Triassic blueschist from northern California and north-central Oregon

Abstract

Four samples of blueschist from the eastern Klamath Mountains near Yreka, northern California, and from north-central Oregon near Mitchell have radiometric ages of approximately 220 m.y. (Middle Triassic). In the Klamath Mountains, there are two types of blueschist occurrence: (1) tectonic blocks of mafic composition in a phyllitic quartzite and siliceous phyllite terrane, which has also undergone blueschist-facies meta-morphism and lies between a belt of serpentinite and an upper Paleozoic or lower Mesozoic greenstone-chert terrane, and (2) discontinuous layers and tectonic blocks in phyllitic rocks of the greenschist metamorphic facies, which occur beneath a thrust plate of lower Paleozoic sedimentary rocks. In north-central Oregon, blueschist blocks occur in strongly sheared upper Paleozoic metasedimentary and metavolcanic rocks overlain by Cretaceous sedimentary rocks. The blueschist blocks in the Klamath Mountains and north-central Oregon are inferred to have formed during subduction that accompanied widespread Middle Triassic tectonism in the western Cordillera.