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A distinctive set of andesitic dikes crops out in the Klamath Mountains province south of ∼41°15′. These dikes are characterized by phenocrysts of Al-rich amphibole, Ca-rich almandine-pyrope garnet, plagioclase, and, commonly, quartz, set in a fine-grained groundmass of plagioclase, quartz, alkali feldspar, and amphibole. Conspicuous microphenocryst/accessory phases are allanite/epidote, zircon, and apatite. A variety of thermobarometric methods plus comparison with published experimental data indicates the phenocryst assemblage was stable at pressure >7 kb and at temperatures between 850 °C and the solidus. The best estimate of pressure of phenocryst equilibration is 8–9 kb and ∼800 °C, which corresponds to depths...

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