INITIATION AND EARLY EVOLUTION OF A SUBDUCTION ZONE: T-T-D HISTORY OF THE EASTON METAMORPHIC SUITE, NORTHWEST WASHINGTON STATE

Exhumed subduction complexes provide unparalleled opportunities to investigate the tectonic and metamorphic evolution of convergent margins. In the Northwest Cascades, results from previous workers show the Easton Metamorphic Suite contains an inverted metamorphic sequence with structurally high ultramafic rocks underlain by amphibolite and high-T blueschist that are juxtaposed above a regional package of HP/LT rocks (Shuksan greenschist-blueschist metabasites and Darrington Phyllite metapelites). Amphibole and white mica ⁴⁰Ar/Ar³⁹ geochronology is combined with cation exchange thermometry to constrain the T-t-D history of the subduction zone. Amphibolite facies fabrics record two deformation events at an estimated pressure of 10 kbar. The oldest amphibolite facies fabric formed at 760 ± 50 ̊C prior to 167.4 ± 1.9 Ma. A second fabric formed at 590 ± 70 ̊C prior to 164.4 ± 1.5 Ma. High-T blueschist associated with the amphibolite crystallized at 530 ± 50 ̊C and yields a white mica cooling age of 165.3 ± 1.4 Ma. A blueschist facies overprint on the amphibolite occurred at or before 162.5 ± 0.9 Ma. Post-tectonic mica in metasomatized amphibolite crystallized at 160.1 ± 1.3 Ma at 350 ± 90 ̊C. Low-T assemblages in the S₂ fabric of Shuksan blueschist and Darrington Phyllite crystallized at 320 ± 50 ̊C at approximately 7 kbar, and white mica ages span 148-136 Ma. Our results demonstrate that subduction began prior to 167 Ma, as recorded by amphibolite facies metamorphism of mafic protoliths that were underthrust beneath hanging wall ultramafic rocks. Subduction initiation was followed by cooling of amphibolite facies rocks below the hornblende closure temperature (~500˚C) by 164 Ma, and the formation of high-T blueschist prior to 165 Ma, during progressive cooling of the subduction zone. Early high-grade rocks were exhumed from ~10 to ~7 kbar and retrograded to lower-T blueschist assemblages between ~165 and 162 Ma. Temperature decreased to ≤350 ̊C by ~162 Ma and stayed approximately constant at ~320 ̊C from 148-136 Ma during underthrusting and possible exhumation of Shuksan Greenschist and Darrington Phyllite. There is a 12 m.y. gap in ages from 160 to 148 Ma between retrograded amphibolite and Shuksan/Darrington rocks that may reflect incomplete sampling or a lack of preserved accreted rocks.