, University of Cambridge, Cambridge, CB2 3EQ, UK b NERC National Isotope Geoscience Laboratories, Kinglsey Dunhem Centre, Keyworth, Nottingham, NG12 5GG, UK article info abstract

Article history: Eclogite facies metamorphic rocks provide critical information pertaining to the timing of continental collision Received 15 December 2010 in zones of plate convergence. Despite being amongst Earth's best studied orogens, little is understood about Received in revised form 28 March 2011 the rates of Alpine metamorphism within the Eastern Alps. We present LA–MC–ICPMS and ID–TIMS U–Pb Accepted 29 March 2011 ages of metamorphic allanite from the Eclogite Zone, Tauern Window, which when coupled with rare earth Available online 30 April 2011 element analysis and thermobarometric modelling, demonstrate that the European continental margin was – ﬁ Editor: R.W. Carlson subducted to between 8 and 13 kbar (30 45 km) by 34.2±3.6 Ma. These data de ne: (i.) an upper limit on the timing of eclogite facies metamorphism at 26.2±1.8 kbar (70–80 km) and 553±12 °C, (ii.) plate velocity −1 Keywords: (1–6 cm·a ) exhumation of the Eclogite Zone from mantle to mid-crustal depths, and (iii.) a maximum eclogite duration of 10 Ma (28–38 Ma) for juxtaposition of Alpine upper-plate and European basement units and Barrovian metamorphism subsequent conductive heating thought to have driven regional Barrovian (re)crystallisation at ca. 30 Ma. U–Pb geochronology One-dimensional thermal modelling of tectonically thickened crust shows that conductive heating is too slow conductive heat transfer to account for Tauern Barrovian conditions (550 °C at 9–13 kbar) within the maximum 10 Ma interval allanite between eclogite exhumation and the thermal peak (28–32 Ma). Given that the Tauern Window is a classic Tauern Window locality for understanding rates of conductive thermal relaxation in tectonically thickened crust, this work raises questions of fundamental importance concerning the length scales of the mechanisms responsible for heat transfer within orogenic crust. © 2011 Elsevier B.V. All rights reserved.

1. Introduction tope geochronology of both (U)HP and Barrovian metamorphic events allows calculation of average exhumation rates of (U)HP bodies. High-pressure (HP) metamorphic rocks are a ubiquitous feature of Recently, this approach has shown that exhumation rates are of the Phanerozoic mountain belts. Their occurrence provides important same order of magnitude to plate velocities—cm·a− 1 (e.g. Baldwin et al., constraints on both the transfer of material between the crust and 2004; Meffan-Main et al., 2004; Parrish et al., 2006; Rubatto and mantle, and also the timing of continental collision within zones of Hermann, 2001). Such rates of exhumation question the dominance of active and past convergence (De Sigoyer et al., 2000; Leech et al., 2005; conductive heat transfer over timescales of tectono-metamorphic Parrish et al., 2006; Rubatto and Hermann, 2001). Whilst subduction evolution in orogenesis. In mountain belts such as the Alps and the provides a plausible mechanism by which tracts of oceanic and Himalayas, where exposed rocks record both Barrovian and (U)HP continental lithosphere are conveyed to upper mantle depths, the metamorphism associated with plate convergence it should be possible processes responsible for exhuming such rocks back to Earth's surface to assess both the duration of thrusting and subsequent conductive are less well understood (e.g. Agard et al., 2009, and references therein). relaxation heating. This is of fundamental importance to understanding Within the typical orogenic cycle (ultra)high-pressure ((U)HP)meta- the rates and mechanisms responsible for driving tectono-metamorphic morphism occurs early, linked to subduction of intervening oceanic evolution in areas of orogenesis. realms, before the inception of continental collision which thickens the The Tauern Window (Fig. 1) exposes the largest coherent eclogite crust and drives subsequent Barrovian metamorphism. This accounts for facies unit within the Eastern Alps. Eclogites senso stricto and eclogite commonly observed amphibolite–greenschist facies overprinting of facies metasediments pertaining to the European margin are exposed blueschist–eclogite facies metamorphic rocks (e.g. De Sigoyer et al., in a nappe which, following HP metamorphism, experienced regional 1997, 2004; England and Richardson, 1977). High-precision radioiso- Barrovian metamorphism associated with emplacement of the Austroalpine nappes. Tauern Barrovian metamorphism provided the impetus behind pioneering studies of the thermal budget of ⁎ Corresponding author. Tel.: +44 1223 333400; fax: +44 1223 333450. tectonically thickened crust (Bickle et al., 1975; England, 1978; E-mail address: [email protected] (A.J. Smye). England and Richardson, 1977; England and Thompson, 1984;

Fig. 1. Tectonic sketch map of the Alps, focusing on the Tauern Window and the Eclogite Zone in particular. Modiﬁed after (Glodny et al., 2005). Collectively, the Venediger cover nappe, the Eclogite Zone, the Rote-Wand/Modereck nappe and the Glockner nappe are referred to as the Schieferhülle nappes.

Oxburgh and England, 1980; Oxburgh and Turcotte, 1974). However, the southern passive margin of the European continent prior to Tertiary despite extensive petrological study (Frank et al., 1987; Getty and subduction and collision (Miller, 1974; Ratschbacher et al., 2004). Selverstone, 1994; Holland, 1979a,b; Miller, 1974) the age of eclogite facies metamorphism and therefore the duration of conductive 2.1. Tectono-metamorphic evolution heating are poorly constrained. Previous geochronological understanding relies heavily on the 40Ar– 39Ar system (Kurz et al., 2008; The Eclogite Zone behaved as a coherent unit within the Alpine Zimmermann et al., 1994), which is highly susceptible to excess 40Ar orogenic episode and preserves petrographical signatures of three contamination under HP conditions (e.g. Sherlock and Kelley, 2002; discrete metamorphic events. Relicts of its prograde history are limited Warren et al., 2010). Further geochronological studies have been to mica, chloritoid and epidote inclusions in garnets. Pseudomorphs hampered by isotopic disequilibrium (Van Breemen and Hawkesworth, after lawsonite, within garnet poikiloblasts, have been inferred by 1980), a paucity of datable accessory phases (Miller et al., 2007) and the several authors (Dachs, 1986; Droop, 1985; Spear, 1986). The HP event is lack of a quantifiable link to P–T conditions (Glodny et al., 2005). As a typified by garnet–omphacite–kyanite–phengite assemblages in mafic result, despite the Eastern Alps being one of Earth's best studied eclogites (Holland, 1979b)andgarnet–chloritoid–kyanite–phengite orogens, the timescales of its evolution are currently uncertain. This parageneses in metapelitic lithologies (Smye et al., 2010). Numerous paper presents results of a combined thermobarometric, geochrono- thermobarometric calculations using peak eclogite-facies mineral logical and thermal modelling study which show that eclogite assemblages yield a range in P–T conditions from 20 to 26 kbar and formation, exhumation and subsequent Barrovian metamorphism all 550–600 °C (Franz and Spear, 1983; Holland, 1979b; Hoschek, 2001; occurred in under 10 Ma—within an order of magnitude quicker than Smye et al., 2010; Stöckhert et al., 1997). During exhumation the previously thought. We use high-precision U(–Th)–Pb allanite geo- Eclogite Zone is thought to have been tectonically inserted between the chronology to illustrate the potential that allanite has as a prograde underlying Venediger nappe and beneath the overlying Glockner nappe geochronometer in high grade metamorphic terranes. (Behrmann and Ratschbacher, 1989) under blueschist-facies conditions, indicated by partial replacement of eclogite-facies amphibole and pyroxene by glaucophane (Holland and Richardson, 1979). Regionally 2. Tauern Window eclogites penetrative greenschist–amphibolite facies metamorphism (ca. 7 kbar and 520–570 °C, Zimmermann et al., 1994) overprinted previous The Tauern Window exposes Penninic nappes of the Alpine orogen's metamorphic assemblages during conductive heating associated with lower plate, which are observed in a geometry reflecting exhumation, isotherm relaxation after emplacement of the Austroalpine nappes post-subduction associated with closure of the Valaisan oceanic realm (Bickle et al., 1975). Numerous radiometric dating studies have shown (Fig. 1). Eclogites and associated HP metasediments outcrop within the that this event occurred ca. 27–32 Ma. Glodny et al. (2005) performed Eclogite Zone (south-central Tauern Window) — a tectonic sliver 15 km Rb–Sr analyses on cogenetic white mica, titanite, amphibole and in length, intercalated between a crystalline basement complex — the plagioclase from a structurally discordant vein assemblage in a Venediger Nappe, and an incomplete ophiolite suite — the Glockner greenschist retrogression aureole within mafic eclogite. They obtained Nappe. The Eclogite Zone comprises abundant metre to hundred metre an isochron age of 31±0.5 Ma (MSWD=0.63). This age is supported by scale eclogite boudins and pods hosted by a metasedimentary matrix, the data of Gleißner et al. (2007), who carried out Rb–Sr isotopic which is dominated by calc-schists, metapelites and quartzites. Both analyses of a pseudomorphic assemblage after blueschist facies law- mafic and sedimentary protoliths have experienced eclogite-facies sonite in the Glockner nappe. A multimineral internal isochron metamorphism. The combination of mafic igneous and sedimentary involving white mica, apatite, epidote, clinozoisite, calcite and albite lithologies supports interpretation that the succession was deposited on yields an age of 29.82±0.54 (MSWD=2.2). The age is interpreted to A.J. Smye et al. / Earth and Planetary Science Letters 306 (2011) 193–204 195 represent syntectonic growth of the greenschist facies matrix assem- considerable portions of the sample's prograde and retrograde P–T blage. Similarly, Inger and Cliff (1997) present white mica, epidote path given the presence of two generations of chloritoid which bracket and whole rock Rb–Sr isotopic data from eleven different localities kyanite growth (Fig. 2). Peak conditions determined using garnet rim, from within the Eclogite Zone and Glockner nappe. All of the ages lie the core of matrix chloritoid blasts and matrix phengite compositions between 27 and 31 Ma and are interpreted to record greenschist coexisting with kyanite, quartz and H2O, are 26.2±1.8 kbar at 553± facies metamorphism concomitant with penetrative deformation, 12 °C (2σ; calculations performed using the average P–T mode of influx of fluids and resetting of Sr isotopes. Phengite 40Ar– 39Ar ages THERMOCALC version 3.33), on the verge of UHP conditions and obtained from Eclogite Zone lithologies range between 32 and 42 Ma consistent with the pseudosection calculations. Sample TH-680 escaped (Kurz et al., 2008; Ratschbacher et al., 2004; Zimmermann et al., pervasive overprinting during the Barrovian metamorphic event. Zoned 1994) and are variably interpreted to represent cooling from high epidotes with allanitic cores and clinozoisite rims are present as pressure metamorphism. elongated lozenges (150–900 μm length; 50–200 μm width) which are

