GEOCHEMISTRY OF PLUTONIC ROCKS IN THE WESTERN CASCADES: RELATIONSHIP BETWEEN CRUSTAL SEGMENTATION & ORE GENESIS

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 is ancestral to modern Cascade magmatism, containing volcanic rocks (~45-25 Ma) and plutonic rocks (~25-10 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. 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, as well as whole rock geochemistry.

New SHRIMP-RG and LA-ICP-MS U-Pb analyses of zircon from ten samples collected along strike of the ancestral arc in Washington and Oregon indicate plutons associated with porphyry Cu-Au and epithermal Au deposits were emplaced between 23 and 13 Ma. Zircons from these intrusions have large negative Eu/Eu* anomalies and differ from the small negative Eu/Eu* anomalies of mineralizing plutons in elsewhere in the Cordillera. Likely, the large negative Eu/Eu* anomalies reflect thin crust and relatively low water contents of the ancestral Cascades arc magmas. We have also identified 23 (inherited) zircon grains from the intrusions that have ages between 63 and 30 Ma consistent with derivation from the Eocene Tyee turbidite sandstones and older western Cascades volcanic rocks. These inherited grains suggest the western Cascades in northern Oregon and southern Washington is built on Siletzia oceanic basalts covered by the 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.