7) 68–88www.elsevier.com/locate/lithos

Lithos 93 (200

Jadeite-gneiss from the Eclogite Zone, Tauern Window, Eastern Alps,Austria: Metamorphic, geochemical and zircon record of a

sedimentary protolith

C. Miller a,⁎, J. Konzett a, M. Tiepolo b, R.A. Armstrong c, M. Thöni d

a Institut für Mineralogie und Petrographie, University of Innsbruck, Innrain 52, A-6020 Innsbruck, Austriab CNR-Istituto di Geosienze e Georisorse-Sede di Pavia, via Ferrata 1, I-27100 Pavia, Italyc Research School of Earth Sciences, Australian National University, Canberra, Australia

d Institut für Geologische Wissenschaften, Geozentrum, University of Vienna, Althanstrasse 14, A-1090 Vienna, Austria

Received 11 November 2005; accepted 22 March 2006Available online 5 June 2006

Abstract

In the Eclogite Zone of the Tauern Window, a layer of strongly retrogressed leucocratic jadeite-bearing gneiss is intercalatedbetween mafic kyanite-eclogites. The jadeite-gneiss consists of garnet+quartz+paragonite±phengite±zoisite+zircon+rutile+apatite+pyrite. Kyanite, jadeite or omphacite are exclusively present as inclusions in garnet. Retrogressive hydration duringexhumation led to a breakdown of matrix jadeite to form pseudomorphs of calcic amphibole+albite. Peak metamorphic conditionsderived from the primary gneiss assemblage are 2.0–2.4 GPa at approximately 640 °C. Major, trace element and isotopiccompositions of the jadeite-gneiss are consistent with a siliciclastic sedimentary protolith. Zircon morphology and zonation patternsreveal a complex history. The presence of fracture-truncated zircons suggests a detrital origin, whereas most internal structures andTh/U ratios are characteristic of zircons from magmatic rocks. In situ LA-ICP-MS and SHRIMP U–Pb geochronology and zircongeochemisty provide evidence of at least three magmatic events in the provenance area. These were dated at 466±2 Ma, 437±2 Maand 288±9 Ma. Older ages ranging from 503 to 691 Ma are preserved in the cores of some zircon grains, suggesting derivationfrom peri-Gondwanan sources. Surprisingly, no firm evidence of the Tertiary high-pressure metamorphic event and subsequentretrograde overprint was seen in any of the studied zircons. However, some zircons show resorbed surfaces suggesting corrosion bya superficial fluid phase undersaturated in zirconium and one extensively altered porous zircon yielded highly discordant206Pb/238U ages in the range 325–109 Ma documenting partial recrystallization by dissolution–reprecipitation of a highly reactivegrain.© 2006 Elsevier B.V. All rights reserved.

Keywords: Tauern window; Eastern Alps; High-pressure metasediment; U–Pb dating; Zircon

1. Introduction

The Alpine orogen is a continental collision beltresulting from the closure of the Neotethys ocean by

⁎ Corresponding author. Tel.: +43 512 507 5508.E-mail address: Christine.Miller@uibk.ac.at (C. Miller).

0024-4937/$ - see front matter © 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.lithos.2006.03.045

south-directed subduction and collision of the Europeanand Apulian plates in the Cretaceous and Tertiary (e.g.Stampfli et al., 1998). During the Alpine orogeny, high-pressure metamorphism affected rocks from differentpalaeogeographic domains, such as the Apulian conti-nental crust, the Piemont–Ligurian oceanic lithosphereand metasediments, the Briançonnais and the European

69C. Miller et al. / Lithos 93 (2007) 68–88

continental margin. In the Western and Central Alps thetiming of this complex geodynamic evolution is wellconstrained by SHRIMP data for zircons: high-pressuremetamorphism of the subducted continental Sesia–Lanzo Zone occurred at approximately 65 Ma, followedby subduction and high-pressure/ultrahigh-pressuremetamorphism of the oceanic crust of the Piemont–Ligurian ocean at approximately 44 Ma and of theAdula–Cima Lunga nappe system and the InternalMassifs 33–35 Ma ago (e.g. Rubatto et al., 1998, 1999;Gebauer, 1999; Lapen et al., 2003; Rubatto andHermann, 2003; Federico et al., 2005). In the EasternAlps, high-pressure metamorphism during Alpinecompression has been dated for the Koralpe–Saualpe–Pohorje region in the eastern parts of the Austroalpinebasement: Sm–Nd garnet and U–Pb zircon ages fromeclogite assemblages cluster at 90±2 Ma (Thöni, 2002;Miller et al., 2005), documenting subduction of thesoutheastern Austroalpine units in the course of thecomplex collision of the Apulian microplate andEurope. In the Eclogite Zone of the Tauern Window,peak pressures were reached shortly before 45 Ma,based on 39Ar/40Ar geochronology (Ratschbacher et al.,2004) whereas Glodny et al. (2005) suggest an

Fig. 1. Simplified tectonic map showing the Tauern Window,

Oligocene age (31.5±0.7 Ma), based on multimineralRb/Sr internal isochrons.

In the Tauern Eclogite Zone, Alpine high-pressuremetamorphism is documented in metabasic eclogitesand metasediments. In this paper we present petrologicaland geochemical data on quartz-rich zircon-bearinggneisses that are interlayered with kyanite-eclogites atthe Steinsteg locality in order to document the impact ofhigh-pressure metamorphism on one of the rare meta-acidic lithologies, and to evaluate the nature of theprotolith. Since the jadeite-gneiss turned out to be ametasediment we also studied the detrital zirconpopulation in order to constrain provenance and theage of the sediment source.

2. Regional geology

The Tauern Window exposes Penninic units ofcontinental and oceanic affinities in the footwall of theAustroalpine nappe system (Fig. 1). The stacking of thisnappe sequence and metamorphism occurred in re-sponse to the closure of the Neotethys and continent–continent collision in late Cretaceous–Tertiary time. Inthe central Tauern Window the Eclogite Zone is

the Eclogite Zone and the Steinsteg sampling locality.

Fig. 2. Backscattered electron images showing (a) a symplectiticintergrowth of albite and calcic amphibole interpreted as pseudomorphafter jadeite or omphacite and (b) inclusions of jadeite and quartz ingarnet of jadeite-gneiss CM73/01.

70 C. Miller et al. / Lithos 93 (2007) 68–88

tectonically sandwiched between European basementunits in the footwall and an imbricate nappe stack in thehanging-wall including the Rote Wand–Moderecknappe, the Glockner nappe, the Matrei Zone andAustroalpine basement nappes. The palaeo-Europeanunits in the footwall of the Eclogite Zone comprise apre-Variscan basement complex intruded by Variscangranitoids, the Palaeozoic volcano-sedimentary Habachformation and a cover sequence of Jurassic meta-carbonates and Cretaceous meta-pelites and meta-psammites. The Eclogite Zone comprises mafic eclo-gites and high-pressure metasediments that represent aMesozoic volcano-sedimentary sequence of a distalcontinental slope (Miller et al., 1980). The eclogitefacies rocks were buried to minimum depths of 65 km(2 GPa, ±600 °C; e.g. Holland, 1979; Franz and Spear,1983; Miller, 1986; Spear and Franz, 1986; Hoschek,2001) and subsequently overprinted by a blueschistfacies metamorphic event (Miller, 1974, 1977; Holland,1979; Frank et al., 1987; Zimmermann et al., 1994; Kurzet al., 1998). Finally, the entire nappe pile underwent aBarrovian-type upper greenschist to lower amphibolitefacies metamorphism (Miller, 1977; Frank et al., 1987;Dachs, 1990; Inger and Cliff, 1994; Zimmermann etal., 1994). The age of the eclogite facies metamorphismis unconstrained: Christensen et al. (1994) suggested ahigh-pressure event older than 62±1.5 Ma, based on Srisotopic zonation of a garnet from the footwall of theEclogite Zone, whereas Ratschbacher et al. (2004)argue for a ∼45 Ma age for the eclogite metamorphismbased on 40Ar/39Ar geochronology. Multi-mineral Rb/Sr internal isochrons with a weighted average of 31.5±0.7 Ma are interpreted by Glodny et al. (2005) to datethe eclogite facies assemblage crystallization. On theother hand, 40Ar/39Ar ages between 32 and 36 Ma ofphengitic micas are thought to date the blueschist faciesoverprint of eclogite-facies lithologies in the course ofthe continent–continent collision between the Adriaticand European plates (Zimmermann et al., 1994).According to Inger and Cliff (1994) the Sr isotopesin the Eclogite Zone metasediments were reset duringthe greenschist facies overprint at 28–30 Ma.

