PENINSULA GEOLOGICAL SOCIETY

(1) LREE-enriched arc tholeiites + alkaline basalts and distal turbidites of the North Fork + Eastern Hayfork terranes were deposited in a subsea environment during Permian(?), Triassic, and earliest Jurassic time. Landward, subduction resulted in production of the high-P Stuart Fork blueschist complex, then its exhumation at ~227 Ma. (2) 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 mélange. These two terranes underwent low-T alteration by seawater at 100-200 °C and <100 MPa; alkali exchange and modest Mg-enrichment were accompanied by increases in greenstone bulk-rock ¹⁸O values from 6 to ~10‰, preceeding 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. (3) Suturing of the North Fork oceanic arc beneath the exhumed, landward Stuart Fork terrane at 165-170 Ma resulted in regional folding and ~sub- to greenschist-facies metamorphism. Pervasive recrystallization took place without substantial chemical or isotopic exchange under conditions of 300-425 °C, 300 ± 100 MPa, higher metamorphic grade in the north, lower in the south. (4) East-descending subduction or transpression continued seaward, and granitoid plutons were emplaced locally during 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 ¹⁸O fluids with intimately intercalated greenstones; ¹⁸O values in metavolcanic rocks locally increased to more than 15‰. Subsequent cooling yielded 150-164 Ma apparent mineral ages for the metamorphic aureoles. (5) 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 at 147 ± 3 Ma, may be associated with this event. Cenozoic exhumation resulted in range-front faulting and erosion. The documented interplay between Phanerozoic convergence/transpression and petrochemical evolution in a suprasubduction zone setting provides an illuminating model for crustal growth.