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| Titel |
In situ 40Ar/39Ar dating of zoned and recrystallized phengite in metasomatized high-pressure rocks |
| VerfasserIn |
Ralf Halama, Matthias Konrad-Schmolke, Masafumi Sudo |
| Konferenz |
EGU General Assembly 2013
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 15 (2013) |
| Datensatznummer |
250078590
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| Zusammenfassung |
| In situ 40Ar/39Ar dating provides spatial resolution that is critical to understand the temporal
information in 40Ar/39Ar dates of metasomatized and partially recrystallized rocks as these
dates can be combined with petrologic and geochemical information about the rock’s history.
Here, we evaluate the effects of deformation and fluid flux on 40Ar/39Ar dates by
investigating a structural profile with strain- and recrystallization gradient across a major
crustal shear zone that separates two tectonometamorphic units in the Sesia-Lanzo Zone
(Western Alps).
Weakly deformed, eclogite-facies samples contain phengites that show significant major
element compositional differences between pristine cores and overprinted rims. In
contrast, samples from the blueschist-facies shear zone contain fine-grained, mylonitic
phengite. To test the hypothesis that increasing deformation and fluid flux promote Ar
transmissivity, we compare in situ 40Ar/39Ar data and geochemical tracers of fluid
flow (B, Li, δ11B) of relict areas with overprinted areas and mylonitized phengites.
Lithium (Li) and boron (B) concentrations and B isotopic data show that retrograde
fluid influx increases from weakly deformed samples towards highly deformed
mylonites. Samples from within the shear zone have the lowest B concentrations and
the highest δ11B values, suggesting equilibration with an external fluid with high
δ11B.
Apparent 40Ar/39Ar laser spot age data were evaluated on isochron and inverse isochron
diagrams to evaluate the initial 40Ar/36Ar ratios and the significance of the dates. Phengite
cores from weakly deformed eclogitic micaschists yield statistically valid ages of about 80±3
Ma that are interpreted as crystallization ages during eclogite-facies conditions. In
contrast, samples that show an intense deformational overprint associated with
mylonitisation yield bimodal apparent ages: Poorly defined ages around 80-90 Ma for large
mica clasts that are surrounded by the foliation and a well-defined age of 65±3
Ma for the mylonitic mica. The latter age can be associated with high fluid flux in
the shear zone an gives the age of fluid-rock interaction, which caused phengite
recrystallization. Overprinted phengite rims in weakly deformed samples give ages of about
75±6 Ma, overlapping with the core ages but also showing a tendency to somewhat
younger ages. These can be interpreted to either reflect incomplete isotopic resetting
during limited fluid influx or a distinct metasomatic event after the eclogite-facies
equilibration.
In summary, fluid-induced resetting of Ar isotopes in phengitic mica is controlled by
mylonitic deformation and associated fluid flux during exhumation and juxtaposition of two
tectonometamorphic segments in the Sesia-Lanzo zone. Our study demonstrates that high
spatial resolution is critical to fully understand the temporal information in 40Ar-39Ar dates
of metasomatized rocks and underlines the importance of metasomatic processes for
geochronology. |
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