Both the Venediger and Glockner nappes appear to have experienced wrapped by the rock's crenulated phengite fabric (S0 +S1), which a common metamorphic history with the Eclogite Zone subsequent to additionally wraps around garnet poikiloblasts. Proximal to fold hinges, the blueschist event (Holland and Ray, 1985). Our geochronology is allanite grains are aligned parallel to the limbs of F2 generation micro- based on a single sample collected from the internal portions of the folds. Euhedral allanite grains are also present as rare inclusions within Eclogite Zone (see Supplemental text S.1). both core and rim portions of garnet poikiloblasts (Fig. 3a)—indicative of pre-garnet growth. The textural setting of allanite in TH-680 is notably similar to that described by Hoschek et al. (2010) from a garnet– 2.2. Sample petrography chloritoid–kyanite bearing micaschist collected nearby TH-680, within the Eclogite Zone. Sample TH-680 is a peraluminous garnet–chloritoid–kyanite bearing metapelite collected from the central portion of the Eclogite Zone. Smye et al. (2010) have shown that this assemblage is restricted 2.3. Allanite to a narrow, temperature controlled, stability ﬁeld within the eclogite facies. A psuedosection combined with isopleth calculations using Allanite ((Ca, REE, Y)2 (Al, Fe)3(SiO4)3OH) can incorporate weight THERMOCALC version 3.33 (Holland and Powell, 1998)constrain percents of the LREE inventory and Th in crustal rocks (Hermann, 2002)

g MnNCKFMASHO car + SiO + mu + H O (+ ky) (- ep) (- car) 29 2 2 ky g jd ky g coe jd qtz ep (+ jd) car av.P-T jd 25 75 g g ctd ctd ky ep g (+ jd) car ctd 21 jd ep g Approximate depth (km) ky 60 (- chl) g growth (+ jd/- pa) (+ g)

17 g (+ car) g ky (+ pa) ctd chl

Pressure (kbar) h t chl 45 ctd w o r ep g chl e t g i s ep i (- ma) o ky z 13 o n li chl c g ctd ma Molar % chl (- bi) g SiO 70.31 ma 2 chl Al O 13.76 ep 30 2 3 ma CaO 1.29 9 MgO 3.64 (+ g) FeO 5.55 g g K O 3.33 ky 2 chl Na O 0.24 g bi 2 ma 0.09 ma MnO ep ma O 0.185 bi 5 st 15 (+ ma) (+ bi) (- ep) (+ st) (- chl) (- st) 400(- ctd) 450 500 550 600 650 Temperature (°C)

Fig. 2. P–T pseudosection for sample TH-680. Italicised labels at the end of equilibria denote which phase is consumed or formed up-grade. The shaded transparency marking the stability ﬁeld of clinozoisite growth represents the region which best correlates with calculated ferric contents of clinozoisite rim portions (f(ep)=0.24–0.26 — epidote activity–composition model from(Holland and Powell, 1998)). Similarly, the ellipse representing initiation of garnet growth is constrained by garnet isopleth values of Fe2+/(Fe2++ Ca+Mg+Mn)=0.66(core)–0.62 (rim), and Ca/(Ca+Fe2++Mg+Mn)=0.21(core)–0.18(rim). The P–T path is constrained by aforementioned isopleth calculations, the presence of two generations of chloritoid growth, which bracket kyanite growth and also the appearance of retrograde margarite. 196 A.J. Smye et al. / Earth and Planetary Science Letters 306 (2011) 193–204

Fig. 3. Allanite chemistry: (a) High contrast BSE image of euhedral allanite in garnet poiklioblast. Clinozoisite rims to allanite cores are present in grains included within garnet and also as matrix porphyroblasts. The textural setting of allanite in TH-680 is very similar to that described by Hoschek et al., 2010 of a similar lithology from the Eclogite Zone. (b) False colour X-ray map of Th concentration in allanite 1, TH-680. Euhedral core regions display complex structure relative to Th-poor clinozoisite rims. (c) Chondrite normalised LA–ICPMS REE patterns for allanite core regions and coexisting garnet. Allanite is enriched in LREEs and also HREEs relative to garnet. Note the absence of a significant Eu anomaly in allanite, and also the presence of a positive Ce anomaly. Garnet REE profiles show a progressive depletion of HREEs during growth. (d) Total REE+Th versus Al (cations per formula unit) plot after Petrik et al., 1995, showing the chemical relationship between representative allanite and clinozoisite rim domains. (e) Modelled Y concentration profiles from core to rim, assuming Rayleigh fractionation between Y Y garnet and metamorphic matrix (Hollister, 1966) both before (red solid line; Dgarnet− matrix =83.8;Cmatrix =30.62 ppm) and after (blue solid line; Dgarnet − matrix =83.8;Cmatrix =5.96ppm) allanite growth. Measured profile constructed from LA–ICPMS Y data; 1σ errors are smaller than the width of an analysis point marker.

and forms a solution with epidote and clinozoisite by the coupled spectra also lack a negative Eu anomaly, consistent with growth outside substitution of Ca2++Fe3+/Al3+ respectively for REE3++Fe2+. the stability field of plagioclase (≤10 kbar). Allanite in TH-680 is zoned from core to rim by increasing clinozoisite Assuming that Σ[Si +Al +Ti +Fe +Mn+Mg]=6 c.p.f.u (Ercit, content (Fig. 3b and d; Table S.D.1; Table S.D.2) which is commonly 2002), structural formulae for allanite and clinozoisite were recalcu- ascribed to prograde metamorphic growth (Janots et al., 2007, 2008, lated in order to generate estimates for Fe3+concentrations. Isopleths 2009). Chondrite normalised REE patterns obtained via LA–ICPMS for constant ferric iron concentration, and hence constant epidote– (Fig. 3c; Table S.D.2) show that the allanite is heavily enriched in the clinozoisite ratio, were calculated between 5 and 25 kbar and 400– LREEs relative to HREEs (average La/Yb = 294). Even so, the allanite 600 °C using the epidote activity–composition model of (Holland and is enriched in HREEs relative to garnet (Fig. 3c). Allanite and garnet Powell, 1998). Given the high concentration of REEs present in allanite, dominate the REE budget of sample TH-680 in which phosphate the two component epidote–clinozoisite model may not be appropriate. accessory phases monazite, xenotime and apatite (common sinks However, lower REE concentrations of the rims surrounding allanite for REE elements in metapelitic rocks) are not observed. Yttrium is cores make such portions suitable for application of the epidote activity a trace element with a strong affinity for both allanite and garnet model. Rim epidote–clinozoisite ratios show a comparable range in Y Y (i.e. Dallanite − matrix and Dgarnet − matrixNN1) and mineral Y profiles values to those calculated, between 8 and 13 kbar and 425–500 °C are therefore very sensitive to the relative timing of their growth. (Fig. 2). Further pseudosection calculations imply that garnet growth Rayleigh fractionation modelling of garnet growth within TH-680 initiated between 18 and 21 kbar and 550–580 °C, consistent with (Supplemental text S.3; Table S.D.4) shows that the Y concentra- chemical and petrographic evidence showing that both allanite and tions in garnet growing prior to allanite are calculated to be an clinozoisite fractions grew early along the prograde metamorphic path. order of magnitude higher than measured values (∼2500 ppm versus Y ∼500 ppm, Fig. 3e). Similarly, values of Dgarnet − matrix calculated for TH- 3. Geochronology of metamorphism 680 garnet cores against a TH-680 matrix composition depleted by allanite formation, are close to literature values unlike those estimated Current understanding of the timing of eclogite-facies metamorphism with matrix compositions undepleted by allanite growth (see Appendix within the Tauern Window is based on Rb–Sr geochronology (Glodny A.3). These calculations corroborate the textural evidence that the et al., 2005, 2008). However, the known susceptibility of Rb–Sr to isotopic allanite grew prior to initiation of garnet growth. The allanite REE resetting during deformation and retrogressive metamorphism (Thoni A.J. Smye et al. / Earth and Planetary Science Letters 306 (2011) 193–204 197 and Jagoutz, 1992)meansthatconfirmation from a more robust system is a 0.85 data-point error ellipses are 2 required. The U(–Th)–Pb system is the preferred choice given its high LA-MC-ICP-MS closure temperature (≥600 °C), and the potential to demonstrably link 0.8 ages with P–T conditions. 0.6 Typically allanite core regions contain 2–235 ppm U and have Th/U 0.83 ratios between 1.7 and 21.6. Such low Th/U ratios focused geochro- 0.4 – – – clinozoisite nological study on the U Pb system. We used U Pb Isotope Dilution 0.2

