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