The Altkristallin of the Kreuzeck Mountains, SE Tauern Window, Eastern Alps

83 Pages·2012·6.57 MB·English

©Geol. Bundesanstalt, Wien; download unter www.geologie.ac.at
Jb. Geol. B.-A. ISSN 0016-7800 Band 133 Heft 1 S.5-87 Wien, März 1990
The Altkristallin of the Kreuzeck Mountains,
SE Tauern Window, Eastern Alps -
Basement Crust in a Convergent Plate Boundary Zone
By LEONORE HOKE*)
With 48 Text-Figures, 9 Tables and 5 Plates
Kärnten
Kreuzeck Gruppe
Tauem-Gebiet
Austroa/pin
Allkristallin
Kretazische Metamorphose
Variszische Metamorphose
Osterreichische Karte Extensive Deformalionslone
8/all 181 Geochronologische Allersdaten
Contents
Zusammenfassung 6
Abstract 6
1. Introduction 6
2. Subdivision of the Study Area. .................................. .................... .......... ..... 8
a
3. The Polinik Unit 1
a
3.1. Lithologies in the Polinik Unit 1
a
3.2. Deformation History of the Polinik Unit 1
3.3. Inter O2-03 Metamorphism 12
3.3.1. Mineral Phases 12
3.3.2. PIT Sensitive equilibria 19
4. The Strieden Unit 22
4.1. Introduction 22
4.2. Deformation History of the Strieden Unit 22
4.2.1. 01, Om' On Fabrics 22
4.2.2. Variscan High-Temperature Metamorphism and Do 23
4.2.3. Dp-Dq Fabrics 24
4.2.4. Original Orientation of Sp and Ip 32
4.3. Metamorphism of the Strieden Unit 36
4.3.1. Distribution of Minerals in the Strieden Unit 36
4.3.2. Mineral Chemistry 36
4.3.3. Phase Relations 41
4.3.4. Metamorphic Conditions 42
5. Geological Synthesis 48
5.1. Geochronological Data from the SE Tauern Area 48
5.1.1. General Considerations on the Significance of KlAr Ages 48
5.1.2. KlAr Age Distribution in the Altkristallin Rocks of the Study Area and Adjacent Areas 50
5.1.3. Summary of the Regional Age Pattern 54
5.2. Age of Metamorphism and Deformation in the Polinik Unit 54
5.2.1. Age of Amphibolite Facies Metamorphism in the Polinik Unit 54
5.2.2. Age of 03 ...•....••••....•••.•.....•••......•••••.......•••••...•••..•....••••.•••.••• 55
5.2.3.Age of Pegmatites 56
5.3. Age of Metamorphism and Deformation in the Strieden Unit 56
5.3.1. Age Data 56
5.3.2. Age of Metamorphism in the Strieden Unit 56
5.3.3. Age of Mylonites (Dp) 56
5.3.4. Age of Dq and Later Deformation 58
5.4. Geological History 58
5.4.1. Pre-Variscan and Variscan Events 58
5.4.2. Permian to Jurassic History 60
5.4.3. Cretaceous and Younger Events 60
Acknowledgements 65
Appendix A: Abbreviations Used in the Text 76
Appendix B: Electron Microprobe Specifications and Methods Used in Calculating Fe3+ in Minerals from Probe Analyses. 76
Appendix C: List of Hand SpeCimens and Thin Sections Used in this Text 77
Appendix 0: Selected Microprobe Analyses 81
References 85
*) Author's address: Dr. LEONOREHOKE,Department of Earth Sciences, Parks Road, Oxford, OX1 3PR, England.
5
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Zusammenfassung enced crustal thickening resulting in mid Cretaceous burial
metamorphism, followed by post-metamorphic crustal thin-
In dieser Arbeit werden detailierte Struktur-, Metamorphose ning, which substantially thinned the Cretaceous metamorphic
und geochronologische Daten vom Altkristallin (Kreuzeck- sequence.
