Introduction

A major part of the Alpine orogen consists of pre-Mesozoic basement units which recorded a polyphase tectonic and metamorphic evolution. The Austroalpine Basement to the south of the Tauern Window is subdivided into four lithological groups (Fig.1). Between the southern margin of the Central Tauern Window and the major faults of the Periadriatic Lineament and towards the hangingwall one can distinguish the (1) Northern-Defereggen-Petzeck Group (NDPG); (2) Durreck Muscoviteschist Group (DMG); (3) Defereggen Group (DG) and the (4) Thurnthaler Phyllite Group (TPG) (Schulz and Bombach 2003).

The Northern-Defereggen-Petzeck Group consists of metapsammopelitic rocks with interlayered meta-quartzites and marbles. Within the NDPG four metabasic subgroups appear with different geochemical characteristics (Prijakt, Rotenkogel, Torkogel, Michelbach and Croda Nera Subgroup). Monotonous garnet-muscovite-schists and garnet-muscovite-quartzitic gneisses are the main rock types in the Durreck Muscoviteschist Group. This unit occurs within the NDPG. The Defereggen Group is characterized by monotonous metapsammopelitic banded quartzitic gneisses, paragneisses and micaschists which are interlayered by rare and thin amphibolites, marble, graphitic gneisses and quartz-feldspar gneisses. The Thurntaler Phyllite Group overlies the DG along a partly overturned pre-Alpine contact (Heinisch and Schmidt 1984; Schulz 1991; Kreutzer 1992). It consists of quartz-phyllites and phyllitic micaschists which are interlayered by amphibolites, epidot-amphibolites, chlorite schists, quartzites, graphite phyllites and rare marbles.
The W-E trending Defereggen-Antholz-Vals line (DAV) is a shear zone which subdivides the Austroalpine Basement into two major blocks. As the NDPG and the DMG appear to the north of the DAV the Defereggen Group and the Thurntaler Phyllite Group occur to the south of the DAV. The shear zone shows a sinistral offset and an uplift of the northern block in transpressional regime with NE-SW-directed compression. In the northern block Early- and Late-Alpine ductile deformation overprinted earlier structures. In the southern block a Variscan ductile deformation and an Alpine brittle overprinting are observed.

Methods

For the distinction of the Variscan and Alpine metamorphism and deformation it is necessary to study the macrotectonical features of the metamorphic and meta-magmatic rocks like foliation, lineation and shear sense indicators. Microtectonical observations are possible by studying thin sections which show typical metamorphic mineral assemblages and their deformation in XZ- or in YZ-sections of finite strain. This microtectonical setting is important for reconstructing the relative sequence of deformation and the metamorphic facies as well as for determining the age of metamorphic events by K-Ar, Rb-Sr dating of mica and electron microprobe (EMP) dating of monazite.

Variscan deformation and metamorphism

Pre-Alpine (Variscan) ductile deformation is observed primarily to the south of the Defereggen-Antholz-Vals Line. Paragneisses and micaschists of the Defereggen Group show sporadic cm-thick monomineralic quartz-layers which are considered to represent a foliation S_V1. These S_V1-quartz-layers became subjected to a deformation D_V2. The quartz-layers were deformed into isoclinal folds F_V2 during deformation D_V2. The main foliation S_V2 of the rocks is axial-planar to the F_V2 folds. Among the congruent and isoclinal F_V2 folds occur strongly non-cylindrical sheath folds (cm to dm scale) with strongly curved fold axes (Schulz 1988b). They appear in quartz-, biotite-plagioclase- and calcsilicategneisses as well as in marbles. Calcsilicategneiss bodies also show these tongue-like structures of D_V2. Upper Ordovician magmatites (now orthogneisses) got their first main foliation and stretching lineation which is parallel to S_V2 in the metasedimentary rocks also during D_V2 (Stöckhert 1985). For this reason the main foliation to the south of the DAV should have a Post-Upper-Ordovician age.

