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Subduction initiation and early evolution of the Easton metamorphic suite, northwest Cascades, Washington

The Easton metamorphic suite, in the northwest Cascades of Washington State, preserves an inverted metamorphic sequence with ultramafic rocks underlain by amphibolite and high-temperature blueschist juxtaposed above low-temperature blueschists. The sequence is interpreted as a metamorphic sole and younger accreted rocks that formed during and after the initiation of Farallon plate subduction beneath North America in Jurassic time. Two high-temperature deformation events are recorded in the metamorphic sole at ~10 kbar and ~760 °C to 590 °C between >167 and 164 Ma. High-temperature blueschist partly overprints the amphibolite but may have accreted separately at ~530 °C between ca. 165 and 163 Ma. Retrograde metamorphism and post-tectonic white mica record cooling of the metamorphic sole to ~350 °C by ca. 160 Ma. Subsequent underplating of the Darrington Phyllite occurred at ~7 kbar and ~320 °C prior to ca. 148 Ma until at least ca. 142 Ma. Blueschist-facies conditions and exhumation to ~5 kbar occurred between ca. 140 and 136 Ma during later accretion and deformation of Shuksan greenschist-blueschist. Cooling ages from the high-temperature metamorphic sole require that subduction began prior to 167 Ma, before or during the formation of ophiolite-related rocks within the Northwest Cascades thrust system. Rapid cooling of the metamorphic sole below 400 °C until ca. 157 Ma through combined thermal relaxation of the subduction zone and partial exhumation was followed by at least 26 m.y. of a steady thermal state as younger units were accreted and exhumed. The record of high-pressure– low-temperature metamorphism suggests that the Easton metamorphic suite formed in a large ocean basin rather than an arc-proximal marginal basin. The metamorphic history also argues against previously suggested correlations of the Easton metamorphic suite with units of the Franciscan complex to the south in California. The temperature-time history of the Easton suite is consistent with models for the early evolution of subduction zones. LITHOSPHERE; v. 11; no. 1; p. 44–58; GSA Data Repository Item 2018416 | Published online 12 December 2018 https://doi.org/10.1130/L1009.1