Schuster, R. and Thöni, M. (2001): Austroalpine basement units - version 1. In: Dunkl, I., Balintoni, I., Frisch, W., Janák, M., Koroknai, B., Milovanovic, D., Pamiæ, J., Székely, B. and Vrabec, M. (Eds.): Metamorphic Map and Database of Carpatho-Balkan-Dinaride Area. http://www.met-map.uni-goettingen.de



Austroalpine basement units (AAB)

Version 1

compiled: R. Schuster and M. Thöni (2001)

completed:

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Definition
Age of Protolith, Geochemistry
Lithology, Mineralogy, Metamorphic Grade
      Thermobarometry
Geochronology
Structural Evolution
Summary
Bibliography
Links

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Definition

The Austroalpine basement units represent a complex nappe pile composed of pre-Variscan metasediments with intrusions of pre-Variscan, Variscan, Permo-Triassic and Tertiary magmatic rocks. They experienced Variscan, Permo-Triassic and Eo-Alpine metamorphism of various grade. Transgressive Permo-Mesozoic metasediments are locally preserved (Tollmann, 1977; Frank et al., 1987; Dallmeyer et al., 1998; Hoinkes et al., 1999; Neubauer et al., 1999; Thöni, 1999; Schuster et al., 2001a).

Geographic Position
The Austroalpine basement units form an E-W oriented belt extending between the Central Alps and the West Carpathians. In the Eastern Alps the Austroalpine basement is about 650 km long, and 50 to 150 km wide. The map covers the easternmost part only.

Maps
Overview maps: Geologische Karte von Österreich 1:1.500.000 (Beck-Mannagetta, 1964).

Maps of the Austrian Geological Survey, scale 1:200.000: Wien und Umgebung, Fuchs & Grill (1984); Steiermark, Flügel & Neubauer (1984)

Maps of the Austrian Geological Survey, scale 1:50.000: 104 Mürzzuschlag, 105 Neunkirchen, 106 Aspang, 107-108, Mattersburg-Deutschkreuz, 134 Passail, 137 Oberwart, 162 Köflach, 187 Bad St. Leonhard, 188 Wolfsberg, 189 Wolfsberg

Additional maps: Wieseneder (1971); Dallmeyer (1998); Tollmann (1977), Schuster et al. (2001b).

Boreholes
Most of the unit is exposed in the central Eastern Alps. However, some boreholes have also proven its existence below the Neogene sediments of the Styrian basin and the Pannonian basin (Frank et al., 1996).

Boundaries

To the north the Austroalpine basement units are bordered by the tectonically overlying Palaeozoic metasediments of the Greywacky Zone. In the northeast and east they are covered by Upper Tertiary sediments of the Vienna and Styrian Basin. In the southern part the border shows a complex pattern to the Penninic Rechnitz Window group and to the (Austroalpine) Graz Paleozoic and the Gurktal Nappe system. The border to the south is formed by the Periadriatic lineament.

Structural Position
Within the Alpine (Tertiary to Cretaceous) orogenic belt the Austroalpine units are in a hanging wall position with respect to the Penninic of the Rechnitz window group (RE-WI). Austroalpine Palaeozoic metasedimentary sequences (Palaeozoic of Graz, Greywacky Zone) and Austroalpine Mesozoic sedimentary piles (Northern Calcareous Alps, Stangalm Mesozoics) are overlying tectonically, mainly with thrust contacts. At several places the present-day contacts are formed by (Tertiary or Cretaceous) normal faults or steely dipping strike slip faults.