Thermal Ionisation Mass Spectrometry (ID–TIMS) and U(–Th)–Pb Pb Tera-Wasserburg concordia 0.0 Laser Ablation–Multiple Collector–Inductively Coupled Plasma Mass 206 0 80 160 240 – – 0.81

Spectrometry (LA MC ICPMS) methods to date laser-milled sepa- Pb/ – – rates and in-situ allanite grains respectively. The U( Th) Pb data and 207 analytical methods are given in the Supplemental Data Set (Supple- allanite mental text S.5; Table S.D.5; Table S.D.6). Preliminary laser ablation 0.79 studies were hampered by the absence of a suitable allanite standard. Analysis of Tertiary Alpine allanites previously used as standards 34.6 ± 4.1 Ma (Gregory et al., 2007) reveals protracted crystallisation histories and MSWD = 15 signiﬁcant spread in single grain U(–Th)–Pb contents. Against a 0.77 primary zircon mineral standard, analyses of both core and rim 02468 portions of zoned grains show that non-radiogenic Pb dominates the 238U/206Pb total Pb composition (Pb /Pbc b0.03). In particular, clinozoisite rim 238 206 portions contain negligible radiogenic Pb ( U/ Pbb0.15) and thus b data-point error ellipses are 2 provide an estimate of the common Pb composition of the rock during ID-TIMS allanite growth. Analyses (n=14) of two allanite grains (allanite 1 0.8 0.85 and 7) in thin section X1 provide a Tera-Wasserburg lower intercept 0.6 age normalised to zircon standard 91500 (Wiedenbeck et al., 1995)of 0.4 34.6±4.1 Ma (2σ; MSWD=15)and an initial 207Pb/206Pb composition of 0.8211±0.0015 (95% conf.; Fig. 4a). 0.2 Tera-Wasserburg concordia Given the lack of a matrix-matched primary standard, this age is not Pb 0.83 0.0 robust enough for geological interpretation. In order to circumvent this, 206 clinozoisite 0 80 160 240 – – three allanite grains previously used for LA MC ICPMS were separated Pb/

for U–Pb ID–TIMS analyses. Individual grains were extracted from 207 polished thin sections via laser milling of matrix material and were allanite subsequently broken into 50 μmdiametershards.ID–TIMS measure- 0.81 ments corroborate observations on U–Pb systematics made from LA– MC–ICPMS measurements. Despite the limited spread in 238U/206Pb 34.2 ± 3.6 Ma compositional space (0.5–5), analyses of eight allanite–clinozoisite MSWD = 3.8 ﬁ shards de ne a Tera-Wasserburg regression with a lower concordia 0.79 intercept age of 34.2 ±3.6 Ma (2σ; MSWD=3.8) and an initial 0123456 207Pb/206Pb composition of 0.8330±0.0014 (95% conf.; Fig. 4b), 238U/206Pb which is slightly more radiogenic than the 34 Ma average upper crustal lead composition predicted by Stacey and Kramers (1975) Fig. 4. Tera-Wasserberg concordia plots for (a) LA–MC–ICPMS and (b) ID–TIMS allanite (0.8379). Due to low allanite Th/U values, 207Pb/206Pb and 238U/206Pb data. Colours of error ellipses correspond to individual grains ((a) red=X1 grain 1; ratios corrected for ingrowth of 206Pb from the incorporation of excess green=X1 grain 7; (b) red=X1 grain 2; blue=X1 grain 4; green=X3 grain 3). Inset Tera-Wasserberg plot illustrates the length of extrapolation required to generate an 230Th (Schärer, 1984), yield an age difference within the uncorrected intercept age. All ages were calculated using Isoplot v. 3.00 (Ludwig, 2003). 2σ age uncertainty. Performing both ID–TIMS and LA–MC–ICPMS analyses on the same allanites is a powerful geochronological approach, combining the spatial resolution of laser ablation and the high precision of ID–TIMS. Although the ID–TIMS Tera-Wasserburg age is within error of the LA–MC–ICPMS age, common Pb compositions are subtly different This corroborates the 31.5±0.7 Ma Rb–Sr age of Glodny et al. (2005), (Δ(207Pb/206Pb)≈0.0119). Identiﬁcation of a suitable allanite standard obtained from a range of eclogitic mineral assemblages. The Eclogite is required to constrain matrix dependent isotopic fractionation for Zone was subjected to regionally pervasive Barrovian metamorphism U(–Th)–Pb LA–ICPMS allanite geochronology. between 27 and 32 Ma (Rb–Sr: Inger and Cliff (1997); Glodny et al. (2005)), which places a limit on the time taken to both form and 4. Tectonic implications exhume the eclogite-facies nappe to mid-crustal levels. Therefore, the Eclogite Zone must have experienced ∼60 km of exhumation at rates We interpret the 34.2±3.6 Ma ID–TIMS age as the time at which between ∼1 and 6 cm·a− 1. It is extraordinary, then, that the Eclogite allanitic cores to clinozoisites grew between ∼8 and 13 kbar at 400– Zone is understood to have been subjected to local recrystallisation in 500 °C (see Figs. 2 and 5) prior to garnet growth during ongoing the blueschist facies, prior to Barrovian overprinting as evidenced by subduction. The new geochronological data, combined with previ- the growth of glaucophane at the expense of peak eclogitic omphacite. ously reported data, permit calculation of exhumation rates and also The P–T conditions of blueschist facies metamorphism are constrained timescales for the process of eclogite formation in the Eastern Alps. between 7 and 11 kbar and 400–450 °C (Eremin, 1994; Holland and Taking the angle of subduction as 45° and a descending slab velocity of Ray, 1985). Given that the Eclogite Zone represents transitional crust 5 cm·a− 1, the earliest peak pressure conditions (∼26 kbar) would be located on the southern margin of the European continent (Kurz et al., attained in the subducted margin is ∼2 Ma after allanite growth i.e. by 1998, and references therein), the 29–37 Ma age of eclogite formation ca. 33 Ma. However, given the 2σ uncertainty on the ID–TIMS age, critically provides a maximum estimate for the timing of continental peak pressure conditions are constrained to the interval 29–37 Ma. collision in the Eastern Alps. This means that a maximum of 10 Ma 198 A.J. Smye et al. / Earth and Planetary Science Letters 306 (2011) 193–204

0 units during Alpine orogenesis (Oxburgh, 1968). Previous conductive heating models call for a period of ca. 30 Ma post-emplacement of the 10 and Austroalpine nappes for the European basement to attain peak sill conditions of 520–550 °C at 7–8 kbar (see Fig. 5) — thus invoking ky ages of East Alpine eclogite formation N60 Ma. This is incompatible 20 with our geochronological data, which show that a maximum of 10 Ma passed between eclogite formation and the peak of Barrovian metamorphism. Here we use a one-dimensional thermal model (e.g. 30 Ma 30 (Bickle et al., 1975; Oxburgh and Turcotte, 1974)) to constrain 0 Ma thermo-tectonic scenarios which satisfy rapid (b10 Ma) attainment of Tauern Barrovian conditions. 40 5.2. The model 50 The thermal development of the Tauern Window is modelled by

