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

Early Mesozoic blueschist terranes in the Klamath Mountains (the Stuart Fork) and Sierra Nevada (the Red Ant), are IithologicaUy, petrologicaUy and structuraUy 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-PIT 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 pumpeUyite-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. INTRODUCfION greater extent than previously appreciated, and through their multiple-generation metamorphic parageneses and structural fabThe Klamath Mountains and the Sierra Nevada of the rics, they provide detailed records of convergent-margin processes North American Cordillera contain a variety of rocks interpreted operative during early Mesozoic growth of the Cordillera. as magmatic arcs, oceanic crust, and subduction complexes This chapter compares and contrasts the structural features, formed during different phases and styles of ocean-continent plate phase assemblages, and mineral chemistry of the two terranes. interactions. Magmatic arc rocks in the eastern Klamath MounFurther details may be found in Goodge (1989a, b, 1990) and tains and eastern Sierra Nevada attest to convergence between B. R. Hacker (unpublished data). Textural relations determined continental North America and westward oceanic provinces from by back-scattered electron microscopy constrain the sequence of at least Devonian to Cretaceous time (e.g., Burchfiel and Davis, deformational and metamorphic events, and phase relations and 1981). The Stuart Fork terrane in the Klamath Mountains and mineral compositions determined by electron-probe microanalythe Red Ant terrane in the Sierra Nevada are subduction comsis constrain the PT conditions of metamorphism. The metaplexes that developed oceanward of the more easterly early Mesomorphic evolution of these terranes provides the petrogenetic zoic magmatic arcs. Both are coherent high-pressure belts of framework for a tectonic model linking the late Paleozoic-early Mesozoic history of the Klamath Mountains and the Sierra *Present address: Department of Geology, Stanford University, Stanford, Nevada. California 94305-2115. Hacker, B. R., and Goodge, J. W., 1990, Comparison of early Mesowic high-pressure rocks in the Klamath Mountains and Sierra Nevada, in Harwood, D. S., and Miller, M. M., eds., Paleozoic and early Mesozoic paleogeographic relations; Sierra Nevada, Klamath Mountains, and related terranes: Boulder, Colorado, Geological Society of America Special Paper 255.