During D_V3 the S_V2 and F_V2 structures were refolded and overprinted by F_V3 folds whose fold axes and crenulation fold axes LV3 are oriented mostly parallel to LV2 and the F_V2 fold axes. The axes of the F_V3 folds are steeply plunging in some regions and are minor structures of large-scale syn- and antiforms, named as “Schlingen” structures (Schmidegg 1936). In the eastern and southern parts of the Defereggen Group a more gentle WSW or ENE plunging of the linear structures prevails. A similar orientation of LV2/S_V2 linear-planar fabrics and of F_V3 fold axes as in the DG is observed in the adjacent Thurntaler Phyllite Group (Schulz 1991).
In phyllitic micaschists of the DG in the E a shear band foliation S_V4 with top-to-NE-transport cuts the older S_V2 and bends this fabric sigmoidally (Schulz 1988a). The lithological contact between DG and TPG (labelled Markinkele-Line to the W) was overprinted by mylonitic shear bands S_V5 with NW to W or S directed movement (Heinisch and Schmidt 1984; Schulz 1991; Kreutzer 1992). S_V4 and S_V5 shear bands show typical dynamic recrystallisation of quartz.

In the DG the typical paragenesis of micaschists is biotite + muscovite ± chlorite ± garnet ± staurolite ± kyanite ± oligoclase + quartz. As staurolite and kyanite crystallize after garnet, staurolite encloses the garnet. Furthermore staurolite and kyanite occur partially in microlithons enclosed by S_V2. Both minerals overgrow also F_V3 crenulation folds post-deformatively. Cuts parallel to the crenulation lin-ear show asymmetric S-shaped bended internal foliations S_V1i and doubled inclu-sions spirals in garnets. This indicates a syndeformative growth during simultaneous rotation of porphyroblasts by dominant simple shear. The sheath folds and calcsilicategneiss bodies were also shaped by simple shear during D_V2. The garnets show growth zonations with increasing X_Mg and a systematic variation of X_Ca from core to rim (Schulz 1995). Thermobarometrical studies (Fig.2) point to maximal p-T-conditions of 630°C/6 kbar for Mg rich rims of Ca-variable garnets (Schulz et al. 2001). Depending on the zonation also lower temperatures and pressures occur there . In garnets of phyllitic micaschists to the south of the DG the X_Mg increases whereas the X_Ca is constant. This causes p-T-conditions with increasing temperatures from 400 to 500°C and pressures from 6 to 7 kbar. Staurolite and kyanite are absent in this part of the unit.

Garnets of the Thurntaler Phyllite Group show S-shaped bended internal foliations with syndeformative growth and rotated porphyroblasts just as in the Defereggen Group. There occur furthermore postdeformative unzoned garnets which overgrow S_V2. Thermobarometrical investigations (Fig.2) of Schulz et al. (2001) result in an increasing temperature and pressure from 420°/4 kbar to 500°C/5 kbar for crystallization of syndeformative garnets. The postdeformative garnets crystallize at 530°/6 kbar.

In amphibolites of the TPG, continuously zoned amphiboles recorded prograde p-T-paths from 300°C/2 kbar to 600°C/6 kbar (Schulz 1991). The thermobarometric data signalize minor differences between the DG and the TPG in respect of maximum temperature of metamorphism but major differences in respect of pressure conditions (Fig.2). The p-T-data of the Defereggen and the Thurntaler Phyllite Group characterise a Variscan metamorphism. This is evident from Upper Carboniferous mica cooling ages (Borsi et al. 1978; Schuster et al. 2001). An other argument for a Variscan (resp. Pre-Alpine) metamorphism is given by the already mentioned pervasive dynamic recrystallization of quartz in S_V4 and S_V5 shear bands which are not observed in Alpine deformed Permo-Triassic sandstones of Kalkstein (Guhl and Troll 1987). Moreover the Upper-Ordovician magmatites were subject to D_V2.

