Abstract Field observations and U–Pb zircon data yieldevidence that the Kaintaleck Complex represents a dis-tinct crystalline basement unit of the eastern GreywackeZone and contrasts partly with other pre-Alpine base-ment complexes of the Eastern Alps. The age data indi-cate possible Late Archean rock formation, several mag-matic and metamorphic events in the Early Palaeozoic(ca. 520–490 and ca. 400–360 Ma), and low-grade meta-morphic overprint in the Cretaceous. Zircons from a gar-net gneiss layer in a plagioclase gneiss and amphibolitesequence yield an upper intercept age of ca. 2.55 Gawhich is interpreted as a possible protolith age. The low-er intercept age at 514 Ma represents either (1) a mag-matic event or (2) a metamorphic overprint withinamphibolite facies conditions. Magmatic zircons derivedfrom granitic orthogneiss boulders of the Kalwang Con-glomerate, which covers the crystalline basement, cry-stallised at ca. 500 Ma. The lower discordia intercepts ofthe orthogneiss boulders mark the Cretaceous low-grademetamorphic overprint. The lower intercept age of para-gneiss zircons from another slice of the Kaintaleck Com-plex suggests a likely Devonian metamorphic event atca. 390–400 Ma. The new data demonstrate that theKaintaleck Complex experienced several stages of tec-tonothermal evolutions, which are in contrast to the evo-lution of the main mass of the Austroalpine basement.The data constrain linkages of this part of the Austroal-

pine basement to the Cordillere Ligérienne and Cadomianblock within West-European Variscides.

Keywords Cambrian · Devonian · Eastern Alps · Metamorphism · Ordovician

Introduction

Much progress has been made in deciphering the pre-Alpine evolution of the Austroalpine crystalline rocks ofthe Eastern Alps through the use of geochronology (seereviews and new data of Neubauer and Frisch 1993; Millerand Thöni 1995; Müller et al. 1999; Neubauer et al. 1999;Thöni 1999; Neubauer and Handler 2000). Magmaticprotolith aged 560–480 Ma only exists in a few localitiesin the eastern part of the Austroalpine basement. These areinterpreted to represent the magmatic formation of calc-alkaline and rare alkaline granitoid suites (Frank et al. 1976;Haiss 1990; Müller et al. 1995; Schaltegger et al. 1997),anatexis (Söllner and Hansen 1987; Klötzli-Chowanetz etal. 1997) and the formation of gabbros within ophiolite(Miller and Thöni 1995). Geochronological data show evi-dence for magmatic events between 460 and 425 Ma(granite intrusions: e.g. Cliff 1980; Scharbert 1981). Mostdata, however, reflect the Late Devonian to Carbonifer-ous, Variscan orogenic events at ca. 360–310 Ma withgranite magmatism and a strong thermal overprint on ear-lier structures and (see reviews of Finger et al. 1997;Schermaier et al. 1997; Neubauer et al. 1999; Thöni 1999;Neubauer and Handler 2000). The significance of scarceRb–Sr whole rock and Rb–Sr and 40Ar/39Ar mineral agedata around 400–370 Ma (Hawkesworth 1976; Frimmel1986, 1988; Handler et al. 1999; Müller et al. 1999) ispoorly understood, but they suggest cooling after a majormetamorphic event and the associated felsic plutonism.

The scattered geochronological data raise fundamen-tal questions:

1. Is the composition of the Austroalpine basement uni-form? What is the significance of the Kaintaleck

F. Neubauer (✉ )Institut für Geologie und Paläontologie, Universität, Hellbrunner Strasse 34, Salzburg, Austriae-mail: franz.neubauer@sbg.ac.atFax: +43-662-8044621

W. FrischInstitut für Geologie und Paläontologie, Universität, Sigwartstrasse 10, Tübingen, Germany

B.T. HansenInstitut für Mineralogie, Universität, Corrensstrasse 24, Münster, Germany

Present address:B.T. Hansen, Institut für Geologie und Dynamik der Lithosphäre,Universität, Goldschmidtstrasse 3, Göttingen, Germany

Int J Earth Sci (Geol Rundsch) (2002) 91:775–786DOI 10.1007/s00531-001-0254-7

O R I G I N A L PA P E R

Franz Neubauer · Wolfgang Frisch · Bent T. Hansen

Early Palaeozoic tectonothermal events in basement complexes of the eastern Greywacke Zone (Eastern Alps): evidence from U–Pb zircon data

Received: 14 August 2000 / Accepted: 26 August 2001 / Published online: 2 March 2002© Springer-Verlag 2002

Complex, a metamorphic complex incorporated in theGreywacke Zone, which always shows tectonic con-tacts to the Palaeozoic sequences so that their primaryrelationships remain unclear. In only one locality hasa primary contact between one of the basement slicesand its Palaeozoic sedimentary cover (Kalwang Con-glomerate) been reported (Neubauer 1985).