3. Petrography

A layer of strongly retrogressed jadeite-gneissseveral meters thick shows concordant contact relationswith kyanite-eclogites near Steinsteg, Frosnitztal(N47°04.189´ E12°27.070´, elevation 2005 m). Theeclogites consist of garnet+omphacite+kyanite+ talc+quartz + rutile ± paragonite ± glaucophane ± zoisite ±magnesite. The jadeite-gneiss samples T1404–T1406

and CM65/01–CM73/01 have a medium-grained gran-ular texture and contain garnet, quartz, phengite,paragonite rimmed by clinozoisite and conspicuoussymplectitic intergrowths consisting of albite, quartzand calcic amphibole (Fig. 2a). Accessory mineralsinclude zoisite, dolomite, zircon, rutile, apatite andpyrite. Garnet contains inclusions of quartz+ rutile±jadeite or omphacite±kyanite±paragonite (Fig. 2b).The presence of relic jadeite and omphacite provides anexplanation of the amphibole-albite intergrowths asbreakdown products of jadeite during retrogressivehydration.

4. Analytical procedures

Whole rock major and trace elements were performedthrough the Service d'Analyses des Roches et desMinéraux du CNRS (SARM) in Nancy, using interna-tional geostandards (Govindaraju, 1994), ICP-AES(major elements) and ICP-MS (trace elements). Thecomposition of mineral phases was determined using aJEOL JXA-8100 superprobe (Institut für Mineralogieund Petrogrphie, University of Innsbruck) with

Table 1Chemical composition of jadeite-gneisses, Eclogite Zone, TauernWindow, Eastern Alps

CM65/01 CM66/01 CM67/01

SiO2 81.94 74.54 77.32TiO2 0.21 0.45 0.39Al2O3 7.85 12.81 11.22Fe2O3 2.55 3.06 2.90MnO 0.04 0.03 0.03MgO 0.92 1.57 1.34CaO 3.24 1.01 1.09Na2O 1.38 3.14 3.42K2O 0.45 1.63 0.91P2O5 0.06 <0.03 0.08LOI 1.17 1.56 1.08Total 99.81 99.80 99.77Be 1 2 2Sc 12 10 9V 38 73 59Cr 34 52 46Co 5 7 6Ni 17 33 30Cu 19 27 18Zn 54 44 36Ga 10 16 14Rb 15 51 32Sr 232 102 108Y 10 17 15Zr 103 155 140Nb 4.7 9.9 8.1Ba 57 228 132U 1.38 1.94 1.94Pb 24 14 11Hf 2.7 4.1 3.8Ta 0.45 0.88 0.77Th 4.2 7.4 6.9La 10.2 19.8 20.4Ce 20.4 40.2 40.5Pr 2.43 4.57 4.70Nd 9.3 17.1 17.6Sm 1.95 2.85 3.40Eu 0.50 0.76 0.83Gd 1.69 2.96 2.92Tb 0.26 0.47 0.44Dy 1.55 2.86 2.64Ho 0.31 0.57 0.54Er 0.93 1.66 1.39Tm 0.14 0.25 0.25Yb 1.05 1.72 1.51Lu 0.16 0.26 0.2287Rb/86Sr 0.18 1.46 0.85787Sr/86Sr 0.71 0.71 0.71±2σ ±0.000002 ±00.000002 ±00.000005147Sm/144Nd 0.13 0.12 0.12143Nd/144Nd 0.512203 0.512170 0.512180±2σ ±0.000002 ±0.000002 ±0.000003

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wavelength-dispersive analytical modes with 15 kVacceleration voltage and beam current of 10 to 20 nA.

Nd and Sm concentrations were determined by ICP-MS. Nd and Sr isotopic compositions were measured ona ThermoFinnigan Triton TI multicollector TIMS, usinga Re double filament. Within-run isotope fractionationwas corrected for 146Nd/144Nd=0.7219 (Nd) and86Sr/88Sr=0.1194 (Sr). All errors quoted in Table 1correspond to 2σ. The 143Nd/144Nd and the 86Sr/88Srratios for the La Jolla (Nd) and the NBS 987 (Sr)international standards during the course of thisinvestigation were 0.511849±5 (n=4) and 0.710245±1 (n=4), respectively. The following model parameterswere used for the calculation of depleted mantle (DM)ages: 147Sm/144Nd=0.222, 143Nd/144Nd= 0.513114(Michard et al., 1985). A linear evolution of the Ndisotope composition of the DM is assumed throughoutgeological time, εNd values are calculated relative toCHUR.

For zircon SHRIMP analyses, sections and the grainmounts were cut and combined into epoxy mountstogether with the RSES reference zircons FC1and SL13.Cathodoluminescence (CL) images were used to deci-pher the internal structures of the sectioned grains and totarget specific areas within the zircons for spot analysis.Post-analysis SEM CL imaging was done to confirm thesites of the ion microprobe spots. All U–Pb analyseswere done using SHRIMP II at the Research School ofEarth Sciences, Australian National University with eachanalysis consisting of 6 scans through the relevant massrange. Data were reduced followingWilliams (1998, andreferences therein), using the SQUID Excel Macro ofLudwig (2000) and Pb/U ratios were normalised relativeto a value of 0.1859 for the 206⁎Pb/238U ratio of the FC1reference zircons, equivalent to an age of 1099 Ma(Paces and Miller, 1993). Uncertainties given forindividual analyses (ratios and ages) are at the 1σlevel, however, uncertainties in the calculated weightedmean ages are reported as 95% confidence limits.Concordia plots and weighted mean age calculationswere carried out using Isoplot/Ex (Ludwig, 1999).

Laser ablation (LA)-ICP-MS zircon Pb geochronol-ogy was carried out at the C.N.R. – Istituto diGeoscienze e Georisorse – Sede di Pavia. The instru-ment couples a 193 nm ArF excimer laser (GeoLas102-Microlas) and a high resolution (HR)-ICP-MS typeElement I from ThermoFinnigan, nearly completelyupgraded to the Element II version (CD-torch and fastscanning). For the present work laser spot size was set to25 μm, laser energy density at 13 J/cm2 and repetitionrate at 5 Hz. The whole laser path was fluxed with N2 inorder to increase energy stability. The analytical

procedure is described in Tiepolo (2003). The signalsfrom masses 202Hg, 204Pb, 206Pb, 207Pb, 208Pb, 232Thand 238U were acquired. U–Pb fractionation effects

Fig. 3. Chondrite-normalized (Boynton, 1984) rare earth element plotof jadeite-bearing gneisses from the Tauern Window. The patterns ofPAAS (post-Archean average Australian shale; Taylor and McLennan,1985) and a jadeite quartzite from Dabie Shan (Liou et al., 1997) areshown for comparison. Shaded fields represent ranges of normalizedREE contents found in ophiolitic plagiogranites from Oman (Colemanand Peterman, 1975; Coleman and Donato, 1979; Pallister and Knight,1981) and in trondhjemitic gneisses (Tarney et al., 1979). See text fordiscussion.