Subunits

The Austroalpine basement consists of several subunits which form a complex nappe pile. The individual units are characterised by differences in lithological composition, mineralogy, protolith age and geochemistry, the occurrence of meta-igneous rocks and by different tectonometamorphic evolutions. At present no generally accepted subdivision and nomenclature for the subunits exists. The subdivision used here is based on studies by Faupl (1970), Wieseneder (1971), Becker (1980), Jung (1982), Tollmann (1977), Dallmeyer et al. (1998) and Schuster et al. (2001b) (Fig. 1). In the northeasternmost part the section comprises from north to south and from bottom to top the Wechsel, Waldbach, Semmering, Strallegg and Sieggraben Complex. To the west the succession is formed by the Seckau-Troiseck-Floning, Speik, Wölz, Rappolt, Saualpe-Koralpe and Plankogel Complex. Generally, the contacts between these units are thrust contacts, except the Plankogel Complex, which rests upon the Saualpe-Koralpe Complex with a normal fault contact.

Correlation
The Austroalpine and the Southalpine units are part of the Apulian microplate (Stampfli & Mosar, 1999). Therefore the Southalpine unit shows many similarities with respect to lithology and the pre-Alpine tectonic and metamorphic history.


Fig.1: Tectonic map showing the eastern part of the Austroalpine unit.      Show map in bigger size & higher resolution (1 Mb).



Age of Protolith, Geochemistry

As the Austroalpine basement rocks are transgressively overlain by Permo-Triassic and partly also by Palaeozoic sequences, pre-Variscan protolith ages for the metasedimentary crystalline rocks can generally be assumed.

Several cycles of magmatic activity have been recognised: Pre-Variscan magmatic rocks occur in the Seckau-Troiseck-Floning and Speik Complex. They comprise ultramafic bodies composed of serpentinites, dunites, harzburgites and orthopyroxenites which are interpreted as relics of an ophiolitic sequence. Layered amphibolites and hornblende gneisses are interpreted to be of volcanic and of volcanodetritic origin respectively, probably related to an island arc system. The protolithe ages of the ultramafic and the mafic rocks are unknown. However, they have to be pre-Variscan, because they are affected by Variscan metamorphism. In the Seckau-Troiseck-Floning Complex Variscan S-type metagranites and augengneisses occur (Frank et al., 1976; Scharbert, 1981). Huge masses of porphyric granitegneisses ("Grobgneise") are characteristic in the Semmering Complex. Variscan (Scharbert, 1990; Peindl, 1990) and Permian (Pumhösl et al., submitted) age data have been published. Intense magmatism can be recognised in Permo-Triassic times: Permian quartzporphyric volcanic rocks (Roßkogel Formation) occur in the northern part of the Semmering Complex (Gaal, 1965). Gabbros with N-MORB characteristics occur in the Saualpe-Koralpe Complex (Miller & Thöni, 1997), olivine-gabbros derived from a subcontiental mantle are reported from the Semmering Complex (Pumhösl et al, submitted). In the Wölz Complex of the Wolfsberg window an A-Type granite occurs (Morauf, 1980), whereas S-Type granites are typical in the Strallegg Complex (Scharbert, 1990; Schuster et al., 2001b). Pegmatitic mobilisates are characteistic in the Rappold, Strallegg, Sieggraben and Saualpe-Koralpe Complex (Thöni & Miller, 2000; Schuster et al., 2001a). This magmatic event is thought to be related to Permo-Triassic lithospheric extension (Thöni, 1999; Schuster et al., 2001a). Scarce Eo-Alpine pegmatitic mobilisates formed during exhumation of the Eo-Alpine high-pressure rocks in the Saualpe-Koralpe Complex (Heede, 1997). Oligocene magmatic rocks are present along the Periadriatic lineament (Exner, 1976), whereas Miocene volcanics occur at the margins of the Styrian and Pannonian basin.