2. Are there rock complexes in the Austroalpine base-ment that suffered Silurian/Devonian metamorphismand magmatism as observed in the internal part of theEuropean Variscides (e.g. Matte 1986; Pin 1990; Timmermann et al. 2000)?

Answers to these questions are given by the study of the Kaintaleck Complex incorporated in the easternGreywacke Zone (Fig. 1). The complex has gained littleattention because of its restricted occurrence as smallslices with poor exposure. Here we present the results ofan isotope dilution, multigrain U–Pb zircon study thatwas carried out on rocks of the Kaintaleck Complex and

on boulders from the Kalwang Conglomerate. The agedata have shed light on the Early Palaeozoic evolution ofthis specific Austroalpine sector, which largely differsfrom most other basement units in the Alps. These dataalso bear significance for the correlation between extra-Alpine and Alpine Variscan basement.

Geological setting

According to recent studies the eastern Greywacke Zoneis composed of four Alpine nappes (Figs. 2 and 3). Theseare, from bottom to top (Neubauer et al. 1994), (1) theVeitsch nappe made up of Early Carboniferous to Perm-ian molasse-type formations that escaped Variscan duc-tile deformation (Ratschbacher 1987), (2) the Silbersbergnappe (not exposed in the working area) with EarlyPalaeozoic quartz phyllite and a predominating Permiancover sequence, (3) the Kaintaleck nappe with the Kaint-aleck Complex and a metaconglomerate cover (Fig. 3),which is an equivalent of the Kalwang Conglomerate(Daurer and Schönlaub 1978) elsewhere, and (4) theNoric nappe comprising Late Ordovician to Namurianshelf and slope sediments deformed in the Late Carboni-

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Fig. 1 Location of the eastern Graywacke zone in the EasternAlps. B Bundschuh basement; W Wechsel Window; Ö Ötztal base-ment; S Silvretta basement

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Fig. 2 Simplified geological map of the Greywacke Zone with theKaintaleck Complex near Bruck a.d. Mur and sample locations(after Neubauer et al. 1987). The Ritting, Prieselbauer and Frauen-

berg sl. (slice) denotes distinct klippen units of the KaintaleckComplex with distinct lithologies in an imbricate fan

Fig. 3 Simplified tectonostratigraphy of the eastern Graywacke Zone and previous geochronology (modified from Handler et al. 1999)

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ferous (Nievoll 1987; Schönlaub and Heinisch 1993).The Noric nappe is unconformably covered by Late Car-boniferous to Permian sediments forming the base of theNorthern Calcareous Alps. All units suffered a Creta-ceous metamorphism within the lower greenschist faciesgrade (Dallmeyer et al. 1996, 1998; Hoinkes et al. 1999).The Cretaceous metamorphism was associated with duc-tile strain that resulted in shear zones in amphibolite-grade basement rocks of the Kaintaleck nappe (Neubaueret al. 1987; Handler et al. 1999). The shear zones delimit10–100-m-thick lens-shaped bodies, which are internallynearly undeformed. Previous studies show a Late Devon-ian age of cooling after metamorphism and subsequentfelsic magmatism with Rb–Sr and 40Ar/39Ar muscoviteages of ca. 380–360 Ma (Handler 1994; Handler et al.1999) and a Cretaceous age of ca. 100–90 Ma (40Ar/39Armuscovite) indicates cooling after Alpine metamorphism(Dallmeyer et al. 1998).

