INSIGHTS INTO HAYES VOLCANO (ALASKA) PETROGENESIS FROM MAJOR AND TRACE ELEMENT GLASS CHEMISTRY

Hayes Volcano, the northeastern-most extent of the modern Alaskan arc, erupted two regionally distinctive magmas in at least two pulses from Late Pleistocene to Holocene. Individual glass shard data from each stage of Hayes eruptive history delineate two geochemical groups and provide evidence for diverging differentiation paths of magmas sourced from the same mantle wedge. Group 1 dacitic eruptive products are characterized by higher CaO and FeO* with adakite-like geochemical characteristics (Sr/Y = 24.8-117.5, La/Yb = 6.4 - 75.3). Group 2 materials are more evolved (up to hi-Si rhyolite) with typical calc-alkaline arc signatures (Sr/Y =1.8 - 12.7, La/Yb = 0.6 - 14.2) and lower CaO and FeO*. Despite contrasting major and trace element compositions, these groups are isotopically indistinguishable (⁸⁷Sr/⁸⁶Sr = 0.70389 ± 6, ¹⁴³Nd/¹⁴⁴Nd = 0.512894 ± 33) and consistent with magmas derived from melts of a sediment-modified mantle wedge. Though the adakitic characteristics of Group 1 materials could be attributed to slab melting, enrichments in compatible elements (Cr, Ni, Mg#) resulting from slab melt-mantle interaction are absent in these samples. The Group 1 adakite-like features are interpreted to be the result of hybridization of melts derived from underplated lower crustal metabasalt and mantle wedge sources at ~40 km depth. Variation within this group is consistent with near closed-system fractional crystallization involving amphibole and Fe-Ti oxides. Conversely, Group 2 materials are derived from mantle wedge melts that rose relatively unmodified to upper crustal levels where they underwent fractional crystallization accompanied by mixing with partial melts of mica-bearing plutons producing high-silica (71.9-78.4 wt % SiO₂) and high Ba (745- 3210 ppm) magmas. Geochemical Groups 1 and 2 are persistent throughout Hayes stratigraphy as both homogeneous and bimodal horizons. These results further demonstrate that the same volcano can erupt adakite-like magmas alongside typical calc-alkaline magmas in arcs with thickened crust.