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U‐Pb ages of detrital zircons in Pacheco Pass metagraywackes: Sierran‐Klamath source of mid‐Cretaceous and Late Cretaceous Franciscan deposition and underplating

Using laser ablation ICP‐MS techniques, U‐Pb ages are reported for 338 detrital zircons separated from four Franciscan 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. These Diablo Range clastic sediments were deposited in mid‐Cretaceous and Late Cretaceous time. The highest allochthon, unit I, was deposited after ∼102 Ma, unit IV was deposited after ∼90–102 Ma, and unit V was deposited after ∼86 Ma. Metagraywacke depositional‐accretionary ages within slab IV young upward, but 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 the Sierran‐Klamath calcalkaline arc provided most of the clastic debris. Judging by the volcanic nature of lithic clasts in these 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 eroding comagmatic volcanic arc rather than from the less voluminous Jurassic arc plutons.