HR: 16:15h
AN: V34C-02    [Abstracts]
TI: High-Precision Dating of Migmatites in an Exhumed Continental Arc
AU: * Gordon, S M
EM: gordo204@umn.edu
AF: University of Minnesota, Dept of Geology and Geophysics 310 Pillsbury Dr SE, Minneapolis, MN 55455, United States
AU: Bowring, S
EM: sbowring@mit.edu
AF: MIT, EAPS 77 Massachusetts Ave 54-1120, Cambridge, MA 02139, United States
AU: Whitney, D L
EM: dwhitney@umn.edu
AF: University of Minnesota, Dept of Geology and Geophysics 310 Pillsbury Dr SE, Minneapolis, MN 55455, United States
AU: Miller, R B
EM: rmiller@geosun.sjsu.edu
AF: San Jose State University, Dept of Geology, San Jose, CA 95192, United States
AU: McLean, N
EM: nmclean@mit.edu
AF: MIT, EAPS 77 Massachusetts Ave 54-1120, Cambridge, MA 02139, United States
AB: Migmatites play an important role in the evolution of mountain systems by inducing rheological contrasts and focusing strain. To understand the role of partially molten crust in tectonometamorphic processes, it is important to determine how much of the crust was partially molten for how long, and to link the conditions, timing, and consequences of partial melting to tectonic processes at different crustal levels during construction and collapse of orogens. The Skagit Gneiss, in the high-grade core of the North Cascade range of Washington and BC, contains abundant migmatites generated in a continental magmatic arc. In one outcrop near the E margin of the Skagit Gneiss, leucosomes occur with a variety of textures (e.g., layer-parallel/discordant; fine- grained/pegmatitic). Zircons from different leucosomes were analyzed using the chemical abrasion (CA-TIMS) technique to determine migmatite crystallization ages. Zircon dates range from ca. 53 Ma for a deformed, cross- cutting pegmatitic leucosome that envelops boudins of garnet-bearing metagraywacke, to 47 Ma for a strongly lineated biotite gneiss. A ~ 6 cm thick layer-parallel, coarse-grained leucosome was dated between the pegmatite and biotite gneiss at ca. 51 Ma. Other leucosomes in different textural and structural settings of the Skagit Gneiss yield older zircon crystallization ages (ca. 69-60 Ma), suggesting at least two pulses of migmatite crystallization during the evolution of the magmatic arc. The ca. 50 Ma ages are similar to Ar cooling ages from much of the Skagit Gneiss and to the timing of Eocene extensional basin formation and volcanic activity in the region. Furthermore, the ca. 53-47 Ma dates are at the young end of the age spectrum for the North Cascades and indicate that magmatism, metamorphism, and partial melting were active until the latest stages of orogeny.
DE: 1115 Radioisotope geochronology
DE: 3625 Petrography, microstructures, and textures
DE: 3690 Field relationships (1090, 8486)
DE: 8110 Continental tectonics: general (0905)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting