Geochemistry, Metamorphic Assemblages, and Microstructures in Small Ultramafic Bodies from the Northern Nason Terrane, Washington

Abstract:

Ultramafic bodies ranging from <1 to 2500 m in length occur in multiple settings across the northern part of the Nason Terrane in the North Cascade Mountains of Washington State. Within the Wenatchee Ridge Orthogneiss (WRO) the bodies are approximately equidimensional, ranging from dm-scale metasomatized lenses up to an exposed diameter of (typically) approximately 40 m. Some bodies are completely serpentinized, but others include dunite, harzburgite, and rare seams of pyroxenite. Many are rimmed by blackwall (talc, phlogopite, tremolite, chlorite, serpentine) coinciding with the Late Cretaceous metamorphism. The Napeequa Ultramafic Body (NUB), cut through and well exposed by the Napeequa River west of Lake Wenatchee, is within the White River Shear Zone (WRSZ, Magloughlin & McEwan, 1988). Though highly variable, it consists of dunite and peridotite and is variably serpentinized with common Mg-amphibole. Assemblages include serp+carb+chl+talc+opq and suggest upper greenschist to low amphibolite facies overprinting. Rare high-strain zones resemble relict pseudotachylyte veins. More common are ultramylonitic zones with olivine grain sizes of <5 microns, suggesting terrane-boundary paleostresses of >250 MPa.

The Nine Mile Creek Ultramafic Body (NMCUB) and Grave Ultramafic Body (GUB) are the largest bodies outside of the White River Shear Zone, and are approximately 300 m and 800 long, respectively. Both are characterized by ol+talc+amph along with chlorite pseudomorphs, commonly cut by <40 micron thick ribbons of calcite, and rarely containing S_i defined by chromite. These tectonites contain fine-grained olivine, but post-deformational, metamorphic cummingtonite and tremolite. In both bodies, a moderate to strong foliation is developed. It is suggested both bodies are retrogressed garnet peridotites.

An interesting problem is why the possible retrogressed garnet peridotite bodies are present south of the WRSZ and surrounded by the metatonalites of the WRO, but none are present within the WRSZ. The extreme variability of the ultramafic bodies south of the WRSZ, and their presence within the WRO pluton, suggests a possible stoping mechanism from a heterogeneous ultramafic-rich sole of the WRSZ, or a possibly now-eroded splay of that shear zone.