Timescales of migmatization, melt crystallization, and cooling in a Cordilleran gneiss dome: Valhalla complex, southeastern British Columbia

Abstract

Integration of in situ and conventional ion microprobe U‐Th‐Pb zircon and monazite geochronology with 40Ar/39Ar hornblende, biotite, and muscovite thermochronology provides a temporal record of metamorphism, partial melting, fluids, and cooling of exhumed midcrustal to lower crustal rocks. The Valhalla complex, southeastern British Columbia, is one of a series of elongate gneiss domes within the Omineca belt of the Canadian Cordillera. We have performed U‐Th‐Pb and 40Ar/39Ar age measurements on migmatitic gneisses from a range of structural levels and settings within the Valhalla complex in an effort to better document the timing of migmatization and exhumation. Stromatic migmatites and leucosomes crystallized in boudin necks yield concordant U‐Pb zircon ages that cluster near 60 Ma and Th‐Pb monazite ages of circa 60–50 Ma. In situ analyses of monazite in samples that contain <10% leucosome yield a bimodal distribution at 70–65 and 62–57 Ma. The younger ages are similar to migmatite zircon ages. 40Ar/39Ar analyses from the same rocks document cooling at 51–49 Ma. These new data, combined with field, structural, and petrologic data from this and previous studies of the Omineca domes, indicate that a large region (thousands of km2) of orogenic crust may have been partially molten during the Early Tertiary (>60–50 Ma). However, the combined data also suggest that melt crystallization (circa 60 Ma) in the Valhalla complex occurred rapidly and before the other domes within the Omineca belt. Cooling at 49 Ma throughout the Omineca is associated with regional extension and exhumation of migmatites in the domes. The presence of abundant migmatites and the similarity in timing of melt crystallization and cooling in domes along the 400‐km‐long Omineca belt is consistent with localization of exhumation at the boundary between the flowing crust of the hinterland and the rigid, colder crust of the Rocky Mountain foreland.