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Titel |
Brittle-ductile deformation effects on zircon crystal-chemistry and U-Pb ages: an example from the Finero Mafic Complex (Ivrea-Verbano Zone, western Alps) |
VerfasserIn |
Antonio Langone, José Alberto Padrón-Navarta, Alberto Zanetti, Maurizio Mazzucchelli, Massimo Tiepolo, Tommaso Giovanardi, Mattia Bonazzi |
Konferenz |
EGU General Assembly 2016
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Medientyp |
Artikel
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Sprache |
en
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Digitales Dokument |
PDF |
Erschienen |
In: GRA - Volume 18 (2016) |
Datensatznummer |
250126341
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Publikation (Nr.) |
EGU/EGU2016-6049.pdf |
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Zusammenfassung |
A detailed structural, geochemical and geochronological survey was performed on zircon
grains from a leucocratic dioritic dyke discordantly intruded within meta-diorites/gabbros
forming the External Gabbro unit of the Finero Mafic Complex. This latter is nowadays
exposed as part of a near complete crustal section spanning from mantle rocks to upper
crustal metasediments (Val Cannobina, Ivrea-Verbano Zone, Italy). The leucocratic dyke
consists mainly of plagioclase (An18−24Ab79−82Or0.3−0.7) with subordinate amounts of
biotite, spinel, zircon and corundum. Both the leucocratic dyke and the surrounding
meta-diorites show evidence of ductile deformation occurred under amphibolite-facies
conditions.
Zircon grains (up to 2 mm in length) occur mainly as euhedral grains surrounded by
fine grained plagioclase-dominated matrix and pressure shadows, typically filled
by oxides. Fractures and cracks within zircon are common and can be associated
with grain displacement or they can be filled by secondary minerals (oxides and
chlorite).
Cathodoluminescence (CL) images show that zircon grains have internal features typical
of magmatic growth, but with local disturbances. However EBSD maps on two selected
zircon grains revealed a profuse mosaic texture resulting in an internal misorientation of ca.
10o. The majority of the domains of the mosaic texture are related to parting and fractures,
but some domains show no clear relation with brittle features. Rotation angles related to the
mosaic texture are not crystallographically controlled. In addition, one of the analysed
zircons shows clear evidence of plastic deformation at one of its corners due to
indentation. Plastic deformation results in gradual misorientations of up to 12o, which are
crystallographically controlled.
Trace elements and U-Pb analyses were carried out by LA-ICP-MS directly on
petrographic thin sections and designed to cover the entire exposed surface of selected grains.
Such investigations revealed a strong correlation between internal zircon structures,
chemistry, U-Pb isotope ratios and mylonitic fabric.
U-Pb data return highly discordant and variable ages: in particular, the 206Pb/238U ages
range from Carboniferous to Triassic within the same zircon grain. The youngest
206Pb/238U data derive from narrow axial stripes oriented parallel or at low angle
with respect to the foliation planes. These stripes are characterized by an overall
HREE, Y, U and Th enrichment possibly reflecting deformation of the grain in
presence of interstitial fluid phases, likely related to a concomitant magmatic activity.
Deformation related structures (cracks and fractures) within zircon grains acted as
fast-diffusion pathways allowing fluids to modify the geochemistry and isotopic systems of
zircon.
Our results suggest that fluid-assisted brittle-ductile deformation can severely modify the
trace elements and isotopic composition of zircon with unexpected patterns constrained by
stress regime. In similar cases, our observations suggest that, for a more appropriate
interpretation of the petrologic evolution and age variability, a direct characterization of the
internal structures of zircons still placed in their microtextural site is highly recommended. |
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