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Subduction generated a NW-trending Andean arc along the Californian margin beginning at ~175 Ma. Coeval high-pressure (HP) transformation of oceanic crust in an east-dipping, subduction zone probably accompanied plate convergence, but recovered Franciscan eclogite and garnet blueschist blocks chiefly possess younger recrystallization ages of ~165-150 Ma. These Jurassic HP metamafic rocks were sequestered in a low-temperature environment well into Cretaceous time. Fragments of actinolitic rinds partially surround many such high-grade blocks. Only slightly younger than the HP metamorphism, these rinds reflect metasomatic exchange between metabasalt and serpentinized harzburgite along the dynamic oceanic crust-mantle hanging wall during storage of the mafic rocks at moderate depth. High-grade tectonic blocks later were brought toward the surface in circulating, low-density, sheared mud-matrix mélange and/or in buoyant serpentinite bodies. Most exotic HP metamafic blocks occur in mélanges of the Franciscan Central Belt, reflecting tectonic insertion within the subduction zone-not near-surface additions to the clastic section. However, rare, high-grade clasts in feebly recrystallized Franciscan conglomerates suggest erosion and sedimentary deposition for some HP blocks. The addition of dense metabasaltic olistoliths to the mid- and Upper Cretaceous section requires that these HP material were carried surfaceward first as tectonic fragments, perhaps immersed in low-density serpentinite or mud-matrix mélange, then eroded and transported into the trench. HP rocks are conspicuously lacking in coeval Great Valley strata. Whatever the origin of particular high-grade rocks, widespread post-depositional shearing has largely obliterated their original natures, but all dense metamafic blocks of Jurassic recrystallization age must have been supplied to the Cretaceous Franciscan accretionary complex by entrainment in a low-density, circulating muddy matrix or serpentinite body. The vast majority of exotic HP blocks resides in Central Belt mélanges, and appears to be tectonic rather than olistostromal in origin.

american mineralogist

Franciscan geologic history constrained by tectonic/olistostromal high-grade metamafic blocks in the iconic California Mesozoic-Cenozoic accretionary complex