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| Titel |
Deformation of quartz and feldspar at mid-crustal depths in an extensional normal fault (Viveiro Fault, NW Spain) |
| VerfasserIn |
M. A. López-Sánchez, S. Llana-Fúnez, A. Marcos, F. J. Martínez |
| Konferenz |
EGU General Assembly 2012
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250064066
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| Zusammenfassung |
| Metamorphic reactions, deformation mechanism and chemical changes during mylonitization
and ultramylonitization of granite affected by a crustal-scale shear zone are investigated using
microstructural observations and quantitative analysis.
The Vivero Fault (VF) is a large extensional shear zone (>140Km) in NW of Iberia that
follows the main Variscan trend dipping 60Ë toward the West. The movement
accumulated during its tectonic history affects the major lithostratigraphic sequence
of Palaeozoic and Neoproterozoic rocks and the metamorphic facies developed
during Variscan orogenesis. Staurolite, and locally, andalucite plus biotite grew in
the hangingwall during the development of VF, overprinted the previous regional
Variscan greenschist facies metamorphism. Andalusite growth took place during
the intrusion of syntectonic granitic bodies, such as the deformed granite studied
here.
The Penedo Gordo granite is coarse-grained two-mica biotite-rich granite intruding the
VF and its hangingwall. This granite developed a localized deformation consisting of a set of
narrow zones (mm to metric scales) heterogeneously distributed subsequently to its intrusion.
Based on pseudosections for representative hangingwall pelites hosting the granite and the
inferred metamorphic evolution, the shear zone that outcrops at present-day erosion surface
was previously active at 14,7-17 km depth (390-450 MPa). Temperature estimates during
deformation reach at least the range 500-600Ë C, implying a local gradient of 35±6Ë
C/km.
Microstructures in the mylonites are characterized by bulging (BLG) to subgrain rotation
(SGR) recristallization in quartz with the increasing of deformation. Albitisation,
flame-perthite and tartan twining are common in K-feldspar at the early stage of deformation.
The inferred dominant deformation mechanisms are: i) intracrystalline plasticity in quartz, ii)
cataclasis with syntectonic crystallisation of very fine albite-oligoclase and micas in
K-feldspar, and iii) cataclasis with precipitation of K-feldspar in fractures and other
dilatational sites in plagioclase.
Ultramylonites consist of a matrix mainly containing feldspar, quartz and micas (mainly
biotite) with an average grain size below 15 μm, usually featuring some quartz pods and
small feldspar porphyroclast. Quartz pods disintegrate into polycrystalline aggregates, and the
resultant grains are mixed into the surrounding matrix reaching its average grain size. In the
matrix, grain size is uniform and the distribution of mineral phases tends to be
homogeneous.
Mass balance analysis based on major elements indicates that the deformation process
was not isochemical for some elements. Preliminary XRF results show that the
mylonitic/ultramylonitic samples are depleted in Na and Mn and enriched in K and Ca
respect to the original protolith, while others remains stable (Si, Al or Fe). This data suggests
a large-scale transport of some components, and therefore, that fluids were involved during
deformation.
Similar feldspar microstructures in mylonites, implying cataclasis and neocrystallisation,
have been previously reported in natural rocks where the temperature was estimated between
250 to 450ºC (see Fitz-Gerald and Stünitz 1993, Hippertt 1998 or Ree et al. 2005). In
opposition to this, petrological and mineralogical thermometry data indicate that
temperatures during deformation of FV reached at 500-600ºC, extending the temperature
range previously reported. |
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