The structure of the Greywacke Zone in the study ar-ea is complicated by Alpine top-W-directed thrust imbri-cation (Neubauer et al. 1987, 1994; Ratschbacher andNeubauer 1989). The Kaintaleck Complex occurs thereat three different structural levels that are characterisedby distinct lithologies (Figs. 2 and 3). These include,structurally upwards:

1. The basal Ritting unit composed of garnet–zoi-site–amphibolite and mica–schist with lenses of ser-pentinite and marble. The assemblage is interpreted asa metamorphosed and dismembered incomplete op-hiolite suite (Neubauer et al. 1989). It is tectonicallyoverlain by phyllites of the Noric nappe (Neubauer etal. 1994). A sedimentary contact between a metacon-glomerate (Kalwang Conglomerate according to Neubauer 1985) and the garnet zoisite amphibolitewas temporarily exposed during the construction of aroad tunnel. The metaconglomerate contains decime-tre-sized orthogneiss boulders. Sample 168/1 is suchan orthogneiss boulder from the tunnel locality(Fig. 2).

2. The Prieselbauer unit is composed of mica-rich mig-matitic paragneiss cut by discordant, but internally fo-liated aplite. One paragneiss sample (GR20G) andone sample of an aplite (GR22G) were taken for geo-chronological dating from an entirely unretrogressedportion.

3. The Frauenberg unit is composed of several imbricateparts with different lithologies: The main part is composed of plagioclase amphibolite with a primary stratigraphic contact with metaconglomerate(Kalwang Conglomerate) in an inverted structural po-sition (Fig. 2). Another basement imbricate sheet iscomposed only of paragneiss. One sample of a 20-cm-thick hornblende–garnet–gneiss layer (GR115G)within the plagioclase amphibolite and one sample ofan orthogneiss boulder (RAD) from the metacon-glomerate were studied. The hornblende garnet gneisssuffered HP–HT metamorphism followed by staticdecompression (Neubauer and Frisch 1993).

The metaconglomerates covering the Ritting and Frauen-berg units belong to the Kalwang Conglomerate Forma-tion, which is widespread in the Noric nappe (Daurer andSchönlaub 1978; Loeschke et al. 1990). The sampled or-thogneiss boulders are identical with respect to modaland chemical composition.

U–Pb analytical techniques

Concentration of the zircon crystals was achieved bymeans of a Wilfley table, a magnetic separator and heavyliquids. The zircons were separated carefully by hand-picking according to size, colour, turbidity and shape.The isotopic analyses were performed in the Geo-chronologisches Zentrallabor, University of Münster,Germany, during 1985–1987. Aliquots of 1–10 mg of dif-ferent zircon fractions were dissolved following the stan-dard low contamination procedure of Krogh (1973). Uand Pb concentrations and Pb isotopic compositions weremeasured using an NBS-type Teledyne mass spectrome-ter. The mass fractionation of lead was corrected linearlyby 0.12% per atomic mass unit. The Pb isotopic data (Ta-ble 1) were corrected for initial Pb and analytical blank.The isotopic composition of the common lead in the zir-cons was corrected according to the age-related model ofStacey and Kramers (1975). A total analytical blank ofca. 0.1–0.5 ng was measured during the analytical work.

The IUGS-recommended constants (Steiger and Jäger1977) were used for age calculation. The calculation oferrors and error propagation of ratios was carried outwith an updated version of the program of Ludwig(1980, 1991), taking into account the errors of measuredisotopic ratios, the uncertainties of the U/Pb ratio in themixed spike, the error propagation caused by thespike/sample ratio, and errors arising from uncertaintiesof Pb blank and common Pb correction (Mattinson1987). The error assigned to the 207Pb/204Pb and206Pb/204Pb initial and blank Pb isotopic compositions is<0.3%. The correlation factor for initial and blank Pb is0.7. The error ellipses on the concordia diagrams weredrawn for a 95% confidence level. The discordias werecalculated according to the least squares method of York(1969), using individual errors and correlation factors forthe 206Pb/238U and 207Pb/235U isotopic ratios.

Correlation with the geological time scale is based oncalibrations of Harland et al. (1989) and Gradstein andOgg (1996).

Results

Frauenberg slice: hornblende garnet gneiss

The hornblende garnet gneiss contains numerous zirconcrystals distinguishable in three types of zircons: (1)more than 90% are slightly red/pink-coloured, apparent-ly rounded zircons with smooth surfaces (Fig. 4a). Inthe scanning electron microscope, smooth crystal sur-

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faces and rounded edges are observed on all grains. (2)Colourless zircons showing similar shapes and surfacesas described above. They occur only in fine grain-sizefractions. (3) Brown, metamict zircons make up about0.5%.