Variscan metamorphism is also proved to the north of the DAV. The already mentioned S_V1 quartz layers occur as sheared off hinges of isoclinal F_V2 folds. These folds lie within the axial-planar foliation S_V2. Garnets from the Durreck Muscoviteschist Group affect in remaining S_V2 main foliation or isolated in microlithons. The inclusion spirals show no connection to the external foliation. They are assumed as remnants of S_V1. The S_V2 foliation in muscoviteschists is partially overgrown by garnet. This indicates a crystallization of garnet seams after D_V2. Either the garnet grew during a static Late-Variscan or during the Alpine metamorphism. In paragneisses and amphibolites of the northern NDPG coarse grained muscovite and biotite as well as amphibole form S_V2. The crystallization of garnet in metapelites and tschermakite in amphibolites is correlated with the formation of S_V1 and S_V2 foliations under prograde conditions within the amphibolite facies. After Schulz et al. (2001) the crystallization of garnet seams (post D_V2) is assign to the Variscan metamorphism. The garnets are continuously zoned with increasing XMg from core to rim. An Alpine overprint with a new growth of garnet would have caused prominent varieties in the zonation profiles.
The pressure-temperature-paths (Fig.2) of the eclogitic amphibolites within the Prijakt Subgroup show a prograde metamorphic evolution from 550-600°C/14 kbar to 600-650°C/15 kbar (Schulz 1993). Amphiboles out of the eclogitic amphibolites display a retrograde evolution from 730°C/10 kbar to 600°C/6 kbar (Schulz et al. 2005). The authors mentioned above state maximal temperatures 650°C/7 kbar and maximal pressures of 650°C/7 kbar for metapelites of the NDPG. Both the metapelites and the eclogitic amphibolites show p-T-conditions of amphibolite facies.

The question concerning dominant Variscan or Alpine metamorphism north to the DAV arises also with respect to the eclogitic amphibolites of the Prijakt Subgroup. Micaschists associated to the eclogitic amphibolites contain monazite which occurs often with staurolite. The Th-U-Pb-dating of these monazites by electron microprobe analysis yielded mostly Variscan (350-320 Ma) and Permian (290-260 Ma) ages (Schulz et al. 2005). One sample contains monazite grains at the age of 95 ± 11 Ma, whereas also Permian monazites occur. Another sample shows a change from Pre-Alpine to Eo-Alpine ages from core to rim. However, the majority of monazites crystallized during a Pre-Alpine (Variscan-Permian) metamorphism. The Eo-Alpine age indicates a low grade Alpine overprint of the dominant Variscan metamorphic rocks.

Alpine deformation and metamorphism

The Austroalpine Basement to the north of the Defereggen-Antholz-Vals line is mainly characterized by ductile Alpine deformation. The Variscan S_V2 foliation within the NDPG and DMG is overprinted by the first Alpine deformation S_A1. A second foliation S_A2 is parallel to S_A1. S_A2 is the axial-planar foliation of FA2 folds and dips steeply to moderate to SSE. Within the DMG also N dipping directions occur. S_A2 is locally folded by FA3. Furthermore ECC-type-foliations S_A3ECC as well as mylonitic foliations S_Amyl occur on the northern margin of the DAV.
Garnets in micaschists of the DMG show typical double circular inclusion figures in cuts parallel to the lineation respectively rotation axis. This is ascribed to a passive rotation of older garnets during the Alpine deformation. Sigma shaped pressure shadows of garnets in S_A2 domains indicate a top-to-west transport.

Variscan and Alpine deformation and metamorphism to the north of the DAV can be distinguished by pegmatites intruded mostly parallel to S_V2. The pegmatites intruded both to the north (e.g. Prijakt Subgroup of the NDPG) and the south of the DAV. The 262 ± 5 Ma old Permian pegmatites (Borsi et al. 1980) to the south of the DAV are overprinted in a cataclastic to pseudotachylitic manner (brittle deformation) whereas the pegmatites to the north of the DAV show incomplete to complete foliation (ductile deformation). This deformation took place after the Permian intrusion. Therefore it must be an Alpine deformation age (S_A1). Schönhofer (1999) proved a top-to-west transport during the first Alpine deformation on the basis of alignments of quartz-c-axes.