Lithology, Mineralogy, Metamorphic grade

The lowermost Wechsel Complex consists of polyphase greenschist facies metamorphic gneisses which are overlain by prograde metamorphic phyllites. Medium-grade hornblende gneisses, augengneisses and phyllitic micaschists with a greenschist facies overprint are dominating the Waldbach Complex. The Semmering Complex is composed of granitegneisses, micaschists and phyllitic micaschists. Leucophyllites composed of white mica + leuchtenbergite + quartz developed from orthogneisses by metasomatic processes (Huber, 1994). In the Seckau-Troiseck-Floning Complex hornblende gneisses and biotite plagioclase gneisses are widespread. The Speik Complex comprises ultramafic bodies overlain by amphibolites. The Wölz Complex mainly consists of prograde metamorphosed garnet-micaschists with intercalated layers of amphibolites, paragonite-bearing amphibolites and marbles. Staurolite-garnet micaschists of the Rappold Complex exhibit a polyphase metamorphic evolution. They are associated with marbles, amphibolites and pegmatitic mobilisates. Polyphase alumosilicate-bearing gneisses, associated with pegmatitic mobilisates are characteristic lithologies of the Strallegg, Saualpe-Koralpe and Sieggraben Complexes. Based on the microfabrics an older HT/Lp assemblage, including andalusite and sillimanite, can be recognised, which in turn is transformed into kyanite-bearing gneisses during a later Hp/LT event. Mylonitic gneisses up to several hundred meters thick ("Plattengneis") and eclogite lenses occur in the Sieggraben and Saualpe-Koralpe Complex. The Plankogel Complex consists of polyphase garnet-micaschists characteristic associated with Mn-quarzites, serpentinite slices and marbles. It is interpreted as a remnant of a pre-Alpine suture zone (Frisch et al., 1989).

Three metamorphic events can be recognised in the Austroalpine basement units:

In the Wechsel, Waldbach, Seckau-Troiseck-Floning, and Speik Complexes a Variscan metamorphic imprint reached greenschist to amphibolite facies conditions at medium to high pressures. Eclogite facies rocks have been preserved in the Hochgrössen massif in the northwesternmost part of the Speik Complex (Faryad et al., 2001).

A Permo-Triassic HT/Lp event with a geothermal field gradient of more than 45°C/km shows increasing conditions from the north to structurally higher units in the south. In parts of the Wölz and Plankogel Complexes almandine-rich garnet was formed at greenschist facies conditions (Lichem et al., 1997; Schuster & Frank, 2000). In the Rappold, Saualpe-Koralpe, Strallegg and Sieggraben Complexes amphibolite to ganulite facies conditions with local anatexis have been reached (Habler & Thöni, 2001; Schuster et al., 2001a; Schuster et al, 2001b). Typical assemblages are characterised by andalusite, sillimanite and K-feldspar.

The Eo-alpine event exhibits increasing conditions from north to south (Hoinkes et al., 1999). In the Wechsel and Waldbach Complex lowermost greenschist facies conditions have been reached. The Semmering and Seckau-Troiseck-Floning Complex exhibit greenschist facies metamorphism, whereas amphibolite facies conditions are indicated by staurolite in the Wölz and Strallegg Complex. In the Saualpe-Koralpe and Sieggraben Complex an eclogite facies event is overprinted by upper amphibolite facies conditions during exhumation (Thöni & Miller, 1996; Stüwe & Powell, 1995). The uppermost Plankogel Complex experienced lower amphibolite facies metamorphism.

Thermobarometry
1) Variscan collisional event: Variscan eclogites (T = 700 °C, p = 18 kbar) of the Hochgrössen massif (Speik Complex) are characterised by mineral parageneses consisting of clinopyroxene (Jd£ 39), garnet (Py15-19) and hornblende (Faryad et al., 2001).

2) Permo-Triassic extensional event:

The metamorphic evolution of the Strallegg complex during the HT/LP event is estimated by a characteristic succsession of reactions: paragonite + quartz = sillimanite + albite + H2O (T = 550 °C, p = 3.2-4.8 kbar), staurolite + muscovite + quartz = andalusite + biotite + H2O, muscovite + quartz = sillimanite + K-feldspar + H2O (T = 640-710 °C, p = 2.2-3.8 kbar (Draganits, 1998; Török, 1999)

Calculations from the assemblage garnet + biotite + plagioclase + muscovite + sillimanite yielded peak metamorphic conditions of 600 °C at 4 kbar for the Saualpe-Koralpe Complex (Habler & Thöni, 2001).

