SEDIMENTARY SERPENTINITE IN FRANCISCAN COMPLEX UNITS, CALIFORNIA: UPDATES FROM MULTIPLE STRUCTURAL HORIZONS

Sedimentary (clastic) serpentinite in within the Franciscan subduction complex is distributed within multiple accreted units in the Franciscan spanning ages of accretion from ca. 130 Ma to at least as young as ca. 80 Ma. The most visible clastic serpentinite units are mélange horizons of up to 200 m thick that extend for up to 5 km in map length, made up of almost entirely clastic serpentinite ranging in clast size from mud to about 10 m with minor siliciclastic components. Blocks in such horizons, excluding serpentinite blocks (up to 10 m), are similar to those found in siliciclastic mélanges, and include siliciclastic, volcanic, chert blocks up to 100s of m in size and spanning a wide range of metamorphic grade. Some siliciclastic or mixed serpentinite-siliciclastic horizons are present within otherwise dominantly clastic sedimentary serpentinite mélanges and a significant volume fraction of the clastic serpentinite lacks non-serpentinite blocks. Siliciclastic rocks of the Franciscan commonly include a minor component of clastic serpentinite. The fraction of detrital serpentinite is usually <5% but is locally higher. When serpentinite makes up more than half of the rock, the rock has the outcrop appearance of serpentinite. Relatively few samples of apparent clastic serpentinite have revealed siliciclastic components based on petrographic inspection. Some clasts or blocks of serpentinite in siliciclastic rocks are themselves clastic serpentinite that formed as a clastic rock prior to exhumation and deposition within otherwise siliciclastic detritus. Such clasts or blocks have internal blocks or clasts of other lithologies including high-grade metamorphic clasts/blocks (amphibolite, eclogite, and variable overprinting of such lithologies by blueschist facies assemblages). Many serpentinite clasts in otherwise siliciclastic rocks have been altered to chlorite, and some may have been metasomatized to talc, chromian phengite (fuchsite), or jadeite-lawsonite-oxide intergrowths. In rare cases altered serpentinite clasts may at times be identified by having chromian spinel within them, but they are more commonly identified by their texture which pseudomorphs the original serpentinite.