Geochemistry and 40Ar/39Ar geochronology of pseudotachylyte associated with UHP whiteschists from the Dora Maira massif, Italy

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State: Public
Version: Final published version
Serval ID
serval:BIB_8F9BC912A052
Type
Article: article from journal or magazin.
Collection
Publications
Institution
Title
Geochemistry and 40Ar/39Ar geochronology of pseudotachylyte associated with UHP whiteschists from the Dora Maira massif, Italy
Journal
Tectonophysics
Author(s)
Cosca M.A., Caby R., Bussy F.
ISSN-L
0040-1951
Publication state
Published
Issued date
2005
Peer-reviewed
Oui
Volume
402
Pages
93-110
Language
english
Abstract
In situ UV-Iaser ablation Ar-40/(39) Ar geochronological and geochemical
data, together with rock and mineral compositional data, have been
determined from pseudotachylyte and surrounding mylonitic gneiss
associated with the UHP whiteschists of the Dora Maira Massif, Italy.
Several generations of fresh pseudotachylyte occur as irregular veins up
to a few cur thick both parallel and at high angles to the foliation.
Whole rock XRF data collected from representative lithologies of
mylonitic gneiss are uniformly consistent with a mildly alkalic granitic
protolith. Minimal compositional variation is observed between the
pseudotachylyte and its surrounding mylonitic gneiss. The
pseudotachylyte contains newly crystallized grains of biotite and
K-feldspar in a matrix of glass with partially fused grains of quartz,
zircon, apatite, and titanite. Electron microprobe analyses of the glass
show significant compositional variation that is probably strongly
influenced by micrometer-scale changes in mineralogy. UV-Iaser ablation
ICP-MS traverses across the mylonitic gneiss-pseudotachylyte contact are
consistent with cataclastic communition of REE carriers such as epidote,
monazite, allanite, zircon, and apatite before melting as an efficient
mechanism of REE homogenization in the pseudotachylyte. The 40Ar/39Ar
data from one band of pseudotachylyte indicate formation at 20.1 +/- 0.5
Ma, when the mylonitic gneisses were already in a near surface position.
The variable effects of top-to-the-west shear deformation within
outcrops of the coesite-bearing unit are reflected in localized zones of
protomylonite, cataclasite, ultracataclasite, and pseudotachylyte.
Preservation of several generations of pseudotachylyte suggests that
seismic events may have played a significant role in triggering late
unroofing of the UHP rocks. It is speculated that deeper crustal seismic
events potentially played a role in the unroofing of the UHP rocks at
earlier stages in their exhumation history. (c) 2005 Elsevier B.V. All
rights reserved.
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01/10/2012 20:07
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20/08/2019 15:53
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