- The volcanic (~45-10 Ma) and plutonic rocks (~37-12 Ma) comprising the Western Cascades extend from northernmost California to southern British Columbia and are ancestral to modern arc magmatism. The ancestral arc hosts a series of small plutons that are locally associated with porphyry (Cu-Mo) and epithermal (Au) ore deposits. Three crustal segments identified by Schmidt et al. (2008, 2013) in the modern arc are potentially reflected in the geochemistry of the ancestral Cascades as well: Paleozoic-Mesozoic accreted terranes, metamorphic rocks, and granites to the north; thin Paleocene Siletzia oceanic crust of the Columbia Embayment in the center; and Paleozoic-Mesozoic ultramafic sheets and marine arc-related volcanic and sedimentary rocks of the Klamath Terrane to the south. Therefore, the Western Cascades of Washington and Oregon provide a field laboratory to examine the chemical compositions and ages of granitoid intrusions associated with a variety of magmatic-hydrothermal ore deposits, and to compare the compositions with the along-arc variation of the age, composition and thickness of the underlying crust.
The majority of the 15 new zircon U-Pb ages reported in this study are 27 to 12 Ma with the exception of the ~37 Ma Snoqualmie North Fork intrusions of central WA. A total of 610 zircons were analyzed, of which 118 have ages older than the main population and are considered to be xenocrystic or inherited. The north segment contains the oldest (up to 67 Ma) and most continuous inherited population. The absence of inherited grains older than 67 Ma suggests that neither the North Fork nor White River districts overlie old crystalline crust, but instead overlie Paleocene-Eocene volcanics that are the likely source of inherited zircons. Districts of the Columbia segment have sparse inherited zircon populations (n = 38 of 350 total), ranging from 54 to 20 Ma. The dearth of inherited zircons in the center of the arc suggests limited contamination by a source no older than 55 Ma, likely the dominant Eocene sources of detrital zircon found within the Tyee Formation. Districts overlying the Klamath Terrane have slightly more substantial inherited zircon populations than districts overlying Siletzia but still decidedly few inherited grains (a total of 25 out of 85 grains analyzed, ranging from 44 to 19 Ma); these grains are likely sourced from similar contaminants to those underlying the central segment of the arc, instead of from accreted Mesozoic rocks of the Klamath Terrane.
The hypabyssal plutonic rocks represent a small area (~1%) of exposures in the Western Cascades, and range in composition from diorites to granodiorites and minor granite. Fe-Ti oxides, where preserved, include magnetite and ilmenite in proportion of ~2:1, and together with presence of hornblende and biotite are suggestive of modest oxidation states of ~ ΔNNO of 0 to 1 (Carmichael & Nicholls, 1967). Abundant hornblende is observed in 31 of 36 available petrographic sections. Ba/Nb values are not obviously correlated with SiO₂ content from any given district, but tend to increase at any given SiO₂ content from north to south. Th/Ta ratios notably increase with SiO₂, and are lowest in the mid-latitude districts (North Santiam, Detroit Dam, Quartzville, and Blue River). While increased slab fluid could increase Ba relative to Nb, the greater abundance of Ba, Th, and Th/Ta southward at given SiO₂ are more consistent with an increased role of crustal contamination. Dy/Yb ratios decrease with increasing SiO₂ contents with the exception of the North Fork District. V/Sc ratios decrease with increasing SiO₂ with the exception of samples of the Spirit Lake Pluton, and slightly increase from south to north at any given SiO₂ content.
Zircons in the Western Cascades plutonic rocks have characteristically large negative Eu anomalies (Eu/Eu* < 0.5) and small positive Ce anomalies, correlated with relatively reduced oxidation states and low water contents, even when directly associated with moderately economic porphyry Cu deposits. Although these magmas are sufficiently water-rich to abundantly crystallize and fractionate amphibole (at least 3 wt. % H₂O), it is evident that plagioclase likely crystallized early and was not suppressed by high water contents (> 3 wt. % H₂O). There is no evidence that Western Cascade magmas were strongly oxidized (> NNO +1).
Although crustal thickness is poorly constrained in the north and south segments, it is evidently variable along-arc (and may thicken slightly to the south), but is likely relatively thin. I therefore suggest that crustal thickness and lithology substantially control ore potential within the Western Cascade Arc.