Actinolitic rinds on low-T mafic blueschist blocks in the Franciscan shale-matrix mélange near San Simeon: Implications for metasomatism and tectonic history

In the mélange terrane of the Franciscan accretionary prism, Mg-rich actinolite/tremolite “rinds” have long been known to be present on many high-T, garnet ± epidote-bearing, mafic blueschist and eclogite blocks, but they were thought to be absent on low-T lawsonite + chlorite blueschist blocks. By analogy with rinds on high-T blocks, these rinds on low-T blocks must have been produced by metasomatic reaction with ultramafic rock. The rinds on high-T blocks have led to the speculation that the high-T Franciscan blueschists were uplifted to the surface in serpentinite diapirs, while other processes exhumed the volumetrically more numerous low-T blocks. This paper reports the discovery of actinolitic rinds on the margins of nine low-T, lawsonite ± epidote Franciscan blueschists that are, or were, clearly encased in the shale-matrix mélange exposed in seacliffs and wavecut benches near San Simeon, California. Petrographic and geochemical analyses show these Mg-rich rinds are similar to those found on high-T garnet-bearing blocks. lk composition indicates the protolith of the blueschist blocks was mafic oceanic crust. The high Fe3+/∑Fe contents indicate it was probably the uppermost hydrated volcanic part of the crust. The block/ultramafic rock association was created by faulting during subduction initiation as fragments from the top of the subducting ocean crust were imbricated with mantle materials at the base of the ophiolitic leading edge of the North American plate. The chemistry of a block margin altered to actinolitic rind indicates Si, Na, Pb, Cu, and Sr were lost from the blocks while K, Rb, Cs, and Ba were added. Most pieces of actinolitic rind are Mg- and Ni-rich because they are metasomatized ultramafic material that was enriched in Si, K, Rb, Cs, and Ba. These metasomatic changes indicate the fluids that hydrated the mantle and altered the mafic blocks ascended from subducting and/or underplated sediments. Both high-T and low-T blueschists that have actinolitic rinds were uplifted to near the surface after they sank or were plucked from the base of the overriding plate by subduction-generated mélange that upwelled from depths of > 15   km .