Initiation and Evolution of Subduction: T-t-D History of the Easton Metamorphic Suite, Northwest Washington State

In the Northwest Cascades, results from previous workers show the Easton Metamorphic Suite contains an inverted metamorphic sequence with ultramafic rocks underlain by amphibolite and high-temperature blueschist that are juxtaposed above high-pressure low-temperature Shuksan Greenschist and Darrington Phyllite. In this study, amphibole and white mica 40Ar/39Ar geochronology is combined with cation exchange thermometry to constrain the temperature-time-deformation history of the suite. Amphibolite facies fabrics record two deformation events at 10 kbar. The oldest fabric formed at ~760 ̊C prior to 167.4 Ma (2σ). A second fabric formed at ~590 ̊C prior to 164.4 Ma. Na-amphibole schist associated with the amphibolite crystallized at ~530 ̊C and yields a white mica cooling age of 165.3 Ma. A blueschist overprint on amphibolite occurred at 162.5 ± 0.9 Ma. Post-tectonic mica in metasomatized amphibolite crystallized at 160.1 Ma at ~350 ̊C. Assemblages in the S2 fabric of Shuksan Greenschist and Darrington Phyllite crystallized at ~325 ̊C at 7 kbar, and white mica ages span 148-136 Ma. Results demonstrate subduction began prior to 167 Ma, as recorded by amphibolite facies metamorphism of mafic protoliths underthrust beneath ultramafic rocks. Subduction initiation was followed by cooling of amphibolite below ~500˚C by 167 Ma, and formation of high-temperature blueschist by 165 Ma. Early high-grade rocks were exhumed from ~10 to ~7 kbar and retrograded to blueschist between ~162 and 160 Ma. Temperature decreased to ≤350 ̊C by ~162 Ma and stayed approximately constant from 148-136 Ma during underthrusting and exhumation of Shuksan Greenschist and Darrington Phyllite