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| Titel |
Protracted weakening during lower crustal shearing along an extensional shear zone |
| VerfasserIn |
Giulia Degli Alessandrini, Luca Menegon, Francesco Giuntoli |
| Konferenz |
EGU General Assembly 2017
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 19 (2017) |
| Datensatznummer |
250143749
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| Publikation (Nr.) |
 EGU/EGU2017-7502.pdf |
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| Zusammenfassung |
| This study investigates grain-scale deformation mechanisms in the mafic lower continental
crust, with particular focus on the role of syn-kinematic metamorphic reactions and their
product – symplectites – in promoting grain size reduction, phase mixing and thus strain
localization. The investigated extensional shear zone is hosted in the Finero mafic-ultramafic
complex in the Italian Southern Alps.
Field and microstructural observations indicate that strain partitioned in gabbroic layers
where the primary mineralogical assemblage contained amphibole, forming ultramylonites.
These ultramylonites are characterized by isolated porphyroclasts of amphibole, garnet,
clinopyroxene and orthopyroxene, embedded in a matrix of plagioclase (ca. 39 vol%) +
amphibole (25 vol%) + clinopyroxene (18 vol%) + orthopyroxene (11 vol%) + Fe-Ti oxides
(6 vol%) ± apatite (<1 vol%). Matrix grain-size is consistently below 30 μm for all
phases.
EBSD results are consistent with deformation by grain-size sensitive creep. Amphibole
shows a CPO with [001] axes preferentially aligned parallel to the stretching lineation, which
we interpret as oriented grain growth during heterogeneous nucleation of amphibole.
Pyroxenes and plagioclase lack a CPO and evidence for dislocation creep and dynamic
recrystallization. Protracted shearing was initiated by syn-kinematic metamorphic reactions:
garnet porphyroclasts formed orthopyroxene + plagioclase symplectites and amphibole
porphyroclasts formed pyroxene + plagioclase symplectites. The latter reaction indicates that
strain localization initiated with dehydration reactions leading to primary amphibole
breakdown into pyroxene and plagioclase, now preserved in the ultramylonite.
Geothermobarometry using plagioclase-amphibole pairs in the ultramylonites indicate
temperature conditions of ca. 800˚ C and pressures from 8 to 6kbar. This suggests that
protracted shearing in the ultramylonites occurred at decreasing pressure and nearly constant
T. We suggest that the fluids released during the dehydration reaction were channelized in the
ultramylonites and subsequently assisted amphibole nucleation in dilatant sites during
creep cavitation, as shearing protracted at P, T conditions at which amphibole was
stable again. The addition of fluid to the system, combined with chemically-driven
grain-size reduction, promoted deformation by diffusion-accommodated grain boundary
sliding.
This study highlights the importance of dehydration reactions for grain size reduction and
strain localization in the lower crust, as well as the possibility that fluids can be
channelized in discrete shear zones during protracted tectono-metamorphic events. |
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