Thermal and structural history of underplated rocks from subduction initiation to thermal steady state: Easton Metamorphic Suite and related units, WA, USA
Authors: Schermer Elizabeth , Cordova Jeremy , Mulcahy Sean 
Published: March 2020

&lt;p&gt;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.&amp;#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 &gt;30 m.y. at a thermal steady state.&lt;/p&gt;&lt;p&gt;The earliest deformation event in the metamorphic sole at ~10 kbar, 760 &amp;#176;C formed S&lt;sub&gt;1&lt;/sub&gt;&lt;sup&gt;A&lt;/sup&gt; 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&lt;sub&gt;1&lt;/sub&gt;&lt;sup&gt;A&lt;/sup&gt; fabric in homogeneous blocks of amphibolite. Metasomatism due to contact with hot hangingwall rocks may have occurred before, during, and after S&lt;sub&gt;1&lt;/sub&gt;&lt;sup&gt;A&lt;/sup&gt;, 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&lt;sub&gt;2&lt;/sub&gt;&lt;sup&gt;A&lt;/sup&gt;) at 590&amp;#176;C, &gt;165 Ma.&amp;#160; S&lt;sub&gt;2&lt;/sub&gt;&lt;sup&gt;A&lt;/sup&gt; 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.&amp;#160; The S&lt;sub&gt;2&lt;/sub&gt;&lt;sup&gt;A&lt;/sup&gt; event overlaps with the earliest metamorphism and strong deformation (S&lt;sub&gt;1&lt;/sub&gt;&lt;sup&gt;N&lt;/sup&gt;) of high-grade Na-amphibole schist at ~530&amp;#176;C, 10 kbar, which cooled below 400&amp;#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&amp;#176;C, 7 kbar by 157 Ma is evidenced by a crenulation cleavage in the Na-amphibole schist (S&lt;sub&gt;2&lt;/sub&gt;&lt;sup&gt;N&lt;/sup&gt;) during brittle deformation in the amphibolite and metasomatic schist evidenced by glaucophane-filled fractures in hornblende.&amp;#160;&lt;/p&gt;&lt;p&gt;Younger accretion and exhumation events occurred as HP/LT conditions persisted, including underplating of regional phyllite at ~7 kbar, ~320&amp;#176;C from ~154-142 Ma and metavolcanic greenschist-blueschist at ~7 kbar, 360&amp;#176;C at ~140 Ma.&amp;#160; Exhumation to ~5 kbar, &amp;#8804;300&amp;#730;C occurred between ~140-125 Ma during later deformation of greenschist-blueschist and underplating of structurally lower metagraywacke and greenstone. &amp;#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.&amp;#160; Strain partitioning at this stage was high, with non-coaxial strain focused in phyllite and flattening fabric dominant in metagraywacke.&amp;#160; No deformation is evident in the high grade rocks at this time, showing the locus of strain had stepped to lower structural levels.&amp;#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.&lt;/p&gt;