Abstract
New U-Pb zircon and monazite dates for the Okanogan Range batholith, a suite of trondhjemitic plutonic rocks in north-central Washington, are interpreted to reflect minimum crystallization ages of 114-111 Ma. These new data suggest that the Okanogan Range batholith may represent an older southerly segment of the late Albian Spences Bridge Group, which crops out in southern British Columbia. This would constitute a >300-km-long, north-northwest-trending belt of 130-100 Ma volcanic and plutonic rocks along the western margin of the Intermontane belt, herein named the Okanogan-Spences Bridge arc. The Okanogan-Spences Bridge arc and the Gambier arc in the Coast belt of British Columbia were coeval, widely separated Early Cretaceous magmatic belts in the southern Canadian-northern United States Cordillera. The cbronologic data require reevaluation of models proposing one versus two east-dipping subduction zones to explain the origin of these two parallel magmatic belts. Neither model adequately reconciles all geologic observations.