Depth (km) a one-dimensional finite-difference approximation to the heat equation 60 following the approaches of Oxburgh and Turcotte, (1974), Bickle et al., (1975) and England, (1978) inwhichthethermalprofile following overthrusting is calculated for different values of total 70 heat generation (radiogenic heat production + mantle heat flow). Additionally, we consider the effect of thrust sheet thickness on the evolving thermal profile. Numerical details of the model are presented 80 u = 0.9 mm.a-1 -3 in the (Supplemental text S.6). Az0 = 5.0 µW.m q = 27 mW.m2 m The initial thermal profile is divided into two units representing κ = 31.54 km2.Ma-1 90 Inception of erosion at 30 Ma the allochthonous Austroalpine complex and lower plate Penninic basement and cover nappes, including the Eclogite Zone. Diffusivity (κ) is kept constant throughout the profile at 31.54 km2·Ma− 1 − 100 (∼1mm2·s 1) as temperature dependent feedbacks (Fig. 3 Bickle 0 100 200 300400 500 600 700 800 900 1000 et al., 1975; Whittington et al., 2009) are only significant over the Temperature (°C) investigated length- and timescales in the presence of concentrated Fig. 5. Often cited model of conductive relaxation heating (Bickle et al., 1975; England heat sources such as magmatism and shear heating, which are not and Thompson, 1984), which assumes a 30 Ma hiatus prior to the initiation of erosion. applicable to the Tauern. The initial temperature profile of the overthrust Dots indicate 5 Ma increments; grey box delimits Tauern Barrovian metamorphic sheet is insignificant to the thermal evolution of the system (Bickle et al., conditions. 1975) and therefore, we use identical equilibrium geotherm relations (see Appendix A5) for both hanging- and footwall units. The surface temperature is 20 °C for all time steps and we assume a basement- separated emplacement of the Austroalpine nappes and attainment of top initial temperature of 20 °C, justified by its 1000 m thick cover peak Barrovian conditions at 27–32 Ma. sequence (Kurz et al., 1998). The lower boundary condition is defined by a constant temperature gradient at 150 km depth. Constraints on 5. Thermal modelling the thickness of the Austroalpine complex are limited due to internal imbrication and the absence of palaeo-horizontal markers. However, 5.1. Background an estimate of the sheet's minimum thickness at the time of overthrusting is obtained from peak pressures experienced within the The thermal diffusivity of continental lithologies determines that Zentralgneiss of the Venediger nappe, during Barrovian metamorphism: conductive heating driven by relaxation of isotherms in regions of 6–14 kbar equating to 20–45 km (this study). These thicknesses are over-thickened crust operates on timescales of the order of tens of slightly greater than assumed by previous studies (15–30 km — Bickle millions of years (England and Thompson, 1984). However, high- et al. (1975); England (1978); England and Thompson (1984)), precision isotope geochronology has recently shown that in some reflecting advances in thermodynamic data used in calculating regional metamorphic terranes, peak Barrovian conditions are metamorphic pressures. Radiogenic elements in the Pennine basement attained within much shorter durations (Ague and Baxter, 2007; are strongly concentrated within the Zentralgneiss—a Variscan calc- Baxter et al., 2002; Oliver et al., 2000). Disparity between predicted alkaline granitoid body. Measurements by Hawkesworth (1974b) are and observed timescales of heating necessitates additional mecha- consistent with an exponential distribution of radiogenic matter nisms to conductive heating. These include: elevated mantle heat flow (Lachenbruch, 1968, 1970) with a characteristic drop-off length (D) of − 3 (Oxburgh and Turcotte, 1974; Stüwe and Sandiford, 1995), increased 8 km and a surface radiogenic heat production (Az0)of5.02μW·m . radiogenic heat production (Huerta et al., 1999; Jamieson et al., 1998), Assuming such a distribution, Bickle et al. (1975) calculate an estimate of shear heating (Grujic et al., 1996; Searle et al., 2008; Whittington the total radiogenic heat production (Az0·D) possible from the basement et al., 2009), magmatism (Lyubetskaya and Ague, 2010), advective of 50.16 mW·m− 2 (1.19 HFU). Given the lack of constraints, mantle heat − 2 transport of heat by fluids (Bickle and McKenzie, 1987) and advection flow (qm) is allowed to vary between 20 and 100 mW·m (0.47– of heat by tectonics (Burg and Gerya, 2005). 2.39 HFU), encompassing values characteristic of stable continental crust The Tauern Window is a classic locality for modelling the thermal to rifting margins (Sclater et al., 1980, 1981). It is important to note that evolution of overthrust terrains (Bickle et al., 1975; Cornelius et al., the contribution of heat from the mantle is the single most important 1939; England, 1978; England and Richardson, 1977; England and parameter affecting the thermal evolution of the system (England, Thompson, 1984; Oxburgh and England, 1980; Oxburgh and Turcotte, 1978). Previous models invoke an erosional hiatus between emplace- 1974; Richardson and England, 1979). Tauern Barrovian metamor- ment of the Austroalpine at ca. 65 Ma and inception of erosion within the phism is understood to have been driven by conductive heating Oligocene (Fig. 6; Bickle et al. (1975); England (1978); Oxburgh and following northward emplacement of allochthonous Austroalpine Turcotte (1974)). However, our new geochronological constraints on the A.J. Smye et al. / Earth and Planetary Science Letters 306 (2011) 193–204 199 timing of thrusting suggest that erosion was active immediately post- Consequently, we consider the range of values for overthrust sheet thrusting. Therefore, we use a constant erosion rate of 0.9 mm·a− 1 thickness, radiogenic heat production and mantle heat flow in which (0.9 km·Ma− 1). Shear heating is not considered because the plate conditions of 9–13 kbar and 550 °C are attained in the upper 6 km of interface zone within the Tauern Window is dominated by low-strength the lower-plate within 10 Ma of thrust sheet emplacement. carbonate lithologies. Shear heating in these rocks will be negligible above 300 °C as shown by the calculations of Bickle et al. (1975),who 5.3. Model solutions calculated shear stress at the base of a 20 km thick overthrust sheet as function of temperature for Yule marble (Heard, 1963) assuming 5.3.1. Overthrust sheet thickness ductile failure in a 4 km shear zone. Similarly, the effect of syn- We calculate Depth–T–t curves for the basement-top for a range of metamorphic magmatism or fluid advection are not taken into overthrust sheet thicknesses between 20 and 60 km and a constant account as there is no Tertiary igneous activity known from within − 2 mantle heat flow of 27 mW·m (0.64 HFU). Results are displayed in the Tauern Window; neither are extensive fluid vein networks of the Fig. 6. The P–T evolution of the upper-basement is characterised by an magnitude required by England (1978) observed. initial phase of rapid heating (≤2Ma), driven by equilibration of Peak temperature of Tauern Barrovian metamorphism is well temperatures across the sawtooth-shaped temperature depression of constrained by conventional thermodynamic methods to between the plate interface. Subsequent heating is exponentially slower until 500 and 570∘C(Bickle, 1973; Dachs, 1990; Dachs and Proyer, 2001; the basement is exhumed via erosion through colder isotherms. The Hoschek, 2001) within the south-central Tauern. Variations in basement top only attains Tauern temperatures within 10 Ma of temperature estimates from the Schieferhülle nappes (Fig. 1) are overthrusting if the thrust sheet is greater than ∼55 km thick. Such a generally within uncertainty of each other. Pressure estimates are less scenario would require peak basement pressures of 14–16 kbar. Peak well constrained due to fewer relevant equilibria and uncertainties in temperatures for overthrust sheets of 55 and 60 km thickness of activity–composition relations. We performed average P–T calcula- 548 °C at 11.2 kbar and 579 °C at 11 kbar respectively, are reached tions (THERMOCALC version 3.33, Holland and Powell, 1998)ona between 20 and 25 Ma post thrusting if rapid exhumation begins after metatonalite (TH-519—see Supplemental text S.1 and Table S.D.7) 5 Ma. Interestingly, emplacement of a 30 km thick thrust sheet — the collected from the upper-most levels of the Zentralgneiss (Fig. 1) preferred value of Bickle et al. (1975), heats the upper-basement to a within the Großvenediger region. Equilibria generated from end- peak temperature of 339 °C in 13 Ma — 200 °C short of Tauern members of garnet, plagioclase, K-feldspar, biotite, muscovite and conditions. For thrust sheets greater than 27 km thickness, erosion epidote yield an average pressure of 13.1±2.1 kbar at 601±38 °C − 1 must operate more rapidly than 0.9 mm·a . σ ( fit =1.66). Although loosely constrained, the estimate is corrobo- rated by pressures calculated from overprinted calc-schists of the fl Eclogite Zone, 2–3 km structurally above the Zentralgneiss, (Hoschek, 5.3.2. Crustal heat production and mantle heat ow 2001) between 9 and 10.2 kbar. The temperature attained by the basement-top following 10 Ma conductive heating under a 30 km thick overthrust sheet is plotted

against total heat contribution (A0·D) from the basement (Fig. 7). 0 Erosion is not active and mantle heat flow varies between 0.2 and 2.0 HFU (8.36–83.68 mW·m− 2). Saliently, the calculations show 35 Ma that for Tauern radiogenic heat production values 0.55–1.2 HFU 2 (Hawkesworth, 1974a,b), Barrovian conditions will only be attained 30 Ma fl N – − 2 – 10 within 10 Ma if mantle heat ow is 62.76 37.66 mW·m (1.5 and 0.9 HFU) respectively, at the time of emplacement. Such values of 25 Ma sill 4 heat flow into the lithosphere correspond to conductive mantle ky temperature gradients of 25.10–15.06 °C·km− 1, implying lithospheric 20 Ma 20 6

