TESTING COMPETING MODELS FOR THE FORMATION OF A SUBDUCTION-RELATED INVERTED METAMORPHIC SEQUENCE WITHIN THE EASTON METAMORPHIC SUITE, WASHINGTON

The Easton metamorphic suite in northwest Washington preserves an inverted metamorphic sequence, or metamorphic sole, related to Jurassic-Cretaceous subduction that is critical to understanding early subduction zone evolution. The inverted metamorphic sequence consists of a high-grade (630-700 C) amphibolite unit with 183 Ma Grt ages structurally above a lower grade (500-550 C) Na-amphibole schist that was metamorphosed prior to 165 Ma. Earlier models interpreted the sequence as a continuous thermal gradient formed by conductive heating of a down going slab during subduction initiation. New field and petrographic observations, however, suggest that the two units have distinct structural and metamorphic histories.

The Grt-amphibolite is associated with a map-scale ultramafic unit and contains Tlc-Tr/Act schist at the contact with associated serpentinite. The Grt-amphibolite has two fabrics defined by different populations of aligned amphibole. Poles to the dominant foliation define a shallow-moderately plunging SW fold.

The Na-amphibole schist consists of interlayered metavolcanic and metachert, and generally lacks ultramafic rocks. The Na-amphibole schist preserves a prominent S₁ foliation defined by coarse Gln-Ep-Wm. Poles to S₁ define a moderate to steeply E-plunging fold.

At the contact between the Grt-amphibolite and Na-amphibole schist, the amphibolite is overprinted by a later Gln-Chl±Ms blueschist facies assemblage. In addition, the Na-amphibole schist has a well-developed S₂ foliation parallel to the amphibolite contact. However, the deformation event associated with S₂ in the Na-amphibole schist is currently uncertain and could be related to younger events including regional folding or juxtaposition of structurally lower units.

The above observations combined with existing geochronology suggest that Grt-amphibolite and Na-amphibole schist are distinct units. We interpret the blueschist overprint on the amphibolite to have formed during juxtaposition with the Na-amphibole schist. The inverted metamorphic sequence in the Easton metamorphic suite therefore formed by progressive underplating of discrete units rather than conductive heating of a single unit. Pending garnet Lu-Hf and ⁴⁰Ar/³⁹Ar ages from the Na-amphibole schist will help further test this hypothesis.