Fig. 4. Simplified pseudosection in the KNCFMASH system forjadeite-bearing gneiss CM67/01 calculated with the DOMINOprogram (De Capitani, 1994). PT-fields for peak metamorphicassemblages are outlined in bold or shaded; PT-fields surroundingthose of the peak metamorphic assemblages all contain Grt+Phe+Omp+Qtz+H2O (abbreviations according to Kretz, 1983) in additionto the phases labelled.

72 C. Miller et al. / Lithos 93 (2007) 68–88

were corrected using zircon 91500 (Wiedenbeck et al.,1995) as external standard and adopting the same spotsize and integration intervals on both unknown andstandard sample. Data reduction, isotope ratios andapparent age calculation were carried out with theGLITTER© software (Macquarie Research Ltd, 2001)developed by Van Achterbergh et al. (1999). Concor-dant ages were calculated using the Isoplot/Ex softwareby Ludwig (1999). For each sample the time resolvedsignals and U/Pb ratios were carefully inspected in orderto detect perturbations related to inclusions, mixing ofdifferent age domains or fractures. Integration intervalswere then selected accordingly. The signal of 204Pb,depurated from the interference of Hg, was monitoredfor the presence of common Pb.

Selected zircons were analysed for trace elementcompositions with a laser ablation (LA)-ICP-MSinstrument coupling a 266 nm laser microprobe and aquadrupole ICP-MS (DRCe, PerkinElmer). Spot analy-sis (20 μm in size) was placed as close as possible to thespots previously selected for Pb-geochronology in orderto have the best correspondence between age and traceelement composition. Laser power was set at 0.5 mJ andrepetition at 10 Hz. Nist 612 and Si were used as externaland internal standards, respectively.

5. Geochemical characteristics of thejadeite-bearing gneiss

Three jadeite-gneiss samples were analysed forchemical, and Sr–Nd isotopic compositions in order to

place constraints on the nature of the protolith (Table 1).Bulk compositions are highly silicic but low in K2O(0.45–1.63 wt.%), resulting in CIPW-normative com-positions containing 45.2–63.5 wt.% quartz, 26–34.4 wt.% plagioclase (Ab47–85), 2.7–9.8 wt.% ortho-clase, 0–4.1 wt.% corundum and 4.3–6.9 wt.%hypersthene. The gneiss samples show moderate Rband Sr contents, ranging from 15 to 51 ppm, and from102 to 232 ppm, respectively. Ba contents are 57–228 ppm, Zr contents are moderate at 103–155 ppm andTh contents are low at 4–7 ppm. Chondrite-normalizedrare earth element (REE) patterns (Fig. 3) are moder-ately enriched in light REE (LaN/YbN=7.8–9.0), withflat heavy REE (HREE) distributions (TbN/YbN=1.1–1.3) and negative Eu anomalies (Eu/Eu⁎=0.73–0.83).

Sm–Nd isotopic analysis for three samples yieldedε(0)Nd values of between −8.5 and −9.1, f Sm/Nd

ranging from −0.35 to −0.41 and Depleted Mantlemodel ages of 1.35–1.46 Ga. The present-day 87Sr/86Srratios range between 0.709782–0.710428 (Table 1).

6. Mineral compositions

6.1. Garnet

The garnets are almandine (54–65%), pyrope (16–26%), grossular (15–22%) and spessartine (2–3%) solidsolutions and display a continuous and prograde zoningwith slightly increasing pyrope and grossular, anddecreasing almandine component from core to rim.

73C. Miller et al. / Lithos 93 (2007) 68–88

6.2. Clinopyroxene

Both jadeite and omphacite were found as rareinclusions in garnet only. End-member components werecalculated on the basis of Jd=AlVI/(Na+Ca), Aeg(aegirine)= (Na−AlVI)/(Na+Ca), and Aug=Ca / (Na+Ca). The analysed jadeites show a compositional rangeof Jd89–94Aeg1–6Aug3–5, similar to jadeites reported froman ultrahigh-pressure quartzite from Dabie Shan (Liou etal., 1997) and from high-pressure metapelites from theAdula nappe (Meyre et al., 1999). Omphacite relicsdisplay a compositional range of 59–61 mol% jadeite.

Fig. 5. Cathodoluminescence (CL) images of zircon grains in thin sections owith oscillatory and sector zoning; (b) igneous zircon with apatite-inclusion;zoning partially obliterated by bright Cl-domain; (e) an igneous zircon cut binternal structure and two quartz inclusions.

6.3. White mica

Paragonite contains muscovite and margarite com-ponents of approximately 7 and 2 mol%, respectively.Coexisting phengite has Si-contents of 3.40–3.44 apfu,about 4 mol% paragonite and up to 1 mol% margaritecomponents.

6.4. Zoisite/Clinozoisite

Zoisite grains in the matrix are characterized by Fe3+/(Fe3+ +Al) ratios ranging from0.016 to 0.033. In contrast,

f jadeite-gneiss from the Tauern Window showing: (a) igneous zircon(c) zircon with xenocrystic core; (d) broken zircon with relict growthy a recrystallized bright-CL domain; (f) complex zircon with chaotic

74 C. Miller et al. / Lithos 93 (2007) 68–88

clinozoisite associated with paragonite has much higherFe3+/(Fe3++Al) ratios ranging from 0.12 to 0.31.

6.5. Jadeite-pseudomorphs

Amphiboles in pseudomorphs after jadeite (Fig. 2a)are quite variable in composition, and range fromferri-barroisite, magnesio-hornblende to actinolite (no-menclature after Leake, 1978). Plagioclase coexistingwith Ca-amphibole, quartz and minor clinozoisitewithin the pseudomorphs after jadeite is almost purealbite (Ab98–99).

7. Conditions of metamorphism

The presence of nearly pure jadeite and quartz asinclusions in garnet provides a minimum pressureestimate of approximately 1.6 GPa at 600 °C (Holland,1980) and clearly shows that this felsic gneiss hasexperienced a high-pressure event. Temperature esti-mates using the Krogh Ravna (2000) calibration of thegarnet-clinopyroxene Fe–Mg exchange thermometeryield 636 and 645 °C for two omphacite inclusions ingarnet with Fe3+ calculated by charge balance andassuming a pressure of 2 GPa.

Fig. 6. Backscattered electron (a, c) and CL (b, d) images of two zircon gcontaining curvilinear tubes filled with quartz.

