PETROLOGY OF EOCENE DIKE SWARM IN CORBALEY CANYON, CENTRAL CASCADES, WASHINGTON

The 47-48 Ma Corbaley Canyon dike system, exposed along a two-mile section of Hwy 2 east of Wentachee, WA, is notable for its remarkable range of compositions ranging from basalt to rhyolite. The dikes were originally studied by Waters (1927), who attributed their diversity to differentiation of a common magma body. Goals of the present study are: (1) to determine the mechanism(s) responsible for the wide range of rock compositions, and (2) to shed insight on the tectonic setting in which the dikes formed.

The dikes vary widely in width (<1 - 70 m) and orientation; most are porphyritic with the dominant phenocrysts being clinopyroxene, amphibole, biotite, quartz, plagioclase and alkali feldspar. Twenty two samples chosen to represent the range of rock types were analyzed by XRF; a subset of the samples was also analyzed by ICP-MS and SEM. The samples range from 49 – 79 wt.% SiO₂, classify as high-K, and are transitional between calc-alkaline and tholeiitic in character. They are moderately LREE-enriched (La/Yb_N = 2-13) and on spidergrams display LILE enrichments and HFSE depletions characteristic of arc magmas.

The chemical data support fractional crystallization as the dominant process of magma diversification. Harker plots that show curved trends for compatible elements and peaks for Ti, P, and Fe are incompatible with a binary magma mixing process. Incompatible element enrichments require an average of ~80% fractional crystallization. Preliminary least squares modeling further supports the fractional crystallization process.

The tectonic setting of these dikes remains enigmatic. Spidergrams, calc-alkaline tendencies, and the presence of amphibole phenocrysts all suggest a subduction environment. However, the age of these dikes and their geographic location correspond closely with the inferred location of the Kula-Farallon slab as it migrated beneath this region, suggesting the dikes could be linked to this event.