15 Ma

Pressure (kbar) Pressure 1.8 8 10 Ma 1.6

30 5 Ma Not applicable 1.4 10

Depth (km) 0 Ma 1.2 0.2 0.4 0.6 H.F.U0.8 1.0 1.2 1.4 1.6 1.8 2.0

40 12 1 .D (H.F.U) 0

A 0.8 14 0.6 50 Not applicable 16 0.4

0.2 200 300 400 500 600 700 800 18 60 T after 10 Ma. (oC) 0 100 200 300 400 500 600 o Temperature ( C) Fig. 7. Temperature attained after 10 Ma conductive heating of the basement after emplacement of a 30 km thick thrust sheet plotted against total heat generation within Fig. 6. Numerically calculated Depth–T–t curves for variable thickness of overthrust the basement complex. The plot is contoured for solutions of constant mantle heat flow sheet. Circles represent 5 Ma increments after thrusting. Mantle heat flow constant at (HFU); the shaded band delineates peak Tauern metamorphic temperatures; stippled − 2 − 3 27 mW·m (0.95 HFU), crustal heat production is 5.0 μW·m (11.95 HFU) and region corresponds to the probable range in A0·D for the Zentralgneiss, of 0.55–1.2 HFU erosion rate is 0.9 mm·a− 1. Grey box constrains metamorphic conditions at the peak of (Hawkesworth, 1974a,b). For simplicity these solutions omit the effect of erosion. Alpine Barrovian metamorphism. Thermal properties of the thrust sheet are identical to those of the basement. 200 A.J. Smye et al. / Earth and Planetary Science Letters 306 (2011) 193–204 thinning and asthenospheric upwelling (Sclater et al., 1980). However, the short timescales investigated. Given that the thermal time constant helium isotopic ratios within East Alpine groundwaters show a virtual (τ=a2/(π2κ)) of a 4 km slab under such conditions is 0.16 Ma, the slab 3 3 4 3 4 absence of mantle-derived He (( He/ He)/( He/ He)air b0.2, Marty attains Barrovian conditions rapidly, within ca. 1.2 Ma. However, in et al., 1992) and there is a notable lack of extensive Alpine magmatism order to reconcile tectonic emplacement with eclogitization between 28 within the Eastern Alps. Both observations are inconsistent with and 38 Ma requires subduction to be active for 30 Ma post-emplacement elevated mantle heat flows. In the absence of erosion, these results of the Austroapline nappes at ca. 65 Ma. provide lower-bound estimates as erosion will shorten the time interval over which conductive heating is operative. Erosion would displace the constant mantle heat flow curves of Fig. 8 towards higher temperatures. 5.4.2. Syn-thrusting heating Oxburgh (1968) suggests that roughly 100 km of displacement has occurred between the Penninic nappes of the southern Tauern 5.4. Alternative solutions Window and the overlying Austroalpine during Alpine convergence. If the maximum duration of thrusting is 10 Ma (38–28 Ma), then 5.4.1. Tectonic emplacement thrust sheet emplacement must have occurred at rates greater than Assuming that the Austroalpine nappes were emplaced ca. 65 Ma, 1 cm·a− 1, akin to plate convergence rates. At high rates of thrusting we now consider the scenario in which the Peripheral Schieferhülle the upper-basement will experience rapid heating due to the constant nappes (Eclogite Zone, Rote-Wand and Glockner nappes) were replenishment of heat by the allochthonous sheet — effectively tectonically inserted into a heated crustal pile ca. 35 Ma. This end- maintaining a constant thermal gradient within the upper plate, member tectonic scenario accounts for the lack of petrological evidence analogous to the action of a hot iron. However, for this scenario to be that the basement experienced blueschist-facies metamorphism prior plausible, the basement must be located proximal to the inception of to Barrovian conditions, in addition to accounting for the Barrovian thrusting so as to avoid conductive cooling of the thrust sheet. Fig. 9 metamorphism by conductive heating alone. Fig. 8 shows the thermal shows the thermal evolution of a crustal pile undergoing synthrusting evolution of a crustal pile in which a 4 km thick slab is inserted at plate heating for a thrusting rate of 1 cm·a− 1. It is clear that syn-thrusting interface depths, into a geotherm produced after 30 Ma of conductive heating would form inverted isotherms within the footwall during heating beneath a 30 km thick thrust sheet. Initial temperature of the early stages of thrusting — as observed in the Himalaya. With slab is taken as a constant 400 °C, corresponding to temperatures of the subsequent conductive relaxation and radiogenic heat production, blueschist-facies event (Holland and Ray, 1985). All other thermal footwall isotherms are smoothed and attain similar values to the base parameters are as in Fig. 5 except that erosion is not considered due to of the overthrust sheet within 10 Ma.

25 0 0 Thrusting rate (r) = 10 km.Ma-1 20 Lbase.< 39 km 10 Displacement = 100 km 10 15 tectonic emplacement 10 20

20 Pressure (kbar) 5 continual thrusting emplacement 30 Ma 30 post-thrusting 300 400 500 600 30 o Temperature ( C) 40

0 Ma 246810 40 50 Depth (km) 50 60

100 ) Depth km

-1 Displacement = 100 km 70 80 κ = 31.54 km2.Ma-1 60 √ κ τ ) Lper = ( . per 60 80 70 40

20 0.4 Ma 4 Ma 90 Thrusting rate - r (km.Ma Exposed basement 0 80 01020 30 40 50 60 emplacement Depth from plate interface - Lper (km) 100 0 100200 300 400 500 600 700 800 900 90 Temperature (°C)

Fig. 9. The ‘Hot Iron’ scenario. One-dimensional conductive heating of a basement 100 − 2 − 3 complex with qm = 27 mW·m ,Az =5.0 μW·m and D=8 km continually over- 0 200 400 600 800 0 thrust by a 35 km thick thrust sheet; erosion is neglected; 100 km displacement o Temperature C occurs at 10 km·Ma− 1 (1 cm·a− 1). Horizontal distance from inception of thrusting

(Lbase) ≤39 km. Grey box reflects Tauern metamorphic conditions. Inset plot shows Fig. 8. One-dimensional conductive relaxation profile of a 4 km thick slab, collectively the depth a thermal perturbation (i.e. a sawtooth geotherm of an overthrust sheet) representing the Glockner nappe and the Eclogite Zone, inserted into a crustal pile with would penetrate against rate of thrusting over 100 km displacement. Graph calculated pffiffiffiffiffiffiffiffiffiffiffiffiffi a geotherm representing 30 Ma post-thrusting (doubling of thickness) conductive from thermal time constant relations: Lper = κ:τper where Lper =depth of penetra- heating. The model represents the Eclogite Zone's insertion into a blueschist facies tion, κ =diffusivity and τper isthetimeoverwhichthetemperatureperturbation nappe stack and subsequent heating and exhumation to the upper-greenschist facies, exists (i.e. duration of thrusting). Grey region delimits the range of basement depths ∼550 °C (highlighted on P–T path of insert). Time steps are 0.4 Ma duration. exposed within the Tauern Window. A.J. Smye et al. / Earth and Planetary Science Letters 306 (2011) 193–204 201

