ZIRCON GEOCHRONOLOGY AND GEOCHEMISTRY OF PLUTONIC ROCKS OF THE WESTERN CASCADES, WASHINGTON AND OREGON

Plutonic rocks of the western Cascades in Washington and Oregon are the focus of this study. The western Cascades arc extends from northernmost California to southern British Columbia and represents ancestral Cascade magmatism active between ~45-8 Ma. The modern Cascades arc lies to the east and was apparently built on Pre-Eocene crust that is segmented along the arc from north to south into four blocks of unique thicknesses and compositions. The ancestral arc overlies the same crust, and hosts a series of epizonal plutons that are locally associated with porphyry (Cu-Mo) and epithermal (Au) ore deposits. We present U-Pb zircon ages and trace element compositions of plutonic zircons.

New SHRIMP-RG and LA-ICP-MS U-Pb analyses of zircon from seven samples collected in Washington and Oregon indicate plutons associated with porphyry Cu-Au and epithermal Au deposits were emplaced between 24 and 14 Ma. In southern Washington, two phases of the Spirit Lake Pluton associated with the Mt. Margaret porphyry deposit near Mt. St. Helens have ages of 23.0 ± 0.2 Ma and 22.6 ± 0.6 Ma respectively; two samples from the Washougal district have ages of 22.4 ± 1.6 Ma and 21.9 ± 1.7 Ma. In central Oregon, the Detroit Dam Stock is 14.2 ± 1.3 Ma; the Yellowbottom Stock near Quartzville is 19.1 ± 1.1 Ma; and a granodiorite porphyry dike to the south at Boulder Creek is 16.4 ± 0.2 Ma.

Zircons from these intrusions have large negative Eu/Eu* anomalies and differ from the small negative Eu/Eu* anomalies documented in mineralizing plutons elsewhere in the Cordillera. Likely, the large negative Eu/Eu* anomalies reflect thin crust and relatively low water content in the ancestral Cascades arc magmas. We have also identified five zircon grains from the Detroit Dam and Washougal intrusions that have ages between 63 and 45 Ma. These ages are consistent with derivation from the Eocene Tyee turbidite sandstones. These inherited grains suggest the western Cascades in northern Oregon and southern Washington is built on Siletzia oceanic basalts covered by Tyee.

The zircon data suggest that the Miocene ancestral Cascade arc was built on thin and immature crust, which was locally assimilated by the arc magmas. Thin crust and low water contents of magmas are globally associated with small magmatic-hydrothermal ore deposits, as in the Cascades.