High-grade metamafic blocks in Franciscan mélanges--olistostromal and/or tectonic?

At ~175 Ma, onset of transpressive plate underflow generated an Andean-type arc along the Californian margin. The east-descending oceanic crust was metamorphosed to eclogite, garnet-blueschist, and amphibolite facies high-pressure (HP) assemblages in an inboard subduction zone at ~165-150 Ma. Except for the fragmentary Red Ant blueschists that crop out in the northern Sierran Foothills, most such metamafic rocks apparently were stored in a low-T environment well into mid- and Late Cretaceous time. Many of the high-grade metamafic rocks returned surfaceward transported as HP tectonic blocks in buoyant Franciscan serpentinite or in subducting, circulating, low-density mud-matrix mélange. However, the rare occurrence of high-grade clasts in relatively feebly recrystallized Franciscan metaconglomerates reflects a sedimentary (including olistostromal) deposition for some HP metamafic blocks. Actinolitic rinds partially encircle many high-grade blocks of either tectonic or olistostromal putative origin. These rinds are only very slightly younger than the subduction-zone metamorphism, suggesting that an early stage of metasomatic exchange took place between HP metabasalt and serpentinized harzburgite along the dynamic oceanic crust-mantle hanging-wall junction, probably while the metamafic rocks were sequestered at moderately shallow mantle depths. For HP metabasaltic rocks to have been added to the Cretaceous Franciscan clastic section as olistoliths, they must first have been carried surfaceward as tectonic blocks immersed in a low-density lithology--probably serpentinite diapirs--then eroded and transported into the trench. Whatever their origin, due to widespread post-depositional convergent-margin shearing, the original natures of many such HP metamafic blocks have been obliterated. In any case, these dense, high-grade rocks of latest Jurassic recrystallization age must have been supplied to the Cretaceous Franciscan accretionary prism by a stage of entrainment in a low-density, ascending serpentinite body or a muddy mélange.