Dating of muscovite yielded early Alpine K-Ar ages (100 and 80 Ma), dating of biotite yielded late Alpine Rb-Sr ages (15 to 28 Ma) (Borsi et al. 1978, Stöckhert 1985). Furthermore Alpine-Variscan mixing ages occure there. These mixing ages in combination with the datings and microtectonic observations show an Alpine overprinting of the pervasively Variscan deformed rocks of the Austroalpine basement. The Alpine influence also was provided by Schulz et al. 2006 on the basis of monazite dating (95 ± 11 Ma).
The metamorphism and deformation took place under greenschist-facies-conditions (Fig.2). The amphibolite facies with maximal temperatures of 500°C and pressures of 4 kbar was attained locally (calculated on the basis of amphibolites of the Penninic Unit, Schulz et al. 2006).
The intrusion of the Oligocene (30 Ma, Borsi et al. 1979) Rieserferner tonalite pluton along the western DAV caused a contact metamorphism (Fig.2) with maxi-mal conditions of 620°C/3 kbar (Schulz 1994). Contact metamorphic blasts like andalusite, sillimanite and staurolite are observed. The original foliation survived as layered anisotropy.

Geodynamic model

A geodynamic model of the Variscan deformation and metamorphism (Fig.3) is presented in Schulz et al. 2006. The Variscan orogeny begins with the collision of Gondwana terranes (and later Gondwana itself) with Laurussia during Devon and Carbon. Furthermore the subduction of Palaeotethys begins. The associated polymetamorphism can be explained by a “overstacking” model (Schulz 1990, Schulz 1995, Schönhofer 1999). During an early phase of the Variscan metamorphism the northern crustal segments underwent burial and subsequent uplift. This resulted in high pressure amphibolite-facies conditions. The process of burial and uplift progressed toward the south and included the units in the hangingwall. The DG and TPG are therefore characterized by lower maximal pressures. The subsequent late Variscan uplift of the rocks took place at a dextral transpression system with NW-SE directed compression. Indications are given by “Schlingen” structures F_V3 and the S_V4 und S_V5 shear bands.

Postcollisional pegmatites intruded in the Variscan orogen during the Permian. The Permian monazite ages and crystallization of andalusite in metapelites nearby the pegmatites signalize re-increasing temperatures at low pressures during the general decompression path. Altogether, the post-Variscan evolution is dominated by crustal extension and thinning, followed by the Mesozoic sedimentation.

In the course of subduction of the European under the Adriatic plate the Alpine orogeny took place (Upper Cretaceous – Paleogene). To the south of the present Tauern Window the Defereggen-Antholz-Vals line developed. The block to the north of the sinistral DAV was uplifted at the oblique reverse fault relatively to the southern block (Borsi et al. 1978, Schulz 1989). The vertical displacement amounts 5 km to the W, 2 – 3 km at the Staller Sattel and < 1 km to the E. The horizontal displacement amounts 30 km (Schulz 1989). The uplift caused an erosional exposure of the late-Alpine ductile deformation structures in the northern block.

References

Borsi S, Del Moro A, Sassi FP, Zanferrari A, Zirpoli G (1978) New geopetrologic and radiometric data on the Alpine history of the Austridic continental margin south of the Tauern Window. Mem Ist Geol Min Univ Padova 32:1-17

Borsi S, Del Moro A, Sassi FP, Zirpoli G (1979) On the age of the Vedrette de Ries (Rieserferner) massif and its geodynamic significance. Geol Rdsch 68: 41-60

Borsi S, Del Moro A, Sassi FP, Visona D, Zirpoli G. (1980) On the existence of Hercynian aplites and pegmatites in the lower Aurina Valley (Ahrntal, Austrides, Eastern Alps). N Jb Min Mh 1980: 501-514

Guhl M, Troll G (1987) Die Permotrias von Kalkstein im Altkristallin der südlichen Deferegger Alpen (Österreich). Jb Geol Bundesanst 130: 37-60

Heinisch H, Schmidt K (1984) Zur Geologie des Thurntaler Quarzphyllits und des Altkri-stallins südlich des Tauernfensters (Ostalpen, Südtirol). Geol Rdsch 73:113-129

