AGE AND TIMING OF MAGMATISM OF THE OKANOGAN RANGE BATHOLITH, CENTRAL WASHINGTON

The Okanogan Range Batholith (ORB) is a suite of peraluminous tonalitic to granitic intrusions in central Washington interpreted as part of the Cretaceous Okanogan-Spences Bridge arc. The southern contact of the ORB is juxtaposed with the Chelan Migmatite Complex (CMC), an assemblage of partially melted mafic-intermediate intrusions, along a poorly-defined contact that appears intrusive. Geochronologic data and field relations from the ORB and the CMC suggest contemporaneous magmatism and partial melting, respectively.

This study characterized the composition and timing of intrusion of rocks across the ORB. Whole rock geochemistry on seven samples show Na and Ta depletion and a positive Pb anomaly. All samples show a depletion in HREE and positive europium anomalies from 2.91-23.23. Eight samples from the ORB were dated using Laser Ablation Split Stream geochronology at the University of California Santa Barbara, allowing for the determination of both age and geochemistry on the same zircon volume. These data suggest that the ORB was intruded between approximately 118-105 Ma. Several samples contain zircons with inherited Jurassic cores, between 150-170 Ma that are geochronologically similar to zircons from the CMC. Together with field observations, these data suggest that migmatization in the CMC may have contributed to the ORB.