HR: 0800h
AN: V21A-0588    [Abstracts]
TI: Brittle-like Deformation and Coeval Glass Formation in Mid- to Lower Continental Crust
AU: * Korhonen, F
EM: korh0011@umn.edu
AF: University of Minnesota, Department of Geology and Geophysics 310 Pillsbury Dr. SE, Minneapolis, MN 55455 United States
AU: Stout, J H
EM: jstout@umn.edu
AF: University of Minnesota, Department of Geology and Geophysics 310 Pillsbury Dr. SE, Minneapolis, MN 55455 United States
AB: An unaltered, glass-like isotropic phase with a composition close to stoichiometric cordierite (wt.% Al₂O₃ ~35%; wt.% SiO₂ ~ 40%; X_Mg~ 0.72) has been observed within a migmatite terrane near Wilson Lake in the Grenville Province of central Labrador. This phase typically has an amoeboid shape 10-20 μm in width and is commonly interconnected over distances of 40-100 μm parallel to foliation. Unlike most pseudotachylites elsewhere, there is no evidence of deformation or flow, and only sparse evidence for devitrification, in this case to a kaolinite-like phase. There is relict cordierite within the isotropic phase, but textural evidence suggests that K-feldspar and biotite are also consumed in the production of this isotropic phase and new sillimanite. Field observations show that the isotropic phase is common within a few millimeters of undeformed kyanite- and phengite-bearing dikes. Previous petrologic studies have constrained P-T conditions of emplacement of these dikes to 750-850°C and ≥ 10 kbar. The presence of unreacted sillimanite in the host rocks and decompression melt textures in phengite suggest that the terrane was rapidly uplifted after a very short residence time in the kyanite zone. The conditions of the isotropic phase formation and preservation are consistent with a more regional isobaric cooling path that was terminated by dike emplacement and rapid uplift at about 800°C. The formation and preservation of this isotropic phase (glass?) provides geologic evidence for fossil earthquakes in the normally ductile region of continental crust. Diaplectic cordierite glass is known experimentally from shock experiments at 250 kbar (Ogilvie et al., 2004), but field and petrographic evidence from Wilson Lake do not support this model of glass formation. An alternative model is thermal melting possibly due to frictional heating or plastic faulting associated with earthquakes. This interpretation is more consistent with petrographic evidence and the overall tectonometamorphic evolution of the Wilson Lake terrane, as well as other studies of lower crustal faulting. Thus we conclude that the observed isotropic phase is likely a quenched melt producted by rapid, brittle-like failure under mid- to lower crustal conditions.
DE: 3625 Petrography, microstructures, and textures
DE: 3660 Metamorphic petrology
DE: 8031 Rheology: crust and lithosphere (8159)
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005