6. Discussion rates for the 100 km of displacement between upper and lower plates to ≤10 km·Ma− 1—similar to rates calculated by Dewey et al. (1989) A maximum interval of 10 Ma separating thrust sheet emplacement and Schmid et al. (2004) via plate tectonic and palinspastic restoration and Barrovian metamorphism in the Eastern Alps raises profound respectively. Therefore a range of thrusting rates between 10 and questions over the length scales of heating responsible for regional 100 km·Ma− 1 is representative of Eocene–Oligocene convergence metamorphism. The classic conductive relaxation model shows that the within the Eastern Alps. Synthrusting heating initially affects the length scale of conductive heating required to attain peak conditions in upper-most levels of the basement, forming inverted isotherms b10 Ma is incompatible with Barrovian pressure estimates and detrital which are subsequently smoothed by conductive heating. Fig. 9 sediment loads (see beneath). Elevated mantle heat flow explains the shows that synthrusting heating beneath a 30 km thick thrust sheet rapid heating, but lacks supporting geochemical evidence. Tectonic displaced at rates between 10 and 100 km·Ma− 1 could feasibly insertion of the Schieferhülle nappes along the plate interface accounts penetrate the upper 20 km of basement—consistent with Barrovian for rapid (b2 Ma) heating from blueschist to Barrovian metamorphic pressure estimates between 9 and 13 kbar calculated from basement conditions. However, this scenario requires a mechanism to exhume the rocks. Eclogite Zone from 70–80 km to ∼30 km depth at plate tectonic rates of Overthrusting of a 30 km thick allochthon across the Eastern Alps 1–6cm·a− 1, between ca. 38 Ma and Barrovian metamorphism ca. 27– (1.8×104 km2 — Oxburgh, 1968) would provide roughly 1.45×1018 kg 32 Ma. Similarly, concomitant thrusting and heating explain the b10 Ma of sediment to circum-East Alpine basins in the interval 40 Ma– timescale of heating, but demand temperatures at the base of the present day, assuming a 10 km present day average thickness of the overthrust sheet close to those of peak metamorphism and also a pulse Austroalpine nappes, a crustal density of 2700 kg·m− 3 and an in detrital sediment supply to circum-East Alpine basins. average erosion rate of 1.1 mm·a− 1. Compared to England (1981)'s The Eclogite Zone is bounded by mylonite zones at its contacts estimate of 1.1× 1018 kg for the East Alpine detrital sediment load, with the Glockner and Venediger nappes. The basal contact is this suggests inconsistency between the detrital sediment record and characterised by thrust-sense, top-to-the-NNE deformation, whilst metamorphic pressures, which could be accounted for by a hidden its sense of movement relative to the hanging wall is less clear, Eocene–present age sediment trap or an overestimate of the metamor- showing both top-to-the-N and S-side-down pre-syn Barrovian phic pressures i.e. a thinner allochthon. The flysch–molasse transition shear indicators. Nevertheless, it is likely that these high strain within circum-Alpine basins occurred across the Eocene–Oligocene zones accommodated between 50 and 70 km of displacement during boundary (Kuhlemann, 2000; Kuhlemann et al., 2006; Sinclair, 1997), exhumation of the nappe back up the subduction channel, prior to which strongly supports contemporaneous thrust sheet emplacement. juxtaposing the Eclogite Zone between the basement and Glockner Plate velocity overthrusting of the Austroalpine nappes provides an nappes as an extrusion wedge (Behrmann and Ratschbacher, 1989; elegant explanation for equally rapid exhumation of the Eclogite Zone as Glodny et al., 2008). Rapid exhumation of the nappe could have been the same basal thrust system would be responsible for both processes facilitated by a buoyancy contrast between the subducted crustal (Fig. 10). Similarly, the scenario accounts for continued subduction post- material and surrounding mantle (Δρ=300 kg·m− 3, England and eclogite formation, explaining blueschist facies overprinting prior to Holland, 1979), following detachment from the slab (Smye et al., Barrovian conditions. 2010). However, the presence of kilometre-scale stratigraphic Syntectonic growth of Barrovian assemblages would provide strong continuity in both marbles and quartzite bands of the Eclogite Zone evidence in support of footwall heating due to rapid overthrusting of hot suggests that the nappe was exhumed as a coherent unit (Behrmann rock, as noted in the Lesser Himalaya (Caddick et al., 2007). However, and Ratschbacher, 1989). As fluid-assisted granular flow (Stöckhert, the relationship between Barrovian mineral growth and fabric forma- 2002)orpower–law deformation of aragonite is expected to have tion in the Penninic units of the Eastern Alps is less clear. Kurz et al., controlled the rheology, this raises considerable doubt over whether (1996) state that westward directed shearing initiated synchronous to the transitional crust would be strong enough to support such the Barrovian peak, whilst Droop (1985) and Inger and Cliff (1997) state buoyancy forces throughout 70–80 km exhumation (e.g. Renner that the metamorphic peak post-dated regional fabric formation. et al., 2001). Alternatively, return flow of low viscosity material Furthermore, evidence for Barrovian conditions at the base of the within a confined subduction channel has been postulated to account Austroalpine overthrust sheet is lacking; temperatures were high for the exhumation of (U)HP rocks (Cloos, 1982; Gerya and Stöckhert, enough to facilitate Ar diffusion in biotite, chloritisation and develop- 2006; Gerya et al., 2002; Warren et al., 2008). High viscosity of the ment of low-temperature quartz microstructures ca. 30 Ma. (Wallis, subducted margin provides an explanation for the coherent nature 1988; Waters, 1976), i.e. ≥300–350 °C. Under these conditions, of the Eclogite Zone. synthrusting heating would not account for the rapid timescales of Despite plausible exhumation mechanics, insertion of the Eclogite Barrovian metamorphism. However, due to the heavily reworked and Zone as an extrusive wedge into a heated crustal pile requires that anhydrous nature of the polymetamorphic basement to the Austroal- continental collision occurred ∼30 Ma prior to eclogite formation pine nappes (the Altkristallin; e.g. Siegesmund et al., 2007), it is between 38 and 28 Ma, effectively refrigerating subducted oceanic uncertain whether pervasive development of Barrovian mineralogy crust for up to 30 Ma. This is implausible given that temperatures of the would occur under higher grade conditions, particularly given the short blueschist overprint (200–450 °C) are indicative of an active subduction timescales of heating. regime between 38 and 28 Ma. Furthermore, shear sense indicators In the absence of an advective component to heating, syn-thrusting along the plate interface are strongly thrust-sense, supporting late-stage heating provides a theoretical explanation for the rapid (b10 Ma) emplacement of the Austroalpine nappes as a tectonic ‘lid’ (Bachmann tectono-metamorphic evolution of the Eastern Alps, but lacks directly et al., 2009; Bickle and Hawkesworth, 1978; Liu et al., 2001; Wallis, supporting field evidence. Tectonic replenishment of heat to footwall 1988; Wallis and Behrmann, 1996). In light of these issues regarding units may be a fundamental mechanism to other overthrust terranes, the mechanism of tectonic insertion we do not consider it a feasible such as the Himalaya, where syn-kinematic Barrovian assemblages are explanation to the problem. observed. Concomitant heating and thrusting are often overlooked in thermal modelling studies. However, our calculations show that they provide 7. Conclusions the impetus for rapid heating of footwall units close to the inception of thrusting (Lbase≤r.τ, where Lbase =distance over which rapid heating 1. We present the first U–Pb geochronological evidence for the age of occurs; r=thrusting rate and τ=thermal time constant of the thrust eclogite-facies metamorphism within the Eclogite Zone, Tauern sheet; Fig. 9). Our allanite geochronology constrains average thrusting Window. Allanite–clinozoisite grew during the prograde portion 202 A.J. Smye et al. / Earth and Planetary Science Letters 306 (2011) 193–204

Fig. 10. Tectonic schematic of scenario in which concomitant syn-thrusting heating of the Penninic basement and plate velocity exhumation of the Eclogite Zone occur. This is possible because the same basal thrust is responsible for the rates of both thrust sheet emplacement and exhumation. Isotherms are approximate and represent an early thermal architecture associated with oceanic subduction. Continental collision would raise isotherms to shallower crustal levels.

oftheEclogiteZone'sP–T–t path, between 8 and 13 kbar at 34.2± Ague, J., Baxter, E., 2007. Brief thermal pulses during mountain building recorded by sr diffusion in apatite and multicomponent diffusion in garnet. Earth Planet. Sci. Lett. 3.6 Ma, prior to initiation of garnet growth. Combined with – – 261, 500 516. previously reported Barrovian metamorphism at 27 32 Ma, this Bachmann, R., Glodny, J., Oncken, O., Seifert, W., 2009. Abandonment of the South places a maximum 10 Ma period between ∼28 and 38 Ma during Penninic–Austroalpine palaeosubduction zone, Central Alps, and shift from which the peak of East Alpine eclogite-facies metamorphism subduction erosion to accretion: constraints from Rb/Sr geochronology. J. Geol. Soc. 166, 217. occurred. Eclogites facies rocks were exhumed at plate velocity Baldwin, S., Monteleone, B., Webb, L., Fitzgerald, P., Grove, M., Hill, E., 2004. Pliocene − rates, between 1 and 6 cm·a 1, over a vertical distance of ∼50 km. eclogite exhumation at plate tectonic rates in eastern Papua New Guinea. Nature 2. These new geochronological data drastically shorten the duration of 431, 263–267. Baxter, E., Ague, J., Depaolo, D., 2002. Prograde temperature–time evolution in the time available for conductive relaxation of isotherms following thrust Barrovian type-locality constrained by Sm/Nd garnet ages from Glen Clova, sheet emplacement, from ∼30 Ma to ≤10 Ma. We show that the Scotland. J. Geol. Soc. 159, 71. thermal evolution of the Tauern Window cannot be explained by Behrmann, J., Ratschbacher, L., 1989. Archimedes revisited: a structural test of eclogite emplacement models in the Austrian Alps. Terra Nova 1, 242–252. conductive heating of overthickened crust on lithosphere with Bickle, M., 1973. Studies across the Matrei Zone. Ph.D. thesis. Oxford. normal continental thermal gradients. Either the thermal evolution Bickle, M., Hawkesworth, C., 1978. Deformation phases and the tectonic history of the took place with a significant thermal contribution from the mantle, Eastern Alps. Geol. Soc. Am. Bull. 89, 293. fl for which there is little supporting evidence or heating occurred Bickle, M., McKenzie, D., 1987. The transport of heat and matter by uids during metamorphism. Contrib. Mineral. Petrol. 95, 384–392. during thrusting with heat provided by an overlying thrust sheet. It is Bickle, M., Hawkesworth, C., England, P., Athey, D., 1975. A preliminary thermal model possible that syn-thrusting heating by rapid emplacement of a hot for regional metamorphism in the Eastern Alps. Earth Planet. Sci. Lett. 26, 13. overthrust sheet, overlooked in previous thermal models, could Burg, J., Gerya, T., 2005. The role of viscous heating in barrovian metamorphism of fl collisional orogens: thermomechanical models and application to the lepontine plausibly have played an in uential role in the tectono-metamorphic dome in the Central Alps. J. Metamorphic Geol. 23, 75–95. evolution of the Eastern Alps and in overthrust terrains in general. Caddick, M., Bickle, M., Harris, N., Holland, T., Horstwood, M., Parrish, R., Ahmad, T., 2007. Burial and exhumation history of a lesser Himalayan schist: recording the formation of an inverted metamorphic sequence in NW India. Earth Planet. Sci. Lett. Supplementary materials related to this article can be found online 264, 375–390. at doi:10.1016/j.epsl.2011.03.037. Cloos, M., 1982. Flow melanges: numerical modeling and geologic constraints on their origin in the Franciscan Subduction Complex, California. Geol. Soc. Am. Bull. 93, 330. Cornelius, H., Clar, E., für Bodenforschung, R., 1939. Geologie des Grossglocknerge- Acknowledgements bietes. Zweigstelle Wien der Reichstelle für Bodenforschung früher Geologische Bundesanstalt. Dachs, E., 1986. High-pressure mineral assemblages and their breakdown-products in A.S further acknowledges support of NERC studentship NE/ metasediments south of the Grossvenediger, Tauern Window, Austria. Schweiz. F007647/1. Matt Horstwood (NIGL) is thanked for directing LA–ICP– Min. Petr. Mitt. 66, 145–161. MC–MS work. We thank John Cottle and Vannessa Paschley (NIGL) for Dachs, E., 1990. Geothermobarometry in metasediments of the southern Grossvene- – – – – diger area (Tauern Window, Austria). J. Metamorphic Geol. 8, 217 230. guidance and advice with LA ICP MC MS work. Peter Nabelek Dachs, E., Proyer, A., 2001. Relics of high-pressure metamorphism from the (University of Missouri) is thanked for discussions regarding thermal Grossglockner region, Hohe Tauern, Austria: paragenetic evolution and PT-paths modelling. Clare Warren (Open University) provided helpful reviews of retrogressed eclogites. Eur. J. Mineral. 13, 67. fi De Sigoyer, J., Guillot, S., Lardeaux, J., Mascle, G., 1997. Glaucophane-bearing eclogites in of an earlier version of the manuscript and assisted with eldwork. the Tso Morari Dome (eastern Ladakh, NW Himalaya). Eur. J. Mineral. 9, 1073. Mark Caddick (ETHZ) is thanked for advice and discussion both within De Sigoyer, J., Chavagnac, V., Blichert-Toft, J., Villa, I., Luais, B., Guillot, S., Cosca, M., the field and in the lab. Max Wigly ran bulk rock ICPMS analyses. Mascle, G., 2000. Dating the Indian continental subduction and collisional thickening in the northwest Himalaya: multichronology of the Tso Morari eclogites. Constructive reviews by two anonymous reviewers served to Geology 28 10, 0091–7613. highlight additional areas of importance and clarify the manuscript. De Sigoyer, J., Guillot, S., Dick, P., 2004. Exhumation of the ultra high-pressure Tso Morari unit in eastern Ladakh (NW Himalaya): a case study. Tectonics 23, 3. Dewey, J., Helman, M., Knott, S., turco, E., Hutton, D., 1989. Kinematics of the western References Mediterranean. In: Coward, M.P., Dietrich, D. (Eds.), Alpine Tectonics vol. 45. Geological Society London Special Publications, London, pp. 265–283. Agard, P., Yamato, P., Jolivet, L., Burov, E., 2009. Exhumation of oceanic blueschists and Droop, G., 1985. Alpine metamorphism in the south-east Tauern Window, Austria: 1 P–T eclogites in subduction zones: timing and mechanisms. Earth-Sci. Rev. 92, 53–79. variations in space and time. J. Metamorphic Geol. 3, 371–402. A.J. Smye et al. / Earth and Planetary Science Letters 306 (2011) 193–204 203

