Field, geochronologic, and geochemical constraints on the early Mesozoic paleogeographic and tectonic evolution of the central Klamath Mountains, California

Terranes of the Klamath Mountains are primarily the products of convergent margin processes that span the mid-Devonian to Jurassic time interval. The addition of oceanic material through accretion was an important process attending construction of the Cordilleran convergent/transpressive margin of North America. A component of this orogenic belt is the early Mesozoic, NS-trending, medially situated North Fork terrane. NW-SE-striking structures in the southern North Fork terrane are regionally consistent with the rest of the belt, ophiolitic rocks are variably deformed (including a serpentinite matrix melange), and the terrane is dominated by mafic volcanic rocks and very fine-grained clastic metasediments interbedded with chert; sediments have a component of continental material. Sediments of the central and southern North Fork terrane had a similar source of Mesozoic zircons, variably mixed with zircons from older Eastern Klamath terranes. The North Fork terrane represents a forearc basin positioned seaward of the early Mesozoic continental arc of the Eastern Klamath terrane and landward of the coeval, Late Triassic or younger, Eastern Hayfork accretionary melange.; Dominated by fine-grained terrigenous debris and exotic blocks of oceanic origin, the Eastern Hayfork melange also contains blocks of quartzofeldspathic metasandstone with ∼1800-2600 Ma detrital zircons, and a total lack of Phanerozoic zircons. This unit has a continental source and textural immaturity of the sandstone precludes long sedimentary transport distances. The metasandstone blocks were tectonically incorporated into the oceanic melange from a protolith locally adjacent to the margin, likely the Antelope Mountain Quartzite of the Yreka terrane or an equivalent. The source must have been along the leading edge of the Cordilleran continental margin during Early Mesozoic melange formation, and was obscured by subsequent erosion, deposition, and/or deformation.; Rodingites and metabasalts are present in a serpentinite matrix melange in the North Fork terrane. Low-grade metabasalts and low-Mg rodingites both have strong MORB trace element signatures. The most significant chemical changes that occurred during Ca-metasomatism are an increase in the concentration of CaO between 100 to 290% relative to the average metabasalt found in the melange, and the loss of K2O, Na2O, Ba, Rb, Cs, and Sr at levels close to 100%.