3) Eo-Alpine collisional event:

3a) Eo-Alpine subduction-related HP/LT metamorphism:

For the eclogites of the Sieggraben Complex conditions of T = 670-750 °C at p = 14-15 kbar have been determined, based on assemblages of clinopyroxene (Jd£ 30), garnet and hornblende (Neubauer et al., 1999; Putis et al., 2000).

Eclogites of the Saualpe-Koralpe Complex are composed of garnet + omphacite (Jd40) + kyanite + quartz + rutile + phengite ± amphibole (Miller, 1990; Miller & Thöni, 1997; Habler, 1999). Metamorphic conditions of T = 580-650 °C and p = 18-20 kbar have been calculated. For eclogites from the southern part of the Koralpe conditions of T = 700-750 °C and p ³ 16-17 kbar are reported (Lichem et al., 1997).

Calculations for metapelite assemblages of garnet + kyanite + muscovite + biotite + plagioclase + quartz from the Saualpe-Koralpe Complex yielded T = 700 °C, p = 14-15 kbar for the Koralpe (Stüwe & Powell, 1995; Gregurek, 1995) and T = 680-690 °C at 20 kbar for the Saualpe (Thöni & Miller, 1996; Habler, 1999).

3b) Eo-Alpine regional metamorphism:

Metamorphic conditions for the gabbros of the Strallegg Complex yielded T = 530–568 °C at p = 8-11 kbar, calculated from garnet + amphibole + plagioclase + quartz and garnet + paragonite + amphibole assemblages (Pumhösl et al., submitted).

A typical feature in the coarse grained micaschists of the Rappold Complex is the breakdown of staurolite by the reaction staurolite + muscovite = kyanite + garnet + biotite + H2O. Conditions based on this reaction are T = 650-700 °C, p = 10.5 kbar (Lichem et al., 1997).

Equilibrium parageneses of garnet + staurolite + kyanite + chloritoid + chlorite + paragonite + muscovite + quartz from the Plankogel Complex gave T = 550-600 °C, p = 8-11 kbar (Gregurek, 1995)).

 

Geochronology

Numerous age data for rocks and minerals of metamorphic as well as meta-igneous origin have been published, mainly over the past 20 years, based on the K-Ar, Rb-Sr, Sm-Nd and U-Th-Pb systems. Additionally, few FT-data are also available (Thöni, 1999). The data are compiled in Tab. 1.

Wechsel Complex

Lithology

Mean (±Ma)

Range(n) (±Ma)

Source

Rb-Sr phengitic Ms WR

gneisses

360±30

331-386 (6)

Müller et al. (1999)

Ar-Ar phengitic Ms

gneisses

326±20

303-339 (3)

Müller et al. (1999)

Rb-Sr paragonitic Ms-WR

phyllites

260±20

240-271 (3)

Müller et al. (1999)

Ar-Ar paragonitic Ms

phyllites

248±1

(1)

Müller et al. (1999)

Ar-Ar muscovite-WR

mylonite

86±12

(1)

Müller et al. (1999)

FT apatite

 

 

6-15 (6)

Dunkl (1992)

Waldbach Complex

Lithology

Mean (±Ma)

Range(n) (±Ma)

Source

Ar-Ar muscovite

Permo-Triassic cover

82±1

(1)

Dallmeyer et al. (1998)

Semmering Complex

Lithology

Mean (±Ma)

Range(n) (±Ma)

Source

Rb-Sr WR errorchron

orthogneisses (6)

338±12

(1)

Scharbert (1990)

Rb-Sr WR errorchron

orthogneisses (13)

253±19

(1)

Schuster et al. (2001a)

U-Pb zircon

orthogneisses

270±10

270-275

Pumhösl et al. (submitted)

Sm-Nd Cpx-Pl-WR

olivine-gabbro

264±8

(1)