Gruppe), SE Tauern Gebiet präsentiert. Die Untersuchungen Inthe study areatwo units can be distinguished, which dif-
zeigen, daß sich das Altkristallin während der Alpinen Oroge-
fer greatly in lithologies, structural and metamorphic history
nese nicht als einheitliche, steife Deckeneinheit verhalten hat, and geochronological age data. In the north the Polinik Unit
sondern eine Krustenverdickung erfahren hat, die zu einer was affected by crustal thickening (0 deformation) prior to
Versenkungsmetamorphose in der mittleren Kreideführte, ge- the burial metamorphism which reache2d peaktemperatures of
folgt von einer Krustenverdünnung, während der die kretazi- 620:!:60°C at 6.25:!:1.25kb pressure at ca. 100Ma. In the
sche Metamorphose-Abfolge stark ausgedünnt wurde. south, inthe Strieden Unit, the Cretaceous metamorphism did
Im Untersuchungsgebiet, in der nördlichen Kreuzeck-Grup- not exceed lower greenschist facies conditions, and over-
pe, treten zwei Einheiten auf. Sie unterscheiden sich in Ge- prints aVariscan high temperature metamorphism. A discrete
steinszusammensetzung, struktureller und metamorpher Ent- zone of high strain deformation (Dp deformation), charac-
wicklung und unterschiedlichen geochronologischen Altersda- terised by the wide-spread occurrence of mylonites, which
ten. In der nördlichen Polinik-Einheit hat eine Phaseder Kru- formed under greenschist facies conditions (main mylonite
stenverdickung (D2-Deformation) zu einer Versen- zone - MMZ), separates the two units and forms the base of
kungsmetamorphose geführt, die maximaleTemperaturen von the Strieden Unit. Kinematic indicators, metamorphic evi-
620:!:60°C bei 6,25:!:1,25kb Druck um ca. 100Ma, erfahren dence and geochronological data, suggest that the Strieden
hat. In der südlich gelegenen Strieden-Einheit hat die kretazi- Unit movedoverthe Polinik Unit inaNWdirection, cutting out
sche Metamorphose die Bedingungen der Grünschieferfazies 2-8 km of the Cretacous metamorphic sequence. This occur-
nicht überschritten und überprägt eine variszische Hochtem- red at and following the time when the two units passed
peratur-Metamorphose. Einevon hohenStrain-Raten unddem through the KlAr mica blocking temperatures at 90-80 Ma,
Vorkommen von Myloniten gekennzeichnete Deformationszo- and continued until 60Ma.
ne (Dp-Deformation) trennt die beiden Einheiten, und befindet These results are presented and discussed within the wider
sich an der Basis der Strieden-Einheit. Bewegungsanzeiger, context of the Alpine orogeny.
Metamorphose-Sachverhalte und geochronologische Daten
deuten darauf hin, daßdie Strieden-Einheit die Polinik-Einheit
in NW-Richtung überfahren hat, während die beiden Einheiten
von der kretazischen Metamorphose abkühlten (90-60 Ma).
1. Introduction
Dabeiwurden 2-8 km der kretazischen Metamorphoseabfolge
herausgeschnitten.
Diese Ergebnissewerden präsentiert und im größeren Rah- This publication presents a detailed study of part of
men der alpinen geologischen Entwicklung in den Ostalpen a large allochthonous crystalline basement thrust
diskutiert. sheet, referred to as the Altkristallin, in the Eastern
Alps of Austria. The structural and metamorphic history
Abstract of this substantial tectonic unit has important implica-
tions for our understanding of the behaviour of deeper
This study presents detailed structural, metamorphic and
crustal levels in zones of continental collision.
geochronological data from the Altkristallin (Kreuzeck Moun-
tains) in the SE Tauern area. Evidence is presented which The Eastern Alps form part of a belt of intense defor-
shows that theAltkristallin did not behaveasarigid body dur- mation between the European Plate and the Adriatic
ing Alpine convergence in the Cretaceous, but has experi- region (Adria). It is not clear whether Adria acted as an
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independent microplate or as a promontary of the Afri- 2. The study area
can plate (PLAn et aI., 1989; DEWEYet aI., 1973, 1989).
