Tectono-metamorphic Evolution of the Eastern Alps - jsg

268 Pages·2013·8.58 MB·English

The Tectono-metamorphic Evolution
of the Tauern Window Eclogites,
Eastern Alps
Andrew Smye
Department of Earth Sciences
University of Cambridge
A thesis submitted for the degree of
Doctor of Philosophy
September 4, 2011
This dissertation is the result of my own work and includes nothing which is the
outcome of work done in collaboration except where specifically indicated in the text.
Andrew J. Smye, September 4, 2011
Acknowledgements
It would not have been possible to write this thesis without the financial support of a
NERCstudentship(NE/007647/1),aNERC,NIGL-steering-committeegrant(NE/1068/1108)
and a Magdalene College travel grant.
IamindebtedtoTimandMikefortheirforesightinproposingsuchaninterestingPhD
title and their patient supervision throughout the duration of the thesis. Collectively,
they helped me foster an independence of thought and kindled my curiosity in orogenic
processes. In particular, Tim has graciously taught me the wonders and intricacies of
metamorphic petrology and how it can be used as a tool to investigate tectonic prob-
lems. Mike has encouraged me to think critically and to question the fundamental
tenets of a scientific problem; in particular Mike’s knowledge of thermal modeling and
geochronology was hugely appreciated. Both Tim and Mike are friends, in addition to
supervisors, for which i am very thankful.
The thesis has benefited from fruitful collaboration with Clare Warren (Open Univer-
sity) and Mark Caddick (ETHZ), both of whom have provided wise advice and stim-
ulating discussion concerning the Eastern Alps. Specifically, I am thankful to Clare
who introduced me to the world of 40Ar/39Ar geochronology, and to Mark, who guided
my thoughts concerning thermal modeling and helped me with numerous programming
bugs. Furthermore, Clare o↵ered constructive comments on the 40Ar/39Ar chapter.
I am hugely grateful to Randy Parrish, Dan Condon and Matt Horstwood at NIGL
for directing U–Th–Pb geochronology. Without Randy’s insight into dealing with non-
conventionalpetrochronometerssuchasallanite,andDan’sTIMSexpertise,theproject
would not have succeeded. Vanessa Paschley and John Cottle are also thanked for
countless hours of LA–ICPMS supervision.
Marian Holness and Alan Smith are thanked for their roles as academic friends. Keith
GrayandIanMillar(Kingston)providedhigh-qualitythinsectionsonwhichtheproject
is based. Probe work was made possible by Chiara Petrone and Andy Buckley; Martin
Walker kindly tutored me in use of the Department’s SEM. My o�ce mates Niko and
Ben, in addition to a host of other Department members, are thanked for providing
much fun and interesting discussion. Ben Veitch is thanked for help with MATLAB.
Thanks, also, to the sta↵ at Magdalene College who provided a superb home from
which to study.
Summers spent in the Tauern were made so much more enjoyable by the company of
thefollowingfieldassistants, who, inmanycases, notonlyservedtohelpwiththeprac-
ticalities of fieldwork, but contributed scientifically: Owen Weller, Lucy Greenwood,
Dave Wilson, Mike Kember and Alan Chetwynd. Frau D¨orer and her family provided
much needed hospitality after many a hostile day in the Venedigergru¨ppe.
Finally, special mention to Katie who graciously endured the brunt of my PhD expe-
rience and who continues to challenge me to see work in life’s perspective. Thank you
all.
Abstract
The Tauern Window exposes the largest continuous tract of eclogite-facies Penninic
crust in the Eastern Alps – the Eclogite Zone. Despite being amongst Earth’s best
studied orogens, little is understood about the rates of Alpine metamorphism within
the Eastern Alps. Recent Rb–Sr geochronology suggests East-Alpine eclogitization
occurred within the early Oligocene, whereas thermal modeling studies require HP-
metamorphic ages > 60 Ma, allowing a period of 30 Ma for isotherm re-equilibration
⇠
following thrust-sheet emplacement. Given that the Tauern Window is a classic local-
ity for understanding rates of conductive thermal relaxation in tectonically thickened
crust, this paradox raises profound questions over the length scales of the mechanisms
responsible for heat transfer within orogenic crust.
