VOLCANIC STRATIGRAPHY OF THE ~2.4 MA EUCHER MOUNTAIN VOLCANO, WRANGELL ARC, ALASKA (U.S.A.)

The Wrangell arc initiated ~26 Ma due to flat-slab subduction of the Yakutat microplate and includes numerous <5 Ma volcanoes. 7 km west of Chisana, AK, Euchre Mountain is a ~2.4 Ma volcano and is truncated to the west by the southeast-striking Totschunda fault, an active right-lateral strike-slip fault. At least 20 lavas spanning ~600 m of stratigraphic thickness are exposed along the southern flank of Euchre Mountain. We collected and analyzed 10 samples (including the basal and capping lavas) from a ~500-m-thick section to characterize the exposed stratigraphy with the goal of documenting the eruptive products of the volcano. Bulk rock XRF geochemistry shows that all samples are basaltic andesite to dacite and are dominantly calc-alkalic. The stratigraphically lowest lavas are basaltic andesite and stratigraphically higher lavas are more evolved. Corresponding mineralogical variations exist; for example, olivine phenocrysts (1-2 mm) are present in the lowermost lavas and olivine becomes less prevalent upsection as orthopyroxene phenocrysts become more prominent. Some olivines have orthopyroxene rims, suggestive of reactions between olivine+melt during fractional crystallization. The samples generally lack disequilibria textures and the overall geochemistry is consistent with fractional crystallization playing a dominant role in petrogenesis. TiO₂, Sr/Y (23-33), Y (15-24 ppm), and (Sr/P)_n (2.4-3.0) of the samples place most in trend 2a of Preece and Hart (2004): calc-alkaline magmas erupted due to subduction. However, two of the basal basaltic andesites have wt. % TiO₂ >1.15, which places them into Preece and Hart (2004) Trend 1, a suite associated with local intra-arc extension. All rocks are chemically similar to products of the <5 Ma Skookum Creek Volcanic Complex, which is located ~55 km northwest on the opposite (west) side of the Totschunda Fault. Extrapolating Holocene-Quaternary slip rates along the Totschunda fault, suggest that Euchre Mountain was translated ~12-17 km southeastward away from the SCVC since ~2.4 Ma; larger amounts of displacement are possible if slip rates were faster during early Pleistocene time. Our results are consistent with tectonic models that invoke shallow-slab subduction of the Yakutat microplate beneath eastern Alaska and dextral slip along intra-arc faults.