From the Permian to early-mid Cretaceous, extension The study area lies in the SE corner of the Tauern
in the Tethyan region produced a broad zone of Window and is entirely within the Altkristallin rocks of
oceanic and thinned continental crust, referred to as the Kreuzeck mountains (Fig.3). In the north, a major
the Pennine domain, between the two major plates subvertical NWtrending fault, which follows part of the
(FRISCH,1979). During subsequent convergence be- Möll valley (Mölltal fault zone), separates the Altkristal-
tween Adria and Europe, the basement and its lin from the Pennine Units, exposed in the Tauern Win-
sedimentary cover were shortened, with the emplace- dow. South (15-20 km) of the edge of the Tauern Win-
ment of the southern continental margin (Austroalpine dow, the Altkristallin is overlain by Palaeozoic phyllites,
nappes including the Altkristallin) onto the oceanic which in turn are unconformable overlain by the Perm-
Pennine domain (Fig.1). Continental collision between ian and Mesozoic sediments of the Drauzug.
Adria and the European continental margin (Bohemian The previous work in the SE corner of the Tauern
massif), marked by the onset of deformation in sedi- Window has been mainly geochronological studies,
ments onthe stable Europeanforeland, occurred inthe both within the Altkristallin and also across the margin
Tertiary (OBERHAUSER19,80; PLAn, 1987). of the Tauern Window (OXBURGHet. aI., 1966; BREWER,
Allochthonous and para-allochthonous units can be 1969, 1970; LAMBERT1,970; HAWKESWORT1H9,76; WA-
studied best in tectonic windows, such as the Tauern TERS,1976; DEUTSCH1,984).The pattern of geochrono-
Window, in the axial zone of the Eastern Alps. The logical ages will be described in detail in Part 5.
largest of these allochthonous sheets is the Altkristal-
lin, consisting of pre-Mesozoic crystalline rocks, and Major structural subdivisionof the Study area
forming a more or less continuous E-W trending belt, Lithological layering, prominent schistosity, major
ca. 500 km long and 50-130 km wide, extending the fault and fold structures trend Eto SE.The prominent
entire length of the Eastern Alps into Switzeriand foliation generally dips towards the south, so that on a
(Fig.2). Mesozoic metasedimentary rocks extend be- km-scale the northern parts of the areaare at structur-
neath the Altkristallin for at least 120km in a N-S di- ally deeper levels, with respect to the foliation, than
rection. the southern parts.
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R.T.F.Z. Ragga Teuchl fault zone
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Phyllites
Text-Fig.3.
Generalizedgeological mapof the south-east TauemWindow,showing location of the study area.
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An important zone of deformation within the Altkris- Unusual amphibolite pods are found along the Mör-
tallin, characterised by the widespread occurrence of nigköpfe ridge, containing the texturally equilibrated
mylonites (Fig. 4), trends WNW and marks the bound- mineral assemblage gt - cpx (di) - hb - qz :t plag.
ary between two distinct units which differ greatly in li- Amphibolites, which do not contain clinopyroxene,
thology, deformation and thermal history, with different contain garnet and have relict textures (plagioclase-
KlAr ages. Concordant KlAr mica ages of c. 80 Ma are hornblende symplectites after clinopyroxene and min-
found north of the mylonites, whereas within them and erai zoning), suggesting a derivation from eclogite am-
further south, ages become progressively older to- phibolites.
wards the south, ranging between 100- 320 Ma
Pegmatites
(Fig. 41). The northern unit is called the Polinik Unit,
Pegmatites are rich in muscovite and plagioclase
named after the prominent mountain, Mt. Polinik
and occur sporadically throughout the Polinik Unit.
(2784 m). The southern unit is called the Strieden Unit
They are intensely deformed, forming sheet-like bodies
after Mt. Strieden (2682 m). The boundary between the
or irregular-shaped pods (up to 5 m wide), and may be
two units is at the northern limit of the mylonites. This
locally concordant with the schistosity of the host rock,
is marked in the study area by subvertical and east-
though cross-cutting on a large scale (Plate 1 - Fig. 6).
trending, predominantly brittle, fault zones, which re-
In the core of metre thick pegmatite bodies, muscovite
work the mylonites along their length. These are refer-
books, up to 5 cm across and 0.5 em thick, are em-
red to as the Ragga - Teuchl fault zone in the east and
bedded in a coarse-grained plagioclase and quartz
the Strugenkopf fault zone in the west (Fig. 4).
matrix which shows no shape fabric. Towards the mar-
The organisation of this paper is based on the struc-
gins, the grainsize progressively decreases and miner-
tural division described above. The geology of the
als have a strong planar fabric.