Pumhösl et al. (submitted)

U-Th-Pb EMP xenotime

lazulithe-quartz vein

246±23

(1)

Bernhard et al. (1998)

Rb-Sr muscovite-WR

orthogneisses

203±80

111-277 (9)

Scharbert (1990), Müller (1994)

Ar-Ar muscovite

phyllitic micaschists

140±20

132-164 (2)

Berka (1998), Müller et al. (1999)

Rb-Sr muscovite-WR

Permo-Triassic cover

86±3

84-88 (2)

Müller et al. (1999)

K-Ar, Ar-Ar muscovite

orthogneisses

85±15

77-148 (10)

Scharbert (1981), Huber (1994), Dallmeyer et al. (1998),

 

 

 

 

Müller et al. (1999), Pumhösl et al. (2001)

K-Ar biotite

orthogneisses

83±2

82-84 (2)

Scharbert (1981)

Ar-Ar muscovite

leucophyllites

80±5

71-94 (10)

Huber (1994)

Ar-Ar muscovite

Permo-Triassic cover

81±3

79-84 (6)

Schmidt (1998), Berka (1998), Müller et al. (1999)

Rb-Sr biotite-WR

orthogneisses

71±10

59-83 (7)

Scharbert (1990), Müller et al. (1999)

FT zircon

 

 

70-80 (3)

Dunkl (1992)

FT apatite

 

56±15

40-70 (18)

Balogh & Dunkl (pers. comm.)

Strallegg Complex

Lithology

Mean (±Ma)

Range(n) (±Ma)

Source

Sm-Nd garnet-WR

pegmatite-granite

286±4

(1)

Schuster et al. (2001b)

Sm-Nd garnet-WR

metapelite

270±7

263-276 (2)

Schuster et al. (2001b)

Rb-Sr muscovite-WR

metapelite

290±7

283-296 (2)

Berka (1998), Schuster et al. (2001b)

Rb-Sr WR errorchron

granitgneisses (9)

240±10

(1)

Scharbert (1990)

Rb-Sr muscovite-WR

pegmatite

230±15

214-244 (3)

Scharbert (1990), Draganits (1996), Berka (1998)

Ar-Ar muscovite

pegmatite

240±2

(1)

Berka (1998)

Ar-Ar muscovite

gneisses

90±20

94-124 (4)

Berka (1998), Schuster et al. (2001b)

Rb-Sr biotite-WR

gneisses (N' part)

133±70

53-209 (6)

Draganits (1996), Berka (1998)

Rb-Sr biotite-WR

gneisses (S' part)

66±4

66 (2)

Scharbert (1990)

Sieggraben Complex

Lithology

Mean (±Ma)

Range(n) (±Ma)

Source

U/Pb zircon, monazite

granitic dyke

313±8

(1)

Putis et al. (2000)

U/Pb zircon, monazite

granitic dyke

103±14

(1)

Putis et al. (2000)

Ar-Ar hornblende

eclogite

122±14

108-136 (3)

Neubauer et al. (1999)

Ar-Ar muscovite

gneisses

80±3

78-82 (2)

Dallmeyer et al. (1998)

K-Ar muscovite

pegmatite

103±8

102-104 (2)

Milota (1983)

K-Ar muscovite

gneisses

86±10

76-95 (3)

Milota (1983), Putis et al. (2000)

K-Ar biotite

gneisses

78±6

(1)

Putis et al. (2000)

FT apatite

 

 

10-80 (3)

Dunkl (1992)

Troiseck-Floning Complex

Lithology

Mean (±Ma)

Range(n) (±Ma)

Source

Ar-Ar hornblende

amphibolites

358±100

458-260 (3)

Handler (1994), Schmidt (1998)

Ar-Ar muscovite

metapelites

310±10

314-265 (4)

Handler (1994), Schmidt (1998)

Ar-Ar muscovite

pegmatites

275±20

255-295 (2)

Schmidt (1998)

