U-PB AGES AND PROVENANCE OF DETRITAL ZIRCONS FROM EASTERN HAYFORK (EHF) META-ARGILLITES, SAWYERS BAR AREA, CENTRAL KLAMATH MOUNTAINS, NW CALIFORNIA

The EHF trench mélange accreted seaward of the North Fork ophiolitic arc. It consists of fine-grained, quartzose meta-argillite, boudinaged metachert layers, and blocks derived from both oceanic and inboard arc-margin sources. Among the blocks are coarse feldspathic metasandstones, from which Scherer et al. (2010) reported detrital zircon SHRIMP U-Pb ages ranging from latest Archean to early Proterozoic; Phanerozoic zircons were absent. Scherer et al. concluded that the blocks were olistostromal, and sourced from a thrust sheet outlier of the Proterozoic Antelope Mountain Quartzite, Eastern Klamath belt. To determine provenance and time of deposition of the muddy matrix hosting these old blocks, we separated zircons from three very fine-grained, intimately interstratified, feldspar-poor meta-argillites. LA-MC-ICPMS zircon U-Pb data indicate maximum sedimentary ages for the meta-argillites as ~272, ~211, and ~201 Ma. Thus the muddy strata were deposited near the end of Triassic time. The meta-argillites contain rare Proterozoic and Paleozoic-early Mesozoic detrital zircons, indicating local inboard Klamath sources, including Antelope Mountain Quartzite. Distal turbidites in the landward North Fork terrane contain Paleozoic and Triassic-Early Jurassic (~188 Ma) zircons plus scattered Precambrian grains like those in the EHF. The EHF docked against the North Fork terrane by ~174-173 Ma because the amalgamated, unmetamorphosed terrane contact was invaded by the English Peak pluton beginning at ~172 Ma (Ernst et al., 2016). Dynamothermal metamorphism took place throughout the central Klamaths during the Siskiyou transpressive accretionary event at ~170-168 Ma. Accordingly, oceanic crustal rocks recrystallized in an east-dipping subduction zone from ~170-155 Ma. By ~165 Ma continuing to ~150-140 Ma, erosion supplied superjacent volcanogenic debris to proximal Mariposa-Galice and Myrtle overlap strata. At ~140, before onset of Franciscan and Great Valley Group deposition, the Klamath salient migrated ~80-100 km westward relative to the Sierran arc, trapping oceanic crust on the south as the Coast Range Ophiolite.