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| Titel |
High-precision geobarometry across the Plattengneiss shear zone in the Koralpe, Eastern Alps, Austria |
| VerfasserIn |
Tobias Eberlei, Richard White, Tim Johnson, Kurt Stüwe, Cornelia Roffeis |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250046008
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| Zusammenfassung |
| Eclogite-facies metamorphism is a common feature of crustal rocks in collisional orogens.
The occurence of such rocks implies subduction of crustal material to mantle depths.
However, questions still exist regarding the mechanisms by which eclogite facies rocks are
exhumed. In many cases, erosion, buoyancy-driven wedge extrusion, crustal extension or
exhumation as a core complex alone can not explain the process of exhumation.
FROITZHEIM ET AL. (2003) introduced the model of an extracting slab as a new
mechanism for the exhumation of the Adula nappe in the western Alps. This new model
has been favoured for exhumation of the Pohorje-Massif in Slovenia, which once
formed a single fragment of crust together with the Koralpe-Saualpe region further
north.
The Koralpe region not only hosts the type locality for eclogites, but also one of the largest
shear zones in the Alps, the Plattengneiss. This shear zone is between 250 and 600 meters
thick and about 600 km2 in areal extent. The shear zone is flat lying and eclogites occur
as lenses and pods, above, below and within the shear zone. It is likely that the
Plattengneiss played an important role in the exhumation of the high-pressure rocks
and that it represents some kind of an “extraction-fault“, which may have formed
during downward extraction of a mantle wedge above the subducted Austroalpine
crust.
In this study, metapelitic gneisses from selected locations in the hangingwall and the
footwall of the Plattengneiss were collected for mineral chemical analysis in order to
constrain the flattening of the shear zone using barometry. Petrographic investigations
show peak metamorphic assemblages containing garnet + plagioclase + muscovite
+ biotite + quartz + rutile ± ilmenite ± kyanite in the metapelitic gneisses. The
software THERMOCALC v3.33 (HOLLAND & POWELL, 1998) was used to perform
detailed relative thermobarometric calculations on the sampled rocks according to
the method of WORLEY & POWELL (2000). The investigations also include the
evaluation of the effects of variable H2O activities on average pressure calculations and
constraints on the P—T paths via pseudosection modelling in the NCKFMASHTO
system.
The results show that pressure differences between the hangingwall and the footwall are in
the order of 1.5–2 kbar, suggesting volume loss. Investigations of the P—T path on a sample
from the hangingwall of the Schwanberger synform suggest near-isothermal decompression
from 18 kbar and 670-C to conditions of about 12 kbar and 700-C modelled using the
metapelitic sample. The model calculations imply a subsequent decrease in both pressure and
temperature to 8–9 kbar and 600–630-C. This assumption is supported by the late growth of
staurolite in the metapelite.
The results can be interpreted to indicate: (a) removal of material during deformation; (b)
diachronous metamorphic equilibration recorded in the hangingall and the footwall or; (c) a
strong flattening component after equilibration. Removal of material is preferred in this study
with or without diachronous metamorphism.
References
FROITZHEIM, N., PLEUGER, J., ROLLER, S. AND NAGEL, T., 2003, Exhumation of high-
and ultrahigh-pressure metamorphic rocks by slab extraction: Geology, v.31, p.
925—928
HOLLAND, T.J.B. AND POWELL, R., 1998, An internally consistent thermodynamic
dataset for phases of petrological interest: Journal Metamorphic Geology, 16, p.
309—344
WORLEY, B. AND POWELL, R., 2000, High-precision relative thermobarometry: theory and
a worked example: Journal Metamorphic Geology, 18, p. 91—101 |
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