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-pressurelow-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.
<p>Rocks of the Easton Metamorphic Suite and San Juan Islands preserve 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. Thermobarometry, Ar/Ar dating, and structural observations constrain a relatively continuous deformation history and the rheology of rocks during subduction.&#160; The data suggest HT metamorphism and accretion of oceanic crust at the initiation of subduction was followed by rapid cooling, underplating, exhumation, and later underplating and HP/LT metamorphism that persisted for >30 m.y. at a thermal steady state.</p><p>The earliest deformation event in the metamorphic sole at ~10 kbar, 760 &#176;C formed S<sub>1</sub><sup>A</sup> in amphibolite followed by cooling through hornblende closure temperature by 167 Ma. Strain was variable, with high strain in amphibolite interlayered with quartzite and quartz-mica schist and weaker S<sub>1</sub><sup>A</sup> fabric in homogeneous blocks of amphibolite. Metasomatism due to contact with hot hangingwall rocks may have occurred before, during, and after S<sub>1</sub><sup>A</sup>, as locally preserved blackwall assemblages occur at the contact of relatively undeformed amphibolite and ultramafic rocks, but metasomatic assemblages also overprint hornblende-dominated fabrics. Recrystallization during isoclinal folding of amphibolite formed a second fabric (S<sub>2</sub><sup>A</sup>) at 590&#176;C, >165 Ma.&#160; S<sub>2</sub><sup>A</sup> is mylonitic where amphibolite blocks are in contact with quartzite, quartz-mica schist, and tremolite schist; foliation in the schists is discordant to and wraps blocks.&#160; The S<sub>2</sub><sup>A</sup> event overlaps with the earliest metamorphism and strong deformation (S<sub>1</sub><sup>N</sup>) of high-grade Na-amphibole schist at ~530&#176;C, 10 kbar, which cooled below 400&#176;C by 165 Ma. We interpret the Na-amphibole schist to have been underplated as a lower metamorphic sole during this event. Retrograde metamorphism, cooling, and partial uplift to ~350&#176;C, 7 kbar by 157 Ma is evidenced by a crenulation cleavage in the Na-amphibole schist (S<sub>2</sub><sup>N</sup>) during brittle deformation in the amphibolite and metasomatic schist evidenced by glaucophane-filled fractures in hornblende.&#160;</p><p>Younger accretion and exhumation events occurred as HP/LT conditions persisted, including underplating of regional phyllite at ~7 kbar, ~320&#176;C from ~154-142 Ma and metavolcanic greenschist-blueschist at ~7 kbar, 360&#176;C at ~140 Ma.&#160; Exhumation to ~5 kbar, &#8804;300&#730;C occurred between ~140-125 Ma during later deformation of greenschist-blueschist and underplating of structurally lower metagraywacke and greenstone. &#160;Low-T fabrics are characterized by early pressure solution cleavage followed by tight to isoclinal folding and local shearing with weak to strong recrystallization in the second cleavage.&#160; Strain partitioning at this stage was high, with non-coaxial strain focused in phyllite and flattening fabric dominant in metagraywacke.&#160; No deformation is evident in the high grade rocks at this time, showing the locus of strain had stepped to lower structural levels.&#160; Meso-scale and microstructures throughout the deformation history are consistent with initial high-T deformation and limited rheological differences between lithologies, rapidly followed by weakening of metasomatized rocks and lower-T ductile and ductile-brittle deformation where strong strength contrasts favored strain partitioning into weaker units.</p>
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