Kreutzer S (1992) Zur Geologie des östlichen Thurntaler Quarzphyllitkomplexes und seiner tektonischen Einbindung in das Ostalpin der südöstlichen Deferegger Alpen, Osttirol. Thesis RWTH Aachen

Schulz B (1988a) Deformation und Metamorphose im ostalpinen Altkristallin südlich des Tauernfensters (südliche Deferegger Alpen, Österreich). Schweiz Mineral Petrogr Mitt 68: 397-406

Schulz B (1988b) Quarz- und Mikrogefüge zonierter Kalksilikatgneis-Körper im ostalpinen Altkristallin (südliche Deferegger Alpen, Österreich). – Erlanger geol Abh 116: 117-122

Schulz B (1989) Jungalpidische Gefügeentwicklung entlang der Defereggen-Antholz-Vals-Linie, Osttirol, Österreich. Jb Geol Bundesanst 132: 775-789

Schulz B (1990) Prograde-retrograde P-T-t-deformation path of Austroalpine micaschists during Variscan continental collision (Eastern Alps). J metam Geol 8: 629-643

Schulz B (1994) Microstructural evolution of metapelites from the Austroalpine basement north of the Staller Sattel during pre-Alpine and Alpine deformation and metamorphism (Eastern Tyrol, Austria). Jb Geol Bundesanst 137: 197-212

Schulz B (1991) Deformation und Metamorphose im Thurntaler Komplex (Ostalpen). Jb Geol Bundesanst 134: 369-391

Schulz B (1995) Rekonstruktion von P-T-t-d-Pfaden der Metamorphose: Mikrostrukturell kontrollierte Geothermobarometrie in Metapeliten und Metabasiten der variskischen Internzone (Ostalpen, Nordost-Bayern, Aiguilles Rouges Massif, Massif Central). Erlanger geol Abh 126: 1-222 (Habilitationsschrift)

Schulz B, Schönhofer, R (2000) Variscan vs Alpine metamorphism in the Austroalpine basement to the south of the central Tauern Window, Lasoerling, eastern Tyrol, Aus-tria; Geology 2000. Terra Nostra 2000-1 pp. 104

Schulz B, Siegesmund S, Steenken A, Schönhofer R, Heinrichs T (2001) Geologie des ostalpinen Kristallins südlich des Tauernfensters zwischen Virgental und Pustertal. Z dt geol Ges 152: 261-307

Schulz B, Bombach K (2003) Single zircon Pb-Pb geochronology of the Early-Palaeozoic magmatic evolution in the Austroalpine basement to the south of the Tauern Window. Jb Geol Bundesanst 143: 303-321

Schulz B, Finger F, Krenn E (2005) Auflösung variskischer, permischer und alpidischer Ereignisse im polymetamorphen ostalpinen Kristallin südlich der Tauern mit EMS-Datierung von Monazit. Arbeitstagung 2005 der Geologischen Bundesanstalt Öster-reich 141-153

Schulz B, Siegesmund S, Steenken A (2006) Geodynamic evolution of an Alpine terrane – the Austroalpine basement to the south of the Tauern Window as a part of the Adriatic plate (Eastern Alps). in: Tectonic aspects of the Alpine-Dinaride-Carpathian system, Geological Society Special Publication (submitted)

Schmidegg O (1936) Steilachsige Tektonik und Schlingenbau auf der Südseite der Tiroler Zentralalpen. Jb Geol Bundesanst 86: 115-149

Schuster R, Scharbert S, Abart R, Frank W (2001) Permo-Triassic extension and related HT/LP metamorphism in the Austroalpine – Southalpine realm. Mitt Ges Geol Bergbaustud Österr 45: 111-141

Stöckhert B (1985) Pre-Alpine history of the Austridic basement to the south of the western Tauern Window (Southern Tyrol, Italy): Caledonian versus Hercynian event. N. Jb. Geol. Paläont. Mh. 1985: 618-642

(Dieser Text ist am 21.11.2006 auf der alten Version von geoberg.de erschienen und wurde übernommen.)

Tags: alpine, Alps, Austroalpine Basement, Metamorphism, Tauern Window, Tauernfenster, variscan

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