Computer Science
Scientific paper
May 2005
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005e%26psl.233..375m&link_type=abstract
Earth and Planetary Science Letters, Volume 233, Issue 3-4, p. 375-390.
Computer Science
7
Scientific paper
Once dated with adequate precision, intra grain recrystallization microstructures in deformed minerals provide valuable information about the tectonic and kinematic history of shear zones and entire orogens. We present combined in situ UV-laser 40Ar/39Ar geochronology and stable isotope data to decipher the relationships between intra grain recrystallization microstructures and 40Ar/39Ar ages in muscovite fish from extensional mylonite of the Porsgrunn Kristiansand Shear Zone, Southern Norway. Texturally controlled UV-laser 40Ar/39Ar dating of isotopically and compositionally distinct cores, rims, and tips of muscovite fish resolves intra grain age variations on the micrometer scale. Twenty-eight in situ UV-laser 40Ar/39Ar analyses from two muscovite cores yield a weighted mean age of 891.2 ± 2.9 Ma, interpreted to date synkinematic neocrystallization of muscovite. The integrated 880.2 ± 2.8 Ma (n = 30 analyses) weighted mean age of rims and tips is in good agreement with observed recrystallization relationships which suggest that rims and tips of the muscovite fish record the final increments of the ductile deformation history. Diffusion considerations based on published argon diffusion rates and peak temperature estimates from oxygen isotope thermometry show that the 40Ar/39Ar ages are unlikely to be disturbed during cooling. The 11.0 ± 4.0 my core-to-rim age difference together with the stable isotope data are consistent with protracted ductile deformation, extensional detachment faulting, and meteoric fluid infiltration occurring over this time interval. This approach combining X-ray element mapping, stable isotope geochemistry, and in situ UV-laser 40Ar/39Ar dating allows well-characterized microstructures to be analyzed with high precision and permits direct determination of the time scales of deformation in crustal-scale shear zones.
Andresen Andreas
Cosca Michael A.
Fiebig Jens
Mulch systems determined by high-spatial resolution in situ UV-laser A
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