To provide additional constraints on peak metamor-phic conditions, an equilibrium phase diagram in theKNCFMASH system was calculated with THERIAK-DOMINO (De Capitani, 1994) using the database ofBerman (1988) with solution models as follows: garnet(Berman, 1990), phengite (Massonne and Szpurka,1997), omphacite and amphibole (Meyre et al., 1997).The resulting equilibrium phase diagram shows a largestability field for the assemblage garnet+omphacite+phengite+paragonite+quartz at temperatures in excessof 590 °C and pressures between 1.9 and 2.4 GPa (Fig.4). The assemblage garnet +omphacite+phengite+paragonite+kyanite+quartz that is stable to onlyslightly higher pressures is thought to represent peakmetamorphic conditions for eclogitic gneiss CM67/01.The upper pressure limit for this bulk composition isgiven by the stability of kyanite at the expense ofparagonite. PT conditions are thus consistent with datafrom the literature on associated mafic eclogites(Holland, 1979; Hoschek, 2001), kyanite–zoisite mar-bles, garnet–chloritoid–quartz–mica schists (Franz andSpear, 1983; Spear and Franz, 1986), and Mg–chloritoid- and talc-bearing eclogites (Miller, 1986).

These studies, along with that of Kurz et al. (1998)also document a complex exhumation history with

rains in a thin section of jadeite-gneiss CM75/01 (Tauern Window)

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decompression and a blueschist facies overprint atapproximately 1 GPa, followed by a second overprint atupper greenschist to lower amphibolite facies condi-tions. Decompression reactions in the studied jadeite-bearing gneiss samples are documented by ubiquitousjadeite pseudomorphs and minor overgrowths of biotiteon phengite. The replacement of nearly pure jadeite by

Fig. 7. Micro-textures of zircon from jadeite-gneiss, Eclogite Zone, Tauern Wizircons within thin sections of samples CM75/01 (a, e), CM65/01 (b, f), CM7oscillatory zoning characteristics of igneous zircon. However, what appears tfracture-truncated, oscillatory zoned zircon cores are observed in detrital zirc

albite, Ca-amphibole quartz±minor clinozoisite musthave occurred at a temperature of approximately 560 °C,based on the Ca-amphibole-plagioclase thermometer ofHolland and Blundy (1994) as the pressure droppedbelow 1.46 to 1.69 GPa, calculated for the reactionjadeite=albite+quartz with THERMOCALCv3.1 andactivities obtained by ax01 (Holland and Powell, 1998).

ndow. BSE and SE images document the presence of fracture-truncated5/01 (c, g), and CM69/01 (d, f). CL images (i–l) of these grains showo have been the core of these grains is now exposed at the rim. Similaron in modern sediments (Hanchar and Miller, 1993).

76 C. Miller et al. / Lithos 93 (2007) 68–88

8. Zircon morphology and trace elementgeochemistry

106 zircons from seven samples were investigated forthis study. Zircon ranges from 5 to 160 μm in size. 28zircon grains recovered from the jadeite-gneiss arefrequently euhedral, clear and colourless or shaded pink,whereas others are non-transparent with a white colour.The grain surface may be flat, pitted or show signs ofdissolution. CL images reveal complex internal structuressimilar to those observed in 78 zircon grains studied inthin section: many zircon grains have euhedral externalshapes, and both fine-scale oscillatory and sector zoning(Fig. 5a, b). Zoning patterns and inclusions of apatite,xenotime and quartz suggest that they come from anigneous source. Some oscillatory zoned grains havexenocrystic cores (Fig. 5c). Others have a relict growthbanding that, in brightly luminescent domains, becomesextinguished (Fig. 5d). Transgressive recrystallization(Fig. 5e) or irregular, strongly contrasting low-CL andbright-CL domains unrelated to crystal margins (Fig. 5f)are evidence for within-grain recrystallization afterigneous growth. Fig. 6a on the other hand, shows an

Table 2LA-ICP-MS trace element analyses (ppm) of zircon from jadeite-gneiss CM

Element C1#2 rim C1#3 core C1#4 core C1 #6 rim C2#2 rim

Age (Ma) 469 435 442 350 503Li 59.02 <0.94 2.45 4.25 <16.39B <6.16 <8.87 <7.00 <5.48 <5.81Na <12.78 <17.61 8.41 45.74 <2.16Sc 310 336 353 655 266Ti <6.84 16.38 17.02 13.61 <5.95Sr 0.75 <0.152 0.21 0.27 <0.14Y 2050 1376 1863 2763 946Nb 3.89 4.94 4.22 1.97 3.87La 0.08 <0.136 0.10 0.20 <0.03Ce 1.33 1.98 2.14 0.75 0.3Pr 0.09 <0.054 <0.061 <0.042 <0.04Nd 1.21 3.23 0.8 1.79 0.3Sm 4.23 4.53 3.49 3.87 0.5Eu 0.19 0.29 <0.108 <0.044 <0.09Gd 31.48 30.31 30.79 15.26 15.81Tb 11.54 9.7 10.83 13.93 7.07Dy 178.1 123.3 159.1 217.8 87.3Ho 66.4 43.8 62.5 81.5 27.2Er 319.1 209.2 306.0 426.0 94.1Tm 69.9 44.4 68.4 119.6 17.1Yb 626.9 423.5 622.9 1290.7 147.9Lu 125.3 85.1 125.0 242.3 28.7Hf 11,840 10,359 11,994 17,523 14,722Ta 0.56 0.91 0.98 0.73 0.42Pb 4.53 3.5 3.67 1.8 0.86Th 79.6 78.0 80.4 31.0 26.3U 234 182 253 1096 250

example of a complexly altered zircon where quartzpenetrates the interior of a zoned zircon, resulting in achaotic, patchy CL domain. Fig. 6b illustrates a partiallyresorbed and recrystallized zircon grain containingcurvilinear tubes filled with quartz. In thin section,some zircons are fractured fragments of larger grains(Fig. 7a–l), with oscillatory-zoned cores exposed at theedge of the grains (Fig. 7i–l). This fact clearly reflectsresidence of the zircon in a supracrustal environmentprior to high-pressure metamorphism.

The ten zircon domains analysed by LA-ICP-MSfor trace elements (Table 2) have variable U and Thcontents, resulting in Th/U ratios between 0.1 and 0.7except for the rim domain of grain 1#6 that is rich inU (1096 ppm) and poor in Th (31 ppm) with a verylow Th/U ratio of 0.03. The chondrite-normalizedREE patterns (Fig. 8) are characterized by a distinctenrichment in HREE (LuN/SmN=80–862), with posi-tive Ce (Ce/Ce⁎=1.8–267) and negative Eu (⁎=0.02–0.52) anomalies. Anomalous Ce and Eu abundancesare a feature of all igneous zircon (Hoskin andSchaltegger, 2003), with negative Eu anomaliesindicating plagioclase fractionation during or prior to

67/01, Eclogite Zone, Tauern Window, Eastern Alps

C2#2 core C3#3 core C3 #3 rim C4#2 core C4#3 core

642 457 436 293 291<0.88 <2.78 <0.83 <0.66 1.0610.44 <17.50 <7.02 <5.00 <6.494.59 <2.96 7.07 69.95 17.8

180 275 283 228 1818.55 17.83 12.37 <12.91 10.36

3 0.28 <0.26 0.31 0.45 <0.119332 1824 1265 1757 796<0.72 6.46 3.04 7.45 4

4 <0.102 0.08 0.05 0.20 <0.05311.2 13.8 0.61 18.19 3.42

7 <0.00 0.06 <0.062 0.26 0.17<0.40 2.55 0.91 3.84 1.55<0.26 7.57 2.08 8.48 4.38

8 <0.154 0.59 0.09 1.48 <0.0893.09 33.09 17.65 39.42 19.211.52 12.08 6.66 12.53 6.4522.4 151.5 101.2 150.8 77.89.5 60.7 39.2 60.1 28.857.8 277.4 208.4 274.3 122.713.9 61.0 47.0 60.9 25.6151.3 526.3 438.3 586.5 229.537.0 100.5 89.5 122.8 44.7

15,259 12,148 12,516 9634 12,1751.26 1.19 0.38 2.15 0.452.68 5.96 1.6 12.6 2.653.6 109.6 29.0 481.3 83.9169 295 174 874 124

77C. Miller et al. / Lithos 93 (2007) 68–88

zircon crystallization. ∑REE (309–2414 ppm), Hf(9634–17523 ppm) and Y abundances (332–2050 ppm)are within the range commonly reported for igneouszircon suites (Hoskin and Schaltegger, 2003).