Polinik and Strieden Units is described in sections 3
and 4 respectively. In a concluding section, this is dis-
cussed within the wider context of the geological his-
tory of the Eastern Alps. 3.2. Deformation History
of the Polinik Unit
Detailed studies in the Polinik Unit have made it pos-
sible to relate metamorphic and microstructural data to
3. The Polinik Unit, large scale structures and the tectonic evolution of the
its Structural and Metamorphic History area.
Sets of structures have been correlated, using
3.1. Lithologies criteria such as orientation, style and position in the
in the Polinik Unit locally established deformation sequence.
Five major groups of regional structures can be dis-
Medium grained (1-5 mm) quartzo-feldspathic tinguished (01 - 05)' F1' F3and F4regional fold phases
schists and gneisses, referred to as the Polinik Schists, correspond with the sequence F1' F2' F3' established
predominate. They contain discontinuous bodies of
amphibolite, augengneiss and pegmatite. Garnet,
staurolite and kyanite-bearing schists (aluminous
metapelites) are rare, found only in the Mt.Polinik and
Griessbergalm areas.
Augengneisses
Biotite augengneisses contain large (up to 5 cm long)
and numerous K-feldspar augen, aligned within the
foliation. They have been found in only one locality,
about 500 m south of the Weissenstein Alm, forming
tens of metres of exposure along a forest road, and
apparently are concordant with the main schistosity.
seM
Amphibolites
Amphibolites can be subdivided into two groups, de-
pending on whether they contain clinopyroxene (cpx).
Cpx-rich amphibolites contain abundant garnet, which
are enclosed in pale green symplectites after
clinopyroxene, with relict omphacite. These rocks have
quartz-feldspathlc rods
been derived from eclogites with the original mineral
assemblage omph - gt - rut - qz (see appendix A for F1 or older
mineral abbreviations), preserved as relicts in the cores Fa mica crenulatlon hinges
of amphibolite pods, where the rock is massive and are not recrystalllsed
contains randomly oriented pale green em-sized col- Text-Fig. 5.
Sketch of structural elements in quartzo-feldspathic schist, in the Ragga-
umnar pseudomorphs of omphacite, surrounded by
aim area (sample 7),showing refolding of 01 fabrics during O2and 03defor-
dark greeen amphibole, plagioclase and irregularly
mation.
shaped garnet (0.2-1 em in diametre). Margins of am- An early quartzo-feldspathic and mica-rich foliation (5,) is crenulated by F2
phibolite pods are rich in biotite and oligoclase blasts folds. F1 quartz fold hinges define alineation 12which issubparallel to the F2
which overgrow the contact with the surrounding crenulation lineation. Subsequent crenulation occurred during 03, and micas
have not recrystallised in F3fold hinges.
schists and gneisses.
10
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by OXBURGH (1966) and WATERS (1976) in rock units was deformed into a km-scale recumbent fold, closing
equivalent to the Polinik Unit. The deformation history to the south. It was during or after the final stages of
and major structures are summarised in Table 1 and in 03 that the Polinik Unit was juxtaposed with the Pen-
a cross-section (Fig. 9). nine tectonic units. However, Tertiary movement along
0, deformation resulted in the earliest regional struc- the Mölltal fault zone has obscured the original contact
tures, though evidence for earlier deformation has relationships. Younger deformation (DeOs) resulted in
been recognised locally. Variscan pegmatites were in- regional warping and brittle faulting.
truded after 0, deformation and are intensely deformed
by O2, O2 deformation is post-dated by a Cretaceous
amphibolite facies burial metamorphism, which was
followed soon after by 03, During 03, the Polinik Unit
coarse-
grained!
mica-rich schist ~
foliated
.1,' pegmatite
---:--'. --'- ,-' _::.: -t'-.':-"
feldspathi~.- / mica-rich
schist schist
1m
@
appearance of
cross-cutting
relationship
1m
N b)
tourmalines
• poles to Splanes
1cm
x13crenulatlon lineations
o quartzo-feldspathlc rods (F, or F2)
Text-Fig.6. Xaverage F3fold axis
Sketches of structural elements in Polinik Schists (ca. 1km SWof Mt. Pali.
nikl, at a variety of scales. Text-Fig.7.