England, P., 1978. Some thermal considerations of the Alpine metamorphism—past, Kuhlemann, J., 2000. Post-collisional sediment budget of circum-Alpine basins (central present and future. Tectonophysics 46, 21–40. Europe). Memorie degli Istituti di Geologia e Mineralogia dell' Universita di Padova, England, P., 1981. Metamorphic pressure estimates and sediment volumes for the v. 52p. 1–91. alpine orogeny: an independent control on geobarometers? Earth Planet. Sci. Lett. Kuhlemann, J., Dunkl, I., Brügel, A., Spiegel, C., Frisch, W., 2006. From source terrains of 56, 387–397. the Eastern Alps to the Molasse basin: detrital record of non-steady-state England, P., Holland, T., 1979. Archimedes and the Tauern eclogites: the role of exhumation. Tectonophysics 413, 301–316. buoyancy in the preservation of exotic eclogite blocks. Earth Planet. Sci. Lett. 44, Kurz, W., Neubauer, F., Genser, J., 1996. Kinematics of Penninic nappes (Glockner nappe 287–294. and basement-cover nappes) in the Tauern Window (Eastern Alps, Austria) during England, P., Richardson, S., 1977. The influence of erosion upon the mineral fades of subduction and Penninic–Austroalpine collision. Eclogae geol. Helv. 89, 573–605. rocks from different metamorphic environments. J. Geol. Soc. 134, 201. Kurz, W., Neubauer, F., Genser, J., Dachs, E., 1998. Alpine geodynamic evolution of England, P., Thompson, A., 1984. Pressure–temperature–time paths of regional passive and active continental margin sequences in the Tauern Window (Eastern metamorphism I. Heat transfer during the evolution of regions of thickened Alps, Austria, Italy): a review. Geol. Rundsch. 87, 225–242. continental crust. J. Petrol. 25, 894. Kurz, W., Handler, R., Bertoldi, C., 2008. Tracing the exhumation of the Eclogite Zone Ercit, T., 2002. The mess that is “allanite”. Can. Mineral. 40, 1411. (Tauern Window, Eastern Alps) by 40Ar/39Ar dating of white mica in eclogites. Eremin, K., 1994. Metamorphic Conditions within the Eclogite Zone, Tauern Window, Swiss J. Geosci. 101, 191–206. Austria. Ph.D. thesis. University of Cambridge. Lachenbruch, A., 1968. Preliminary geothermal model of the Sierra Nevada. J. Geophys. Frank, W., Höck, V., Miller, C., 1987. Metamorphic and tectonic history of the central Res. 73, 6977–6989. Tauern Window. In: Flügel, H.W., Faupl, P. (Eds.), Geodynamics of the Eastern Alps. Lachenbruch, A., 1970. Crustal temperature and heat production: implication of the Franz Deuticke, Vienna, 1987. linear heat flow relation. J. Geophys. Res. 75, 3291–3300. Franz, G., Spear, F., 1983. High pressure metamorphism of siliceous dolomites from the Leech, M., Singh, S., Jain, A., Klemperer, S., Manickavasagam, R., 2005. The onset of central Tauern Window, Austria. Am. J. Sci. 283, 396–413. India–Asia continental collision: early, steep subduction required by the timing of Gerya, T., Stöckhert, B., 2006. Two-dimensional numerical modeling of tectonic and UHP metamorphism in the western Himalaya. Earth Planet. Sci. Lett. 234, 83–97. metamorphic histories at active continental margins. Int. J. Earth. Sci. 95, 250–274. Liu, Y., Genser, J., Handler, R., Friedl, G., Neubauer, F., 2001. 40Ar/39Ar muscovite ages Gerya, T., Stöckhert, B., Perchuk, A., 2002. Exhumation of high-pressure metamorphic from the Penninic–Austroalpine plate boundary, Eastern Alps. Tectonics 20, rocks in a subduction channel: a numerical simulation. Tectonics 21, 1056. 526–547. Getty, S., Selverstone, J., 1994. Stable isotopic and trace element evidence for restricted Ludwig, K., 2003. Isoplot/ex Version 3.00: A Geochronological Toolkit for Microsoft fluid migration in 2 GPa eclogites. J. Metamorphic Geol. 12, 747. Excel. Berkeley Geochronological Center, Berkeley, CA. Gleißner, P., Glodny, J., Franz, G., 2007. Rb–Sr isotopic dating of pseudomorphs after Lyubetskaya, T., Ague, J., 2010. Modeling metamorphism in collisional orogens intruded lawsonite in metabasalts from the Glockner nappe, Tauern Window, Eastern Alps. by magmas: I. Thermal evolution. Am. J. Sci. 310, 427. Eur. J. Mineral. 19, 723–734. Marty, B., O'Nions, R., Oxburgh, E., Martel, D., Lombardi, S., 1992. Helium isotopes in Glodny, J., Ring, U., Kühn, A., Gleissner, P., Franz, G., 2005. Crystallization and very rapid alpine regions. Tectonophysics 206, 71–78. exhumation of the youngest Alpine eclogites (Tauern Window, Eastern Alps) from Meffan-Main, S., Cliff, R., Barnicoat, A., Lombardo, B., Compagnoni, R., 2004. A tertiary Rb/Sr mineral assemblage analysis. Contrib. Mineral. Petrol. 149, 699–712. age for alpine high-pressure metamorphism in the Gran Paradiso massif, western Glodny, J., Ring, U., Kühn, A., 2008. Coeval high-pressure metamorphism, thrusting, Alps: a Rb–Sr microsampling study. J. Metamorphic Geol. 22, 267–281. strike–slip, and extensional shearing in the Tauern Window, Eastern Alps. Miller, C., 1974. On the metamorphism of the eclogites and high-grade blueschists from Tectonics 27. the Penninic terrane of the Tauern Window. Austria. Schweiz. Min. Petr. Mitt 54, Gregory, C., Rubatto, D., Allen, C., Williams, I., Hermann, J., Ireland, T., 2007. Allanite 371–384. micro-geochronology: a LA–ICP–MS and SHRIMP U–Th–Pb study. Chem. Geology Miller, C., Konzett, J., Tiepolo, M., Armstrong, R., Thöni, M., 2007. Jadeite-gneiss from the 245, 162–182. Eclogite Zone, Tauern Window, Eastern Alps, Austria: metamorphic, geochemical Grujic, D., Casey, M., Davidson, C., Hollister, L., Kündig, R., Pavlis, T., Schmid, S., 1996. and zircon record of a sedimentary protolith. Lithos 93, 68–88. Ductile extrusion of the higher Himalayan crystalline in Bhutan: evidence from Oliver, G., Chen, F., Buchwaldt, R., Hegner, E., 2000. Fast tectonometamorphism and quartz microfabrics. Tectonophysics 260, 21–43. exhumation in the type area of the Barrovian and Buchan zones. Geology 28, 459. Hawkesworth, C., 1974a. Vertical distribution of heat production in the basement of the Oxburgh, E., 1968. An outline of the geology of the central Eastern Alps. Proceedings of Eastern Alps. Nature 249, 435–436. the Geologists' Association, 79, pp. 1–46. Hawkesworth, C., 1974a. Geochemical studies in and around the southest corner of the Oxburgh, E., England, P., 1980. Heat flow and the metamorphic evolution of the Eastern Tauern Window. Ph.D. thesis. Oxford. Alps. Eclogae geol. Helv. 73, 379–398. Heard, H., 1963. Effect of large changes in strain rate in the experimental deformation of Oxburgh, E., Turcotte, D., 1974. Thermal gradients and regional metamorphism in yule marble. J. Geology 71, 162. overthrust terrains with special reference to the Eastern Alps. Schweiz. Min. Petr. Hermann, J., 2002. Allanite: thorium and light rare earth element carrier in subducted Mitt. 54, 642–662. crust. Chem. Geology 192, 289–306. Parrish, R., Gough, S., Searle, M., Waters, D., 2006. Plate velocity exhumation of Holland, T., 1979a. Experimental determination of the reaction paragonite=jadeite+ ultrahigh-pressure eclogites in the Pakistan Himalaya. Geology 34, 989.

