TIMESCALES OF MIGMATIZATION, MELT CRYSTALLIZATION, AND COOLING IN CORDILLERAN GNEISS DOMES

A series of elongated gneiss domes occurs within the Omineca belt of the Canadian Cordillera. From N to S, these are the Frenchman's Cap, Thor-Odin, and Valhalla Complexes, which are comprised of migmatitic core rocks mantled by metasedimentary rocks and leucogranite. Previous studies have documented high-grade metamorphism of Valhalla rocks at 70-60 Ma, with peak conditions of 820 ºC, 8 kbar. To determine the timing and duration of migmatization in relation to metamorphism and other thermotectonic processes, we analyzed monazite and zircon from 10 Valhalla samples with conventional and in situ ion microprobe techniques at UCLA. Samples were collected from the dome core to the bounding detachment fault and contain variable amounts of leucosome (5 – 30%). We utilized the in situ method for some samples to specifically target monazite included in garnet, grains in the biotite matrix, and monazite in cm- to mm-scale leucocratic layers. Stromatic migmatites and leucosomes crystallized in boudin necks have concordant U-Pb zircon ages that cluster near 60 Ma and Th-Pb monazite ages of 57-49 Ma. In comparison, in situ analyses of monazite in samples that contain <10% leucosome yielded a bimodal distribution, with one age grouping at 70-65 Ma and one at 62-57 Ma. The older age grouping is consistent with previous investigations of biotite gneiss in the dome core, whereas the younger ages are similar to zircon ages from the migmatites. Ar-Ar analyses for 6 biotite separates from the same rocks document rapid cooling at 53-49 Ma following leucosome crystallization. Geo/thermochronology data from the other Omineca domes are similar: leucosome crystallization at 56 Ma, followed by rapid cooling at 49 Ma. The new geochronometric data from Valhalla, combined with field, structural, and petrologic data from this and previous studies of the Omineca domes, show that a large region of orogenic crust in this part of the Cordillera was partially molten in the early Tertiary (60-50 Ma). Rapid cooling at 49 Ma is associated with extension and exhumation of migmatites in the domes. The similarity in timing of melt crystallization and cooling along 200 km of the eastern margin of the Canadian Cordillera is consistent with collapse of an orogenic plateau at the boundary between the flowing hinterland and the rigid, colder crust of the Rocky Mountain foreland.