The U content of the red/pink zircons ranges from249 to 435 ppm (Table 1), whereas that of the colourlesszircons is lower within the same grain-size fraction (Ta-ble 1). The zircon populations are highly discordant, butplot on a common discordia with an upper intercept at2,545 +54/–51 Ma and the lower intercept at 514±43 Ma(Fig. 5). The upper intercept may be interpreted as theage of zircon crystallisation in a magmatic rock or as theinherited zircons in a magmatic rock that crystallised atca. 514 Ma. Consequently, the lower intercept indicateseither (1) the diffusive loss of radiogenic lead during a

strong thermal overprint in Early Palaeozoic or (2) mag-matic crystallisation.

The U contents of the brown zircons are higher thanthose of the other populations (Table 1). The three analy-sed fractions are situated close to the same discordia asthe red zircons. But they define a separate discordia withan upper intercept at ca. 2,800 Ma and an ill-definedlower intercept within the Proterozoic, suggesting amulti-stage evolution of the brown zircons.

The hornblende–garnet gneiss includes mainly plagio-clase (>60 vol%), with quartz, garnet and amphibole asminor constituents, and biotite, phengitic muscovite andgraphite as accessories. The chemical composition is an-desitic to dacitic, slightly corundum-normative, which ismirrored in the high garnet content (own unpublished da-ta). For the correct interpretation of the zircon data, it is

Table 1 U–Pb analytical data of zircon concentrates of the Kaintaleck metamorphic complex, Eastern Alps

Fractions Sample Concentrations Isotopic ratios Apparent agesweight(mg) U Pbcom Pbrad

206Pb/ 206Pb/ 207Pb/ 207Pb/ (Ma)(ppm) (ppm) (ppm) 204Pb 238U 235U 206Pb

206Pb/ 207Pb/ 207Pb/238U 235U 206Pb

Garnet gneiss GR1151. 57–80 µm clear 2.8 395 0.44 57.9 6,544 0.14044 2.220 0.11466 847 1,187 1,8742. 80–100 µm clear 4.0 270 0.06 38.6 11,978 0.13469 2.007 0.10807 815 1,118 1,7673. 80–100 µm red 5.2 435 0.24 77.3 14,326 0.17011 2.988 0.12739 1,013 1,405 2,0624. 100–125 µm red 8.0 435 0.13 75.6 25,166 0.16641 2.954 0.12876 992 1,396 2,0815. 125–160 µm red 4.35 249 0.15 40.7 11,162 0.15147 2.422 0.11595 909 1,249 1,8956. >160 µm red 2.6 393 0.21 83.4 19,077 0.19817 3.702 0.13549 1,165 1,572 2,1707. 80–100 µm brown 1.6 797 0.55 194.6 14,941 0.23695 4.655 0.14247 1,371 1,759 2,2578. 100–125 µm brown 3.45 797 0.39 200.1 20,866 0.24246 4.858 0.14531 1,399 1,795 2,2929. >60 µm brown 0.95 484 0.43 135.8 8,503 0.26925 5.713 0.15390 1,537 1,933 2,390

Orthogneiss boulder RAD1. 40–63 µm white 3.7 4,068 42.2 170.0 279 0.04354 0.329 0.05471 275 289 4022. 63–80 µm white 3.4 4,663 51.7 201.6 270 0.04496 0.339 0.05466 284 296 3983. 80–100 µm white 2.6 4,571 61.9 203.4 231 0.04644 0.352 0.05489 293 306 4084. 63–80 µm red 1.8 2,035 7.7 133.8 1,069 0.06809 0.532 0.05663 425 433 4775. 80–100 µm red 6.3 3,200 20.6 204.4 656 0.06620 0.518 0.05675 413 423 4796. 100–125 µm red 1.9 1,917 18.5 127.3 453 0.06885 0.538 0.05664 429 437 477

Orthogneiss b. 168/11. 40–63 µm red 6.4 1,530 2.10 96.9 2,961 0.06605 0.517 0.05671 412 423 4802. 63–80 µm red 7.6 3,694 4.92 224.5 2,942 0.06356 0.495 0.05647 397 408 4713. 80–100 µm red 7.6 1,957 2.77 120.9 2,816 0.06466 0.505 0.05661 404 415 4764. 100–125 µm red 5.4 1,671 4.17 104.6 2,889 0.06557 0.510 0.05644 409 419 4705. 125–160 µm red 6.2 1,898 2.54 118.9 2,890 0.06571 0.513 0.05659 410 420 4756. >160 µm red 2.7 1,798 9.76 118.4 3,971 0.06874 0.538 0.05676 429 437 482

Aplite gneiss GR22G1. 40–63 µm 1.4 6,077 86.9 201.4 173 0.03551 0.266 0.05429 225 239 3812. 63–80 µm 1.7 4,266 68.1 140.3 154 0.03512 0.263 0.05433 220 234 3793. 80–100 µm 2.1 5,249 74.4 183.4 179 0.03653 0.274 0.05430 231 245 3824. >100 µm 2.45 4,144 76.8 197.8 152 0.03964 0.296 0.05418 251 262 364

Paragneiss GR20G1. 40–63 µm 3.4 1,691 1.21 99.3 5,263 0.06349 0.511 0.05838 397 419 5442. 63–80 µm 4.3 1,074 0.98 65.4 4,167 0.06565 0.550 0.06075 410 445 6303. 80–100 µm 1.3 1,257 1.03 74.4 3,846 0.06367 0.538 0.06129 398 437 6504. 100–180 µm 4.85 1,348 2.03 93.5 2,570 0.06584 0.641 0.07065 411 503 947

important to establish whether the protolith was, for in-stance, a true magmatic rock, or a reworked rock such asa volcanogenic greywacke. The relatively uniform popu-lation of slightly pink-coloured zircons and the highamount of plagioclase suggest a weathered magmaticrock, possibly a tuff as a precursor rock. The high garnetcontents result in high alumina contents typically forrocks that have undergone a weathering cycle.

Orthogneiss boulders of the Kalwang Conglomerate

Orthogneiss boulders (20–50 cm in diameter) dominatethe Kalwang Conglomerate, which also includes amphi-bolite, quartzite and low-grade metamorphic rocks. Theorthogneiss boulders comprise plagioclase, chess boardalbite (after K-feldspar), quartz and neither muscovitenor mafic minerals. Only sample RAD contains some

muscovite along foliation planes that gave an 40Ar/39Arage of 386 Ma (Handler et al. 1999).

Zircons of the orthogneiss boulders in the metacon-glomerate from both localities (Ritting unit, sample168/1, and Frauenberg unit, sample RAD) have similaroptical and isotopic characteristics. Both samples show apredominance of elongated euhedral red crystals withequally well-developed (100) and (110) prisms, and apredominance of the (101) over the (211) pyramids(Fig. 4c). In some cases, oscillatory magmatic zoning isvisible in the microscope. Abundant translucent inclu-sions were identified as apatite by energy-dispersive X-ray spectrometry. Zircon crystals with optically discern-ible inclusions were omitted from isotopic analysis. Euhedral milky white zircon is much less abundant thanthe red zircons and occurs predominantly in the sizefractions below 80 µm of sample 168/1. Nearly all whitezircons contain inclusions of red zircon and frequentlygrow homoaxially along one side of the red zircon.These relationships show that the red zircons were thefirst to crystallise.

Six fractions were studied isotopically from eachsample (Table 1). The U contents of the red zircon fromsample 168/1 range from 1,530 to 3,700 ppm. The data

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Fig. 4a–e Zircons from rocks of the Kaintaleck Complex andfrom orthogneiss boulders of the Kalwang Conglomerate: a Horn-blende garnet gneiss (sample GR115); b paragneiss (sampleGR20G); c orthogneiss boulder (sample RAD), d, e aplite gneiss(sample GR22G)

points define a discordia with an upper intercept of 498+28/–63 Ma and a lower intercept of 116 +132/–159 Ma(Fig. 5).

Three fractions of red zircon and three fractions ofwhite zircon were analysed in sample RAD. Red zirconsshow similar 206Pb/238U, 207Pb/235U and 207Pb/206Pb

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Fig. 5 Concordia plots of zircon isotopic data from the gar-net–hornblende gneiss (sample GR115), orthogneiss bouldersfrom the Kalwang Conglomerate (samples RAD and 168/1), para-gneiss (sample GR20G) and aplite gneiss (sample GR22G). Notethat the upper intercept age of sample GR22G is probably mean-ingless (see text for explanation)

model ages as zircons in sample 168/1 (Fig. 5) whereasthe white zircons are much more discordant. Together,the two populations define an upper intercept age 502(+17/–15) and an ill-defined lower intercept with139±18 Ma.