a) Sketch of atight, m-scale F2fold, defined byadeformed quartzo-feldspa- a) Block diagram of adeformed pegmatite, intruding mica schist in the Poli-
thic layer. Mica schists contain anaxial planar cleavage (52)'which iscre- nik Unit, NNE of Bodensee. A pronounced foliation in the pegmatite (52)'
nulated (F3). subparallel to the pegmatite margins, is folded by F3cascading subhori-
b) Sketch of crenulated mica schist inthe cores ofminor F3folds. Quartz lay- zontal folds. See Plate 1- Fig.6.
ers on F3fold limbs are attenuated and are not crenulated. b) Equal area lower hemisphere stereographic plot of poles to 52foliation in
c) Thin section of similar style F3fold, showing mm-scale F3crenulation of the deformed pegmatite andthe enclosing schists, and lineations. Poles to
mica schist. 52 foliation lie on great circle with pole (X) consistent with F3fold axes.
11
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01 04 and younger deformation
01 fabrics are defined by a metamorphic segregation Ouring 04, the whole area was warped by upright,
into mica and/or amphibole and quartzo-feldspathic open, E-trending folds (Fig. 9), forming open antiforms
layers (Sl), which are deformed by O2 structures. A and tightly pinched synforms, developed on a metre to
strong linear fabric (11), best developed in gneisses, is a kilometre scale. Steep fault zones, which have ac-
defined by the preferred orientation of stretched and commodated several kilometres of both strike slip and
broken garnets, quartzo-feldspathic rods and quartz- dip slip displacement, cut 0 and all earlier structures.
4
ribbons . .Dismembered fold hinges (rods) also define a These faults resulted in both N-S-shortening and E-W-
strong linear anisotropy (11 or older, Fig. 5). extension. Pseudotachylite was generated along some
fault surfaces (HOKE,in prep.).
O
2
O2caused a crenulation and substantial modification
of Sl (Figs. 5, 6). On a metre to 10's of metre-scale, Sl
is folded throughout the Polinik Unit into conspicuous
'M', 'Z' and 'S'-shaped tight folds (F2),(Fig. 8, Plate 1 3.3. Inter 02-03
- Figs. 3-5), with subhorizontal fold axes which are Cretaceous
nearly parallel to 11 (within :t20°, Fig. 9B). However, no Amphibolite Facies Metamorphism
large scale O2 folds have been found, though the
change in minor fold asymmetry suggests a series of PIT conditions during the inter O2-03 metamorphism
km-scale folds with subhorizontal E-trending fold axes. have been estimated from microprobe chemical and
A variably developed crenulation cleavage, S2' locally textural analyses of staurolite, garnet and kyanite-bear-
transposes the Sl foliation and becomes the dominant ing schists (aluminous metapelites). Samples were col-
foliation in less competent mica-rich lithologies. Eclo- lected from a section between Mt. Polinik and the
gite amphibolite pods are aligned parallel to the S2foli- Polinik Alpine Hut (samples 68, 83, 81.3, 81.1, 81, 8,
ation. 77; Tables A,B).
A pronounced lineation (12),which is either a crenula-
tion lineation or the intersection of S2with Sl' parallels
3.3.1. Mineral phases
F2 fold axes. The S2 foliation is found in pegmatites,
Garnets
particularly at their margins (Fig. 7A, Plate 1 - Fig. 6).
Three generations of garnets have been recognised
This is the earliest deformation fabric in the pegma-
(Fig. 10, Plate 2 - Fig. 1) and all garnets may show
tites, and therefore the pegmatites must have been
emplaced after 01, but prior to O2. Rb/Sr two-mineral some degree of chemical zoning.
isochron ages from the coarse-grained centre of these - In some garnets, cores (garnet I) can be optically
pegmatites give ages of ca. 254 Ma (CLIFF,pers. com.). distinguished from their rims (garnet II), (Fig. 10).
