Thermal and structural history of underplated rocks from subduction initiation to thermal steady state: Easton Metamorphic Suite and related units, WA, USA

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.  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.

The earliest deformation event in the metamorphic sole at ~10 kbar, 760 °C formed S₁^A 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 S₁^A fabric in homogeneous blocks of amphibolite. Metasomatism due to contact with hot hangingwall rocks may have occurred before, during, and after S₁^A, 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 (S₂^A) at 590°C, >165 Ma.  S₂^A 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.  The S₂^A event overlaps with the earliest metamorphism and strong deformation (S₁^N) of high-grade Na-amphibole schist at ~530°C, 10 kbar, which cooled below 400°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°C, 7 kbar by 157 Ma is evidenced by a crenulation cleavage in the Na-amphibole schist (S₂^N) during brittle deformation in the amphibolite and metasomatic schist evidenced by glaucophane-filled fractures in hornblende. 

Younger accretion and exhumation events occurred as HP/LT conditions persisted, including underplating of regional phyllite at ~7 kbar, ~320°C from ~154-142 Ma and metavolcanic greenschist-blueschist at ~7 kbar, 360°C at ~140 Ma.  Exhumation to ~5 kbar, ≤300˚C occurred between ~140-125 Ma during later deformation of greenschist-blueschist and underplating of structurally lower metagraywacke and greenstone.  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.  Strain partitioning at this stage was high, with non-coaxial strain focused in phyllite and flattening fabric dominant in metagraywacke.  No deformation is evident in the high grade rocks at this time, showing the locus of strain had stepped to lower structural levels.  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.