First 40Ar-39Ar Ages from Low-T Mafic Blueschist Blocks in a Franciscan Mélange near San Simeon: Implications for Initiation of Subduction

Abstract

The Franciscan Complex of California is a type example of an accretionary prism with widespread high-P/T subduction-zone metamorphism. Low-T, fine-grained, lawsonite-bearing mafic blueschists encased in a shale-matrix mélange near San Simeon have been dated for the first time. 40Ar-39Ar ages were obtained for phengite separated from four blocks and from an actinolitic rind on the margin of one of the blocks. The three blocks with lawsonite + epidote yield ages of between 154 and Ma, while the actinolitic rind yields an age of Ma. These ages are from the part of the Franciscan Complex west of the San Andreas Fault that moved northward at least 300 km with respect to the extensive Franciscan exposures east of the fault. The ages obtained in this study show that some fine-grained, low-T (lawsonite + epidote) Franciscan blueschist blocks formed at the same time as coarse-grained, high-T (garnet ± epidote) Franciscan blueschist blocks. These dated rocks indicate that both high- and low-T mafic blueschists are coeval and probably formed along 1000+ km of the North American Plate margin at ∼160–155 Ma. The episode of dynamic blueschist metamorphism was soon followed by high-Mg rind formation and a long period of nearly static high-P/T conditions.