Field-based structural measurements are consistent with the interpretation that the
Eclogite Zone was tectonically juxtasposed between the European basement complex
– the Venediger nappe, and a dismembered ophiolite sequence – the Glockner nappe,
during exhumation. Conventional thermobarometry and pseudosection calculations on
a garnet–chloritoid–kyanite-bearing metasediment collected from the Eclogite Zone,
show that the Eclogite Zone was subducted to the verge of UHP conditions: 26 kbar
⇠
and 540 C. Furthermore, such assemblages are shown to be common throughout
�
⇠
(U)HP terranes and representative of a state of enhanced detachment for slab-top ma-
terial. During exhumation the Eclogite Zone was subjected to partial recrystallisation
in the blueschist facies before its upper-structural layers were penetratively a↵ected by
regional Barrovian metamorphism. Coexisting jadeitic-pyroxene and glaucophane pre-
servedasraresymplectitecoresshowthattheGlocknernappeexperiencedasimilarP–
T path from blueschist facies metamorphism between 9–13 kbar and and 350–450 C
�
⇠
toBarrovianconditionsof 5–8kbarat450–550 C.TheVenedigernappelacksdemon-
�
⇠
strative evidence for subduction-related HP metamorphism. Phase-diagram modeling
of sub-millimetric garnet, biotite, muscovite and epidote, which grew at the expense
of plutonic plagioclase in the crystalline basement complex, shows that the internal
Venedigernappeexperiencedhigh-gradeBarrovianmetamoprhism,closetosoliduscon-
ditions: 10–13 kbar and 550–620 C. Collectively, these data are interpreted as showing
�
that the Penninic-nappe stack was assembled under blueschist-facies conditions prior
to behaving as a coherent unit during concomitant decompression and heating to Bar-
rovian conditions.
LA–MC–ICPMSandID–TIMSU–Th–PbagesofmetamorphicallanitefromtheEclog-
ite Zone, which when coupled with rare earth element analysis and thermobarometric
modeling, demonstrate that the European continental margin was subducted to be-
tween 8–13 kbar (30-45 km) by 34.2 3.6 Ma. These data define: (i.) an upper limit on
±
thetimingofeclogitefaciesmetamorphismat26.2 1.8kbar(70–80km)and553 12 C,
�
± ±
(ii.) plate velocity (1–6 cm.a 1) exhumation of the Eclogite Zone from mantle to mid-
�
crustal depths, and (iii.) a maximum duration of 10 Ma (28–38 Ma) for juxtaposition
of Alpine upper-plate and European basment units and subsequent conductive heating
thought to have driven regional Barrovian (re)crystallisation at ca. 30 Ma.
U–Th–Pb geochronologic data are augmented by 40Ar/39Ar single-grain fusion data
from individual muscovite and paragonite collected from representative lithologies of
each of the Pennine nappes. Apparent ages range from 23 Ma to 90 Ma – generally
older than the U–Th–Pb allanite age. Textural and chemical evidence show that the
majority of micas grew or equilibrated close to the peak of HP metamorphism. Nu-
merical modeling of open-system Ar di↵usion in white mica subjected to nappe-specific
T–t paths suggest that 40Ar/39Ar ages constraining the timing of cooling during ex-
humation should lie between 26 and 28 Ma. The discrepancy between measured- and
model-40Ar/39Ar ages confirms the spatially-variably presence of excess-Ar. Modeling
ofopenandclosedsystemmica–fluid40Arexchangeshowsthatintra-sample40Ar/39Ar
age di↵erences are best reproduced by a finite volume grain boundary network with
centimeter-scale connectivity. Mafic rocks show larger intra-sample age ranges because
they do not de-volatise during burial.
One-dimensional modeling shows that the thermal evolution of the Tauern Window
cannot be explained by conductive heating of overthickened crust on lithosphere with
normal continental thermal gradients. Either the thermal evolution took place with a
significant thermal contribution from the mantle, for which there is little supporting
evidence, or heating occurred during thrusting with heat provided by an overlying
thrust sheet. It is possible that syn–thrusting heating by rapid emplacement of a hot
overthrust sheet, overlooked in previous thermal models, could plausibly have played
an influential role in the tectono–metamorphic evolution of the Eastern Alps and in
overthrust terrains in general.