Ar-Ar muscovite

diaphthoritic micaschists

260±25

314-265 (2)

Handler (1994), Schmidt (1998)

Ar-Ar muscovite

metapelites

145±25

119-170 (2)

Schmidt (1998)

Ar-Ar muscovite

Permo-Triassic cover

89±3

87-93 (4)

Handler (1994)

Ar-Ar muscovite

pegmatite mylonite

81±1

(1)

Dallmeyer et al. (1998)

Rb-Sr muscovite-WR

pegmatites

253±30

219-284 (3)

Handler (1994), Schmidt (1998)

Rb-Sr muscovite-WR

metapelites

305±25

273-332 (4)

Handler (1994), Schmidt (1998)

Rb-Sr biotite-WR

metapelites

89±15

75-134 (7)

Handler (1994), Schmidt (1998)

Seckau Complex

Lithology

Mean (±Ma)

Range(n) (±Ma)

Source

Rb-Sr WR-errorchrone

metavolcanic gneisses (5)

500±45

(1)

Frank et al. (1976)

Rb-Sr muscovite

pegmatite

329±12

(1)

Scharbert (1981)

Rb-Sr muscovite

granite

331±7

(1)

Scharbert (1981)

K-Ar muscovite

orthogneiss

105±4

(1)

Scharbert (1981)

Rb-Sr muscovite-WR

paragneiss

84±9

(1)

Frank et al. (1976)

Ar-Ar biotite

paragneisses

91±2

(3)

Kolenprat (1997)

K-Ar biotite

orthogneiss

91±2

(1)

Scharbert (1981)

Ar-Ar muscovite

Permo-Triassic cover

89±3

(3)

Kolenprat (1997)

Rb-Sr biotite-WR

orthogneisses

74±4

70-77 (3)

Scharbert (1981)

Rb-Sr biotite-WR

paragneiss

77±3

72-80 (4)

Frank et al. (1976), Kolenprat (1997)

FT-apatite

 

52±10

44-60 (2)

Hejl (1997)

Speik Complex

Lithology

Mean (±Ma)

Range(n) (±Ma)

Source

Ar-Ar hornblende

eclogite

397±8

(1)

Faryad et al. (2001)

Ar-Ar hornblende

amphibolites

101±8

96-109 (6)

Neubauer et al. (1995), Schuster et al. (1999)

Ar-Ar muscovite

augengneisses

86±2

84-87 (2)

Frank (unpubl. data)

Ar-Ar muscovite

paragneiss

84±1

(1)

Neubauer et al. (1995)

Ar-Ar biotite

augengneisses, gneisses

89±5

84-93 (3)

Kolenprat (1997), Frank (unpubl. data)

Rb-Sr biotite-WR

paragneiss

80±1

(1)

Kolenprat (1997)

FT sphene

amphibolite

70±9

(1)

Neubauer et al. (1995)

FT zircon

amphibolite

61±6

(1)

Neubauer et al. (1995)

FT apatite

amphibolite

34±6

(1)

Neubauer et al. (1995)

Wölz Complex

Lithology

Mean (±Ma)

Range(n) (±Ma)

Source

Rb-Sr WR errorchron

orthogneisses (6)

258±11

(1)

Morauf (1980)

Sm-Nd garnet(core)-WR

micaschists

269±4

(1)

Schuster & Frank (2000)

Sm-Nd garnet-WR

micaschists

90±10

84-95 (2)

Schuster & Frank (2000)

Ar-Ar muscovite

micaschists

88±10

81-115 (3)

Frank (unpubl. data)

K-Ar muscovite

micaschists, orthogneisses

87±5

75-95 (18)

Morauf (1980), Hejl (1984), Schuster & Frank (2000)

K-Ar biotite

micaschists, orthogneisses

92±20

76-131 (8)

Morauf (1980), Hejl (1984), Schuster (1994)

Rb-Sr muscovite-WR

orthogneisses

79±3

79-80 (7)

