Mesozoic petrotectonic development of the Sawyers Bar suprasubduction-zonearc, central Klamath Mountains, northern California

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 fin e-grained quartzofeldspathic argillites, interstratified with and overlain by two mafic igneous, largely extrusive suites-North Fork (sensu stricto) m ildly alkaline basalts, and Salmon River basaltic-diahasic-gabbroic are tho leiites; and (3), the cher ty, Eastern Hayfork metagraywacke melange terran e west of the minor, high-angle Twin Sisters fault. Mineral and bulk-rock e lemental and isotopic data, integrated with geologic mapping, document defo rmation and fluid-rock interaction in the upper few kilometers of a suprasu bduction-zone basaltic are during tectonic accretion to the western margin of North America or a nearby offshore are. The following history is advance d. Light rare earth element (REE) enriched are 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 Mi ddle Jurassic time, producing the nest-facing North Fork oceanic are and ad jacent, tectonically disrupted Eastern Hayfork melange. These two terranes underwent low-temperature alteration by seawater at 100-200 degrees C and < 100 MPa; alkali exchange and modest Mg enrichment were accompanied by incre ases in greenstone bulk-rock delta(18)O values from 6 parts per thousand to similar to 10 parts per thousand, preceding and during initial stages of i sland-are formation at 175-200 Ma, By the end of this time, the outboard We stern Hayfork calc-alkaline are had accreted to the Eastern Hayfork terrane , Suturing of the North Fork oceanic are 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 300-4 25 degrees 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 granitoi d plutons were emplaced locally 159-164 Ma, heating adjacent wall rocks to a maximum of similar to 500-600 degrees C at pressures of 200-300 MPa, The temperature increase caused devolatilization of metasediments and the excha nge of high delta(18)O fluids with intimately intercalated greenstones; del ta(18)O values in metavolcanic rocks locally increased to >15 parts per tho usand. 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 mu scovite porphyry felsite dikes transected the Stuart Fork-North Fork thrust contact; formation of hydrothermal gold-bearing quartz veins, dated as 147 +/- 3 Ma, mag be associated with this event. Cenozoic exhumation resulted in range-front faulting and erosion. The documented interplay between Phane rozoic convergence and/or transpression and petro-chemical evolution in a s uprasubduction-zone setting provides an illuminating model for growth of th e sialic crust.