RUTILE PRECIPITATES IN GARNET FROM A GARNET-KYANITE-STAUROLITE-GEDRITE AMPHIBOLITE FROM THE NASON TERRANE, WASHINGTON: POSSIBLE INDICATION OF EARLY HIGH-PRESSURE METAMORPHISM

A unusual amphibolite from the Nason Terrane, Washington appears to have a unique geologic and metamorphic history that does not fit well into the regional geologic history, and may instead contain indicators of high-P and/or high-T metamorphism. The amphibolite contains ~2 cm fractured garnets in a magnesiohornblende-chlorite matrix. Petrographic observations reveal abundant rutile, with minor plagioclase (≤An₃₀), biotite, epidote, and ilmenite. In the matrix, large ~4 mm euhedral amphibole-habit pseudomorphs contain chlorite and sericite remnant gedrite. Staurolite and kyanite form small inclusions in garnet. Textural relationships indicate a range of metamorphic conditions and partial overprinting. An early high pressure phase involving, minimally, gt+ru+st+ky, with later amphibolite facies gt+hb+ged+pla+bio+ru, and finally a lower amphibolite facies overprint gt+hb+chl+pla+ilm+epi. Garnets contain ∼500 µm rutile inclusions as well as ∼3x100 μm needles aligned in a <111> orientation relative to garnet. This may be the product of open system precipitation from Ti-rich garnet during exhumation, similar to what is observed in high to UHP terrains (Proyer et al., 2013; Axler & Ague 2015). EMP analyses indicate staurolite has a high X_Mg of up to ~0.40; garnet is Alm₆₃, Pyr₂₇, Grs₇, Sps₄ in the rim and Alm₅₅, Pyr₃₅, Grs₆, Sps₂ in the core. Bulk rock composition shows slightly lower SiO₂ and higher Al₂O₃ compared to other Nason Terrane amphibolites, and considerably higher MgO (~12 wt%). Chondrite-normalized REE patterns shows a strong LREE (La ∼2-4x chondrite) and modest MREE depletion, but HREE levels are comparable to other Nason Terrane amphibolites at ∼2x MORB (Magloughlin, 1993). These relationships may indicate the MORB was derived from a relatively more depleted mantle source. Discriminant analysis using four procedures places the amphibolite within or on the margin of the MORB field, similar to other Nason Terrane amphibolites. At this point, it is unclear whether this amphibolite merely records more of the tectonic history of the terrane than ‘typical’ amphibolites, indicating the Nason Terrane experienced subsequently overprinted high pressure metamorphism, or whether it records a separate tectonometamorphic history, possibly involving HP metamorphism early in its metamorphic history.