Origin and Metamorphism of Blueschist Blocks In the Franciscan Melange, San Simeon, California

One of the best exposures of blueschist-bearing Franciscan Mélange, where contact relationships between blocks and matrix can be observed, is found along 6 km of seacliffs near San Simeon. In order to determine their origin and evolution petrographic, electron microprobe, XRD, and geochemical analyses were carried out in 34 blueschist blocks.

The blueschists have a common mafic protolith. The source contained a small volume of sedimentary layers, as indicated by the presence of qtz and/or mica-rich lenses and layers, and associated metasedimentary graphite-schists. Two main mineral assemblages were recognized among mafic blueschists: 1) Lws + Na-amp + Chl + Pmp + Phen, recrystallized at 7-9 kb and 250-350°C, and 2) Lws + Na-amp + Ep + Chl + Pmp + Phen, formed under higher temperatures (350-375°C). About half of the blocks contain 1-3 mm layers richer in Lws + Pmp ± Chl suggesting a primary compositional layering. Similar layers rich in Cal are foliation-parallel veins. At least two stages of folding and veining record a complex pre-mélange deformational history.

A pre-blueschist facies metamorphic event under lower-P and equal or higher-T conditions is recorded by calcic cores overprinted by Na-amp rims in half of the blocks. Rims with a higher Fe3+ content probably developed due to the breakdown of Ep during a decrease in T. Metasomatic alteration to Chl ± Pmp occurred along cataclastic zones coeval with boudinaging. The preservation of aragonite in veins in 11 blocks indicates T was less than 150°C when decompression reached 4 kb.

The geochemistry of the mafic blocks is consistent with that of MORB. The probable protolith is the layered upper volcanic part of oceanic crust with thin interlayered sediment. The blueschist blocks were plucked from the base of the overriding plate and incorporated into the upwelling mélange subduction channel shear zone.