Morauf (1980)

Rb-Sr biotite-WR

micaschists, orthogneisses

76±2

74-78 (7)

Morauf (1980); Schuster & Frank (2000)

FT apatite

 

17±3

14-20 (5)

Hejl (1997)

Rappold Complex

Lithology

Mean (±Ma)

Range(n) (±Ma)

Source

Sm-Nd garnet-WR

pegmatite

276±12

264-288 (2)

Schuster & Frank (2000), Schuster et al. (2001a)

Rb-Sr muscovite-WR

pegmatite

235±30

205-248 (5)

Frank et al. (1983), Schuster et al. (2001a)

Sm-Nd garnet-WR

micaschist

286±3

(1)

Schuster et al. (2001)

Ar-Ar muscovite

pegmatite

86±3

84-88 (4)

Frank (unpubl. data)

Ar-Ar muscovite

metapelites

86±2

84-87 (3)

Frank (unpubl. data)

FT apatite

 

33±4

(1)

Hejl (1997)

Saualpe-Koralpe Complex

Lithology

Mean (±Ma)

Range(n) (±Ma)

Source

Sm-Nd Cpx-Pl

gabbro

257±15

247-275 (4)

Thöni & Jagoutz (1992), Miller & Thöni (1997)

U-Pb Zircon

spodumene pegmatite

240±1.5

(1)

Heede (1997)

Rb-Sr spodumene-WR

spodumene pegmatite

241±5

239-243 (2)

Thöni & Miller (2000)

Sm-Nd garnet-WR

pegmatites

245±20

225-264 (3)

Thöni & Miller (2000)

Sm-Nd garnet(core)-WR

micaschist

267±17

(1)

Schuster et al. (2001a)

Sm-Nd Grt-Cpx-WR

eclogites

98±10

91-109 (6)

Thöni & Jagoutz (1992), Miller & Thöni (1997),

 

 

 

 

Lichem et al. (1997)

Sm-Nd garnet-WR

metapelites

90±4

86-94 (9)

Thöni & Miller (1996), Miller & Thöni (1997)

Rb-Sr muscovite-WR

pegmatites (corse gr.)

294±120

232-428 (7)

Frank et al. (1983)

Rb-Sr muscovite-WR

pegmatites (fine grained)

152±50

117-205 (4)

Frank et al. (1983)

Rb-Sr phengite-WR

pegmatiodes in eclogites

92±10

84-102 (4)

Thöni & Jagoutz (1992), Heede (1995)

K-Ar, Ar-Ar hornblende

eclogites

301±150

163-536 (9)

Rittmann (1984)

K-Ar, Ar-Ar hornblende

pegmatiodes in eclogites

150±40

109-190

Rittmann (1984)

Ar-Ar phengite

pegmatiodes in eclogites

85±3

83-88 (2)

Rittmann (1984)

K-Ar muscovite

pegmatites

90±15

79-116 (14)

Morauf (1980), Morauf (1981)

K-Ar muscovite

metapelites

85±3

82-88 (5)

Morauf (1980)

K-Ar biotite

metapelites

87±9

79-109 (6)

Morauf (1980)

Rb-Sr biotite-WR

metapelites

87±9

79-109 (6)

Morauf (1980)

FT apatite

 

42±15

26-53 (5)

Hejl (1997)

Plankogel Complex

Lithology

Mean (±Ma)

Range(n) (±Ma)

Source

Sm-Nd garnet-WR

micaschist

275±10

264-285 (2)

Lichem et al. (1997), Thöni (2002)

Table1: Geochronological age data from the Austroalpine basement units.