Contents
1 Introduction 1
1.1 Project Aims . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.2 The Alps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.2.1 Lithotectonic units . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.3 The Problem . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.4 Fieldwork . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.5 Thesis structure. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2 Geology of the Tauern Window 8
2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.2 Geologic Setting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.3 Field Area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
2.4 Litho-tectonic units. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
2.4.1 Venediger nappe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
2.4.1.1 Lithologies of the Venediger nappe . . . . . . . . . . . . . . . . . . . . . . . . 11
2.4.1.2 Structures of the Venediger nappe . . . . . . . . . . . . . . . . . . . . . . . . 14
2.4.2 Eclogite Zone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
2.4.2.1 Lithologies of the Eclogite Zone . . . . . . . . . . . . . . . . . . . . . . . . . 16
2.4.2.2 Structures of the Eclogite Zone . . . . . . . . . . . . . . . . . . . . . . . . . . 19
2.4.3 Rote-Wande nappe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
2.4.4 Glockner nappe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
2.4.4.1 Lithologies of the Glockner nappe . . . . . . . . . . . . . . . . . . . . . . . . 23
2.4.4.2 Structures of the Glockner nappe . . . . . . . . . . . . . . . . . . . . . . . . 24
2.4.5 Matrei Zone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
2.4.6 Altkristallin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
2.5 Fabric correlation across units . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
3 Conditions of Metamorphism 33
3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
3.1.1 Application of equilibrium thermodynamics . . . . . . . . . . . . . . . . . . . . . . . . 33
3.1.2 Pseudosection modeling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
3.1.3 Average P–T . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
3.2 Eclogite Zone P–T evolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
3.2.1 Previous work. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
vii
3.2.2 Sample petrography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
3.2.3 Mineral chemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
3.2.4 Metamorphic modeling of TH-680 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
3.2.4.1 Activity–composition models . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
3.2.4.2 TH–680 P–T pseudosection . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
3.2.4.3 Average P–T calculations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
3.2.4.4 Influence of Garnet a–X models . . . . . . . . . . . . . . . . . . . . . . . . . 47
3.2.4.5 P–T evolution of TH–680 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
3.3 Garnet–chloritoid–kyanite assemblages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
3.3.1 Metamorphic modeling of garnet + chloritoid + kyanite assemblages . . . . . . . . . . 52
3.3.1.1 The KFMASH subsystem . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
3.3.1.2 The NCKFMASH subsystem . . . . . . . . . . . . . . . . . . . . . . . . . . . 54
3.3.1.3 The NCKFMASHO subsystem . . . . . . . . . . . . . . . . . . . . . . . . . . 54
3.3.1.4 The MnNCKFMASHO system . . . . . . . . . . . . . . . . . . . . . . . . . . 54
3.3.2 Bulk compositional controls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56
3.3.3 Comparison of peak P–T conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58
3.3.4 Tectonic implications. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60
3.4 Glockner nappe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62
3.4.1 Previous work. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62
3.4.2 Field relations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63
3.4.3 Petrology of breakdown textures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64
3.4.4 Mineral chemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65
3.4.5 Metamorphic modeling of gl+jd assemblages . . . . . . . . . . . . . . . . . . . . . . . 66
3.4.5.1 a–X models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68
3.4.5.2 Estimating a reactive bulk composition . . . . . . . . . . . . . . . . . . . . . 69
3.4.5.3 Breakdown reactions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
3.4.5.4 Calibrating the clinopyroxene model . . . . . . . . . . . . . . . . . . . . . . . 75
3.4.5.5 Peak conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76
3.4.5.6 Average P–T . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77
3.4.6 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
3.4.7 Tectonic implications. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81
3.5 Venediger nappe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83
3.5.1 Previous work. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83
3.5.2 Sample petrography and mineral chemistry . . . . . . . . . . . . . . . . . . . . . . . . 84
3.5.2.1 Samples TH–519 and ASA–41a . . . . . . . . . . . . . . . . . . . . . . . . . . 84
3.5.2.2 Sample SP614 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86
3.5.2.3 Sample ASA–38a. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86
3.5.2.4 Sample ASA–42a. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
3.5.3 Average P–T . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
3.5.3.1 Sample TH–519 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
3.5.3.2 Sample SP614 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88
3.5.3.3 Sample ASA–38a. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88
3.5.3.4 Sample ASA–42a. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88
3.5.4 Metamorphic modeling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89
viii
3.5.5 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90
3.5.6 Tectonic implications. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93
3.6 P–T evolution of the Pennine nappes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95
3.7 Chapter Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96
4 U–Th–Pb Isotope Geochronology 99
4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99
4.2 Aims . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100
4.3 Previous work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100
4.4 Geochronology of high–P metamorphism. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102
4.5 U-Th-Pb geochronology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104
4.5.1 Principles and methodology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104
4.5.2 Accessory minerals in Eclogite Zone . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105
4.5.2.1 Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105
4.5.2.2 Mafic lithologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106
4.5.2.3 Metasedimentary lithologies . . . . . . . . . . . . . . . . . . . . . . . . . . . 107
4.6 Allanite and REE-rich epidote . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109
4.7 Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111
4.7.1 Grain selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111
4.7.2 LA–ICPMS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112
4.7.3 ID–TIMS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113
4.8 Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114
4.8.1 Allanite LA–ICPMS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114
4.8.2 Allanite ID–TIMS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119
4.9 Allanite petrogenesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122
4.9.1 Textural observations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122
4.9.2 REE+Y modeling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122
4.9.3 Metamorphic modeling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126
4.10 Metamorphic rates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128
4.11 Tectonic Implications. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129
4.12 Chapter Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131
5 40Ar/39Ar Geochronology 133
5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133
5.2 40Ar/39Ar geochronology in the Tauern Window . . . . . . . . . . . . . . . . . . . . . . . . . 134
5.3 Methodology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 136
5.3.1 Sample description and mica chemistry . . . . . . . . . . . . . . . . . . . . . . . . . . 136
5.3.1.1 Venediger nappe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137
5.3.1.2 Eclogite Zone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137
5.3.1.3 Rote-Wand nappe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139
5.3.1.4 Glockner nappe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 140
5.3.2 Step-heating versus single grain fusion data . . . . . . . . . . . . . . . . . . . . . . . . 140
5.4 40Ar/39Ar analytical technique . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 142
5.5 Results. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143
ix