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Titel |
Strain localization in brittle–ductile shear zones: fluid-abundant vs. fluid-limited conditions (an example from Wyangala area, Australia) |
VerfasserIn |
L. Spruzeniece, S. Piazolo |
Medientyp |
Artikel
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Sprache |
Englisch
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ISSN |
1869-9510
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Digitales Dokument |
URL |
Erschienen |
In: Solid Earth ; 6, no. 3 ; Nr. 6, no. 3 (2015-07-24), S.881-901 |
Datensatznummer |
250115501
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Publikation (Nr.) |
copernicus.org/se-6-881-2015.pdf |
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Zusammenfassung |
This study focuses on physiochemical processes occurring in a
brittle–ductile shear zone at both fluid-present and fluid-limited
conditions. In the studied shear zone (Wyangala, SE Australia), a
coarse-grained two-feldspar–quartz–biotite granite is transformed into a
medium-grained orthogneiss at the shear zone margins and a fine-grained
quartz–muscovite phyllonite in the central parts.
The orthogneiss displays cataclasis of feldspar and crystal-plastic
deformation of quartz. Quartz accommodates most of the deformation and is
extensively recrystallized, showing distinct crystallographic preferred
orientation (CPO). Feldspar-to-muscovite, biotite-to-muscovite and
albitization reactions occur locally at porphyroclasts' fracture surfaces
and margins. However, the bulk rock composition shows very little change in
respect to the wall rock composition. In contrast, in the shear zone centre
quartz occurs as large, weakly deformed porphyroclasts in sizes similar to
that in the wall rock, suggesting that it has undergone little deformation.
Feldspars and biotite are almost completely reacted to muscovite, which is
arranged in a fine-grained interconnected matrix. Muscovite-rich layers
contain significant amounts of fine-grained intermixed quartz with random
CPO. These domains are interpreted to have accommodated most of the strain.
Bulk rock chemistry data show a significant increase in SiO2 and
depletion in NaO content compared to the wall rock composition.
We suggest that the high- and low-strain microstructures in the shear zone
represent markedly different scenarios and cannot be interpreted as a simple
sequential development with respect to strain. Instead, we propose that the
microstructural and mineralogical changes in the shear zone centre arise
from a local metasomatic alteration around a brittle precursor. When the
weaker fine-grained microstructure is established, the further flow is
controlled by transient porosity created at (i) grain boundaries in
fine-grained areas deforming by grain boundary sliding (GBS) and (ii) transient
dilatancy sites at porphyroclast–matrix boundaries. Here a growth of
secondary quartz occurs from incoming fluid, resulting in significant
changes in bulk composition and eventually rheological hardening due to the
precipitation-related increase in the mode and grain size of quartz. In
contrast, within the shear zone margins the amount of fluid influx and
associated reactions is limited; here deformation mainly proceeds by dynamic
recrystallization of the igneous quartz grains.
The studied shear zone exemplifies the role of syn-deformational fluids and
fluid-induced reactions on the dominance of deformation processes and
subsequent contrasting rheological behaviour at micron to metre scale. |
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