HR: 0800h
AN: T11B-0579    [Abstracts]
TI: Petrology of sapphirine-bearing gedrite-cordierite gneiss, Okanogan dome, Washington USA, and implications for gneiss dome tectonics
AU: * Kruckenberg, S C
EM: kruc0030@umn.edu
AF: University of Minnesota - Twin Cities, 108 Pillsbury Hall, 310 Pillsbury Dr SE, Minneapolis, MN 55455, United States
AU: Whitney, D L
EM: dwhitney@umn.edu
AF: University of Minnesota - Twin Cities, 108 Pillsbury Hall, 310 Pillsbury Dr SE, Minneapolis, MN 55455, United States
AB: The northern Cordilleran migmatite domes (Thor-Odin, Valhalla, Okanogan) contain Mg-Al-rich (Si-poor) orthoamphibole-cordierite gneiss as layers and lenses surrounded by quartzofeldspathic migmatite. The Mg-Al- rich rocks contain assemblages and reaction textures that provide information about metamorphic conditions during the tectonic evolution of the migmatite dome. In the Okanogan dome, gedrite + anorthite + cordierite + spinel + sapphirine +/- kyanite +/- hornblende assemblages of the Tunk Creek Amphibolite indicate T > 700- 800 degrees C. This Okanogan unit structurally overlies a migmatite domain dominated by diatexite. In contrast to gedrite-cordierite gneiss in other northern Cordilleran domes, the Okanogan rocks occur in a discontinuous kilometer-scale unit rather than small pods; are more calcic; and lack garnet. In addition, kyanite did not transform to sillimanite, and spinel occurs most commonly as a blocky matrix phase rather than as vermicules in symplectite. The differences in textures of gedrite-cordierite gneiss in the Cordilleran domes may in part be related to differences in bulk composition, but are also likely related to differences in tectonic evolution. The Okanogan dome is located at a greater distance from the Rocky Mountain foreland-hinterland boundary, a major crustal boundary that localized and therefore maximized vertical flow (decompression) in domes adjacent to it. The Okanogan dome, ~200 km west of the boundary, may have experienced less isothermal decompression during dome emplacement compared to domes located closer to the boundary, and therefore contains relict kyanite and only minor corona/symplectite development.
DE: 3625 Petrography, microstructures, and textures
DE: 3660 Metamorphic petrology
SC: Tectonophysics [T]
MN: 2007 Fall Meeting