9. Results of U–Pb dating of zircons

9.1. SHRIMP age determinations

Three zircon grains from sample T1404 wereanalysed: one single large zircon extracted from thecrushed whole rock and subsequently mounted inepoxy and two zircons in a polished thin section. Theepoxy-mounted single zircon with 190 μm in lengthappears to be a fragment of a larger grain, with a singlechisel-shaped euhedral termination preserved. Thegrain has a longitudinal fracture and an internal

Fig. 8. REE patterns for different zircon domains normalized tochondrite values of Boynton (1984): (a) magmatic zircon withconcordant ages between 435 and 469 Ma (open symbols) andmetamorphic rim domain 1#6 (solid diamond); (b) magmatic zirconwith concordant ages of 291–293 Ma (solid symbols) and grain 2#2(open symbols) with concordant ages of 669 Ma (core) and 503 Ma(rim). See Fig. 12 for location of spots.

structure with a core, which is embayed by subsequentgrowth. This inherited core is enveloped by whatappears to be a typical magmatic zircon with concentriccompositional zoning. Six analyses were done on thisgrain, with the actual sites shown in Fig. 9. The datashow some scatter, with significant apparent agedifferences between the core, embayment and theouter, zoned portions (Table 3, Fig. 10). The twoanalyses sited in the core have lost variable amounts ofradiogenic Pb and give 206⁎Pb/238U ages of 606 and678 Ma. The oldest date is the best estimate of the ageof this core, although it is probably still a minimumage. The single analysis of the embayment structure(CM5.3) has an apparent 206⁎Pb/238U date of 579 Ma.Three analyses (CM5.2, CM5.5, CM5.6) of the zonedouter zircon, interpreted as magmatic growth, can becombined to give a weighted mean 206⁎Pb/238U date of484±29 Ma (95% confidence limits, MSWD=2.5;probability=0.081).

Zircons CM1 and CM2a were measured in thepolished thin section. Zircon CM1 is subhedral, about80 μm in length, and CL imaging shows concentriccompositional zoning throughout the grain (Fig. 11a).Five analyses were done on this grain, with the data givenin Table 3 and plotted in Fig. 10. One analysis had over40% common Pb and was excluded from any furtherconsideration, but the remaining analyses gave aweightedmean 206⁎Pb/238U date of 459± 13 Ma (95% confidencelimits, MSWD=1.4; probability=0.23). Zircon CM2a isvery different from the previous two zircons analysedfrom this sample. It is anhedral and has clearly beenextensively altered. The CL imaging shows a blotchypattern with no obvious original zoning preserved (Fig.11b). This zircon has a low-U rim (showing as brightCL zones) that is evidence of reworking of themargins and some parts of the grain interior during ametamorphic episode. It is interesting that this zirconis not sited far from the previously described grain, inthe same thin section, and yet they are so different incharacter and state of preservation. Only threeanalyses were done on this grain (Table 3): all havevery high U contents (1814–2706 ppm) and areclearly highly discordant, showing a range of206⁎Pb/238U dates between 325 and 109 Ma.

9.2. LA-ICP-MS dating of zircons

The external structures of all zircon grains separatedfrom the jadeite-gneiss that were used for dating areillustrated in Fig. 12. Some crystals are euhedral andshort to long-prismatic (C3#3). Well-developed oscilla-tory zoning, often around an inherited core (Fig. 12:

Fig. 9. CL image of zircon grain CM5 extracted from jadeite-gneissT1404 showing the six spots analysed by SHRIMP. The grain is about190 μm in length.

78 C. Miller et al. / Lithos 93 (2007) 68–88

C1#1, C2#2, C2#3, C3#1, C3#4, C4#1) suggests growthin a melt phase, although modification of pre-existingtextures in some zircons (Fig. 12: C1#2, C1#5, C1#6)clearly indicates a disturbance of the U–Pb system. Thecore domains of grains C1#6, C3#2, C3#4 containinclusions of quartz and apatite.

With one exception, no significant common Pb wasobserved and most of the data (Table 4) are concordant.The 206Pb/238U and 207Pb/235U determinations yieldedthree concordant age clusters with weighted averages of466±2 Ma, 437±2 Ma and 288±9 Ma (Fig. 13). Inaddition, concordant age data ranging from 503–691 Ma were obtained, usually on zircon cores (Figs.12: C12: C2#2, C2#3, C3#1, C4#1; Fig. 13). The U–Pb

Table 3SHRIMP U–Pb zircon data for jadeite-gneiss T1404, Tauern Window, Easte

Grainspot++

(1) %206Pbc

U(ppm)

Th(ppm)

Th/U 206Pb⁎

(ppm)(1) 206Pb/238U age

(1) 2

206Pb

CM2a (thin section)2A1.1 3.97 2706 154 0.06 41.1 108.6 ±1.6 1832A1.2 2.33 1814 58 0.03 41.9 167.1 ±2.4 3642A1.3 1.23 2324 267 0.11 104 324.8 ±3.2 393M1 (thin section)1.1 2.68 244 127 0.52 15.8 457.1 ±7.5 5531.3 4.34 318 143 0.45 21.3 463.4 ±5.8 5631.4 0.91 263 76 0.29 17.0 462.7 ±5.4 4951.5 3.78 422 133 0.32 27.2 449.5 ±5.2 794CM5 (epoxy mount)CM5.1c 0.00 401 122 0.30 38.2 678.0 ±10.6 700CM5.2 0.16 258 72 0.28 17.5 487.8 ±7.3 488CM5.3e 0.33 330 105 0.32 26.7 579.0 ±7.6 567CM5.4c 0.15 397 137 0.35 33.6 606.1 ±9.8 629CM5.5 0.00 150 57 0.38 0.77 472.3 ±6.9 489CM5.6 0.21 122 39 0.32 8.35 494.4 ±7.6 569

Errors are 1-sigma; Pbc and Pb⁎ indicate the common and radiogenic portioError in standard calibration was 0.45% (not included in above errors but re(1) Common Pb corrected using measured 204Pb. ++ c = core, e = embayme

data of discordant zircon core (Fig. 12: C1#1), on theother hand, suggest ages in excess of 1.8 Ga.

A direct comparison between the LA-ICP-MS andSHRIMP age results is hampered by the unavailabilityof analyses performed on the same zircon grains.However, both techniques yielded similar age patterns,suggesting that the results are comparable. SHRIMPages of zircon CM1 and of the rim domain of zirconCM5 are within error with the age cluster of 466±2 Maobtained by LA-ICP-MS. The 206⁎Pb/238U SHRIMPage (579 Ma) of the embayment domain CM5.3 can becompared with the LA-ICP-MS age of 573±12 Ma ofthe rim domain of grain C4#1. In addition, bothtechniques revealed the presence of an inheritanceolder than 500 Ma.

10. Discussion

10.1. Metamorphism and nature of the protolith

At the Steinsteg locality, mafic kyanite-eclogitesand quartzo-feldspathic rocks are essentially concor-dant. As pointed out by Heinrich (1982), retrogressionof rocks of pelitic composition occurs faster comparedto mafic eclogite facies assemblages due to kineticreasons, resulting in an apparent facies contrast. Rarerelics of jadeite, omphacite and kyanite in the studiedsamples, however, clearly document that both lithologiesunderwent a common high-pressure metamorphism anda subsequent intermediate-pressure overprint. In

rn Alps07Pb/age

% Dis-cordant

(1) 207Pb⁎/206Pb⁎±%

(1) 207Pb⁎/235U±%

(1) 206Pb⁎/238U±%

Errcorr

±177 41 0.04973 7.6 0.117 7.8 0.0170 1.5 0.194±70 54 0.05383 3.1 0.195 3.4 0.0263 1.5 0.427±95 17 0.05450 4.2 0.388 4.4 0.0517 1.0 0.233

±206 17 0.05863 9.4 0.594 9.6 0.0735 1.7 0.179±200 18 0.05890 9.2 0.605 9.3 0.0745 1.3 0.140±100 6 0.05710 4.5 0.586 4.7 0.0744 1.2 0.256±150 43 0.06560 7.2 0.653 7.3 0.0722 1.2 0.165

±44 3 0.06276 2.1 0.960 2.6 0.1109 1.6 0.624±42 0 0.05691 1.9 0.617 2.4 0.0786 1.6 0.636±57 −2 0.05900 2.6 0.764 2.9 0.0940 1.4 0.468±88 4 0.06070 4.1 0.825 4.4 0.0986 1.7 0.383±62 3 0.05690 2.8 0.597 3.2 0.0760 1.5 0.479±82 13 0.05900 3.8 0.649 4.1 0.0797 1.6 0.391

ns, respectively.quired when comparing data from different mounts).nt.

Fig. 10. Wetherill U–Pb Concordia plot of the SHRIMP data for threezircons from jadeite-gneiss T1404. Black error ellipses represent thedata for grain CM5, dark grey for grain CM1 and light grey for grainCM2a.

79C. Miller et al. / Lithos 93 (2007) 68–88

addition, phase relations in the non-mafic rock providean upper pressure limit of approximately 2.4 GPa by thepresence of kyanite in a garnet–omphacite–paragonite–phengite-bearing assemblage (Fig. 4).

The protolith of the jadeite-bearing gneiss is not easyto identify as pervasive deformation has destroyed anyprimary features. In a ternary anorthite–albite–ortho-clase diagram (Barker, 1979) the normative composi-tions of the analysed samples plot in the trondhjemite,plagiogranite and tonalite fields (Fig. 14). However,silica is higher, and total alkalis are lower than introndhjemites, plagiogranites and tonalites. The Al2O3

versus MgO diagram (Fig. 15) after Marc (1992) clearlyshows that all samples fall in the paragneiss field. Inaddition, trace element (Fig. 16) and isotopic character-istics (Table 1) argue against a plagiogranitic ortrondhjemitic precursor rock. REE patterns (Fig. 3),Th/Sc (0.4–0.8), La/Sc (0.9–2.3) and La/Th (2.5–3.0)ratios are similar to those of quartz-intermediate grey-

Fig. 11. CL images of zircon CM1 (a) and CM2a (b) analysed by SHRIMP inare just 5 mm apart. See text for discussion.

wackes (Taylor and McLennan, 1985), suggesting thatthese rocks may represent highly metamorphosedgreywacke or immature sandstones. CIA (chemicalindex of alteration; Nesbitt and Young, 1982) valuesrange between 58 and 62, suggesting negligiblechemical weathering in the source area.

Since trace element characteristics of sediments arecontrolled by the nature of the source rocks, the REEpatterns and the negative Eu anomalies of the jadeite-gneiss samples are regarded as evidence for a differen-tiated source, similar to granite (Taylor and McLennan,1985). Photomicrographs (Fig. 7) illustrate the fact thatsome zircon crystals are fractured fragments of largergrains. This is also consistent with the interpretation ofthis rock as a metasediment since fracturing is commonin zircons from modern sediments and unmetamor-phosed sedimentary rocks (Hanchar and Miller, 1993).

10.2. Implications of the new geochronological data

Detrital zircons in sedimentary rocks reflect thehistory of zircon growth in the provenance area and U–Pb zircon ages usually reflect the age at which newzircon grew by either crystallization from a silicate meltor during metamorphism. The U–Pb LA-ICP-MS andSHRIMP ages are within error and suggest that thejadeite-gneiss precursor rock represents reworked pre-Alpine crustal material. Its age of deposition must post-date the age of the youngest detrital zircon. The onlyages younger than 200 Ma (Fig. 10) are part of theSHRIMP data set of the highly altered porous zircongrain CM2a. The three ages obtained on this crystal(Table 3) are highly discordant and scattering, and likelyaffected by Pb loss. This suggests that the age numbersare geologically meaningless. Therefore the youngestage recorded by the detrital zircon population rangefrom 280–293 Ma (Table 4). Based on zircon chemistry(Table 2) and zoning patterns (Fig. 12, grains C4#2 and

a polished thin section of jadeite-gneiss T1404. Note that these grains

80 C. Miller et al. / Lithos 93 (2007) 68–88

C4#3) these ages date a Late Carboniferous to earlyPermian magmatic event. This in turn implies that thehigh-pressure metamorphic overprint was indeed causedby subduction during Alpine convergence.

Igneous zircon is usually characterized by oscillatoryand/or sector zoning, whereas metamorphic zircon hasfeatures that differ from igneous zircon, such as blurredor convoluted zoning and transgressive recrystallization(Hoskin and Schaltegger, 2003). With one exception(grain 1#6), the external morphology, mineral inclu-sions, CL patterns, trace element abundances and Th/Uratios of the zircon grains suggest a magmatic origin(e.g. Rubatto, 2002; Hoskin and Schaltegger, 2003).This is further supported by the normalized REEpatterns with positive Ce and negative Eu anomaliescharacteristic for igneous zircon (e.g. Hoskin andSchaltegger, 2003). According to the classification andregression scheme proposed by Belousova et al. (2002),the zircons should be derived from granitoid sourcerocks. Therefore, the ages obtained from these domainsindicate provenance from multiple igneous sources.

The slightly discordant 206Pb/238U SHRIMP age of678±11 Ma of the optically visible and chemicallyresorbed zircon core (Fig. 9) could be regarded asminimum crystallization age related to a Neoproter-ozoic magmatic event. The concordant U–Pb LA-ICP-MS ages in the range 630–691 Ma preserved inoscillatory zoned zircon cores (Fig. 12, Table 4) alsosuggest a pre-Cambrian magmatic event in the sourceregion. The SHRIMP spots located on oscillatory zoneddomains of zircon grains 1 and 5 (Fig. 9, 11a) yielded206Pb/238U ages of 459±13 and 484±29 Ma (Fig. 10)that are thought to date Ordovician magmatism basedon Th/U ratios ranging from 0.28 to 0.52. Theconcordant U–Pb LA-ICP-MS ages of 466±2 and437±2 Ma obtained from other detrital zircon grainswith oscillatory zoning and Th/U ratios >0.17 (Fig. 12,Table 4) also suggest Ordovician magmatic sourcerocks. The preservation of inherited Neoproterozoiccores in some of the analysed detrital zircon grainssuggests that the magmatic rim zones precipitated fromperaluminous melts generated by crustal melting duringthe Ordovician. Since zircon solubility is extremelysensitive to temperature (Watson and Harrison, 1983),restitic zircon may survive in relatively low-tempera-ture granitic melts (e.g., Zeck and Williams, 2002).

