TIMING AND DURATION OF PARTIAL MELTING IN A CORDILLERAN GNEISS DOME - CORE COMPLEX: THE VALHALLA COMPLEX, SE BRITISH COLUMBIA

During orogeny, partially molten crust is significant for heat and mass transfer by both vertical and lateral flow during crustal thickening and collapse. Evidence of former melt is commonly exposed in migmatite-cored gneiss domes and core complexes, but we don't yet know the timescale and duration of melting, the relationship between melting and crustal flow, and how melting and flow couples with other processes (e.g., extension). To determine the timing and duration of partial melting and evaluate the role of melt in the evolution of a gneiss dome/core complex, we dated zircon and monazite from 6 migmatite samples of the Valhalla complex using the UCLA ion microprobe. The Valhalla complex is the southernmost in a series of migmatite domes along the E detachment of the Shuswap metamorphic core complex, southeast BC. As in the other Shuswap domes, Valhalla consists of a migmatitic core mantled by gneiss and granite. The complex includes 2 domes: Valhalla and Passmore. Previous geochronometric investigations of Valhalla have inferred prograde metamorphism at ca. 70 Ma, with a peak at 60 Ma and conditions of 820ºC, 8 kbar. In our results, leucosome crystallized in a boudin neck from the central Passmore dome has U-Pb zircon ages of 61- 57 Ma. From the same locality, a grt-sil-kfs schist yielded 2 in situ Th-Pb monazite ages of 64.2 ± 0.7 and 59.5 ± 0.7 Ma. A migmatitic gneiss located between the 2 domes has a Th-Pb monazite age of 57.5 ± 0.6 Ma. A grt-sil schist from the Valhalla dome yielded an in situ Th-Pb monazite age of 59.9 ± 1.9 Ma. Also from the Valhalla dome, a migmatite gave younger monazite ages from 54.7 to 49.7 Ma. Finally, a boudin neck leucosome from the E detachment revealed U-Pb zircon ages from 62 to 59 Ma and a single Th-Pb monazite age of 52.9 ± 0.3 Ma. The ages from the metasedimentary rocks are consistent with previous investigations, supporting peak metamorphism at ca. 60 Ma. Our results further show that partial melting was coeval with high-grade metamorphism and continued for nearly 10 m.y. until ca. 50 Ma, when the rocks cooled rapidly. The range of dates from the complex, including observations that melt was involved in deformation both within the domes and in the E detachment, suggest a protracted history of metamorphism and melting that terminated with crystallization at 50 Ma, marking the end of orogeny in the region.