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| Titel |
40Ar/39Ar systematics in an exhumed ultra-high pressure terrane: implications for the timing of exhumation |
| VerfasserIn |
C. J. Warren, S. P. Kelley, S. C. Sherlock, C. S. McDonald |
| 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 |
250065940
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| Zusammenfassung |
| 40Ar/39Ar dating of white mica is commonly used to elucidate the timing of cooling of
metamorphic rocks. Single grain fusion and spot muscovite and/or phengite data from mafic
eclogites and their host gneisses in the Nordfjord region of the Western Gneiss Complex,
Norway, show that apparent ages may vary significantly both between and within grains from
within the same sample.
Calculated 40Ar/39Ar phengite ages of 440-750 Ma from mafic eclogites are significantly
older than previously reported U-Pb zircon constraints on the timing of peak pressure
metamorphism in the region (ca. 400-405 Ma). These apparent “old” ages are readily
attributable to excess argon contamination and metamorphic evolution of the white mica in an
environment in which efficient removal of Ar from the grain boundary is hindered by a lack
of fluid and/or suitable permeability.
The calculated 40Ar/39Ar age range in phengites and muscovites from the felsic host
gneisses is from ca. 385 to 420 Ma, spanning the timing of peak metamorphism. Numerical
modeling of Ar diffusion in an open system, constrained by previously reported
pressure-temperature-time data for the Nordfjord region, suggests that a 2-4 Ma age range
should be expected for the measured grain size distribution (0.5-2mm diameter) and expected
uncertainties on the cooling rate (10-50°C Ma-1). The 15-35 Ma range of calculated ages in
each sample is instead most readily reconciled with a model of limited within-sample
permeability during exhumation, causing heterogeneously distributed Ar grain boundary
concentrations and differences in apparent 40Ar/39Ar age on a |
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