The similar results for the two orthogneisses from twodifferent localities suggest a similar origin of both sam-ples. Their upper intercepts of ca. 500 Ma are interpretedas the age of magmatic zircon crystallisation. The lowerintercepts are interpreted in terms of recrystallisation andPb loss during Cretaceous low grade metamorphism andare inferred to date the thermal peak applying the modelof Köppel et al. (1980). This model links lead loss dur-ing metamorphism under greenschist metamorphic con-ditions to thermal stabilisation of metamict zircon. Thelower intercept ages are close to 40Ar/39Ar whole rockand muscovite ages of the region (Dallmeyer et al.1998).

Prieselbauer slice: paragneiss (GR20G) and aplite gneiss(GR22G)

The paragneiss of the Prieselbauer slice contains well-rounded reddish zircons with overgrowth shells a fewmicrons thick (Fig. 4b). The overgrowth is best visible

on the acute edges of the elongated crystals. The U con-tents of these zircons are variable and range between1,074 and 1,691 ppm. The zircon analyses plot near theconcordia (Fig. 5). Three fractions define a discordiawith a lower intercept at 391 +20/–34 Ma. The fourthfraction plots slightly apart from this discordia. The up-per intercept is badly constrained and within the Arche-an. We interpret the lower intercept to likely date the ageof metamorphism in the Kaintaleck Complex.

The zircons of the discordant aplite gneiss show un-usual shapes. They are red-coloured, very elongated(Fig. 4e), and generally broken at their ends most likelybecause of the extraction procedure. Parallel coalescencepatterns of crystals are frequent (Fig. 4d). The U con-tents of these zircons are high (4,143–6,077 ppm). Thehigh amount in non-radiogenic common lead (Table 1)and the very low 206Pb/204Pb ratios (152–179) cause highanalytical uncertainties. Although highly discordant (ca.60%), the four fractions measured plot on a discordiawith an upper intercept age of 365 (+10/–35) Ma. Thelower intercept cuts the origin of the diagram.

The upper intercept age is probably geologicallymeaningless because the207Pb/206Pb ages of three analy-sed zircon fractions are between 379 and 382 Ma. The207Pb/206Pb age of 379–382 Ma may represent a betterestimate of the age of zircon crystallization.

The lower intercept of the paragneiss (GR20G) is in-terpreted to represent Devonian metamorphism. Field re-lations show that the aplite veins are younger than theformation of the predominant foliation in the paragneiss.The upper intercept of the aplite sample is interpreted asthe age of zircon crystallisation in the protolith magma.Grade and duration of the Cretaceous metamorphismwas not sufficient to significantly reset the isotopic sig-nature. The high U contents suggest lead loss after thediffusion model of Wasserburg (1963; see discussions ofvarious models in Mezger and Krogstadt 1997).

Discussion

The new data from the Kaintaleck Complex result fromisotope dilution U–Pb age dating using large zircon frac-tions. Although extreme care was taken in the selectionof uniform zircon populations, the results must be con-sidered with some caution. Nevertheless, field relation-ships, confirmation by other geochronological data andsimilar ages as found in other Austroalpine and Penninicbasement units suggest that the data have some signifi-cance.

The new U–Pb data from the Kaintaleck Complexand from orthogneiss boulders of the Kalwang Conglom-erate show that these metamorphic successions in theeastern part of the Greywacke Zone constrain severalgeological events.

The data from the garnet gneiss of the Frauenbergunit (GR115G) and the orthogneiss boulders in the meta-conglomerate are evidence for strong thermal overprintand magmatism at ca. 500 Ma (ca. 514±43 Ma). A

40Ar/39Ar muscovite concentrate from the orthogneissboulder RAD yielded a plateau age of 386 Ma interpret-ed to record cooling after penetrative, pre-depositionalmetamorphism in the source region (Handler et al.1999). The age of deposition of the Kalwang Conglom-erate, therefore, must be younger than 386 Ma (Ear-ly/Middle Devonian). This excludes the possibility thatthe Kalwang Conglomerate represents the stratigraphicbasement of the Ordovician–Carboniferous succession ofthe Noric unit.

There is no geochronological evidence for a Carboni-ferous (Variscan) overprint within the Kaintaleck Com-plex. The sedimentary record of the Noric nappe reflectsan extensional geodynamic environment with continuoussubsidence of the continental crust between the Middle/Late Ordovician and Early Carboniferous (e.g. Loeschke1989; Neubauer and Handler 2000). Variscan orogeny re-sulted in thin-skinned foreland thrust-and-fold tectonics.

