THERMOBAROMETRIC CONSTRAINTS ON THE CHELAN MIGMATITE COMPLEX, NORTH CASCADES, WASHINGTON

The Early Cretaceous Chelan Migmatite Complex (CMC) and the Late Triassic Twentyfive Mile Creek unit crop out in the eastern North Cascade Range, Washington. The CMC is a metaplutonic migmatite that intruded the supracrustal Twentyfive Mile Creek unit, which is pervasively invaded by trondhjemite dikes from the CMC near the contact. Thermobarometry performed on four samples of metatonalite orthogneiss from the CMC and on two samples of garnet amphibolite gneiss from the Twentyfive Mile Creek unit indicate that both equilibrated to similar conditions of ca. 700-800°C and 7-9 kbar. Therefore, these conditions record final juxtaposition of the CMC with the surrounding supracrustal rocks at a depth of ~30 km, and previously determined crystallization ages of 100-120 Ma for the CMC likely also approximate the age of this metamorphism. Combination of previous thermochronology results with these new thermobarometric constraints suggests the CMC and Twentyfive Mile Creek unit yields an average exhumation rate of ~0.5 mm/a from peak metamorphic conditions until surface exposure at 45 Ma.