These are assumed to be emplacement ages and Garnet I tends to be full of quart.z and tiny highly
therefore post-date 01 deformation, but pre-date O2 birefringent inclusions, but generally has a uniform
deformation. chemical composition: almandine 73-75 mol.-%;
spessartine 1.5 -3.6 mol.-%; pyrope 14-20 mol.-%;
grossular 2-6 mol.-%. A chemical zonation is only
developed near garnet I rims, where there is a slight
Inter O2 - 03 amphibolite facies metamorphism
decrease in Mg, and increase in Fe, while Ca and
Extensive growth of staurolite, kyanite, plagioclase
Mn show no significant variation.
and locally garnet in metapelites post-dates O2 and
Garnet II is the most commonly found garnet gene-
pre-dates 03. Staurolite, kyanite and individual mica
ration, developed either as a clear rim around gar-
laths cross-cut the S2foliation and plagioclase heliciti-
net Ior as euhedral or spongy grains less than 1 mm
cally overgrows the earlier fabrics (Plate 2 - Figs. 1-3).
in size. Garnet II often contains elongate rutile inclu-
K/Ar radiometric ages suggests that this mineral
sions, and is found intergrown with staurolite, kya-
growth occurred in the Cretaceous (ca. 100 Ma). The
nite, biotite and muscovite.
PIT conditions during metamorphism are discussed in
Garnet II is characterised by higher Mg (pyrope
section 3.3.
18-25 mol.-%) and lower Fe contents than garnet I.
03 The Mg/Mg+Fe ratio is higher and can show a
Subhorizontal cascading asymmetrical folds (F3) marked variation, suggesting changing equilibrium
occur throughout the Polinik Unit (Fig. 9, Plate 1 - conditions between ferromagnesian minerals as a
Figs. 1-5), trending E-NE, parallel to 11 and 12, and result of increasing temperature and probably pres-
forming coaxial interference folds with F2(Fig. 7B, 9B). sure during garnet growth.
A distinct variation in fold asymmetry suggests the Garnet III does not contain inclusions and is difficult
existence of a km- scale recumbent fold, closing to the to distinguish optically from garnet II, though it
south (Fig. 9). Crenulation of S2 defines a crenulation forms chemically distinctive, narrow, 100-300!.l
lineation (13), which is parallel to F3axes. In crenulation wide rims around garnet II, or more rarely occurs as
hinges, staurolite, kyanite and micas are bent and bro- swarms of tiny euhedral garnets in a quartz and
ken (Plate 2 - Figs. 4-6) and have been mechanically mica-rich matrix. Garnet III is commonly charac-
rotated and transposed into the foliation. terised by a zonation showing a decrease in Mg and
In areas of intense 03 deformation, kyanite and increase in Fe content (overall reduction in Mg/
staurolite are partly altered to fine-grained aggregates Mg+Fe ratio compared to garnet II), and an increase
of sericite, margarite and chlorite; garnets are partly in both Mn and Ca, towards the rim. This suggests
replaced by chlorite. This alteration may have occurred growth during the waning stages of metamorphism
during or after 03 deformation. (HESS, 1971; TRACY, 1979; KERR, 1981). Garnet III
12
©Geol. Bundesanstalt, Wien; download unter www.geologie.ac.at
Text-Fig.8. N
Sketch of m-scale, gently plunging, fold interference pat-
terns, observed in the Polinik Schists, in the Mt. Polinik
area, produced by superposition of F3and F2 folds.
Sketches are drawn as if viewed towards the west.
a) "S"-shaped F3folds on "M"-shaped F2 folds.
b) "S"-shaped F3folds on "Z"-shaped F2 folds.
Compare with Plate 1.
N Text-Fig. 9.
Sketch of N-S cross-section through the Polinik Unit, showing an interpre-
tation of the relation between structural elements.
A km-scale recumbent F3fold isfolded into an F4antiformal structure. South
of Mt. Polinik the structures aretruncated by the Teuchl fault zone, with the
formation ofasubvertical shear fabric. Dismembered folds (possibly F3)and a
pronounced cleavage (53or younger) occur near the base of the Altkristallin.
Thebasal contact isnot preserved inthe study area, because it hasbeen dis-
placed by movement along the steeply dipping Mölltal fault zone. This cross-
section suggests that the exposed ,structural thickness of the Polinik Unit,
measured perpendicular to the prominent foliation, is not more than 2km.
Orientation data from Polinik Schists in the Mt. Polinik area are shown in an
equal area lower hemisphere stereogram.
• poles to 8 and 8
1 2
@ axis of best fit
great circle
5
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N
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13
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