U-PB AGES OF DETRITAL ZIRCONS IN PACHECO PASS METAGRAYWACKES: SIERRAN-KLAMATH SOURCE FOR EASTERN BELT FRANCISCAN TRENCH DEPOSITION AND UNDERPLATING

Using laser-ablation ICP-MS techniques, U-Pb ages are reported for 338 detrital zircons separated from four metagraywacke samples from imbricate thrust sheets cropping out in the Pacheco Pass area, east-central California Coast Ranges. Structurally higher slabs are clastic quartz ± albite-bearing rocks typified by neoblastic lawsonite + jadeitic pyroxene; the lowest exposed slab lacks jadeitic pyroxene but contains traces of newly grown pumpellyite and lawsonite in addition to abundant quartz + albite. Studied specimens contain moderate amounts of phengite + titanite ± chlorite, stilpnomelane, carbonate, iron oxides, rock fragments, and carbonaceous matter. The highest allochthon, unit I, was deposited after ~102 Ma, unit IV after ~90-102 Ma, and unit V after ~86 Ma. Metagraywacke depositional ages within slab IV young upward, but depositional-accretionary ages of the slabs decrease progressively downward. In the four-sample aggregate, most zircons have igneous ages falling in the 135-170 Ma range, with a smaller population at 85-120 Ma. The rocks contain rare zircons of Middle Proterozoic-Late Archean ages. A minor source in northern California and NW Nevada seems likely for the Pacheco Pass metagraywackes, but provenance was dominated by debris shed from the Sierran-Klamath calcalkaline arc. Judging by the volcanic nature of lithic clasts in Franciscan Eastern Belt metagraywackes, and because massive Sierran plutonism occurred at ~85-120 Ma, the abundant 135-170 Ma zircons probably were derived chiefly from the superjacent, comagmatic volcanic arc rather than from the less voluminous Jurassic arc plutons.