MAFIC BLUESCHIST AND ASSOCIATED GRAPHITE-SCHIST BLOCKS IN THE FRANCISCAN MELANGE, SAN SIMEON, CALIFORNIA

The 6 km of seacliff exposure of the Franciscan mélange near San Simeon contains a small volume of mafic blueschist and graphite-schist blocks. The other inclusions that define the block-in-matrix fabric are mostly graywacke, greenstone, and chert. The interlayered blueschist and lawsonite-bearing graphite-schist are related pieces of a cryptic terrane.

Two main mineral assemblages are common among 32 mafic blocks: 1) Lws + Na-amp + Chl + Pmp + Phen, recrystallized at 5-7 kb and 225-275°C, and 2) Lws + Na-amp + Ep + Chl + Pmp + Phen, which formed under slightly higher temperatures (275-300°C). 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 high-P breakdown of Ep during a decrease in T. Geochemical analysis of 12 blueschist blocks indicates that 9 are similar to MORB while three have similarities to OIB.

Graphite-schist blocks have not been described before. 30 blocks were studied and all contain Qtz + Mus + Ab + graph. There are two types: 13 blocks have relict sedimentary textures with a weak foliation defined by graphite and pressure solution seams. 17 blocks have a compositional layering containing well recrystallized quartz. Nine of these blocks also contain lawsonite within the more graphitic layers.

The mafic blocks are pieces of ocean crust with at least one seamount. The graphite schists are masses of sediment that was probably deposited on top of the ocean crust. These materials were imbricated and dynamically metamorphosed soon after the initiation of Franciscan subduction. The lawsonite-bearing blueschists were underplated forming an aureole that cooled at the base of the North American plate at ~20 km depth. Exhumation to depths of ~10 km is postulated to have occurred by hanging wall extension that was driven by the late Cretaceous decrease in Farallon plate dip. These metamorphic rocks were plucked from the base of the hanging wall when undercompacted, shale-rich sediment upwelled along the top of the subduction shear zone. The strong recrystallized rocks became cataclastically deformed as shearing caused them to boudinage. The final stage of deformation occurred as the mélange dewatered and compacted very near the trench slope surface.