Comparison of early Mesozoic high-pressure rocks in the Klamath Mountains and Sierra Nevada

Bradley R. Hacker, John W. Goodge

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Early Mesozoic blueschist terranes in the Klamath Mountains (the Stuart Fork) and Sierra Nevada (the Red Ant), are lithologically, petrologically and structurally similar. Both contain common crossite-epidote assemblages formed during subduction of oceanic sedimentary and volcanic protoliths that had previously been metamorphosed at an ocean ridge. Both are remnants of a high-P/T metamorphic belt paired with a more easterly magmatic arc along the western margin of North America. These similarities imply that the Mesozoic blueschists in both mountain belts shared a common early history. Important differences, however, indicate that their later postsubduction histories were different: blueschists in the Klamath Mountains were preserved without overprinting by higher T assemblages, whereas blueschists in the Sierra Nevada were overprinted by pumpellyite-actinolite facies metamorphism. From these relations we infer that the Stuart Fork rocks may have been lifted upward more rapidly than the Red Ant terrane, or that the Red Ant terrane was incompletely subducted and not subject to the cooling effect of continuing subduction. The Stuart Fork blueschists were also affected by a contact metamorphic event during widespread Middle to Late Jurassic plutonism, which is not manifest in the northern Sierra Nevada blueschist terrane.

Original languageEnglish (US)
Pages (from-to)277-295
Number of pages19
JournalSpecial Paper of the Geological Society of America
StatePublished - 1990


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