Fig. 12. CL images of zircon grains separated from jadeite-gneiss (Tauern Wlocation of spots analysed by LA-ICP-MS. Zircon trace element data are listeC1#4, C2#3, C3#3, C4#1 and dark CL-domains in grains C1#3 and C3#4contrast. The core domains of grains C1#6, C3#2, and C3#4 contain inclusionshow embayments filled with quartz.

The three age data ranging from 280±7.2 to 293±7.5 Ma (Fig. 12, C4#2, C4#3; Table 4) are thought todocument a Late Carboniferous to Permian magmaticevent in the provenance area.

The zircon rim domain (spot 10) of grain 1#6 has acomposition similar to magmatic zircon, including anegative Eu anomaly, but very low Th/U ratio of 0.03typical of metamorphic zircons (Rubatto, 2002). Thesechemical characteristics suggest that it could havecrystallized in equilibrium with a partial melt during ahigh-temperature metamorphic episode about 350 Maago (Figs. 8 and 12, Table 4).

Attempts to constrain the age of the Alpine high-pressure metamorphism by dating the resetting of theU–Pb isotopic system in these zircons have failedbecause most of them have preserved a pre-Variscan orVariscan age. Possibly, the only evidence for Alpinereworking is seen in the anhedral and extensively alteredzircon depicted in Fig. 10b that yielded highlydiscordant 206Pb/238U ages ranging from 325 to109 Ma. The resultant discordia has an upper interceptat 429 Ma and a lower intercept at 65 Ma, both howeverwith large errors. The rounded shape, high U contentssometimes coupled with variable low to very low Th/Uratios (Table 2) and the chaotic internal texture of thisgrain suggest metamorphic reworking (Hoskin andSchaltegger, 2003). In addition, this zircon is porous,suggesting recrystallization by a dissolution–reprecipi-tation mechanism. This and the fact that some zirconsshow resorbed surfaces could indicate corrosion by aninterstitial supercritical fluid phase undersaturated inzirconium. As discussed by Tomaschek et al. (2003), ahigh-pressure environment would be particularly effec-tive for such a reaction and mobility of Zr duringsubduction is supported by the presence of baddeleyiteas a daughter phase in fluid inclusions of high-pressurevein minerals (e.g., Philippot and Selverstone, 1991).Since this zircon has by far the highest U content of allanalysed grains, one might suspect that in a chemicallyheterogeneous detrital zircon population only highlymetamict grains and/or zircons with very high traceelement contents are particularly reactive and wereaffected by this process (e.g. Hoskin and Schaltegger,2003). However, alteration and partial age resettingcould also have happened during the retrogradeoverprint of the jadeite-gneiss.

indow, Eclogite Zone) showing the morphology, internal structure andd in Table 2, age results in Table 4. Bright CL-domains in grains C1#2,also show growth zones when imaged with different brightness ands of quartz and apatite, whereas the rim domains of grains 1#3 and 4#2

81C. Miller et al. / Lithos 93 (2007) 68–88

Fig. 12 (continued)

82 C. Miller et al. / Lithos 93 (2007) 68–88

Table 4Single zircon LA-ICP-MS U–Pb data from jadeite-gneiss CM67/01, Tauern Window, Eastern Alps

Grain spot Th/U

207Pb/206Pb

1σ 208Pb/232Th

1σ 207Pb/235U

1σ 206Pb/238U

1σ rho Concordant

Age 2σ 206Pb/238U

1σ 207Pb/235U

1σ 208Pb/232Th

1σ 207Pb/206Pb

1σ

C1 #1 core 5 0.188 0.002 0.076 0.0008 5.800 0.061 0.223 0.003 – – 1299 14 1946 9 1473 15 2727 14C1 #1 core 6 0.158 0.001 0.087 0.0009 7.194 0.078 0.330 0.004 0.990 – 1837 19 2136 10 1681 17 2435 15C1 #2 core 7 0.067 0.001 0.026 0.0004 0.740 0.010 0.080 0.001 0.953 – 495 6 562 6 524 7 846 24C1 #2 rim 8 0.34 0.057 0.001 0.024 0.0004 0.588 0.010 0.075 0.001 0.728 469 11 469 5 469 6 478 8 472 35C1 #3 core 9 0.42 0.057 0.001 0.021 0.0003 0.542 0.009 0.069 0.001 0.715 435 9.9 432 5 440 5 422 5 478 27C1 #3 core 10 0.055 0.001 0.020 0.0002 0.494 0.008 0.065 0.001 0.731 404 9.2 404 5 408 4 410 5 430 25C1 #4 core 11 0.32 0.057 0.001 0.020 0.0002 0.556 0.010 0.071 0.001 0.653 442 10 440 5 449 5 399 5 492 26C1 #4 rim 12 0.057 0.001 0.024 0.0004 0.581 0.009 0.074 0.001 0.903 465 11 489 6 502 5 473 6 560 27C1 #5 13 0.059 0.001 0.020 0.0003 0.547 0.008 0.067 0.001 0.882 – 419 5 443 5 396 5 572 27C1 #5 14 0.056 0.001 0.023 0.0003 0.579 0.009 0.075 0.001 0.815 465 11 466 6 464 6 465 6 452 31C1 #6 core 15 0.056 0.001 0.027 0.0007 0.579 0.010 0.075 0.001 0.800 466 12 468 6 464 7 529 15 448 35C1 #6 rim 16 0.03 0.053 0.001 0.017 0.0003 0.419 0.005 0.057 0.001 0.990 350 4.4 358 4 356 4 334 6 345 22C2 #1 core 22 0.056 0.001 0.018 0.0002 0.537 0.010 0.070 0.001 0.641 434 9.9 433 5 437 4 367 4 452 22C2 #1 core 23 0.054 0.001 0.018 0.0002 0.532 0.011 0.071 0.001 0.565 443 10 444 5 433 4 365 4 372 24C2 #2 core 24 0.32 0.063 0.001 0.032 0.0004 0.939 0.015 0.109 0.001 0.754 669 15 666 8 672 8 646 8 693 32C2 #2 core2 25 0.061 0.001 0.032 0.0005 0.882 0.013 0.105 0.001 0.820 642 14 643 7 642 7 637 9 638 29C2 #2 rim 26 0.11 0.057 0.001 0.026 0.0004 0.642 0.010 0.081 0.001 0.762 503 12 503 6 504 6 523 8 504 34C2 #3 core 27 0.062 0.001 0.033 0.0004 0.979 0.013 0.115 0.001 0.905 691 13 699 8 693 7 662 8 671 26C2 #3 rim 28 0.056 0.001 0.024 0.0004 0.573 0.009 0.074 0.001 0.799 459 10 458 5 460 6 479 7 468 32C3 #1 core 29 0.059 0.002 0.030 0.0007 0.722 0.022 0.088 0.001 0.506 546 16 545 8 552 13 589 14 583 64C3 #1 rim 30 0.057 0.001 0.021 0.0004 0.610 0.012 0.078 0.001 0.636 482 11 481 6 484 7 428 7 490 42C3 #2 rim 31 0.056 0.001 0.018 0.0002 0.570 0.011 0.074 0.001 0.784 458 12 459 6 458 7 360 4 460 37C3 #3 core 32 0.37 0.056 0.002 0.023 0.0005 0.571 0.019 0.074 0.001 0.464 457 14 457 7 459 12 453 11 466 73C3 #3 rim 33 0.17 0.055 0.001 0.021 0.0004 0.532 0.007 0.070 0.001 0.800 436 9.2 439 5 433 5 415 7 403 30C3 #4 core 34 0.060 0.001 0.030 0.0003 0.861 0.011 0.104 0.001 0.886 630 12 637 7 631 6 602 6 611 26C3 #4 rim 35 0.056 0.001 0.024 0.0004 0.584 0.010 0.076 0.001 0.804 468 11 469 6 467 6 489 8 458 32C4 #1 med 36 0.061 0.001 0.032 0.0004 0.898 0.015 0.107 0.001 0.680 655 14 658 7 651 8 628 7 630 35C4 #1 rim 37 0.059 0.001 0.029 0.0004 0.758 0.012 0.093 0.001 0.714 573 12 573 6 573 7 577 7 575 34C4 #1 core 38 0.070 0.001 0.033 0.0004 0.760 0.015 0.079 0.001 0.612 – 489 6 574 8 651 9 928 39C4 #2 rim 39 0.050 0.001 0.013 0.0002 0.310 0.007 0.045 0.001 0.628 280 7.2 281 4 274 4 260 4 208 28C4 #2 core 40 0.55 0.054 0.001 0.013 0.0002 0.344 0.007 0.046 0.001 0.662 293 7.5 293 5 300 9 268 4 354 65C4 #3 core 41 0.67 0.053 0.001 0.015 0.0002 0.339 0.007 0.046 0.001 0.534 291 6.5 290 3 296 6 309 4 345 48

