75-10 Mineralogy & texture analysis of a region of lenticular nephrite within the Darrington-Devils Mountain Fault Zone, Western Washington

Session: Mineralogy, Geochemistry, Petrology, and Volcanology Student Session (Posters)

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Abstract:

Characterization of texture and mineralogy within the Helena Haystack Mélange, a largely understudied Jurassic block in the NW Cascades, provides useful context for the formation conditions of ophiolitic deposits of serpentinite and nephrite. Located along Darrington-Devils Mountain Fault Zone, just South of Darrington, Washington, there is a serpentinite outcrop with lensing nephrite that is exposed along the lower portion of Helena Ridge. This research includes a rough mapping of the outcrop, as well as petrography of in situ material, hand samples, and thin sections. Two thin sections have been selected for electron microprobe analysis (EMPA) to mineralogically identify unknown minerals. The nephrite host is unprecedented in its occurrence due to the inclusion of green garnets, which are hypothesized to be Ugrandite series garnets because of their chrome green color. Textural analysis through petrography helps characterize the presence of multiple species of garnets within the nephrite. EMPA provides context for the chemistry of these green garnets, which point towards origin in a deeper facies, such as the amphibole or granulite. A possible mechanism of delivery for these garnets is from metasomatism along the accretionary wedge, resulting in their movement into the greenschist facies. Alternatively, the presence of red garnets in nephrite (which are hypothesized to be almandine or grossular) calls into question what could alter this material if they have not transitioned facies. Initial observations indicate that the red garnets would have occurred along with the original nephrite fabric and altered throughout subsequent metamorphic events. Textures within the nephrite surrounding the garnets, observed through petrographic analysis, indicate a variety of alterations that may have caused the garnets to deform as they were solidifying. Multiple textural mechanisms are visible within the nephrite, indicative of further movement of the fluid material as the garnets cooled along with the host nephrite. This implies more complex mechanisms of fluid movement visible through the laminar movement of the fluid nephrite pre-cooling that is uncommon in most other metamorphic settings and should be explored further.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

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