PETROGENESIS OF SERPENTINITE FROM THE INGALLS OPHIOLITE COMPLEX, CENTRAL CASCADES, WASHINGTON. FURTHER EVIDENCE FOR A SUPRA-SUBDUCTION ZONE SETTING

Like the “Troodos Massif” the Ingalls ophiolite complex was originally classified geochemically as MORB. With further study, the interpretation for the Ingalls evolved into a polygenetic ophiolite with an Early Jurassic seamount structurally below a Late Jurassic supra subduction zone ophiolite. Also like the Troodos, the Ingalls has been interpreted to be cut by a fracture zone. Unlike the Troodos, the fracture zone in the Ingalls has created a large-scale mélange, with large crustal blocks within a serpentinite matrix mélange. Whole-rock and mineral geochemistry of the massive serpentinite was done to better understand the tectonic history of this ophiolite. The majority of samples analyzed are strongly serpentinized, while a few were moderately to weakly altered. Whole-rock major and trace elements of the massive serpentinite are similar to modern peridotites. Ca, Mg, and Al suggest these rocks formed from serpentinization of harzburgite and dunite with minor lherzolite. All samples have positive Eu/Eu* suggesting the hydrothermal fluids reacted with plagioclase bearing rocks. Serpentinites plot in fields defined by modern abyssal and forearc peridotites. Trace elements suggests the protoliths underwent variable amounts of mantel depletion (5–20%). Serpentine and relic igneous minerals in the serpentinites were analyzed by EPMA. The serpentine dose not chemically display brucite mixing, has minor substitution of Fe, Ni, and Cr for Mg, and minor Al and Fe³⁺ substitution for Si. Mg# of the serpentine ranges from 92 to 98. Mineral chemistry, X-ray diffraction, and Raman spectrometry suggest lizardite is the primary serpentine polymorph, with minor chrysotile also occurring. Rare heazlewoodite and cobaltpentlandite were identified in the serpentinites. Relic Al-chromite and Cr-spinel commonly have Cr-magnetite rims. Most spinels plot in overlap fields defined by abyssal and arc peridotite. Relic olivine are Fo₉₀–Fo₉₂ and plot along the mantle array. Relic pyroxene are primarily enstatite, with lesser high-Ca varieties. Relic minerals plot near fields defined by unaltered Ingalls peridotite. The massive serpentinite likely formed by low T, and possibly low pressure, hydrothermal alteration of Ingalls peridotite under reducing conditions. The serpentinite protoliths were variably depleted mantle residues from a possible supra-subduction zone setting.