Submarine and superimposed contact metamorphic oxygen isotopic exchange in an oceanic arc, Sawyers Bar area, central Klamath Mountains, California, USA

Abstract

New bulk-rock oxygen isotope data indicate a complicated history of fluid-rock interactions in the upper few kilometers of a basaltic arc, attending its Mesozoic accretion to the western margin of North America. Folded, multiply recrystallized, weakly metasomatized mafic volcanics and interstratified sediments of the Sawyers Bar terrane, an eastern segment of the western Triassic and Paleozoic belt, were investigated. The following scenario can now be reconstructed: (1) island-arc tholeiites (IATs), ocean-island basalts (OIBs), and distal turbidites were deposited in a subsea environment during Permian and Early Mesozoic time (170–245 Ma). Basaltic rocks underwent low-temperature alteration by seawater; recrystallization occurred at 100–200°C and < 1 kbar. Alkali exchange and variable Mg-enrichment were accompanied by increases in bulk-rock δ 18O values of the greenstones from 6 to approximately 10%o, preceeding initial stages of island-arc formation. (2) Middle Jurassic (165–170 Ma) suturing of the seaward oceanic arc structurally beneath a landward, 227 Ma blueschist terrane resulted in regional deformation and greenschist-facies metamorphism. Pervasive overprinting took place without important chemical or isotopic exchange under conditions of 300–425°C, 3 ± 1 kbar. (3) Granitoid plutons, emplaced during late-Middle Jurassic time (160–165 Ma), heated adjacent wallrocks to ∼500–600°C at pressures of approximately 2–3 kbar; thermal upgrading resulted in devolatilization of isotopically heavy metasediments and in the exchange of high δ 18O fluids with intercalated greenstones. δ 18O values in IAT and OIB metavolcanics increased from 9 to 10% along axial portions of NS-trending folds to more than 15%o where metabasalts are intimately interlayered with the metasediments. Construction of this sector of the continental margin through the assembly of a weakly deformed and polymetamorphosed oceanic arc was attended by a sequence of aqueous fluid-rock isotopic exchanges, reflecting the P- T-time evolution of the complex.