GEOCHEMISTRY OF RELIC SPINEL AND SILICATE MINERALS FROM SERPENTINITE OF THE INGALLS OPHIOLITE COMPLEX, CENTRAL CASCADES, WASHINGTON

The Ingalls ophiolite complex, central Cascades, Washington, primarily consists of ultramafic rocks. The ultramafic rocks consists of lherzolite to the north that is separated from dunite and harzburgite to the south by mylonitic lherzolite and hornblende peridotite. This mylonitic ultramafic zone has been interpreted to be a fracture zone. Serpentinite is ubiquitous in the ultramafic rocks. Relic spinel is commonly preserved in serpentinites, with lesser pyroxene and olivine also being preserved. Relic minerals were analyzed by electron probe micro-analyzer, at the Florida Center for Analytical Electron Microscopy, to better understand the lithology and original tectonic setting of the serpentinite protoliths. Serpentinites have mesh, hourglass, and interlocking textures in thin section. Relic olivine and pyroxene have hypidiomorphic-granular textures. Spinels can be well preserved in serpentinites, or, totally replaced by magnetite. We were carful not to analyze replacement magnetite. Relic spinels (n=44) have very low SiO₂ and Fe³⁺, indicating serpentinization did not affect the data. The spinels range in composition from Al-chromite to Cr-spinel. Cr# range between 38 and 85, Mg# between 24 and 68, Al₂O₃ between 12 and 35, while TiO₂ is between 0.01 and 0.21. Spinels from two samples plot in arc fields on discriminations diagrams, while all other samples plot in arc and abyssal peridotite overlap fields. Relic pyroxene are primarily enstatite (n=13), with minor diopside (n=3) and augite (n=1). Relic olivine (n=17) are forsterite, have Fo₉₀ to Fo₉₂, NiO from 0.38 to 0.43, and plot along the olivine mantle array. Relic spinels have compositions similar to the northern lherzolite and southern dunite. They also plot near or in fields defined by the northern lherzolite and southern dunite on discrimination diagrams. Relic silicate minerals have compositions similar to the northern lherzolite. Although limited by relic minerals persevered well enough for analysis, we conclude the northern lherzolite and southern dunite were the protoliths for the serpentinite we analyzed. The original tectonic setting for these relic minerals was transitional between arc and mid-ocean ridge settings. This tectonic setting is similar to the Late Jurassic crustal rocks from this ophiolite.