Precambrian record

The Late Archean to Early Proterozoic upper interceptage of the hornblende garnet gneiss is interpreted interms of early protolith formation, and suggests that anold basement is incorporated in the Austroalpine conti-nental crust. This age needs confirmation by single graindating. A possible old age is also suggested by similarages of detrital zircon from phyllite regions belonging tothe Palaeozoic sequence elsewhere in the Eastern Alpsand from old memories in paragneiss zircons (Grauert etal. 1973, 1974; Söllner and Hansen 1987). The data alsoagree in a broad sense with similar records of LateArchean and Early Proterozoic zircons in sedimentaryand magmatic rocks reported from various sites of themid-European Variscides (e.g. Grauert and Arnold 1968;Grauert et al. 1973, 1974; Kröner et al. 1988; Gebauer etal. 1988, 1989; Gebauer 1993; and Köppel 1993; Wendtet al. 1993 for reviews).

Early Palaeozoic events

Zircons from both orthogneiss samples from the KalwangConglomerate record ages of zircon crystallisation at ca.500 Ma. These are interpreted to indicate a major epi-sode of granite intrusion in a region close to the meta-morphic complex of the Frauenberg slice.

The tectonothermal event at about 520–495 Ma con-firms the well-known evolution of the Middle Austroal-pine crystalline basement. Similar magmatic ages ofgranitoid and gabbro formation were reported from theSivretta and the Ötztal regions in the western Austroal-pine units and in Penninic units exposed in the Tauernwindow (Müller et al. 1995; Poller 1997; Schaltegger etal. 1997; Eichhorn et al. 1995, 1999). Similar ca. 500-Ma-U–Pb zircon ages of felsic intrusions and lava extru-sions were reported from the Western Alps (Guilot et al.1991; Bussy et al. 1996; Barféty et al. 1997). In the

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Middle Austroalpine basement, a high-grade metamor-phic event occurred between 490–470 Ma (e.g. Scharbert1981; Söllner and Hansen 1987; Klötzli-Chowanetz et al.1997; Hoinkes et al. 1997; Thöni 1999, with references)and was followed by granitoid magmatism between 460and 425 Ma (Thöni 1999, with references). A secondhigh-grade metamorphic event and associated granitemagmatism occurred between 360 and 310 Ma (Peindl1990; Miller and Thöni 1995; Finger et al. 1997; Schermaier et al. 1997; Thöni 1999).

On the other hand, geochronological data suggesting(possible back arc) ophiolite formation during the same520–485 Ma time interval are reported from other Alpineregions (Pin and Carme 1987; Menot et al. 1988; Vavraand Hansen 1991; von Quadt 1992; Eichhorn et al. 1995,1999; Miller and Thöni 1995). These data suggest theexistence of an orogenic environment with subductionand/or collision in the Late Cambrian/Early Ordovician.

The new U–Pb data of the Kaintaleck Complex sug-gest a Devonian age of metamorphic and magmaticevents (in the Prieselbauer slice). These ages are sup-ported by slightly younger or similar 40Ar/39Ar andRb–Sr muscovite ages (375–360 Ma) from metamorphicand magmatic (pegmatite, aplite) rocks. Therefore, thecrystalline rocks of the Kaintaleck Complex cannot bepart of the basement of the Noric(-Tirolic) nappe, whichis characterised by a carbonate platform and pelagicshelf sedimentation that is active throughout the Devon-ian and, thus, has a contrasting tectonic evolution.

A basement for the Veitsch nappe is unknown. Themarine to deltaic carbonate and clastic sediments of theVeitsch nappe are classified as molasse-type by Rats-chbacher (1987). Granite pebbles and heavy mineralspostulate a plutonic source region for the clastic sedi-ments. This source region has generally been assumed tobe the Middle Austroalpine basement (Schönlaub 1982;Ratschbacher 1987; Handler et al. 1997) which, howev-er, was deeply buried and experienced amphibolite fa-cies-grade metamorphism in the Early Carboniferous.Terrains that underwent Devonian metamorphism andmagmatism are widespread in the internal part of theVariscides (Gebauer et al. 1988, 1989; Paquette et al1989; Pin 1990; Gebauer 1993; von Raumer et al.,1993), and are a more realistic source region for the Veitsch nappe. The sediments of the Veitsch nappe,therefore, are the logical candidates to represent the cov-er of such an Early Devonian basement complex. TheKaintaleck Complex is considered to be a part of thisbasement. A primary relationship between metamorphicrocks and Carboniferous sediments of this zone is pre-served in the Western Carpathians (e.g. Krist et al. 1992;Dallmeyer et al. 1996). These relationships indicate thatbasement sequences recording intra-Devonian tectono-thermal events were more widespread in the Alpine–Car-pathian belt than previously thought.

