The Sawyers Bar area of the central Klamath Mountains, California, consists of three tectonically juxtaposed supracrustal units: (1) on the east, the Stuart Fork metabasalt-metachert-metagraywacke terrane above the low-angle, east-dipping Soap Creek Ridge thrust; (2) the medial North Fork ophiolitic terrane, composed of intercalated St. Clair Creek laminated cherts and fine-grained quartzofeldspathic argillites, interstratified with and overlain by two mafic igneous, largely extrusive suites—North Fork (sensu stricto) mildly alkaline basalts, and Salmon River basaltic-diabasic-gabbroic arc tholeiites; and (3), the cherty, Eastern Hayfork metagraywacke melange terrane west of the minor, high-angle Twin Sisters fault. Mineral and bulk-rock elemental and isotopic data, integrated with geologic mapping, document deformation and fluid-rock interaction in the upper few kilometers of a suprasubduction-zone basaltic arc during tectonic accretion to the western margin of North America or a nearby offshore arc. The following history is advanced.

Light rare earth element (REE) enriched arc tholeiites and alkaline basalts and distal turbidites of the North Fork and Eastern Hayfork terranes were deposited in a subsea environment during Permian(?), Triassic, and earliest Jurassic time. Toward land, subduction resulted in production of the high-pressure Stuart Fork blueschist complex, then its exhumation ca. 227 Ma.

Submarine eruption and sedimentation continued outboard during Early and Middle Jurassic time, producing the west-facing North Fork oceanic arc and adjacent, tectonically disrupted Eastern Hayfork melange. These two terranes underwent low-temperature alteration by seawater at 100–200 °C and <100 MPa; alkali exchange and modest Mg enrichment were accompanied by increases in greenstone bulk-rock δ18O values from 6‰ to ∼10‰, preceding and during initial stages of island-arc formation at 175–200 Ma. By the end of this time, the outboard Western Hayfork calc-alkaline arc had accreted to the Eastern Hayfork terrane.

Suturing of the North Fork oceanic arc beneath the exhumed landward Stuart Fork terrane at 165–170 Ma resulted in regional folding and subgreenschist to greenschist facies metamorphism. Pervasive recrystallization took place without substantial chemical or isotopic exchange under conditions of 300–425 °C and 300 ± 100 MPa; metamorphic grade is higher in the north and lower in the south.

East-descending subduction or transpression continued seaward, and granitoid plutons were emplaced locally 159–164 Ma, heating adjacent wall rocks to a maximum of ∼500∼600 °C at pressures of 200–300 MPa. The temperature increase caused devolatilization of metasediments and the exchange of high δ18O fluids with intimately intercalated greenstones; δ18O values in metavolcanic rocks locally increased to >15‰. Subsequent cooling yielded 150–164 Ma apparent mineral ages for the metamorphic aureoles.

Minor intrusion took place at the end of Jurassic time, when distinctive muscovite porphyry felsite dikes transected the Stuart Fork–North Fork thrust contact; formation of hydrothermal gold-bearing quartz veins, dated as 147 ± 3 Ma, may be associated with this event. Cenozoic exhumation resulted in range-front faulting and erosion. The documented interplay between Phanerozoic convergence and/or transpression and petrochemical evolution in a suprasubduction-zone setting provides an illuminating model for growth of the sialic crust.

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