rho = ratio between the relative standard deviation between 207/235 and 206/235 ratios.

83C.Miller

etal.

/Lithos

93(2007)

68–88

Fig. 13. Wetherill U–Pb Concordia plots of the LA-ICP-MS data for(a) all zircon grains analysed from jadeite-gneiss CM67/01 and detailsof concordant populations at (b) 465±1.9 Ma and (c) at 437.5±2.2 Ma.

Fig. 14. Composition of jadeite quartzites (solid squares) from theEclogite Zone of the Tauern Window plotted on the normative Ab–An–Or classification diagram of granitic rocks (Barker, 1979). Shadedfield represents ophiolitic plagiogranites from Oman (Coleman andPeterman, 1975; Coleman and Donato, 1979; Pallister and Knight,1981). Jadeite quartzites (solid circles) from Dabie Shan (Liou et al.,1997) are shown for comparison.

Fig. 15. Composition of jadeite quartzites (solid squares) from theEclogite Zone of the TauernWindow plotted on the Al2O3 versus MgOdiscrimination diagram after Marc (1992). Jadeite quartzites (solidcircles) from Dabie Shan (Liou et al., 1997) are shown for comparison.

84 C. Miller et al. / Lithos 93 (2007) 68–88

10.3. Provenance of the jadeite-gneiss precursor rock

Detrital zircon geochronology is a powerful tool forprovenance studies and understanding of crustal evolu-tion. However, natural complexities do not always

permit an unbiased evaluation of the provenance area.The fact that the zircon grains in the jadeite-gneiss aremostly euhedral argues against a distal source andsuggests a first-cycle origin, whereas rounded zirconsmay be derived from sedimentary rocks (Asiedu et al.,2000).

Igneous rocks that could have yielded Cambrianand Ordovician zircons are documented by SHRIMPzircon ages (Eichhorn et al., 1999, 2001) in the pre-Variscan Tauern Window basement rocks (InnerSchieferhülle, Fig. 1) that represent the Europeanmargin (Lammerer, 1986; Frisch et al., 1993). Late

Fig. 16. Trondhjemite Rb–Sr variation diagram (after Coleman andDonato, 1979) showing that the analysed jadeite quartzites from theEclogite Zone of the TauernWindow have compositions different fromthose of continental trondhjemites and oceanic plagiogranites.

85C. Miller et al. / Lithos 93 (2007) 68–88

Carboniferous to Early Permian magmatism in theZentralgneiss (Fig. 1) that was SHRIMP dated for therange 299±4 to 271±4 Ma by Eichhorn et al. (2000)could have yielded the source rocks for the 288±9 Madetrital zircon of the jadeite-gneiss. On the other hand,the Neoproterozoic magmatic protolith ages reported byVon Quadt (1992) for various varieties of the Basisam-phibolit (a part of the Inner Schieferhülle, Fig. 1) wereshown to be in error by Kebede et al. (2005) who datedthis unit at 351±2 to 341±1 Ma by single-grain ID-TIMS U–Pb zircon geochronology. Therefore, sourcerocks for the Precambrian zircon cores and grain C4#1are not yet known from the basement units of the TauernWindow, whereas the widespread Late Devonian toEarly Carboniferous magmatic events (Eichhorn et al.,2000; Kebede et al., 2005) seen in the Basisamphibolitand Zentralgneiss domains were not detected in thejadeite-gneiss zircon population.

According to Stampfli and Borel (2002) the pre-Variscan rocks exposed in the Tauern Window were apart of the European Hunic terrane that detachedalong the northern margin of Gondwana during sub-duction of the Rheic ocean and opening of Palaeo-Tethys in the early Palaeozoic. The widespread LateCarboniferous–Early Permian granitoid magmatism inthe Tauern basement (Finger et al., 1993) and inother Variscan Alpine domains is related to thenorthward subduction of the Palaeo-Tethys (Stampfliand Borel, 2002). The lithology of the Penninicnappes in the Tauern Window suggests an evolutionfrom a stable continental environment consolidatedby the Variscan orogeny to a rifting environment andspreading of the Alpine Tethys. The Tauern Windowbasement nappes probably belonged to the southernEuropean shelf, situated north of the Alpine Tethys

(Lammerer, 1986; Frisch et al., 1993; Stampfli andBorel, 2002). However, at present, the ages recordedby the detrital zircons of the jadeite-gneiss do notpermit an unambiguous identification of the sourceregion of the precursor sediment since Neoproter-ozoic, Ordovician and Early Permian zircon crystal-lization ages are also known from Austroalpinebasement lithologies (Thöni, 1999 and referencestherein) formerly belonging to the southern margin ofthe Alpine Tethys.

11. Conclusions

The peak metamorphic assemblage of the jadeite-gneiss consists of jadeite or omphacite+garnet+quartz+phengite+paragonite±kyanite suggesting high-pressuremetamorphic conditions of approximately 640 °C at 2–2.4 GPa. Geochemical data indicate that the precursorrock of the jadeite-gneiss was a siliciclastic sedimentmainly derived from felsic igneous sources. CL imagesrevealed the presence of a heterogeneous zircon popu-lation containing euhedral grains and fractured frag-ments with cores exposed at the rim.

Inclusion assemblages, CL imagery, HREE profilesand negative Eu anomalies suggest that most zirconscrystallized from granitoid melts. The U–Pb datademonstrate input from sources with 288±9, 437±2,466±2, 503–691 Ma and >1.8 Ga zircon ages,suggesting peri-Gondwanan sources. Surprisingly, nofirm evidence for recrystallization and mobilization ofzircon during the Alpine high-pressure metamorphismand subsequent retrograde overprint of the jadeite-gneiss was detected in the analysed zircon population,with the possible exception of some grains that showsigns of resorption/dissolution (Fig. 6; Fig. 12: C4#2)and the single porous zircon CM2a grain that yieldedhighly discordant ages in the range 325–109 Ma and alower intercept age of 65 Ma.

Acknowledgements

We are indebted to M. Jelenc, University ofVienna, for the help with the Sr and Nd isotopeanalytical work. Three anonymous reviews helped toimprove the manuscript. The financial support by aUniversity of Innsbruck research grant is gratefullyacknowledged.

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