Unlike the other Austroalpine basement units, theKaintaleck Complex is part of a zone affected by Devonianorogenic events that extends from the West EuropeanCordillere Ligerienne and Helvetic zones in the Western

Alps to the Penninic/Austroalpine zones in the EasternAlps. Remarkably, in the West Alpine Helvetic and inthe East Alpine Penninic zones, a similar relationship be-tween Devonian and 500-Ma ages as within the KaintaleckComplex is reported (Vavra and Hansen 1991; von Raumeret al. 1993).

Extent of Alpine resetting

A matter of systematic interest is the difference in reac-tion of the zircon U–Pb systems of the orthogneiss boul-ders in the metaconglomerate and the aplitic vein in thePrieselbauer slice to Cretaceous metamorphism. Where-as the zircons of the orthogneiss boulders reflect Creta-ceous metamorphism, those of the aplite gneiss show ap-parently no record of this event. The U contents of thezircons of all these samples are high, but the U contentsof zircons from the aplite (sample GR22G), which showsno Cretaceous influence, is higher than that of the ortho-gneiss boulders. Following the diffusion model of Wasserburg (1963), different temperatures and/or dura-tion of heating may be the cause for the different behav-iour (for full discussion, see Mezger and Krogstadt1997). A possible explanation, therefore, is that thePrieselbauer slice was less intensely affected by Creta-ceous metamorphism than the Ritting and Frauenbergunits. This interpretation is supported by preliminary ob-servations on retrogressed mineral parageneses of allthree basement slices. However, 40Ar/39Ar muscoviteand whole rock ages from Permian to Mesozoic coverrocks range from 99 Ma (hanging wall) to 85 Ma (foot-wall of the Kaintaleck Complex) and are significantlyyounger than the 139±18-Ma lower intercept age (Handler 1994; Dallmeyer et al. 1996, 1998). On the oth-er hand, K–Ar and Rb–Sr muscovite ages from units ex-posed in the hanging wall of the Kaintaleck Complexfurther west range between ca. 145 and 120 Ma (Kralik1983; Kralik et al. 1987). These Late Jurassic to EarlyCretaceous ages and the similar lower intercept age,therefore, are interpreted according to the model of iso-topic resetting during recrystallisation of Köppel et al.(1980), to represent the first Alpine thermal peak duringongoing burial associated with Alpine nappe stacking.

Conclusions

The following conclusions are drawn from field observa-tions and the U–Pb zircon study of the amphibolite-gradeKaintaleck Complex and the overlying Kalwang Con-glomerate. Some caution must be noted as the study isbased on isotope dilution multi-grain U–Pb dating.

1. The basement rocks possess a memory of Late Arche-an to Early Proterozoic protolith formation.

2. The basement rocks were probably affected by meta-morphism and granitoid magmatism in the 520- to495-Ma interval.

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3. Paragneisses probably suffered Devonian metamor-phism and, therefore, are considered to be part of thebasement of the molasse-type Early Carboniferous toPermian sequences now exposed within the Veitschnappe.

4. The principal orogenic evolution links the KaintaleckComplex with the West European Cordillere Ligerienne.

5. The evolutions of the Kaintaleck Complex contrast withthat of the main mass of the Middle Austroalpine base-ment. Thus, the different Austroalpine basement zoneshave been accreted at different times. The significantdifferences with respect to the influence of Cretaceousmetamorphism favour an Alpine accretion of varioustectonic units that now form the Kaintaleck Complex.

Acknowledgements We gratefully acknowledge careful reviewsby Corfu and an anonymous reviewer to an earlier version of themanuscript. These helped to clarify the text. Financial support wasgiven by the Deutsche Forschungsgemeinschaft (project Fr610/1-1).The isotopic analyses were performed in the GeochronologischesZentrallaboratorium at the University of Münster, Germany. F.N.gratefully acknowledges advice and help with the analytical workfrom Borwin Grauert, Dieter Buhl and Stefan Teufel, all at Münsterduring that time. Susanne Borchert corrected the English text of anearlier version of the manuscript.

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