TESTING MODELS OF EARLY SUBDUCTION ACCRETION IN THE EASTON METAMORPHIC SUITE, NORTHWEST CASCADES, WASHINGTON

The Easton metamorphic suite in the North Cascades of Washington preserves subduction-related metamorphic rocks that record ≥ 40 m.y. of subduction and accretion. The exhumed rocks of the Easton metamorphic suite are composed of a high- and low-temperature metamorphic sole which overlies younger regional blueschist units that include the metasedimentary Darrington phyllite and Mount Josephine semi-schist and the metavolcanic Shuksan greenschist/blueschist. Within these regional blueschist units, the relative ages of the different protoliths and the nature of the contacts between the Darrington phyllite and Shuksan greenschist/blueschist remain debated.

Regional mapping and microstructural analysis support models that interpret the Darrington phyllite as older than the Shuksan greenschist and the contact between the two units as tectonic. The Darrington has three generations of folds and fabrics, and a shallowly NE plunging regional fold orientation. The coarser unit, the Mount Josephine semi schist, has a shallowly SE plunging regional fold orientation. The Shuksan greenschist unit has two fabrics and a similar fold orientation to the semi schist unit. All fold generations in the regional blueschist units occurred at blueschist/greenschist facies. Deformation intensity in the Darrington phyllite increases towards contact with the Shuksan, and foliations are locally truncated at the contact. The phyllite/semi schist units and the greenschist unit are both folded by the same regional folding event, so their juxtaposition pre-dates folding. Thus, the different orientation of folds, increasing strain intensity towards boundaries, higher number of fabrics, and overprinting of foliations suggest that the Shuksan greenschist was accreted after the Darrington phyllite in the Easton’s early subduction history. Ongoing U-Pb geochronology will provide further insights into the protolith age and structural relationships.