Structural Evolution

In Eoalpine times the Austroalpine basement units were affected by several deformation events (Ratschbacher, 1986; Ratschbacher et al., 1989; Genser & Neubauer, 1989). The collisional event prior to 100 Ma is characterised by W-WNW directed thrust tectonics. It is responsible for the decoupling of the sedimentary cover series and burial of the crystalline rocks. Exhumation of the high pressure rocks started at about 90± 3 Ma (Thöni, 1999). In the eastern part of the basement units it is characteried by a first period of northwest to north directed thrusting and south directed normal faulting (Frank et al., 1983; Krohe, 1987), followed by southeast directed extension (Habler, 1999). It started close to the thermal peak of metamorphism by ductile deformation and continued into the regime of brittle deformation in the lower grade metamorphic units. In Miocene times lateral extrusion of the orogen caused block faulting (Ratschbacher et al., 1989; Genser & Neubauer, 1989; Peresson & Decker, 1997).

Summary

The Austroalpine represents a part of the northern Apulian microplate. Prior to the Variscan collisional event it held an island arc environment position, and magmatic rocks developed. During the Variscan collisional event parts of the Unit experienced a Lower Carboniferous HP/LP metamorphism (390-350 Ma) and a subsequent thermal overprint in Upper Carboniferous times (340-300 Ma). In Permian time an extensional regime developed. Lithospheric extension caused a HT/Lp metamorphic imprint and related magmatic activity. Peak metamorphic conditions were reached at about 260 Ma. Subsequently, the lithosphere cooled down and huge Triassic sedimentary piles were deposited on top (245-200 Ma). Since the Jurassic the Austroalpine realm was affected by strike slip faulting and compressional tectonics. In Cretaceous times the Austroalpine acted as the tectonic lower plate during the closure of the Meliata-Hallstatt Ocean. The peak of the suduction-related HP/LT metamorphism occurred at about 100± 10 Ma, followed by a medium-pressure thermal overprint during exhumation (90-65 Ma).

According to Stampfli et al. (1998) the Austroalpine was a part of the Hun Superterrane in early Palaeozoic times. Due to subduction processes the Austroalpine stayed in an island arc environment and pre-Variscan intrusives and volcanic rocks developed (Neubauer et al., 1999).

In late Palaeozoic time the Austroalpine was affected by the Variscan event, induced by the collision of Africa, Baltica, Laurentia and the intervening microplates (Tait et al., 1997). Parts of the Unit experienced a Lower Carboniferous HP/LP event (390-350 Ma) and a subsequent thermal overprint in Upper Carboniferous times (340-300 Ma) (Thöni, 1999; Faryad et al. 2001,).

After a period of orogenic collapse and erosion, an extensional regime developed between the European and the Apulian plate. As an element in between the Austroalpine unit was affected by a HT/LP metamorphic imprint and magmatic activity, due to lithospheric extension. Peak metamorphic conditions were reached at about 260 Ma. Ongoing extension resulted in the opening of the Triassic Meliata-Hallstatt Ocean to the south. The Austroalpine lithosphere cooled down and huge Triassic sedimentary piles were deposited on top (245-200 Ma) (Schuster et al. 2001a) .

Opening of the Atlantic and Penninic Ocean in Jurassic time leads to sinistral strike slip faults in Europe and a reorganisation of the westernmost part of the Tethyan realm. The Austroalpine realm was also affected by strike slip faulting and compressional tectonics (Gawlik, et al., 1999).

The change of motion between the African and Eurasian Plate since lower Cretaceous time caused shortening in the area resulting in the complex scenario of the Eo-Alpine and Neo-Alpine orogeny. In Cretaceous times the Austroalpine acted as the tectonic lower plate during the closure of the Meliata-Hallstatt Ocean. The peak of the suduction related HP/LT metamorphism occurred at about 100± 10 Ma, followed by a medium-pressure thermal overprint during exhumation (90-65 Ma) (Thöni, 1999). During the following subduction of the Peninnic ocean the Austroalpine formed the tectonic upper plate and suffered only minor thermal influence.

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Links

Descriptions of the neighbouring Units:

Paleozoic of Graz (PZG)

Greywacky Zone (GWZ)

Penninic of the Rechnitz window group (REW)

Transdanubian Central Range (TCR)

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