Structural, metamorphic and geochronologic constraints on the origin of the Condrey Mountain schist, north central Klamath Mountains, northern California

The Condrey Mountain Schist (CMS) occupies a window through Late Triassic amphibolite facies melange in the north central Klamath Mountains in northern California and southwest Oregon. The schists owe their present level of exposure to a large structural dome centered on the Condrey Mountain Window. Transitional blueschist-greenschist facies assemblages are widespread in mafic schists in the structurally lowest levels of the window; structurally higher CMS near the window margins contains medium- to high-pressure greenschist facies parageneses. An ⁴⁰Ar/³⁹Ar crossite age indicates a late Middle Jurassic age of metamorphism. All subunits of the CMS contain evidence of progressive, polyphase deformational and metamorphic histories. The styles and geometries of minor structures in the central part of the window suggest that early folding and transposition was the result of noncoaxial deformation, and that rotational strains were replaced by irrotational flattening strains with time. Rotational strains were accompanied by the development of epidote-crossite assemblages and the growth of deerite in meta-ironstones; irrotational flattening strains were accompanied and followed by the growth of albite, actinolite, spessartine, and the Ba-silicate, cymrite. Pressure-temperature estimates, the relative ages of mineral growth and deformation, and strain geometries are consistent with, but not restricted to, a subduction zone environment. High shear strains may reflect descent and burial, whereas flattening and late, static mineral growth occur during uplift. Pressure-temperature estimates for the overlying CMS greenschists suggest temperatures similar to those in the central part of the window, but at slightly lower pressures. Thrusting of the overlying amphibolites at 150-156 Ma occurred while the amphibolites were above about 500°C. Stretching lineations indicate a movement vector of about N45W. Comparisons of the sequence and timing of metamorphic and structural events, radiometric ages, and movement directions during thrusting indicate the CMS does not represent an inlier of Klamath Western Jurassic Belt flysch but is instead an older, isolated thrust plate. Similarities with the age of metamorphism and plutonism in the overlying amphibolites suggest the two plates may be remnants of the same Middle Jurassic paired metamorphic belt.