kyanite+H2O, and internally consistent thermodynamic data for part of the Petrik, I., Broska, I., Lipka, J., Siman, P., 1995. Granitoid allanite–(ce) substitution system Na2O–Al2O3–SiO2–H2O, with applications to eclogites and blueschists. relations, redox conditions and REE distributions (on an example of I-type Contrib. Mineral. Petrol. 68, 293–301. granitoids, western Carpathians, Slovakia). Geol. Carpathica 46, 79–94. Holland, T., 1979b. High water activities in the generation of high pressure kyanite Ratschbacher, L., Dingeldey, C., Miller, C., Hacker, B., McWilliams, M., 2004. Formation, eclogites of the Tauern Window, Austria. J. Geology 87, 1–27. subduction, and exhumation of Penninic oceanic crust in the Eastern Alps: time Holland, T., Powell, R., 1998. An internally consistent thermodynamic data set for constraints from 40Ar/39Ar geochronology. Tectonophysics 394, 155–170. phases of petrological interest. J. Metamorphic Geol. 16, 309–344. Renner, J., Stöckhert, B., Zerbian, A., Röller, K., Rummel, F., 2001. An experimental study Holland, T., Ray, N., 1985. Glaucophane and pyroxene breakdown reactions in the into the rheology of synthetic polycrystalline coesite aggregates. J. Geophys. Res. Pennine units of the Eastern Alps. J. Metamorphic Geol. 3, 417–438. 106, 19411. Holland, T., Richardson, S., 1979. Amphibole zonation in metabasites as a guide to the Richardson, S., England, P., 1979. Metamorphic consequences of crustal eclogite evolution of metamorphic conditions. Contrib. Mineral. Petrol. 70, 143–148. production in overthrust orogenic zones. Earth Planet. Sci. Lett. 42, 183–190. Hollister, L., 1966. Garnet zoning: an interpretation based on the Rayleigh fractionation Rubatto, D., Hermann, J., 2001. Exhumation as fast as subduction? Geology 29, 3. model. Science 154, 1647–1651. Schärer, U., 1984. The effect of initial 230Th disequilibrium on young U–Pb ages: the Hoschek, G., 2001. Thermobarometry of metasediments and metabasites from the Makalu case, Himalaya. Earth Planet. Sci. Lett. 67, 191–204. Eclogite Zone of the Hohe Tauern, Eastern Alps, Austria. Lithos 59, 127–150. Schmid, S., Fügenschuh, B., Kissling, E., Schuster, R., 2004. TRANSMED transects iv, v and Hoschek, G., Konzett, J., Tessadri, R., 2010. Phase equilibria in quartzitic garnet–kyanite– vi: three lithospheric transects across the Alps and their forelands. In: Cavazza, W., chloritoid micaschist from the Eclogite Zone, Tauern Window, Eastern Alps. Eur. J. Roure, F., Spakman, W., Stampfli, G., Ziegler, P. (Eds.), The TRANSMED Atlas: the Mineral. 22, 721–732. Mediterranean Region from crust to mantle. Springer Verlag, Berlin, p. 44. Huerta, A., Royden, L., Hodges, K., 1999. The effects of accretion, erosion and radiogenic Sclater, J., Jaupart, C., Galson, D., 1980. The heat flow through oceanic and continental heat on the metamorphic evolution of collisional orogens. J. Metamorphic Geol. 17, crust and the heat loss of the Earth. Rev. Geophys. 18, 269–311. 349–366. Sclater, J., Parsons, B., Jaupart, C., 1981. Oceans and continents: similarities and Inger, S., Cliff, R., 1997. Timing of metamorphism in the Tauern Window, Eastern Alps: differences in the mechanisms of heat loss. Journal of Geophysical Research 86, Rb–Sr ages and fabric formation. J. Metamorphic Geol. 12, 695–707. 11535. Jamieson, R., Beaumont, C., Fullsack, P., Lee, B., 1998. Barrovian metamorphism: where's Searle, M., Law, R., Godin, L., Larson, K., Streule, M., Cottle, J., Jessup, M., 2008. Defining the heat? Geol. Soc. Lon. Spec. Pub. 138, 23–51. the Himalayan main central thrust in Nepal. J. Geol. Soc. 165, 523. Janots, E., Brunet, F., Goffe, B., Poinssot, C., Burchard, M., Cemič, L., 2007. Thermochem- Sherlock, S., Kelley, S., 2002. Excess argon evolution in hp-lt rocks: a uvlamp study of istry of monazite-(La) and dissakisite-(La): implications for monazite and allanite phengite and k-free minerals, NW Turkey. Chem. Geology 182, 619–636. stability in metapelites. Contributions to Mineralogy and Petrology 154, 1–14. Siegesmund, S., Heinrichs, T., Romer, R., Doman, D., 2007. Age constraints on the Janots, E., Engi, M., Berger, A., Allaz, J., Schwarz, J., Spandler, C., 2008. Prograde evolution of the Austroalpine basement to the south of the Tauern Window. Int. J. metamorphic sequence of ree minerals in pelitic rocks of the Central Alps: implications Earth. Sci. 96, 415–432. for allanite–monazite–xenotime phase relations from 250 to 610 °C. J. Metamorphic Sinclair, H., 1997. Flysch to molasse transition in peripheral foreland basins: the role of Geol. 26, 509–526. the passive margin versus slab breakoff. Geology 25, 1123. Janots, E., Engi, M., Rubatto, D., Berger, A., Gregory, C., Rahn, M., 2009. Metamorphic Smye, A., Greenwood, L., Holland, T., 2010. Garnet–chloritoid–kyanite assemblages: rates in collisional orogeny from in situ allanite and monazite dating. Geology 37, eclogite facies indicators of subduction constraints in orogenic belts. J. Metamor- 11. phic Geol. 28, 753–768. 204 A.J. Smye et al. / Earth and Planetary Science Letters 306 (2011) 193–204

Spear, F., 1986. PT evolution of metasediments from the Eclogite Zone, south-central Wallis, S., Behrmann, J., 1996. Crustal stacking and extension recorded by tectonic Tauern Window, Austria. Lithos 19, 219–234. fabrics of the SE margin of the Tauern Window, Austria. J. Struct. Geol. 18, Stacey, J., Kramers, J., 1975. Approximation of terrestrial lead isotope evolution by a 1455–1470. two-stage model. Earth Planet. Sci. Lett. 26, 207–221. Warren, C., Beaumont, C., Jamieson, R., 2008. Deep subduction and rapid exhumation: Stöckhert, B., 2002. Stress and deformation in subduction zones: insight from the record role of crustal strength and strain weakening in continental subduction and of exhumed metamorphic rocks. In: De Meer, S., Drury, M.R., De Bresser, J.H.P., ultrahigh-pressure rock exhumation. Tectonics 27, 28. Pennock, G.M. (Eds.), Current issues and new developments in deformation Warren, C., Sherlock, S., Kelley, S., 2010. Interpreting high-pressure phengite 40 Ar/39 mechanisms, rheology and tectonics, vol.200. Geological Society London Special Ar laserprobe ages: an example from Saih Hatat, NE Oman. Contrib. Mineral. Petrol. Publications, London, pp. 207–221. 1–19. Stöckhert, B., Massonne, H., Nowlan, E., 1997. Low differential stress during high- Waters, D., 1976. Structural, metamorphic and geochronological studies in the pressure metapelite from the Eclogite Zone, Tauern Window, Eastern Alps. Lithos southeast Tauern. Ph.D. thesis. University of Oxford. 41, 103–118. Whittington, A., Hofmeister, A., Nabelek, P., 2009. Temperature-dependent thermal Stüwe, K., Sandiford, M., 1995. Mantle-lithospheric deformation and crustal metamor- diffusivity of the Earth's crust and implications for magmatism. Nature 458, 319–321. phism with some speculations on the thermal and mechanical signiﬁcance of the Wiedenbeck, M., Alle, P., Corfu, F., Grifﬁn, W., Meier, M., Oberli, F., Quadt, A., Roddick, J., Tauern event, Eastern Alps. Tectonophysics 242, 115–132. Spiegel, W., 1995. Three natural zircon standards for U–Th–Pb, Lu–Hf, trace Thoni, M., Jagoutz, E., 1992. Some new aspects of dating eclogites in orogenic belts: Sm– element and REE analyses. Geostandards Geoanalytical Res. 19, 1–23. Nd, Rb–Sr, and Pb–Pb results from the Austroalpine Saualpe and Koralpe type- Zimmermann, R., Hammerschmidt, K., Franz, G., 1994. Eocene high pressure locality (Carinthia/Styria, SE Austria). Geochim. Cosmochim. Acta 56, 347–368. metamorphism in the Penninic units of the Tauern Window (Eastern Alps): Van Breemen, O., Hawkesworth, C., 1980. Sm–Nd isotopic study of garnets and their evidence from 40Ar–39Ar dating and petrological investigations. Contrib. Mineral. metamorphic host rocks. Trans. Roy. Soc. Edinburgh 71, 97–102. Petrol. 117, 175–186. Wallis, S., 1988. The structural and kinematic development of the Austroalpine Pennine boundary in the S.E.Tauern, E.Alps. Ph.D. thesis. University of Oxford.