Abstract

Lawsonite-bearing blueschist metamorphic rocks along the North Fork Yuba River in the northern Sierra Nevada consist of three lithologic types. Most abundant is quartz-muscovite phyllite that possibly originated as part of a chert and shale sequence. These rocks rarely contain blueschist minerals. Metavolcanic rocks including massive lava and volcanic breccia are second in abundance and are typified by fracture-filling growths of crossite. Rare metaclastic rocks contain abundant lawsonite and quartz and minor glaucophane.

The rocks were deformed inhomogeneously in three principal phases. Blueschist metamorphic recrystallization accompanied the earliest deformation. The later structural events, possibly related to the Late Jurassic Nevadan orogeny, deformed the blueschist minerals. Despite uncertainty regarding time(s) of Ar loss, K-Ar dates indicate that the high-pressure metamorphism occurred before 174 m.y. B.P. and possibly before 190 m.y. B.P. (Bajocian time or earlier), during Andean-type magmatic arc activity in eastern California and western Nevada. The blueschist rocks provide the first direct evidence for pre-Franciscan subduction in the Sierra Nevada region.

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