NATIVE IRON, AWARUITE, AND POLYGONAL SERPENTINE IN VEINS CUTTING METAPERIDOTITE TECTONITES, WHITE RIVER SHEAR ZONE, WASHINGTON

Investigation into ~2-5 mm thick veins in metaperidotite from the White River Shear Zone in the North Cascade Mountains, Washington, has been undertaken to evaluate the relative roles of fluid-assisted dilatant vein growth versus brittle faulting (cataclasis, friction melting). In addition to being present within highly tectonized metaperidotite (some are ultramylonites) in a terrane-bounding shear zone, as well as cutting micro-scale ultramylonite zones, veins resemble both cataclasite and pseudotachylyte in having sharp margins, narrow alteration (damage?) zones, rare clasts of olivine and chromite, and a flow-banded-like appearance.

Veins typically have an isotropic center with chrysotile fibers radiating inward from the boundary with outer part of the vein, defined by an increase in Fe. The outer parts of the veins are more complex, commonly with relatively large antigorite, chlorite, and possibly phlogopite flakes. Awaruite is common, with an average composition of Co_0.02Fe_0.26Ni_0.71, indicating extremely reducing conditions. There are also highly unusual, opaque, extremely narrow, curving, spiral, and straight 'swaths' up to about 100 microns long of Fe₂O₃ after native iron.

XRD analysis suggests all three serpentine polymorphs are present, especially lizardite and chrysotile. TEM analysis reveals chrysotile in a lizardite groundmass, antigorite (~40 Å layering), and 30 sector povlen-type (polygonal) serpentine in fibers up to ~300 nm in diameter.

ICP-MS analysis of the immediate host rock reveals about 53% SiO₂, which is not surprising considering the presence of talc and tremolite. The vein, however, has a crudely serpentine major element composition, with an LOI of 12.6%, and is enriched in nearly all divalent cations, but very few with other valences. The chondrite-normalized REE patterns are low, mostly between 0.01 and 1.0; the host shows a positive Eu anomaly whereas the vein shows a corresponding negative anomaly.

The veins likely formed by dominantly repeated syntaxial fracturing and fluid flow with subordinate cataclastic deformation. Cu, Ba, and Li show large enrichments in the vein and may have been externally derived. Oxygen fugacity was extremely low (awaruite and native iron), the latter in wire-like habit within the chrysotile-lizardite matrix of the vein.