The alpine-type peridotite in the area of Vulcan Peak, Oregon, is part of the larger Josephine ultramafic complex in the Klamath Mountains geologic province. Partially serpentinized, foliated harzburgite with 15 to 30 percent orthopyroxene makes up approximately 90 percent of the body. The remaining 10 percent is dunite that occurs in the harzburgite as concordant and discordant layers and as irregular bodies. In general, the peridotite at Vulcan Peak is similar in structure, texture, mineralogy, and chemistry to the peridotite at Burro Mountain, California. Structures, textures, and compositions of coexisting phases are consistent with high-temperature (1,000° to 1,200°C) deformation and recrystallization in the upper mantle, and tectonic emplacement into its present crustal position. Evidence to indicate whether the peridotite originated as a refractory residue during partial fusion processes that produced mafic melt or by crystallization from an ultramafic or picritic magma remains inconclusive; poikilitic clinopyroxene enclosing olivine in some dunites, and certain chromitite textures, may represent relict igneous features suggesting a magmatic stage in the history of the peridotite.

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