Regional detrital zircon provenance of exotic metasandstone blocks, Eastern Hayfork terrane, Western Paleozoic and Triassic belt, Klamath Mountains, California

摘要

The addition of oceanic material through accretion-underplating was an important process in construction of the convergent/transpressive Cordilleran margin of North America. According to radiolarian ages, the Late Triassic or younger Eastern Hayfork terrane represents trench deposits that accreted outboard of the coeval North Fork immature island arc. It consists chiefly of chert and argillite but contains tectonic and olistostromal blocks of preexisting rock units derived from exotic sources that include inboard Klamath terranes. Fine-grained terrigenous debris was shed from these eastern terranes, but the Eastern Hayfork mélange also contains blocks of quartzofeldspathic metasandstone of uncertain provenance. New detrital zircon U-Pb ages from five blocks of immature, variably metamorphosed, arkosic to subarkosic sandstone range from Late Proterozoic to Late Archean, with modes of ～1804-1843, ～1970, ～2300, and ～2550-2585 Ma. The adjacent, inboard North Fork terrane contains zircons with Phanerozoic ages, indicating derivation from eastern Klamath lithotectonic belts; however, no zircon grains with Phanerozoic ages were identified in the investigated Eastern Hayfork metasandstones. Petrographic analysis indicates that sand grains of the Eastern Hayfork blocks were initially derived from a transitional continental province whose textural immaturity precludes long sedimentary-transport distances. Thus, the metasandstone blocks were most likely tectonically incorporated into the oceanic mélange from a local protolith and were not deposited in the trench as sand. Single-grain detrital zircon age spectra for the Eastern Hayfork metasandstone blocks are similar to those for several Cordilleran terranes, precluding clear discrimination among them regarding provenance. Although no currently exposed source is identical to the mélange blocks, the Eastern Hayfork zircons show similarities to Laurentian passive-margin clastics in terms of Paleoproterozoic and Neoarchean ages. The presence of an outlier of the Antelope Mountain Quartzite of the Yreka terrane near the Eastern Hayfork terrane, however, indicates that this was a likely source of the exotic blocks. Evidently, this preexisting unit cropped out along the leading edge of the Cordilleran continental margin during Early Mesozoic mélange formation, but its areal extent has been diminished by subsequent erosion, deposition, and/or deformation.