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| Titel |
The microstructural record of porphyroclasts and matrix of partly serpentinized peridotite mylonites – from brittle and crystal-plastic deformation to dissolution–precipitation creep |
| VerfasserIn |
J. Bial, C. A. Trepmann |
| 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 ; 4, no. 2 ; Nr. 4, no. 2 (2013-10-11), S.315-330 |
| Datensatznummer |
250084935
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| Publikation (Nr.) |
 copernicus.org/se-4-315-2013.pdf |
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| Zusammenfassung |
| We present microfabrics in high-pressure,
metamorphic, partly serpentinized peridotite mylonites from the Voltri
Massif, in which porphyroclasts and matrix record independent deformation
events. The microfabrics are analysed using polarization microscopy and
electron microscopy (SEM/EBSD, EMP). The mylonites contain diopside and
olivine porphyroclasts originating from the mantle protolith embedded in a
fine-grained matrix consisting mainly of antigorite and minor olivine and
pyroxene. The porphyroclasts record brittle and crystal-plastic deformation
of the peridotite at upper-mantle conditions and differential stresses of a
few hundred MPa. After the peridotites became serpentinized, deformation
occurred mainly by dissolution–precipitation creep resulting in a pronounced
foliation of the antigorite matrix, crenulation cleavages and newly
precipitated olivine and pyroxene from the pore fluid at sites of dilation,
i.e. in strain shadows next to porphyroclasts and folded fine-grained
antigorite layers. Antigorite reveals a pronounced associated shape
preferred orientation (SPO) and crystallographic preferred orientation (CPO)
with the basal (001) cleavage plane oriented in the foliation plane. In
monomineralic antigorite aggregates at sites of stress concentration around
porphyroclasts, a characteristically reduced grain size and deflecting CPO
as well as sutured grain boundaries indicate also some contribution of
crystal-plastic deformation and grain-boundary migration of antigorite. In
contrast, the absence of any intragranular deformation features in newly
precipitated olivine in strain shadows reveals that stresses were not
sufficiently high to allow for significant dislocation creep of olivine at
conditions at which antigorite is stable. The porphyroclast microstructures
are not associated with the microstructures of the mylonitic matrix, but are
inherited from an independent earlier deformation. The porphyroclasts record
a high-stress deformation of the peridotite with dislocation creep of
olivine in the upper mantle probably related to rifting processes, whereas
the serpentinite matrix records dominantly dissolution–precipitation creep
and low stresses during subduction and exhumation. |
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