DETRITAL SERPENTINITE IN PALEO FOREARC BASIN AND SUBDUCTION COMPLEX ROCKS: RECORD OF MANTLE ROCK EXHUMATION AND DERIVATIVE DEPOSITION

Detrital (sedimentary) serpentinite units up to at least 1 km in thickness, with included blocks of a variety of lithologies, make up part of the comparatively unmetamorphosed basal strata of the Great Valley Group (GVG) paleo-forearc basin deposits of California. Similar, but thinner (≤300 m thick) horizons of detrital serpentinite crop out in the structurally subjacent Franciscan subduction complex of the California Coast Ranges, whose subduction and accretion was coeval with GVG deposition. Burial of Franciscan sedimentary serpentinite horizons reached prehnite-pumpellyite to blueschist facies conditions on the basis of metamorphism of flanking strata and the lowest-grade metamorphism recorded in included blocks. Detrital serpentinite also makes up a minor component of siliciclastic rocks in the Franciscan Complex. Collectively sedimentary serpentinite in the paleoforearc basin and subduction complex records exhumation of mantle-derived material in the paleoforearc region. Timing of trench-fill serpentinite deposition recorded in Franciscan units differs from that of GVG. GVG sedimentary serpentinite horizons range in age from ca. 150 to 130 Ma. The oldest detrital serpentinite in the Franciscan may be <100 thick horizons in the structurally high Skaggs Springs schist, which has yielded a maximum depositional age of ca. 145 Ma and Ar-Ar phengite ages of ca. 132 Ma. Detrital serpentinite deposition recorded in Franciscan rocks apparently persisted until at least ca. 80 Ma, based on outcrops in the structurally lowest units of the Franciscan Central Belt in the northern Coast Ranges. Some have proposed that sedimentary serpentinite in the basal GVG represent serpentinite mud volcano deposits sourced from serpentinite diapirs, but larger blocks (hundreds of m to km), including some recording high-pressure metamorphism, exceed the probable maximum size of diapiric transport, requiring derivation from coherent exhumed rocks. The primary submarine exposures of source rocks for detritus may have been submarine fault scarps (normal faults), but this does not preclude significant volumes of serpentinite sourced from diapirs/mud volcanoes as well. The younger ages of most Franciscan detrital serpentinite compared to the GVG may reflect sedimentary reworking of basal GVG strata into the paleo trench.